PPAR-γ signalling as a key mediator of human hair follicle physiology and pathology

Adipose transcriptome in the scalp of androgenetic alopecia

Previous studies have shown how adipocytes can modulate the activity of hair follicle stem cells. However, the role of adipocytes in the pathogenesis of androgenetic alopecia (AGA) remains unknown. We aimed to determine signaling pathways related to the adipose tissue changes in the human scalp with AGA through RNA-seq analysis. RNA was isolated from the adipose tissues derived from the bald (frontal) and normal (occipital) scalps of male patients with AGA (n = 4). The pooled RNA extracts from these samples were subjected to RNA sequencing, followed by differential gene expression and pathway analysis. Our gene expression analysis identified 1,060 differentially expressed genes, including 522 upregulated and 538 downregulated genes in the bald AGA scalp. Biological pathways pertaining to either adipose tissue metabolism or the hair cycle were generated in our pathway analysis. Downregulation of the peroxisome proliferator-activated receptor (PPAR) signaling pathway was noted to be significant in the bald scalp. Expression of adipogenic markers (e.g., PPARG, FABP4, PLN1, and ADIPOQ) was also decreased in the bald site. These findings imply that adipogenesis becomes downregulated in AGA, specifically within the bald scalp adipose. Our results lead to the hypothesis that PPARγ-mediated adipogenesis in the scalp adipose, via crosstalk with signaling pathways involved in hair cycling, might play a role in AGA.

mTOR signaling in hair follicle and hair diseases: recent progress

Mammalian target of rapamycin (mTOR) signaling pathway is a major regulator of cell proliferation and metabolism, playing significant roles in proliferation, apoptosis, inflammation, and illness. More and more evidences showed that the mTOR signaling pathway affects hair follicle circulation and maintains the stability of hair follicle stem cells. mTOR signaling may be a critical cog in Vitamin D receptor (VDR) deficiency-mediated hair follicle damage and degeneration and related alopecia disorders. This review examines the function of mTOR signaling in hair follicles and hair diseases, and talks about the underlying molecular mechanisms that mTOR signaling regulates.

The effects of GPR40 agonists on hair growth are mediated by ANGPTL4

GPR40 is found primarily in pancreatic β cells, and is well known to regulate insulin secretion. Despite numerous studies on GPR40, the role and functions of GPR40 related to hair growth are not yet known. The current study investigated hair growth promoting effect of the GPR40 agonists and its mechanism of action using various bio-informatics tools, in vitro and animal experiments. GPR40 may affect the hair cycle, according to clustering and Gene Set Enrichment Analysis (GSEA). Hair growth effect of GPR40 was validated by telogen-to-anagen transition and vibrissae organ culture in the mouse. GPR40 was predominantly expressed in the outer root sheath (ORS) in anagen stage, suggesting that ORS cell is the target of GPR40 agonists. To investigate the mechanism of action for GPR40 agonists' hair growth effect, Gene Ontology (GO) enrichment analysis was performed and it revealed that GPR40 agonists were associated with angiogenesis. ANGPTL4, known for promoting angiogenesis, was highly up-regulated after GPR40 agonists treatment in the hORS cells, and also increased the proliferation and migration. Furthermore, GPR40 agonists promoted hair growth by inducing angiogenesis via ANGPTL4 in the animal experiment. GPR40 agonists activated MAPK and peroxisome proliferator-activated receptors (PPARγ) pathway in hORS cells, while the inhibition of MAPK pathway attenuated ANGPTL4 expression. Finally, GPR40 agonists increased hair growth via autocrine effects in the ORS cells, and induced angiogenesis through paracrine effects by upregulating ANGPTL4 via p38 and PPARγ pathways. As a result, GPR40 agonists have potential as a therapeutic drug for hair loss treatment.

Stimulation with THBS4 activates pathways that regulate proliferation, migration and inflammation in primary human keratinocytes

As in other mammalian tissues, the extracellular matrix (ECM) of skin functions as mechanical support and regulative environment that guides the behavior of the cells. ECM is a gel-like structure that is primarily composed of structural and nonstructural proteins. While the content of structural proteins is stable, the level of nonstructural ECM proteins, such as thrombospondin-4 (THBS4), is dynamically regulated. In a previous work we demonstrated that THBS4 stimulated cutaneous wound healing. In this work we discovered that in addition to proliferation, THBS4 stimulated the migration of primary keratinocytes in 3D. By using a proteotransciptomic approach we found that stimulation of keratinocytes with THBS4 regulated the activity of signaling pathways linked to proliferation, migration, inflammation and differentiation. Interestingly, some of the regulated genes (eg IL37, TSLP) have been associated with the pathogenesis of atopic dermatitis (AD). We concluded that THBS4 is a promising candidate for novel wound healing therapies and suggest that there is a potential convergence of pathways that stimulate cutaneous wound healing with those active in the pathogenesis of inflammatory skin diseases.

