LXRα Enhances Lipid Synthesis in SZ95 Sebocytes

The primary role of sebum in the pathophysiology of acne vulgaris and its therapeutic relevance in acne management

BACKGROUND

Sebum physiology and its contributions to acne vulgaris (AV) pathophysiology have been long debated. Within the pilosebaceous unit, androgens drive sebocyte production of sebum, comprising mono-, di-, and triglycerides (the latter converted to fatty acids); squalene; cholesterol; cholesterol esters; and wax esters. Upon release to the skin surface, human sebum has important roles in epidermal water retention, antimicrobial defenses, and innate immune responses.

AIMS

Alterations in sebum alone and with other pathogenic factors (inflammation, follicular hyperkeratinization, and Cutibacterium acnes [C. acnes] proliferation) contribute to AV pathophysiology. Androgen-driven sebum production, mandatory for AV development, propagates C. acnes proliferation and upregulates inflammatory and comedogenic cascades.

RESULTS

Some sebum lipids have comedogenic effects in isolation, and sebum content alterations (including elevations in specific fatty acids) contribute to AV pathogenesis. Regional differences in facial sebum production, coupled with patient characteristics (including sex and age), help exemplify this link between sebum alterations and AV lesion formation.

CONCLUSIONS

To date, only combined oral contraceptives and oral spironolactone (both limited to female patients), oral isotretinoin and topical clascoterone (cortexolone 17α-propionate) modulate sebum production in patients with AV. A better understanding of mechanisms underlying sebaceous gland changes driving AV development is needed to expand the AV treatment armamentarium.

Correlation between meibomian gland dysfunction and sebaceous adenitis in dogs

BACKGROUND

Sebaceous adenitis (SA) is an immune-mediated disease targeting the sebaceous glands. Meibomian gland dysfunction is a disease affecting meibomian glands with inflammatory features. Sebaceous and meibomian glands share anatomical, physiological and embryological similarities. The involvement of meibomian glands in dogs with SA is currently unknown.

OBJECTIVES

To evaluate meibomian glands in dogs affected by SA and compare them with healthy dogs.

ANIMALS

Eighteen dogs were enrolled. Nine dogs with SA were retrospectively identified from clinical records and represented the case group. Nine healthy, breed- and age-matched dogs were prospectively enrolled in the control group.

MATERIALS AND METHODS

Both groups underwent dermatological examination, Schirmer tear test-1 (STT-1), tear meniscus height (TMH), slit-lamp biomicroscopy, interferometry (INT) and noncontact infrared meibography (NIM).

RESULTS

One third of SA dogs presented subepithelial crystalline opacities. No significant difference between groups was observed in TMH (p = 0.944) and STT-1 values (p = 0.066). INT (p = 0.016) and NIM grades (p = 0.010) were significantly higher and lower in the SA group compared to the control group, respectively. INT values decreased with age (η = 0.930), while NIM scores (η = 0.935) increased.

CONCLUSIONS

Clinical Relevance: Subepithelial crystalline opacities in SA dogs might reflect a reduced tear film quality. In the absence of standardised methods, INT and NIM proved to be noninvasive and useful methods to examine meibomian glands. Dogs with SA showed a thinner lacrimal lipid layer and more severe meibomian gland abnormalities than control dogs, which seemed to progress with age.

Thymic stromal lymphopoietin induces IL-4/IL-13 from T cells to promote sebum secretion and adipose loss

BACKGROUND

The cytokine TSLP promotes type 2 immune responses and can induce adipose loss by stimulating lipid loss from the skin through sebum secretion by sebaceous glands, which enhances the skin barrier. However, the mechanism by which TSLP upregulates sebaceous gland function is unknown.

OBJECTIVES

This study investigated the mechanism by which TSLP stimulates sebum secretion and adipose loss.

METHODS

RNA-sequencing analysis was performed on sebaceous glands isolated by laser capture microdissection and single-cell RNA-sequencing analysis was performed on sorted skin T cells. Sebocyte function was analyzed by histological analysis and sebum secretion in vivo and by measuring lipogenesis and proliferation in vitro.

RESULTS

This study found that TSLP sequentially stimulated the expression of lipogenesis genes followed by cell death genes in sebaceous glands to induce holocrine secretion of sebum. TSLP did not affect sebaceous gland activity directly. Rather, single-cell RNA-sequencing revealed that TSLP recruited distinct T-cell clusters that produce IL-4 and IL-13, which were necessary for TSLP-induced adipose loss and sebum secretion. Moreover, IL-13 was sufficient to cause sebum secretion and adipose loss in vivo and to induce lipogenesis and proliferation of a human sebocyte cell line in vitro.

CONCLUSIONS

This study proposes that TSLP stimulates T cells to deliver IL-4 and IL-13 to sebaceous glands, which enhances sebaceous gland function, turnover, and subsequent adipose loss.

Nuclear receptors for epidermal lipid barrier: Advances in mechanisms and applications

The skin plays an essential role in preventing the entry of external environmental threats and the loss of internal substances, depending on the epidermal permeability barrier. Nuclear receptors (NRs), present in various tissues and organs including full-thickness skin, have been demonstrated to exert significant effects on the epidermal lipid barrier. Formation of the lipid lamellar membrane and the normal proliferation and differentiation of keratinocytes (KCs) are crucial for the development of the epidermal permeability barrier and is regulated by specific NRs such as PPAR, LXR, VDR, RAR/RXR, AHR, PXR and FXR. These receptors play a key role in regulating KC differentiation and the entire process of epidermal lipid synthesis, processing and secretion. Lipids derived from sebaceous glands are influenced by NRs as well and participate in regulation of the epidermal lipid barrier. Furthermore, intricate interplay exists between these receptors. Disturbance of barrier function leads to a range of diseases, including psoriasis, atopic dermatitis and acne. Targeting these NRs with agonists or antagonists modulate pathways involved in lipid synthesis and cell differentiation, suggesting potential therapeutic approaches for dermatosis associated with barrier damage. This review focuses on the regulatory role of NRs in the maintenance and processing of the epidermal lipid barrier through their effects on skin lipid synthesis and KC differentiation, providing novel insights for drug targets to facilitate precision medicine strategies.

Research progress on the role of macrophages in acne and regulation by natural plant products

Acne vulgaris is one of the most common skin diseases. The current understanding of acne primarily revolves around inflammatory responses, sebum metabolism disorders, aberrant hormone and receptor expression, colonization by Cutibacterium acnes, and abnormal keratinization of follicular sebaceous glands. Although the precise mechanism of action remains incompletely understood, it is plausible that macrophages exert an influence on these pathological features. Macrophages, as a constituent of the human innate immune system, typically manifest distinct phenotypes across various diseases. It has been observed that the polarization of macrophages toward the M1 phenotype plays a pivotal role in the pathogenesis of acne. In recent years, extensive research on acne has revealed an increasing number of natural remedies exhibiting therapeutic efficacy through the modulation of macrophage polarization. This review investigates the role of cutaneous macrophages, elucidates their potential significance in the pathogenesis of acne, a prevalent chronic inflammatory skin disorder, and explores the therapeutic mechanisms of natural plant products targeting macrophages. Despite these insights, the precise role of macrophages in the pathogenesis of acne remains poorly elucidated. Subsequent investigations in this domain will further illuminate the pathogenesis of acne and potentially offer guidance for identifying novel therapeutic targets for this condition.

