Rat preputial sebocyte differentiation involves peroxisome proliferator-activated receptors

Propionibacterium acnes and sebaceous lipogenesis: a love-hate relationship?

In this issue, Iinuma et al. show that Propionibacterium acnes (P. acnes)-conditioned medium and formalin-killed P. acnes augment intracellular lipid formation in hamster sebocytes by increasing the de novo synthesis of triacylglycerols. This commentary summarizes the current knowledge of the association of P. acnes with sebaceous lipogenesis, inflammation, and innate immunity, and points out the concurrent evidence that P. acnes-induced lipids may represent a recruitment of allies and/or enemies of the human skin.

Age-related changes in the meibomian gland

The purpose of this study was to characterize the age-related changes of the mouse meibomian gland. Eyelids from adult C57Bl/6 mice at 2, 6, 12 and 24 months of age were stained with specific antibodies against peroxisome proliferator activated receptor gamma (PPARgamma) to identify differentiating meibocytes, Oil Red O (ORO) to identify lipid, Ki67 nuclear antigen to identify cycling cells, B-lymphocyte-induced maturation protein-1 (Blimp1) to identify potential stem cells and CD45 to identify immune cells. Meibomian glands from younger mice (2 and 6 months) showed cytoplasmic and perinuclear staining with anti-PPARgamma antibodies with abundant ORO staining of small, intracellular lipid droplets. Meibomian glands from older mice (12 and 24 months) showed only nuclear PPARgamma localization with less ORO staining and significantly reduced acinar tissue (p < 0.04). Acini of older mice also showed significantly reduced (p < 0.004) numbers of Ki67 stained nuclei. While Blimp1 appeared to diffusely stain the superficial ductal epithelium, isolated cells were occasionally stained within the meibomian gland duct and acini of older mice that also stained with CD45 antibodies, suggesting the presence of infiltrating plasmacytoid cells. These findings suggest that there is altered PPARgamma receptor signaling in older mice that may underlie changes in cell cycle entry/proliferation, lipid synthesis and gland atrophy during aging. These results are consistent with the hypothesis that mouse meibomian glands undergo age-related changes similar to those identified in humans and may be used as a model for age-related meibomian gland dysfunction.

Involvement of Propionibacterium acnes in the augmentation of lipogenesis in hamster sebaceous glands in vivo and in vitro

Propionibacterium acnes is considered to be involved in the aggravation of acne vulgaris, but it remains unclear whether P. acnes directly influences lipogenesis in sebaceous glands. In this study, we showed that a culture medium of P. acnes (acnes-CM) and formalin-killed P. acnes (F-acnes) prepared from P. acnes strains, JCM6473 and JCM6425, intracellularly augmented lipid droplet formation and triacylglycerol (TG) synthesis in undifferentiated and insulin-differentiated hamster sebocytes. Acnes-CM and F-acnes prepared from four clinical P. acnes strains elicited the same lipogenesis augmentation. The augmented TG production resulted from an increase in the diacylglycerol acyltransferase activity. Topical application of acnes-CM to the skin of hamster auricles every day for 4 weeks revealed that sebum accumulation was augmented in sebaceous glands and ducts. Furthermore, both acnes-CM and F-acnes increased the production of 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), a cytochrome P450 (CYP)-linked sebaceous lipogenic factor, in differentiated sebocytes. A CYP inhibitor, SKF-525A, decreased the acnes-CM- and F-acnes-augmented production of TG and 15d-PGJ(2). Thus, to our knowledge these results provide previously unreported evidence that P. acnes directly participates in the augmentation of sebaceous lipogenesis through a proposed mechanism in which an increase of 15d-PGJ(2) production through the CYP pathway is closely associated with the enhancement of TG production.

Hair follicle stem cell-specific PPARgamma deletion causes scarring alopecia

Primary cicatricial or scarring alopecias (CA) are a group of inflammatory hair disorders of unknown pathogenesis characterized by the permanent destruction of the hair follicle. The current treatment options are ineffective in controlling disease progression largely because the molecular basis for CA is not understood. Microarray analysis of the lymphocytic CA, Lichen planopilaris (LPP), compared to normal scalp biopsies identified decreased expression of genes required for lipid metabolism and peroxisome biogenesis. Immunohistochemical analysis showed progressive loss of peroxisomes, proinflammatory lipid accumulation, and infiltration of inflammatory cells followed by destruction of the pilosebaceous unit. The expression of peroxisome proliferator-activated receptor (PPAR) gamma, a transcription factor that regulates these processes, is significantly decreased in LPP. Specific agonists of PPARgamma are effective in inducing peroxisomal and lipid metabolic gene expression in human keratinocytes. Finally, targeted deletion of PPARgamma in follicular stem cells in mice causes a skin and hair phenotype that emulates scarring alopecia. These studies suggest that PPARgamma is crucial for healthy pilosebaceous units and it is the loss of this function that triggers the pathogenesis of LPP. We propose that PPARgamma-targeted therapy may represent a new strategy in the treatment of these disorders.

Peroxisome Proliferator-Activated Receptor beta/delta in the Brain: Facts and Hypothesis

peroxisome proliferator-activated receptors (PPARs) are nuclear receptors acting as lipid sensors. Besides its metabolic activity in peripheral organs, the PPAR beta/delta isotype is highly expressed in the brain and its deletion in mice induces a brain developmental defect. Nevertheless, exploration of PPARβ action in the central nervous system remains sketchy. The lipid content alteration observed in PPARβ null brains and the positive action of PPARβ agonists on oligodendrocyte differentiation, a process characterized by lipid accumulation, suggest that PPARβ acts on the fatty acids and/or cholesterol metabolisms in the brain. PPARβ could also regulate central inflammation and antioxidant mechanisms in the damaged brain. Even if not fully understood, the neuroprotective effect of PPARβ agonists highlights their potential benefit to treat various acute or chronic neurological disorders. In this perspective, we need to better understand the basic function of PPARβ in the brain. This review proposes different leads for future researches.

Transient receptor potential vanilloid-1 signaling as a regulator of human sebocyte biology

Transient receptor potential vanilloid-1 (TRPV1), originally described as a central integrator of nociception, is expressed on human epidermal and hair follicle keratinocytes and is involved in regulation of cell growth and death. In human pilosebaceous units, we had shown that TRPV1 stimulation inhibits hair shaft elongation and matrix keratinocyte proliferation, and induces premature hair follicle regression and keratinocyte apoptosis. In the current study, we have explored the role of TRPV1-mediated signaling in sebaceous gland (SG) biology, using a human sebocyte cell culture model (SZ95 sebocytes). Demonstrating that human skin SG in situ and SZ95 sebocytes in vitro express TRPV1, we show that the prototypic TRPV1 agonist, capsaicin, selectively inhibits basal and arachidonic acid-induced lipid synthesis in a dose-, time-, and extracellular calcium-dependent and a TRPV1-specific manner. Low-dose capsaicin stimulates cellular proliferation via TRPV1, whereas higher concentrations inhibit sebocyte growth and induce cell death independent of TRPV1. Moreover, capsaicin suppresses the expression of genes involved in lipid homeostasis and of selected proinflammatory cytokines. Collectively, these findings support the concept that TRPV1 signaling is a significant, previously unreported player in human sebocyte biology and identify TRPV1 as a promising target in the clinical management of inflammatory SG disorders (for example, acne vulgaris).

