Endocrine Reviews 21 (4): 363-392
Copyright © 2000 by The Endocrine Society
Role of Hormones in Pilosebaceous Unit Development
Dianne Deplewski and
Robert L. Rosenfield
Departments of Medicine and Pediatrics, The University of Chicago
Pritzker School of Medicine, Chicago, Illinois 60637-1470
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Abstract
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Androgens are required for sexual hair and sebaceous gland development.
However, pilosebaceous unit (PSU) growth and differentiation require
the interaction of androgen with numerous other biological factors. The
pattern of PSU responsiveness to androgen is determined in the embryo.
Hair follicle growth involves close reciprocal epithelial-stromal
interactions that recapitulate ontogeny; these interactions are
necessary for optimal hair growth in culture. Peroxisome
proliferator-activated receptors (PPARs) and retinoids have recently
been found to specifically affect sebaceous cell growth and
differentiation. Many other hormones such as GH, insulin-like growth
factors, insulin, glucocorticoids, estrogen, and thyroid hormone play
important roles in PSU growth and development. The biological and
endocrinological basis of PSU development and the hormonal treatment of
the PSU disorders hirsutism, acne vulgaris, and pattern alopecia are
reviewed. Improved understanding of the multiplicity of factors
involved in normal PSU growth and differentiation will be necessary to
provide optimal treatment approaches for these disorders.
- I. Introduction
- II. Embryology and Molecular Genetics of PSU Differentiation
- III. Postnatal Growth and Development of the PSU
- A. Hair follicle
- B. Sebaceous gland
- IV. Growth and Development of the PSU in Vitro
- A. Organ culture
- B. Monolayer culture
- V. Androgen Mechanism of Action in the PSU
- VI. Role of Peroxisome Proliferator-Activated Receptors in Sebocyte
Development
- VII. Retinoid Effects on the PSU
- VIII. Roles of Nonandrogenic Hormones in PSU Development
- IX. PSU Pathophysiology in Hirsutism, Acne Vulgaris, and Pattern Alopecia
- A. Hirsutism
- B. Acne vulgaris
- C. Pattern alopecia
- D. PSU sensitivity to androgen
- X. The Role of Hormonal Treatment in PSU Disorders
- XI. Conclusions
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I. Introduction
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ANDROGENS are a prerequisite for sexual hair and sebaceous
gland development (1 1A ). The importance of androgens in human hair
growth was first established by Hamilton (2), who observed that
castration before puberty prevented beard and axillary hair growth,
while castration after puberty reduced hair growth in both areas.
Furthermore, patients with androgen insensitivity typically have no
pubic or axillary hair. Androgens have been shown to increase the size
of the hair follicle, the diameter of the hair fiber, and the
proportion of time that terminal hairs spend in anagen (3). Androgens
are also important for sebaceous gland growth and differentiation as
acne vulgaris, a disorder of the sebaceous gland, has been shown to be
dependent upon the pubertal rise in androgen levels (4).
The pilosebaceous unit (PSU) consists of a piliary component and a
sebaceous component. Each PSU has the capacity to differentiate into
either a terminal hair follicle (in which a large medullated hair
becomes the prominent structure) or a sebaceous follicle (in which the
sebaceous gland becomes prominent and the hair remains vellus) (Fig. 1a
). Androgens play a key role in the
development of the PSU in most areas of the body. In androgen-sensitive
areas before puberty, the hair is vellus and the sebaceous glands are
small. In response to increasing levels of androgens, PSUs become large
terminal hair follicles (sexual hairs) in sexual hair areas or they
become sebaceous follicles (sebaceous glands) in sebaceous areas.
Androgens appear to promote sexual hair growth by recruiting a
population of PSUs to switch from producing vellus hairs to initiating
terminal hair growth. PSU disorders, namely acne vulgaris, hirsutism,
and pattern alopecia, do not occur until after the processes of puberty
begin (5). However, it is clear that the pathogenesis of these
disorders involves more than androgen (1, 3). For one thing, although
the development of acne normally parallels the rise in androgen with
pubertal progression, acne wanes in the late teenage years while blood
androgen levels remain stable. For another, PSUs respond differently to
androgen depending on their location; e.g., sexual hairs
grow only in certain areas of the body, while hairs on the scalp
undergo regression from a terminal to a vellus type in genetically
susceptible individuals. In addition, acne, hirsutism, and alopecia are
variably expressed manifestations of androgen action, and the severity
of acne or hirsutism is quite variable for a given degree of androgen
excess. Furthermore, some women will develop acne or hirsutism at
normal levels of androgen (idiopathic acne or hirsutism), while at the
other extreme some women will have no manifestations of androgen excess
(cryptic hyperandrogenemia). All of these considerations indicate that
factors other than androgen play major roles in PSU development and in
PSU disorders.
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Figure 1. Role of androgen in the development of the
pilosebaceous unit. Solid lines indicate effects of
androgens; dotted lines indicate effects of antiandrogens.
Hairs are depicted only in the anagen (growing) phase of the growth
cycle. In balding scalp (bracketed area), terminal hairs not
previously dependent on androgen regress to vellus hairs under the
influence of androgen. [Reprinted with permission from R. L.
Rosenfield and D. Deplewski: Am J Med 98:80S–88S, 1995
(1A ) © Excerpta Medica Inc.]
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II. Embryology and Molecular Genetics of PSU Differentiation
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PSU differentiation occurs in the embryo between 2 and 4 months
gestation and requires precisely timed and localized interactions
between the fetal epidermis and dermis (3, 6). Each PSU acquires an
intrinsically determined morphology and pattern of behavior during its
development, which may be modulated by hormones (7). The difference in
the apparent density of sexual hair between men and women is due to a
different density of terminal hairs rather than a difference in the
number of PSUs, which is established before birth (8). Studies of
glucose-6-phosphate dehydrogenase mosaicism have demonstrated that
hairs originate as a clone from a pool of about five primitive
epidermal cells (9). The respective roles of the epidermis and dermis
in PSU formation have been elicited with tissue recombination
experiments by which the epidermis and dermis are separated, and then
the epidermis and dermis of different ages, locations, and species are
combined and the formed appendages are studied (10).
PSU differentiation begins with formation of a dermal mesenchymal
condensation that sends a signal to the overlying embryonic epithelium
to "make an appendage here" (Fig. 2)
(6, 11). This results in downward growth of an epidermal plug to form a
skin appendage (6, 7). This initial message from the dermis to
epidermis is common to all classes of vertebrates. The epidermis
determines the type of appendage, directs its cephalo-caudal polarity,
and determines species specificity of keratin composition. For example,
mouse dermis can instruct the development of feather follicles in chick
epidermis. Some authors postulate that the epidermis sends the first
signal to the dermis and is responsible for the patterning of skin
appendages (12). Lymphoid-enhancing factor-1 (LEF-1), a DNA binding
molecule that acts by bringing together other DNA-bound transcription
factors, is expressed in the epidermis just before the formation of the
dermal mesenchymal condensations. Altering the expression of LEF-1 in
transgenic mice results in abnormal hair follicle distribution and
orientation (13, 14).
