The hair follicle as an estrogen target and source

Androgenetic alopecia and insulin resistance in young men

BACKGROUND

Epidemiological studies have associated androgenetic alopecia (AGA) with severe young-age coronary artery disease and hypertension, and linked it to insulin resistance. We carried out a case-control study in age- and weight-matched young males to study the link between AGA and insulin resistance using the homeostasis model assessment of insulin resistance (HOMA-IR) index or metabolic syndrome clinical manifestations.

METHODS

Eighty young males, 18-35 years old, with AGA > or = stage III in the Hamilton-Norwood classification, and 80 weight- and age-matched controls were included. Alopecia, glucose, serum insulin, HOMA-IR index, lipid profile and androgen levels, as well as metabolic syndrome criteria, were evaluated.

RESULTS

The HOMA-IR index was significantly higher in cases than controls. Nonobese cases had a higher mean diastolic blood pressure and a more frequent family history of AGA than nonobese controls. A borderline difference in the HOMA-IR index was found in obese AGA cases vs. obese controls [P = 0.055, 95% confidence interval (CI) 2.36-4.20 vs. 1.75-2.73]. Free testosterone values were significantly higher in controls than cases, regardless of body mass index (BMI). A statistically significant additive effect for obesity plus alopecia was found, with significant trends for insulin, the HOMA-IR index, lipids and free testosterone when BMI and alopecia status were used to classify the participants.

CONCLUSIONS

Our results support the recommendation for assessing insulin resistance and cardiovascular-related features and disorders in all young males with stage III or higher AGA, according to the Hamilton-Norwood classification.

Cellular targets of estrogen signaling in regeneration of inner ear sensory epithelia

Estrogen signaling in auditory and vestibular sensory epithelia is a newly emerging focus propelled by the role of estrogen signaling in many other proliferative systems. Understanding the pathways with which estrogen interacts can provide a means to identify how estrogen may modulate proliferative signaling in inner ear sensory epithelia. Reviewed herein are two signaling families, EGF and TGFbeta. Both pathways are involved in regulating proliferation of supporting cells in mature vestibular sensory epithelia and have well characterized interactions with estrogen signaling in other systems. It is becoming increasingly clear that elucidating the complexity of signaling in regeneration will be necessary for development of therapeutics that can initiate regeneration and prevent progression to a pathogenic state.

The hair follicle as a dynamic miniorgan

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. The hair shaft consists of terminally differentiated keratinocytes that are produced by the hair follicle. Hair follicle development takes place during fetal skin development and relies on tightly regulated ectodermal-mesodermal interactions. After birth, mature and actively growing hair follicles eventually become anchored in the subcutis, and periodically regenerate by spontaneously undergoing repetitive cycles of growth (anagen), apoptosis-driven regression (catagen), and relative quiescence (telogen). Our molecular understanding of hair follicle biology relies heavily on mouse mutants with abnormalities in hair structure, growth, and/or pigmentation. These mice have allowed novel insights into important general molecular and cellular processes beyond skin and hair biology, ranging from organ induction, morphogenesis and regeneration, to pigment and stem cell biology, cell proliferation, migration and apoptosis. In this review, we present basic concepts of hair follicle biology and summarize important recent advances in the field.

Androgens and hair growth

Hair's importance in human communication means that abnormalities like excess hair in hirsutism or hair loss in alopecia cause psychological distress. Androgens are the main regulator of human hair follicles, changing small vellus follicles producing tiny, virtually invisible hairs into larger intermediate and terminal follicles making bigger, pigmented hairs. The response to androgens varies with the body site as it is specific to the hair follicle itself. Normally around puberty, androgens stimulate axillary and pubic hair in both sexes, plus the beard, etc. in men, while later they may also inhibit scalp hair growth causing androgenetic alopecia. Androgens act within the follicle to alter the mesenchyme-epithelial cell interactions, changing the length of time the hair is growing, the dermal papilla size and dermal papilla cell, keratinocyte and melanocyte activity. Greater understanding of the mechanisms of androgen action in follicles should improve therapies for poorly controlled hair disorders like hirsutism and alopecia.

