The vitamin D receptor is a Wnt effector that controls hair follicle differentiation and specifies tumor type in adult epidermis

1,25-(OH)2D3 promotes hair growth by inhibiting NLRP3/IL-1β and HIF-1α/IL-1β signaling pathways

Vitamin D is a crucial vitamin that participates in various biological processes through the Vitamin D Receptor (VDR). While there are studies suggesting that VDR might regulate hair growth through ligand-independent mechanisms, the efficacy of Vitamin D in treating hair loss disorders has also been reported. Here, through in vivo experiments in mice, in vitro organ culture of hair follicles, and cellular-level investigations, we demonstrate that 1,25-(OH)2D3 promotes mouse hair regeneration, prolongs the hair follicle anagen, and enhances the proliferation and migration capabilities of dermal papilla cells and outer root sheath keratinocytes in a VDR-dependent manner. Transcriptome analysis of VDR-knockout mouse skin reveals the involvement of HIF-1α, NLRP3, and IL-1β in these processes. Finally, we confirm that 1,25-(OH)2D3 can counteract the inhibitory effects of DHT on hair growth. These findings suggest that 1,25-(OH)2D3 has a positive impact on hair growth and may serve as a potential therapeutic agent for androgenetic alopecia (AGA).

VDRA downregulate β-catenin/Smad3 and DNA damage and repair associated with improved prognosis in ccRCC patients

The clear cell renal cell carcinoma (ccRCC) spotlighted the poorest survival, while chromophobe renal cell carcinoma (chRCC) was associated with the best survival. Earlier studies corroborated vitamin D receptor (VDR) was a promising molecular for improving the prognosis of RCC. In contrast to VDRA, the one of VDR isoforms, VDRB1 (VDR isoform B1) has an N-terminal extension of 50 amino acids and is less ligand-dependent. However, the functional differences between VDRA and VDRB1, and their roles in the prognosis of ccRCC and chRCC, have not been investigated. In the present study, we uncovered that the transcripts related to vitamin D pathway and cellular calcium signaling were effectively decreased in the context of ccRCC, yet failed to exert a comparable effect within chRCC. Specially, minimally levels of VDRA wherein kidneys of patients suffering from ccRCC predict shorter survival time. In addition, the protein expressions for β-catenin/Smad3 pathway and DNA damage and repair pathways were obviously impeded in VDRA-overexpressed ccRCC cells, yet this inhibitory effect was conspicuously absent in enable VDRB1 cells. Our results provide a new idea to improve the prognosis of ccRCC via VDRA upregulation.

Co-expression of PADI isoforms during progenitor differentiation enables functional diversity

Protein isoforms, generated through alternative splicing or promoter usage, contribute to tissue function. Here, we characterize the expression of predicted Padi3α and Padi3β isoforms in hair follicles and describe expression of Padi2β, a hitherto unknown PADI2 isoform, in the oligodendrocyte lineage. Padi2β transcription is initiated from a downstream intronic promoter, generating an N-terminally truncated, unstable, PADI2β. By contrast to the established role of the canonical PADI2 (PADI2α) (Falcao et al. 2019 Cell Rep. 27, 1090-1102.e10. (doi:10.1016/j.celrep.2019.03.108)), PADI2β inhibits oligodendrocyte differentiation, suggesting that PADI2 isoforms exert opposing effects on oligodendrocyte lineage progression. We localize Padi3α and Padi3β to developing hair follicles and find that both transcripts are expressed at low levels in progenitor cells, only to increase in expression concomitant with differentiation. When expressed in vitro, PADI3α and PADI3β are enriched in the cytoplasm and precipitate together. Whereas PADI3β protein stability is low and PADI3β fails to induce protein citrullination, we find that the enzymatic activity and protein stability of PADI3α is reduced in the presence of PADI3β. We propose that PADI3β modulates PADI3α activity by direct binding and heterodimer formation. Here, we establish expression and function of Padi2 and Padi3 isoforms, expanding on the mechanisms in place to regulate citrullination in complex tissues. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Vitamin D receptor cross-talk with p63 signaling promotes epidermal cell fate

The vitamin D receptor with its ligand 1,25 dihydroxy vitamin D3 (1,25D3) regulates epidermal stem cell fate, such that VDR removal from Krt14 expressing keratinocytes delays re-epithelialization of epidermis after wound injury in mice. In this study we deleted Vdr from Lrig1 expressing stem cells in the isthmus of the hair follicle then used lineage tracing to evaluate the impact on re-epithelialization following injury. We showed that Vdr deletion from these cells prevents their migration to and regeneration of the interfollicular epidermis without impairing their ability to repopulate the sebaceous gland. To pursue the molecular basis for these effects of VDR, we performed genome wide transcriptional analysis of keratinocytes from Vdr cKO and control littermate mice. Ingenuity Pathway analysis (IPA) pointed us to the TP53 family including p63 as a partner with VDR, a transcriptional factor that is essential for proliferation and differentiation of epidermal keratinocytes. Epigenetic studies on epidermal keratinocytes derived from interfollicular epidermis showed that VDR is colocalized with p63 within the specific regulatory region of MED1 containing super-enhancers of epidermal fate driven transcription factor genes such as Fos and Jun. Gene ontology analysis further implicated that Vdr and p63 associated genomic regions regulate genes involving stem cell fate and epidermal differentiation. To demonstrate the functional interaction between VDR and p63, we evaluated the response to 1,25(OH)2D3 of keratinocytes lacking p63 and noted a reduction in epidermal cell fate determining transcription factors such as Fos, Jun. We conclude that VDR is required for the epidermal stem cell fate orientation towards interfollicular epidermis. We propose that this role of VDR involves cross-talk with the epidermal master regulator p63 through super-enhancer mediated epigenetic dynamics.

Role of vitamin D and calcium signaling in epidermal wound healing

PURPOSE

This review will discuss the role of vitamin D and calcium signaling in the epidermal wound response with particular focus on the stem cells of the epidermis and hair follicle that contribute to the wounding response.

METHODS

Selected publications relevant to the mechanisms of wound healing in general and the roles of calcium and vitamin D in wound healing in particular were reviewed.

RESULTS

Following wounding the stem cells of the hair follicle and interfollicular epidermis are activated to proliferate and migrate to the wound where they take on an epidermal fate to re-epithelialize the wound and regenerate the epidermis. The vitamin D and calcium sensing receptors (VDR and CaSR, respectively) are expressed in the stem cells of the hair follicle and epidermis where they play a critical role in enabling the stem cells to respond to wounding. Deletion of Vdr and/or Casr from these cells delays wound healing. The VDR is regulated by co-regulators such as the Med 1 complex and other transcription factors such as Ctnnb (beta-catenin) and p63. The formation of the Cdh1/Ctnn (E-cadherin/catenin) complex jointly stimulated by vitamin D and calcium plays a critical role in the activation, migration, and re-epithelialization processes.

CONCLUSION

Vitamin D and calcium signaling are critical for the ability of epidermal and hair follicle stem cells to respond to wounding. Vitamin D deficiency with the accompanying decrease in calcium signaling can result in delayed and/or chronic wounds, a major cause of morbidity, loss of productivity, and medical expense.

A Hairy Cituation - PADIs in Regeneration and Alopecia

In this Review article, we focus on delineating the expression and function of Peptidyl Arginine Delminases (PADIs) in the hair follicle stem cell lineage and in inflammatory alopecia. We outline our current understanding of cellular processes influenced by protein citrullination, the PADI mediated posttranslational enzymatic conversion of arginine to citrulline, by exploring citrullinomes from normal and inflamed tissues. Drawing from other stem cell lineages, we detail the potential function of PADIs and specific citrullinated protein residues in hair follicle stem cell activation, lineage specification and differentiation. We highlight PADI3 as a mediator of hair shaft differentiation and display why mutations in PADI3 are linked to human alopecia. Furthermore, we propose mechanisms of PADI4 dependent fine-tuning of the hair follicle lineage progression. Finally, we discuss citrullination in the context of inflammatory alopecia. We present how infiltrating neutrophils establish a citrullination-driven self-perpetuating proinflammatory circuitry resulting in T-cell recruitment and activation contributing to hair follicle degeneration. In summary, we aim to provide a comprehensive perspective on how citrullination modulates hair follicle regeneration and contributes to inflammatory alopecia.

