Endocrine controls of primary adult human stem cell biology: thyroid hormones stimulate keratin 15 expression, apoptosis, and differentiation in human hair follicle epithelial stem cells in situ and in vitro

Associations of serum keratin 1 with thyroid function and immunity in Graves' disease

BACKGROUND

Autoimmune thyroid disease (AITD) can cause enormous health burdens; however, trustworthy biomarkers in identifying the onset and progression of AITD are limited. In this study, we attempted to discover new potential serum biomarkers to discriminate AITD using mass spectrometry (MS).

METHODS

In the biomarker study cohort, 20 patients with Graves' disease (GD), 20 patients with Hashimoto's thyroiditis (HT), and 20 healthy controls were enrolled for a liquid chromatographic-tandem MS assessment. A novel biomarker, keratin 1 (KRT1), was selected for further evaluation in the validation cohort, including 125 patients with GD, 34 patients with HT, and 77 controls. Relationships of serum KRT1 with AITD-related immunomodulatory cytokines were also analyzed using enzyme-linked immunosorbent assays (ELISAs).

RESULTS

In the MS analysis, KRT1 was the single marker overexpressed in GD, while it was underexpressed in HT. In the ELISA analysis of the validation cohort, KRT1 was consistently upregulated in GD, while it was not downregulated in HT. There were significant associations of KRT1 levels with thyroid function in GD, AITD, and overall subjects. Additionally, a significant association of KRT1 levels with thyroid-stimulating hormone receptor antibody (TSHRAb) levels was observed. Moreover, there were significant associations of KRT1 with osteopontin (OPN) and B-cell activating factor (BAFF) levels in GD.

CONCLUSIONS

Serum KRT1 levels were upregulated in GD and were associated with thyroid function and TSHRAb levels. Moreover, KRT1 was correlated with the BAFF and OPN levels in GD patients. Further molecular-based research to elucidate the role of KRT1 in the pathogenesis of AITD is needed.

Upregulated keratin 15 links to the occurrence of lymphovascular invasion, stromal cervical invasion as well as unfavorable survival profile in endometrial cancer patients

Keratin 15 (KRT15) overexpression links with tumor initiation, metastasis, and poor survival in several solid carcinomas. While its clinical relevance is scarcely reported in endometrial cancer (EC). Therefore, the current study aimed to investigate the abnormal expression of KRT15 and its correlation with clinical characteristics, survival in EC patients. Totally, 135 surgical EC patients were enrolled. KRT15 protein expression in formalin-fixed and paraffin-embedded tumor and adjuvant tissues was detected by immunohistochemical staining; meanwhile, KRT15 mRNA expression in fresh-frozen tumor and adjacent tissues was detected by reverse transcription-quantitative polymerase chain reaction. KRT15 protein and mRNA expressions were higher in tumor tissue compared with adjacent tissue (both P < .001). Elevated KRT15 protein expression was correlated with the occurrence of lymphovascular invasion (P = .010) and more advanced International Federation of Gynecology and Obstetrics stage (P = .018); meanwhile, elevated KRT15 mRNA expression was linked with more advanced International Federation of Gynecology and Obstetrics stage (P = .038) and marginally associated with the occurrence of stromal cervical invasion (P = .052). Besides, KRT15 protein and mRNA expressions were not correlated with other clinical features (all P > .05). KRT15 protein high was marginally correlated with poor accumulating disease-free survival (DFS) (P = .091) and overall survival (OS) (P = .059); meanwhile, the correlation of KRT15 mRNA expression with accumulating DFS (P = .212) and OS (P = .092) was even weaker. However, multivariate Cox's regressions showed that tumor KRT15 protein (high vs low) was independently correlated with poor DFS (P = .045) and OS (P = .043). KRT15 is abnormally increased in EC tissue, meanwhile, its upregulation links to the occurrence of lymphovascular invasion, stromal cervical invasion, and poor prognosis in EC patients.

Use of facial care products and frontal fibrosing alopecia: Coincidence or true association?