Insulin and the sebaceous gland function

Insulin affects metabolic processes in different organs, including the skin. The sebaceous gland (SG) is an important appendage in the skin, which responds to insulin-mediated signals, either directly or through the insulin growth factor 1 (IGF-1) axis. Insulin cues are differently translated into the activation of metabolic processes depending on several factors, including glucose levels, receptor sensitivity, and sebocyte differentiation. The effects of diet on both the physiological function and pathological conditions of the SG have been linked to pathways activated by insulin and IGF-1. Experimental evidence and theoretical speculations support the association of insulin resistance with acne vulgaris, which is a major disorder of the SG. In this review, we examined the effects of insulin on the SG function and their implications in the pathogenesis of acne.

Linoleic Acid Induced Changes in SZ95 Sebocytes-Comparison with Palmitic Acid and Arachidonic Acid

Linoleic acid (LA) is an essential omega-6 polyunsaturated fatty acid (PUFA) derived from the diet. Sebocytes, whose primary role is to moisturise the skin, process free fatty acids (FFAs) to produce the lipid-rich sebum. Importantly, like other sebum components such as palmitic acid (PA), LA and its derivative arachidonic acid (AA) are known to modulate sebocyte functions. Given the different roles of PA, LA and AA in skin biology, the aim of this study was to assess the specificity of sebocytes for LA and to dissect the different roles of LA and AA in regulating sebocyte functions. Using RNA sequencing, we confirmed that gene expression changes in LA-treated sebocytes were largely distinct from those induced by PA. LA, but not AA, regulated the expression of genes related to cholesterol biosynthesis, androgen and nuclear receptor signalling, keratinisation, lipid homeostasis and differentiation. In contrast, a set of mostly down-regulated genes involved in lipid metabolism and immune functions overlapped in LA- and AA-treated sebocytes. Lipidomic analyses revealed that the changes in the lipid profile of LA-treated sebocytes were more pronounced than those of AA-treated sebocytes, suggesting that LA may serve not only as a precursor of AA but also as a potent regulator of sebaceous lipogenesis, which may not only influence the gene expression profile but also have further specific biological relevance. In conclusion, we have shown that sebocytes are able to respond selectively to different lipid stimuli and that LA-induced effects can be both AA-dependent and independent. Our findings allow for the consideration of LA application in the therapy of sebaceous gland-associated inflammatory skin diseases such as acne, where lipid modulation and selective targeting of AA metabolism are potential treatment options.

The Critical Role of Galectin-12 in Modulating Lipid Metabolism in Sebaceous Glands

Sebaceous glands play an important role in maintaining the skin barrier function by producing lipids. Dysregulated lipid production in these glands may contribute to the pathogenesis of human skin diseases. Galectin-12, a member of the β-galactoside‒binding lectin family, is preferentially expressed in adipocytes, where it regulates adipogenesis and functions as an intrinsic negative regulator of lipolysis. It is also expressed by sebocytes and contributes to the proliferation of this cell type. In this study, we show the association between galectin-12 expression and sebocyte differentiation. Galectin-12 knockdown in a human sebocyte cell line reduced lipogenesis and decreased the production of cholesteryl esters, triglycerides, free fatty acids, and cholesterol. Metabolomic analysis of skin surface lipids showed that the levels of the lipids mentioned earlier decreased in sebaceous gland‒specific galectin-12‒knockout mice compared with that in wild-type mice. In addition, galectin-12 positively regulated peroxisome proliferator‒activated receptor-γ transcriptional activity in sebocytes stimulated with fatty acids. Downregulating galectin-12 suppressed the expression of peroxisome proliferator‒activated receptor-γ target genes-acetyl-coenzyme A synthetase 2 gene ACS2 and diacylglycerol O-acyltransferase 1 gene DGAT1-that are required for fatty acid activation and cholesterol and triglyceride biosynthesis. In conclusion, galectin-12 is a positive regulator of sebaceous lipid metabolism with a potential role in the maintenance of skin homeostasis.

Botulinum Neurotoxin Type A Directly Affects Sebocytes and Modulates Oleic Acid-Induced Lipogenesis

Excess sebum (seborrhea) results in oily skin and is associated with large pore size and acne. Studies in healthy, seborrheic volunteers have reported that intradermal injection of commercial preparations of botulinum neurotoxin type A (BoNT/A) (onabotulinumtoxinA, abobotulinumtoxinA, and incobotulinumtoxinA) reduced sebum production, and thus, skin oiliness and pore size. The mechanism for these effects has not been fully elucidated; however, several theories involving direct or indirect effects of BoNT/A on neuronal and/or dermal cells (e.g., sebocytes) have been proposed. In the present study, we evaluated the direct effect of native research grade BoNT/A complex, a commercial preparation of BoNT/A (onabotA), and BoNT/A variants on sebocyte lipogenesis using an in vitro sebocyte cell model. We show that picomolar concentrations of BoNT/A (BoNT/A complex: half maximal effective concentration [EC₅₀] = 24 pM; BoNT/A 150 kDa: EC₅₀ = 34 pM) modulate sebocyte lipogenesis and reduce oleic acid-induced sebocyte differentiation, lipogenesis, and holocrine-like secretion. Comparative studies with the binding domain of BoNT/A, which lacks enzymatic activity, show that this effect is independent of the enzymatic activity of BoNT/A and likely occurs via sebocyte cell surface receptors (e.g., fibroblast growth factor receptors). Overall, these results shed light on the potential mechanism of action and rationale for use of BoNT/A for treatment of sebum-related conditions.

Ferredoxin reductase regulates proliferation, differentiation, cell cycle and lipogenesis but not apoptosis in SZ95 sebocytes

Ferredoxin reductase (FDXR), a mitochondrial membrane-associated flavoprotein, is essential for electron transfer and modulates p53-dependent apoptosis in cancer cells.FDXR may be implicated in epidermal and sebocytic differentiation, but its explicit function in sebocytes remains to be elucidated. In the present study, immunohistochemistry revealed that FDXR expression was increased in sebaceous cells of acne lesions. FDXR, PPARγ, LXRα/β, SREBP1 and Sox9 expression was incremental during sebocyte differentiation. FDXR overexpression induced by Ad-GFP-FDXR infection enhanced differentiation, reactive oxygen species (ROS), lipogenesis and PPARγ expression, and consequnently inhibited proliferation in SZ95 sebocytes. Flow cytometry showed that FDXR overexpression induced significant blockade of G2/M phase but had no effect on sub-G1 (apoptotic) sebocytes. Insulin-like growth factor-1 (IGF-1)-induced FDXR and PPARγ expression and lipogenesis were abolished by pretreatment with PI3K inhibitor LY294002. These results suggest that FDXR overexpression might promote differentiation and lipogenesis via ROS production and suppress proliferation via G2/S blockade in SZ95 sebocytes. IGF-1 could facilitate differentiation and lipogenesis through PI3K/Akt/FDXR pathway. FDXR could serve as a potential marker of advanced sebaceous differentiation, and its overexpression may be involved in the development of acne lesions.