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.

Frontiers in Lichen Planopilaris and Frontal Fibrosing Alopecia Research: Pathobiology Progress and Translational Horizons

Lichen planopilaris (LPP) and frontal fibrosing alopecia (FFA) are primary, lymphocytic cicatricial hair loss disorders. These model epithelial stem cell (SC) diseases are thought to result from a CD8⁺ T-cell‒dominated immune attack on the hair follicle (HF) SC niche (bulge) after the latter has lost its immune privilege (IP) for as yet unknown reasons. This induces both apoptosis and pathological epithelial‒mesenchymal transition in epithelial SCs, thus depletes the bulge, causes fibrosis, and ultimately abrogates the HFs' capacity to regenerate. In this paper, we synthesize recent progress in LPP and FFA pathobiology research, integrate our limited current understanding of the roles that genetic, hormonal, environmental, and other factors may play, and define major open questions. We propose that LPP and FFA share a common initial pathobiology, which then bifurcates into two distinct clinical phenotypes, with macrophages possibly playing a key role in phenotype determination. As particularly promising translational research avenues toward direly needed progress in the management of these disfiguring, deeply distressful cicatricial alopecia variants, we advocate to focus on the development of bulge IP and epithelial SC protectants such as, for example, topically effective, HF‒penetrating and immunoinhibitory preparations that contain tacrolimus, peroxisome proliferator-activated receptor-γ, and/or CB1 agonists.

Hair Follicle-Related MicroRNA-34a Serum Expression and rs2666433A/G Variant in Patients with Alopecia: A Cross-Sectional Analysis

Alopecia areata (AA) is a type of immune-mediated alopecia. Recent studies have suggested microRNAs’ (miRNAs) implication in several cellular processes, including epidermal and hair follicle biology. Single nucleotide polymorphisms (SNPs) can modify gene expression levels, which may induce an autoimmune response. This case−control study included 480 participants (240 for each case/control group). MicroRNA-34a gene (MIR-34A) rs2666433A/G variant was genotyped using real-time allelic discrimination polymerase chain reaction (PCR). Additionally, circulatory miR-34a levels were quantified by quantitative reverse transcription PCR (qRT-PCR). On comparing between alopecia and non-alopecia cohorts, a higher frequency of A variant was noted among patients when compared to controls—A allele: 28 versus 18% (p < 0.001); A/A genotype: 9 versus 2%; A/G genotype: 39 versus 32% (p < 0.001). A/A and A/G carriers were more likely to develop alopecia under heterozygote comparison (OR = 1.83, 95% CI = 1.14−2.93), homozygote comparison (OR = 4.19, 95% CI = 1.33−13.1), dominant (OR = 2.0, 95% CI = 1.27−3.15), recessive (OR = 3.36, 95% CI = 1.08−10.48), over-dominant (OR = 1.65, 95% CI = 1.04−32.63), and log additive (OR = 1.91, 95% CI = 1.3−2.82) models. Serum miR-34a expression levels were upregulated in alopecia patients with a median and quartile fold change of 27.3 (1.42−2430). Significantly higher levels were more pronounced in A/A genotype patients (p < 0.01). Patients carrying the heterozygote genotype (rs2666433 * A/G) were two times more likely to develop more severe disease grades. Stratified analysis by sex revealed the same results. A high expression level was associated with concomitant autoimmune comorbidities (p = 0.001), in particular SLE (p = 0.007) and vitiligo (p = 0.049). In conclusion, the MIR34A rs2666433 (A/G) variant is associated with AA risk and severity in the studied population. Furthermore, high miR-34a circulatory levels could play a role in disease pathogenesis.

Targeting mitochondria in dermatological therapy: Beyond oxidative damage and skin aging

INTRODUCTION

The analysis of the role of the mitochondria in oxidative damage and skin aging is a significant aspect of dermatological research. Mitochondria generate most reactive oxygen species (ROS); however, excessive ROS are cytotoxic and DNA-damaging and promote (photo-)aging. ROS also possesses key physiological and regulatory functions and mitochondrial dysfunction is prominent in several skin diseases including skin cancers. Although many standard dermatotherapeutics modulate mitochondrial function, dermatological therapy rarely targets the mitochondria. Accordingly, there is a rationale for "mitochondrial dermatology"-based approaches to be applied to therapeutic research.

AREAS COVERED

This paper examines the functions of mitochondria in cutaneous physiology beyond energy (ATP) and ROS production. Keratinocyte differentiation and epidermal barrier maintenance, appendage morphogenesis and homeostasis, photoaging and skin cancer are considered. Based on related PubMed search results, the paper evaluates thyroid hormones, glucocorticoids, Vitamin D3 derivatives, retinoids, cannabinoid receptor agonists, PPARγ agonists, thyrotropin, and thyrotropin-releasing hormone as instructive lead compounds. Moreover, the mitochondrial protein MPZL3 as a promising new drug target for future "mitochondrial dermatology" is highlighted.

