The enhanced hypercalcemic response to 20-epi-1,25-dihydroxyvitamin D3 results from a selective and prolonged induction of intestinal calcium-regulating genes

Epigenetic regulation during 1,25-dihydroxyvitamin D3-dependent gene transcription

Multiple evidence accumulated over the years, demonstrates that vitamin D-dependent physiological control in vertebrates occurs primarily through the regulation of target gene transcription. In addition, there has been an increasing appreciation of the role of the chromatin organization of the genome on the ability of the active form of vitamin D, 1,25(OH)₂D₃, and its specific receptor VDR to regulate gene expression. Chromatin structure in eukaryotic cells is principally modulated through epigenetic mechanisms including, but not limited to, a wide number of post-translational modifications of histone proteins and ATP-dependent chromatin remodelers, which are operative in different tissues during response to physiological cues. Hence, there is necessity to understand in depth the epigenetic control mechanisms that operate during 1,25(OH)₂D₃-dependent gene regulation. This chapter provides a general overview about epigenetic mechanisms functioning in mammalian cells and discusses how some of these mechanisms represent important components during transcriptional regulation of the model gene system CYP24A1 in response to 1,25(OH)₂D₃.

Role of Neural Stem Cells and Vitamin D Receptor (VDR)-Mediated Cellular Signaling in the Mitigation of Neurological Diseases

Specific stem cell-based therapies for treating Alzheimer's disease, Parkinson's disease, and schizophrenia are gaining importance in recent years. Accumulating data is providing further support by demonstrating the efficacy of neural stem cells in enhancing the neurogenesis in the aging brain. In addition to stem cells, recent studies have shown the efficacy of supplementing vitamin D in promoting neurogenesis and neuronal survival. Studies have also demonstrated the presence of mutational variants and single-nucleotide polymorphisms of the vitamin D receptor (VDR) in neurological disorders; however, implications of these mutations in the pathophysiology and response to drug treatment are yet to be explored. Hence, in this article, we have reviewed recent reports pertaining to the role of neural stem cells and VDR-mediated cellular signaling cascades that are involved in enhancing the neurogenesis through Wnt/β-catenin and Sonic Hedgehog pathways. This review benefits neurobiologists and pharmaceutical industry experts to develop stem cell-based and vitamin D-based therapies to better treat the patients suffering from neurological diseases.

Specificity of the Redox Complex between Cytochrome P450 24A1 and Adrenodoxin Relies on Carbon-25 Hydroxylation of Vitamin-D Substrate

Metabolic deactivation of 1,25(OH)₂D3 is initiated by modification of the vitamin-D side chain, as carried out by the mitochondrial cytochrome P450 24A1 (CYP24A1). In addition to its role in vitamin-D metabolism, CYP24A1 is involved in catabolism of vitamin-D analogs, thereby reducing their efficacy. CYP24A1 function relies on electron transfer from the soluble ferredoxin protein adrenodoxin (Adx). Recent structural evidence suggests that regioselectivity of the CYP24A1 reaction may correlate with distinct modes of Adx recognition. Here we used nuclear magnetic resonance (NMR) spectroscopy to monitor the structure of ¹⁵N-labeled full-length Adx from rat while forming the complex with rat CYP24A1 in the ligand-free state or bound to either 1,25(OH)₂D3 or the vitamin-D supplement 1α(OH)D3. Although both vitamin-D ligands were found to induce a reduction in overall NMR peak broadening, thereby suggesting ligand-induced disruption of the complex, a crosslinking analysis suggested that ligand does not have a significant effect on the relative association affinities of the redox complexes. However, a key finding is that, whereas the presence of primary CYP24A1 substrate was found to induce NMR peak broadening focused on the putative recognition site α-helix 3 of rat adrenodoxin, the interaction in the presence of 1α(OH)D3, which is lacking the carbon-25 hydroxyl, results in disruption of the NMR peak broadening pattern, thus indicating a ligand-induced nonspecific protein interaction. These findings provide a structural basis for the poor substrate turnover of side-chain-modified vitamin-D analogs, while also confirming that specificity of the CYP24A1-ligand interaction influences specificity of CYP24A1-Adx recognition. SIGNIFICANCE STATEMENT: Mitochondrial cytochrome P450 enzymes, such as CYP24A1 responsible for catabolizing vitamin-D and its analogs, rely on a protein-protein interaction with a ferredoxin in order to receive delivery of the electrons required for catalysis. In this study, we demonstrate that this protein interaction is influenced by the enzyme-ligand interaction that precedes it. Specifically, vitamin-D missing carbon-25 hydroxylation binds the enzyme active site with high affinity but results in a loss of P450-ferredoxin binding specificity.