Peroxisome proliferator-activated receptors (PPARs) and the human skin: importance of PPARs in skin physiology and dermatologic diseases

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily that regulate lipid, glucose, and amino acid metabolism. More recently, PPARs and corresponding ligands have been shown in skin and other organs to regulate important cellular functions, including cell proliferation and differentiation, as well as inflammatory responses. These new functions identify PPARs and corresponding ligands as potential targets for the treatment of various skin diseases and other disorders. It has been shown that in inflammatory skin disorders, including hyperproliferative psoriatic epidermis and the skin of patients with atopic dermatitis, the expression of both PPARalpha and PPARgamma is decreased. This observation suggests the possibility that PPARalpha and PPARgamma activators, or compounds that positively regulate PPAR gene expression, may represent novel NSAIDs for the topical or systemic treatment of common inflammatory skin diseases such as atopic dermatitis, psoriasis, and allergic contact dermatitis. Moreover, recent findings indicate that PPAR-signaling pathways may act as a promising therapeutic target for the treatment of hyperproliferative skin diseases including skin malignancies. Studies in non-diabetic patients suggest that oral thiazolidinediones, which are synthetic ligands of PPARgamma, not only exert an antidiabetic effect but also may be beneficial for moderate chronic plaque psoriasis by suppressing proliferation and inducing differentiation of keratinocytes; furthermore, they may even induce cell growth arrest, apoptosis, and terminal differentiation in various human malignant tumors. It has been reported that PPARalpha immunoreactivity is reduced in human keratinocytes of squamous cell carcinoma (SCC) and actinic keratosis (AK), while PPARdelta appears to be upregulated. Additionally, the microvessel density is significantly higher in AK and SCC that express high levels of PPARdelta. PPARdelta has been demonstrated to have an anti-apoptotic role and to maintain survival and differentiation of epithelial cells, whereas PPARalpha and PPARgamma activators induce differentiation and inhibit proliferation and regulate apoptosis. In melanoma, the growth inhibitory effect of PPARgamma activation is independent of apoptosis and seems to occur primarily through induction of cell cycle arrest in the G1 phase of the cell cycle or induction of re-differentiation. PPARalpha activation causes inhibition of migration of melanoma cells and anchorage-independent growth, whereas primary tumor growth remains unaltered. In clinical trials of gemfibrozil, a PPARalpha ligand, significantly fewer patients treated with this lipid-lowering drug were diagnosed with melanoma as compared to those in the control group. In conclusion, an increasing body of evidence indicates that PPAR signaling pathways may represent interesting therapeutic targets for a broad variety of skin disorders, including inflammatory skin diseases such as psoriasis and atopic dermatitis, and skin malignancies.

Thematic review series: skin lipids. Sebaceous gland lipids: friend or foe?

Sebaceous glands are intriguing glands that are found throughout the human body except on the palms of the hands and soles of the feet. The true function of these glands has yet to be determined, but there are several theories, including antioxidant effects, antibacterial effects, and transport of pheromones. Sebaceous glands produce lipids that are involved in the pathogenesis of one of the most prevalent diseases of adolescence, acne. Although the majority of lipids produced by the sebaceous gland are also produced in other areas of the body, there are two that are characteristic of the sebaceous gland, wax esters and squalene. This review seeks to present an update on the physiology of the sebaceous glands, with particular emphasis on the production of sebaceous lipids.

Present concepts and future outlook: function of peroxisome proliferator-activated receptors (PPARs) for pathogenesis, progression, and therapy of cancer

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily of transcriptional regulators that regulate lipid, glucose, and amino acid metabolism. In recent studies it also has been shown that these receptors are implicated in tumor progression, cellular differentiation, and apoptosis and modulation of their function is therefore considered as a potential target for cancer prevention and treatment. PPAR ligands and other agents influencing PPAR signalling pathways have been shown to reveal chemopreventive potential by mediating tumor suppressive activities in a variety of human cancers and could represent a potential novel strategy to inhibit tumor carcinogenesis and progression. This review summarizes the currently available data on the roles of PPARs in relation to the processes of cell differentiation and carcinogenesis as well as their role as promising future therapeutic targets.

Abuse of anabolic-androgenic steroids and bodybuilding acne: an underestimated health problem

Abuse of anabolic-androgenic steroids (AAS) by members of fitness centers and others in Germany has reached alarming dimensions. The health care system provides the illegal AAS to 48.1 % of abusers. Physicians are involved in illegal prescription of AAS and monitoring of 32.1 % of AAS abusers. Besides health-threatening cardiovascular, hepatotoxic and psychiatric long-term side effects of AAS, acne occurs in about 50 % of AAS abusers and is an important clinical indicator of AAS abuse, especially in young men 18-26 years of age. Both acne conglobata and acne fulminans can be induced by AAS abuse. The dermatologist should recognize bodybuilding acne, address the AAS abuse, and warn the patient about other potential hazards.

Peroxisome Proliferator-Activated Receptors Increase Human Sebum Production

Sebum production is key in the pathophysiology of acne, an extremely common condition, which when severe, may require treatment with isotretinoin, a known teratogen. Apart from isotretinoin and hormonal therapy, no agents are available to reduce sebum. Increasing our understanding of the regulation of sebum production is a milestone in identifying alternative therapeutic targets. Studies in sebocytes and human sebaceous glands indicate that agonists of peroxisome proliferator-activated receptors (PPARs) alter sebaceous lipid production. The goal of this study is to verify the expression and activity of PPARs in human skin and SEB-1 sebocytes and to assess the effects of PPAR ligands on sebum production in patients. To investigate the contribution of each receptor subtype to sebum production, lipogenesis assays were performed in SEB-1 sebocytes that were treated with PPAR ligands and isotretinoin. Isotretinoin significantly decreased lipogenesis, while the PPARalpha agonist-GW7647, PPARdelta agonist-GW0742, PPARalpha/delta agonist-GW2433, PPARgamma agonist rosiglitazone, and the pan-agonist-GW4148, increased lipogenesis. Patients treated with thiazolidinediones or fibrates had significant increases in sebum production (37 and 77%, respectively) when compared to age-, disease-, and sex-matched controls. These data indicate that PPARs play a role in regulating sebum production and that selective modulation of their activity may represent a novel therapeutic strategy for the treatment of acne.

