Scarring Alopecia and the PPAR-γ Connection

Study of the Thyroid Profile of Patients with Alopecia

Thyroid hormones are required for the physiological growth and maintenance of hair follicles. We aim to study the thyroid profile of patients with alopecia. This is a narrative review. PubMed literature was searched from 2013 to 2022. We followed different types of alopecia: alopecia areata (AA), androgenic alopecia in males and females, telogen effluvium (TE), frontal fibrosing alopecia (FFA), lichen planopilaris, and alopecia neoplastica (AN). AA shares a common autoimmune background with autoimmune thyroid diseases, either sporadic or belonging to autoimmune polyglandular syndromes. Some data suggested that AA is more severe if thyroid anomalies are confirmed, including subclinical dysfunction or positive antithyroid antibodies with normal hormone values. However, routine thyroid screening for patients with AA, if the patients are asymptomatic from a thyroid point of view and they have negative personal and family history of autoimmunity, remains controversial. TE, apart from the autoimmune type, associates thyroid anomalies of a hormonal assay (between 5.7% and 17%). FFA, mostly a postmenopausal entity (however, not exclusive), associates a higher prevalence of thyroid conditions (up to 50%) than the general population. However, these might have an age-dependent pattern, thus the association may be incidental since there are a limited number of studies. Overall, alopecia remains a very challenging condition for patients and physicians; a multidisciplinary team is required to improve the outcome and quality of life. The common autoimmune background is suggestive of some types of alopecia and thyroid disorders, yet, the underlying mechanisms are still a matter of debate. AA, TE, FFA, LPP, and, potentially, female pattern hair loss have been found to be connected with thyroid entities, thus a state of awareness from a dual perspective, of trichology and endocrinology, is helpful.

Frontal fibrosing alopecia: A review of disease pathogenesis

Frontal fibrosing alopecia (FFA) is a primary patterned cicatricial alopecia that mostly affects postmenopausal women and causes frontotemporal hairline regression and eyebrow loss. Although the incidence of FFA has increased worldwide over the last decade, its etiology and pathology are still unclear. We cover the latest findings on its pathophysiology, including immunomodulation, neurogenic inflammation, and genetic regulation, to provide more alternatives for current clinical treatment. A persistent inflammatory response and immune privilege (IP) collapse develop and lead to epithelial hair follicle stem cells (eHFSCs) destruction and epithelial-mesenchymal transition (EMT) in the bulge area, which is the key process in FFA pathogenesis. Eventually, fibrous tissue replaces normal epithelial tissue and fills the entire hair follicle (HF). In addition, some familial reports and genome-wide association studies suggest a genetic susceptibility or epigenetic mechanism for the onset of FFA. The incidence of FFA increases sharply in postmenopausal women, and many FFA patients also suffer from female pattern hair loss in clinical observation, which suggests a potential association between FFA and steroid hormones. Sun exposure and topical allergens may also be triggers of FFA, but this conjecture has not been proven. More evidence and cohort studies are needed to help us understand the pathogenesis of this disease.

Lichen Planopilaris: The first biopsy layer microbiota inspection

Lichen Planopilaris (LPP) is a lymphatic disease affecting the scalp that is characterized by a chronic and destructive inflammation process, named as ‘cicatricial alopecia’ in which the hair follicles are targeted and may involve predominantly lymphocytes or neutrophils. Scalp and biopsy layers have never been used to investigate microbial community composition and its relative taxa abundances in LPP. We sought to examine the significant taxa of this chronic relapsing inflammatory skin disease, together with inspect the existing connections with metabolic pathways featuring this microbial community. We used a multilevel analysis based on 16S rRNA marker sequencing in order to detect OTU abundances in pathologic/healthy samples, real time PCR for measuring the levels of IL-23 interleukin expression and urinary metabolomics to find out volatile organic metabolites (VOMs). By using a linear regression model, we described peculiar taxa that significantly differentiated LPP and healthy samples. We inspected taxa abundances and interleukin mRNA levels and the Microbacteriaceae family resulted negatively correlated with the IL-23 expression. Moreover, starting from 16S taxa abundances, we predicted the metabolic pathways featuring this microbial community. By inspecting microbial composition, sample richness, metabolomics profiles and the relative metabolic pathways in a cohort of LPP and healthy samples we deepened the contribution of significant taxa that are connected to inflammation maintenance and microbiota plasticity in LPP pathology.

