Lymphoid enhancer-binding factor-1 (LEF1) interacts with the DNA-binding domain of the vitamin D receptor

Decreased progenitor TCF1 + T-cells correlate with COVID-19 disease severity

COVID-19, caused by SARS-CoV-2, can lead to a severe inflammatory disease characterized by significant lymphopenia. However, the underlying cause for the depletion of T-cells in COVID-19 patients remains incompletely understood. In this study, we assessed the presence of different T-cell subsets in the progression of COVID-19 from mild to severe disease, with a focus on TCF1 expressing progenitor T-cells that are needed to replenish peripheral T-cells during infection. Our results showed a preferential decline in TCF1+ progenitor CD4 and CD8+ T-cells with disease severity. This decline was seen in various TCF1+ subsets including naive, memory and effector-memory cells, and surprisingly, was accompanied by a loss in cell division as seen by a marked decline in Ki67 expression. In addition, TCF1+ T-cells showed a reduction in pro-survival regulator, BcL2, and the appearance of a new population of TCF1 negative caspase-3 expressing cells in peripheral blood from patients with severe disease. The decline in TCF1+ T-cells was also seen in a subgroup of severe patients with vitamin D deficiency. Lastly, we found that sera from severe patients inhibited TCF1 transcription ex vivo which was attenuated by a blocking antibody against the cytokine, interleukin-12 (IL12). Collectively, our findings underscore the potential significance of TCF1+ progenitor T-cells in accounting for the loss of immunity in severe COVID-19 and outline an array of markers that could be used to identify disease progression.

Characterization of Rickets Type II Model Rats to Reveal Functions of Vitamin D and Vitamin D Receptor

Vitamin D has been known to exert a wide range of physiological effects, including calcemic, osteogenic, anticancer, and immune responses. We previously generated genetically modified (GM) rats and performed a comparative analysis of their physiological properties to elucidate the roles of vitamin D and vitamin D receptor (VDR). In this study, our primary goal was to investigate the manifestations of type II rickets in rats with the VDR(H301Q) mutation, analogous to the human VDR(H305Q). Additionally, we created a double-mutant rat with the VDR(R270L/H301Q) mutation, resulting in almost no affinity for 1,25-dihydroxy-vitamin D3 (1,25D3) or 25-hydroxy-vitamin D3 (25D3). Notably, the plasma calcium concentration in Vdr(R270L/H301Q) rats was significantly lower than in wild-type (WT) rats. Meanwhile, Vdr(H301Q) rats had calcium concentrations falling between those of Vdr(R270L/H301Q) and WT rats. GM rats exhibited markedly elevated plasma parathyroid hormone and 1,25D3 levels compared to those of WT rats. An analysis of bone mineral density in the cortical bone of the femur in both GM rats revealed significantly lower values than in WT rats. Conversely, the bone mineral density in the trabecular bone was notably higher, indicating abnormal bone formation. This abnormal bone formation was more pronounced in Vdr(R270L/H301Q) rats than in Vdr(H301Q) rats, highlighting the critical role of the VDR-dependent function of 1,25D3 in bone formation. In contrast, neither Vdr(H301Q) nor Vdr(R270L/H301Q) rats exhibited symptoms of alopecia or cyst formation in the skin, which were observed in the Vdr-KO rats. These findings strongly suggest that unliganded VDR is crucial for maintaining the hair cycle and normal skin. Our GM rats hold significant promise for comprehensive analyses of vitamin D and VDR functions in future research.

Gene therapy for alopecia in type II rickets model rats using vitamin D receptor-expressing adenovirus vector

Type II rickets is a hereditary disease caused by a mutation in the vitamin D receptor (VDR) gene. The main symptoms of this disease are bone dysplasia and alopecia. Bone dysplasia can be ameliorated by high calcium intake; however, there is no suitable treatment for alopecia. In this study, we verified whether gene therapy using an adenoviral vector (AdV) had a therapeutic effect on alopecia in Vdr-KO rats. The VDR-expressing AdV was injected into six 7-week-old female Vdr-KO rats (VDR-AdV rats). On the other hand, control-AdV was injected into 7-week-old female rats (control-AdV rats); non-infected Vdr-KO rats (control rats) were also examined. The hair on the backs of the rats was shaved with hair clippers, and VDR-AdV or control-AdV was intradermally injected. Part of the back skin was collected from each rat after AdV administration. Hair follicles were observed using hematoxylin and eosin staining, and VDR expression was examined using immunostaining and western blotting. VDR-AdV rats showed significant VDR expression in the skin, enhanced hair growth, and low cyst formation, whereas control-AdV and non-infected rats did not show any of these effects. The effect of VDR-AdV lasted for nearly 60 days. These results indicate that gene therapy using VDR-AdV may be useful to treat alopecia associated with type II rickets, if multiple injections are possible after a sufficient period of time.