Ligand-Independent Actions of the Vitamin D Receptor: More Questions Than Answers

Our predominant understanding of the actions of vitamin D involve binding of its ligand, 1,25(OH)D, to the vitamin D receptor (VDR), which for its genomic actions binds to discrete regions of its target genes called vitamin D response elements. However, chromatin immunoprecipitation‐sequencing (ChIP‐seq) studies have observed that the VDR can bind to many sites in the genome without its ligand. The number of such sites and how much they coincide with sites that also bind the liganded VDR vary from cell to cell, with the keratinocyte from the skin having the greatest overlap and the intestinal epithelial cell having the least. What is the purpose of the unliganded VDR? In this review, I will focus on two clear examples in which the unliganded VDR plays a role. The best example is that of hair follicle cycling. Hair follicle cycling does not need 1,25(OH)₂D, and Vdr lacking the ability to bind 1,25(OH)₂D can restore hair follicle cycling in mice otherwise lacking Vdr. This is not true for other functions of VDR such as intestinal calcium transport. Tumor formation in the skin after UVB radiation or the application of chemical carcinogens also appears to be at least partially independent of 1,25(OH)₂D in that Vdr null mice develop such tumors after these challenges, but mice lacking Cyp27b1, the enzyme producing 1,25(OH)₂D, do not. Examples in other tissues emerge when studies comparing Vdr null and Cyp27b1 null mice are compared, demonstrating a more severe phenotype with respect to bone mineral homeostasis in the Cyp27b1 null mouse, suggesting a repressor function for VDR. This review will examine potential mechanisms for these ligand‐independent actions of VDR, but as the title indicates, there are more questions than answers with respect to this role of VDR. © 2021 The Author. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Wnt and Vitamin D at the Crossroads in Solid Cancer

Simple Summary

The Wnt/β-catenin signaling pathway is aberrantly activated in most colorectal cancers and less frequently in a variety of other solid neoplasias. Many epidemiological and experimental studies and some clinical trials suggest an anticancer action of vitamin D, mainly against colorectal cancer. The aim of this review was to analyze the literature supporting the interference of Wnt/β-catenin signaling by the active vitamin D metabolite 1α,25-dihydroxyvitamin D₃. We discuss the molecular mechanisms of this antagonism in colorectal cancer and other cancer types. Additionally, we summarize the available data indicating a reciprocal inhibition of vitamin D action by the activated Wnt/β-catenin pathway. Thus, a complex mutual antagonism between Wnt/β-catenin signaling and the vitamin D system seems to be at the root of many solid cancers.

Abstract

Abnormal activation of the Wnt/β-catenin pathway is common in many types of solid cancers. Likewise, a large proportion of cancer patients have vitamin D deficiency. In line with these observations, Wnt/β-catenin signaling and 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), the active vitamin D metabolite, usually have opposite effects on cancer cell proliferation and phenotype. In recent years, an increasing number of studies performed in a variety of cancer types have revealed a complex crosstalk between Wnt/β-catenin signaling and 1,25(OH)₂D₃. Here we review the mechanisms by which 1,25(OH)₂D₃ inhibits Wnt/β-catenin signaling and, conversely, how the activated Wnt/β-catenin pathway may abrogate vitamin D action. The available data suggest that interaction between Wnt/β-catenin signaling and the vitamin D system is at the crossroads in solid cancers and may have therapeutic applications.

Vitamin D modulates E-cadherin turnover by regulating TGF-β and Wnt signalings during EMT-mediated myofibroblast differentiation in A459 cells

Vitamin D (VitD) has an anti-fibrotic effect on fibrotic lungs. It reduces epithelial-mesenchymal transition (EMT) on tumors. We aimed to investigate target proteins of VitD for the regression of EMT-mediated myofibroblast differentiation. A group of A549 cells were treated with 5 % cigarette smoke extract (CSE) and 5 %CSE + TGF-β (5 ng/ml) to induce EMT. The others were treated with 50 nM VitD 30 min before %5CSE and TGF-β treatments. All cells were collected at 24, 48 and 72 h following 5 %CSE and TGF-β administrations. The expression of p120ctn and NEDD9 proteins acted on E-cadherin turnover in addition to activations of TGF-β and Wnt pathways were examined in these cells and fibrotic human lungs. CSE and TGF-β induced EMT by reducing E-cadherin, p-VDR, SMAD7 and DKK1, increasing α-SMA, p120ctn, Kaiso, NEDD9 and stimulating TGF-β and Wnt/β-catenin signalings in A549 cells. VitD administration reversed these alterations and regressed EMT. Co-immunoprecipitation analysis revealed p-VDR interaction with β-catenin and Kaiso in fibrotic and non-fibrotic human lungs. VitD pre-treatments reduced TGF-β and Wnt/β-catenin signalings by increasing p-VDR, protected from E-cadherin degradation and led to the regression of EMT in A549 cells treated with CSE and TGF-β. Finally, VitD supplementation combined with anti-fibrotic therapeutics can be suggested for treatment of pulmonary fibrosis, which may be developed by smoking, in cases of VitD deficiency.

Vitamin D Effects on Cell Differentiation and Stemness in Cancer

Vitamin D₃ is the precursor of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), a pleiotropic hormone that is a major regulator of the human genome. 1,25(OH)₂D₃ modulates the phenotype and physiology of many cell types by controlling the expression of hundreds of genes in a tissue- and cell-specific fashion. Vitamin D deficiency is common among cancer patients and numerous studies have reported that 1,25(OH)₂D₃ promotes the differentiation of a wide panel of cultured carcinoma cells, frequently associated with a reduction in cell proliferation and survival. A major mechanism of this action is inhibition of the epithelial–mesenchymal transition, which in turn is largely based on antagonism of the Wnt/β-catenin, TGF-β and EGF signaling pathways. In addition, 1,25(OH)₂D₃ controls the gene expression profile and phenotype of cancer-associated fibroblasts (CAFs), which are important players in the tumorigenic process. Moreover, recent data suggest a regulatory role of 1,25(OH)₂D₃ in the biology of normal and cancer stem cells (CSCs). Here, we revise the current knowledge of the molecular and genetic basis of the regulation by 1,25(OH)₂D₃ of the differentiation and stemness of human carcinoma cells, CAFs and CSCs. These effects support a homeostatic non-cytotoxic anticancer action of 1,25(OH)₂D₃ based on reprogramming of the phenotype of several cell types.

Vitamin D and calcium signaling in epidermal stem cells and their regeneration

Epidermal stem cells (SCs) residing in the skin play an essential role for epidermal regeneration during cutaneous wound healing. Upon injury, distinct epidermal SCs residing in the interfollicular epidermis and/or hair follicles are activated to proliferate. Subsequently, SCs and progeny migrate, differentiate and restore the epidermis. We review a role of the vitamin D signaling through its receptor of vitamin D receptor (Vdr) in these processes. Vdr conditional knockout (cKO) mouse skin experiences a delay in wound re-epithelialization under low dietary calcium conditions, stimulating our efforts to examine a cooperative role of Vdr with calcium signaling through the calcium sensing receptor in the epidermis. We review the role of vitamin D and calcium signaling in different processes essential for injury induced epidermal regeneration during cutaneous wound repair. First, we discuss their roles in self-renewal of epidermal SCs through β-catenin signaling. Then, we describe epidermal remodeling, in which SCs and progeny migrate and differentiate to restore the epidermis, events controlled by the E-cadherin mediated adherens junction signaling. Finally, we discuss the potential mechanisms for vitamin D and calcium signaling to regulate injury induced epidermal regeneration mutually and interdependently.