Loss of the acyl-CoA binding protein (Acbp) results in fatty acid metabolism abnormalities in mouse hair and skin

Proper fatty acid metabolism is critical for hair and skin development and maintenance. The acyl-CoA binding protein (Acbp) is a widely expressed protein that binds long-chain fatty acyl-CoA esters and plays a role in fatty acyl-CoA transport and pool formation. However, loss of function of Acbp in the whole animal has not been investigated. Here, we show that deletion of Acbp in mouse results in sebocyte hyperplasia and sparse, matted hair with a greasy appearance. Consistent with these gross abnormalities, Acbp is highly expressed in the pilosebaceous units of mouse skin as determined by Northern analysis and in situ hybridization. Loss of Acbp also results in fatty acid metabolism abnormalities, with hair lipid profiles showing altered levels of triacylglycerols and nearly co-migrating lipids. These data suggest that Acbp plays a role in triacylglycerol biosynthesis, and that regulation of this process is important for proper hair and skin development and maintenance in the mouse.

Insulin-Like Growth Factor-1 Induces Lipid Production in Human SEB-1 Sebocytes Via Sterol Response Element-Binding Protein-1

An understanding of the molecular signaling involved in sebaceous gland lipid production is needed to develop therapeutic targets to improve acne. Treatment with methylisobutylxanthine, dexamethasone, and a high dose of insulin (MDI) has been shown to differentiate 3T3-L1 preadipocytes into adipocytes, a differentiation marked by an increase in lipid production. The present study has the following aims: (1) Since high doses of insulin, as found in MDI, will activate the IGF-1 receptor, we sought to determine if IGF-1 is capable of reproducing the lipogenic effect seen with MDI treatment, and (2) to determine if the sterol response element-binding protein-1 (SREBP-1) pathway mediates the increase in lipogenesis. Here we report that MDI increases lipogenesis and that this effect can be attributed wholly to the high-dose insulin in SEB-1 cells. Further, we show that a physiologically relevant dose of IGF-1 or high-dose (1 microM) insulin induces an increase in SREBP-1 mRNA, protein, and total lipid production; while 100 nM insulin induces lipogenesis yet the SREBP protein levels remain unchanged. These data indicate that activation of the IGF-1 receptor increases lipogenesis in SEB-1 cells through both SREBP-dependent and SREBP-independent pathways.

From molecular action to physiological outputs: peroxisome proliferator-activated receptors are nuclear receptors at the crossroads of key cellular functions

Peroxisome proliferator-activated receptors (PPARs) compose a family of three nuclear receptors which act as lipid sensors to modulate gene expression. As such, PPARs are implicated in major metabolic and inflammatory regulations with far-reaching medical consequences, as well as in important processes controlling cellular fate. Throughout this review, we focus on the cellular functions of these receptors. The molecular mechanisms through which PPARs regulate transcription are thoroughly addressed with particular emphasis on the latest results on corepressor and coactivator action. Their implication in cellular metabolism and in the control of the balance between cell proliferation, differentiation and survival is then reviewed. Finally, we discuss how the integration of various intra-cellular signaling pathways allows PPARs to participate to whole-body homeostasis by mediating regulatory crosstalks between organs.

Augmentation of Lipogenesis by 15-Deoxy-Δ12,14-Prostaglandin J2 in Hamster Sebaceous Glands: Identification of Cytochrome P-450-mediated 15-Deoxy-Δ12,14-Prostaglandin J2 Production

Prostaglandins (PGs) play important roles in the regulation of cutaneous cell functions under physiological and pathological conditions. In this study, we examined the involvement of PGs in sebocyte lipogenesis using non-steroidal anti-inflammatory drugs in vivo and in vitro. Hamster auricle sebocytes spontaneously differentiated to accumulate intracellular triacylglycerol (TG), under which the relative levels of 15-deoxy-Delta(12,14)-PGJ2 (15d-PGJ2) to PGF(2alpha) and PGE2 increased. 15d-PGJ2 was found to augment the formation of lipid droplets, which was because of an increase of TG synthesis by diacylglycerol acyltransferase (DGAT). Furthermore, sebocytes constitutively produced cyclooxygenase 2 (COX-2), but not COX-1, in vivo and in vitro. When sebocytes were treated with COX inhibitors such as indomethacin, diclofenac, or NS-398, the production of PGF(2alpha) and PGE2 decreased. The production of 15d-PGJ2, however, was increased in these inhibitor-treated sebocytes. In addition, indomethacin, diclofenac, and NS-398 augmented the synthesis of TG along with the increase in DGAT activity. Similarly, topical administration of indomethacin to hamster auricles caused the development of sebaceous glands with the augmentation of sebum deposition in vivo. Furthermore, indomethacin and NS-398-augmented 15d-PGJ2 production and TG synthesis were suppressed by a non-selective cytochrome P-450 (CYP) inhibitor, SKF-525A. A ligand activator of peroxisome proliferation activating receptor gamma (PPARgamma), troglitazone-induced synthesis of TG, however, was not altered even in the presence of SKF-525A. These results suggest that 15d-PGJ2 is a crucial stimulator of sebocyte lipogenesis by augmenting DGAT-mediated synthesis of TG. In addition to the COX-2-dependent pathway of PG synthesis, our findings suggest a sebocyte-specific pathway of 15d-PGJ2 production by CYP, the activity of which may be evoked by inhibiting COX-2.

Expression of perilipin A on the surface of lipid droplets increases along with the differentiation of hamster sebocytes in vivo and in vitro

To clarify the involvement of perilipin, a lipid-droplet-surface protein associated with adipocytes and steroidogenic cells, in the differentiation of sebocytes, we investigated the expression of perilipin in sebaceous glands in vivo and in vitro. Perilipin was expressed in sebaceous glands of the hamster auricle in vivo and was localized at the surface of intracellular lipid droplets in differentiated hamster sebocytes in vitro. Western blot analysis showed that perilipin with a molecular weight of approximately 57 kDa, which was identical to that in differentiated mouse 3T3-L1 adipocytes, was detected in cultured sebocytes, indicating that sebaceous glands expressed perilipin A. In addition, the production of perilipin A in cultured sebocytes was transcriptionally augmented by sebocytic-lipogenesis stimulators, insulin, and 5alpha-dihydrotestosterone, whereas it was decreased by a suppressor of sebocytic differentiation, epidermal growth factor. Furthermore, hamster sebocytes were found to express peroxisome proliferation-activating receptor alpha and gamma1, the activation of which by WY14643 and troglitazone, respectively, caused the transcriptional augmentation of perilipin A expression along with an increase in levels of triacylglycerols in lipid droplets in sebocytes. Therefore, these results provide novel evidence that the expression of perilipin A increases on the surface of intracellular lipid droplets augmented along with the differentiation of hamster sebocytes.