Human female hair follicles are a direct, nonclassical target for thyroid-stimulating hormone

Pituitary thyroid-stimulating hormone (TSH) regulates thyroid hormone synthesis via receptors (TSH-R) expressed on thyroid epithelial cells. As the hair follicle (HF) is uniquely hormone-sensitive and, hypothyroidism with its associated, increased TSH serum levels clinically can lead to hair loss, we asked whether human HFs are a direct target for TSH. Here, we report that normal human scalp skin and microdissected human HFs express TSH-R mRNA. TSH-R-like immunoreactivity is limited to the mesenchymal skin compartments in situ. TSH may alter HF mesenchymal functions, as it upregulates alpha-smooth muscle actin expression in HF fibroblasts. TSH-R stimulation by its natural ligand in organ culture changes the expression of several genes of human scalp HFs (for example keratin K5), upregulates the transcription of classical TSH target genes and enhances cAMP production. Although the functional role of TSH in human HF biology awaits further dissection, these findings document that intracutaneous TSH-Rs are fully functional in situ and that HFs of female individuals are direct targets for nonclassical, extrathyroidal TSH bioregulation. This suggests that organ-cultured scalp HFs provide an instructive and physiologically relevant human model for exploring nonclassical functions of TSH, in and beyond the skin.

Development of alopecia areata is associated with higher central and peripheral hypothalamic-pituitary-adrenal tone in the skin graft induced C3H/HeJ mouse model

The relationship of the stress response to the pathogenesis of alopecia areata (AA) was investigated by subjecting normal and skin graft-induced, AA-affected C3H/HeJ mice to light ether anesthesia or restraint stress. Plasma corticosterone (CORT), adrenocorticotropic hormone (ACTH), and estradiol (E2) levels were determined by RIA, whereas gene expression in brains, lymphoid organs, and skin was measured by quantitative RT-PCR for corticotropin-releasing hormone (Crh), arginine vasopressin (Avp), proopiomelanocortin (Pomc), glucocorticoid receptor (Nr3c1), mineralocorticoid receptor (Nr3c2), corticotropin-releasing hormone receptor types 1 and 2 (Crhr1, Crhr2), interleukin-12 (Il12), tumor necrosis factor-alpha (Tnf alpha), and estrogen receptors type-1 (Esr1) and type-2 (Esr2). AA mice had a marked increase in hypothalamic-pituitary-adrenal (HPA) tone and activity centrally, and peripherally in the skin and lymph nodes. There was also altered interaction between the adrenal and gonadal axes compared with that in normal mice. Stress further exacerbated changes in AA mouse HPA activity both centrally and peripherally. AA mice had significantly blunted CORT and ACTH responses to acute ether stress (physiological stressor) and a deficit in habituation to repeated restraint stress (psychological stressor). The positive correlation of HPA hormone levels with skin Th1 cytokines suggests that altered HPA activity may occur as a consequence of the immune response associated with AA.

Modulation of basal and squamous cell carcinoma by endogenous estrogen in mouse models of skin cancer

Patched1 heterozygous mice (Ptch1(+/-)) are useful for basal cell carcinoma (BCC) studies, being remarkably susceptible to BCC induction by ultraviolet or ionizing radiation. Analogously, skin carcinogenesis-susceptible (Car-S) mice are elective for studies of papilloma and squamous cell carcinoma (SCC) induction. We previously reported a striking effect of gender on BCC induction in Ptch1(+/-) mice, with total resistance of females; likewise, Car-S females show increased skin tumor resistance relative to males. Here, we investigated the protective role of endogenous estrogen in skin keratinocyte tumorigenesis. Control (CN) and ovariectomized Ptch1(+/-) or Car-S females were irradiated for BCC induction or topically treated with chemical carcinogens for SCC induction. Susceptibility to BCC or SCC was dramatically increased in ovariectomized Ptch1(+/-) and Car-S females and restored to levels observed in males. Remarkably, progression of initially benign papillomas to malignant SCC occurred only in ovariectomized Car-S females. We explored the mechanisms underlying tumor progression and report overexpression of estrogen receptor (ER)-alpha, downregulation of ERbeta and upregulation of cyclin D1 in papillomas from ovariectomized Car-S relative to papillomas from CN females. Thus, an imbalanced ERalpha/ERbeta expression may be associated with estrogen-mediated modulation of non-melanoma skin carcinogenesis, with a key role played by cyclin D1. Our findings underscore a highly protective role of endogenous estrogen against skin tumorigenesis by diverse agents in two independent mouse models of skin cancer.

A human folliculoid microsphere assay for exploring epithelial- mesenchymal interactions in the human hair follicle

The search for more effective drugs for the management of common hair growth disorders remains a top priority, both for clinical dermatology and industry. In this pilot study, we report a pragmatic organotypic assay for basic and applied hair research. The patented technique produces microdroplets, which generate human folliculoid microspheres (HFMs), consisting of human dermal papilla fibroblasts and outer root sheath keratinocytes within an extracellular matrix that simulates elements of the hair follicle mesenchyme. Studying a number of different markers (for example, proliferation, apoptosis, cytokeratin-6, versican), we show that these HFMs, cultured under well-defined conditions, retain several essential epithelial-mesenchymal interactions characteristic for human scalp hair follicle. Selected, recognized hair growth-modulatory agents modulate these parameters in a manner that suggests that HFMs allow the standardized preclinical assessment of test agents on relevant human hair growth markers under substantially simplified in vitro conditions that approximate the in vivo situation. Furthermore, we show by immunohistochemistry, reverse transcriptase-PCR, and DNA microarray techniques that HFMs also offer a useful discovery tool for the identification of target genes and their products for candidate hair drugs. HFM thus represent an instructive modern experimental and screening tool for basic and applied hair research in the human system.