Epidermal Growth Factor Modulates Palmitic Acid-Induced Inflammatory and Lipid Signaling Pathways in SZ95 Sebocytes

Epidermal growth factor (EGF) acts as a paracrine and autocrine mediator of cell proliferation and differentiation in various types of epithelial cells, such as sebocytes, which produce the lipid-rich sebum to moisturize the skin. However, sebum lipids via direct contact and by penetrating through the epidermis may have regulatory roles on epidermal and dermal cells as well. As EGF receptor (EGFR) is expressed throughout the proliferating and the lipid-producing layers of sebaceous glands (SGs) in healthy and acne-involved skin, we investigated the effect of EGF on SZ95 sebocytes and how it may alter the changes induced by palmitic acid (PA), a major sebum component with bioactive roles. We found that EGF is not only a potent stimulator of sebocyte proliferation, but also induces the secretion of interleukin (IL)6 and down-regulates the expression of genes involved in steroid and retinoid metabolism. Importantly, when applied in combination with PA, the PA-induced lipid accumulation was decreased and the cells secreted increased IL6 levels. Functional clustering of the differentially regulated genes in SZ95 sebocytes treated with EGF, PA or co-treated with EGF+PA further confirmed that EGF may be a potent inducer of hyperproliferative/inflammatory pathways (IL1 signaling), an effect being more pronounced in the presence of PA. However, while a group of inflammatory genes was up-regulated significantly in EGF+PA co-treated sebocytes, PA treatment in the absence of EGF, regulated genes only related to cell homeostasis. Meta-analysis of the gene expression profiles of whole acne tissue samples and EGF- and EGF+PA -treated SZ95 sebocytes showed that the EGF+PA co-activation of sebocytes may also have implications in disease. Altogether, our results reveal that PA-induced lipid accumulation and inflammation can be modulated by EGF in sebocytes, which also highlights the need for system biological approaches to better understand sebaceous (immuno)biology.

Histone Deacetylase 1 Reduces Lipogenesis by Suppressing SREBP1 Transcription in Human Sebocyte Cell Line SZ95

Proper regulation of sebum production is important for maintaining skin homeostasis in humans. However, little is known about the role of epigenetic regulation in sebocyte lipogenesis. We investigated histone acetylation changes and their role in key lipogenic gene regulation during sebocyte lipogenesis using the human sebaceous gland cell line SZ95. Sebocyte lipogenesis is associated with a significant increase in histone acetylation. Treatment with anacardic acid (AA), a p300 histone acetyltransferase inhibitor, significantly decreased the lipid droplet number and the expression of key lipogenic genes, including sterol regulatory-binding protein 1 (SREBP1), fatty acid synthase (FAS), and acetyl-CoA carboxylase (ACC). In contrast, treatment with trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor, increased the expression of these genes. Global HDAC enzyme activity was decreased, and HDAC1 and HDAC2 expression was downregulated during sebaceous lipogenesis. Interestingly, HDAC1 knockdown increased lipogenesis through SREBP1 induction, whereas HDAC1 overexpression decreased lipogenesis and significantly suppressed SREBP1 promoter activity. HDAC1 and SREBP1 levels were inversely correlated in human skin sebaceous glands as demonstrated in immunofluorescence images. In conclusion, HDAC1 plays a critical role in reducing SREBP1 transcription, leading to decreased sebaceous lipogenesis. Therefore, HDAC1 activation could be an effective therapeutic strategy for skin diseases related to excessive sebum production.

Effect of Genetic Polymorphisms of Human SLC22A3 in the 5'-flanking Region on OCT3 Expression and Sebum Levels in Human Skin

BACKGROUND

Human organic cation transporter 3 (OCT3,SLC22A3) mediates the uptake of many important endogenous substances and basic drugs, and has been identified as one of the transporters that are highly expressed in human skin. However, the mechanisms responsible for variability in mRNA expression, and the role of SLC22A3 in human skin is not clear.

OBJECTIVE

We examined the effects of the single nucleotide polymorphisms ofSLC22A3 on the variability in SLC22A3 expression and sebum levels in humans.

METHODS

Immunostaining of OCT3 in human skin was performed. We analyzed the association of promoter variants with the SLC22A3 mRNA expression levels in human skins. Luciferase, knockdown, chromatin immunoprecipitation (ChIP), electrophoretic mobility shift assay were employed to investigate transcriptional regulation of SLC22A3 expression. Effects of the identified variant on sebum levels were evaluated in healthy volunteers.

RESULTS

Immunohistochemistry revealed marked expressions of OCT3 in the basal epidermis, sebaceous glands, hair follicles, and sweat glands of human skin. SLC22A3 mRNA levels were significantly lower in skin samples with homozygotes for -1603A/A than in those for -1603 G/G. The analysis of p53 binding to -1603 G > A in the promoter ofSLC22A3 suggested that -1603 G > A down-regulates SLC22A3 gene expression by decreased p53 binding in the vicinity of the -1603 site. In humans, squalene levels in samples from the back at the baseline were significantly lower in homozygotes for -1603A/A than in those for -1603 G/G.

CONCLUSION

These results suggest that the genetic variant contributes to the variability of expression and activities of OCT3 in human skin.

Establishment and characterization of a canine sebaceous epithelial cell line derived from an eyelid mass

Little is known about the pathological roles of sebaceous glands in canine skin diseases, as most examinations have been conducted with cultured human sebaceous epithelial cell lines. To our knowledge, there is no available canine sebaceous epithelial cell line. The purpose of this study was to establish a canine sebaceous epithelial cell line and characterize it. An eyelid mass in a dog was surgically resected for treatment, and it was histologically diagnosed as sebaceous epithelioma. Collected tissue was conducted for culture, and the growing epithelial-like cells were passaged. The cells showed continuous proliferation for over 6 months. After 40 passages, the cells were named CMG-1. Lipid droplets in the cytoplasm of CMG-1 cells were confirmed by Oil Red O staining. As reported in studies with human sebaceous epithelial cell lines, lipogenesis in CMG-1 cells was promoted by linoleic acid, whereas transforming growth factor-β (TGF-β) suppressed it. Additionally, real-time PCR revealed that the expression levels of chemokines and cytokines, including CC chemokine ligand (CCL)-2, CCL-20, CXCL-10, Tumor necrosis factor-α (TNF-α), Interleukin (IL)-1α, IL-1β, and IL-8, were significantly increased in CMG-1 cells following treatment with lipopolysaccharide. In conclusion, we successfully established a new canine sebaceous epithelial cell line. Our data indicated that lipogenesis and inflammatory responses were quantitatively evaluable in this cell line. CMG-1 cells could be useful for the pathological analysis of sebaceous gland diseases in dogs.

A strategic review on the involvement of receptors, transcription factors and hormones in acne pathogenesis

Acne vulgaris is a very common pilosebaceous inflammatory disease occurring primarily on the face and also rare on the upper arms, trunk, and back, which is caused by Propionibacterium, Staphylococcus, Corynebacterium, and other species. Pathophysiology of acne comprises of irregular keratinocyte proliferation, differentiation, increased sebum output, bacterial antigens and cytokines induced inflammatory response. Treatment of acne requires proper knowledge on the pathophysiology then only the clinician can come out with a proper therapeutic dosage regimen. Understanding the pathophysiology not only includes the mechanism but also involvement of receptors. Thus, this review is framed in such a way that the authors have focused on the disease acne vulgaris, pathophysiology, transcription factors viz. the Forkhead Box O1 (FoxO1) Transcription Factor, hormones like androgens and receptors such as Histamine receptors, Retinoic receptor, Fibroblast growth factor receptors, Toll like receptor, Androgen receptor, Liver X-receptor, Melanocortin receptor, Peroxisome proliferator-activated receptor and epidermal growth factor receptors involvement in the progression of acne vulgaris.

Increased Lipid Accumulation under Hypoxia in SZ95 Human Sebocytes

BACKGROUND

Excessive sebum is produced by specialized cells called sebocytes and is considered a cause or consequence of acne, sebaceous cysts, hyperplasia, and sebaceous adenoma.

OBJECTIVE

To report changes in lipid accumulation in human sebocytes under hypoxia, which occurs under conditions of seborrhea.