Functions of peroxisome proliferator-activated receptors (PPAR) in skin homeostasis

The peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors that belong to the nuclear hormone receptor family. Three isotypes (PPAR alpha, PPAR beta or delta, and PPAR gamma) with distinct tissue distributions and cellular functions have been found in vertebrates. All three PPAR isotypes are expressed in rodent and human skin. They were initially investigated for a possible function in the establishment of the permeability barrier in skin because of their known function in lipid metabolism in other cell types. In vitro studies using specific PPAR agonists and in vivo gene disruption approaches in mice indeed suggest an important contribution of PPAR alpha in the formation of the epidermal barrier and in sebocyte differentiation. The PPAR gamma isotype plays a role in stimulating sebocyte development and lipogenesis, but does not appear to contribute to epidermal tissue differentiation. The third isotype, PPAR beta, regulates the late stages of sebaceous cell differentiation, and is the most effective isotype in stimulating lipid production in these cells, both in rodents and in humans. In addition, PPAR beta activation has pro-differentiating effects in keratinocytes under normal and inflammatory conditions. Finally, preliminary studies also point to a potential role of PPAR in hair follicle growth and in melanocyte differentiation. By their diverse biological effects on cell proliferation and differentiation in the skin, PPAR agonists or antagonists may offer interesting opportunities for the treatment of various skin disorders characterized by inflammation, cell hyperproliferation, and aberrant differentiation.

Peroxisome Proliferator-Activated Receptors and their Ligands

Glucocorticoids have remained one of the most frequently used classes of drugs for the treatment of skin diseases since their introduction more than 50 years ago. As a result of the discovery of new members of the nuclear hormone receptor (NR) superfamily, alternative therapeutic interventions that target retinoid and vitamin D receptors have been developed. Peroxisome proliferator-activated receptors (PPARs) comprise another important NR subfamily, consisting of three different isotypes: PPARalpha, PPARdelta (PPARbeta) and PPARgamma. These NRs are activated by a variety of natural and synthetic ligands such as fatty acids, eicosanoids, and antidiabetic and antihyperlipidaemic agents. While these receptors are established as regulators of gene expression in lipid and glucose homeostasis, evidence is now accumulating that PPARs also play a crucial role in cutaneous biology. Results from in vitro and in vivo studies have indicated the involvement of PPARs in epidermal maturation, proliferation and differentiation, as well as in immune and inflammatory responses, carcinogenesis, hyperpigmentation and skin wound healing. Furthermore, treatment of psoriatic patients with PPARgamma activators (thiazolidinediones) has been shown to induce beneficial effects. However, the effects of PPAR ligands should be carefully evaluated to determine whether they are in fact mediated via PPAR-dependent mechanisms. Nonetheless, PPARs seem to have significant potential as therapeutic targets in skin inflammatory disorders.

Modulation of androgen receptor transcriptional activity by anti-acne reagents

BACKGROUND

To study the potential anti-androgenic activity of roxithromycin (RXM), we previously used human dermal fibroblasts transiently transfected with the expression vector of androgen receptor (AR) coactivator ARA55 as the in vitro model reflecting the end-organ hypersensitivity.

OBJECTIVE

To examine the potential anti-androgenic activity of anti-acne therapeutic agents, nadifloxacin (NDFX), RXM, all-trans retinoic acid (atRA), and glycolic acid (GA), we carried out the transient transfection assays using the CV-1 cells as a more sensitive assay system.

RESULTS

The result showed that 5 microg/ml of RXM suppress 10(-9)M R1881-induced AR transcriptional activity by 21.2%. 50 microg/ml of NDFX can suppress AR transcriptional activity to 29.8%. Furthermore, the assays with treatment of 1, 5, 10, or 50 microg/ml NDFX in the presence of 1 microg/ml RXM showed that 5, 10, or 50 microg/ml NDFX inhibits the AR transactivity by 32.7, 31.1 or 61.0%, respectively, indicating the synergistic effect of NDFX and RXM. Besides 10(-5)M atRA suppressed the R1881-induced luciferase activity by 50%, but GA did not alter AR transactivity.

CONCLUSIONS

We demonstrated that anti-acne agents available in the clinical practice can exert anti-androgenic effects in the treatment of acne.

Peroxisome proliferator-activated receptor and farnesoid X receptor ligands differentially regulate sebaceous differentiation in human sebaceous gland organ cultures in vitro

BACKGROUND

Nuclear hormone receptors are important in the regulation of epidermal differentiation and have been implicated in lipid metabolism. In particular, there is evidence suggesting that the activation of peroxisome proliferator-activated receptors (PPARs) is an important factor in the regulation of sebocyte lipogenesis.

OBJECTIVES

To determine the role of PPARs, farnesoid X receptor (FXR) and other orphan nuclear hormone receptors in sebaceous gland function in vitro by investigating the biochemical effects of appropriate ligands, and by establishing the RNA and protein expression patterns of a number of nuclear receptors in sebaceous glands ex vivo.

METHODS

Human chest sebaceous glands were maintained in vitro as freshly isolated and as 7-day cultured whole organs. We then studied the effects of appropriate ligands on the glandular rates of lipogenesis and DNA synthesis, as well as determining the mRNA (reverse transcription-polymerase chain reaction) and protein expression patterns (immunohistochemistry/immunoblotting) of the nuclear hormone receptors of interest.

RESULTS

PPAR ligands, but not FXR ligands, inhibited sebaceous lipogenesis, in particular the PPARalpha ligands LY 171883 and WY 14643, and the PPARgamma ligands BRL 49653 and 15-deoxy-Delta-12,14-prostaglandin J(2). We detected RNA expression of PPARalpha, PPARbeta, PPARgamma, retinoid X receptor alpha, liver X receptor alpha (LXRalpha) and pregnane X receptor but not FXR in freshly isolated and 7-day maintained sebaceous glands. PPARalpha, PPARbeta, PPARgamma and LXRalpha protein were detected in nuclear extracts of sebaceous glands.

CONCLUSIONS

We conclude that activation of nuclear hormone receptors, in particular activation of PPARalpha and PPARgamma, can regulate lipogenesis in human sebaceous glands. As suppression of sebum secretion is associated with reduced acne activity, the nuclear hormone receptors involved may open new avenues in the development of novel acne treatments.

