The vitamin D response element of the involucrin gene mediates its regulation by 1,25-dihydroxyvitamin D3

CYP11A1‑derived vitamin D hydroxyderivatives as candidates for therapy of basal and squamous cell carcinomas

Hydroxyderivatives of vitamin D3, including classical 1,25(OH)2D3 and novel CYP11A1‑derived hydroxyderivatives, exert their biological activity by acting as agonists on the vitamin D receptor (VDR) and inverse agonists on retinoid‑related orphan receptors (ROR)α and γ. The anticancer activities of CYP11A1‑derived hydroxyderivatives were tested using cell biology, tumor biology and molecular biology methods in human A431 and SCC13 squamous (SCC)‑ and murine ASZ001 basal (BCC)‑cell carcinomas, in comparison with classical 1,25(OH)2D3. Vitamin D3‑hydroxyderivatives with or without a C1α(OH) inhibited cell proliferation in a dose‑dependent manner. While all the compounds tested had similar effects on spheroid formation by A431 and SCC13 cells, those with a C1α(OH) group were more potent in inhibiting colony and spheroid formation in the BCC line. Potent anti‑tumorigenic activity against the BCC line was exerted by 1,25(OH)2D3, 1,20(OH)2D3, 1,20,23(OH)3D3, 1,20,24(OH)3D3, 1,20,25(OH)3D3 and 1,20,26(OH)3D3, with smaller effects seen for 25(OH)D3, 20(OH)D3 and 20,23(OH)2D3. 1,25(OH)2D3, 1,20(OH)2D3 and 20(OH)D3 inhibited the expression of GLI1 and β‑catenin in ASZ001 cells. In A431 cells, these compounds also decreased the expression of GLI1 and stimulated involucrin expression. VDR, RORγ, RORα and CYP27B1 were detected in A431, SCC13 and ASZ001 lines, however, with different expression patterns. Immunohistochemistry performed on human skin with SCC and BCC showed nuclear expression of all three of these receptors, as well as megalin (transmembrane receptor for vitamin D‑binding protein), the level of which was dependent on the type of cancer and antigen tested in comparison with normal epidermis. Classical and CYP11A1‑derived vitamin D3‑derivatives exhibited anticancer‑activities on skin cancer cell lines and inhibited GLI1 and β‑catenin signaling in a manner that was dependent on the position of hydroxyl groups. The observed expression of VDR, RORγ, RORα and megalin in human SCC and BCC suggested that they might provide targets for endogenously produced or exogenously applied vitamin D hydroxyderivatives and provide excellent candidates for anti‑cancer therapy.

CYP11A1-derived vitamin D3 products protect against UVB-induced inflammation and promote keratinocytes differentiation

UVB radiation mediates inflammatory responses causing skin damage and defects in epidermal differentiation. 1α,25-Dihydroxyvitamin D3 (1,25(OH)2D3) interacts with the vitamin D3 receptor (VDR) to regulate inflammatory responses. Additionally, 1,25(OH)2D3/VDR signaling represents a potential therapeutic target in the treatment of skin disorders associated with inflammation and poor differentiation of keratinocytes. Since the protective effect of 1,25(OH)2D3 against UVB-induced skin damage and inflammation is recognized, CYP11A1-derived vitamin D3-hydroxyderivatives including 20(OH)D3, 1,20(OH)2D3, 20,23(OH)2D3 and 1,20,23(OH)3D3 were tested for their anti-inflammatory and skin protection properties in UVB-irradiated human epidermal keratinocytes (HEKn). HEKn were treated with secosteroids for 24 h pre- and post-UVB (50 mJ/cm2) irradiation. Secosteroids modulated the expression of the inflammatory response genes (IL-17, NF-κB p65, and IκB-α), reducing nuclear-NF-κB-p65 activity and increasing cytosolic-IκB-α expression as well as that of pro-inflammatory mediators, IL-17, TNF-α, and IFN-γ. They stimulated the expression of involucrin (IVL) and cytokeratin 10 (CK10), the major markers of epidermal differentiation, in UVB-irradiated cells. We conclude that CYP11A1-derived hydroxyderivatives inhibit UVB-induced epidermal inflammatory responses through activation of IκB-α expression and suppression of NF-kB-p65 activity and its downstream signaling cytokines, TNF-α, and IFN-γ, as well as by inhibiting IL-17 production and activating epidermal differentiation.

Shedding light on the effects of 1,25-dihydroxyvitamin D3 on epidermal lipid barrier formation in three-dimensional human skin equivalents

Human skin equivalents (HSEs) are three dimensional models resembling native human skin (NHS) in many aspects. Despite the manifold similarities to NHS, a restriction in its applications is the altered in vitro lipid barrier formation, which compromises the barrier functionality. This could be induced by suboptimal cell culturing conditions, which amongst others is the diminished activation of the vitamin D receptor (VDR) signalling pathway. The active metabolite of this signalling pathway is 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). An interacting role in the formation of the skin barrier has been ascribed to this pathway, although it remains unresolved to which extent this pathway contributes to the (mal-)formation of the epidermal barrier in HSEs. Our aim is to study whether cell culture medium enriched with 1,25(OH)₂D₃ affects epidermal morphogenesis and lipid barrier formation in HSEs. Addition of 20 nM 1,25(OH)₂D₃ resulted in activation of the VDR signalling pathway by inducing transcription of VDR target genes (CYP24A and LL37) in keratinocyte monocultures and in HSEs. Characterization of HSEs supplemented with 1,25(OH)₂D₃ using immunohistochemical analyses revealed a high similarity in epidermal morphogenesis and in expression of lipid processing enzymes. The barrier formation was assessed using state-of-the art techniques analysing lipid composition and organization. Addition of 1,25(OH)₂D₃ did not alter the composition of ceramides. Additionally, the lateral and lamellar organization of the lipids was similar, irrespective of supplementation. In conclusion, epidermal morphogenesis and barrier formation in HSEs generated in presence or absence of 1,25(OH)₂D₃ leads to a similar morphogenesis and comparable barrier formation in vitro.