METHODS

Sebocytes from the immortalized human gland cell line SZ95 were cultured under conditions of hypoxia for 48 h; lipid formation was confirmed by Nile red and Oil Red O staining. To investigate whether HIF-1α plays a role in lipid accumulation, SZ95 cells transfected or treated with dimethyloxalylglycine (DMOG) were assessed by Nile red. For protein expression of the sterol regulatory element-binding protein-1 (SREBP-1) and perilipin 2 (PLIN2), Western blot analysis was performed. Differentially expressed genes (DEGs) in SZ95 sebocytes under hypoxia were revealed by RNA-Seq analyses, and the statistical significance of the correlation between hypoxic and acne/non-acne skin was evaluated using gene set enrichment analysis.

RESULTS

Hypoxia induces lipid accumulation in SZ95 sebocytes. In addition, the levels of SREBP-1 and PLIN2 were regulated by HIF-1α in SZ95 sebocytes under hypoxia. RNA-Seq analyses of DEGs in SZ95 sebocytes under hypoxia revealed 256 DEGs, including several lipid droplet-associated genes. DEGs between acne and non-acne skin are significantly enriched in hypoxia gene sets. We also detected 93 differentially expressed inflammatory mediators.

CONCLUSIONS

To the best of our knowledge, this study is the first to show that a hypoxic microenvironment can increase lipogenesis and provides a link between seborrhea and inflammation.

Differentiation Model Establishment and Differentiation-Related Protein Screening in Primary Cultured Human Sebocytes

Sebocyte differentiation is a continuous process, but its potential molecular mechanism remains unclear. We aimed to establish a novel sebocyte differentiation model using human primary sebocytes and to identify the expression profiles of differentiation-associated proteins. Primary human sebocytes were cultured on Sebomed medium supplemented with 2% serum for 7 days. Flow cytometry showed that S phase cells were decreased time-dependently, while G1 and subG1 (apoptosis) phase cells increased under serum starvation. Transmission electron microscopy and Oil Red O staining revealed a gradual increase of intracellular lipid accumulation. Expression of proliferation marker was diminished, while expression of differentiation, apoptosis, and lipogenic markers elevated gradually during 7-day culture. iTRAQ analysis identified 3582 expressed proteins in this differentiation model. Compared with day 0, number of differentially expressed proteins was 132, 54, 321, and 96 at days 1, 3, 5, and 7, respectively. Two overexpressed proteins (S100 calcium binding protein P and ferredoxin reductase) and 2 downexpressed proteins (adenosine deaminase and keratin 10) were further confirmed by Western blot and immunohistochemistry.

Sebaceous-immunobiology is orchestrated by sebum lipids

The major role of sebaceous glands in mammals is to produce sebum, which coats the epidermis and the hair providing waterproofing, thermoregulation and photoprotection. However, as the need for these functions decreased along the evolutionary changes in humans, a relevant question has been raised: are sebaceous glands and sebum the remnants of our mammalian heritage or do they have overtaken a far more complex role in human skin biology? Trying to provide answers to this question, this review introduces the evolving field of sebaceous immunobiology and puts into the focus the pathways that sebum lipids use to influence the immune milieu of the skin. By introducing possible modifiers of sebaceous lipogenesis and discussing the – human-specific – alterations in composition and amount of sebum, the attribute of sebum as a sensitive tool, which is capable of translating multiple signalling pathways into the dermal micro environment is presented. Further their interaction with macrophages and keratinocytes involves sebum lipid fractions into disease pathogenesis, which could lead – on the other side – to the development of novel sebum-based therapeutic strategies.

Immunohistochemical Expression of Cyclo-oxygenase 2 and Liver X Receptor-α in Acne Vulgaris

Introduction

Acne Vulgaris (AV) is a common inflammatory disease of pilosebaceous units. Liver X Receptor-α (LXR-α) is a ligand activated transcription factor. It controls transcription of genes involved in lipid and fatty acid synthesis. Cyclo-oxygenase 2 (COX2) is a rate limiting enzyme in prostaglandin synthesis. It plays important role in inflammation.

Aim

To evaluate the immunohistochemical expression of LXR-α and COX2 in acne vulgaris skin biopsies to explore their possible pathogenic role in this disease.

Materials and Methods

Sixty five subjects were included (45 cases with AV and 20 age and gender-matched healthy controls). Skin biopsies were taken from lesional and perilesional skin of cases and from site-matched areas of control subjects. The evaluation of LXR-α and COX2 was done using immunohistochemical technique. Data were collected, tabulated and statistically analysed using a personal computer with "(SPSS) version 11" program. Chi-square test was used to study the association between qualitative variables. Mann-Whitney test was used for comparison between quantitative variables. Student's t-test was used for comparison between two groups having quantitative variables. Spearman's coefficient was used to study the correlation between two different variables. Differences were considered statistically significant with p<0.05.

Results

COX2 was upregulated in lesional skin compared with peilesional and control skin both in epidermis and pilosebaceous units (p<0.001 for all). Higher epidermal COX2% was significantly associated with papulopustular acne (p=0.009) and higher acne score (p=0.018). Higher pilosebaceous units COX2% was significantly associated with papulopustular acne (p=0.04). LXR-α was upregulated in lesional skin compared with peilesional and control skin both in epidermis and pilosebaceous units (p<0.001 for all). Higher LXR-α % in epidermis and pilosebaceous units was significantly associated with papulopustular acne (p=0.01 for both) and higher acne score (p=0.03 for both). Significant positive correlation was detected between COX2% and LXR-α % in epidermis (p=0.001, r=0.87) and pilosebaceous units (p=0.001, r=0.65).

Conclusion

Both LXR-α and COX-2 play a role in the pathogenesis of acne vulgaris through their effects on cellular proliferation, inflammation and lipid synthesis. Research for new therapeutic modalities based on their inhibition is needed. More understanding of the interaction between LXR-α, COX2 and acne lesions may lead to effective interference, possibly directed toward specific cell types or steps within inflammatory pathways.

Beyond acne: Current aspects of sebaceous gland biology and function

The sebaceous gland is most commonly found in association with a hair follicle. Its traditional function is the holocrine production of sebum, a complex mixture of lipids, cell debris, and other rather poorly characterized substances. Due to the gland's central role in acne pathogenesis, early research had focused on its lipogenic activity. Less studied aspects of the sebaceous gland, such as stem cell biology, the regulation of cellular differentiation by transcription factors, the significance of specific lipid fractions, the endocrine and specially the neuroendocrine role of the sebaceous gland, and its contribution to the innate immunity, the detoxification of the skin, and skin aging have only recently attracted the attention of researchers from different disciplines. Here, we summarize recent multidisciplinary progress in sebaceous gland research and discuss how sebaceous gland research may stimulate the development of novel therapeutic strategies targeting specific molecular pathways of the pathogenesis of skin diseases.