EXPERT OPINION

Future dermatological therapeutic research should have a mitochondrial medicine emphasis. Focusing on selected lead agents, protein targets, in silico drug design, and model diseases will fertilize a mito-centric approach.

Do epidermal keratinocytes have sensory and information processing systems?

It was long considered that the role of epidermal keratinocytes is solely to construct a water-impermeable protective membrane, the stratum corneum, at the uppermost layer of the skin. However, in the last two decades, it has been found that keratinocytes contain multiple sensory systems that detect environmental changes, including mechanical stimuli, sound, visible radiation, electric fields, magnetic fields, temperature and chemical stimuli, and also a variety of receptor molecules associated with olfactory or taste sensation. Moreover, neurotransmitters and their receptors that play crucial roles in the brain are functionally expressed in keratinocytes. Recent studies have demonstrated that excitation of keratinocytes can induce sensory perception in the brain. Here, we review the sensory and information processing capabilities of keratinocytes. We discuss the possibility that epidermal keratinocytes might represent the earliest stage in the development of the brain during the evolution of vertebrates.

PPARγ signaling protects hair follicle stem cells from chemotherapy-induced apoptosis and epithelial-mesenchymal transition

BACKGROUND

Permanent chemotherapy-induced alopecia (pCIA), for which preventive interventions remain limited, can manifest with scarring. While the underlying pathomechanisms of pCIA are unclear, depletion of epithelial hair follicle (HF) stem cells (eHFSCs) is likely to play a role.

OBJECTIVES

To explore the hypothesis that eHFSCs undergo pathological epithelial-mesenchymal transition (EMT) besides apoptosis in pCIA, thus explaining the scarring phenotype. Furthermore, we tested whether a PPARγ modulator can prevent pCIA-associated pathomechanisms.

METHODS

Organ-cultured human scalp HFs were treated with the cyclophosphamide metabolite, 4-hydroperoxycyclophosphamide (4-HC). Additionally, HFs were pre-treated with the agnostic PPARγ modulator, N-Acetyl-GED-0507-34-Levo (NAGED), which we had previously shown to promote K15 expression and antagonize EMT in eHFSCs.

RESULTS

In accordance with anticipated hair bulb cytotoxicity, dystrophy and catagen induction, 4-HC promoted apoptosis along with increased p53 expression, DNA damage and pathological EMT in keratin 15+ (K15) bulge eHFSCs, as evidenced by decreased E-cadherin expression and the appearance of fibronectin- and vimentin-positive cells in the bulge. Pre-treatment with NAGED protected from 4-HC-induced hair bulb cytotoxicity/dystrophy, and halted apoptosis, p53 up-regulation, and EMT in the bulge, thereby significantly preventing the depletion of K15+ human eHFSCs ex vivo.

CONCLUSIONS

A cyclophosphamide metabolite alone suffices to damage and deplete human scalp eHFSCs by promoting apoptosis, DNA damage, and EMT ex vivo. Therefore, pCIA-therapeutic strategies need to target these pathological processes. Our data introduce the stimulation of PPARγ signaling as a novel intervention strategy for the prevention of pCIA, given the ability of NAGED to prevent chemotherapy-induced eHFSCs damage ex vivo.

Mitochondrially localized MPZL3 emerges as a signaling hub of mammalian physiology

MPZL3 is a nuclear-encoded, mitochondrially localized, immunoglobulin-like V-type protein that functions as a key regulator of epithelial cell differentiation, lipid metabolism, ROS production, glycemic control, and energy expenditure. Recently, MPZL3 has surfaced as an important modulator of sebaceous gland function and of hair follicle cycling, an organ transformation process that is also governed by peripheral clock gene activity and PPARγ. Given the phenotype similarities and differences between Mpzl3 and Pparγ knockout mice, we propose that MPZL3 serves as a signaling hub that is regulated by core clock gene products and/or PPARγ to translate signals from these nuclear transcription factors to the mitochondria to modulate circadian and metabolic regulation. Conservation between murine and human MPZL3 suggests that human MPZL3 may have similarly complex functions in health and disease. We summarize current knowledge and discuss future directions to elucidate the full spectrum of MPZL3 functions in mammalian physiology.