Genome-wide association study of 25(OH) Vitamin D concentrations in Punjabi Sikhs: Results of the Asian Indian diabetic heart study

Vitamin D deficiency is implicated in multiple disease conditions and accumulating evidence supports that the variation in serum vitamin D (25(OH)D) levels, including deficiency, is under strong genetic control. However, the underlying genetic mechanism associated with vitamin 25(OH)D concentrations is poorly understood. We earlier reported a very high prevalence of vitamin D deficiency associated with an increased risk for type 2 diabetes and obesity in a Punjabi Sikh diabetic cohort as part of the Asian Indian diabetic heart study (AIDHS). Here we have performed the first genome-wide association study (GWAS) of serum 25(OH)D on 3538 individuals from this Punjabi Sikh population. Our discovery GWAS comprised of 1387 subjects followed by validation of 24 putative SNPs (P<10(-4)) using an independent replication sample (n=2151) from the same population by direct genotyping. A novel locus at chromosome 20p11.21 represented by rs2207173 with minor allele frequency (MAF) 0.29, [β=-0.13, p=4.47×10(-9)] between FOXA2 and SSTR4 was identified to be associated with 25(OH)D levels. Another suggestive association signal at rs11586313 (MAF 0.54) [β=0.90; p=1.36×10(-6)] was found within the regulatory region of the IVL gene on chromosome 1q21.3. Additionally, our study replicated 3 of 5 known GWAS genes associated with 25(OH)D concentrations including GC (p=0.007) and CYP2R1 (p=0.019) reported in Europeans and the DAB1 (p=0.003), reported in Hispanics. Identification of novel association signals in biologically plausible regions with 25(OH)D metabolism will provide new molecular insights on genetic drivers of vitamin D status and its implications in health disparities.

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.

Vitamin D levels in multiple sclerosis patients: Association with TGF-β2, TGF-βRI, and TGF-βRII expression

AIM

A variety of evidence suggests that vitamin D can prevent the development of multiple sclerosis (MS). TGF-β pathway genes also play important roles in MS. Here, we aim to study whether vitamin D affects TGF-β pathway gene expression and Expanded Disability Status Scale (EDSS) scores in MS patients.

MAIN METHODS

A randomized clinical trial was conducted on 31 relapsing-remitting (RR) MS patients. Using real-time RT-PCR, we tested the levels of TGF-β2, TGF-βRI and TGF-βRII mRNAs in the RRMS patients before and after 8 weeks of supplementation with vitamin D.

KEY FINDINGS

Expression of TGF-β2 mRNA increased 2.84-fold, while TGF-βRI and TGF-βRII mRNA levels did not change after vitamin D treatment. In addition, these results revealed no correlation between the normalized expression of TGF-β2, TGF-βRI, or TGF-βRII and EDSS scores.

SIGNIFICANCE

Here, we demonstrate new evidence for the complex role of vitamin D in the pathogenesis, activity and progression of MS through the TGF-β signaling pathway.

Compound screening and transcriptional profiling in human primary keratinocytes: a brief guideline

Cultured human primary keratinocytes constitute suitable targets for in-depth evaluation of the proliferative or differentiative potential of compounds. There is, however, a double-edged and intrinsically inseparable transition from biological activity to cytotoxicity for any agent under investigation. For that reason, we here first of all present an established protocol for the isolation, cultivation, and analysis of primary foreskin-derived keratinocytes. Taking calcitriol as example, we then reveal how a straightforward photometric cell culture assay can be exploited to assess overall cell viability in response to increasing compound doses. With predetermined cellular cytotoxicity at hand, physiologically meaningful (sub-toxic) compound concentrations for subsequent stimulation of cells can be readily selected, and, in doing so, differentially expressed genes with biological significance can be reliably identified.

Role of the calcium-sensing receptor in calcium regulation of epidermal differentiation and function

The epidermis is a stratified squamous epithelium composed of proliferating basal and differentiated suprabasal keratinocytes. It serves as the body's major physical and chemical barrier against infection and harsh environmental insults, as well as preventing excess water loss from the body into the atmosphere. Calcium is a key regulator of the proliferation and differentiation in keratinocytes. Elevated extracellular Ca(2+) concentration ([Ca(2+)]o) raises the levels of intracellular free calcium ([Ca(2+)]i), promotes cell-cell adhesion, and activates differentiation-related genes. Keratinocytes deficient in the calcium-sensing receptor fail to respond to [Ca(2+)]o stimulation and to differentiate, indicating a role for the calcium-sensing receptor in transducing the [Ca(2+)]o signal during differentiation. The concepts derived from in vitro gene knockdown experiments have been evaluated and confirmed in three mouse models in vivo.