Affinity-based protein profiling identifies vitamin D3 as a heat shock protein 70 antagonist that regulates hedgehog transduction in murine basal cell carcinoma

Vitamin D3 (VD3) is a seco-steroid that inhibits the Hedgehog (Hh) signaling pathway. Initial studies suggested its anti-Hh activity results from direct inhibition of Smoothened, a seven-transmembrane cell surface receptor that is a key regulator of the Hh signaling cascade. More recently, a role for the Vitamin D Receptor in mediating inhibition of Hh-signaling by seco-steroid has been suggested. Herein, an affinity-based protein profiling study was carried out to better understand the cellular proteins that govern VD3-mediated anti-Hh activity. We synthesized a novel biotinylated VD3 analogue (8) for use as a chemical probe to explore cellular binding targets of the seco-steroidal scaffold. Through a series of pull-down experiments and follow up mass spectrum analyses, heat shock protein 70 (Hsp70) was identified as a primary binding protein of VD3. Hsp70 was validated as a binding target of VD3 through a series of biochemical and cellular assays. VD3 bound with micromolar affinity to Hsp70. In addition, both selective knockdown of Hsp70 expression and pharmacological inhibition of its activity with known Hsp70 inhibitors suppressed Hh-signaling transduction in murine basal cell carcinoma cells, suggesting that Hsp70 regulates proper Hh-signaling. Additional cellular assays suggest that VD3 and its seco-steroidal metabolites inhibit Hh-signaling through different mechanisms.

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.

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).

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.

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.

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.

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.

Vitamin D and calcium regulation of epidermal wound healing

Wound healing is essential for survival. This is a multistep process involving a number of different cell types. In the skin wounding triggers an acute inflammatory response, with the innate immune system contributing both to protection against invasive organisms and to triggering the invasion of inflammatory cells into the wounded area. These cells release a variety of cytokines and growth factors that stimulate the proliferation and migration of dermal and epidermal cells to close the wound. In particular, wounding activates stem cells in the interfollicular epidermis (IFE) and hair follicles (HF) to proliferate and send their progeny to re-epithelialize the wound. β-catenin and calcium signaling are important for this activation process. Mice lacking the VDR when placed on a low calcium diet have delayed wound healing. This is associated with reduced β-catenin transcriptional activity and proliferation in the cells at the leading edge of wound closure. These data suggest that vitamin D and calcium signaling are necessary components of the epidermal response to wounding, likely by regulating stem cell activation through increased β-catenin transcriptional activity.

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.

1,25-Dihydroxyvitamin D and Klotho: A Tale of Two Renal Hormones Coming of Age

1,25-Dihydroxyvitamin D3 (1,25D) is the renal metabolite of vitamin D that signals through binding to the nuclear vitamin D receptor (VDR). The ligand-receptor complex transcriptionally regulates genes encoding factors stimulating calcium and phosphate absorption plus bone remodeling, maintaining a skeleton with reduced risk of age-related osteoporotic fractures. 1,25D/VDR signaling exerts feedback control of Ca/PO4 via regulation of FGF23, klotho, and CYP24A1 to prevent age-related, ectopic calcification, fibrosis, and associated pathologies. Vitamin D also elicits xenobiotic detoxification, oxidative stress reduction, neuroprotective functions, antimicrobial defense, immunoregulation, anti-inflammatory/anticancer actions, and cardiovascular benefits. Many of the healthspan advantages conferred by 1,25D are promulgated by its induction of klotho, a renal hormone that is an anti-aging enzyme/coreceptor that protects against skin atrophy, osteopenia, hyperphosphatemia, endothelial dysfunction, cognitive defects, neurodegenerative disorders, and impaired hearing. In addition to the high-affinity 1,25D hormone, low-affinity nutritional VDR ligands including curcumin, polyunsaturated fatty acids, and anthocyanidins initiate VDR signaling, whereas the longevity principles resveratrol and SIRT1 potentiate VDR signaling. 1,25D exerts actions against neural excitotoxicity and induces serotonin mood elevation to support cognitive function and prosocial behavior. Together, 1,25D and klotho maintain the molecular signaling systems that promote growth (p21), development (Wnt), antioxidation (Nrf2/FOXO), and homeostasis (FGF23) in tissues crucial for normal physiology, while simultaneously guarding against malignancy and degeneration. Therefore, liganded-VDR modulates the expression of a "fountain of youth" array of genes, with the klotho target emerging as a major player in the facilitation of health span by delaying the chronic diseases of aging.