Acne update: 2004

PURPOSE OF REVIEW

Acne vulgaris is a common skin disorder among children and young adults that carries enormous financial and psychosocial impact. Contemporary therapies attempt to address factors underlying acne as a disorder of the pilosebaceous unit. These longstanding paradigms regarding pathogenesis and treatment continue to evolve in light of recent work on this ubiquitous disease.

RECENT FINDINGS

This review focuses on new literature that has emerged regarding the biology of the folliculosebaceous unit, the identification of particular mediators responsible for inflammatory acne, the use of topical and systemic retinoids in acne therapy, and approaches to address the emergence of antibiotic-resistant Propionibacterium acnes strains. In addition, the use of several novel therapeutic avenues is discussed, including combination therapies, lipoxygenase inhibitors, and lasers.

SUMMARY

As the understanding of the factors that initiate and exacerbate acne vulgaris continues to increase, so does the diversity of therapeutic options. Rational use of available treatment options based on the type and severity of acne lesions is a key component of successful acne therapy and allows the physician who treats adolescents with acne to provide optimum care.

Current concepts of the pathogenesis of acne: implications for drug treatment

The pathogenesis of acne is complex, with strong evidence supporting the involvement of sebaceous hyperplasia, follicular hyperkeratinisation, bacterial hypercolonisation, as well as immune reactions and inflammation. High sebum concentrations and follicular hyperkeratinisation lead to a change of the follicular milieu with consecutive proliferation of bacteria, chiefly Propionibacterium acnes. This leads to further increased production of the pro-inflammatory cytokines interleukin-1alpha and tumour necrosis factor alpha by T cells and keratinocytes, leading to proliferation of both cell types. Follicular keratinocytes fail to differentiate by apoptosis and produce hypergranulosis similar to the impermeable skin outer layer, resulting in the formation of microcomedones. Further inflammatory responses lead to the development of increasing degrees of severity in inflammatory forms of acne. Retinoids aid the differentiation and reduce the hyperproliferation of keratinocytes, and can inhibit the migration of leucocytes. Combination therapy using retinoids plus benzoyl peroxide or antibacterials can treat existing acne lesions faster than the individual agents alone and can also prevent the development of new lesions. The new retinoids (e.g. adapalene) have not only the typical potent comedolytic activity but also anti-inflammatory effects. When added to antibacterial therapy, topical retinoids demonstrate faster and significantly greater reduction of inflammatory acne lesions and comedones than antibacterials alone.

Expression of peroxisome proliferator-activated receptor and CCAAT/enhancer binding protein transcription factors in cultured human sebocytes

Lipid synthesis and accumulation represent a major step in sebocyte differentiation and it may be of importance for sebocytes to express two families of transcription factors, CCAAT/enhancer binding proteins (c/EBPs) and peroxisome proliferator-activated receptors (PPARs), which were found to play a crucial role in the differentiation of adipocytes. Using the immortalized human sebaceous gland cell line SZ95 we examined the expression of the molecules before and after treatment with testosterone, 5alpha-dihydrotestosterone, dexamethasone, 17beta-estradiol and genistein, at 6, 12, 24, and 48 h, respectively. Reverse transcription-PCR analysis showed expression of peroxisome proliferator-activated receptors -alpha, -delta, -gamma1, -gamma2 and CCAAT/enhancer binding proteins-alpha, -beta, -gamma-delta in native SZ95 sebocytes. In western blot studies, high levels of CCAAT/enhancer binding proteins-alpha and -beta, and peroxisome proliferator-activated receptors-gamma were expressed at 6, 24, and 12 h, respectively. Immunostaining of the cultured sebocytes showed the CCAAT/enhancer binding proteins-alpha and -beta mainly localized within nuclei, whereas peroxisome proliferator-activated receptors-gamma in the cytoplasm. Strong staining of sebocytes was immunohistochemically revealed in the basal layer of sebaceous glands in human scalp and sebaceous nevus. Genistein down-regulated the expression of CCAAT/enhancer binding proteins-alpha and -beta, and peroxisome proliferator-activated receptors-gamma on the protein level. Treatment with linoleic acid for 48 h induced further differentiation of sebocytes leading to abundant lipid synthesis.

Update and future of systemic acne treatment

Systemic treatment is required in patients with moderate-to-severe acne, especially when acne scars start to occur. Antibiotics with anti-inflammatory properties, such as tetracyclines (oxytetracycline, tetracycline chloride, doxycycline, minocycline and limecycline) and macrolide antibiotics (erythromycin and azithromycin) are the agents of choice for papulopustular acne, even though the emerging resistant bacterial strains are minimizing their effect, especially regarding erythromycin. Systemic antibiotics should be administered during a period of 8-12 weeks. In severe papulopustular and in nodulocystic/conglobate acne, oral isotretinoin is the treatment of choice. Hormonal treatment represents an alternative regimen in female acne, whereas it is mandatory in resistant, severe pubertal or post-adolescent forms of the disease. Compounds with anti-androgenic properties include estrogens combined with progestins, such as ethinyl estradiol with cyproterone acetate, chlormadinone acetate, desogestrel, drospirenone, levonogestrel, norethindrone acetate, norgestimate, and other anti-androgens directly blocking the androgen receptor (flutamide) or inhibiting androgen activity at various levels, corticosteroids, spironolactone, cimetidine, and ketoconazole. After 3 months of treatment control of seborrhea and acne can be obtained. Low-dose corticosteroids (prednisone, prednisolone, or dexamethasone) are indicated in patients with adrenal hyperandrogenism or acne fulminans. New developments and future trends represent low-dose long-term isotretinoin regimens, new isotretinoin formulations (micronized isotretinoin), isotretinoin metabolites, combination treatments to reduce toxicity, insulin-sensitizing agents, 5alpha-reductase type 1 inhibitors, antisense oligonucleotide molecules, and, especially, new anti-inflammatory agents, such as lipoxygenase inhibitors.

Peroxisome proliferator-activated receptors in cutaneous biology

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that regulate the expression of target genes involved in many cellular functions including cell proliferation, differentiation and immune/inflammation response. The PPAR subfamily consists of three isotypes: PPAR alpha, PPAR beta/delta and PPAR gamma, which have all been identified in keratinocytes. PPAR beta/delta is the predominant subtype in human keratinocytes, whereas PPAR alpha and PPAR gamma are expressed at much lower levels and increase significantly upon keratinocyte differentiation. PPAR beta/delta is not linked to differentiation, but is significantly upregulated upon various conditions that result in keratinocyte proliferation, and during skin wound healing. In vitro and in vivo evidence suggests that PPARs appear to play an important role in skin barrier permeability, inhibiting epidermal cell growth, promoting epidermal terminal differentiation and regulating skin inflammatory response by diverse mechanisms. These proprieties are pointing in the direction of PPARs being key regulators of skin conditions characterized by hyperproliferation, inflammatory infiltrates and aberrant differentiation such as psoriasis, but may also have clinical implications in inflammatory skin disease (e.g. atopic dermatitis), proliferative skin disease, wound healing, acne and protease inhibitor associated lipodystrophia.