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Figure 2. Embryonic development of the pilosebaceous unit.
The stages shown correspond to the respective stages at which (a)
mesenchymal cells signal the overlying epithelium to initiate follicle
differentiation; (b) the epithelium signals the mesenchyme to form a
dermal papilla; and (c) the dermal papilla then signals for formation
of the pilosebaceous unit. [Adapted with permission from R. L.
Rosenfield and D. Deplewski: Am J Med 98:805–885, 1995 (1A )
© Excerpta Medica Inc.]
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After the initial signal, the differentiating epithelium of the hair
plug then sends a less well defined but species-specific signal back to
the mesenchyme to "make a dermal papilla" (3). The dermal papilla
subsequently sends a message back to the adjacent epidermal placode to
"make a PSU." This message is species specific and cannot be
interpreted by epithelial cells from other classes of vertebrates. In
response, the PSU forms a hair bulb, a bulge region [site of
attachment of the arrector pili muscle and presumptive location and
source of stem cells that support regrowth of the follicle at the
beginning of anagen (6)], and a sebaceous gland. Then the rapidly
proliferating matrix cells at the base of the bulb grow rapidly
downward, giving rise to all the inner layers of the hair.
The cells in the sebaceous anlagen are identical to those in the basal
layer of the epidermis and the piliary canal. Most sebaceous glands
arise in a cephalo-caudal sequence from hair follicles (15). The future
common excretory duct, around which the acini of the sebaceous gland
attach, begins as a solid cord. The cells composing the cord are filled
with sebum, and eventually they lose their integrity, rupture, and form
a channel that establishes the first pilosebaceous canal. Fetal
sebaceous cells are quite large and functional and probably contribute
to vernix caseosa.
Epithelial and mesenchymal cells appear to communicate during
morphogenesis, and these interactions seem to involve molecules or
"morphogens" that play a regulatory role in development. Likely
morphogens include growth factors, cell adhesion molecules,
extracellular matrix molecules, intracellular signaling molecules such
as ß-catenin and LEF-1, hormones, cytokines, enzymes and retinoids,
together with their receptors (16, 17). Growth factors such as
epidermal growth factor (EGF), transforming growth factor 
(TGF),
transforming growth factor ß (TGFß) and fibroblast growth factor
(FGF) affect the proliferation and differentiation of the cells of the
PSU during development (18). These growth factors appear to exert their
effects via autocrine or paracrine pathways between cell types. EGF was
the first growth factor to be implicated in hair development when it
was shown that its administration to newborn mice delayed hair follicle
development (19), and this effect occurred over the entire coat.
Furthermore, growth of the first coat of hair in newborn mice is
accelerated by the administration of antibodies to EGF (20). The EGF
peptide has been found in the outer root sheath and sebaceous gland in
later stages of follicular development in sheep skin (21). The EGF
receptor has been found in embryonic skin by autoradiography and
immunohistochemistry; however, it is present in the adjacent
interfollicular epidermis rather than the placode and hair germ (22, 23). In later development, the EGF receptors are expressed in the outer
root sheath and sebaceous epithelium, and in some species in the hair
bulb, but no EGF receptors have been demonstrated in the dermal papilla
(3). The specific distribution of EGF in skin and throughout follicle
morphogenesis suggests that this growth factor has a more important
role in differentiation than in proliferation (18). TGF, which is in
the EGF family and binds to the same receptor as EGF, has also been
found to inhibit murine hair growth (24). Several members of the TGFß
family (TGFß-1, ß-2, ß-3, bone morphogenetic protein-2, and bone
morphogenetic protein-4) have also been localized to various regions of
the developing PSU using in situ hybridization (25, 26). FGF
was also found to affect hair follicle initiation and development, but
the effects were confined to the area of treatment since FGF is not
readily diffusible in the skin (18). The FGF receptor 2 is likely to be
important in sebaceous gland development in humans, as a somatic
activating mutation of this receptor has been associated with localized
acne (27).
Cell adhesion molecules such as the cadherins, neural cell adhesion
molecule (N-CAM), intercellular cell adhesion molecule (I-CAM), and
tenascin are also thought to play a prominent role in PSU
differentiation. Both E-cadherin and P-cadherin have been detected in
developing follicles by immunohistochemistry (28). Whereas P-cadherin
is expressed throughout the epithelium, E-cadherin is confined to cells
in the presumptive matrix region. In studies of cultured lip skin, the
addition of antibodies against E-cadherin and P-cadherin caused
disruption of follicular development, and dispersal of the mesenchymal
aggregate (3). Although both embryonic and fetal keratinocytes express
E-cadherin, only embryonic keratinocytes express N-CAM, which is
localized in the initial mesenchymal aggregate (6); N-CAM is probably
important in cell adhesion and furthering cell aggregation. It is found
in the dermal papilla and dermal sheath in the adult PSU (6). I-CAM is
transiently expressed in the outer layer of the follicular cells,
perhaps as a result of a signal from the condensing mesenchymal cells
(6). Tenascin, an extracellular matrix protein, has been found to be
expressed in the basement membrane underneath the hair germ, but not in
the basement membrane between follicles (6). Tenascin is considered to
be a marker for epithelial-mesenchymal interactions, but the exact
function it plays in PSU development is not known.
Another molecule that may be important for PSU differentiation is
epimorphin, which is a mesenchymal signal factor. Epimorphin is found
in mesenchymal aggregates in embryonic rat skin and lung and may
function in aggregative behavior of immature cells (29). Studies have
shown that epimorphin can be detected in cell suspensions that have
been aggregated by centrifugation, whereas it is not present in the
same cells grown in monolayer. Other studies have shown that hair
follicles fail to develop in embryonic skin cells cultured in the
presence of antibodies to epimorphin (3). Other morphogens such as the
wingless homolog Wnt and sonic hedgehog also seem important for the
development and pattern of hair follicles (17, 30, 31).
Although more is being learned about the various molecules involved in
cell-cell interaction within the PSU, these cells must be organized in
a precise spatial and temporal order for proper function. The overall
complexity of PSU morphogenesis indicates the involvement of multiple
genes in a coordinated fashion, which suggests a role for homeobox
(HOX) genes. HOX genes have been found to control the developmental
fate of embryonic cells by encoding regulatory transcription factors
that either induce or repress effector genes, which in turn are
responsible for the position and development of each particular cell
(32, 33).
The HOX genes are aligned in tandem as clusters arranged in a colinear
fashion on four different chromosomes (Fig. 3). They are transcribed in
"lock-step" with the first set of HOX genes being expressed
anteriorly (34). In humans, 39 HOX genes have been identified (33). The
HOX genes contain a homeobox, which is a highly conserved 180-bp DNA
sequence. Point mutations within the homeobox were discovered to be the
cause of "homeotic" malformations in fruit flies, i.e.,
malformations in which one body part develops looking like another. The
HOX genes encode monomer proteins with three -helices, with the
second and third helices being arranged in a helix-turn-helix
configuration. The homeobox encodes the highly conserved homeodomain,
which is thought to bind to specific areas of DNA of both HOX and
non-HOX genes to regulate transcription.