Deciphering the calcitriol-induced transcriptomic response in keratinocytes: presentation of novel target genes

Numerous studies to date have been aimed at unraveling the large suite of calcitriol (1α,25-dihydroxyvitamin D(3)) response genes in diverse tissues including skin, where this hormone is involved in regulating keratinocyte proliferation, differentiation, permeability barrier formation, innate immunity promotion, antimicrobial peptide production, and wound healing. However, the various approaches differ considerably in probed cell types, scale, throughput, and statistical reliability and do, of note, not reveal much overlap. To further expand our knowledge on presently elusive targets and characterize the extent of fragmentation of existing datasets, we have performed whole-transcriptome microarray examinations of calcitriol-treated human primary keratinocytes. Out of 28,869 genes investigated, we uncovered 86 differentially expressed (67 upregulated and 19 downregulated) candidates that were functionally clustered into five annotation categories: response to wounding, protease inhibition, secondary metabolite biosynthesis, cellular migration, and amine biosynthetic processes. A complementary RTq-PCR study of 78 nominees selected thereof demonstrated significant differential expression of 55 genes (48 upregulated and seven downregulated) within biological replicates. Our hit list contains nine previously authenticated targets (16.36%, proof of concept) and 46 novel genes (83.6%) that have not yet been explicitly described as being differentially regulated within human primary keratinocytes. Direct vitamin D receptor response element predictions within the regulatory promoter regions of 50 of the RTq-PCR-validated targets agreed with known biological functionality and corroborated our stringent data validation pipeline. Altogether, our results indicate the value of continuing these kinds of gene expression studies, which contribute to an enhanced comprehension of calcitriol-mediated processes that may be dysregulated in human skin pathophysiology.

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: an ancient hormone

Vitamin D has been produced by plants and animals almost from the time life began. The ability to transport and metabolize vitamin D to more active forms evolved as the structures of plants and animals became more complex, and the cells within these organisms took on more specialized functions. In higher-order animals, the vitamin D receptor (VDR) is found in nearly every cell, and the ability of the cell to produce the active hormone, 1,25(OH)2D, is also widely distributed. Furthermore, the physiological functions with which vitamin D signalling is now associated are as diverse as the tissues in which the VDR is located. Why is this, and is there a common theme? This viewpoint article argues that there is. All cells maintain a fairly constant and submicromolar concentration of free calcium. Calcium is an important regulator of many processes within the cell. The ebb and flow of calcium within cells is controlled by calcium pumps, antiporters and channels. Animals with calcified exo- or endoskeletons have an additional need for calcium, a need that changes during the life cycle of the organism. In this article, I make the case that vitamin D signalling evolved to enable the organism to effectively regulate calcium flux, storage and signalling and that such regulation is critical for the evolutionary process.

PKC-delta and -eta, MEKK-1, MEK-6, MEK-3, and p38-delta are essential mediators of the response of normal human epidermal keratinocytes to differentiating agents

Previous studies suggest that the novel protein kinase C (PKC) isoforms initiate a mitogen-activated protein kinase (MAPK) signaling cascade that regulates keratinocyte differentiation. However, assigning these functions has relied on treatment with pharmacologic inhibitors and/or manipulating kinase function using overexpression of wild-type or dominant-negative kinases. As these methods are not highly specific, an obligatory regulatory role for individual kinases has not been assigned. In this study, we use small interfering RNA knockdown to study the role of individual PKC isoforms as regulators of keratinocyte differentiation induced by the potent differentiating stimulus, 12-O-tetradecanoylphorbol-13-acetate (TPA). PKC-delta knockdown reduces TPA-activated involucrin promoter activity, nuclear activator protein-1 factor accumulation and binding to DNA, and cell morphology change. Knockdown of PKC downstream targets, including MEKK-1, MEK-6, MEK-3, or p38-delta, indicates that these kinases are required for these responses. Additional studies indicate that knockdown of PKC-eta inhibits TPA-dependent involucrin promoter activation. In contrast, knockdown of PKC-alpha (a classical PKC isoform) or PKC-epsilon (a novel isoform) does not inhibit these TPA-dependent responses. Further studies indicate that PKC-delta is required for calcium and green tea polyphenol-dependent regulation of end responses. These findings are informative as they suggest an essential role for selected PKC and MAPK cascade enzymes in mediating a range of end responses to a range of differentiation stimuli in keratinocytes.

Arsenite suppression of involucrin transcription through AP1 promoter sites in cultured human keratinocytes

While preserving keratinocyte proliferative ability, arsenite suppresses cellular differentiation markers by preventing utilization of AP1 transcriptional response elements. In present experiments, arsenite had a dramatic effect in electrophoretic mobility supershift analysis of proteins binding to an involucrin promoter AP1 response element. Without arsenite treatment, binding of JunB and Fra1 was readily detected in nuclear extracts from preconfluent cultures and was not detected a week after confluence, while c-Fos was detected only after confluence. By contrast, band shift of nuclear extracts from arsenite treated cultures showed only JunB and Fra1 binding in postconfluent as well as preconfluent cultures. Immunoblotting of cell extracts showed that arsenite treatment prevented the loss of Fra1 and the increase in c-Fos proteins that occurred after confluence in untreated cultures. Chromatin immunoprecipitation assays demonstrated substantial reduction of c-Fos and acetylated histone H3 at the proximal and distal AP1 response elements in the involucrin promoter and of coactivator p300 at the proximal element. Alteration of AP1 transcription factors was also examined in response to treatment with four metal containing compounds (chromate, vanadate, hemin, divalent cadmium) that also suppress involucrin transcription. These agents all influenced transcription at AP1 elements in a transcriptional reporter assay, but exhibited less effect than arsenite on binding activity assessed by mobility shift and chromatin immunoprecipitation and displayed variable effects on AP1 protein levels. These findings help trace a mechanism by which transcriptional effects of arsenite become manifest and help rationalize the unique action of arsenite, compared to the other agents, to preserve proliferative ability.