(Neuro-)endocrinology of epithelial hair follicle stem cells

The hair follicle is a repository of different types of somatic stem cells. However, even though the hair follicle is both a prominent target organ and a potent, non-classical site of production and/or metabolism of numerous polypetide- and steroid hormones, neuropeptides, neurotransmitters and neurotrophins, the (neuro-)endocrine controls of hair follicle epithelial stem cell (HFeSC) biology remain to be systematically explored. Focussing on HFeSCs, we attempt here to offer a "roadmap through terra incognita" by listing key open questions, by exploring endocrinologically relevant HFeSC gene profiling and mouse genomics data, and by sketching several clinically relevant pathways via which systemic and/or locally generated (neuro-)endocrine signals might impact on HFeSC. Exemplarily, we discuss, e.g. the potential roles of glucocorticoid and vitamin D receptors, the hairless gene product, thymic hormones, bone morphogenic proteins (BMPs) and their antagonists, and Skg-3 in HFeSC biology. Furthermore, we elaborate on the potential role of nerve growth factor (NGF) and substance P-dependent neurogenic inflammation in HFeSC damage, and explore how neuroendocrine signals may influence the balance between maintenance and destruction of hair follicle immune privilege, which protects these stem cells and their progeny. These considerations call for a concerted research effort to dissect the (neuro-)endocrinology of HFeSCs much more systematically than before.

Possibilities and potential roles of estrogen in the pathogenesis of proliferation hemangiomas formation

Hemangiomas, often categorized as angiogenic diseases, are the most common tumors of infancy, the life span of which is generally divided into proliferating phase, involuting phase, and involuted phase. Despite their high prevalence, the mechanism leading to proliferation hemangiomas formation is poorly understood and the best approach to their management remains controversial. None of the current therapeutic modalities is ideal, partly because the pathogenesis of hemangioma and the mechanism of its proliferation are far from clear. Many clues reveal that estrogen has an important role in developing the vascular system, experimental and clinical evidences accumulated in recent years also suggest the potential for estrogen to influence neovascularization. Based on those, we hypothesize that estrogen play a potential role in the development of hemangiomas, mainly by regulating some key angiogenic factors, including MMP-9, EPCs, VEGF, NO, etc. Accepting the hypothesis to be correct, a therapy that identify estrogen as a potential target for the design of new, more specific treatments can be used to prevent the proliferation hemangiomas formation. The hypothesis may lead a new direction in the study of mechanisms for proliferation hemangiomas formation, and further study of the precise mechanisms for estrogen-induced hemangiomas will produce effective antiestrogens and estrogen receptor antagonists as new medication for the very difficult problem.

Melatonin and the hair follicle

Melatonin, the chief secretory product of the pineal gland, has long been known to modulate hair growth, pigmentation and/or molting in many species, presumably as a key neuroendocrine regulator that couples coat phenotype and function to photoperiod-dependent environmental and reproductive changes. However, the detailed effects and mechanisms of this surprisingly pleiotropic indole on the hair follicle (HF) regarding growth control and pigmentation have not yet been completely understood. While unspecific melatonin binding sites have long been identified (e.g., in goat and mouse HFs), specific melatonin membrane MT2 receptor transcripts and both protein and mRNA expression for a specific nuclear melatonin binding site [retinoid-related orphan receptor alpha (RORalpha)] have only recently been identified in murine HFs. MT1, known to be expressed in human skin cells, is not transcribed in mouse skin. After initial enzymologic data from hamster skin related to potential intracutaneous melatonin synthesis, it has recently been demonstrated that murine and human skin, namely human scalp HFs in anagen, are important sites of extrapineal melatonin synthesis. Moreover, HF melatonin production is enhanced by catecholamines (as it classically occurs in the pineal gland). Melatonin may also functionally play a role in hair-cycle control, as it down-regulates both apoptosis and estrogen receptor-alpha expression, and modulates MT2 and RORalpha expression in murine skin in a hair-cycle-dependent manner. Because of melatonin's additional potency as a free radical scavenger and DNA repair inducer, the metabolically and proliferatively highly active anagen hair bulb may also exploit melatonin synthesis in loco as a self-cytoprotective strategy.