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Figure 3. Homeobox (HOX) gene clusters. The four sets of
homeobox genes are organized in tandem on four different chromosomes.
Previous nomenclature of the HOX genes is shown in
parentheses, and the chromosomal location in mice is
likewise shown in parentheses. The genes are numbered
according to their anterior-posterior sequence and are expressed in a
"lock-step" manner with genes in the 3'-end of the clusters being
transcribed earlier in embryonic development than genes in the 5'-ends
of the clusters. Anticipated HOX genes are represented by
unnumbered boxes. HOX genes thought to be important for
PSU morphogenesis include A4, A5, C4, C6, C8, and D4. [Adapted with
permission from R. L. Rosenfield and D. Deplewski: Am J Med
98:805–885, 1995 (1A ) © Excerpta Medica Inc.]
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HOX genes appear to be important in PSU morphogenesis. Chuong et
al. (35) demonstrated HOX gene expression in the chick feather
bud, which is an analog of the developing hair follicle. They showed
that the HOX C6 and D4 genes are expressed in a pattern that is
position specific (strongest expression in the anterior-proximal region
of skin appendages) and that a homeoprotein gradient existed within the
feather buds. Retinoic acid (RA) disrupted the normal pattern of the
expression of these HOX genes. Bieberich et al. (36) studied
the expression of HOX genes in murine hair development. They showed
that the HOX C8 gene was expressed in skin in an ascending gradient
from anterior to posterior. They also linked a HOX C8 clone to a
ß-galactosidase gene and demonstrated localization of this gene to
the dermal papillae of anagen hair follicles. Recently, Stelnicki
et al. (37) studied the expression of HOX genes during human
fetal skin development. The HOX genes appeared to be expressed in a
relatively conserved temporal and spatial pattern in developing skin
and hair follicles. HOX A4 gene expression was found in both developing
hair follicle (in the epidermal layer) and in sebaceous glands. In
developing hair follicles, HOX C4 expression was also detected in the
epidermal layer, while HOX A5 expression was limited to the inner root
sheath.
Retinoic acid also plays an important role in PSU morphogenesis (see
below), and the effects of retinoic acid on PSU development may be
partly due to regulation of the pattern of expression of HOX genes by
retinoic acid. Retinoic acid excess during a critical stage of mouse
embryogenesis has been shown to cause abnormal development of hair
follicles between the follicle peg and the bulbous follicle peg stage
(38). Furthermore, retinoic acid stimulates sebaceous gland development
and causes formation of a metaplastic branching tubular duct system
from the developing follicle. Since retinoic acid alters the pattern of
expression of HOX genes (34, 39), differential retinoic acid action on
HOX gene expression within the PSU during embryogenesis may play a role
in the subdivision of the PSU into its separate hair follicle and
sebaceous gland components.
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III. Postnatal Growth and Development of the PSU
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A. Hair follicle
The hair follicle is composed of epithelial components (the
matrix, medulla, inner root sheath, cortex, cuticle, and outer root
sheath) and dermal components (the dermal papilla and connective tissue
sheath) (40). During embryogenesis, the dermal papilla, upper outer
root sheath [including the bulge area where hair follicle stem cells
are thought to reside (3, 41)], and sebaceous gland are permanently
established (11). Postnatally, the remainder of the follicle undergoes
repetitive cycles of growth that recapitulate embryogenesis (1, 11, 42). Hair grows cyclically by passing from telogen (resting), to anagen
(growth), and through the phase of catagen (shortening), back to the
telogen phase to begin a new cycle. (Fig. 4). The PSU at the telogen-anagen
transitional phase morphologically resembles the embryonic bulbous hair
peg. The dynamics of the hair growth cycle vary between species,
between different body sites in the same species, and between different
follicle types in the same body site (3). It is likely that hair
follicles have an intrinsic rhythmic behavior that is modulated by
systemic factors (3, 16). In man, these follicle cycles occur
independently, for the most part, with a superimposed modest summertime
peak of sexual and scalp hair growth, which may reflect changes in
androgen levels (42). The duration of anagen is the major determinant
of the length to which a hair grows, and it varies with the location of
the hair follicle. Mustache hairs grow for approximately 4 months and
scalp hairs for 3 yr. In contrast, the percentage of time scalp and
beard hairs spend in anagen only differs by about one-third. Other
factors influencing the amount of hair growth in various areas of the
body include the linear growth rate of the hair fiber, as well as the
diameter and density of the terminal hairs. Whereas shaving does not
induce hair growth, plucking a resting (telogen) hair causes an
advancement in the onset of anagen and thus induces hair regrowth (3, 43).
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Figure 4. The hair growth cycle. Hair follicles progress
through repetitive cycles of growth, from anagen (active phase of
growth which is the longest phase in the hair cycle), through catagen
(shortening of the hair follicle), to telogen (resting phase of the
hair cycle), after which the club hair is shed, and the follicle begins
a new hair cycle again.
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The postnatal hair follicle appears to retain the capability for
reciprocal interactions between hair epithelial cells and dermal
papilla cells, similar to the embryonic hair follicle. The hair growth
cycle is under the ultimate control of the dermal papilla. Studies by
Oliver and his colleagues (7, 44) demonstrated that the dermal papilla
must be present for regeneration of the hair follicle, and that cells
from the dermal sheath serve as a source of a new papilla. In a classic
series of experiments, he showed that removal of the dermal papilla
from the base of a rat whisker (vibrissa) caused cessation of hair
follicle growth, and reimplantation of dermal papilla stimulated the
growth of generations of whiskers. Dermal papilla cells were also
capable of inducing follicle formation when implanted elsewhere, with
the hair type specificity being determined by the source of the dermal
papilla. Cultured papilla cells from early passages retained the
capacity to induce the differentiated growth of the hair follicle (45).
Although the dermal papilla is a key factor in controlling the hair
follicle growth, other components of the hair follicle also play a
role. If only the dermal papilla is removed, after a lag time, a new
dermal papilla will eventually regenerate and induce growth of normal
whiskers. If the lower third of the hair follicle is removed (leaving
the dermal sheath intact), the dermal papilla will regenerate, but the
whiskers that grow will be proportional to the length of follicle used
for the regeneration experiment. If more than the lower third of the
follicle is removed including the portion of the follicle about the
sebaceous gland and its outlet, the dermal papilla will not regenerate
(44). Recently, Reynolds et al. (46) demonstrated that the
transplantation of a few hundred cells from human dermal-sheath tissue
from the scalp of an adult male into the skin of a genetically
unrelated female induced the formation of a new dermal papilla and hair
follicle.