Transgenic Expression of the Vitamin D Receptor Restricted to the Ileum, Cecum, and Colon of Vitamin D Receptor Knockout Mice Rescues Vitamin D Receptor-Dependent Rickets

Although the intestine plays the major role in 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] action on calcium homeostasis, the mechanisms involved remain incompletely understood. The established model of 1,25(OH)2D3-regulated intestinal calcium absorption postulates a critical role for the duodenum. However, the distal intestine is where 70% to 80% of ingested calcium is absorbed. To test directly the role of 1,25(OH)2D3 and the vitamin D receptor (VDR) in the distal intestine, three independent knockout (KO)/transgenic (TG) lines expressing VDR exclusively in the ileum, cecum, and colon were generated by breeding VDR KO mice with TG mice expressing human VDR (hVDR) under the control of the 9.5-kb caudal type homeobox 2 promoter. Mice from one TG line (KO/TG3) showed low VDR expression in the distal intestine (<50% of the levels observed in KO/TG1, KO/TG2, and wild-type mice). In the KO/TG mice, hVDR was not expressed in the duodenum, jejunum, kidney, or other tissues. Growth arrest, elevated parathyroid hormone level, and hypocalcemia of the VDR KO mice were prevented in mice from KO/TG lines 1 and 2. Microcomputed tomography analysis revealed that the expression of hVDR in the distal intestine of KO/TG1 and KO/TG2 mice rescued the bone defects associated with systemic VDR deficiency, including growth plate abnormalities and altered trabecular and cortical parameters. KO/TG3 mice showed rickets, but less severely than VDR KO mice. These findings show that expression of VDR exclusively in the distal intestine can prevent abnormalities in calcium homeostasis and bone mineralization associated with systemic VDR deficiency.

Vitamin D Signaling Modulators in Cancer Therapy

The antiproliferative and pro-apoptotic effects of 1α,25-dihydroxycholecalciferol (1,25(OH)2D3, 1,25D3, calcitriol) have been demonstrated in various tumor model systems in vitro and in vivo. However, limited antitumor effects of 1,25D3 have been observed in clinical trials. This may be attributed to a variety of factors including overexpression of the primary 1,25D3 degrading enzyme, CYP24A1, in tumors, which would lead to rapid local inactivation of 1,25D3. An alternative strategy for improving the antitumor activity of 1,25D3 involves the combination with a selective CYP24A1 inhibitor. The validity of this approach is supported by numerous preclinical investigations, which demonstrate that CYP24A1 inhibitors suppress 1,25D3 catabolism in tumor cells and increase the effects of 1,25D3 on gene expression and cell growth. Studies are now required to determine whether selective CYP24A1 inhibitors+1,25D3 can be used safely and effectively in patients. CYP24A1 inhibitors plus 1,25D3 can cause dose-limiting toxicity of vitamin D (hypercalcemia) in some patients. Dexamethasone significantly reduces 1,25D3-mediated hypercalcemia and enhances the antitumor activity of 1,25D3, increases VDR-ligand binding, and increases VDR protein expression. Efforts to dissect the mechanisms responsible for CYP24A1 overexpression and combinational effect of 1,25D3/dexamethasone in tumors are underway. Understanding the cross talk between vitamin D receptor (VDR) and glucocorticoid receptor (GR) signaling axes is of crucial importance to the design of new therapies that include 1,25D3 and dexamethasone. Insights gained from these studies are expected to yield novel strategies to improve the efficacy of 1,25D3 treatment.

Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects

1,25-Dihydroxvitamin D3 [1,25(OH)2D3] is the hormonally active form of vitamin D. The genomic mechanism of 1,25(OH)2D3 action involves the direct binding of the 1,25(OH)2D3 activated vitamin D receptor/retinoic X receptor (VDR/RXR) heterodimeric complex to specific DNA sequences. Numerous VDR co-regulatory proteins have been identified, and genome-wide studies have shown that the actions of 1,25(OH)2D3 involve regulation of gene activity at a range of locations many kilobases from the transcription start site. The structure of the liganded VDR/RXR complex was recently characterized using cryoelectron microscopy, X-ray scattering, and hydrogen deuterium exchange. These recent technological advances will result in a more complete understanding of VDR coactivator interactions, thus facilitating cell and gene specific clinical applications. Although the identification of mechanisms mediating VDR-regulated transcription has been one focus of recent research in the field, other topics of fundamental importance include the identification and functional significance of proteins involved in the metabolism of vitamin D. CYP2R1 has been identified as the most important 25-hydroxylase, and a critical role for CYP24A1 in humans was noted in studies showing that inactivating mutations in CYP24A1 are a probable cause of idiopathic infantile hypercalcemia. In addition, studies using knockout and transgenic mice have provided new insight on the physiological role of vitamin D in classical target tissues as well as evidence of extraskeletal effects of 1,25(OH)2D3 including inhibition of cancer progression, effects on the cardiovascular system, and immunomodulatory effects in certain autoimmune diseases. Some of the mechanistic findings in mouse models have also been observed in humans. The identification of similar pathways in humans could lead to the development of new therapies to prevent and treat disease.

Mechanisms of Enhancer-mediated Hormonal Control of Vitamin D Receptor Gene Expression in Target Cells

The biological actions of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are mediated by the vitamin D receptor (VDR), whose expression in bone cells is regulated positively by 1,25(OH)2D3, retinoic acid, and parathyroid hormone through both intergenic and intronic enhancers. In this report, we used ChIP-sequencing analysis to confirm the presence of these Vdr gene enhancers in mesenchyme-derived bone cells and to describe the epigenetic histone landscape that spans the Vdr locus. Using bacterial artificial chromosome-minigene stable cell lines, CRISPR/Cas9 enhancer-deleted daughter cell lines, transient transfection/mutagenesis analyses, and transgenic mice, we confirmed the functionality of these bone cell enhancers in vivo as well as in vitro. We also identified VDR-binding sites across the Vdr gene locus in kidney and intestine using ChIP-sequencing analysis, revealing that only one of the bone cell-type enhancers bound VDR in kidney tissue, and none were occupied by the VDR in the intestine, consistent with weak or absent regulation by the 1,25(OH)2D3 hormone in these tissues, respectively. However, a number of additional sites of VDR binding unique to either kidney or intestine were present further upstream of the Vdr gene, suggesting the potential for alternative regulatory loci. Importantly, virtually all of these regions retained histone signatures consistent with those of enhancers and exhibited unique DNase I hypersensitivity profiles that reflected the potential for chromatin access. These studies define mechanisms associated with hormonal regulation of the Vdr and hint at the differential nature of VDR binding activity at the Vdr gene in different primary target tissues in vivo.

Mouse and human BAC transgenes recapitulate tissue-specific expression of the vitamin D receptor in mice and rescue the VDR-null phenotype

The biological actions of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are mediated by the vitamin D receptor (VDR), which is expressed in numerous target tissues in a cell type-selective manner. Recent studies using genomic analyses and recombineered bacterial artificial chromosomes (BACs) have defined the specific features of mouse and human VDR gene loci in vitro. In the current study, we introduced recombineered mouse and human VDR BACs as transgenes into mice and explored their expression capabilities in vivo. Individual transgenic mouse strains selectively expressed BAC-derived mouse or human VDR proteins in appropriate vitamin D target tissues, thereby recapitulating the tissue-specific expression of endogenous mouse VDR. The mouse VDR transgene was also regulated by 1,25(OH)2D3 and dibutyryl-cAMP. When crossed into a VDR-null mouse background, both transgenes restored wild-type basal as well as 1,25(OH)2D3-inducible gene expression patterns in the appropriate tissues. This maneuver resulted in the complete rescue of the aberrant phenotype noted in the VDR-null mouse, including systemic features associated with altered calcium and phosphorus homeostasis and disrupted production of parathyroid hormone and fibroblast growth factor 23, and abnormalities associated with the skeleton, kidney, parathyroid gland, and the skin. This study suggests that both mouse and human VDR transgenes are capable of recapitulating basal and regulated expression of the VDR in the appropriate mouse tissues and restore 1,25(OH)2D3 function. These results provide a baseline for further dissection of mechanisms integral to mouse and human VDR gene expression and offer the potential to explore the consequence of selective mutations in VDR proteins in vivo.