The role of PPARγ-mediated signalling in skin biology and pathology: new targets and opportunities for clinical dermatology

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that modulate the expression of multiple different genes involved in the regulation of lipid, glucose and amino acid metabolism. PPARs and cognate ligands also regulate important cellular functions, including cell proliferation and differentiation, as well as inflammatory responses. This includes a role in mediating skin and pilosebaceous unit homoeostasis: PPARs appear to be essential for maintaining skin barrier permeability, inhibit keratinocyte cell growth, promote keratinocyte terminal differentiation and regulate skin inflammation. They also may have protective effects on human hair follicle (HFs) epithelial stem cells, while defects in PPARγ-mediated signalling may promote the death of these stem cells and thus facilitate the development of cicatricial alopecia (lichen planopilaris). Overall, however, selected PPARγ modulators appear to act as hair growth inhibitors that reduce the proliferation and promote apoptosis of hair matrix keratinocytes. The fact that commonly prescribed PPARγ-modulatory drugs of the thiazolidine-2,4-dione class can exhibit a battery of adverse cutaneous effects underscores the importance of distinguishing beneficial from clinically undesired cutaneous activities of PPARγ ligands and to better understand on the molecular level how PPARγ-regulated cutaneous lipid metabolism and PPARγ-mediated signalling impact on human skin physiology and pathology. Surely, the therapeutic potential that endogenous and exogenous PPARγ modulators may possess in selected skin diseases, ranging from chronic inflammatory hyperproliferative dermatoses like psoriasis and atopic dermatitis, via scarring alopecia and acne can only be harnessed if the complexities of PPARγ signalling in human skin and its appendages are systematically dissected.

Emodin isolated from Polygoni Multiflori Ramulus inhibits melanogenesis through the liver X receptor-mediated pathway

Melanogenesis is a physiological process that results in the synthesis of melanin pigments, which play a crucial protective role against skin photocarcinogenesis. We investigated the effects of a Polygoni Multiflori Ramulus extract on melanogenesis and isolated emodin from Polygoni Multiflori as an active compound. In addition, the possible mechanisms of action were examined. We found that emodin inhibited both melanin content and tyrosinase activity concentration and time dependently. Tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2 mRNA levels decreased following emodin treatment. However, while the mRNA levels of microphthalmia-associated transcription factor (MITF) were not affected by emodin, emodin reduced MITF protein levels. Furthermore, expression of the liver X-receptor (LXR) α gene, but not the LXR β gene was upregulated by emodin. Moreover, emodin regulated melanogenesis by promoting degradation of the MITF protein by upregulating the LXR α gene. The emodin effects on MITF was found to be mediated by phosphorylation of p42/44 MAPK. Taken together, these findings indicate that the inhibition of melanogenesis by emodin occurs through reduced MITF protein expression, which is mediated by upregulation of the LXR α gene and suggest that emodin may be useful as a hyperpigmentation inhibitor.

Insulin and insulin-like growth factor-1 can modulate the phosphoinositide-3-kinase/Akt/FoxO1 pathway in SZ95 sebocytes in vitro

A recent hypothesis suggests that a high glycaemic load diet-associated increase of insulin-like growth factor-1 (IGF-1) and insulin may promote acne by reducing nuclear localization of the forkhead box-O1 (FoxO1) transcription factor via activation of the phosphoinositide-3-kinase (PI3K)/Akt pathway. Using SZ95 sebocytes as a model, we investigated the effect of the most important insulinotropic western dietary factors, IGF-1 and insulin on acne. SZ95 sebocytes were stimulated with different concentrations of IGF-1 and insulin (0.001, 0.01, 0.1 and 1 μM) for 15 to 120 min ± PI3K inhibitor LY294002 (50 μM). Cytoplasmic and nuclear protein expression of p-Akt and p-FoxO1 as well as FoxO transcriptional activity was analysed. In addition, the proliferation and differentiation of sebocytes and their TLR2/4 expression were determined. We found that high concentrations of IGF-1 and insulin differentially stimulate the PI3K/Akt/FoxO1 pathway by an early up-regulation of cytoplasmic p-Akt and delayed up-regulation of p-FoxO1 resulting in FoxO1 shift to the cytoplasm and the reduction of FoxO transcriptional activity, physiological serum concentration had no effect. IGF-1 at concentrations of 0.1 and 1 μM significantly reduced proliferation but increased differentiation of sebocytes to a greater extent than insulin (0.1 and 1 μM), but up-regulated TLR2/4 expression to comparable extent. These data provide the first in vitro evidence that FoxO1 principally might be involved in the regulation of growth-factor-stimulatory effects on sebaceous lipogenesis and inflammation in the pathological condition of acne. However, the in vivo significance under physiological conditions remains to be elucidated.

Possible intermediary role of ghrelin in seborrhea

Seborrhea, or oily skin, is a very common condition, especially among young people, caused by the increased secretion of sebum by sebaceous glands in the skin. Based on today's knowledge, a regulatory role of various hormones especially androgens is considered for sebaceous gland secretion; but despite significant evidences emphasis on the effects of the gastrointestinal disorders on coetaneous manifestations, the role of gastrointestinal problems in sebum secretion has not been emphasized yet. This study aimed to explain hormonal changes occurring in gastroesophageal reflux disease which may cause changes in sebaceous gland secretion so that, by explaining these communication mechanisms, common investigations between gastroenterology and dermatology can be performed to evaluate the accuracy of this hypothesis.

Linking diet to acne metabolomics, inflammation, and comedogenesis: an update

Acne vulgaris, an epidemic inflammatory skin disease of adolescence, is closely related to Western diet. Three major food classes that promote acne are: 1) hyperglycemic carbohydrates, 2) milk and dairy products, 3) saturated fats including trans-fats and deficient ω-3 polyunsaturated fatty acids (PUFAs). Diet-induced insulin/insulin-like growth factor (IGF-1)-signaling is superimposed on elevated IGF-1 levels during puberty, thereby unmasking the impact of aberrant nutrigenomics on sebaceous gland homeostasis. Western diet provides abundant branched-chain amino acids (BCAAs), glutamine, and palmitic acid. Insulin and IGF-1 suppress the activity of the metabolic transcription factor forkhead box O1 (FoxO1). Insulin, IGF-1, BCAAs, glutamine, and palmitate activate the nutrient-sensitive kinase mechanistic target of rapamycin complex 1 (mTORC1), the key regulator of anabolism and lipogenesis. FoxO1 is a negative coregulator of androgen receptor, peroxisome proliferator-activated receptor-γ (PPARγ), liver X receptor-α, and sterol response element binding protein-1c (SREBP-1c), crucial transcription factors of sebaceous lipogenesis. mTORC1 stimulates the expression of PPARγ and SREBP-1c, promoting sebum production. SREBP-1c upregulates stearoyl-CoA- and Δ6-desaturase, enhancing the proportion of monounsaturated fatty acids in sebum triglycerides. Diet-mediated aberrations in sebum quantity (hyperseborrhea) and composition (dysseborrhea) promote Propionibacterium acnes overgrowth and biofilm formation with overexpression of the virulence factor triglyceride lipase increasing follicular levels of free palmitate and oleate. Free palmitate functions as a "danger signal," stimulating toll-like receptor-2-mediated inflammasome activation with interleukin-1β release, Th17 differentiation, and interleukin-17-mediated keratinocyte proliferation. Oleate stimulates P. acnes adhesion, keratinocyte proliferation, and comedogenesis via interleukin-1α release. Thus, diet-induced metabolomic alterations promote the visible sebofollicular inflammasomopathy acne vulgaris. Nutrition therapy of acne has to increase FoxO1 and to attenuate mTORC1/SREBP-1c signaling. Patients should balance total calorie uptake and restrict refined carbohydrates, milk, dairy protein supplements, saturated fats, and trans-fats. A paleolithic-like diet enriched in vegetables and fish is recommended. Plant-derived mTORC1 inhibitors and ω-3-PUFAs are promising dietary supplements supporting nutrition therapy of acne vulgaris.