There is evidence that PSU pluripotential stem cells reside in the
bulge area of the outer root sheath, just beneath the sebaceous duct,
and are capable of repopulating the hair matrix to the point where it
will recapitulate ontogeny by reinducing the dermal papilla (47). This
appears to be why the portion of the follicle about the sebaceous gland
outlet is important for hair regeneration (48). Basal cells of the
bulge form an outgrowth pointing away from the hair shaft and are
therefore safeguarded against accidental loss due to plucking (41).
Changes take place in the dermal papilla during the hair growth cycle
in terms of cell morphology, vascularization, and in composition and
volume of the extracellular matrix (3). During late telogen the dermal
papilla is pulled upward toward the bulge area. If the dermal papilla
fails to ascend upward toward the bulge area during this phase, the
follicle stops cycling and the hair is lost (49). This was deduced
because mutations of the hairless gene, which encodes a transcription
factor important for movement of the dermal papilla to the bulge area,
results in permanent alopecia (50, 51). Stem cells of the bulge area
are thought to be activated by dermal papilla cells, to which they
respond by proliferating and growing down to push the dermal papilla
away (41). Once the dermal papilla is pushed away, the bulge area
becomes quiescent again. During anagen, the dermal papilla enlarges and
develops an extensive extracellular matrix (3). At a given time, the
anagen follicle receives a signal that terminates this phase and
initiates catagen (see below). The catagen (regression) phase involves
apoptosis (16, 52), which is associated with a decrease in volume of
the extracellular matrix. In telogen the dermal papilla becomes a
condensed ball of cells with almost nonexistent extracellular matrix
located immediately below the lower pole of the follicular epithelium
(3). There is a decline, and eventual cessation of mitotic activity in
the extracellular matrix during telogen, and the matrix cells adjacent
to the dermal papilla convert to lower outer root sheath cells. The
hair becomes a club hair, which is eventually shed from the follicle to
make room for new hair growth. The extracellular matrix becomes
organized again around the papilla at the start of the next hair growth
cycle (7). The dermal papilla has its own blood supply, and the
capillary loops present in the dermal papilla in anagen are lost in
telogen (3).
Multiple growth factors are ultimately involved with hair follicle
growth and normal cycling including insulin-like growth factor-I
(IGF-I), FGF-7 (also known as keratinocyte growth factor), FGF-5, and
EGF. IGF expression is stimulated by androgen in dermal papilla cells,
and IGF-I has been demonstrated to stimulate hair follicle growth
in vitro (53). This suggests that some of the trophic
effects of androgens on the hair follicle are mediated through growth
factors such as IGF. IGF-I has also been shown to slow hair follicle
entry into the catagen phase, which suggests that it is an important
factor in control of the hair growth cycle (54). FGF-7 production has
been found in the dermal papilla, and its receptor has been found in
nearby matrix cells (55). When the FGF-7 receptor is disrupted in mice,
the morphology of the hair follicles is abnormal and there are 60–80%
fewer hair follicles present than in control mice (56). FGF-5 knock-out
mice and mice with nonfunctional EGF receptors have long, fine
angora-like hairs, due to an extended anagen phase, which suggests that
these growth factors are potential signals that cause anagen to
terminate and catagen to begin (43, 57, 58).
Hair grows through keratinocyte cell division, which takes place in the
hair bulb close to the dermal papilla. The cells differentiate to form
the various layers as they move up the follicle (59). Hair can thus be
considered to be the holocrine secretion of the hair bulb. The mature
hair consists of medulla, cortex, cuticle, inner root sheath (three
layers), and outer root sheath. As matrix cells divide, they form
keratin microfibrils, which mature in daughter cells in the upper bulb.
At this point the keratin is 46–58 kDa in size. The hair shaft comes
to consist essentially of solid packages of hard keratin fibrils
embedded in an amorphous matrix. Accompanying this terminal
differentiation of hair is the formation of larger keratins (53 and 63
kDa) in the shaft.
Keratins comprise more than 90% of hair proteins. Keratins are a group
of water-insoluble, cystine-containing proteins. Each hair fibril
consists of a bundle of coiled, low-sulfur keratins, which are in turn
bundled in an -helical pattern forming a coiled coil. The amorphous
matrix into which these bundles are imbedded contains high-sulfur,
lower molecular mass keratins. The molecular and biochemical
basis for the ultrastructural differences among the layers of hair is
unknown. Unlike scalp hairs, sexual hairs are curled around their axes.
Racial differences also affect such diverse features of hair as shape
and medullation. The concept of "donor dominance" was elucidated in
classic hair transplant studies (60), which indicated that these
structural differences were inherent in the PSU, i.e. hairs
retain the characteristics of their area of origin.
Before puberty, the androgen-dependent PSU consists of a prepubertal
vellus follicle, which consists of a virtually invisible hair and a
tiny sebaceous gland component (Fig. 1
) (1A ). Under the influence of
pubertal amounts of androgens, PSUs in sexual hair areas differentiate
in a distinctive pattern which depends on their location. Sexual hair
development is normally not seen before age 9 in girls (average age of
stage 3 sexual hair development, 12 yr) and age 10 in boys (average age
of stage 3 sexual hair development, 13 yr) (61). In the sexual hair
areas, a terminal hair follicle develops and the sebaceous gland
develops only moderately. In the balding-prone area of scalp, PSUs
respond to androgen in yet a different manner in individuals
predisposed to pattern alopecia. Terminal hair follicles that
previously grew without androgen gradually change with each growth
cycle to an intermediate kind of follicle in which the hair component
reverts to the vellus state, leaving an adult vellus follicle (Fig. 1).
These phenomena are reversed by antiandrogens: both types of
androgen-dependent PSUs revert toward the prepubertal state.
The most direct evidence that androgens are the principal hormones
controlling sexual hair growth is that androgens stimulate hair growth
in eunuchs and castration reduces it. The latter classic observation
illustrates the plastic nature of the PSU response to androgens,
i.e., the reversion from terminal to vellus follicles. The
sensitivity of sexual hair follicles to androgen is determined by their
pattern of distribution (1A ) and generally wanes from pubis to head
(Fig. 5), or from posterior to anterior
considering the embryogenesis of these PSUs. Thus, rising androgen
levels (such as occur either normally during puberty or abnormally in
hyperandrogenic states) recruit an increasing proportion of PSUs in a
given area to initiate the growth of terminal hair follicles, each in
accordance with its preset genetic sensitivity to androgen. The
apparent dose-response curve to androgen is fairly steep, with a
mustache typically appearing at plasma testosterone levels just
slightly above the upper limits of normal for women and the beard
requiring 10-fold higher levels for full growth. There is considerable
individual variability.
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Figure 5. Relationship of stages of sexual hair development
to testosterone as a representative plasma androgen. Note logarithmic
scale for testosterone. A, Prepubertal; B, stage 3 pubic hair; C, stage
5 pubic hair; D, moderate hirsutism; E adult male. [Reprinted with
permission from R. L. Rosenfield: Clin Endocrinol Metab
15:341–362, 1986 (1 ) © W. B. Saunders Co.]