Regulation of VDR by deltaNp63alpha is associated with inhibition of cell invasion

The p63 transcription factor has a pivotal role in epithelial morphogenesis. Multiple transcripts of the TP63 gene are generated because of alternative promoter usage and splicing. DeltaNp63alpha is the predominant isoform of p63 observed during epithelial morphogenesis and in human cancers. Loss of DeltaNp63alpha expression has been shown to promote invasiveness in a subset of human cancer cell lines. Here, we studied whether the regulation of VDR by DeltaNp63alpha controls the invasiveness of an epidermoid cancer cell line. We demonstrate that VDR expression is induced by all p63 isoforms, including DeltaNp63alpha. Endogenous DeltaNp63alpha protein was observed to bind to the VDR promoter, and silencing of endogenous DeltaNp63alpha resulted in diminished VDR expression. Although silencing of p63 inhibits VDR expression leading to an increase in cell migration, overexpression of p63 or VDR results in reduced cell migration as a result of increased VDR expression. Therefore, it is conceivable that p63 inhibits cell invasion by regulating VDR expression. Finally, we observed that expression of p63 and VDR overlaps in the wild-type mouse skin, but a reduced or complete absence of VDR expression was observed in skin from p63-null mice and in p63-null mouse embryonic fibroblasts. In conclusion, we demonstrate a direct transcriptional regulation of VDR by DeltaNp63alpha. Our results highlight a crucial role for VDR in p63-mediated biological functions.

Genetic architecture of mouse skin inflammation and tumour susceptibility

Germline polymorphisms in model organisms and humans influence susceptibility to complex trait diseases such as inflammation and cancer. Mice of the Mus spretus species are resistant to tumour development, and crosses between M. spretus and susceptible Mus musculus strains have been used to map locations of genetic variants that contribute to skin cancer susceptibility. We have integrated germline polymorphisms with gene expression in normal skin from a M. musculus x M. spretus backcross to generate a network view of the gene expression architecture of mouse skin. Here we demonstrate how this approach identifies expression motifs that contribute to tissue organization and biological functions related to inflammation, haematopoiesis, cell cycle control and tumour susceptibility. Motifs associated with inflammation, epidermal barrier function and proliferation are differentially regulated in backcross mice susceptible or resistant to tumour development. The intestinal stem cell marker Lgr5 is identified as a candidate master regulator of the hair follicle, and the vitamin D receptor (Vdr) is linked to coordinated control of epidermal barrier function, inflammation and tumour susceptibility.

Bag-1 proteins in oral squamous cell carcinoma

Bag-1 is an anti-apoptotic protein that exhibits altered expression in many malignancies, including oral squamous cell carcinoma. The bag-1 gene gives rise to different protein products with different subcellular localisations through alternative translational initiation sites. In oral squamous cell carcinoma, cytoplasmic expression has been associated with metastasis to regional lymph nodes and poor prognosis. In contrast, the longest Bag-1 isoform is nuclear and may regulate differentiation in oral epithelium. In this review, the functions of the three isoforms of Bag-1 expressed in oral epithelial cells are discussed in relation to their contribution to oral carcinogenesis.

20-Hydroxyvitamin D3, a product of vitamin D3 hydroxylation by cytochrome P450scc, stimulates keratinocyte differentiation

It has been shown that mammalian cytochrome P450scc can metabolize vitamin D3 to 20-hydroxyvitamin D3 (20(OH)D3) and 20,22(OH)2D3. To define the biological significance of this pathway, we tested the effects of 20(OH)D3 on the differentiation program of keratinocytes and on the expression of enzymes engaged in vitamin D3 metabolism. Immortalized HaCaT and adult human epidermal keratinocytes were used as a model and the effects of 20(OH)D3 were compared with those of 25(OH)D3 and 1,25(OH)2D3. 20(OH)D3 inhibited proliferation and caused G2/M arrest. 20(OH)D3 stimulated involucrin and inhibited cytokeratin 14 expression. The potency of 20(OH)D3 was comparable to that of 1,25(OH)2D3. 20(OH)D3 decreased the expression of cytochrome P450 enzyme (CYP)27A1 and CYP27B1, however, having only slight effect on CYP24. The effect of 20(OH)D3 was dependent on the vitamin D receptor (VDR). As shown by electrophoretic mobility shift assay, 20(OH)D3 stimulated the binding of nuclear proteins to the VDRE. Transfection of cells with VDR-specific siRNA decreased 20(OH)D3-stimulated transcriptional activity of the VDRE promoter and the expression of involucrin and CYP24 mRNA. Therefore, the above studies identify 20(OH)D3 as a biologically active secosteroid that induces keratinocyte differentiation. These data imply that the previously unreported pathway of vitamin D3 metabolism by P450scc may have wider biological implications depending, for example, on the extent of adrenal gland or cutaneous metabolism.