Frontal Fibrosing Alopecia: A Review

Frontal fibrosing alopecia is a scarring alopecia, the prevalence of which is increasing worldwide since its first description in 1994. The reason for this emerging epidemic may be a higher exposure to an unknown trigger, although its aethiology and pathogenesis still remain enigmatic. Clinical, trichoscopic, sonographic, and histopathologic findings are allowing clinicians to understand more aspects about this type of cicatricial alopecia. Several treatments have been used in frontal fibrosing alopecia, although the 5-alpha reductase inhibitors seem to be the most promising. The aim of this report is to provide a compilation about the published data regarding frontal fibrosing alopecia in a narrative review.

Frontal Fibrosing Alopecia. An Example of Disrupted Aryl Hydrocarbon Receptor-Mediated Immunological Homeostasis in the Skin?

Sun protection has been recommended by public health authorities to reduce the risk of keratinocyte and melanocyte tumors, yet some sun exposure is required for vitamin D synthesis. Frontal fibrosing alopecia (FFA) is a cicatricial alopecia that has been reported to have an association with facial photoprotection. The brief review proposes the hypothesis that FFA arises as a result of excessive facial photo-protection with a resultant disturbance in immunological homeostasis mediated via the aryl hydrocarbon receptor-kynurenine pathway axis (AHR/KP) leading to the collapse of immune privilege at the hair bulge.

Pathomechanisms of immune-mediated alopecia

The hair follicle (HF) is a complex mini-organ that constantly undergoes dynamic cycles of growth and regression throughout life. While proper progression of the hair cycle requires homeostatic interplay between the HF and its immune microenvironment, specific parts of the HF, such as the bulge throughout the hair cycle and the bulb in the anagen phase, maintain relative immune privilege (IP). When this IP collapses, inflammatory infiltrates that aggregate around the bulge and bulb launch an immune attack on the HF, resulting in hair loss or alopecia. Alopecia areata (AA) and primary cicatricial alopecia (PCA) are two common forms of immune-mediated alopecias, and recent advancements in understanding their disease mechanisms have accelerated the discovery of novel treatments for immune-mediated alopecias, specifically AA. In this review, we highlight the pathomechanisms involved in both AA and CA in hopes that a deeper understanding of their underlying disease pathogenesis will encourage the development of more effective treatments that can target distinct disease pathways with greater specificity while minimizing adverse effects.

Experimentally Induced Epithelial-Mesenchymal Transition of Human Hair Follicle Stem Cells as a Model of Scarring Alopecia Ex Vivo

Primary cicatricial alopecia is characterized by a permanent "scarring" alopecia. This condition is characterized by the irreversible loss of hair follicles (HF) as a result of apoptosis and epithelial-mesenchymal transition (EMT) of epithelial stem cells localized in the HF bulge.We here report the procedure for experimentally induced EMT in healthy human epidermal stem cells (eSCs) using full-length HF organ culture ex vivo. The present model can be used to recapitulate the complex processes observed in scarring alopecia patient tissues, to further investigate the mechanisms involved in EMT transformation of HFeSCs, and to test substances that could prevent and/or rescue HFeSCs from EMT for the management of scarring alopecias.

Precision Medicine and the Practice of Trichiatry: Adapting the Concept

Evidence-based medicine (EBM) aims for the ideal that healthcare professionals make conscientious, explicit, and judicious use of the best available evidence gained from the scientific method to clinical decision-making. It seeks to assess the strength of the evidence for benefits of diagnostic tests and treatments, using techniques from science, engineering, and statistics, such as the systematic review of medical literature, meta-analysis, risk-benefit analysis, and randomized controlled trials. The limited success rate of EBM therapies suggests that the complex nature of hair loss may be inadequately served by the present levels of evidence, and that physicians treating hair loss may have fallen short of adequately researching a robust evidence to underpin their practices. Against this backdrop, the concept of precision medicine (PM) is evolving. PM refers to the customization of medical care to the patient's individual characteristics based on the patient's genetic background and other molecular or cellular analysis, while classifying patients into subpopulations that differ in their susceptibility to a particular medical condition, in the biology or prognosis of those medical conditions, or in their response to a specific treatment. With the advances in hair research, the powerful tools of molecular biology and genetics, and innovative technologies, we have the robust scientific data and tools to adapt the concept of PM to the practice of trichiatry. Finally, databases pertaining to the development and efficacy of PM must be analyzed and be used to form the basis of evidence-based personalized trichiatry.