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B. Sebaceous gland
The sebaceous gland is composed of acini, which are attached to a
common excretory duct composed of cornifying, stratified squamous
epithelium that is continuous with the wall of the piliary canal and,
indirectly, with the surface of the epidermis (15). The life cycle of
sebaceous cells (sebocytes) begins at the periphery of the gland in the
highly mitotic basal layer. As sebaceous cells differentiate, they
accumulate increasing amounts of lipid and migrate toward the central
duct. Eventually, the most mature sebocytes burst and their lipid is
extruded into the ducts of the sebaceous gland as the holocrine
secretion sebum (1, 15). Sebaceous lipid is different from other skin
surface lipid in that it is composed of 12% squalene and 26% wax
esters in addition to the cholesterol, cholesterol esters, and
triglyceride common to both kinds of epithelial secretions (1). The
cells of sebaceous glands turn over more rapidly than those of hairs,
as they are normally completely renewed every month (62). The sebaceous
gland is thought to play an active role in processing of the sheath of
terminal hair shafts. The shaft does not separate normally from the
sheath in the absence of the sebaceous gland (63). The exact nature of
this component is not known.
In acne-prone areas, androgen causes the prepubertal vellus follicle to
develop into a sebaceous follicle in which the hair remains vellus and
the sebaceous gland enlarges tremendously. The sensitivity of sebaceous
glands to androgens seems to follow a different dose-response curve
than the hair follicle, with most sebaceous glands being highly and
similarly sensitive to testosterone. Sebum production is at its nadir
at about 4 yr of age and begins to increase at about 8 yr of age.
Microcomedones (1 mm or less in diameter), which form when desquamated
cornified cells of the upper canal of the sebaceous follicle become
exceptionally adherent and form a plug in the follicular canal, make
their appearance in about 40% of 8–10 yr olds. Thus, sebaceous gland
function begins before true puberty, at levels of testosterone below
those ordinarily required for the initiation of pubic hair growth (Fig. 6). This development corresponds with
adrenarche, the "adrenal puberty" marked by increasing production
of the adrenal androgen dehydroepiandrosterone sulfate (DHEA-sulfate).
Seventy-five percent of the normal male amount of sebaceous gland
function is achieved at androgen levels normal for women. As for hair
growth, there is considerable individual variability in the degree of
sebum production to a given level of androgen. Although the apparent
dose-response curves above are given in terms of the major circulating
form of androgen, testosterone, other plasma androgens contribute to a
greater or lesser extent, as will be discussed.
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Figure 6. Relationship between sebum output and testosterone
as a representative plasma androgen. Note logarithmic scale for
testosterone. Dotted lines show the normal range of
sebum excretion. A, 4 yr-old children, computed from data on
composition of sebum assuming epidermal lipid secretion rate of 10
µg/cm2/3 h; B, 7- to 11-yr-old prepubertal children; C,
castrated men; D, normal adult women, 20–40 yr of age; E, normal adult
men, 20–40 yr of age; *, average sebum level of normal 15-
to 19 yr-old boys and girls. [Reprinted with permission from R. L.
Rosenfield: Clin Endocrinol Metab 15:341–362, 1986 (1 ) ©
W. B. Saunders Co.]
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Retinoids antagonize the effects of androgen on the sebaceous gland.
They appear to inhibit the proliferation and differentiation of
sebocytes. This results in atrophy of sebaceous glands and decreased
sebum production in man (64, 65, 66, 67) and animals (68, 69, 70).
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IV. Growth and Development of the PSU in Vitro
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Stromal-epithelial interaction is an important feature of the
growth and differentiation of the epithelial cells of the skin and its
PSU appendages in vitro just as it is in vivo.
This is like the situation in other glands that are targets of sex
steroid action (71).
A. Organ culture
Organ culture has permitted the short-term study of growth and
development of hair follicles and sebaceous glands in vitro
without disturbing the natural close relationship between the stromal
and epithelial components of these structures (40, 67). Human hair
follicles isolated by microdissection have less stringent requirements
for maintenance and growth in culture than sebaceous glands (40).
Cortisol and insulin are necessary for optimal growth (72). Hair
follicles can be maintained in short-term organ culture while
maintaining their in situ morphology and growth at a normal
rate of about 0.3 mm/day (72, 73, 74). By day 14 in culture, the dermal
papilla rounds up and the follicle no longer produces a
keratinized hair fiber.
Human sebaceous glands isolated by microdissection can be maintained
for up to 7 days in organ culture with full retention of their in
situ morphology, rates of lipogenesis, and responses to steroid
hormones (72, 75). Although they continue to form new cells at a normal
rate until 14 days of culture, they do not differentiate normally after
7 days in culture unless phenol-red, an estrogen, is removed from the
medium (67, 75). Insulin, cortisol, T3, and
bovine pituitary extract are required for optimal maintenance of the
human sebaceous gland in organ culture (72).
B. Monolayer culture
Monolayer culture has been used to study the specific factors
involved in the growth and development of hair and sebaceous epithelial
cells. However, monolayer culture is known to be incompatible with the
normal differentiation of skin epithelial cells (76). When epidermal
cells are grown in monolayer, they progress almost directly from basal
to a thin squamous cell layer without the intervening cell stages. On
the other hand, when epidermal cells are grown on an artificial dermis
(composed of 3T3 fibroblasts in a collagen lattice) lifted on a raft to
the air-liquid interface, development is virtually normal. The normal
balance between epidermal growth and differentiation in the lifted raft
system has been attributed to a retinoic acid gradient being
established at the epidermal-dermal junction (77). The nanomolar
concentration of retinoic acid in FCS inhibits normal orderly cell
maturation and the biochemical changes characteristic of terminal
differentiation in a submerged raft system. When rafts are lifted to
the surface of the medium, the level of retinoic acid falls in the
suprabasal layers, so differentiation progresses normally. If, however,
the retinoic acid concentration in the medium is raised in the lifted
raft system, epidermal differentiation becomes disturbed like that in
monolayer culture (78).
Hair and sebaceous epithelial cells are grown in epidermal type
monolayer culture systems. Traditionally, this requires maintaining
them in close contact with a stromal feeder layer consisting of
3T3-fibroblasts. For epidermal cells, the stromal growth factors have
been identified and the requirements for growth in culture simplified:
a collagen matrix or fibronectin are necessary for good plating
efficiency, and insulin or IGF-I plus keratinocyte growth factor
(FGF-7) are necessary for growth (79, 80). Hair and sebaceous epithelia
have more stringent requirements for growth in primary monolayer
culture. For sustained proliferation, most systems require a stromal
support system and medium containing insulin, hydrocortisone, and
cAMP-amplifying agent such as choleratoxin (72, 73, 81). Stromal
support systems include 3T3 cells, nitrocellulose filters (72, 74), 3T3
cells mixed with collagen (76), and gelatin sponge supports (73).
Although most previous studies have been done in the presence of serum,
serum has been found to inhibit growth of hair follicles and to inhibit
differentiation of sebaceous cells in culture (40, 73, 82). This may be
due, in part, to inhibitory factors within serum such as TGFß (73).