The increasing incidence of frontal fibrosing alopecia (FFA) suggests that environmental factors may be related to the pathogenesis. Recent studies reported potential association between the use of facial care products and FFA. The aim of this study was to investigate the possible association between the use of facial care products and FFA in Asian females. A total of 250 females (50 FFA patients, 100 pattern hair loss [PHL] patients, and 100 normal controls) were recruited and completed a questionnaire to obtain information approximately facial care products and various environmental factors. Our study revealed the use of moisturizer to be significantly higher in the FFA group compared to normal controls (p < 0.001), and sunscreen use was significantly higher in the PHL group than in the control group (adjusted p < 0.001). Subjects with FFA or PHL reported significantly higher use of both sunscreen and moisturizer compared to normal controls (p < 0.001). This study focused on Asian populations. Our results revealed a high frequency of moisturizer and sunscreen use in both FFA and PHL among Asian females. Therefore, the use of facial care products appears not to be linked to the true disease mechanism of FFA, but rather to appearance-related concerns of patients.

Human epithelial stem cell survival within their niche requires "tonic" cannabinoid receptor 1-signalling-Lessons from the hair follicle

The endocannabinoid system (ECS) regulates multiple aspects of human epithelial physiology, including inhibition/stimulation of keratinocyte proliferation/apoptosis, respectively. Yet, how the ECS impacts on human adult epithelial stem cell (eSC) functions remains unknown. Scalp hair follicles (HFs) offer a clinically relevant, prototypic model system for studying this directly within the native human stem cell niche. Here, we show in organ-cultured human HFs that, unexpectedly, selective activation of cannabinoid receptor-1 (CB1)-mediated signalling via the MAPK (MEK/Erk 1/2) and Akt pathways significantly increases the number and proliferation of cytokeratin CK15+ or CK19+ human HF bulge eSCs in situ, and enhances CK15 promoter activity in situ. In striking contrast, CB1-stimulation promotes apoptosis in the differentiated progeny of these eSCs (CK6+ HF keratinocytes). Instead, intrafollicular CB1 gene knockdown or CB1 antagonist treatment significantly reduces human HF eSCs numbers and stimulates their apoptosis, while CB1 knockout mice exhibit a reduced bulge eSCs pool in vivo. This identifies "tonic" CB1 signalling as a required survival stimulus for adult human HF eSCs within their niche. This novel concept must be taken into account whenever the human ECS is targeted therapeutically.

The biology of human hair greying

Hair greying (canities) is one of the earliest, most visible ageing-associated phenomena, whose modulation by genetic, psychoemotional, oxidative, senescence-associated, metabolic and nutritional factors has long attracted skin biologists, dermatologists, and industry. Greying is of profound psychological and commercial relevance in increasingly ageing populations. In addition, the onset and perpetuation of defective melanin production in the human anagen hair follicle pigmentary unit (HFPU) provides a superb model for interrogating the molecular mechanisms of ageing in a complex human mini-organ, and greying-associated defects in bulge melanocyte stem cells (MSCs) represent an intriguing system of neural crest-derived stem cell senescence. Here, we emphasize that human greying invariably begins with the gradual decline in melanogenesis, including reduced tyrosinase activity, defective melanosome transfer and apoptosis of HFPU melanocytes, and is thus a primary event of the anagen hair bulb, not the bulge. Eventually, the bulge MSC pool becomes depleted as well, at which stage greying becomes largely irreversible. There is still no universally accepted model of human hair greying, and the extent of genetic contributions to greying remains unclear. However, oxidative damage likely is a crucial driver of greying via its disruption of HFPU melanocyte survival, MSC maintenance, and of the enzymatic apparatus of melanogenesis itself. While neuroendocrine factors [e.g. alpha melanocyte-stimulating hormone (α-MSH), adrenocorticotropic hormone (ACTH), ß-endorphin, corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH)], and micropthalmia-associated transcription factor (MITF) are well-known regulators of human hair follicle melanocytes and melanogenesis, how exactly these and other factors [e.g. thyroid hormones, hepatocyte growth factor (HGF), P-cadherin, peripheral clock activity] modulate greying requires more detailed study. Other important open questions include how HFPU melanocytes age intrinsically, how psychoemotional stress impacts this process, and how current insights into the gerontobiology of the human HFPU can best be translated into retardation or reversal of greying.

Topical L-thyroxine: The Cinderella among hormones waiting to dance on the floor of dermatological therapy?