The future of vitamin D analogs

The active form of vitamin D₃, 1,25-dihydroxyvitamin D₃, is a major regulator of bone and calcium homeostasis. In addition, this hormone also inhibits the proliferation and stimulates the differentiation of normal as well as malignant cells. Supraphysiological doses of 1,25-dihydroxyvitamin D₃ are required to reduce cancer cell proliferation. However, these doses will lead in vivo to calcemic side effects such as hypercalcemia and hypercalciuria. During the last 25 years, many structural analogs of 1,25-dihydroxyvitamin D₃ have been synthesized by the introduction of chemical modifications in the A-ring, central CD-ring region or side chain of 1,25-dihydroxyvitamin D₃ in the hope to find molecules with a clear dissociation between the beneficial antiproliferative effects and adverse calcemic side effects. One example of such an analog with a good dissociation ratio is calcipotriol (Daivonex®), which is clinically used to treat the hyperproliferative skin disease psoriasis. Other vitamin D analogs were clinically approved for the treatment of osteoporosis or secondary hyperparathyroidism. No vitamin D analog is currently used in the clinic for the treatment of cancer although several analogs have been shown to be potent drugs in animal models of cancer. Transcriptomics studies as well as in vitro cell biological experiments unraveled basic mechanisms involved in the antineoplastic effects of vitamin D and its analogs. 1,25-dihydroxyvitamin D₃ and analogs act in a cell type- and tissue-specific manner. Moreover, a blockade in the transition of the G0/1 toward S phase of the cell cycle, induction of apoptosis, inhibition of migration and invasion of tumor cells together with effects on angiogenesis and inflammation have been implicated in the pleiotropic effects of 1,25-dihydroxyvitamin D₃ and its analogs. In this review we will give an overview of the action of vitamin D analogs in tumor cells and look forward how these compounds could be introduced in the clinical practice.

Regulation of mouse Cyp24a1 expression via promoter-proximal and downstream-distal enhancers highlights new concepts of 1,25-dihydroxyvitamin D(3) action

CYP24A1 functions in vitamin D target tissues to degrade 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)). Thus, the concentration of this enzyme and the regulation of its expression is a primary determinant of the overall biological activity of 1,25(OH)(2)D(3) within cells. The principle regulator of CYP24A1 expression is 1,25(OH)(2)D(3) itself, which functions through the vitamin D receptor to upregulate the transcriptional activity of the Cyp24a1 gene. In this report, we explore the mechanism of this regulation using recently developed ChIP-chip and ChIP-seq techniques that permit an unbiased search for enhancer elements that participate in this transcriptional control. Our studies both confirm a regulatory region defined earlier and located proximal to the transcriptional start site (TSS) of mouse Cyp24a1 (-160 and -265nt) and identify a novel intergenic region located downstream of the transcription unit that contains two enhancers (+35 and +37kb) that facilitate 1,25(OH)(2)D(3)-dependent upregulation of Cyp24a1 expression. Interestingly, while C/EBPβ also binds under basal conditions to a site located immediately upstream of the Cyp24a1 promoter (-345nt), occupancy by this factor is strikingly increased following 1,25(OH)(2)D(3) treatment. The locations and activities of these regulatory regions that mediate 1,25(OH)(2)D(3) actions were confirmed in mice in vivo. We conclude that the mechanism through which 1,25(OH)(2)D(3) induces the CYP24A1 enzyme, thereby autoregulating its own destruction, involves both promoter-proximal as well as downstream-distal enhancers. These findings highlight new concepts regarding the molecular mechanism of action of 1,25(OH)(2)D(3) and other hormonal regulators.

Vitamin D analogs

Vitamin D has gone through a renaissance with the association of vitamin D deficiency with a wide array of common diseases including breast, colorectal and prostate cancers, cardio-vascular disease, autoimmune conditions and infections. Vitamin D analogs constitute a valuable group of compounds which can be used to regulate gene expression in functions as varied as calcium and phosphate homeostasis, as well as cell growth regulation and cell differentiation of a wide spectrum of cell types. This review will discuss the full range of vitamin D compounds currently available, some of their possible uses, and potential mechanisms of action.