Nukleäre Hormonrezeptoren: Interventionsmöglichkeiten jenseits von Glukokortikoiden, Retinoiden und Vitamin D

Nuclear hormone receptors are ligand-dependent transcription factors. The class of nuclear hormone receptors that form heterodimers with retinoid X receptor includes members that are well established targets of current dermatological therapeutics such as the retinoic acid receptor (RAR) and the vitamin D receptor (VDR) as well as more recently discovered receptors including the peroxisome proliferator-activated receptors (PPAR) and the liver X receptor (LXR). After ligand activation (often lipid metabolites), these intracellular receptors exert their functions by binding to specific response elements in regulatory sequences of target genes, preferentially those involved in differentiation, energy expenditure and lipid metabolism. A number of selective activators has been developed by combinational chemistry, initially for their anti-diabetic and lipid lowering properties as well as their ability to regulate bile acid and drug metabolism. However, these activators also have marked effects on cutaneous homeostasis. Therefore, these compounds have important implications for dermatological therapy. In this review, the clinical implications of the more recently discovered members of the nuclear hormone receptor family are discussed.

Genomic organization of the mouse peroxisome proliferator-activated receptor beta/delta gene: alternative promoter usage and splicing yield transcripts exhibiting differential translational efficiency

Peroxisome proliferator-activated receptor (PPAR) beta/delta is ubiquitously expressed, but the level of expression differs markedly between different cell types. In order to determine the molecular mechanisms governing PPARbeta/delta gene expression, we have isolated and characterized the mouse gene encoding PPARbeta/delta. The gene spans approx. 41 kb and comprises 11 exons of which the six exons located in the 3'-end of the gene are included in all transcripts. Primer-extension and 5'-rapid amplification of cDNA ends experiments revealed the presence of multiple transcription start points and splice variants, originating from the use of at least four different promoters. One of these transcription start points was found to be used predominantly in all tissues examined. Initiation from this major transcription start point gives rise to a transcript with a 548 nt 5'-untranslated leader containing eight upstream AUG codons. We show that the presence of the 548 nt leader resulted in a low translational efficiency of the corresponding PPARbeta/delta mRNA and propose, based on structural features of the 5'-untranslated region, that translational initiation may be mediated via an internal ribosome entry site-dependent mechanism.

Peroxisome proliferator-activated receptors and skin development

PPARs are nuclear hormone receptors. PPAR subtypes (alpha, gamma, delta, the latter a xPPARbeta homologue) were initially investigated in skin because of their known role in regulating lipid metabolism. Studies adding specific PPAR ligand activators to cultured skin or skin cells are compatible with the concepts that PPARalpha activation mediates early lipogenic steps common to the function of both skin epidermal cells (keratinocytes) and sebaceous cells (sebocytes), PPARgamma activation plays a unique role in stimulating sebocyte lipogenesis, and PPARdelta activation may contribute to lipid biosynthesis in both sebocytes and keratinocytes under certain circumstances. Epidermal keratinocytes appear to express small amounts of PPARalpha and PPARdelta mRNA and a trace of PPARgamma mRNA which is up-regulated with differentiation. Sebocytes express all subtypes; PPARgamma gene expression excedes that in epidermis. The emerging data on PPAR protein expression suggests that epidermis normally expresses predominantly PPARalpha, while sebocytes express more PPARgamma than PPARalpha. These expression patterns may change during hyperplasia, differentiation and inflammation. Gene disruption studies in mice are compatible with a contribution of PPARalpha to skin barrier function, suggest that PPARgamma is necessary for sebocyte differentiation, and indicate that PPARdelta can ameliorate inflammatory responses in skin. PPARs appear to play a role in keratinocyte synthesis of the lipids that they export to the intercellular space to form the skin permeability barrier. They also appear to be important for sebocyte formation of the intracellular fused lipid droplets that constitute the holocrine secretion of the sebaceous gland. In addition, they may play roles in keratinocyte growth and differentiation and the inhibition of skin inflammation by diverse mechanisms not necessarily related to fat metabolism.

Sebaceous epithelial cell differentiation requires cyclic adenosine monophosphate generation

The cyclic adenosine monophosphate (cAMP) generator choleratoxin is known to promote the growth of sebaceous epithelial cells (sebocytes) in monolayer culture in classical serum-containing media. Now that sebocytes can be grown in serum-free medium, we have examined whether choleratoxin or other cAMP generators are required for differentiation of rat preputial sebocytes in response to specific ligand activators of peroxisome proliferator-activated receptors (PPARs). Unexpectedly, choleratoxin reduced sebocyte proliferation. However, sebocyte differentiation in response to specific PPARalpha and PPARgamma agonists required a cAMP generator such as choleratoxin, and this response was suppressed by a protein kinase A inhibitor. In contrast, the stable prostacyclin analog, carbaprostacyclin (cPGI2), a PPARalpha,delta agonist that also generates cAMP, stimulated differentiation independently of choleratoxin. Furthermore, unlike the selective PPARalpha and PPARgamma agonists, cPGI2 stimulated both sebocyte DNA synthesis and proliferation. These data are compatible with the evidence that prostacyclin has the additional effect of generating cAMP. In addition, we addressed the possibility that choleratoxin may act as a surrogate for beta-adrenergic catecholamines in generating cAMP. In contrast with choleratoxin, both alpha- and beta-adrenergic catecholamines stimulated sebocyte growth and interfered with the choleratoxin effect on differentiation. These data suggest ligand-dependent, complex interactions between cAMP and the other signal transduction pathways involved in sebocyte growth and development.