Liver X receptor activation induces apoptosis of melanoma cell through caspase pathway

Liver X receptors (LXRs) are nuclear receptors that function as ligand-activated transcription factors regulating lipid metabolism and inflammation. Recent discoveries found LXRs could regulate tumor growth in a variety of cancer cell lines. In this study, we investigated the effect of LXR activation on melanoma cell proliferation and apoptosis both in vitro and in vivo. Treatment of B16F10 and A-375 melanoma cells with synthetic LXR agonist T0901317 significantly inhibited the proliferation of melanoma cells in vitro. Meanwhile, T0901317 induced the apoptosis of B16F10 melanoma cells in a dose-dependent manner. Furthermore, western blot assay showed that the pro-apoptotic effect of T0901317 on B16F10 melanoma cells was mediated through caspase-3 pathway. Oral administration of T0901317 inhibited the growth of B16F10 melanoma in C56BL/6 mice. Altogether, this study demonstrates the critical role of LXRs in the regulation of melanoma growth and presents the LXR agonist T0901317 as a potential anti-melanoma agent.

Role of PPAR, LXR, and PXR in epidermal homeostasis and inflammation

Epidermal lipid synthesis and metabolism are regulated by nuclear hormone receptors (NHR) and in turn epidermal lipid metabolites can serve as ligands to NHR. NHR form a large superfamily of receptors modulating gene transcription through DNA binding. A subgroup of these receptors is ligand-activated and heterodimerizes with the retinoid X receptor including peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR) and pregnane X receptor (PXR). Several isotypes of these receptors exist, all of which are expressed in skin. In keratinocytes, ligand activation of PPARs and LXRs stimulates differentiation, induces lipid accumulation, and accelerates epidermal barrier regeneration. In the cutaneous immune system, ligand activation of all three receptors, PPAR, LXR, and PXR, has inhibitory properties, partially mediated by downregulation of the NF-kappaB pathway. PXR also has antifibrotic effects in the skin correlating with TGF-beta inhibition. In summary, ligands of PPAR, LXR and PXR exert beneficial therapeutic effects in skin disease and represent promising targets for future therapeutic approaches in dermatology. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Potential role of FoxO1 and mTORC1 in the pathogenesis of Western diet-induced acne

Acne in adolescents of developed countries is an epidemic skin disease and has currently been linked to the Western diet (WD). It is the intention of this viewpoint to discuss the possible impact of WD-mediated nutrient signalling in the pathogenesis of acne. High glycaemic load and dairy protein consumption both increase insulin/insulin-like growth factor-1 (IGF-1) signalling (IIS) that is superimposed on elevated IGF-1 signalling of puberty. The cell's nutritional status is primarily sensed by the forkhead box transcription factor O1 (FoxO1) and the serine/threonine kinase mammalian target of rapamycin complex 1 (mTORC1). Increased IIS extrudes FoxO1 into the cytoplasm, whereas nuclear FoxO1 suppresses hepatic IGF-1 synthesis and thus impairs somatic growth. FoxO1 attenuates androgen signalling, interacts with regulatory proteins important for sebaceous lipogenesis, regulates the activity of innate and adaptive immunity, antagonizes oxidative stress and most importantly functions as a rheostat of mTORC1, the master regulator of cell growth, proliferation and metabolic homoeostasis. Thus, FoxO1 links nutrient availability to mTORC1-driven processes: increased protein and lipid synthesis, cell proliferation, cell differentiation including hyperproliferation of acroinfundibular keratinocytes, sebaceous gland hyperplasia, increased sebaceous lipogenesis, insulin resistance and increased body mass index. Enhanced androgen, TNF-α and IGF-1 signalling due to genetic polymorphisms promoting the risk of acne all converge in mTORC1 activation, which is further enhanced by nutrient signalling of WD. Deeper insights into the molecular interplay of FoxO1/mTORC1-mediated nutrient signalling are thus of critical importance to understand the impact of WD on the promotion of epidemic acne and more serious mTORC1-driven diseases of civilization.

Nuclear hormone receptor functions in keratinocyte and melanocyte homeostasis, epidermal carcinogenesis and melanomagenesis

Skin homeostasis is maintained, in part, through regulation of gene expression orchestrated by type II nuclear hormone receptors in a cell and context specific manner. This group of transcriptional regulators is implicated in various cellular processes including epidermal proliferation, differentiation, permeability barrier formation, follicular cycling and inflammatory responses. Endogenous ligands for the receptors regulate actions during skin development and maintenance of tissue homeostasis. Type II nuclear receptor signaling is also important for cellular crosstalk between multiple cell types in the skin. Overall, these nuclear receptors are critical players in keratinocyte and melanocyte biology and present targets for cutaneous disease management.

Liver X receptor activation inhibits melanogenesis through the acceleration of ERK-mediated MITF degradation

Liver X receptors (LXRs) are nuclear receptors that act as ligand-activated transcription factors regulating lipid metabolism and inflammation. In the skin, activation of LXRs stimulates differentiation of keratinocytes and augments lipid synthesis in sebocytes. However, the function of LXRs in melanocytes remains largely unknown. We investigated whether LXR activation would affect melanogenesis. In human primary melanocytes, MNT-1, and B16 melanoma cells, TO901317, a synthetic LXR ligand, inhibited melanogenesis. Small interfering RNA (siRNA) experiments revealed the dominant role of LXRβ in TO901317-mediated antimelanogenesis. Enzymatic activities of tyrosinase were unaffected, but the expression of tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2 was suppressed by TO901317. Expressions of microphthalmia-associated transcription factor (MITF), a master transcriptional regulator of melanogenesis, and cAMP-responsive element-binding activation were not affected. It is noteworthy that the degradation of MITF was accelerated by TO901317. Extracellular signal-regulated kinase (ERK) contributed to TO901317-induced antimelanogenesis, which was evidenced by recovery of melanogenesis with ERK inhibitor. Other LXR ligands, 22(R)-hydroxycholesterol (22(R)HC) and GW3965, also activated ERK and suppressed melanogenesis. The intermediary role of Ras was confirmed in TO901317-induced ERK phosphorylation. Finally, antimelanogenic effects of TO901317 were confirmed in vivo in UVB-tanning model in brown guinea pigs, providing a previously unreported line of evidence that LXRs may be important targets for antimelanogenesis.

Topically applied L-carnitine effectively reduces sebum secretion in human skin

BACKGROUND

  Oily skin condition is caused by an excessive sebaceous gland activity, resulting in an overproduction of sebum, giving the skin an undesired shiny, oily appearance.

AIMS

  To identify an active substance that reduces sebum production in human sebaceous glands by regulating fat metabolism in a natural way.

PATIENTS/METHODS

  The effects of L-carnitine on β-oxidation and intracellular lipid content were investigated in vitro using the human sebaceous cell line SZ95. Penetration experiments utilizing pig skin as a model system were performed with a cosmetic formulation containing radioactively labeled L-carnitine. To determine the in vivo effects, a vehicle-controlled, randomized study was carried out using a cosmetic formulation containing 2%l-carnitine for 3 weeks. Sebum production was investigated utilizing the lipid-absorbent Sebutape(®).

RESULTS

  SZ95 cells treated with 0.5% or 1% L-carnitine demonstrated a significant concentration-dependent increase in β-oxidation compared to control cells. Following the treatment with L-carnitine, intracellular lipid concentrations decreased significantly in a dose-dependent manner compared with untreated control cells. In skin penetration experiments, topically applied L-carnitine reached the dermis. In addition, topical in vivo application of a formulation containing 2% L-carnitine for 3 weeks significantly decreased the sebum secretion rate compared to the treatment with vehicle.