Vitamin D deficiency as a potential risk factor for accelerated aging, impaired hippocampal neurogenesis and cognitive decline: a role for Wnt/β-catenin signaling

Vitamin D is an essential fat-soluble vitamin that participates in several homeostatic functions in mammalian organisms. Lower levels of vitamin D are produced in the older population, vitamin D deficiency being an accelerating factor for the progression of the aging process. In this review, we focus on the effect that vitamin D exerts in the aged brain paying special attention to the neurogenic process. Neurogenesis occurs in the adult brain in neurogenic regions, such as the dentate gyrus of the hippocampus (DG). This region generates new neurons that participate in cognitive tasks. The neurogenic rate in the DG is reduced in the aged brain because of a reduction in the number of neural stem cells (NSC). Homeostatic mechanisms controlled by the Wnt signaling pathway protect this pool of NSC from being depleted. We discuss in here the crosstalk between Wnt signaling and vitamin D, and hypothesize that hypovitaminosis might cause failure in the control of the neurogenic homeostatic mechanisms in the old brain leading to cognitive impairment. Understanding the relationship between vitamin D, neurogenesis and cognitive performance in the aged brain may facilitate prevention of cognitive decline and it can open a door into new therapeutic fields by perspectives in the elderly.

New aspects of vitamin D metabolism and action - addressing the skin as source and target

Vitamin D has a key role in stimulating calcium absorption from the gut and promoting skeletal health, as well as many other important physiological functions. Vitamin D is produced in the skin. It is subsequently metabolized to its hormonally active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D), by the 1-hydroxylase and catabolized by the 24-hydroxylase. In this Review, we pay special attention to the effect of mutations in these enzymes and their clinical manifestations. We then discuss the role of vitamin D binding protein in transporting vitamin D and its metabolites from their source to their targets, the free hormone hypothesis for cell entry and HSP70 for intracellular transport. This is followed by discussion of the vitamin D receptor (VDR) that mediates the cellular actions of 1,25(OH)₂D. Cell-specific recruitment of co-regulatory complexes by liganded VDR leads to changes in gene expression that result in distinct physiological actions by 1,25(OH)₂D, which are disrupted by mutations in the VDR. We then discuss the epidermis and hair follicle, to provide a non-skeletal example of a tissue that expresses VDR that not only makes vitamin D but also can metabolize it to its hormonally active form. This enables vitamin D to regulate epidermal differentiation and hair follicle cycling and, in so doing, to promote barrier function, wound healing and hair growth, while limiting cancer development.

The Vitamin D Receptor as Tumor Suppressor in Skin

Cutaneous malignancies including melanomas and keratinocyte carcinomas (KC) are the most common types of cancer, occurring at a rate of over one million per year in the United States. KC, which include both basal cell carcinomas and squamous cell carcinomas, are substantially more common than melanomas and form the subject of this chapter. Ultraviolet radiation (UVR), both UVB and UVA, as occurs with sunlight exposure is generally regarded as causal for these malignancies, but UVB is also required for vitamin D synthesis in the skin. Keratinocytes are the major cell in the epidermis. These cells not only produce vitamin D but contain the enzymatic machinery to metabolize vitamin D to its active metabolite, 1,25(OH)₂D, and express the receptor for this metabolite, the vitamin D receptor (VDR). This allows the cell to respond to the 1,25(OH)₂D that it produces. Based on our own data and that reported in the literature, we conclude that vitamin D signaling in the skin suppresses UVR-induced epidermal tumor formation. In this chapter we focus on four mechanisms by which vitamin D signaling suppresses tumor formation. They are inhibition of proliferation/stimulation of differentiation with discussion of the roles of hedgehog, Wnt/β-catenin, and hyaluronan/CD44 pathways in mediating vitamin D regulation of proliferation/differentiation, regulation of the balance between oncogenic and tumor suppressor long noncoding RNAs, immune regulation, and promotion of DNA damage repair (DDR).

The what, why and how of curly hair: a review

An attempt to understand and explain a peculiarity that was observed for curly fibres during experimentation revealed disparate literature reporting on several key issues. The phenotypical nature of curly fibres is only accurately understood within the larger scope of hair fibres, which are highly complex biological structures. A brief literature search produced thousands of research items. Besides the large amount of information on the topic, there was also great variability in research focus. From our review, it appeared that the complexity of hair biology, combined with the variety of research subtopics, often results in uncertainty when relating different aspects of investigation. During the literature investigation, we systematically categorized elements of curly hair research into three basic topics: essentially asking why fibres curl, what the curly fibre looks like and how the curly fibre behaves. These categories were subsequently formalized into a curvature fibre model that is composed of successive but distinctive tiers comprising the elements in curly hair research. The purpose of this paper is twofold: namely to present (i) a literature review that explores the different aspects of curly human scalp hair and (ii) the curvature fibre model as a systemized approach to investigating curly hair.

Wnt Signaling Pathways in Keratinocyte Carcinomas

The skin functions as a barrier between the organism and the surrounding environment. Direct exposure to external stimuli and the accumulation of genetic mutations may lead to abnormal cell growth, irreversible tissue damage and potentially favor skin malignancy. Skin homeostasis is coordinated by an intricate signaling network, and its dysregulation has been implicated in the development of skin cancers. Wnt signaling is one such regulatory pathway orchestrating skin development, homeostasis, and stem cell activation. Aberrant regulation of Wnt signaling cascades not only gives rise to tumor initiation, progression and invasion, but also maintains cancer stem cells which contribute to tumor recurrence. In this review, we summarize recent studies highlighting functional evidence of Wnt-related oncology in keratinocyte carcinomas, as well as discussing preclinical and clinical approaches that target oncogenic Wnt signaling to treat cancers. Our review provides valuable insight into the significance of Wnt signaling for future interventions against keratinocyte carcinomas.

CRISPR/Cas9-mediated VDR knockout plays an essential role in the growth of dermal papilla cells through enhanced relative genes

Background

Hair follicles in cashmere goats are divided into primary and secondary hair follicles (HFs). HF development, which determines the morphological structure, is regulated by a large number of vital genes; however, the key functional genes and their interaction networks are still unclear. Although the vitamin D receptor (VDR) is related to cashmere goat HF formation, its precise effects are largely unknown. In the present study, we verified the functions of key genes identified in previous studies using hair dermal papilla (DP) cells as an experimental model. Furthermore, we used CRISPR/Cas9 technology to modify the VDR in DP cells to dissect the molecular mechanism underlying HF formation in cashmere goats.

Results

The VDR expression levels in nine tissues of Shaanbei white cashmere goats differed significantly between embryonic day 60 (E60) and embryonic day 120 (E120). At E120, VDR expression was highest in the skin. At the newborn and E120 stages, the VDR protein was highly expressed in the root sheath and hair ball region of Shaanbei cashmere goats. We cloned the complete CDS of VDR in the Shaanbei white cashmere goat and constructed a VDR-deficient DP cell model by CRISPR/Cas9. Heterozygous and homozygous mutant DP cells were produced. The growth rate of mutant DP cells was significantly lower than that of wild-type DP cells (P < 0.05) and VDR mRNA levels in DP cells decreased significantly after VDR knockdown (P < 0.05). Further, the expression levels of VGF, Noggin, Lef1, and β-catenin were significantly downregulated (P < 0.05).

Conclusions

Our results indicated that VDR has a vital role in DP cells, and that its effects are mediated by Wnt and BMP4 signaling.