Contrasting expression patterns of CCAAT/enhancer-binding protein transcription factors in the hair follicle and at different stages of the hair growth cycle

Hair follicles undergo repeated cycles of growth and regression, throughout the entire life of the organism. These dynamic changes require closely co-ordinated regulation of gene expression. The CCAAT/enhancer-binding proteins are a family of basic region/leucine zipper transcription factors that regulate gene transcription in various tissues. They have been implicated in epidermal differentiation and may therefore play an important role in the hair follicle. We have investigated the localization of four members of this family--CCAAT/enhancer-binding protein-alpha, -beta, and -delta, and Gadd153--in both human and murine hair follicles by immunohistochemistry. Furthermore, we examined CCAAT/enhancer-binding protein-alpha, -beta, and -delta immunoreactivity at different stages of the depilation-induced murine hair growth cycle. Distinct immunoreactivity patterns for CCAAT/enhancer-binding protein-alpha, -beta, and -delta, and Gadd153 were observed in the outer root sheath, sebaceous gland, dermal papilla, and connective tissue sheath of human anagen hair follicles. In murine follicles, CCAAT/enhancer-binding protein-alpha was expressed in the outer root sheath, sebaceous gland, and dermal papilla, whereas CCAAT/enhancer-binding protein-beta expression was confined to the matrix, sebaceous gland, and inner and outer root sheaths. Both CCAAT/enhancer-binding protein-alpha and -beta were upregulated during anagen, then downregulated in catagen follicles. In contrast, CCAAT/enhancer-binding protein-delta showed no hair cycle-dependent variation in immunoreactivity. These data suggests that the expression of CCAAT/enhancer-binding protein-alpha and -beta may, in turn, play a part in regulating hair cycle-dependent gene expression. Moreover, as CCAAT/enhancer-binding protein-alpha, -beta, and -delta are crucial in the regulation of adipocyte differentiation and lipid metabolism, their expression in sebocytes suggests they may also play a similar role in differentiation and lipid metabolism of the sebaceous gland.

Limited cooperation between peroxisome proliferator-activated receptors and retinoid X receptor agonists in sebocyte growth and development

We have found that sebaceous epithelial cell (sebocyte) differentiation is induced by cognate ligand-agonists of either peroxisome proliferator-activated receptors (PPARs) or retinoid X receptors (RXRs). In this study, we tested the hypothesis that PPAR-RXR cooperation is used in sebocytes as was reported to occur in gene transfection systems and liposarcoma cells through PPAR-RXR heterodimerization. PPAR agonists at maximally effective concentrations were tested in combination with a specific RXR agonist (the rexinoid CD2809) at doses ranging from submaximal to maximal in a primary rat preputial cell monolayer culture system. We evaluated ligand-agonists of PPARalpha (WY-14643 = WY), PPARgamma (troglitazone = TRO), and PPARdelta,alpha (carbaprostacyclin = cPGI2). Cell differentiation was determined by analysis of lipid staining and proliferation by cell counting. The RXR agonist induced a more diffuse and granular pattern of lipid staining throughout colonies than did PPAR agonists. The PPAR ligands WY, TRO, and cPGI2 induced 37, 35, and 59% lipid-forming colonies (LFCs), respectively (P < 0.05 vs controls, which averaged 19%). Low-dose rexinoid (10(-8) M) alone exerted no significant effect but amplified the effect of cPGI2 (P < 0.05). Middose rexinoid (10(-7) M), which alone induced about 40% LFCs, had an additive effect on differentiation with WY, TRO, and cPGI2 (71, 48, and 83% LFCs respectively, P < 0.05 vs each agonist alone). Proliferation was enhanced significantly by either rexinoid or cPGI2, but there was no change in growth when the two were added together. The greater effectiveness of cPGI2 than the other PPAR agonists may be explained by the predominance of PPARdelta gene expression in cultured sebocytes, as demonstrated by RNase protection assay. These studies demonstrate that a submaximal dose of RXR agonist augmented the stimulation of sebocyte differentiation by PPAR agonists, as expected from PPAR-RXR heterodimerization. However, the evidence for PPAR-RXR cooperativity is limited. The pattern of lipid staining is compatible with an independent effect of rexinoid on sebocyte differentiation. Furthermore, since there is no enhancement of the growth-promoting effects of RXR agonist and cPGI2 when they are combined, this effect also does not appear to be mediated by PPAR-RXR interaction.

Epidermal growth factor and 1alpha,25-dihydroxyvitamin D3 suppress lipogenesis in hamster sebaceous gland cells in vitro

We have previously reported the establishment of a culture system of hamster auricular sebocytes. Although their morphologic and biochemical properties are very similar to those of human sebocytes, the regulation of lipogenesis is not clear. Therefore, we investigated the effect of epidermal growth factor, all-trans retinoic acid, 1alpha,25-dihydroxyvitamin D3, and androgens such as testosterone and 5alpha-dihydrotestosterone on lipogenesis in cultured hamster sebocytes. Intracellular lipid droplets detected with Oil-Red-O staining were observed in 5 d cultures and increased in a time-dependent manner; 40.7% +/- 1.11% of 2 wk cultured cells tested lipid-positive by flow cytometric analysis. When the hamster sebocytes were cultured in the presence of epidermal growth factor, all-trans retinoic acid, or 1alpha,25-dihydroxyvitamin D3, the intracellular lipid droplets were diminished by all-trans retinoic acid and epidermal growth factor, and slightly by 1alpha,25-dihydroxyvitamin D3. The intracellular lipid droplets consisted mainly of triglycerides (71.8%) and, as minor components, cholesterol (18.0%), wax esters (3.6%), and free fatty acids (6.6%). Epidermal growth factor and all-trans retinoic acid decreased the intracellular accumulation of triglycerides (92.6% and 96.1% inhibition, respectively) and free fatty acids (54.3% and 62.6% inhibition, respectively) in the sebocytes. In addition, 1alpha,25-dihydroxyvitamin D3 decreased the triglyceride level (34.3% inhibition), but augmented the accumulation of wax esters (30% increase). There was no difference in the level of cholesterol as a result of these treatments, however. In contrast, 5alpha-dihydrotestosterone augmented the formation of intracellular lipid droplets along with an increase in the accumulation of triglycerides in hamster sebocytes. Our findings that regulation of lipogenesis by all-trans retinoic acid and androgen in hamster sebocytes is identical to regulation in humans suggest that hamster sebocytes are useful for the elucidation of sebaceous function at the cellular level. Furthermore, this is the first evidence that epidermal growth factor and 1alpha,25-dihydroxyvitamin D3 may act as suppressors in the regulation of lipogenesis in hamster sebocytes in vitro.

Chronological ageing and photoageing of the human sebaceous gland

The human sebaceous gland undergoes both extrinsic and intrinsic ageing. The latter is associated with morphological changes and alteration in the sebaceous gland activity. The high androgen-dependent sebum secretion in neonates falls during childhood, starts to rise again during puberty and reaches its maximum in young adults. While the number of sebaceous glands remains the same during life, sebum levels tend to decrease after menopause in females, whereas no major changes appear until the eighth decade of life in men. Reduced androgen levels in aged individuals lead to a slow cellular turnover in the sebaceous glands resulting in hyperplasia of the facial sebaceous glands in advanced age. Ultraviolet radiation and immune suppression (cyclosporin A with corticosteroids) represent cofactors for the development of sebaceous gland hyperplasia. Current molecular findings indicate that overexpression of the ageing-associated gene Smad7 and parathormone-related protein correlate with sebaceous gland hyperplasia, whereas c-myc overexpression is associated with enhanced sebum production. On the other hand, down-regulation of the mismatch repair genes hMLH-1 and hMSH-2 may promote the development of sebaceous gland carcinoma. In addition to spontaneous single tumours, sebaceous gland carcinomas have been reported in immune-suppressed transplant recipients (azathiorpine, cisplatin, cyclosporin A) and in association with the Muir-Torre syndrome. Microsatellite instability with a loss of the mismatch repair gene hMSH-2 has been detected in immune suppressed patients and under photo-induced DNA damage. Topical and systemic oestrogens offer treatment options for skin xerosis in menopausal females. A combination of isotretinoin and interferon-alpha may prevent tumour development in patients with Muir-Torre syndrome.