Topical fulvestrant solution has no effect on male and postmenopausal female androgenetic alopecia: results from two randomized, proof-of-concept studies

BACKGROUND

Androgenetic alopecia (pattern baldness) affects approximately half of all white-skinned men and women over the age of 40 years. Based on preclinical studies in mice in which topical fulvestrant (ICI182,780, an anti-oestrogen) caused telogen hair follicles to enter anagen, thereby causing hair growth, a topical formulation of fulvestrant was developed for the potential treatment of androgenetic alopecia.

OBJECTIVES

To evaluate the efficacy of fulvestrant solution in stimulating hair growth in men and postmenopausal women with androgenetic alopecia in two randomized, phase II, minoxidil- and/or vehicle-controlled studies.

METHODS

One hundred and two white-skinned men aged 18-50 years with Norwood/Hamilton grades III, IIIv, IV, V or Va androgenetic alopecia received topical fulvestrant 70 mg mL(-1) solution, vehicle or minoxidil 2% solution twice daily for 16 weeks. Seventy postmenopausal women with Ludwig grade 1 or 2 androgenetic alopecia received topical fulvestrant 70 mg mL(-1) solution or vehicle twice daily for 16 weeks. The endpoints in both studies were hair density, cumulative hair thickness and hair growth rate, measured by TrichoScan analysis of digital images.

RESULTS

There were no statistically significant differences favouring fulvestrant over vehicle at study end (day 113) for any of the efficacy parameters in men or women. Statistically significant differences in favour of minoxidil over fulvestrant were seen from day 57 onwards for hair density, cumulative hair thickness and hair growth rate in men.

CONCLUSIONS

These results indicate a lack of effect of topical fulvestrant in the treatment of subjects with androgenetic alopecia. The reasons for this lack of effect remain unclear.

Dissecting the impact of chemotherapy on the human hair follicle: a pragmatic in vitro assay for studying the pathogenesis and potential management of hair follicle dystrophy

Chemotherapy-induced alopecia represents one of the major unresolved problems of clinical oncology. The underlying molecular pathogenesis in humans is virtually unknown because of the lack of adequate research models. Therefore, we have explored whether microdissected, organ-cultured, human scalp hair follicles (HFs) in anagen VI can be exploited for dissecting and manipulating the impact of chemotherapy on human HFs. Here, we show that these organ-cultured HFs respond to a key cyclophosphamide metabolite, 4-hydroperoxycyclophosphamide (4-HC), in a manner that resembles chemotherapy-induced HF dystrophy as it occurs in vivo: namely, 4-HC induced melanin clumping and melanin incontinence, down-regulated keratinocyte proliferation, massively up-regulated apoptosis of hair matrix keratinocytes, prematurely induced catagen, and up-regulated p53. In addition, 4-HC induced DNA oxidation and the mitochondrial DNA common deletion. The organ culture system facilitated the identification of new molecular targets for chemotherapy-induced HF damage by microarray technology (eg, interleukin-8, fibroblast growth factor-18, and glypican 6). It was also used to explore candidate chemotherapy protectants, for which we used the cytoprotective cytokine keratinocyte growth factor as exemplary pilot agent. Thus, this novel system serves as a powerful yet pragmatic tool for dissecting and manipulating the impact of chemotherapy on the human HF.

Targeted skin overexpression of the mineralocorticoid receptor in mice causes epidermal atrophy, premature skin barrier formation, eye abnormalities, and alopecia

The mineralocorticoid receptor (MR) is a transcription factor of the nuclear receptor family, activation of which by aldosterone enhances salt reabsorption in the kidney. The MR is also expressed in nonclassical aldosterone target cells (brain, heart, and skin), in which its functions are incompletely understood. To explore the functional importance of MR in mammalian skin, we have generated a conditional doxycycline-inducible model of MR overexpression, resulting in double-transgenic (DT) mice [keratin 5-tTa/tetO-human MR (hMR)], targeting the human MR specifically to keratinocytes of the epidermis and hair follicle (HF). Expression of hMR throughout gestation resulted in early postnatal death that could be prevented by antagonizing MR signaling. DT mice exhibited premature epidermal barrier formation at embryonic day 16.5, reduced HF density and epidermal atrophy, increased keratinocyte apoptosis at embryonic day 18.5, and premature eye opening. When hMR expression was initiated after birth to overcome mortality, DT mice developed progressive alopecia and HF cysts, starting 4 months after hMR induction, preceded by dystrophy and cycling abnormalities of pelage HF. In contrast, interfollicular epidermis, vibrissae, and footpad sweat glands in DT mice were normal. This new mouse model reveals novel biological roles of MR signaling and offers an instructive tool for dissecting nonclassical functions of MR signaling in epidermal, hair follicle, and ocular physiology.