Towards broadening Forensic DNA Phenotyping beyond pigmentation: Improving the prediction of head hair shape from DNA

Human head hair shape, commonly classified as straight, wavy, curly or frizzy, is an attractive target for Forensic DNA Phenotyping and other applications of human appearance prediction from DNA such as in paleogenetics. The genetic knowledge underlying head hair shape variation was recently improved by the outcome of a series of genome-wide association and replication studies in a total of 26,964 subjects, highlighting 12 loci of which 8 were novel and introducing a prediction model for Europeans based on 14 SNPs. In the present study, we evaluated the capacity of DNA-based head hair shape prediction by investigating an extended set of candidate SNP predictors and by using an independent set of samples for model validation. Prediction model building was carried out in 9674 subjects (6068 from Europe, 2899 from Asia and 707 of admixed European and Asian ancestries), used previously, by considering a novel list of 90 candidate SNPs. For model validation, genotype and phenotype data were newly collected in 2415 independent subjects (2138 Europeans and 277 non-Europeans) by applying two targeted massively parallel sequencing platforms, Ion Torrent PGM and MiSeq, or the MassARRAY platform. A binomial model was developed to predict straight vs. non-straight hair based on 32 SNPs from 26 genetic loci we identified as significantly contributing to the model. This model achieved prediction accuracies, expressed as AUC, of 0.664 in Europeans and 0.789 in non-Europeans; the statistically significant difference was explained mostly by the effect of one EDAR SNP in non-Europeans. Considering sex and age, in addition to the SNPs, slightly and insignificantly increased the prediction accuracies (AUC of 0.680 and 0.800, respectively). Based on the sample size and candidate DNA markers investigated, this study provides the most robust, validated, and accurate statistical prediction models and SNP predictor marker sets currently available for predicting head hair shape from DNA, providing the next step towards broadening Forensic DNA Phenotyping beyond pigmentation traits.

Genome-wide discovery of lincRNAs with spatiotemporal expression patterns in the skin of goat during the cashmere growth cycle

Background

Long intergenic noncoding RNAs (lincRNAs) have been recognized in recent years as key regulators of biological processes. However, lincRNAs in goat remain poorly characterized both across various tissues and during different developmental stages in goat (Capra hircus).

Results

We performed the genome-wide discovery of the lincRNAs in goat by combining the RNA-seq dataset that were generated from 28 cashmere goat skin samples and the 12 datasets of goat tissues downloaded from the NCBI database. We identified a total of 5546 potential lincRNA transcripts that overlapped 3641 lincRNA genes. These lincRNAs exhibited a tissue-specific pattern. Specifically, there are 584 lincRNAs expressed exclusively in only one tissue, and 91 were highly expressed in hair follicle (HF). In addition, 2350 protein-coding genes and 492 lincRNAs were differentially expressed in the skin of goat. The majority exhibited the remarkable differential expression during the transition of the goat skin from the May–June to August–October time point, which covered the different seasons. Fundamental biological processes, such as skin development, were significantly enriched in these genes. Furthermore, we identified several lincRNAs highly expressed in the HF, which exhibited not only the co-expression pattern with the key factors to the HF development but also the activated expression in the August to October time point. Intriguingly, one of spatiotemporal lincRNAs, linc-chig1598 could be a potential regulator of distal-less homeobox 3 expression during the secondary hair follicle growth.

Conclusions

This study will facilitate future studies aimed at unravelling the function of lincRNAs in hair follicle development.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4864-x) contains supplementary material, which is available to authorized users.

Vitamin D Receptor Is Required for Proliferation, Migration, and Differentiation of Epidermal Stem Cells and Progeny during Cutaneous Wound Repair

Epidermal stem cells residing in the skin play an essential role in epidermal regeneration. When skin is injured, the stem cells are first activated to proliferate, and subsequently the progeny migrate and differentiate to regenerate the epidermis. Here, we demonstrate that the vitamin D receptor (VDR) is essential for these processes to occur. The requirement for VDR on epidermal stem cell function was revealed in conditional VDR knockout mice, in which VDR was deleted from stem cells and progeny, and mice were maintained on a low calcium diet. First, self-renewal and niche formation of epidermal stem cells were impaired. Wound-induced activation of epidermal stem cells was blunted associated with a reduction of β-catenin signaling. Second, wound induced migration of stem cells and progeny was impaired as shown by lineage tracing and delayed migration of VDR silenced cells. Epidermal differentiation of progeny was impaired at the wounding site associated with reduced E-cadherin expression. Deletion of VDR also changed stem cell fate blunting hair development, increasing sebaceous glands, and altering expression and location of epidermal markers. These results suggest that VDR is required for self-renewal, migration, and differentiation of epidermal stem cells and progeny during cutaneous wound healing.

A role for the vitamin D pathway in non-intestinal lesions in genetic and carcinogen models of colorectal cancer and in familial adenomatous polyposis

Vitamin D is implicated in the etiology of cancers of the gastrointestinal tract, usually characterized by alteration in the APC/β-catenin/TCF tumor suppressor pathway. The vitamin D receptor (VDR) is also implicated in cardiovascular and skin diseases as well as in immunity. Activated VDR can indirectly alter β-catenin nuclear localization and directly suppress β-catenin/TCF mediated transcriptional activity. We treated VDR null mice with the carcinogen azoxymethane (AOM) and generated mice bearing a mutated APC (hypomorph) on a VDR null background (Apc^1638N/+Vdr^−/−). VDR null mice do not develop GI or extra-colonic tumors but loss of VDR decreased intestinal tumor latency and increased progression to adenocarcinoma in both models. AOM treatment of VDR null mice also caused squamous cell carcinoma of the anus. Although levels and distribution of total or activated β-catenin in the epithelial component of tumors were unaffected by loss of VDR, β-catenin dependent cyclin D1 expression was affected suggesting a direct VDR effect on β-catenin co-activator activity. Extra-colonic mucosa manifestations in Apc^1638N/+Vdr^−/− animals included increased nuclear β-catenin in submucosal stromal cells, spleno- and cardiomegaly and large epidermoid cysts characteristic of the FAP variant, Gardner's syndrome. Consistent with this, SNPs in the VDR, vitamin D binding protein and CYP24 as well as mutations in APC distal to codon 850 were strongly associated with Gardners syndrome in a cohort of 457 FAP patients, This work suggests that alterations in the vitamin D/VDR axis are important in Gardner's syndrome, as well as in the etiology of anal cancer.

Wnt Signaling in Adult Epithelial Stem Cells and Cancer

Wnt/β-catenin signaling is integral to the homeostasis and regeneration of many epithelial tissues due to its critical role in adult stem cell regulation. It is also implicated in many epithelial cancers, with mutations in core pathway components frequently present in patient tumors. In this chapter, we discuss the roles of Wnt/β-catenin signaling and Wnt-regulated stem cells in homeostatic, regenerative and cancer contexts of the intestines, stomach, skin, and liver. We also examine the sources of Wnt ligands that form part of the stem cell niche. Despite the diversity in characteristics of various tissue stem cells, the role(s) of Wnt/β-catenin signaling is generally coherent in maintaining stem cell fate and/or promoting proliferation. It is also likely to play similar roles in cancer stem cells, making the pathway a salient therapeutic target for cancer. While promising progress is being made in the field, deeper understanding of the functions and signaling mechanisms of the pathway in individual epithelial tissues will expedite efforts to modulate Wnt/β-catenin signaling in cancer treatment and tissue regeneration.

Functional Analysis of VDR Gene Mutation R343H in A Child with Vitamin D-Resistant Rickets with Alopecia

The functional study of different mutations on vitamin D receptor (VDR) gene causing hereditary vitamin D-resistant rickets (HVDRR) remains limited. This study was to determine the VDR mutation and the mechanisms of this mutation-causing phenotype in a family with HVDRR and alopecia. Phenotype was analyzed, and in vitro functional studies were performed. The proband and his affected sister exhibited typical HVDRR with alopecia, and their biochemical and radiographic abnormalities but not alopecia responded to supraphysiological doses of active vitamin D₃. A novel homozygous missense R343H mutation in the exon 9 of VDR residing in the retinoid X receptor (RXR)-binding domain was identified. The expression level and C-terminal conformation of R343H mutant are not different from the wild-type VDR. This mutant had no effect on the nuclear localization of VDR, VDR-RXR heterodimerization, but it impaired CYP24A1 promoter activity in the presence of 1,25 (OH)₂ vitamin D₃, at least in part, mediated through specific nuclear receptor coactivator. Simulation models revealed the vanished interaction between guanidinium group of R343 and carboxyl group of E269. Without affecting the expression, conformation, nuclear location of VDR or heteridimerization with RXR, VDR-R343H impairs the transactivation activity of VDR on downstream transcription, accounting for HVDRR features with alopecia.