CONCLUSIONS

  Our results show that the treatment of human sebocytes with L-carnitine significantly augments β-oxidation and significantly decreases intracellular lipid content in human sebocytes. Topically applied L-carnitine is bioavailable and leads to a significant sebum reduction in vivo. In conclusion, L-carnitine represents a valuable compound, produced naturally within the body, for the topical treatment of oily skin in humans.

Hormones and the pilosebaceous unit

Hormones can exert their actions through endocrine, paracrine, juxtacrine, autocrine and intracrine pathways. The skin, especially the pilosebaceous unit, can be regarded as an endocrine organ meanwhile a target of hormones, because it synthesizes miscellaneous hormones and expresses diverse hormone receptors. Over the past decade, steroid hormones, phospholipid hormones, retinoids and nuclear receptor ligands as well as the so-called stress hormones have been demonstrated to play pivotal roles in controlling the development of pilosebaceous units, lipogenesis of sebaceous glands and hair cycling. Among them, androgen is most extensively studied and of highest clinical significance. Androgen-mediated dermatoses such as acne, androgenetic alopecia and seborrhea are among the most common skin disorders, with most patients exhibiting normal circulating androgen levels. The "cutaneous hyperandrogenism" is caused by in stiu overexpression of the androgenic enzymes and hyperresponsiveness of androgen receptors. Regulation of cutaneous steroidogenesis is analogous to that in gonads and adrenals. More work is needed to explain the regional difference within and between the androgn-mediated dermatoses. The pilosebaceous unit can act as an ideal model for studies in dermato-endocrinology.

An update on the role of the sebaceous gland in the pathogenesis of acne

The pathogenesis of acne, a disease of the pilosebaceous follicle and one of the most common chronic skin disorders, is attributed to multiple factors such as increased sebum production, alteration of the quality of sebum lipids, inflammatory processes, dysregulation of the hormone microenvironment, interaction with neuropeptides, follicular hyperkeratinisation and the proliferation of Propionibacterium acnes within the follicle. In particular, the sebaceous gland plays an exquisite role in the initiation of the disease as it possesses all the enzyme machinery for the production of hormones and cytokines. In addition, in response to the altered tissue environment in the pilosebaceous follicle as well as in answer to emotional fret, stress response system mechanisms with induction of central and local expression of neuropeptides, are also initiated. This review summarises the latest advances in understanding the role of sebaceous gland cells in the pathomechanism of acne.

Sebaceous gland receptors

Receptors are proteins, embedded in a cell or cytoplasmic membrane, to which a mobile signaling molecule may attach. Receptor ligands may be peptides (such as neurotransmitters), hormones, pharmaceutical drugs and/or a toxins, whereas "binding" ordinarily initiates a cellular response. Human sebocytes are biologically and metabolically very active cells and consequently express numerous receptors. Three of four groups of peptide/neurotransmitter receptors, the so-called serpentine receptor group are present (corticotropin-releasing hormone receptors 1 and 2, melanocortin-1 and 5 receptors, mu-opiate receptors, VPAC receptors, cannabinoid receptors 1 and 2, vascular endothelial growth factor receptor and histamine 1 receptor). The single-transmembrane domain receptors are represented by the insulin-like growth factor-I receptor and the third group, which does not possess intrinsic tyrosine kinase activity, by the growth factor receptor. Nuclear receptors expressed in sebocytes are grouped into two major subtypes. From the steroid receptor family, the androgen receptor and the progesterone receptor are expressed. The thyroid receptor family includes the estrogen receptors (alpha and beta isotypes), the retinoic acid receptors (isotypes alpha and gamma) and retinoid X receptors (isotypes alpha, beta, gamma), the vitamin D receptor, the peroxisome proliferator-activated receptors (isotypes alpha, delta and gamma) and the liver X receptors (alpha and beta isotypes). The vanilloid receptor belongs to the transient ion channels and is expressed in differentiating human sebocytes. Further sebocyte receptors, which may influence their function are fibroblast growth factor receptor 2, epidermal growth factor receptor, c-MET, CD14, Toll-like receptor 2, Toll-like receptor 4 and Toll-like receptor 6. Receptor-ligand interactions control sebocyte proliferation, differentiation and lipid synthesis. However, not every ligand that binds to a sebocyte receptor also activates it, such ligands are receptor antagonists and inverse agonists.

Isotretinoin and FoxO1: A scientific hypothesis

Oral isotretinoin (13-cis retinoic acid) is the most effective drug in the treatment of acne and restores all major pathogenetic factors of acne vulgaris. isotretinoin is regarded as a prodrug which after isomerizisation to all-trans-retinoic acid (ATRA) induces apoptosis in cells cultured from human sebaceous glands, meibomian glands, neuroblastoma cells, hypothalamic cells, hippocampus cells, Dalton's lymphoma ascites cells, B16F-10 melanoma cells, and neuronal crest cells and others. By means of translational research this paper provides substantial indirect evidence for isotretinoin's mode of action by upregulation of forkhead box class O (FoxO) transcription factors. FoxOs play a pivotal role in the regulation of androgen receptor transactivation, insulin/insulin like growth factor-1 (IGF-1)-signaling, peroxisome proliferator-activated receptor-γ (PPArγ)- and liver X receptor-α (LXrα)-mediated lipogenesis, β-catenin signaling, cell proliferation, apoptosis, reactive oxygene homeostasis, innate and acquired immunity, stem cell homeostasis, as well as anti-cancer effects. An accumulating body of evidence suggests that the therapeutic, adverse, teratogenic and chemopreventive effecs of isotretinoin are all mediated by upregulation of FoxO-mediated gene transcription. These FoxO-driven transcriptional changes of the second response of retinoic acid receptor (RAR)-mediated signaling counterbalance gene expression of acne due to increased growth factor signaling with downregulated nuclear FoxO proteins. The proposed isotretinoin→ATRA→RAR→FoxO interaction offers intriguing new insights into the mode of isotretinoin action and explains most therapeutic, adverse and teratogenic effects of isotretinoin in the treatment of acne by a common mode of FoxO-mediated transcriptional regulation.

[The sebaceous gland]

The development and function of the sebaceous gland in the fetal and neonatal periods appear to be regulated by maternal androgens and by endogenous steroid synthesis, as well as by other morphogens. The most apparent function of the glands is to excrete sebum. A strong increase in sebum excretion occurs a few hours after birth; this peaks during the first week and slowly subsides thereafter. A new rise takes place at about age 9 years with adrenarche and continues up to age 17 years, when the adult level is reached. The sebaceous gland is a target organ but also an important formation site of hormones, and especially of active androgens. Hormonal activity is based on an hormone (ligand)-receptor interaction, whereas sebocytes express a wide spectrum of hormone receptors. Androgens are well known for their effects on sebum excretion, whereas terminal sebocyte differentiation is assisted by peroxisome proliferator-activated receptor ligands. Estrogens, glucocorticoids, and prolactin also influence sebaceous gland function. In addition, stress-sensing cutaneous signals lead to the production and release of corticotrophin-releasing hormone from dermal nerves and sebocytes with subsequent dose-dependent regulation of sebaceous nonpolar lipids. Among other lipid fractions, sebaceous glands have been shown to synthesize considerable amounts of free fatty acids without exogenous influence. Atopic dermatitis, seborrheic dermatitis, psoriasis and acne vulgaris are some of the disease on which pathogenesis and severity sebaceous lipids may or are surely involved.