Topical hormone therapy with natural or synthetic ligands of nuclear hormone receptors such as glucocorticoids, vitamin D analogues and retinoids has a long and highly successful tradition in dermatology. Yet the dermatological potential of thyroid hormone receptor (TR) agonists has been widely ignored, despite abundant clinical, cell and molecular biology, mouse in vivo, and human skin and hair follicle organ culture data documenting a role of TR-mediated signalling in skin physiology and pathology. Here, we review this evidence, with emphasis on wound healing and hair growth, and specifically highlight the therapeutic potential of repurposing topical L-thyroxine (T4) for selected applications in future dermatological therapy. We underscore the known systemic safety and efficacy profile of T4 in clinical medicine, and the well-documented impact of thyroid hormones on, for example, human epidermal and hair follicle physiology, hair follicle epithelial stem cells and pigmentation, keratin expression, mitochondrial energy metabolism and wound healing. On this background, we argue that short-term topical T4 treatment deserves careful further preclinical and clinical exploration for repurposing as a low-cost, effective and widely available dermatotherapeutic, namely in the management of skin ulcers and telogen effluvium, and that its predictable adverse effects are well-manageable.

Thyroxine restores severely impaired cutaneous re-epithelialisation and angiogenesis in a novel preclinical assay for studying human skin wound healing under "pathological" conditions ex vivo

Impaired cutaneous wound healing remains a major healthcare challenge. The enormity of this challenge is compounded by the lack of preclinical human skin wound healing models that recapitulate selected key factors underlying impaired healing, namely hypoxia/poor tissue perfusion, oxidative damage, defective innervation, and hyperglycaemia. Since organ-cultured human skin already represents a denervated and impaired perfusion state, we sought to further mimic “pathological” wound healing conditions by culturing experimentally wounded, healthy full-thickness frontotemporal skin from three healthy female subjects for three days in either serum-free supplemented Williams’ E medium or in unsupplemented medium under “pathological” conditions (i.e. hypoxia [5% O₂], oxidative damage [10 mM H₂O₂], absence of insulin, excess glucose). Under these “pathological” conditions, dermal–epidermal split formation and dyskeratosis were prominent in organ-cultured human skin, and epidermal reepithelialisation was significantly impaired (p < 0.001), associated with reduced keratinocyte proliferation (p < 0.001), cytokeratin 6 expression (p < 0.001) and increased apoptosis (p < 0.001). Moreover, markers of intracutaneous angiogenesis (CD31 immunoreactivity and the number of of CD31 positive cells and CD31 positive vessel lumina) were significantly reduced. Since we had previously shown that thyroxine promotes wound healing in healthy human skin ex vivo, we tested whether this in principle also occurs under “pathological” wound healing conditions. Indeed, thyroxine administration sufficed to rescue re-epithelialisation (p < 0.001) and promoted both epidermal keratinocyte proliferation (p < 0.01) and angiogenesis in terms of CD31 immunoreactivity and CD31 positive cells under “pathological” conditions (p < 0.001) ex vivo. This demonstrates the utility of this pragmatic short-term ex vivo model, which recapitulates some key parameters of impaired human skin wound healing, for the preclinical identification of promising wound healing promoters.

Comparative transcriptome analysis to elucidate the therapeutic mechanism of colchicine against atrial fibrillation

In recent years, colchicine has been used to reduce the risk of cardiovascular events; in particular, it has been effectively used for the treatment of atrial fibrillation (AF). We first discovered that colchicine can treat AF in a rat model and that it can reverse the effects of atrial fibrosis. To illustrate the potential therapeutic mechanism of colchicine against AF, we performed comparative transcriptome analyses; our aim was to elucidate the therapeutic effects of colchicine so as to improve treatment and prognoses of AF. Genomics and bioinformatics analyses revealed that the IL-17 signaling pathway, and renin secretion pathway are involved in the mechanism of action of colchicine. Furthermore, there was a significant correlation between overlapping genes in the two groups of differentially expressed genes. The genes encoding Akap4, Pcdha9, Gp2, Cd177, Krt15, Aqp3, Chia, and Bpifb1 were pivotal and possible action sites for the therapeutic mechanisms of colchicine. We conclude that AF involves a multifactorial pathological process. The mechanisms underlying the action of colchicine in the treatment of AF warrant further studies.