Only recently have chemically defined serum-free media become available
that support growth of hair and epithelial cells.
Hair and sebaceous epithelial cells form typical polyhedral epithelial
cell colonies in culture, which resemble epidermal epithelial cell
colonies by light microscopy. Nevertheless, they can be identified as
unique epithelial cell populations by a variety of techniques. For
example, early-passage cells cultured from plucked anagen hairs have
the characteristics of outer root sheath cells according to
ultrastructural analysis and the pattern of expression of hair proteins
(83). They also have a pattern of testosterone metabolism that favors
androgen action (high ratio of 5-reductase to
17ß-hydroxysteroid dehydrogenase activities) compared with
epidermal cells (84). Dermal papilla cells, which themselves have a
distinctive profile in culture (85), are the only type of stroma known
to support the growth and early differentiation of putative hair germ
cells from epidermis (86). Fujie et al. (87) reported
recently that, by using specific growth medium containing bovine
pituitary extract, cells derived from human sebaceous glands could be
maintained in primary culture and serially cultured under serum free
conditions, without a biological feeder layer or specific matrices.
This effect was demonstrated in both explant culture and dispersed cell
culture. Sebocytes obtained from outgrowths (explants) from the
periphery of the gland lobules (88, 89) can be passaged twice in
monolayer culture without a stromal support system before rates of
sebocyte proliferation fall (72). Proliferation of these cells in
vitro has been found to depend inversely on the age of the donor
and also on the specific body site where the skin was isolated (88).
Recently, Zouboulis et al. (90) developed an aneuploid
immortalized human sebaceous gland cell line that maintains the
morphological and functional characteristics of normal differentiated
human sebocytes in the absence of a stromal matrix.
Our studies of sebocyte growth and development have used rat preputial
sebocytes. The preputial glands of the rat are located on either side
of the penis in the male, and the clitoris in the female. The
secretions of the preputial gland are thought to play a role in both
territorial marking and mating behavior. The preputial gland has been
an attractive source of cells for the study of sebaceous cell growth
and differentiation as the paired glands are easy to isolate because
they are large and encapsulated, they are available on demand, and
single cell suspensions at all stages of differentiation can be
prepared for study (91). Although the preputial gland may have
physiological functions beyond that of the human sebaceous glands, the
gland is a holocrine organ, and preputial sebocytes resemble human
sebocytes in many ways (81, 92, 93). Single cell suspensions are
prepared from isolated preputial glands and plated on a mitomycin-C
treated 3T3-J2 feeder layer. After attachment, sebocytes are cultured
in a serum-free, chemically defined cell culture medium that permits
definition of the factors regulating sebocyte proliferation and
differentiation (82).
These sebaceous cells have been shown to exhibit a number of
differentiation characteristics in monolayer culture that are similar
to those of human sebocytes and distinguish them from epidermal cells.
Sebocytes form relatively slow-growing colonies (81, 94). They contain
a variety of keratins, including cytokeratin K4, which is localized to
suprabasal sebocytes and is constitutively expressed in culture (81, 88). Sebocytes also differ from epidermal cells by forming few
cornified envelopes in culture, as in vivo (Fig. 7, top panel) (81, 94). They
respond to ß-adrenergic treatment in vitro, as in
vivo, with a distinctive pattern of cAMP-regulatory subunit
predominance (95, 96). A striking difference between the behavior of
sebocytic and epidermal keratinocytes in culture is in their
differential response to the administration of
all-trans-retinoic acid (1A, 97). Retinoic acid causes
dose-dependent inhibition of sebocyte proliferation but does not have
an effect on epidermal cell growth (Fig. 7, bottom panel).
In contrast to its inhibitory effect on sebocytes, however, retinoic
acid appears to maintain the PSU duct (98). Sebocytes form
sebum-specific lipids such as squalene and wax esters (88). Although
fatty acid synthesis is greater in cultured sebocytes than in cultured
epidermal cells (88, 94), the amount of lipid is not enough to clearly
distinguish them from epidermal cells by light microscopy (99).
Electron microscopy reveals that sebocytes in monolayer culture form
abundant tiny lipid droplets, but they undergo only abortive
differentiation, with coalescence of these droplets in only a very few
cells at the center of colonies (Fig. 8) (1A ).
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Figure 7. Retinoic acid (RA) effects on preputial sebocytes
grown in monolayer culture. Top panel, Epidermal cells
develop more cornified envelopes in culture than sebocytes, and this
development is inhibited by all-trans-RA. Bottom
panel, All-trans-RA inhibits the proliferation
of cultured sebocytes in a dose-related fashion but does not affect the
growth of cultured epidermal cells. P values by Tukey’s
test after two-way ANOVA. [Reprinted with permission from R. L.
Rosenfield and D. Deplewski: Am J Med 98:80S–88S, 1995 (1A )
© Excerpta Medica Inc.]
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Figure 8. Electron photomicrographs of preputial sebocytes
in monolayer culture. Top, Typical early differentiated
preputial cell. The cytoplasm is filled with much smooth
endoplasmic reticulum, a few cisternae of rough endoplasmic reticulum,
mitochondria (M), and small lipid droplets (LD). Several inclusions are
present in the nucleus (N). Scale bar, 2 µm.
Bottom, Maturing preputial cell near center of colony.
The cytoplasm contains coalescing lipid droplets (LD), smooth (SER) and
rough (RER) endoplasmic reticulum, mitochondria (M), and occasional
lysosomes (LY). N, Nucleus. Scale bar, 1 µm.
[Reprinted with permission from R. L. Rosenfield and D. Deplewski:
Am J Med 98:80S–88S, 1995 (1A ) © Excerpta Medica
Inc.]
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V. Androgen Mechanism of Action in the PSU
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Skin is a major site of testosterone formation in women, in whom
half of testosterone production is derived from peripheral conversion
of secreted 17-ketosteroids such as DHEA, DHEA-sulfate, and
androstenedione (5). Although the skin is not capable of de
novo synthesis of androgens from cholesterol, it contains all the
enzymes necessary to convert the prohormones DHEA and androstenedione
into testosterone and the most potent androgen, dihydrotestosterone
(DHT) (1, 100, 101, 102). Serum levels of DHEA-sulfate have been found to
correlate with sebum production in early puberty (103) and with the
presence of acne vulgaris in prepubertal girls (4, 104). The metabolic
pathway involved in forming active androgens from DHEA-sulfate is
illustrated in Fig. 9.