Nanoparticle-mediated local delivery of pioglitazone attenuates bleomycin-induced skin fibrosis

BACKGROUND

Nanoparticle-loaded delivery systems have attracted much attention recently. Poly(lactic-co-glycolic acid) (PLGA) is one of the most successful biodegradable polymers for biomedical applications. There are only a few studies on the treatment of dermal fibrosis with sustained-release drugs. Peroxisome proliferator-activated receptor-γ (PPAR-γ) plays an important role in endogenous anti-fibrotic defense mechanisms. Recent studies have suggested that pioglitazone, a synthetic PPAR-γ activator, has effects beyond reducing blood sugar and it can reduce fibrosis and inflammation when used systemically.

OBJECTIVE

We aimed to assess the effects of local injections of pioglitazone-loaded PLGA nanoparticles (PGN-NP) on an experimental sclerosis and to demonstrate the in vivo pharmacokinetics of subcutaneously administered PLGA nanoparticles.

METHODS

Locally injectable PGN-NP were prepared and subcutaneously administered to bleomycin (BLM)-induced scleroderma model mice. The effect of pioglitazone was also evaluated with cultured fibroblasts. Coumarin-6-loaded fluorescent PLGA nanoparticles (FL-NP) and silicon naphthalocyanine-loaded near-infrared PLGA nanoparticles (NIR-NP) were used to demonstrate in vitro cellular uptake by cultured fibroblasts and the in vivo pharmacokinetics of subcutaneously administered nanoparticles.

RESULTS

Weekly subcutaneous injections of PGN-NP attenuated skin fibrosis in BLM-induced scleroderma model mice. Pioglitazone significantly suppressed migration ability and TGF-β-mediated myofibroblast differentiation in cultured fibroblasts. FL-NP were internalized into cultured fibroblasts within 60 min, and PGN-NP-primed fibroblasts expressed anti-fibrotic phenotypes. Subcutaneously injected NIR-NP remained in the vicinity of the injection site more than non-particulate silicon naphthalocyanine.

CONCLUSION

These results provide a basis for the development of new treatments for dermal fibrosis and a better understanding of the potential of PLGA nanoparticles in dermatology.

A proposed mechanism for central centrifugal cicatricial alopecia

Central centrifugal cicatricial alopecia (CCCA) has an unknown mechanism. Analyzing other scarring diseases (lichen planopilaris, fibrotic kidney disease and scleroderma) may help to clarify the mechanism of scarring in CCCA. These diseases were chosen for comparison due to either their location of disease (skin or scalp specifically), or prominence in patients of African descent. Genetics, possible triggers, an autoimmune lymphocytic response, and epithelial to mesenchymal transition are potentially involved. Possible common pathways in scarring diseases and a better understanding of the CCCA mechanism will lead to further research into the pathogenesis and potential treatments of CCCA.

Hair disorders in cancer survivors

With increasing survival rates across all cancers, survivors represent a growing population that is frequently affected by persistent or permanent hair growth disorders as a result of systemic therapies, radiotherapy, surgical procedures, and therapeutic transplants. These hair disorders include persistent chemotherapy-induced alopecia, persistent radiotherapy-induced alopecia, endocrine therapy-induced alopecia and hirsutism, postsurgery alopecia and localized hypertrichosis, and persistent stem cell transplantation and targeted therapy-induced alopecia. The information contained in this continuing medical education series should facilitate a better understanding on hair disorders in cancer survivors so that adequate support and therapies may be provided.