Activation of LXRα induces lipogenesis in HaCaT cells

The oxysterol nuclear receptors, LXRα (liver X receptor α; NR1H3) and LXRβ (NR1H2), coordinately regulate the expression of genes involved in lipid metabolism, anti-inflammation, and cholesterol transport. Previous studies have demonstrated that ligands of LXRα are important in the maintenance of the normal epidermal barrier function and keratinocyte differentiation. In this study, we examined whether LXRα and its ligands regulate lipid synthesis in HaCaT cells, a spontaneously transformed human keratinocyte cell line. When HaCaT cells were treated with the LXRα ligand TO901317, lipid droplets accumulated in the majority of cells, which were stained by Oil Red O. A luciferase reporter construct containing the LXR response element was activated about fourfold in HaCaT cells by TO901317 treatment, suggesting that LXR has a role in lipid synthesis in these cells. The expression of LXRα target genes, such as those encoding sterol regulatory binding protein and fatty acid synthase, were induced time dependently by TO901317, as measured by RT-PCR and western blotting. The expression of PPAR-α, -β, and -γ which regulate lipid metabolism, was also increased by TO901317 treatment. In contrast, TO901317 reduced the lipopolysaccharide-induced expression of cyclooxygenase 2 and inducible nitric oxide synthase in HaCaT cells. These results indicate that LXRα activation leads to lipogenesis in keratinocytes, which may enhance the epidermal barrier function of the skin.

Culture, immortalization, and characterization of human meibomian gland epithelial cells

PURPOSE

Meibomian gland epithelial cells are essential in maintaining the health and integrity of the ocular surface. However, very little is known about their physiological regulation. In this study, the cellular control mechanisms were explored, first to establish a defined culture system for the maintenance of primary epithelial cells from human meibomian glands and, second, to immortalize these cells, thereby developing a preclinical model that could be used to identify factors that regulate cell activity.

METHODS

Human meibomian glands were removed from lid segments after surgery, enzymatically digested, and dissociated. Isolated epithelial cells were cultured in media with or without serum and/or 3T3 feeder layers. To attempt immortalization, the cells were exposed to retroviral human telomerase reverse transcriptase (hTERT) and/or SV40 large T antigen cDNA vectors, and antibiotic-resistant cells were selected, expanded, and subcultured. Analyses for possible biomarkers, cell proliferation and differentiation, lipid-related enzyme gene expression, and the cellular response to androgen were performed with biochemical, histologic, and molecular biological techniques.

RESULTS

It was possible to isolate viable human meibomian gland epithelial cells and to culture them in serum-free medium. These cells proliferated, survived through at least the fifth passage, and contained neutral lipids. Infection with hTERT immortalized these cells, which accumulated neutral lipids during differentiation, expressed multiple genes for lipogenic enzymes, responded to androgen, and continued to proliferate.

CONCLUSIONS

The results show that human meibomian gland epithelial cells may be isolated, cultured, and immortalized.

FoxO1 - the key for the pathogenesis and therapy of acne?

Five main factors play a pivotal role in the pathogenesis of acne: androgen dependence, follicular retention hyperkeratosis, increased sebaceous lipogenesis, increased colonization with P. acnes, and inflammatory events. This paper offers a solution for the pathogenesis of acne and explains all major pathogenic factors at the genomic level by a relative deficiency of the nuclear transcription factor FoxO1. Nuclear FoxO1 suppresses androgen receptor, other important nuclear receptors and key genes of cell proliferation, lipid biosynthesis and inflammatory cytokines. Elevated growth factors during puberty and persistent growth factor signals due to Western life style stimulate the export of FoxO1 out of the nucleus into the cytoplasm via activation of the phos-phoinositide-3-kinase (PI3K)/Akt pathway. By this mechanism, genes and nuclear receptors involved in acne are derepressed leading to increased androgen receptor-mediated signal transduction, increased cell proliferation of androgen-dependent cells, induction of sebaceous lipogenesis and upregulation of Toll-like-receptor-2-dependent inflammatory cytokines. All known acne-inducing factors exert their action by reduction of nuclear FoxO1 levels. In contrast, retinoids, antibiotics and dietary intervention will increase the nuclear content of FoxO1, thereby normalizing increased transcription of genes involved in acne. Various receptor-mediated growth factor signals are integrated at the level of PI3K/Akt activation which finally results in nuclear FoxO1 deficiency.

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.

New developments in our understanding of acne pathogenesis and treatment

Interest in sebaceous gland physiology and its diseases is rapidly increasing. We provide a summarized update of the current knowledge of the pathobiology of acne vulgaris and new treatment concepts that have emerged in the last 3 years (2005-2008). We have tried to answer questions arising from the exploration of sebaceous gland biology, hormonal factors, hyperkeratinization, role of bacteria, sebum, nutrition, cytokines and toll-like receptors (TLRs). Sebaceous glands play an important role as active participants in the innate immunity of the skin. They produce neuropeptides, excrete antimicrobial peptides and exhibit characteristics of stem cells. Androgens affect sebocytes and infundibular keratinocytes in a complex manner influencing cellular differentiation, proliferation, lipogenesis and comedogenesis. Retention hyperkeratosis in closed comedones and inflammatory papules is attributable to a disorder of terminal keratinocyte differentiation. Propionibacterium acnes, by acting on TLR-2, may stimulate the secretion of cytokines, such as interleukin (IL)-6 and IL-8 by follicular keratinocytes and IL-8 and -12 in macrophages, giving rise to inflammation. Certain P. acnes species may induce an immunological reaction by stimulating the production of sebocyte and keratinocyte antimicrobial peptides, which play an important role in the innate immunity of the follicle. Qualitative changes of sebum lipids induce alteration of keratinocyte differentiation and induce IL-1 secretion, contributing to the development of follicular hyperkeratosis. High glycemic load food and milk may induce increased tissue levels of 5alpha-dihydrotestosterone. These new aspects of acne pathogenesis lead to the considerations of possible customized therapeutic regimens. Current research is expected to lead to innovative treatments in the near future.

Liver X receptor mediates hepatitis B virus X protein-induced lipogenesis in hepatitis B virus-associated hepatocellular carcinoma

Although hepatitis B virus X protein (HBx) has been implicated in abnormal lipid metabolism in hepatitis B virus (HBV)-associated hepatic steatosis, its underlying molecular mechanism remains unclear. Liver X receptor (LXR) plays an important role in regulating the expression of genes involved in hepatic lipogenesis. Here we demonstrate that LXRalpha and LXRbeta mediate HBV-associated hepatic steatosis. We have found that HBx induces the expression of LXR and its lipogenic target genes, such as sterol regulatory element binding protein-1c (SREBP-1c), fatty acid synthase (FAS), and peroxisome proliferator-activated receptor, and this is accompanied by the accumulation of lipid droplets. RNA interference with LXR expression decreases the amount of lipid droplets as well as the expression of the lipogenic genes, and this indicates that HBx-induced lipogenesis is LXR-dependent. LXRalpha and HBx colocalize in the nucleus and are physically associated. HBx induces the transactivation function of LXRalpha by recruiting CREB binding protein to the promoter of the target gene. Furthermore, we have observed that expression of LXR is increased in the livers of HBx-transgenic mice. Finally, there is a significant increase in the expression of LXRbeta (P = 0.036), SREBP-1c (P = 0.008), FAS, and stearoyl-coenyzme A desaturase-1 (P = 0.001) in hepatocellular carcinoma (HCC) in comparison with adjacent nontumorous nodules in human HBV-associated HCC specimens.

CONCLUSION

Our results suggest a novel association between HBx and LXR that may represent an important mechanism explaining HBx-induced hepatic lipogenesis during HBV-associated hepatic carcinogenesis.