FGF23 gene regulation by 1,25-dihydroxyvitamin D: opposing effects in adipocytes and osteocytes

In a closed endocrine loop, 1,25-dihydroxyvitamin D3 (1,25D) induces the expression of fibroblast growth factor 23 (FGF23) in bone, with the phosphaturic peptide in turn acting at kidney to feedback repress CYP27B1 and induce CYP24A1 to limit the levels of 1,25D. In 3T3-L1 differentiated adipocytes, 1,25D represses FGF23 and leptin expression and induces C/EBPβ, but does not affect leptin receptor transcription. Conversely, in UMR-106 osteoblast-like cells, FGF23 mRNA concentrations are upregulated by 1,25D, an effect that is blunted by lysophosphatidic acid, a cell-surface acting ligand. Progressive truncation of the mouse FGF23 proximal promoter linked in luciferase reporter constructs reveals a 1,25D-responsive region between -400 and -200  bp. A 0.6  kb fragment of the mouse FGF23 promoter, linked in a reporter construct, responds to 1,25D with a fourfold enhancement of transcription in transfected K562 cells. Mutation of either an ETS1 site at -346  bp, or an adjacent candidate vitamin D receptor (VDR)/Nurr1-element, in the 0.6  kb reporter construct reduces the transcriptional activity elicited by 1,25D to a level that is not significantly different from a minimal promoter. This composite ETS1-VDR/Nurr1 cis-element may function as a switch between induction (osteocytes) and repression (adipocytes) of FGF23, depending on the cellular setting of transcription factors. Moreover, experiments demonstrate that a 1 kb mouse FGF23 promoter-reporter construct, transfected into MC3T3-E1 osteoblast-like cells, responds to a high calcium challenge with a statistically significant 1.7- to 2.0-fold enhancement of transcription. Thus, the FGF23 proximal promoter harbors cis elements that drive responsiveness to 1,25D and calcium, agents that induce FGF23 to curtail the pathologic consequences of their excess.

Regulation of VDR Expression in Apc-Mutant Mice, Human Colon Cancers and Adenomas

One variable that may affect the ability of vitamin D to reduce colon cancer risk is the expression of its high-affinity receptor, VDR. Here, we show that vitamin D does not reduce tumor formation in Apc(Δ14/+) mice and that VDR expression is lost in the majority of the colon tumor cells. The extent of VDR loss corresponded inversely to the level of β-catenin nuclear localization and could be observed in early lesions composed of just a few crypts. Analysis of reported VDR regulators showed that the repressing class I histone deacetylases (HDAC) were significantly elevated in the tumors (up to 4-fold), whereas the VDR-activating retinoid X receptors (RXR) were downregulated (∼50%). Expression of the Slug repressor was also increased, but was found primarily in stromal cells. Analysis of epigenetically active compounds on colon cell lines and intestinal organoids showed that HDAC inhibitors were particularly adept at stimulating VDR expression. Treatment of tumor-bearing Apc(Δ14/+) mice with the HDAC inhibitor panobinostat increased VDR expression in the tumors and normal mucosa. The RXR agonist bexarotene failed to activate VDR expression, indicating that RXR ligands were not limiting. Analysis of human microarray data indicated that VDR mRNA is frequently downregulated in colon adenomas, which correlated positively with RXRA expression and inversely with HDAC 2 and 8 expression. Human adenomas showed variable VDR protein expression levels, both between and within individual lesions. Determining the mechanisms of VDR regulation in colon neoplasms may significantly enhance our ability to use vitamin D as a cancer prevention agent.

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.

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.

Interaction of vitamin D with membrane-based signaling pathways

Many studies in different biological systems have revealed that 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃) modulates signaling pathways triggered at the plasma membrane by agents such as Wnt, transforming growth factor (TGF)-β, epidermal growth factor (EGF), and others. In addition, 1α,25(OH)₂D₃ may affect gene expression by paracrine mechanisms that involve the regulation of cytokine or growth factor secretion by neighboring cells. Moreover, post-transcriptional and post-translational effects of 1α,25(OH)₂D₃ add to or overlap with its classical modulation of gene transcription rate. Together, these findings show that vitamin D receptor (VDR) cannot be considered only as a nuclear-acting, ligand-modulated transcription factor that binds to and controls the transcription of target genes. Instead, available data support the view that much of the complex biological activity of 1α,25(OH)₂D₃ resides in its capacity to interact with membrane-based signaling pathways and to modulate the expression and secretion of paracrine factors. Therefore, we propose that future research in the vitamin D field should focus on the interplay between 1α,25(OH)₂D₃ and agents that act at the plasma membrane, and on the analysis of intercellular communication. Global analyses such as RNA-Seq, transcriptomic arrays, and genome-wide ChIP are expected to dissect the interactions at the gene and molecular levels.