PPAR-γ Agonists and Their Role in Primary Cicatricial Alopecia

Peroxisome proliferator-activated receptor γ (PPAR-γ) is a ligand-activated nuclear receptor that regulates the transcription of various genes. PPAR-γ plays roles in lipid homeostasis, sebocyte maturation, and peroxisome biogenesis and has shown anti-inflammatory effects. PPAR-γ is highly expressed in human sebaceous glands. Disruption of PPAR-γ is believed to be one of the mechanisms of primary cicatricial alopecia (PCA) pathogenesis, causing pilosebaceous dysfunction leading to follicular inflammation. In this review article, we discuss the pathogenesis of PCA with a focus on PPAR-γ involvement in pathogenesis of lichen planopilaris (LPP), the most common lymphocytic form of PCA. We also discuss clinical trials utilizing PPAR-agonists in PCA treatment.

DHEA and frontal fibrosing alopecia: molecular and physiopathological mechanisms

The transforming growth factor-beta 1 (TGFβ1) promotes fibrosis, differentiating epithelial cells and quiescent fibroblasts into myofibroblasts and increasing expression of extracellular matrix. Recent investigations have shown that PPAR (peroxisome proliferator-activated receptor*) is a negative regulator of fibrotic events induced by TGFβ1. Dehydroepiandrosterone (DHEA) is an immunomodulatory hormone essential for PPAR functions, and is reduced in some processes characterized by fibrosis. Although scarring alopecia characteristically develops in the female biological period in which occurs decreased production of DHEA, there are no data in the literature relating its reduction to fibrogenic process of this condition. This article aims to review the fibrogenic activity of TGFβ1, its control by PPAR and its relation with DHEA in the frontal fibrosing alopecia.

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.

Lichen planopilaris is characterized by immune privilege collapse of the hair follicle's epithelial stem cell niche

Lichen planopilaris (LPP) is a chronic inflammatory disease of unknown pathogenesis that leads to permanent hair loss. Whilst destruction of epithelial hair follicle stem cells (eHFSCs) that reside in an immunologically protected niche of the HF epithelium, the bulge, is a likely key event in LPP pathogenesis, this remains to be demonstrated. We have tested the hypotheses that bulge immune privilege (IP) collapse and inflammation-induced eHFSC death are key components in the pathogenesis of LPP. Biopsies of lesional and non-lesional scalp skin from adult LPP patients (n = 42) were analysed by quantitative (immuno)histomorphometry, real-time quantitative polymerase chain reaction (qRT-PCR), laser capture microdissection and microarray analysis, or skin organ culture. At both the protein and transcriptional level, lesional LPP HFs showed evidence for bulge IP collapse (ie increased expression of MHC class I and II, β2microglobulin; reduced TGFβ2 and CD200 expression). This was accompanied by a Th1-biased cytotoxic T cell response (ie increased CD8(+) GranzymeB(+) T cells and CD123(+) plasmacytoid dendritic cells, with increased CXCR3 expression) and increased expression of interferon-inducible chemokines (CXCL9/10/11). Interestingly, lesional LPP eHFSCs showed both increased proliferation and apoptosis in situ. Microarray analysis revealed a loss of eHFSC signatures and increased expression of T cell activation/binding markers in active LPP, while bulge PPARγ transcription was unaltered compared to non-lesional LPP HFs. In organ culture of non-lesional LPP skin, interferon-γ (IFNγ) induced bulge IP collapse. LPP is an excellent model disease for studying and preventing immune destruction of human epithelial stem cells in situ. These novel findings raise the possibility that LPP represents an autoimmune disease in whose pathogenesis IFNγ-induced bulge IP collapse plays an important role. Therapeutically, bulge IP protection/restoration may help to better manage this highly treatment-resistant cicatricial alopecia.

Mouse model phenotypes provide information about human drug targets

Motivation: Methods for computational drug target identification use information from diverse information sources to predict or prioritize drug targets for known drugs. One set of resources that has been relatively neglected for drug repurposing is animal model phenotype.

Results: We investigate the use of mouse model phenotypes for drug target identification. To achieve this goal, we first integrate mouse model phenotypes and drug effects, and then systematically compare the phenotypic similarity between mouse models and drug effect profiles. We find a high similarity between phenotypes resulting from loss-of-function mutations and drug effects resulting from the inhibition of a protein through a drug action, and demonstrate how this approach can be used to suggest candidate drug targets.

Availability and implementation: Analysis code and supplementary data files are available on the project Web site at https://drugeffects.googlecode.com.