Extraskeletal actions of vitamin D

The vitamin D receptor (VDR) is found in nearly all, if not all, cells in the body. The enzyme that produces the active metabolite of vitamin D and ligand for VDR, namely CYP27B1, likewise is widely expressed in many cells of the body. These observations indicate that the role of vitamin D is not limited to regulation of bone and mineral homeostasis, as important as that is. Rather, the study of its extraskeletal actions has become the major driving force behind the significant increase in research articles on vitamin D published over the past several decades. A great deal of information has accumulated from cell culture studies, in vivo animal studies, and clinical association studies that confirms that extraskeletal effects of vitamin D are truly widespread and substantial. However, randomized, placebo-controlled clinical trials, when done, have by and large not produced the benefits anticipated by the in vitro cell culture and in vivo animal studies. In this review, I will examine the role of vitamin D signaling in a number of extraskeletal tissues and assess the success of translating these findings into treatments of human diseases affecting those extracellular tissues.

Combined Deletion of the Vitamin D Receptor and Calcium-Sensing Receptor Delays Wound Re-epithelialization

When the skin is injured, keratinocytes proliferate, migrate, and differentiate to regenerate the epidermis. We recently showed that ablation of the vitamin D receptor (Vdr) in keratinocytes delays wound re-epithelialization in mice also fed a low-calcium diet, implicating a cooperative role of Vdr and calcium signaling in this process. In this study, we examined the role of vitamin D and calcium signaling in wound healing by deleting their receptors, Vdr and the calcium-sensing receptor (Casr). Gene expression profiling of neonatal epidermis lacking both Vdr and Casr [Vdr and Casr double knockout (DKO)] specifically in keratinocytes revealed that DKO affects a number of pathways relevant to wound healing, including Vdr, β-catenin, and adherens junction (AJ) signaling. In adult skin, DKO caused a significant delay in wound closure and re-epithelialization, whereas myofibroblast numbers and matrix deposition were unaffected. The injury-induced proliferation of epidermal keratinocytes was blunted in both epidermis and hair follicles, and expression of β-catenin target genes was reduced in the DKO. Expression of E-cadherin and desmoglein 1 was reduced in the shortened leading edges of the epithelial tongues re-epithelializing the wounds, consistent with the decreased migration rate of DKO keratinocytes in vitro. These results demonstrate that Vdr and Casr are required for β-catenin-regulated cell proliferation and AJ formation essential for re-epithelialization after wounding. We conclude that vitamin D and calcium signaling in keratinocytes are required for a normal regenerative response of the skin to wounding.

Wnt/β-catenin signaling in adult mammalian epithelial stem cells

Adult stem cells self-renew and replenish differentiated cells in various organs and tissues throughout a mammal's life. Over the last 25 years an ever-growing body of knowledge has unraveled the essential regulation of adult mammalian epithelia by the canonical Wnt signaling with its key intracellular effector β-catenin. In this review, we discuss the principles of the signaling pathway and its role in adult epithelial stem cells of the intestine and skin during homeostasis and tumorigenesis. We further highlight the research that led to the identification of new stem cell markers and methods to study adult stem cells ex vivo.

Absence of vitamin D receptor (VDR)-mediated PPARγ suppression causes alopecia in VDR-null mice

Vitamin D receptor (VDR) mutations in humans and mice cause alopecia. VDR-null (VDR-/-) mice exhibit lack of postmorphogenic hair cycles as a result of impaired keratinocyte stem cell (KSC) function. To identify the molecular basis for abnormal KSC function, RNA sequencing of wild-type (WT) and VDR-/- KSCs was performed. These studies demonstrated that >80% of differentially expressed genes are up-regulated in VDR-/- KSCs; thus, the VDR is a transcriptional suppressor in WT KSCs. Peroxisome proliferator-activated receptor γ (PPARγ), PPARγ coactivator 1β (PGC1β), and lipoprotein lipase (LPL) were among the up-regulated genes identified. Chromatin immunoprecipitation analyses demonstrated that these genes are direct VDR targets in WT keratinocytes. Notably, VDR occupancy of the PPARγ regulatory region precludes PPARγ occupancy of this site, based on the observation that PPARγ interacts with these sequences in VDR-/- but not WT keratinocytes. This contrasts with the VDR and PPARγ co-occupancy observed on PGC1β and LPL gene regulatory regions identified. Studies in mice with keratinocyte-specific PPARγ haploinsufficiency were performed to identify the functional consequences of enhanced PPARγ expression. PPARγ haploinsufficiency normalized PPARγ mRNA levels in VDR-/- keratinocytes and restored anagen responsiveness in vivo in VDR-/- mice, resulting in hair regrowth. Thus, absence of VDR-mediated PPARγ suppression underlies alopecia in VDR-/- mice.-Saini, V., Zhao, H., Petit, E. T., Gori, F., Demay, M. B. Absence of vitamin D receptor (VDR)-mediated PPARγ suppression causes alopecia in VDR-null mice.

Vitamin D receptor is a novel transcriptional regulator for Axin1

BACKGROUND

Axin1 is a scaffold protein in the β-catenin destruction complex, which, if disrupted, contributes to pathogenesis of various human diseases, including colorectal carcinogenesis and inflammatory bowel diseases (IBD). We have previously demonstrated that Salmonella infection promotes the degradation and plasma sequestration of Axin1, leading to bacterial invasiveness and inflammatory responses. Vitamin D and the vitamin D receptor (VDR) appear to be important regulators of IBD and colon cancer. Although VDR and Axin1 are all involved in intestinal inflammation, it remains unclear whether these processes are related or function independently. In the current study, we hypothesize that VDR is an important regulator for the maintenance of physiological level of Axin1.

METHODS

Using the intestinal epithelial conditional VDR knockout mouse model (VDR^ΔIEC) and cultured cell lines, influences of VDR status on the expression of Axin1 was evaluated by Western blots and real-time PCR. Loss- and gain-of-function assays were used to investigate the regulation of VDR on Axin1 at the transcriptional and translational levels. Cells were treated with cycloheximide or actinomycin for molecular mechanistic studies. Candidate genomic VDR binding sites for Axin1 were tested by chromatin immunoprecipitation (ChIP) assay. Physical interactions among VDR, Axin1, and β-catenin were tested by immunoprecipitation. Cellular localization of Axin1 with different VDR status was determined by fractionation and immunohistochemistry.

RESULTS

We found that VDR deletion led to lower protein and mRNA levels of Axin1, whereas knockdown of Axin1 did not change the expression level of VDR protein. Immunoprecipitation data did not support physical interaction between VDR and Axin1. The VDR regulation of Axin1 was through a VDR genomic binding site for Axin1 gene on the regulatory region. Fractionation data showed that cytosolic Axin1 was significantly reduced due to VDR deletion, leaving the nuclear fraction unchanged. In ileum, Axin1 was distributed in the cytosol of apical epithelium and crypts.

CONCLUSION

VDR is important for the maintenance of physiological level of Axin1. The discovery of Axin1 as a VDR target gene provides novel and fundamental insights into the interactions between the VDR and β-catenin signaling pathways.