GSK3β inhibition and LEF1 upregulation in skeletal muscle following a bout of downhill running

Canonical Wnt signaling is important in skeletal muscle repair but has not been well characterized in response to physiological stimuli. The objective of this study was to assess the effect of downhill running (DHR) on components of Wnt signaling. Young, male C57BL/J6 mice were exposed to DHR. Muscle injury and repair (MCadherin) were measured in soleus. Gene and protein expression of Wnt3a, active β-catenin, GSK3β, and LEF1 were measured in gastrocnemius. Muscle injury increased 6 days post-DHR and MCadherin protein increased 5 days post-DHR. Total and active GSK3β protein decreased 3 days (9-fold and 3.6-fold, respectively) post-DHR. LEF1 protein increased 6 days (5-fold) post-DHR. DHR decreased GSK3β and increased LEF1 protein expression, but did not affect other components of Wnt signaling. Due to their applicability, using models of physiological stimuli such as DHR will provide significant insight into cellular mechanisms within muscle.

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.

Protective actions of vitamin D in UVB induced skin cancer

Non-melanoma skin cancers (NMSC) are the most common type of cancer, occurring at a rate of over 1 million per year in the United States. Although their metastatic potential is generally low, they can and do metastasize, especially in the immune compromised host, and their surgical treatment is often quite disfiguring. Ultraviolet radiation (UVR) as occurs with sunlight exposure is generally regarded as causal for these malignancies, but UVR is also required for vitamin D synthesis in the skin. Based on our own data and that reported in the literature, we hypothesize that the vitamin D produced in the skin serves to suppress UVR epidermal tumor formation. In this review we will first discuss the evidence supporting the conclusion that the vitamin D receptor (VDR), with or without its ligand 1,25-dihydroxyvitamin D, limits the propensity for cancer formation following UVR. We will then explore three potential mechanisms for this protection: inhibition of proliferation and stimulation of differentiation, immune regulation, and stimulation of DNA damage repair (DDR).

Discovery of novel vitamin D receptor interacting proteins that modulate 1,25-dihydroxyvitamin D3 signaling

The nuclear vitamin D receptor (VDR) modulates gene transcription in 1,25-dihydroxyvitamin D(3) (1,25D) target tissues such as kidney, intestine, and bone. VDR is also expressed in heart, and 1,25D deficiency may play a role in the acceleration of cardiovascular disease. Employing a yeast two-hybrid system and a human heart library, using both a 1,25D-independent and 1,25D-dependent screen, we discovered six candidate VDR interacting proteins (VIPs). These novel VIPs include CXXC5, FASTK, NR4A1, TPM2, MYL3 and XIRP1. Mammalian two-hybrid assays as well as GST pull-downs were used to confirm VIP-VDR interaction, and the combination of these two assays reveals that CXXC5, XIRP1, FASTK and NR4A1 interactions with VDR may be modulated by 1,25D. The functional effects of these VIPs on 1,25D-mediated gene expression were explored in transcriptional assays employing three separate and distinct 1,25D-responsive element (VDRE)-driven luciferase reporter genes in transfected Caco-2 and HEK-293 cells, and in a C2C12 myoblast line. FASTK and TPM2 activated expression in all cell line and promoter contexts, while CXXC5 and XIRP1 exhibited differing effects depending on the cell line and promoter employed, suggesting promoter and cell-specific effects of these unique VIPs on VDR signaling. Further evaluation of the interaction between CXXC5 and VDR revealed that CXXC5 acts in a dose-dependent manner to stimulate VDR-mediated transcription on select VDREs. Identification of novel heart VIPs and their influence on VDR activity may increase our understanding of how vitamin D impacts cardiac physiology and may facilitate development of VDR/VIP drug analogs to combat heart disease.