Contact:leechuck@leechuck.de or roh25@aber.ac.uk

Supplementary information:Supplementary data are available at Bioinformatics online.

Long-term bezafibrate treatment improves skin and spleen phenotypes of the mtDNA mutator mouse

Pharmacological agents, such as bezafibrate, that activate peroxisome proliferator-activated receptors (PPARs) and PPAR γ coactivator-1α (PGC-1α) pathways have been shown to improve mitochondrial function and energy metabolism. The mitochondrial DNA (mtDNA) mutator mouse is a mouse model of aging that harbors a proofreading-deficient mtDNA polymerase γ. These mice develop many features of premature aging including hair loss, anemia, osteoporosis, sarcopenia and decreased lifespan. They also have increased mtDNA mutations and marked mitochondrial dysfunction. We found that mutator mice treated with bezafibrate for 8-months had delayed hair loss and improved skin and spleen aging-like phenotypes. Although we observed an increase in markers of fatty acid oxidation in these tissues, we did not detect a generalized increase in mitochondrial markers. On the other hand, there were no improvements in muscle function or lifespan of the mutator mouse, which we attributed to the rodent-specific hepatomegaly associated with fibrate treatment. These results showed that despite its secondary effects in rodent’s liver, bezafibrate was able to improve some of the aging phenotypes in the mutator mouse. Because the associated hepatomegaly is not observed in primates, long-term bezafibrate treatment in humans could have beneficial effects on tissues undergoing chronic bioenergetic-related degeneration.

Cicatricial (scarring) alopecias: an overview of pathogenesis, classification, diagnosis, and treatment

Cicatricial (scarring) alopecia forms a group of disorders in which the common final pathway is the destruction of the hair follicle unit that is replaced by fibrous tissue. Hair loss may occur as a primary event when the follicle is the main target of the disease process (primary cicatricial alopecias) or as a secondary event when the follicle act as an 'innocent bystander' in the course of a disease occurring outside of the follicular unit (secondary cicatricial alopecias). Permanent hair loss may also occur in the late phases of some nonscarring alopecias that are called 'biphasic alopecias.' Primary cicatricial alopecia accounts for 5% of all trichologic consultations at the Section of Dermatology, University of Genoa, Genoa, Italy. Considering that hair loss has a strong impact on patients' psychology and quality of life, and that cicatricial alopecias can be associated with underlying systemic implications, it is extremely important that every clinician is familiar with the diagnosis and treatment of the different types of cicatricial alopecia. An accurate clinical assessment integrated with (video) dermatoscopy and histopathologic studies permits a high standard performance of correct diagnoses. A brief review of our current knowledge of disease pathogenesis and the hypothetical disease mechanisms is presented. Some practical considerations for improving the 2001 North American Hair Research Society working classification of the primary cicatricial alopecias are suggested. The aim of treatment is to slow or stop the progression of the inflammatory waves and the scarring process at the earliest phase of involvement. Recommendations for therapy are based upon a literature review, personal experience, expected adverse effects, and some pragmatic considerations such as the cost and patient compliance.

Deciphering the mesodermal potency of porcine skin-derived progenitors (SKP) by microarray analysis

Skin stem cells have an essential role in maintaining tissue homeostasis by dynamically replenishing those constantly lost during tissue turnover or following injury. Multipotent skin derived progenitors (SKP) can generate both neural and mesodermal progeny, representing neural crest-derived progenitors during embryogenesis through adulthood. SKP cells develop into spheres in suspension and can differentiate into fibroblast-like cells (SFC) in adhesive culture with serum. Concomitantly they gradually lose the neural potential but retain certain mesodermal potential. However, little is known about the molecular mechanism of the transition of SKP spheres into SFC in vitro. Here we characterized the transcriptional profiles of porcine SKP spheres and SFC by microarray analysis. We found 305 upregulated and 96 downregulated genes, respectively. The downregulated genes are mostly involved in intrinsic programs like the Dicer pathway and asymmetric cell division, whereas upregulated genes are likely to participate in extrinsic signaling pathways such as ErbB signaling, MAPK signaling, ECM-receptor reaction, Wnt signaling, cell communication, and tumor growth factor (TGF)-β signaling pathways. These intrinsic programs and extrinsic signaling pathways collaborate to mediate the transcription-state transition between SKP spheres and SFC. We speculate that these potential signaling pathways may play an important role in regulating the cell fate transition between SKP spheres and SFC in vitro.