DNA Damage-Inducible Transcript 4 Is an Innate Surveillant of Hair Follicular Stress in Vitamin D Receptor Knockout Mice and a Regulator of Wound Re-Epithelialization

Mice and human patients with impaired vitamin D receptor (VDR) signaling have normal developmental hair growth but display aberrant post-morphogenic hair cycle progression associated with alopecia. In addition, VDR^–/– mice exhibit impaired cutaneous wound healing. We undertook experiments to determine whether the stress-inducible regulator of energy homeostasis, DNA damage-inducible transcript 4 (Ddit4), is involved in these processes. By analyzing hair cycle activation in vivo, we show that VDR^−/− mice at day 14 exhibit increased Ddit4 expression within follicular stress compartments. At day 29, degenerating VDR^−/− follicular keratinocytes, but not bulge stem cells, continue to exhibit an increase in Ddit4 expression. At day 47, when normal follicles and epidermis are quiescent and enriched for Ddit4, VDR^−/− skin lacks Ddit4 expression. In a skin wound healing assay, the re-epithelialized epidermis in wildtype (WT) but not VDR^−/− animals harbor a population of Ddit4- and Krt10-positive cells. Our study suggests that VDR regulates Ddit4 expression during epidermal homeostasis and the wound healing process, while elevated Ddit4 represents an early growth-arresting stress response within VDR^−/− follicles.

Disruption of Vitamin D and Calcium Signaling in Keratinocytes Predisposes to Skin Cancer

1,25 dihydroxyvitamin D (1,25(OH)₂D), the active metabolite of vitamin D, and calcium regulate epidermal differentiation. 1,25(OH)₂D exerts its effects through the vitamin D receptor (VDR), a transcription factor in the nuclear hormone receptor family, whereas calcium acts through the calcium sensing receptor (Casr), a membrane bound member of the G protein coupled receptor family. We have developed mouse models in which the Vdr and Casr have been deleted in the epidermis (^epidVdr^−∕− and ^epidCasr^−∕−). Both genotypes show abnormalities in calcium induced epidermal differentiation in vivo and in vitro, associated with altered hedgehog (HH) and β–catenin signaling that when abnormally expressed lead to basal cell carcinomas (BCC) and trichofolliculomas, respectively. The Vdr^−∕− mice are susceptible to tumor formation following UVB or chemical carcinogen exposure. More recently we found that the keratinocytes from these mice over express long non-coding RNA (lncRNA) oncogenes such as H19 and under express lncRNA tumor suppressors such as lincRNA-21. Spontaneous tumors have not been observed in either the ^epidVdr^−∕− or ^epidCasr^−∕−. But in mice with epidermal specific deletion of both Vdr and Casr (^epidVdr^−∕−/^epidCasr^−∕− [DKO]) tumor formation occurs spontaneously when the DKO mice are placed on a low calcium diet. These results demonstrate important interactions between vitamin D and calcium signaling through their respective receptors that lead to cancer when these signals are disrupted. The roles of the β–catenin, hedgehog, and lncRNA pathways in predisposing the epidermis to tumor formation when vitamin D and calcium signaling are disrupted will be discussed.

Assessment of vitamin D receptors in alopecia areata and androgenetic alopecia

BACKGROUND

Alopecia areata (AA) is a frequent autoimmune disease, the pathogenesis of which is still unknown. Androgenetic alopecia (AGA) is a noncicatricial type of patterned hair loss. Expression of vitamin D receptors (VDRs) on keratinocytes is essential for maintenance of normal hair cycle, especially anagen initiation.

OBJECTIVE

To assess VDRs in the skin and blood of AA and AGA patients, in order to evaluate their possible role in these hair diseases.

METHODS

This study recruited 20 patients with AA, 20 patients with AGA, and 20 healthy controls. Blood samples and lesional scalp biopsies were taken from all participants for detection of VDR levels.

RESULTS

Serum and tissue VDR levels were lower in AA as well as AGA patients when compared to controls (P = 0.000). Serum and tissue VDR were positively correlated in each group. Tissue VDR was significantly lower in female patients with AA than males (P = 0.046) although serum and tissue VDR levels were significantly higher in female AGA patients than males (P = 0.004).

CONCLUSION

This study suggests an important role for VDR in the pathogenesis of AA and AGA through documenting lower serum and tissue VDR levels in AA and AGA patients in comparison with controls.

Compartmentalized Epidermal Activation of β-Catenin Differentially Affects Lineage Reprogramming and Underlies Tumor Heterogeneity

Wnt/β-catenin activation in adult epidermis can induce new hair follicle formation and tumor development. We used lineage tracing to uncover the relative contribution of different stem cell populations. LGR6(+) and LRIG1(+) stem cells contributed to ectopic hair follicles formed in the sebaceous gland upon β-catenin activation, whereas LGR5(+) cells did not. Lgr6, but not Lrig1 or Lgr5, was expressed in a subpopulation of interfollicular epidermal cells that were competent to form new hair follicles. Oncogenic β-catenin expression in LGR5(+) cells led to formation of pilomatricomas, while LRIG1(+) cells formed trichoadenomas and LGR6(+) cells formed dermatofibromas. Tumor formation was always accompanied by a local increase in dermal fibroblast density and transient extracellular matrix remodeling. However, each tumor had a distinct stromal signature in terms of immune cell infiltrate and expression of CD26 and CD44. We conclude that compartmentalization of epidermal stem cells underlies different responses to β-catenin and skin tumor heterogeneity.

The Androgen Receptor Antagonizes Wnt/β-Catenin Signaling in Epidermal Stem Cells

Activation of Wnt/β-catenin signaling in adult mouse epidermis leads to expansion of the stem cell compartment and redirects keratinocytes in the interfollicular epidermis and sebaceous glands (SGs) to differentiate along the hair follicle (HF) lineages. Here we demonstrate that during epidermal development and homeostasis there is reciprocal activation of the androgen receptor (AR) and β-catenin in cells of the HF bulb. AR activation reduced β-catenin-dependent transcription, blocked β-catenin-induced induction of HF growth, and prevented β-catenin-mediated conversion of SGs into HFs. Conversely, AR inhibition enhanced the effects of β-catenin activation, promoting HF proliferation and differentiation, culminating in the formation of benign HF tumors and a complete loss of SG identity. We conclude that AR signaling has a key role in epidermal stem cell fate selection by modulating responses to β-catenin in adult mouse skin.

ZEB1 and TCF4 reciprocally modulate their transcriptional activities to regulate Wnt target gene expression

The canonical Wnt pathway (TCF4/β-catenin) has important roles during normal differentiation and in disease. Some Wnt functions depend on signaling gradients requiring the pathway to be tightly regulated. A key Wnt target is the transcription factor ZEB1 whose expression by cancer cells promotes tumor invasiveness by repressing the expression of epithelial specification markers and activating mesenchymal genes, including a number of Wnt targets such as LAMC2 and uPA. The ability of ZEB1 to activate/repress its target genes depends on its recruitment of corepressors (CtBP, BRG1) or coactivators (p300) although conditions under which ZEB1 binds these cofactors are not elucidated. Here, we show that TCF4 and ZEB1 reciprocally modulate each other's transcriptional activity: ZEB1 enhances TCF4/β-catenin-mediated transcription and, in turn, Wnt signaling switches ZEB1 from a repressor into an activator. In colorectal cancer (CRC) cells with active Wnt signaling, ZEB1 enhances transcriptional activation of LAMC2 and uPA by TCF4/β-catenin. However, in CRC cells with inactive Wnt, ZEB1 represses both genes. Reciprocal modulation of ZEB1 and TCF4 activities involves their binding to DNA and mutual interaction. Wnt signaling turns ZEB1 into an activator by replacing binding of CtBP/BRG1 in favor of p300. Using a mouse model of Wnt-induced intestinal tumorigenesis, we found that downregulation of ZEB1 reduces the expression of LAMC2 in vivo. These results identify a mechanism through which Wnt and ZEB1 transcriptional activities are modulated, offering new approaches in cancer therapy.