The androgen-sensitive skin appendages (sweat gland, hair follicle, and
sebaceous gland) each metabolize androgens in a characteristic pattern;
however, sweat glands and sebaceous glands account for the vast
majority of androgen metabolism in skin (Table 1) (105, 106, 107, 108). 3ß-Hydroxysteroid
dehydrogenase (HSD) is particularly prominent in sebaceous glands (109, 110). Type 2 17ß-HSD mRNA expression has been reported in outer root
sheath cells of cultured human hair, and type 3 17ß-HSD expression in
beard and axillary dermal papilla cells from both sexes (111). The
former favors inactivation of estrogen; the latter favors formation of
testosterone. The predominant 17ß-HSD isozyme expressed in human
sebaceous glands at both the mRNA and protein level is the type 2 form
(112). Furthermore, the oxidative activity (conversion of estrogen and
testosterone to less active precursors) of 17ß-HSD is greater in
sebaceous glands from non-acne-prone skin as compared with acne-prone
regions. A predominance of 5-reductase over 17ß-HSD activity
appears to favor DHT formation in sweat glands and in the outer root
sheath cells of pubic, as compared with scalp, hairs. In addition,
5-reductase is 2 to 4 times more active than 17ß-HSD in sebaceous
glands from facial skin (113). A summary of the localization of the
mediators of androgen signal transduction in the PSU is provided in
Table 2.
The biological activity of testosterone on target tissues is effected
in large part by its conversion to DHT by 5-reductase, which is a
microsomal NADPH-dependent enzyme (114, 115, 116, 117). 5-Reductase was first
suspected to play a key role in androgen action when DHT was found to
be the predominant form of steroid bound to the androgen receptor in
prostate glands after the administration of testosterone (118, 119).
Testosterone and DHT stimulate 5-reductase mRNA and 5-reductase
activity, an effect mediated through the androgen receptor (116, 120, 121, 122). Two forms of 5-reductase exist, which are
differentially expressed in various tissues, likely as a result of
their respective promoters. They have different pH optima and
sensitivity to inhibitors. The two isozymes are approximately 46%
identical in sequence, have similar gene structures, are both
hydrophobic, and share similar substrate preferences (115, 116). The
type 2 isozyme is important for most androgen actions in sexual organs
(123), and a deficiency of 5-reductase type 2 in humans is a cause
of male pseudohermaphroditism (115, 124). However, the type 1 isozyme
is the major form of 5-reductase in skin.
Both 5-reductase isozymes are expressed at variable times in
development. Thigpen et al. (123) used immunoblotting to
demonstrate two waves of expression of the type 1 isozyme in human
skin, the first appearing at birth and lasting through age 2–3 yr and
the second beginning during puberty and continuing throughout life.
This suggested induction by androgens secreted perinatally and at
puberty. In contrast, there was just a single wave of expression of the
type 2 isozyme in skin, beginning at or just before birth and ending
around age 2–3 yr. Since they did not detect 5-reductase type 2
expression in adult skin, they postulated that the tendency to balding
may be programmed by the expression of the 5-reductase type 2 in
early life. However, the 5-reductase type 2 isozyme has been
localized by immunohistochemistry to hair follicles of human scalp;
specifically to the innermost portion of the outer root sheath and the
proximal inner root sheath (125). In addition, the 5-reductase
activity in the dermal papilla from beard resembles that of the type 2
isozyme in having an acidic pH optimum and a lower Michealis-Menten
constant than that of nonsexual hairs (101); this suggests that dermal
papillae of sexual hairs form more DHT than those of nonsexual hairs.
5-Reductase activity has been found in cultured fibroblasts from
sexual and nonsexual skin sites, in a distribution compatible with
regional specialization of mesenchymal cells with respect to this
important determinant of androgen action (1). 5-Reductase activity
is successively greater in fibroblasts cultured from nonsexual, pubic,
and genital skin (Table 3) (108). Further
studies found evidence for regional specialization of androgen
metabolism in sexual and nonsexual epithelial cell types. DHT formation
from testosterone in cells cultured from skin organelles increased in
the following order (%/mg DNA/min): epidermal (0.8%) < scalp
hair (2.8%) < pubic hair (8.1%) < foreskin fibroblasts
(71%) (84).
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Table 3. Relative 5-reductase activity and androgen
receptor content of fibroblasts cultured from sexual and nonsexual
skin sites
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The activity of 5-reductase has been found to be higher in sebaceous
glands of the scalp and facial skin than in other skin areas (106). The
type 1 isozyme is the major form of 5-reductase in the scalp, and
there are no obvious differences in type 1 isozyme expression between
balding vs. nonbalding areas of adult scalp according to
immunohistochemical studies (117, 123, 125). Within the scalp, the type
1 isozyme is localized primarily to the sebaceous glands, with lower
levels present in the hair follicle and dermis. Imperato-McGinley
et al. (126) found sebum production in patients with
5-reductase type 2 deficiency to equal that of normal males. This
suggested that either the male level of testosterone compensated for
the decreased DHT and was capable of sustaining sebum production or
that sebum production was under the control of the type 1 isozyme. The
type 1 isozyme is also the predominant isozyme in rat preputial
sebocytes (127).
Androgens act after binding to the androgen receptor, which is a member
of the subfamily of steroid hormone receptors that includes the
progesterone, mineralocorticoid, and glucocorticoid receptors (128). At
low concentrations, potent agonists of the androgen receptor facilitate
interactions between the amino-terminal and carboxy-terminal regions of
the androgen receptor, which stabilizes the receptor and likely causes
a slowing of ligand dissociation from the receptor (129). Both
testosterone and DHT bind to the same high-affinity androgen receptor,
but they bind with different affinities and dissociation rate
constants, have different efficacy in stabilizing the androgen
receptor, and have different physiological roles (130). Once
testosterone or DHT is bound to the androgen receptor, the
substrate-receptor complex binds to the androgen receptor response
element and regulates gene expression by acting as a transcription
factor. The DHT-receptor complex appears to be the more effective
complex at activating gene transcription (115) and may also be capable
of activating genes that the testosterone-receptor complex cannot
activate.
Androgen receptors in skin are primarily localized to dermal papilla,
sebaceous epithelium, and eccrine sweat epithelium according to
immunohistochemical analysis (131, 132, 133). They are also present in
lesser amounts in basal epidermal cells and scattered reticular dermal
fibroblasts. Successively greater numbers of androgen receptors have
been found in fibroblasts cultured from nonsexual, pubic, and genital
skin (Table 3) (108). It is unclear whether there is specific androgen
receptor immunoreactivity in the hair bulb or outer root sheath. In rat
preputial sebocytes, androgen receptor expression has been found to
increase with sebocyte differentiation (93). Androgen receptor mRNA
abundance seems to approach its maximum at the stage at which sebocytes
achieve competence for their specific pattern of lipid accumulation.
The dermal papilla cells are thought to be the primary target
cells within the hair follicle that mediate the growth-stimulating
signals of androgens, by releasing growth factors that act in a
paracrine fashion on the other cells of the hair follicle (16, 134).