Culture of the human pilosebaceous unit, hair follicle and sebaceous gland

Terence Kealey first pioneered the isolation and organ maintenance of human eccrine and sebaceous glands in the early to mid-1980. This led to subsequent methods describing the isolation and culture of human hair follicles, the human pilosebaceous unit as well as the sebaceous duct. The importance of these models in the study of the biology of human skin glands and appendages has been demonstrated in numerous publications and their importance as models for animal replacement, refinement and reduction (3Rs) is increasingly important. In particular, in vitro (ex vivo) hair follicle culture has played a significant part in helping elucidate the role of signalling molecules in regulating hair growth and hair fibre formation and has been especially useful in understanding metabolic aspects of hair growth. However, obtaining sufficient numbers of hair follicles is becoming increasingly difficult as plastic surgery becomes less invasive and smaller skin samples provided. There is therefore an urgent requirement for the next generation of in vitro models using cell lines and tissue engineering, and this has led to the development of immortalised cell lines as well as attempts to model hair follicle embryogenesis in vitro and development of skin on a chip.

Frontal fibrosing alopecia: An update on the hypothesis of pathogenesis and treatment

Frontal fibrosing alopecia (FFA) is a relatively new scarring alopecia that is considered a variant of lichen planopilaris (LPP) with no recognized promising treatments. In this study, we tried to clarify the underlying signaling pathways and their roles in the pathogenesis and progression of FFA. Because of several differences in clinical manifestations, response to treatments, and pathological findings, these two conditions could be differentiated from each other. Taking into account the already discussed signaling pathways and involved players such as T cells, mast cells, and sebaceous glands, different possible therapeutic options could be suggested. In addition to treatments supported by clinical evidence, such as 5 alpha-reductase inhibitors, topical calcineurin inhibitors, hydroxychloroquine, peroxisome proliferator-activated receptor gamma agonists, and oral retinoid agents, various other treatment strategies and drugs, such as phototherapy, Janus kinase inhibitors, dehydroepiandrosterone, sirolimus, cetirizine, and rituximab, could be suggested to mitigate disease progression. Of course, such lines of treatment need further evaluation in clinical trials.

The inconsistent regulation of HOXC13 on different keratins and the regulation mechanism on HOXC13 in cashmere goat (Capra hircus)

Background

During hair growth, cortical cells emerging from the proliferative follicle bulb rapidly undergo a differentiation program and synthesize large amounts of hair keratin proteins. In this process, HOXC13 is one critical regulatory factor, proved by the hair defects in HOXC13 mutant mice and HOXC13 mutant patients. However, inconsistent conclusions were drawn from previous researches regarding the regulation of HOXC13 on different keratins. Whether HOXC13 has extensive and unified regulatory role on these numerous keratins is unclear.

Results

In this study, firstly, RNA-seq was performed to reveal the molecular mechanism of cashmere cycle including anagen and telogen. Subsequently, combining the sequencing with qRT-PCR and immunofluorescent staining results, we found that HOXC13 showed similar expression pattern with a large proportion of keratins except for KRT1 and KRT2, which were higher in anagen compared with telogen. Then, the regulatory role of HOXC13 on different keratins was investigated using dual-luciferase reporter system and keratin promoter-GFP system by overexpressing HOXC13 in HEK 293 T cells and dermal papilla cells. Our results demonstrated that HOXC13 up-regulated the promoter activity of KRT84 and KRT38, while down-regulated the promoter activity of KRT1 and KRT2, which suggested HOXC13 had an ambivalent effect on the promoters of different KRTs. Furtherly, the regulation on HOXC13 itself was investigated. At transcriptional level, the binding sites of HOXC13 and LEF1 were found in the promoter of HOXC13. Then, through transfecting corresponding overexpression vector and dual-luciferase reporter system into dermal papilla cells, the negative-feedback regulation of HOXC13 itself and positive regulation of LEF1 on HOXC13 promoter were revealed. In addition, melatonin could significantly increase the promoter activity of HOXC13 under the concentration of 10 μM and 25 μM by adding exogenous melatonin into dermal papilla cells. At post-transcriptional level, we investigated whether chi-miR-200a could target HOXC13 through dual-luciferase reporter system. At epigenetic level, we investigated the methylation level of HOXC13 promoter at different stages including anagen, telogen and 60d of embryonic period. As a result, miR-200a and methylation were not regulatory factors of HOXC13. Interestingly, we found two SNPs (c.812A > G and c.929A > C) in the homeodomain of HOXC13 that could deprive the regulatory function of HOXC13 on keratins without changing its protein expression.