Subcellular localisation of BAG-1 and its regulation of vitamin D receptor-mediated transactivation and involucrin expression in oral keratinocytes: implications for oral carcinogenesis

In oral cancers, cytoplasmic BAG-1 overexpression is a marker of poor prognosis. BAG-1 regulates cellular growth, differentiation and survival through interactions with diverse proteins, including the vitamin D receptor (VDR), a key regulator of keratinocyte growth and differentiation. BAG-1 is expressed ubiquitously in human cells as three major isoforms of 50 kDa (BAG-1L), 46 kDa (BAG-1M) and 36 kDa (BAG-1S) from a single mRNA. In oral keratinocytes BAG-1L, but not BAG-1M and BAG-1S, enhanced VDR transactivation in response to 1alpha,25-dihydroxyvitamin D3. BAG-1L was nucleoplasmic and nucleolar, whereas BAG-1S and BAG-1M were cytoplasmic and nucleoplasmic in localisation. Having identified the nucleolar localisation sequence in BAG-1L, we showed that mutation of this sequence did not prevent BAG-1L from potentiating VDR activity. BAG-1L also potentiated transactivation of known vitamin-D-responsive gene promoters, osteocalcin and 24-hydroxylase, and enhanced VDR-dependent transcription and protein expression of the keratinocyte differentiation marker, involucrin. These results demonstrate endogenous gene regulation by BAG-1L by potentiating nuclear hormone receptor function and suggest a role for BAG-1L in 24-hydroxylase regulation of vitamin D metabolism and the cellular response of oral keratinocytes to 1alpha,25-dihydroxyvitamin D3. By contrast to the cytoplasmic BAG-1 isoforms, BAG-1L may act to suppress tumorigenesis.

Mechanism of vitamin D3-induced transcription of phospholipase D1 in HaCat human keratinocytes

1alpha,25-Dihydroxyvitamin D(3) (VitD(3)) increases protein and gene expression of phospholipase D1 (PLD1), but not PLD2, in HaCaT human keratinocytes. We show that VitD(3) increases PLD1 gene expression through a vitamin D responsive element (VDRE) on the 5' PLD1 promoter (-260 bp to -246 bp from exon 1). Similar results were obtained by transfecting VitD(3) receptor (VDR) into HEK293 cells, which are originally VitD(3)-unresponsive. Electrophoresis mobility shift assays (EMSA) and chromatin immunoprecipitation (CHIP) assays showed that the complex of VitD(3), VDR and retinoid X receptor alpha (RXRalpha) binds to the VDRE and increases PLD1 gene expression in HaCaT cells.

An alternative pathway of vitamin D metabolism. Cytochrome P450scc (CYP11A1)-mediated conversion to 20-hydroxyvitamin D2 and 17,20-dihydroxyvitamin D2

We report an alternative, hydroxylating pathway for the metabolism of vitamin D2 in a cytochrome P450 side chain cleavage (P450scc; CYP11A1) reconstituted system. NMR analyses identified solely 20-hydroxyvitamin D2 and 17,20-dihydroxyvitamin D2 derivatives. 20-Hydroxyvitamin D2 was produced at a rate of 0.34 mol x min(-1) x mol(-1) P450scc, and 17,20-dihydroxyvitamin D2 was produced at a rate of 0.13 mol x min(-1) x mol(-1). In adrenal mitochondria, vitamin D2 was metabolized to six monohydroxy products. Nevertheless, aminoglutethimide (a P450scc inhibitor) inhibited this adrenal metabolite formation. Initial testing of metabolites for biological activity showed that, similar to vitamin D2, 20-hydroxyvitamin D2 and 17,20-dihydroxyvitamin D2 inhibited DNA synthesis in human epidermal HaCaT keratinocytes, although to a greater degree. 17,20-Dihydroxyvitamin D2 stimulated transcriptional activity of the involucrin promoter, again to a significantly greater extent than vitamin D2, while the effect of 20-hydroxyvitamin D2 was statistically insignificant. Thus, P450scc can metabolize vitamin D2 to generate novel products, with intrinsic biological activity (at least in keratinocytes).

The distal and proximal regulatory regions of the involucrin gene promoter have distinct functions and are required for in vivo involucrin expression

Involucrin is a marker of human keratinocyte differentiation. Previous studies show that the human involucrin gene promoter has two distinct regulatory regions - the proximal regulatory region (PRR) and the distal regulatory region (DRR). To study the role of these regions in vivo, we have constructed human involucrin promoter transgenic mice and monitored the impact of specific promoter mutations on involucrin gene expression. In this study, we monitor the impact of specific mutations on expression in a range of surface epithelia. We begin by confirming previous observations made in footpad epidermis by showing that the full-length involucrin promoter drives differentiation-appropriate expression in other surface epithelia, including epidermis, cervix, and esophagus. We further show that mutation of the activator protein AP1-5 site in the DRR completely eliminates transgene expression in all of these tissues. In contrast, mutation of the DRR Sp1 site reduces overall expression, but does not alter the differentiation dependence. Additional studies identify a DRR immediate suprabasal element (ISE). Deletion of the ISE results in a loss of transgene expression in the immediate suprabasal layers. Our studies also indicate that the PRR is important for appropriate transgene expression. Mutation of a PRR C/EBP (CCAAT enhancer binding protein) transcription factor binding site results in patchy/discontinuous expression. These studies suggest that AP1, Sp1, and C/EBP transcription factors are required for appropriate differentiation-dependent involucrin expression, and that the mechanism of regulation is similar in most surface epithelia.