Gsdma3 mutation causes bulge stem cell depletion and alopecia mediated by skin inflammation

Primary cicatricial alopecias (PCAs) are a group of permanent hair loss disorders, of which the pathogenesis is still poorly understood. The alopecia and excoriation (AE) mouse strain is a dominant mutant generated from ethyl nitrosourea mutagenesis. AE mice exhibit a progressive alopecia phenotype similar to that seen in PCAs, resulting from a point mutation in the gasdermin A3 gene. Mutant mice begin to show alopecia on the head from postnatal day 22 and experience complete hair loss by the age of 6 months, along with hyperkeratosis and catagen delay. The results of a histological examination showed that bulge stem cells in AE skin are gradually depleted, as indicated by decreased keratin 15 and CD34 expression, and reduced bromodeoxyuridine label-retaining cells in the AE bulge. In addition, AE mice display an inflammatory condition in the skin from postnatal day 7, including elevated tumor necrosis factor-α and monocyte chemotactic protein-1 mRNA levels and significantly increased macrophages and dendritic cell number. Immune privilege in the bulge was also compromised in AE skin. Consistently, after treatment with the immunosuppressive agent, cyclosporine A, immune privilege collapse, stem cell destruction, and alopecia phenotype of AE mice were all rescued. Collectively, our data demonstrate that immune-mediated destruction of bulge stem cells plays a crucial role in the pathogenesis of alopecia in AE mice, and this strain might be an interesting model for PCAs, especially for lichen planopilaris.

Delineating immune-mediated mechanisms underlying hair follicle destruction in the mouse mutant defolliculated

Defolliculated (Gsdma3(Dfl)/+) mice have a hair loss phenotype that involves an aberrant hair cycle, altered sebaceous gland differentiation with reduced sebum production, chronic inflammation, and ultimately the loss of the hair follicle. Hair loss in these mice is similar to that seen in primary cicatricial, or scarring alopecias in which immune targeting of hair follicle stem cells has been proposed as a key factor resulting in permanent hair follicle destruction. In this study we examine the mechanism of hair loss in GsdmA3(Dfl)/+ mice. Aberrant expression patterns of stem cell markers during the hair cycle, in addition to aberrant behavior of the melanocytes leading to ectopic pigmentation of the hair follicle and epidermis, indicated the stem cell niche was not maintained. An autoimmune mechanism was excluded by crossing the mice with rag1-/- mice. However, large numbers of macrophages and increased expression of ICAM-1 were still present and may be involved either directly or indirectly in the hair loss. Reverse transcriptase-PCR (RT-PCR) and immunohistochemistry of sebaceous gland differentiation markers revealed reduced peroxisome proliferator-activated receptor-γ (PPARγ), a potential cause of reduced sebum production, as well as the potential involvement of the innate immune system in the hair loss. As reduced PPARγ expression has recently been implicated as a cause for lichen planopilaris, these mice may be useful for testing therapies.

The mineralocorticoid receptor as a novel player in skin biology: beyond the renal horizon?

The mineralocorticoid receptor (MR) and its ligand aldosterone regulate renal sodium reabsorption and blood pressure and much knowledge has been accumulated in MR physiopathology, cellular and molecular targets. In contrast, our understanding of this hormonal system in non-classical targets (heart, blood vessels, neurons, keratinocytes...) is limited, particularly in the mammalian skin. We review here the few available data that point on MR in the skin and that document cutaneous MR expression and function, based on mouse models and very limited observations in humans. Mice that overexpress the MR in the basal epidermal keratinocytes display developmental and post-natal abnormalities of the epidermis and hair follicle, raising exciting new questions regarding skin biology. The MR as a transcription factor may be an unexpected novel player in regulating keratinocyte and hair physiology and pathology. Because its activating ligand also includes glucocorticoids, that are widely used in dermatology, we propose that the MR may be also involved in the side-effects of corticoids, opening novel options for therapeutical intervention.