Interactions among HCLS1, HAX1 and LEF-1 proteins are essential for G-CSF-triggered granulopoiesis

We found that hematopoietic cell-specific Lyn substrate 1 (HCLS1 or HS1) is highly expressed in human myeloid cells and that stimulation with granulocyte colony-stimulating factor (G-CSF) leads to HCLS1 phosphorylation. HCLS1 binds the transcription factor lymphoid-enhancer binding factor 1 (LEF-1), transporting LEF-1 into the nucleus upon G-CSF stimulation and inducing LEF-1 autoregulation. In patients with severe congenital neutropenia, inherited mutations in the gene encoding HCLS1-associated protein X-1 (HAX1) lead to profound defects in G-CSF-triggered phosphorylation of HCLS1 and subsequently to reduced autoregulation and expression of LEF-1. Consistent with these results, HCLS1-deficient mice are neutropenic. In bone marrow biopsies of the majority of tested patients with acute myeloid leukemia, HCLS1 protein expression is substantially elevated, associated with high levels of G-CSF synthesis and, in some individuals, a four-residue insertion in a proline-rich region of HCLS1 protein known to accelerate intracellular signaling. These data demonstrate the importance of HCLS1 in myelopoiesis in vitro and in vivo.

The nonskeletal effects of vitamin D: an Endocrine Society scientific statement

Significant controversy has emerged over the last decade concerning the effects of vitamin D on skeletal and nonskeletal tissues. The demonstration that the vitamin D receptor is expressed in virtually all cells of the body and the growing body of observational data supporting a relationship of serum 25-hydroxyvitamin D to chronic metabolic, cardiovascular, and neoplastic diseases have led to widespread utilization of vitamin D supplementation for the prevention and treatment of numerous disorders. In this paper, we review both the basic and clinical aspects of vitamin D in relation to nonskeletal organ systems. We begin by focusing on the molecular aspects of vitamin D, primarily by examining the structure and function of the vitamin D receptor. This is followed by a systematic review according to tissue type of the inherent biological plausibility, the strength of the observational data, and the levels of evidence that support or refute an association between vitamin D levels or supplementation and maternal/child health as well as various disease states. Although observational studies support a strong case for an association between vitamin D and musculoskeletal, cardiovascular, neoplastic, and metabolic disorders, there remains a paucity of large-scale and long-term randomized clinical trials. Thus, at this time, more studies are needed to definitively conclude that vitamin D can offer preventive and therapeutic benefits across a wide range of physiological states and chronic nonskeletal disorders.

Vitamin D and the skin: Physiology and pathophysiology

The keratinocytes of the skin are unique in being not only the primary source of vitamin D for the body, but in possessing both the enzymatic machinery to metabolize the vitamin D produced to active metabolites (in particular 1,25(OH)(2)D) and the vitamin D receptor (VDR) that enables the keratinocytes to respond to the 1,25(OH)(2)D thus generated. Numerous functions of the skin are regulated by vitamin D and/or its receptor. These include inhibition of proliferation, stimulation of differentiation including formation of the permeability barrier, promotion of innate immunity, regulation of the hair follicle cycle, and suppression of tumor formation. Regulation of these actions is exerted by a number of different coregulator complexes including the coactivators vitamin D receptor interacting protein (DRIP) complex also known as Mediator and the steroid receptor coactivator (SRC) family (of which SRC 2 and 3 are found in keratincytes), the inhibitor hairless (Hr), and β-catenin whose impact on VDR function is complex. Different coregulators appear to be involved in different VDR regulated functions. This review will examine the various functions of vitamin D and its receptor in the skin, and explore the mechanisms by which these functions are regulated.

Vitamin D resistance and colon cancer prevention

Observational studies have been largely consistent in showing an inverse association between vitamin D and an individual's risk of developing colorectal cancer. Vitamin D protection is further supported by a range of preclinical colon cancer models, including carcinogen, genetic and dietary models. A large number of mechanistic studies in both humans and rodents point to vitamin D preventing cancer by regulating cell proliferation. Counterbalancing this mostly positive data are the results of human intervention studies in which supplemental vitamin D was found to be ineffective for reducing colon cancer risk. One explanation for these discrepancies is the timing of vitamin D intervention. It is possible that colon lesions may progress to a stage where they become unresponsive to vitamin D. Such a somatic loss in vitamin D responsiveness bears the hallmarks of an epigenetic change. Here, we review data supporting the chemopreventive effectiveness of vitamin D and discuss how gene silencing and other molecular changes somatically acquired during colon cancer development may limit the protection that may otherwise be afforded by vitamin D via dietary intervention. Finally, we discuss how understanding the mechanisms by which vitamin D protection is lost might be used to devise strategies to enhance its chemopreventive actions.