Keratinocyte-derived IL-1β induces PPARG downregulation and PPARD upregulation in human reconstructed epidermis following barrier impairment

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear hormone receptors. In skin, PPARs modulate inflammation, lipid synthesis, keratinocyte differentiation and proliferation and thus are important for skin barrier homeostasis. Accordingly, PPAR expression is altered in various skin conditions that entail epidermal barrier impairment, that is atopic dermatitis (AD) and psoriasis. Using human epidermal equivalents (HEEs), we established models of acute epidermal barrier impairment devoid of immune cells. We assessed PPAR and cytokine expression after barrier perturbation and examined effects of keratinocyte-derived cytokines on PPAR expression. We show that acetone or SDS treatment causes graded impairment of epidermal barrier function. Furthermore, we demonstrate that besides IL-1β and TNFα, IL-33 and TSLP are highly relevant markers for acute epidermal barrier impairment. Both SDS- and acetone-mediated epidermal barrier impairment reduce PPARG expression levels, whereas only SDS enhances PPARD expression. In line with findings in IL-1β and TNFα-treated HEEs, abrogation of IL-1 signalling restores PPARG expression and limits the increase of PPARD expression in SDS-induced epidermal barrier impairment. Thus, following epidermal barrier perturbation, keratinocyte-derived IL-1β and partly TNFα modulate PPARG and PPARD expression. These results emphasize a role for PPARγ and PPARβ/δ in acute epidermal barrier impairment with possible implications for diseases such as AD and psoriasis.

Metabolic Disorders/Obesity Is a Primary Risk Factor in Hidradenitis Suppurativa: An Immunohistochemical Real-World Approach

BACKGROUND

Hidradenitis suppurativa (HS) is an inflammatory, potentially scarring disease of the hair follicle, affecting the apocrine gland-bearing skin areas. The major comorbid disorders associated with the occurrence or the aggravation of the disease are obesity and smoking. Numerous efforts to dissociate these factors led to controversial results.

OBJECTIVES

To assess the importance of metabolic disorders/obesity, smoking/environmental toxins, and inflammation in HS by utilizing the differential expression of major relevant protein markers in lesional skin of obese/smoking versus non-obese/non-smoking HS patients.

METHODS

Lesional skin specimens deriving from two groups of HS patients (BMI >30 and smokers, n = 12 vs. BMI <30 and non-smokers, n = 10) were stained with antibodies raised against irisin, PPARγ, and IGF-1R, which correlate with metabolic disorders/obesity, EGFR and AhR, associated with smoking, and IL-17, IL-17R, and S100A8, as markers of inflammation.

RESULTS

Metabolic disorders/obesity-related markers exhibited marked differential expression between the two groups, while smoking-associated markers a limited one. IL-17R expression was stronger in obese/smokers, and S100A8 staining exhibited intense strong immunoreactivity in both groups without significant difference.

CONCLUSIONS

The notion that obesity plays a role in HS development appears to be supported by the prominent regulation of the associated lesional biomarkers. Tobacco smoking might contribute less to HS than previously suspected.

Janus kinase inhibitors in dermatology: Part I. A comprehensive review

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) intracellular signaling pathway is utilized by many proinflammatory molecules to mediate downstream effects and activate gene transcription. Activation of the JAK-STAT pathway contributes to a number of inflammatory dermatoses. Clinical trials and smaller studies have demonstrated the efficacy of JAK inhibitors in the treatment of a variety of dermatologic conditions. Here, we review the use of JAK inhibitors for the treatment of a wide range of dermatologic diseases in a two-part review series.

Topical and nutricosmetic products for healthy hair and dermal antiaging using "dual-acting" (2 for 1) plant-based peptides, hormones, and cannabinoids

One of the side effects of oral antiaging retinoids is increased hair shedding. Retinoids promote the expression of TGF-β2 from fibroblasts, which stimulate collagen expression but silences keratinocytes. Since keratinocytes normally influence differentiation of dermal papilla cells at the base of the hair follicle, retinoids feasibly inhibit hair growth via the increased expression of TGF-β2, which inhibits Wnt/β-catenin signaling. Fortunately, the plant kingdom provides an array of alternatives as dual-acting nutricosmetics and topicals that work independently of TGF-β2 to confer dermal antiaging and hair health effects. These alternatives include "plant hormones" such as cytokinins and phytoestrogens. Many cytokinins are agonists of the G-coupled adenosine receptors. Partial agonism of adenosine receptors promotes collagen synthesis independently of TGF-β2 signaling. Adenosine expression is potentially also the mechanism of minoxidil in promotion of scalp hair growth. Because of crosstalk between adenosine and cannabinoid receptors it makes sense to try combinations of specific CB2 agonists and cytokinins (or phytoestrogens). However, dual-acting cosmetics including peptides with high numbers of positively charged amino acids, such as lysine or arginine, offer real potential as they can be processed from multiple botanical candidates, including almond, fenugreek, pea sprouts, soy, and seaweeds. The current review summarizes much of what is known about retinoid alternatives in the plant kingdom and identifies potentially fruitful new areas of research.