Conclusion

HOXC13 had an inconsistent effect on the promoters of different keratins. Two SNPs (c.812A > G and c.929A > C) in the homeodomain of HOXC13 deprived its function on keratin regulation. Besides, the negative-feedback regulation by HOXC13 itself and positive regulation by LEF1 and melatonin on HOXC13 promoter were revealed. This study will enrich the function of HOXC13 on keratin regulation and contribute to understand the mechanism of hair follicle differentiation.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5011-4) contains supplementary material, which is available to authorized users.

Thyroid Hormone Receptors Regulate the Expression of microRNAs with Key Roles in Skin Homeostasis

BACKGROUND

MicroRNAs (miRNAs) play a unique role in posttranscriptional regulation of gene expression and control different aspects of skin development, homeostasis, and disease. Although it is generally accepted that thyroid hormone signaling is important in skin pathophysiology, the role of their nuclear receptors (TRs) in cutaneous miRNA expression has yet to be explored.

METHODS

RNAseq was used to compare the skin miRnome of wild-type mice and genetically modified mice lacking both TRα1 and TRβ, the main thyroid hormone binding isoforms. Changes in miRNAs with a crucial role in skin physiopathology were confirmed by stem-loop quantitative polymerase chain reaction in both total skin and isolated keratinocytes, and the levels of their target mRNAs were evaluated by real-time polymerase chain reaction.

RESULTS

The skin of TRα1/TRβ knockout mice displays altered levels of >50 miRNAs. Among the downregulated species are several miRNAs, including miR-21, miR-31, miR-34, and miR-203, with crucial roles in skin homeostasis. TRα1 appears to be the main isoform responsible for their regulation. Increased levels of gene transcripts previously shown to be bona fide targets of these miRNAs are also found in the skin and keratinocytes of TR-deficient mice. This suggests that multiple miRNAs that are downregulated in the absence of TRs cooperate to regulate gene expression in the skin.

CONCLUSIONS

The miRNAs reduced in TRα1/TRβ knockout mice are known to play crucial roles in epidermal proliferation, hair cycling, wound healing, stem-cell function, and tumor development, all processes altered in the absence of TRs. These results suggest that their regulation could contribute to the skin defects found in these mice and to the skin disorders associated with altered thyroid status in humans.

Thyroid hormone regulation of intestinal epithelial stem cell biology

The gastrointestinal tract is a well-characterized target of thyroid hormones and thyroid hormone nuclear receptors TRs, as extensively described in the literature. The paradigm is its important remodelling in amphibians during thyroid hormone-dependent metamorphosis. Interestingly, several studies have described the conservation of this hormonal signal during intestinal development in mammals. Additional data suggested that it may also play a role in intestinal homeostasis, stem cell physiology and progenitor commitment as well as in tumour development. It is worth underlining that in the mammalian intestine the functionality of the TRα1 receptor is coordinated and integrated with other signalling pathways, such as Wnt and Notch, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and concepts and discuss this new role for thyroid hormones and the TRα1 receptor in the biology of intestinal epithelial precursor cells.

Thyroxine differentially modulates the peripheral clock: lessons from the human hair follicle