Studies by Itami et al. (135) support this concept. These
workers reported a stimulatory effect of androgen on the growth of
beard hair epithelium in monolayer culture. However, as with outer root
sheath cells grown from other sexual hairs (84), androgen did not
directly stimulate the growth of outer root sheath cells, nor did
androgen affect the growth of beard dermal papilla cells. However, when
beard outer root sheath cells and beard dermal papilla cells were
cocultured, androgen stimulated the growth of the beard epithelial
cells, and antiandrogen countered this effect. Furthermore, dermal
papilla cells cultured from androgen-sensitive (beard) hair follicles
not only have more androgen receptor binding sites than do those from
less androgen-sensitive (scalp) sites (134), but the dermal papilla are
also larger (42). Saturation analysis revealed more androgen receptors
in dermal papilla cells cultured from balding scalp, as compared with
nonbalding scalp, which supports the hypothesis that androgens work via
the dermal papilla cells (136).
The mode of androgen action on sebocyte proliferation is unclear.
Akamatsu et al. (137, 138) reported a direct stimulatory
effect of androgen on the growth of passaged human sebocytes in
monolayer culture. In addition, there is evidence that the effect of
testosterone and DHT on human sebaceous cell proliferation depends on
the area of skin from which the glands are obtained. In one system,
proliferation of sebaceous cells obtained from facial skin was
stimulated up to 50% in a dose-dependent manner by both testosterone
and DHT, whereas in sebaceous cells isolated from the extremity,
testosterone had no effect at all, while DHT had only a small effect
(137, 139). Furthermore, spironolactone, which exhibits antagonistic
activity to androgens at a cellular level, inhibited sebocyte
proliferation, thus supporting a receptor-mediated effect (139). In
another system, the androgen effect on proliferation of facial
sebaceous cells was maximized (50% increase) at about
10-9 M and lost at
10-7 M (87). On the other
hand, androgen has an inhibitory effect on preputial sebocyte
proliferation in primary monolayer culture (140). It is unclear whether
these different effects are due to variances in culture technique or
species differences.
Androgens have been shown to stimulate the differentiation of
sebocytes, although this effect is modest in vitro (75, 141). However, androgen augments the differentiative effect of
peroxisome proliferator activated receptor-
(PPAR). Recent
research also suggests important postreceptor interactions of androgen
with retinoic acid derivatives and GH.
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VI. Role of Peroxisome Proliferator-Activated Receptors in Sebocyte
Development
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Despite the fact that sebaceous gland growth is dependent on
androgen in vivo, androgens have not had a clear effect on
sebocyte differentiation in a variety of in vitro culture
systems (75, 137, 142, 143). Since androgen receptors are present in
sebaceous cells (93), we postulated that a downstream signal
transduction pathway involved in the regulation of lipid metabolism was
not being expressed in cultured sebocytes. We tested the hypothesis
that mechanisms involved in lipogenesis during adipocyte
differentiation may be similarly used in sebocyte differentiation.
We found that PPAR activators induced lipogenesis in rat sebocytes
in vitro (141), although in vivo studies had not
shown the systemic administration of the PPAR activators, clofibric
acid (144) or eicosatetraynoic acid (145), to stimulate sebum activity.
PPARs have been shown to regulate multiple lipid metabolic genes in
peroxisomes, microsomes, and mitochondria by acting on PPAR response
elements (146, 147). PPARs were originally identified as part of a
subfamily of "orphan receptors" within the nonsteroid receptor
family of nuclear hormone receptors (148). There are three PPAR
subtypes: , 
, and . Activation of PPAR and by their
respective specific ligands, the thiazolidinedione rosiglitazone and
the fibrate WY-14643, induced lipid droplet formation in sebocytes but
not in epidermal cells. Linoleic acid and carbaprostacyclin, both
PPAR and ligand-activators, were more effective but less
specific, stimulating lipid formation in both types of cells. Either
was more effective than the combination of PPAR and activation,
suggesting that PPAR is involved in this lipid formation. Linoleic
acid 0.1 mM stimulated significantly more
advanced sebocyte maturation than any other treatment, including
carbaprostacyclin, which was compatible with a distinct role of long
chain fatty acids in the final, terminally differentiated stage of
sebocyte maturation. When DHT was added with the PPAR activators, an
additive effect on lipid droplet formation in sebocytes was seen only
with the combination of DHT and a PPAR activator (Fig. 10). This suggests that PPAR
influences a step in sebocyte differentiation which is related but
distinct from that influenced by androgen.
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Figure 10. Differentiation of preputial sebocytes in primary
culture after treatment with dihydrotestosterone (DHT)
10-6 M and/or the thiazolidinedione
rosiglitazone (BRL-49653) in serum free medium in the presence of
insulin 10-6 M (n = 5). Lipid is stained
with Oil Red O (ORO). Means ± SEMs are shown. DHT
10-6 M has a small but significant effect
(P < 0.05). BRL has a dose-response effect over a
broad range commencing at 10-10 M
(P < 0.01 vs. control, with
10-8 M BRL differing from the higher and lower
doses at the P level shown). DHT is additive in its
effect with BRL = 10-8 M, and the effect
of DHT + BRL 10-6 M is the greatest of all.
[Adapted with permission from R. L. Rosenfield et
al.: J Invest Dermatol 112:226–232, 1999
(141 ).]
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PPAR subtype mRNA expression was also detected in rat sebocytes (141).
PPAR1 mRNA was demonstrated in sebocytes, but not in epidermal
cells; it was more strongly expressed in freshly dispersed than in
cultured sebocytes. In contrast, PPAR mRNA was expressed to a
similarly high extent before and after culture in both sebocytes and
epidermal cells. These findings are compatible with the concept that
androgen-enhanced PPAR1 gene expression plays a unique role in
initiating the differentiation of sebocytes, while activation of
constitutively expressed PPAR by long-chain fatty acids finalizes
sebocyte maturation.
Since increased sebum production is an important element in the
pathogenesis of acne vulgaris (149, 150), the finding that PPARs appear
to mediate sebocyte cytoplasmic lipid accumulation may have
implications for the treatment of acne. It may be feasible to develop
PPAR antagonists that can interfere selectively with sebum formation
without invoking the side-effects of currently available treatment
modalities.
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VII. Retinoid Effects on the PSU
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Retinoic acid derivatives (retinoids), which are analogs of
vitamin A, have an effect on growth and differentiation of diverse
tissues. Retinoids likely play a role in the hair follicle, since, like
androgens, they are involved in epithelialmesenchymal interactions
in morphogenesis and embryological development, and the hair growth
cycle partially recapitulates the embryogenesis of the hair follicle
(42). Furthermore, retinoids alter the expression of HOX genes, which
are likely to be involved in PSU morphogenesis. Retinoids have also
been shown to affect the hair follicle growth-cycle in mice (151, 152, 153),
with topical application increasing the length of the anagen phase, and
decreasing time in telogen.
Retinoids have profound effects on sebaceous gland activity. Whereas
trace amounts promote sebocyte growth and differentiation, larger doses
cause atrophy of sebaceous glands and a decrease in sebum secretion in
both animals and humans (64, 65, 69, 154). Retinoids have been
postulated to inhibit lipid synthesis in sebocytes either directly,
through an inhibition of lipogenic enzymes, or indirectly, by decreased
cell proliferation (15
Top
Abstract
I. Introduction