Polycystic ovary syndrome and insulin-resistant hyperinsulinemia

In a broad sense, polycystic ovary syndrome (PCOS) may be considered to be synonymous with chronic unexplained hyperandrogenemia, which accounts for approximately 95% of hyperandrogenism in women. PCOS comprises a mosaic of classic and nonclassic forms, which may ultimately prove to have distinct genetic determinants. The hyperandrogenism appears to arise from generalized abnormal regulation (dysregulation) of steroidogenesis. This dysregulation seems to result from imbalance among the various extrinsic and intrinsic factors involved in the modulation of trophic hormone action. Hyperinsulinemia seems to be an important extrinsic factor in many cases of PCOS; it results from resistance to the effects of insulin on glucose metabolism. The elevation in insulin levels may precipitate hyperandrogenemia in genetically vulnerable individuals by unmasking latent abnormalities in the regulation of steroidogenesis. One of these may be a polycystic ovary gene that is expressed in the male as pattern baldness. Insulin also seems to be one of many factors that interact with androgen to regulate pilosebaceous unit development. Treatment of PCOS with antidiabetic insulin-lowering agents may improve ovarian function and androgen levels; it remains to be determined whether it will benefit the pilosebaceous unit manifestations of the disorder.

Abnormal differentiation of epidermis in transgenic mice constitutively expressing cyclooxygenase-2 in skin

In prostanoid biosynthesis, the first two steps are catalyzed by cyclooxygenases (COX). In mice and humans, deregulated expression of COX-2, but not of COX-1, is characteristic of epithelial tumors, including squamous cell carcinomas of skin. To explore the function of COX-2 in epidermis, a keratin 5 promoter was used to direct COX-2 expression to the basal cells of interfollicular epidermis and the pilosebaceous appendage of transgenic mouse skin. COX-2 overexpression in the expected locations, resulting in increased prostaglandin levels in epidermis and plasma, correlated with a pronounced skin phenotype. Heterozygous transgenic mice exhibited a reduced hair follicle density. Moreover, postnatally hair follicle morphogenesis and thinning of interfollicular dorsal epidermis were delayed. Adult transgenics showed a body-site-dependent sparse coat of greasy hair, the latter caused by sebaceous gland hyperplasia and increased epicutaneous sebum levels. In tail skin, hyperplasia of scale epidermis reflecting an increased number of viable and cornified cell layers was observed. Hyperplasia was a result of a disturbed program of epidermal differentiation rather than an increased proliferation rate, as reflected by the strong suppression of keratin 10, involucrin, and loricrin expression in suprabasal cells. Further pathological signs were loss of cell polarity, mainly of basal keratinocytes, epidermal invaginations into the dermis, and formation of horn perls. Invaginating hyperplastic lobes were surrounded by CD31-positive vessels. These results demonstrate a causal relationship between transgenic COX-2 expression in basal keratinocytes and epidermal hyperplasia as well as dysplastic features at discrete body sites.

Role of hormones in pilosebaceous unit development

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.

The role of specific retinoid receptors in sebocyte growth and differentiation in culture

Retinoic acid derivatives (retinoids) exert their pleiotropic effects on cell development through specific nuclear receptors, the retinoic acid receptors and retinoid X receptors. Despite recent progress in understanding the cellular and molecular mechanisms of retinoid activity, it is unknown which of the retinoid receptor pathways are involved in the specific processes of sebocyte growth and development. In this study, we investigated the roles of specific retinoid receptors in sebocyte growth and differentiation, by testing the effects of selective retinoic acid receptor and retinoid X receptor ligands at concentrations between 10-10 M and 10-6 M in a primary rat preputial cell monolayer culture system. Cell growth was determined by number of cells and colonies, and cell differentiation by analysis of lipid-forming colonies. All-trans retinoic acid and selective retinoic acid receptor agonists (CD271 = adapalene, an RAR-beta,gamma agonist; CD2043 = retinoic acid receptor pan-agonist; and CD336 = Am580, an RAR-alpha agonist) caused significant decreases in numbers of cells, colonies, and lipid-forming colonies, but with an exception at high doses of all-trans retinoic acid (10-6 M), with which only a small number of colonies grew but they became twice as differentiated as controls (42.2 +/- 4.0% vs 22.6 +/- 2.7%, mean +/- SEM, lipid-forming colonies, p < 0.01). Furthermore, the RAR-beta,gamma antagonist CD2665 antagonized the suppressive effects of all-trans retinoic acid, adapalene, and CD2043 on both cell growth and differentiation. In contrast, the retinoid X receptor agonist CD2809 increased cell growth slightly and lipid-forming colonies dramatically in a clear dose-related manner to a maximum of 73.7% +/- 6.7% at 10-6 M (p < 0. 001). Our data suggest that retinoic acid receptors and retinoid X receptors differ in their roles in sebocyte growth and differentiation: (i) retinoic acid receptors, especially the beta and/or gamma subtypes, mediate both the antiproliferative and antidifferentiative effects of retinoids; (ii) retinoid X receptors mediate prominent differentiative and weak proliferative effects; (iii) the antiproliferative and antidifferentiative effects of all-trans retinoic acid are probably mediated by retinoic acid receptors, whereas its differentiative effect at high dose may be mediated by retinoid X receptors via all-trans retinoic acid metabolism to 9-cis retinoic acid, the natural ligand of retinoid X receptors.

PPAR gamma is required for the differentiation of adipose tissue in vivo and in vitro

The process of adipogenesis is known to involve the interplay of several transcription factors. Activation of one of these factors, the nuclear hormone receptor PPAR gamma, is known to promote fat cell differentiation in vitro. Whether PPAR gamma is required for this process in vivo has remained an open question because a viable loss-of-function model for PPAR gamma has been lacking. We demonstrate here that mice chimeric for wild-type and PPAR gamma null cells show little or no contribution of null cells to adipose tissue, whereas most other organs examined do not require PPAR gamma for proper development. In vitro, the differentiation of ES cells into fat is shown to be dependent on PPAR gamma gene dosage. These data provide direct evidence that PPAR gamma is essential for the formation of fat.