The human hair follicle (HF) exhibits peripheral clock activity, with knock-down of clock genes (BMAL1 and PER1) prolonging active hair growth (anagen) and increasing pigmentation. Similarly, thyroid hormones prolong anagen and stimulate pigmentation in cultured human HFs. In addition they are recognized as key regulators of the central clock that controls circadian rhythmicity. Therefore, we asked whether thyroxine (T4) also influences peripheral clock activity in the human HF. Over 24 hours we found a significant reduction in protein levels of BMAL1 and PER1, with their transcript levels also decreasing significantly. Furthermore, while all clock genes maintained their rhythmicity in both the control and T4 treated HFs, there was a significant reduction in the amplitude of BMAL1 and PER1 in T4 (100 nM) treated HFs. Accompanying this, cell-cycle progression marker Cyclin D1 was also assessed appearing to show an induced circadian rhythmicity by T4 however, this was not significant. Contrary to short term cultures, after 6 days, transcript and/or protein levels of all core clock genes (BMAL1, PER1, clock, CRY1, CRY2) were up-regulated in T4 treated HFs. BMAL1 and PER1 mRNA was also up-regulated in the HF bulge, the location of HF epithelial stem cells. Together this provides the first direct evidence that T4 modulates the expression of the peripheral molecular clock. Thus, patients with thyroid dysfunction may also show a disordered peripheral clock, which raises the possibility that short term, pulsatile treatment with T4 might permit one to modulate circadian activity in peripheral tissues as a target to treat clock-related disease.

Thyroid hormone signaling controls hair follicle stem cell function

In mice lacking thyroid hormone receptors, bulge stem cells of the hair follicles present epigenetic alterations and a functional defect in their mobilization out of the niche. This is related to aberrant activation of Smad signaling and reduced nuclear β-catenin accumulation, an important component of stem cell mobilization.

Observations in thyroid patients and experimental animals show that the skin is an important target for the thyroid hormones. We previously showed that deletion in mice of the thyroid hormone nuclear receptors TRα1 and TRβ (the main thyroid hormone–binding isoforms) results in impaired epidermal proliferation, hair growth, and wound healing. Stem cells located at the bulges of the hair follicles are responsible for hair cycling and contribute to the regeneration of the new epidermis after wounding. Therefore a reduction in the number or function of the bulge stem cells could be responsible for this phenotype. Bulge cells show increased levels of epigenetic repressive marks, can retain bromodeoxyuridine labeling for a long time, and have colony-forming efficiency (CFE) in vitro. Here we demonstrate that mice lacking TRs do not have a decrease of the bulge stem cell population. Instead, they show an increase of label-retaining cells (LRCs) in the bulges and enhanced CFE in vitro. Reduced activation of stem cells leading to their accumulation in the bulges is indicated by a strongly reduced response to mobilization by 12-O-tetradecanolyphorbol-13-acetate. Altered function of the bulge stem cells is associated with aberrant activation of Smad signaling, leading to reduced nuclear accumulation of β-catenin, which is crucial for stem cell proliferation and mobilization. LRCs of TR-deficient mice also show increased levels of epigenetic repressive marks. We conclude that thyroid hormone signaling is an important determinant of the mobilization of stem cells out of their niche in the hair bulge. These findings correlate with skin defects observed in mice and alterations found in human thyroid disorders.

Thyroid hormones and their nuclear receptors: new players in intestinal epithelium stem cell biology?

Thyroid hormones participate in the development and homeostasis of several organs and tissues. It is well documented that they act via nuclear receptors, the TRs, which are transcription factors whose function is modulated by the hormone T3. Importantly, T3-induced physiological response within a cell depends on the specific TR expression and on the T3 bioavailability. However, in addition to this T3-dependent control of TR functionality, increasing data show that the action of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. By focusing on the intestinal epithelium of both amphibians and mammals we summarize here new data in support of a role for thyroid hormones and the TR nuclear receptors in stem cell biology. This new concept may be extended to other organs and have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer.

Human epithelial hair follicle stem cells and their progeny: current state of knowledge, the widening gap in translational research and future challenges

Epithelial hair follicle stem cells (eHFSCs) are required to generate, maintain and renew the continuously cycling hair follicle (HF), supply cells that produce the keratinized hair shaft and aid in the reepithelialization of injured skin. Therefore, their study is biologically and clinically important, from alopecia to carcinogenesis and regenerative medicine. However, human eHFSCs remain ill defined compared to their murine counterparts, and it is unclear which murine eHFSC markers really apply to the human HF. We address this by reviewing current concepts on human eHFSC biology, their immediate progeny and their molecular markers, focusing on Keratin 15 and 19, CD200, CD34, PHLDA1, and EpCAM/Ber-EP4. After delineating how human eHFSCs may be selectively targeted experimentally, we close by defining as yet unmet key challenges in human eHFSC research. The ultimate goal is to transfer emerging concepts from murine epithelial stem cell biology to human HF physiology and pathology.