Targeting cancer stem cells in solid tumors by vitamin D

Cancer stem cells (CSCs) are a small subset of cells that may be responsible for initiation, progression, and recurrence of tumors. Recent studies have demonstrated that CSCs are highly tumorigenic and resistant to conventional chemotherapies, making them a promising target for the development of preventive/therapeutic agents. A single or combination of various markers, such as CD44, EpCAM, CD49f, CD133, CXCR4, ALDH-1, and CD24, were utilized to isolate CSCs from various types of human cancers. Notch, Hedgehog, Wnt, and TGF-β signalingregulate self-renewal and differentiation of normal stem cells andare aberrantly activated in CSCs. In addition, many studies have demonstrated that these stem cell-associated signaling pathways are required for the maintenance of CSCs in different malignancies, including breast, colorectal, prostate, and pancreatic cancers. Accumulating evidence has shown inhibitory effects of vitamin D and its analogs on the cancer stem cell signaling pathways, suggesting vitamin D as a potential preventive/therapeutic agent against CSCs. In this review, we summarize recent findings about the roles of Notch, Hedgehog, Wnt, and TGF-β signaling in CSCs as well as the effects of vitamin D on these stem cell signaling pathways. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Novel mechanisms for the vitamin D receptor (VDR) in the skin and in skin cancer

The VDR acting with or without its principal ligand 1,25(OH)2D regulates two central processes in the skin, interfollicular epidermal (IFE) differentiation and hair follicle cycling (HFC). Calcium is an important co-regulator with 1,25(OH)2D at least of epidermal differentiation. Knockout of the calcium sensing receptor (CaSR) in addition to VDR accelerates the development of skin cancer in mice on a low calcium diet. Coactivators such as mediator 1 (aka DRIP205) and steroid receptor coactivator 3 (SRC3) regulate VDR function at different stages of the differentiation process, with Med 1 essential for hair follicle differentiation and early stages of epidermal differentiation and proliferation and SRC3 essential for the latter stages of differentiation including formation of the permeability barrier and innate immunity. The corepressor of VDR, hairless (HR), is essential for hair follicle cycling, although its effect on epidermal differentiation in vivo is minimal. In its regulation of HFC and IFE VDR controls two pathways-wnt/β-catenin and sonic hedgehog (SHH). In the absence of VDR these pathways are overexpressed leading to tumor formation. Whereas, VDR binding to β-catenin may block its activation of TCF/LEF1 sites, β-catenin binding to VDR may enhance its activation of VDREs. 1,25(OH)2D promotes but may not be required for these interactions. Suppression of SHH expression by VDR, on the other hand, requires 1,25(OH)2D. The major point of emphasis is that the role of VDR in the skin involves a number of novel mechanisms, both 1,25(OH)2D dependent and independent, that when disrupted interfere with IFE differentiation and HFC, predisposing to cancer formation. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

miR-125b can enhance skin tumor initiation and promote malignant progression by repressing differentiation and prolonging cell survival

Zhang et al. show that miR-125b is abundantly expressed, particularly at early stages of malignant progression to squamous cell carcinoma. When elevated in normal murine epidermis, miR-125b promotes tumor initiation and contributes to malignant progression. mir-125b directly represses stress-responsive MAPK genes and indirectly prolongs activated EGFR signaling by repressing Vps4B, encoding a protein implicated in negatively regulating the endosomal sorting complexes that are necessary for the recycling of active EGFR.

Previously, we identified miR-125b as a key regulator of the undifferentiated state of hair follicle stem cells. Here, we show that in both mice and humans, miR-125b is abundantly expressed, particularly at early stages of malignant progression to squamous cell carcinoma (SCC), the second most prevalent cancer worldwide. Moreover, when elevated in normal murine epidermis, miR-125b promotes tumor initiation and contributes to malignant progression. We further show that miR-125b can confer “oncomiR addiction” in early stage malignant progenitors by delaying their differentiation and favoring an SCC cancer stem cell (CSC)-like transcriptional program. To understand how, we systematically identified and validated miR125b targets that are specifically associated with tumors that are dependent on miR-125b. Through molecular and genetic analysis of these targets, we uncovered new insights underlying miR-125b’s oncogenic function. Specifically, we show that, on the one hand, mir-125b directly represses stress-responsive MAP kinase genes and associated signaling. On the other hand, it indirectly prolongs activated (phosphorylated) EGFR signaling by repressing Vps4b (vacuolar protein-sorting 4 homolog B), encoding a protein implicated in negatively regulating the endosomal sorting complexes that are necessary for the recycling of active EGFR. Together, these findings illuminate miR-125b as an important microRNA regulator that is shared between normal skin progenitors and their early malignant counterparts.

Vitamin D receptor, a tumor suppressor in skin

Vitamin D and calcium are well-established regulators of keratinocyte proliferation and differentiation. Therefore, it was not a great surprise that deletion of the vitamin D receptor (VDR) should predispose the skin to tumor formation, and that the combination of deleting both the VDR and calcium sensing receptor (CaSR) should be especially pro-oncogenic. In this review I have examined 4 mechanisms that appear to underlie the means by which VDR acts as a tumor suppressor in skin. First, DNA damage repair is curtailed in the absence of the VDR, allowing mutations in DNA to accumulate. Second and third involve the increased activation of the hedgehog and β-catenin pathways in the epidermis in the absence of the VDR, leading to poorly regulated proliferation with reduced differentiation. Finally, VDR deletion leads to a shift in the expression of long noncoding RNAs toward a more oncogenic profile. How these different mechanisms interact and their relative importance in the predisposition of the VDR null epidermis to tumor formation remain under active investigation.

Reciprocal role of vitamin D receptor on β-catenin regulated keratinocyte proliferation and differentiation

The active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), suppresses the proliferation while promoting the differentiation of keratinocytes through the vitamin D receptor (VDR). β-Catenin, on the other hand, promotes proliferation and blocks epidermal differentiation, although it stimulates hair follicle differentiation. In intestinal epithelia VDR binds β-catenin and blocks its proliferative effects. In this study we investigated the role of 1,25(OH)2D3/VDR on β-catenin regulated gene transcription during keratinocyte proliferation and differentiation. 1,25(OH)2D3 suppressed promoter reporter activity driven by synthetic and natural TCF/β-catenin response elements. Over-expression of VDR further suppressed these TCF/β-catenin promoter activities. 1,25(OH)2D3 also suppressed the mRNA expression of the β-catenin regulated gene Gli1 through VDR. These data were consistent with our previous observations that VDR silencing resulted in keratinocyte hyperproliferation with increased expression of Gli1 in vitro, whereas VDR null skin showed hyperproliferation in vivo. In contrast, 1,25(OH)2D3 induced expression of another β-catenin regulated gene, PADI1, important for both epidermal and hair follicle differentiation. Deletion of VDR resulted in defects in hair differentiation in vivo, with decreased expression of β-catenin regulated hair differentiation genes such as PADI1, hair keratin KRT31 and calcium binding protein S100a3. These genes possess vitamin D response elements (VDRE) adjacent to TCF/β-catenin response elements and are regulated by both VDR and β-catenin signaling. Therefore, we propose that VDR and β-catenin interact reciprocally to promote VDR stimulation of genes involved with differentiation that contain both VDR and β-catenin response elements while inhibiting β-catenin stimulation of genes involved with proliferation. Thus the major finding of this study is that while 1,25(OH)2D3/VDR inhibits the actions of β-catenin to promote keratinocyte proliferation, 1,25(OH)2D3/VDR promotes the ability of β-catenin to stimulate hair follicle differentiation. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.

Wnt some lose some: transcriptional governance of stem cells by Wnt/β-catenin signaling

In mammals, Wnt/β-catenin signaling features prominently in stem cells and cancers, but how and for what purposes have been matters of much debate. In this review, we summarize our current knowledge of Wnt/β-catenin signaling and its downstream transcriptional regulators in normal and malignant stem cells. We centered this review largely on three types of stem cells--embryonic stem cells, hair follicle stem cells, and intestinal epithelial stem cells--in which the roles of Wnt/β-catenin have been extensively studied. Using these models, we unravel how many controversial issues surrounding Wnt signaling have been resolved by dissecting the diversity of its downstream circuitry and effectors, often leading to opposite outcomes of Wnt/β-catenin-mediated regulation and differences rooted in stage- and context-dependent effects.