Hairless suppresses vitamin D receptor transactivation in human keratinocytes

The vitamin D receptor (VDR) and its ligand 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] are required for normal keratinocyte differentiation. Both the epidermis and the hair follicle are disrupted in VDR-null mice. Hairless (Hr), a presumptive transcription factor with no known ligand, when mutated, disrupts hair follicle cycling similar to the effects of VDR mutations. Hr, like VDR, is found in the nuclei of keratinocytes in both epidermis and hair follicle. To investigate the potential interaction between Hr and VDR on keratinocyte differentiation, we examined the effect of Hr expression on vitamin D-responsive genes in normal human keratinocytes. Inhibition of Hr expression in keratinocytes potentiated the induction of vitamin D-responsive genes, including involucrin, transglutaminase, phospholipase C-gamma1, and 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) by 1,25(OH)2D3. Overexpression of Hr in human keratinocytes suppressed the induction of these vitamin D-responsive genes by 1,25(OH)2D3. Coimmunoprecipitation, DNA mobility shift assays, and chromatin immunoprecipitation revealed that Hr binds to VDR in human keratinocytes. Hr binding to the VDR was eliminated by 1,25(OH)2D3, which recruited the coactivator vitamin D receptor-interacting protein 205 (DRIP205) to the VDR/vitamin D response element complex. These data indicate that Hr functions as a corepressor of VDR to block 1,25(OH)2D3 action on keratinocytes.

Vitamin D and skin cancer: a problem in gene regulation

The skin is the major source of Vitamin D(3) (cholecalciferol), and ultraviolet light (UV) is critical for its formation. Keratinocytes, the major cell in the epidermis, can further convert Vitamin D(3) to its hormonal form, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] (calcitriol). 1,25(OH)(2)D(3) in turn stimulates the differentiation of keratinocytes, raising the hope that 1,25(OH)(2)D(3) may prevent the development of malignancies in these cells. Skin cancers (squamous cell carcinoma (SCC), basal cell carcinoma (BCC), and melanomas) are the most common cancers afflicting humans. UV exposure is linked to the incidence of these cancers-UV is thus good and bad for epidermal health. Our focus is on the mechanisms by which 1,25(OH)(2)D(3) regulates the differentiation of keratinocytes, and how this regulation breaks down in transformed cells. Skin cancers produce 1,25(OH)(2)D(3), contain ample amounts of the Vitamin D receptor (VDR), and respond to 1,25(OH)(2)D(3) with respect to induction of the 24-hydroxylase, but fail to differentiate in response to 1,25(OH)(2)D(3). Why not? The explanation may lie in the overexpression of the DRIP complex, which by interfering with the normal transition from DRIP to SRC as coactivators of the VDR during differentiation, block the induction of genes required for 1,25(OH)(2)D(3)-induced differentiation.

Vitamin D and skin cancer

Skin cancer is the most common cancer afflicting humans. These cancers include melanomas and 2 types of malignant keratinocytes: basal-cell carcinomas (BCC) and squamous-cell carcinomas (SCC). UV light exposure is linked to the incidence of these cancers. On the other hand, the skin is the major source of vitamin D-3 (cholecalciferol) and UV light is critical for its formation. Keratinocytes can convert vitamin D-3 to its hormonal form, 1,25 dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] (calcitriol). 1,25(OH)(2)D(3) in turn stimulates the differentiation of keratinocytes, raising the hope that 1,25(OH)(2)D(3) may prevent the development of malignancies in these cells. We identified a number of mechanisms by which 1,25(OH)(2)D(3) regulates the differentiation of keratinocytes and explored where this regulation breaks down in SCCs. 1,25(OH)(2)D(3) regulates gene expression by activating the vitamin D receptor (VDR). When activated, the VDR binds to one of two coactivator complexes: DRIP or p160/SRC. Binding to DRIP occurs in the undifferentiated keratinocyte, but, as the cell differentiates, DRIP(205) levels fall and p160/SRC binding takes over as SRC3 expression increases. SCCs fail to respond to the prodifferentiating actions of 1,25(OH)(2)D(3). These cells have normal levels of VDR and normal binding of VDR to vitamin D response elements. However, they overexpress DRIP(205) such that the p160/SRC complex is blocked from binding to VDR. We hypothesize that failure of 1,25(OH)(2)D(3) to induce differentiation in SCCs lies at least in part with its failure to induce the replacement of the DRIP complex with the SRC complex in the promoters of genes required for differentiation.