Thyroid hormone signaling and homeostasis during aging

Studies in humans and in animal models show negative correlations between thyroid hormone (TH) levels and longevity. TH signaling is implicated in maintaining and integrating metabolic homeostasis at multiple levels, notably centrally in the hypothalamus but also in peripheral tissues. The question is thus raised of how TH signaling is modulated during aging in different tissues. Classically, TH actions on mitochondria and heat production are obvious candidates to link negative effects of TH to aging. Mitochondrial effects of excess TH include reactive oxygen species and DNA damage, 2 factors often considered as aging accelerators. Inversely, caloric restriction, which can retard aging from nematodes to primates, causes a rapid reduction of circulating TH, reducing metabolism in birds and mammals. However, many other factors could link TH to aging, and it is these potentially subtler and less explored areas that are highlighted here. For example, effects of TH on membrane composition, inflammatory responses, stem cell renewal and synchronization of physiological responses to light could each contribute to TH regulation of maintenance of homeostasis during aging. We propose the hypothesis that constraints on TH signaling at certain life stages, notably during maturity, are advantageous for optimal aging.

Mimicking hair disorders by genetic manipulation of organ-cultured human hair follicles

Human hair follicles can be dissected out of scalp skin and cultured in vitro in defined growth medium. Hair follicle organ cultures have previously been used to investigate the molecular and cellular mechanisms through which various factors regulate the maintenance and cycling of adult hair follicles. In this issue, Samuelov et al. transfected organ-cultured human hair follicles with siRNA nucleotides and suppressed the expression of the endogenous P-cadherin gene in follicular keratinocytes. Knocking down the expression of P-cadherin in hair follicles in vitro recapitulated the hair follicle phenotype observed in patients with hypotrichosis with juvenile macular dystrophy (HJMD) and enabled the authors to establish a cause-effect relationship between loss of P-cadherin and suppression of the canonical Wnt signaling pathway and upregulation of TGFβ2 during development of the hair abnormalities observed in HJMD patients.

An Intimate Relationship between Thyroid Hormone and Skin: Regulation of Gene Expression

Skin is the largest organ of the human body and plays a key role in protecting the individual from external insults. The barrier function of the skin is performed primarily by the epidermis, a self-renewing stratified squamous epithelium composed of cells that undergo a well-characterized and finely tuned process of terminal differentiation. By binding to their receptors thyroid hormones (TH) regulate epidermal cell proliferation, differentiation, and homeostasis. Thyroid dysfunction has multiple classical manifestations at skin level. Several TH-responsive genes, as well as genes critical for TH metabolism and action, are expressed at epidermal level. The role of TH in skin is still controversial, although it is generally recognized that TH signaling is central for skin physiology and homeostasis. Here we review the data on the epidermis and its function in relation to TH metabolism and regulation of gene expression. An understanding of the cellular and molecular basis of TH action in epidermal cells may lead to the identification of putative therapeutical targets for treatment of skin disorders.

Damage of hair follicle stem cells and alteration of keratin expression in external radiation-induced acute alopecia

Alopecia is known as a symptom of acute radiation, yet little is known concerning the mechanism of this phenomenon and the alteration of hair protein profiles. To examine this, 6-week-old male C57/BL6 mice were exposed to 6 Gy of X-ray irradiation, which caused acute alopecia. Their hair and skin were collected, and hair proteins were analyzed with liquid chromatography/electrospray-ionization mass spectrometry and immunohistochemistry. No change was observed in the composition of major hair keratins, such as Krt81, Krt83 and Krt86. However, cytokeratin Krt15 and CD34, which are known as hair follicle stem cell markers, were decreased in alopecic mice. Cytokeratin Krt5, which is known as a marker for basal and undifferentiated keratinocytes, was increased in the epidermis of alopecic mice. These findings suggest that radiation damages hair stem cells and the differentiation of keratinocytes in the epidermis. For the evaluation of radiation exposure, chromosomal aberration is considered to be the gold standard, yet our results suggest that Krt5 may be a novel biological marker for acute radiation symptoms.