The vitamin D receptor is required for activation of cWnt and hedgehog signaling in keratinocytes

Alopecia (hair loss) in vitamin D receptor (VDR)-null mice is due to absence of ligand-independent actions of the VDR that are required for initiation of postmorphogenic hair cycles. Investigations were undertaken to determine whether the VDR is required for the induction of signaling pathways that play an important role in this process. The induction of cWnt and hedgehog target genes that characterizes early anagen was found to be dramatically attenuated in VDR(-/-) mice, relative to wild-type (WT) mice. To determine whether this reflects impaired responsiveness to cWnt ligands, in vitro studies were performed in primary keratinocytes. These studies demonstrated impaired induction of cWnt target genes in response to Wnt3a in VDR(-/-) keratinocytes, relative to wild-type keratinocytes. Chromatin immunoprecipitation analyses revealed that the VDR was recruited to the regulatory regions of cWnt and hedgehog target genes in WT keratinocytes but not in VDR(-/-) or Lef1(-/-) keratinocytes. Lef1 was enriched on these same regulatory regions in WT keratinocytes but not in VDR(-/-) keratinocytes. In vivo studies were performed to determine whether activation of the hedgehog pathway could bypass the defect in cWnt signaling observed in the absence of the unliganded VDR. In WT, but not VDR(-/-), mice, hedgehog agonist treatment resulted in an induction of cWnt and hedgehog target genes and the generation of mature anagen hair follicles. Thus, these studies demonstrate that the unliganded VDR interacts with regulatory regions in the cWnt and hedgehog target genes and is required for the induction of these pathways during the postnatal hair cycle.

Signaling involved in hair follicle morphogenesis and development

Hair follicle morphogenesis depends on Wnt, Shh, Notch, BMP and other signaling pathways interplay between epithelial and mesenchymal cells. The Wnt pathway plays an essential role during hair follicle induction, Shh is involved in morphogenesis and late stage differentiation, Notch signaling determines stem cell fate while BMP is involved in cellular differentiation. The Wnt pathway is considered to be the master regulator during hair follicle morphogenesis. Wnt signaling proceeds through EDA/EDAR/NF-κB signaling. NF-κB regulates the Wnt pathway and acts as a signal mediator by upregulating the expression of Shh ligand. Signal crosstalk between epithelial and mesenchymal cells takes place mainly through primary cilia. Primary cilia formation is initiated with epithelial laminin-511 interaction with dermal β-1 integrin, which also upregulates expression of downstream effectors of Shh pathway in dermal lineage. PDGF signal transduction essential for crosstalk is mediated through epithelial PDGF-A and PDGFRα expressed on the primary cilia. Dermal Shh and PDGF signaling up-regulates dermal noggin expression; noggin is a potent inhibitor of BMP signaling which helps in counteracting BMP mediated β-catenin inhibition. This interplay of signaling between the epithelial and dermal lineage helps in epithelial Shh signal amplification. The dermal Wnt pathway helps in upregulation of epithelial Notch expression. Dysregulation of these pathways leads to certain abnormalities and in some cases even tumor outgrowth.

The protective role of vitamin d signaling in non-melanoma skin cancer

Although the epidemiologic evidence that adequate vitamin D nutrition protects against non-melanoma skin cancer (NMSC) is limited, recent evidence that the vitamin D receptor (VDR) is protective is compelling. The role of vitamin D signaling in limiting the proliferation while promoting the differentiation of keratinocytes, the major cell in the epidermis from which NMSC are derived, is well known. However, recent findings that mice lacking the VDR are predisposed to skin cancer has brought to the fore the question of how the VDR is protective. In this review we will look first at the role of vitamin D signaling in regulating the proliferation and differentiation of keratinocytes. We will examine two pathways, β-catenin (CTNNB) and hedgehog (HH), that are regulated by vitamin D signaling and may contribute to the dysregulated proliferation and differentiation in the absence of VDR. We will then examine the failure of VDR deficient keratinocytes to repair DNA damaged by UVB. Finally we will examine the change in long non-coding RNA (LncRNA) expression in VDR null keratinocytes that in other cells is associated with malignant transformation, a potential newly appreciated mechanism by which vitamin D signaling is protective against NMSC.

Vitamin D Is a Multilevel Repressor of Wnt/b-Catenin Signaling in Cancer Cells

The Wnt/b-catenin signaling pathway is abnormally activated in most colorectal cancers and in a proportion of other neoplasias. This activation initiates or contributes to carcinogenesis by regulating the expression of a large number of genes in tumor cells. The active vitamin D metabolite 1a,25-dihydroxyvitamin D3 (1,25(OH)2D3) inhibits Wnt/b-catenin signaling by several mechanisms at different points along the pathway. Additionally, paracrine actions of 1,25(OH)2D3 on stromal cells may also repress this pathway in neighbouring tumor cells. Here we review the molecular basis for the various mechanisms by which 1,25(OH)2D3 antagonizes Wnt/b-catenin signaling, preferentially in human colon carcinoma cells, and the consequences of this inhibition for the phenotype and proliferation rate. The effect of the vitamin D system on Wnt/b-catenin signaling and tumor growth in animal models will also be commented in detail. Finally, we revise existing data on the relation between vitamin D receptor expression and vitamin D status and the expression of Wnt/b-catenin pathway genes and targets in cancer patients.

Vitamin D Is a Multilevel Repressor of Wnt/b-Catenin Signaling in Cancer Cells

The Wnt/β-catenin signaling pathway is abnormally activated in most colorectal cancers and in a proportion of other neoplasias. This activation initiates or contributes to carcinogenesis by regulating the expression of a large number of genes in tumor cells. The active vitamin D metabolite 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) inhibits Wnt/β-catenin signaling by several mechanisms at different points along the pathway. Additionally, paracrine actions of 1,25(OH)₂D₃ on stromal cells may also repress this pathway in neighbouring tumor cells. Here we review the molecular basis for the various mechanisms by which 1,25(OH)₂D₃ antagonizes Wnt/β-catenin signaling, preferentially in human colon carcinoma cells, and the consequences of this inhibition for the phenotype and proliferation rate. The effect of the vitamin D system on Wnt/β-catenin signaling and tumor growth in animal models will also be commented in detail. Finally, we revise existing data on the relation between vitamin D receptor expression and vitamin D status and the expression of Wnt/β-catenin pathway genes and targets in cancer patients.

1α,25(OH)2-dihydroxyvitamin D3/VDR protects the skin from UVB-induced tumor formation by interacting with the β-catenin pathway

Ultra violet (UV) irradiation, in particular UVB, is the single most important carcinogen for skin tumor formation. UVB induces genetic mutations and immune suppression, which lead to abnormal cell proliferation and eventually tumor formation. Previously studies from our group and others demonstrated that both global and epidermal specific VDR knock out mice are predisposed to either chemical (DMBA)- or long-term UVB-induced skin tumor formation, paralleled by an increase in β-catenin signaling. Using primary cultured human keratinocytes, we further demonstrated that 1,25(OH)2-dihydroxyvitamin D3 (1,25(OH)2D3) suppresses cyclin D1 and Gli1 which are regulated by β-catenin/TCF signaling and have a critical role in epidermal carcinogenesis. Blockage of VDR by siRNA resulted in hyperproliferation of keratinocytes, and increased expression of cyclin D1 and Gli1. In addition, we also showed that 1,25(OH)2D3/VDR directly regulates transcriptional activity of β-catenin/TCF signaling using the -catenin reporter TopGlow. Using K14 driven tamoxifen-induced cre recombinase to delete both VDR and β-catenin in keratinocytes of mice following the first hair follicle cycle, we found that ablation of epidermal specific β-catenin cannot rescue VDR null mice from UVB-induced skin tumor formation. Further study using VDR or β-catenin single null mice is necessary to compare with the data from double null mice. This article is part of a Special Issue entitled 'Vitamin D Workshop'.