Regulation of Involucrin Gene Expression

The epidermis is a dynamic renewing structure that provides life-sustaining protection from the environment. The major cell type of the epidermis, the epidermal keratinocyte, undergoes a carefully choreographed program of differentiation. Alteration of these events results in a variety of debilitating and life-threatening diseases. Understanding how this process is regulated is an important current goal in biology. In this review, we summarize the literature regarding regulation of involucrin, an important marker gene that serves as a model for understanding the mechanisms that regulate the differentiation process. Current knowledge describing the role of transcription factors and signaling cascades in regulating involucrin gene expression are presented. These studies describe a signaling cascade that includes the novel protein kinase C isoforms, Ras, MEKK1, MEK3, and a p38delta-extracellular signal regulated kinase 1/2 complex. This cascade regulates activator protein one, Sp1, and CCATT/enhancer-binding protein transcription factor DNA binding to two discrete involucrin promoter regions, the distal- and proximal-regulatory regions, to regulate involucrin gene expression.

Calcium and 1,25(OH)2D: interacting drivers of epidermal differentiation

Both calcium and 1,25(OH)(2)D promote the differentiation of keratinocytes in vitro. The autocrine or paracrine production of 1,25(OH)(2)D by keratinocytes combined with the critical role of the epidermal calcium gradient in regulating keratinocyte differentiation in vivo suggest the physiologic importance of this interaction. The interactions occur at a number of levels. Calcium and 1,25(OH)(2)D synergistically induce involucrin, a protein critical for cornified envelope formation. The involucrin promoter contains an AP-1 site essential for calcium and 1,25(OH)(2)D induction and an adjacent VDRE essential for 1,25(OH)(2)D but not calcium induction. Calcium regulates coactivator complexes that bind to the Vitamin D receptor (VDR). Nuclear extracts from cells grown in low calcium contain an abundance of DRIP(205), whereas calcium induced differentiation leads to reduced DRIP(205) and increased SRC 3 which replaces DRIP in its binding to the VDR. In vivo models support the importance of 1,25(OH)(2)D-calcium interactions in epidermal differentiation. The epidermis of 1alphaOHase null mice fails to form a normal calcium gradient, has reduced expression of proteins critical for barrier function, and shows little recovery of the permeability barrier when disrupted. Thus in vivo and in vitro, calcium and 1,25(OH)(2)D interact at multiple levels to regulate epidermal differentiation.

Identification of vitamin D target genes in human keratinocytes by subtractive screening

1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2) D(3)) imposes cell cycle block in late G1 phase in cultured human keratinocytes. We wanted to identify early vitamin D target genes using a subtractive screening approach. Human foreskin keratinocytes were grown to about 70% confluence, treated with 2 x 10(-7) M 1alpha,25(OH)(2) D(3) or left untreated and RNA from both populations were isolated after 22h of incubation. cDNA was synthesised and cloned into plasmid vectors. For screening of the libraries, cDNA was amplified in vitro using T7 RNA polymerase and then the amplified RNA (driver, control population) and single stranded cDNA (tester) were used for subtractive hybridisation. Heterohybrids were then separated from single stranded nucleotides using a hydroxyapatite column. The radiolabeled single stranded cDNA was used for screening a colony blot. Positive clones were rescreened, plasmid DNA was isolated and used for verifying the results by Northern blot analysis, using RNA isolated from untreated keratinocytes, as well as RNA isolated after 6h, 12h and 24h of vitamin D treatment.

Calcitriol in cancer treatment: From the lab to the clinic

1,25-Dihydroxyvitamin D (calcitriol), the most active metabolite of vitamin D, has significant antineoplastic activity in preclinical models. Several mechanisms of activity have been proposed. These include inhibition of proliferation associated with cell cycle arrest and, in some models, differentiation, reduction in invasiveness and angiogenesis, and induction of apoptosis. Proposed mechanisms differ between tumor models and experimental conditions, and no unifying hypothesis about the mechanism of antineoplastic activity has emerged. Synergistic and/or additive effects with cytotoxic chemotherapy, radiation, and other cancer drugs have been reported. Significantly supraphysiological concentrations of calcitriol are required for antineoplastic effects. Such concentrations are not achievable in patients when calcitriol is dosed daily due to predictable hypercalcemia and hypercalcuria; however, phase I trials have demonstrated that intermittent dosing allows substantial dose escalation and has produced potentially therapeutic peak calcitriol concentrations. Recently, a phase II study reported encouraging levels of activity for the combination of high-dose calcitriol and docetaxel administered on a weekly schedule in patients with androgen-independent prostate cancer. This regimen is now under study in a placebo-controlled randomized trial in androgen-independent prostate cancer and in phase II studies in several other tumor types. Further work is needed to elucidate the molecular mechanisms of antineoplastic activity and optimal clinical applications of calcitriol in cancer.

Vitamin D regulated keratinocyte differentiation

The epidermis is the largest organ in the body. It is comprised primarily of keratinocytes which are arranged in layers that recapitulates their programmed life cycle. Proliferating keratinocytes are on the bottom-the stratum basale. As keratinocytes leave the stratum basale they begin to differentiate, culminating in the enucleated stratum corneum which has the major role of permeability barrier. Calcium and the active metabolite of vitamin D, 1,25(OH)(2)D(3), play important roles in this differentiation process. The epidermis has a gradient of calcium with lowest concentrations in the stratum basale, and highest concentrations in the stratum granulosum where proteins critical for barrier function are produced. Vitamin D is made in different layers of the epidermis, but 1,25(OH)(2)D(3) is made primarily in the stratum basale. Together calcium and 1,25(OH)(2)D(3) regulate the ordered differentiation process by the sequential turning on and off the genes producing the elements required for differentiation as well as activating those enzymes involved in differentiation. Animal models in which the sensing mechanism for calcium, the receptor for 1,25(OH)(2)D(3), or the enzyme producing 1,25(OH)(2)D(3) have been rendered inoperative demonstrate the importance of these mechanisms for the differentiation process, although each animal model has its own phenotype. This review will examine the mechanisms by which calcium and 1,25(OH)(2)D(3) interact to control epidermal differentiation.