Thyroid hormone receptors, cell growth and differentiation

BACKGROUND

Tissue homeostasis depends on the balance between cell proliferation and differentiation. Thyroid hormones (THs), through binding to their nuclear receptors, can regulate the expression of many genes involved in cell cycle control and cellular differentiation. This can occur by direct transcriptional regulation or by modulation of the activity of different signaling pathways.

SCOPE OF REVIEW

In this review we will summarize the role of the different receptor isoforms in growth and maturation of selected tissues and organs. We will focus on mammalian tissues, and therefore we will not address the fundamental role of the THs during amphibian metamorphosis.

MAJOR CONCLUSIONS

The actions of THs are highly pleiotropic, affecting many tissues at different developmental stages. As a consequence, their effects on proliferation and differentiation are highly heterogeneous depending on the cell type, the cellular context, and the developmental or transformation status. Both during development and in the adult, stem cells are essential for proper organ formation, maintenance and regeneration. Recent evidence suggests that some of the actions of the thyroid hormone receptors could be secondary to regulation of stem/progenitor cell function. Here we will also include the latest knowledge on the role of these receptors in proliferation and differentiation of embryonic and adult stem cells.

GENERAL SIGNIFICANCE

The thyroid hormone receptors are potent regulators of proliferation and differentiation of many cell types. This can explain the important role of the thyroid hormones and their receptors in key processes such as growth, development, tissue homeostasis or cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Spermidine promotes human hair growth and is a novel modulator of human epithelial stem cell functions

BACKGROUND

Rapidly regenerating tissues need sufficient polyamine synthesis. Since the hair follicle (HF) is a highly proliferative mini-organ, polyamines may also be important for normal hair growth. However, the role of polyamines in human HF biology and their effect on HF epithelial stem cells in situ remains largely unknown.

METHODS AND FINDINGS

We have studied the effects of the prototypic polyamine, spermidine (0.1-1 µM), on human scalp HFs and human HF epithelial stem cells in serum-free organ culture. Under these conditions, spermidine promoted hair shaft elongation and prolonged hair growth (anagen). Spermidine also upregulated expression of the epithelial stem cell-associated keratins K15 and K19, and dose-dependently modulated K15 promoter activity in situ and the colony forming efficiency, proliferation and K15 expression of isolated human K15-GFP+ cells in vitro. Inhibiting the rate-limiting enzyme of polyamine synthesis, ornithine decarboyxlase (ODC), downregulated intrafollicular K15 expression. In primary human epidermal keratinocytes, spermidine slightly promoted entry into the S/G2-M phases of the cell cycle. By microarray analysis of human HF mRNA extracts, spermidine upregulated several key target genes implicated e.g. in the control of cell adherence and migration (POP3), or endoplasmic reticulum and mitochondrial functions (SYVN1, NACA and SLC25A3). Excess spermidine may restrict further intrafollicular polyamine synthesis by inhibiting ODC gene and protein expression in the HF's companion layer in situ.

CONCLUSIONS

These physiologically and clinically relevant data provide the first direct evidence that spermidine is a potent stimulator of human hair growth and a previously unknown modulator of human epithelial stem cell biology.

A neuroendocrinological perspective on human hair follicle pigmentation

The role of neurohormones and neuropeptides in human hair follicle (HF) pigmentation extends far beyond the control of melanin synthesis by α-MSH and ACTH and includes melanoblast differentiation, reactive oxygen species scavenging, maintenance of HF immune privilege, and remodeling of the HF pigmentary unit (HFPU). It is now clear that human HFs are not only a target of multiple neuromediators, but also are a major non-classical production site for neurohormones such as CRH, proopiomelanocortin, ACTH, α-MSH, ß-endorphin, TRH, and melatonin. Moreover, human HFs have established a functional peripheral equivalent of the hypothalamic-pituitary-adrenal axis. By charting the author's own meanderings through the jungle of hair pigmentation research, the current perspectives essay utilizes four clinical observations - hair repigmentation, canities, poliosis, and 'overnight greying'- as points of entry into the enigmas and challenges of .pigmentary HF neuroendocrinology. After synthesizing key principles and defining major open questions in the field, selected research avenues are delineated that appear clinically most promising. In this context, novel neuroendocrinological strategies to retard or reverse greying and to reduce damage to the HFPU are discussed.