Squamous cell carcinomas fail to respond to the prodifferentiating actions of 1,25(OH)2D: why?

1,25(OH)2D regulates a number of cellular events which contribute to its ability to stimulate differentiation of the keratinocyte. 1,25(OH)2D raises the intracellular calcium (Cai) level in part by increasing the expression of the calcium receptor (CaR). This sensitizes the cell to extracellular calcium, triggering the signaling pathway coupled to the CaR, which results in a rise in Cai. 1,25(OH)2D induces the family of phospholipases C (PLC). These enzymes mediate the hydrolysis of phosphatidyl inositol bisphosphate (PIP2) to form inositol tris phosphate (IP3) and diacylglycerol (DG), which stimulate calcium release from intracellular stores and activate protein kinases C (PKC), respectively. The CaR and other G protein coupled receptors signal through PLC-beta, whereas tyrosine kinase growth factor receptors such as the EGF receptor signal through PLC-gamma. Calcium and PKC regulate the expression of genes in part by controlling the levels and activity of AP-1 transcription factors. 1,25(OH)2D also directly induces structural genes such as involucrin, a substrate for transglutaminase, which crosslinks it to other substrates to form the cornified envelope. 1,25(OH)2D regulates gene expression by activating the vitamin D receptor (VDR), a transcription factor, which, in combination with the retinoid X receptor (RXR) or retinoid A receptor (RAR), binds to its vitamin D response elements (VDRE) in the promoters of genes whose expression it regulates. The VDR also binds to one of two coactivator complexes, Mediator/DRIP (VDR interacting proteins) or p160/SRC (steroid hormone receptor complex), complexes which link the VDR to the RNA polymerase complex. We have recently discovered that the binding of VDR to these complexes is sequential. Binding to Mediator/DRIP occurs in the undifferentiated keratinocyte, but as the cell differentiates, DRIP(205) (the key protein of the DRIP complex binding to the VDR) levels fall, and p160/SRC binding takes over. We hypothesize that this sequential replacement of Mediator/DRIP by p160/SRC is critical for differentiation. Squamous cell carcinomas (SCC) fail to respond to the prodifferentiating actions of 1,25(OH)2D. These cells have normal levels of VDR and normal binding of VDR to VDREs. However, they fail to down-regulate DRIP(205) such that the p160/SRC complex fails to bind to VDR. This lack of sequential binding of these coactivator complexes to the VDR, we believe, maintains the cell in a state of continued proliferation and blocks the ability of 1,25(OH)2D to induce the expression of genes required for the differentiation process.

Vitamin D and the skin

Vitamin D was originally discovered as a factor that regulates calcium and bone metabolism. Recent advances in investigation have shown that vitamin D also functions as a regulator of cellular growth and differentiation in various tissues. The skin is not an exception from such effects of vitamin D; it is regarded as a site of its activation and action. Evidence has accumulated showing that the active form of vitamin D and its analogs suppress growth and stimulate the terminal differentiation of keratinocytes. In psoriatic lesions, epidermal keratinocytes exhibit hyper-proliferation and impaired differentiation triggered by inflammation. Therefore, it is quite reasonable that vitamin D is effective on psoriasis. Indeed, within the past decade, analogs of vitamin D3 have been used as topical therapy for psoriasis. In this review, we summarize the fundamental features of vitamin D and the development of vitamin D therapy for psoriasis. Clinical application to other skin diseases and the future of vitamin D therapy in dermatology are also discussed.

Vitamin D regulated keratinocyte differentiation: role of coactivators

1,25 Dihydroxyvitamin D (1,25(OH)(2)D) regulates the differentiation of keratinocytes. 1,25(OH)(2)D raises intracellular free calcium (Cai) as a necessary early step toward stimulating differentiation. 1,25(OH)(2)D induces the calcium sensing receptor (CaR) in keratinocytes and enhances the calcium response of these cells. Activation of the CaR by calcium increases intracellular free calcium by a mechanism involving phospholipase C (PLC) cleavage of phosphatidylinositolbisphosphate into inositoltrisphosphate (IP(3)) and diacylglycerol (DG). 1,25(OH)(2)D induces the family of PLCs. PLC-gamma1 has a DR6 VDRE in its promoter which binds and is activated by VDR/RAR rather than VDR/RXR. The involucrin gene, which encodes a critical component of the cornified envelope, contains a DR3 VDRE in its promoter that acts in conjunction with a nearby AP-1 site. The sequential regulation of these genes is critical for the differentiation process. In undifferentiated keratinocytes, the VDR binds preferentially to the DRIP complex of coactivators. However, with differentiation DRIP 205 is no longer produced, and the VDR switches partners to the SRC family (SRC2 and 3). These studies suggest that at least part of the sequential activation of genes required during keratinocyte differentiation is regulated by the change (availability) of these different coactivator complexes.