Inhibitory effect of 1alpha-hydroxyvitamin D3 on N-methyl-N'-nitro-N-nitrosoguanidine-induced gastrointestinal carcinogenesis in Wistar rats

Pathogenic roles of alterations in vitamin D and vitamin D receptor in gastric tumorigenesis

Gastric cancer is currently the second leading cause of cancer-related death worldwide, especially in Japan, Korea and China, and the 5-year survival rate of gastric cancer is less than 30%. Thus, it is important to shed more lights on novel agents to prevent gastric cancer or to improve survival rate of the patients. Vitamin D not only maintains calcium and bone homeostasis, but also mostly inhibits tumor genesis, invasion, and metastasis through activation of vitamin D receptor. Although epidemiological results are not consistent, accumulating evidence from gastric cancer cells, animal models, and clinical trials suggest that vitamin D deficiency may increase the risk and mortality of gastric cancer, but vitamin D supplement might be a safe and economical way to prevent or treat gastric cancer. Here, we reviewed the current studies on vitamin D and its receptor and focused on the pathogenic roles of their alterations in gastric tumorigenesis.

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.

A positive feedback signaling loop between ATM and the vitamin D receptor is critical for cancer chemoprevention by vitamin D

Both epidemiologic and laboratory studies have shown the chemopreventive effects of 1α,25-dihydroxyvitamin D(3) (1,25-VD) in tumorigenesis. However, understanding of the molecular mechanism by which 1,25-VD prevents tumorigenesis remains incomplete. In this study, we used an established mouse model of chemical carcinogenesis to investigate how 1,25-VD prevents malignant transformation. In this model, 1,25-VD promoted expression of the DNA repair genes RAD50 and ATM, both of which are critical for mediating the signaling responses to DNA damage. Correspondingly, 1,25-VD protected cells from genotoxic stress and growth inhibition by promoting double-strand break DNA repair. Depletion of the vitamin D receptor (VDR) reduced these genoprotective effects and drove malignant transformation that could not be prevented by 1,25-VD, defining an essential role for VDR in mediating the anticancer effects of 1,25-VD. Notably, genotoxic stress activated ATM and VDR through phosphorylation of VDR. Mutations in VDR at putative ATM phosphorylation sites impaired the ability of ATM to enhance VDR transactivation activity, diminishing 1,25-VD-mediated induction of ATM and RAD50 expression. Together, our findings identify a novel vitamin D-mediated chemopreventive mechanism involving a positive feedback loop between the DNA repair proteins ATM and VDR.

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.

Association of genetic polymorphisms in vitamin D receptor gene and susceptibility to sporadic prostate cancer

Genetic and environmental factors are involved in prostate cancer (PCa) etiology. Single nucleotide polymorphisms (SNPs) may contribute to the PCa pathogenesis. The goal of this study is to determine the role of vitamin D receptor (VDR) gene polymorphisms and haplotypes in the development and progression of sporadic PCa. One hundred and thirty-three PCa patients and 157 age-matched healthy controls were genotyped for the Apa I (rs7975232), Bsm I (rs1544410) and Taq I (rs731236) polymorphisms in VDR gene by using polymerase chain reaction-restriction fragment length polymorphism. An association was observed between the Apa I polymorphism and PCa predisposition (P = 0.03). When compared with AA genotype, there was a highly notable difference in the frequencies of the Aa (P = 0.02), aa (P = 0.026) and Apa I ''a'' allele carriers (Aa + aa) (P = 0.009) genotypes. Furthermore, we found a statistical difference in the allele frequencies of the Apa I polymorphism between the sporadic PCa patients and control subjects (P = 0.013). The genotype distribution for the Bsm I and Taq I polymorphisms were similar between cases and controls (P > 0.05). No clinically significant relationship was found between the three-locus haplotypes and development of sporadic PCa. The genotype frequencies for the three polymorphisms of the VDR gene within subgroups of PCa (defined by tumor stage, Gleason score, PSA levels) were also analyzed, but no statistically noteworthy difference was observed (P > 0.05). As far as we know, this is the first study which investigates the relationship between VDR genotypes and sporadic PCa in the Turkish population. Our findings suggest that the VDR ApaI (rs7975232) polymorphism may play a role in the development of sporadic PCa.

The pleiotropic actions of vitamin D

General knowledge of the role of vitamin D3 in human physiology has been shaped by its discovery as a preventive agent of nutritional rickets, a defect in bone development due to inadequate uptake of dietary calcium. Studies on the function of the hormonal form of vitamin D3, 1alpha,25-dihydroxyvitamin D3, have been greatly accelerated by the molecular cloning and structural analysis of the vitamin D3 receptor, which is a ligand-activated regulator of gene transcription. Molecular genetic techniques including genomics have helped reveal that 1alpha,25-dihydroxyvitamin D3 can control more than calcium homeostasis. It has widespread effects on cellular differentiation and proliferation, and can modulate immune responsiveness, and central nervous system function. Moreover, accumulating epidemiological and molecular evidence suggests that 1alpha,25-dihydroxyvitamin D3 acts as a chemopreventive agent against several malignancies including cancers of the prostate and colon. Here, we survey the most-recent findings and discuss their implications for the potential therapeutic uses of vitamin D analogues.

Expression profiling in squamous carcinoma cells reveals pleiotropic effects of vitamin D3 analog EB1089 signaling on cell proliferation, differentiation, and immune system regulation

The active form of vitamin D3, 1alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3] is key mediator of calcium homeostasis and is a component of the complex homeostatic system of the skin. 1,25-(OH)2D3 regulates cellular differentiation and proliferation and has broad potential as an anticancer agent. Oligonucleotide microarrays were used to assess profiles of target gene regulation at several points over a 48 h period by the low calcemic 1,25-(OH)2D3 analog EB1089 in human SCC25 head and neck squamous carcinoma cells. One hundred fifty-two targets were identified, composed of 89 up- and 63 down-regulated genes distributed in multiple profiles of regulation. Results are consistent with EB1089 driving SCC25 cells toward a less malignant phenotype, similar to that of basal keratinocytes. Targets identified control inter- and intra-cellular signaling, G protein-coupled receptor function, intracellular redox balance, cell adhesion, and extracellular matrix composition, cell cycle progression, steroid metabolism, and more than 20 genes modulating immune system function. The data indicate that EB1089 performs three key functions of a cancer chemoprevention agent; it is antiproliferative, it induces cellular differentiation, and has potential genoprotective effects. While no evidence was found for gene-specific differences in efficacy of 1,25-(OH)2D3 and EB1089, gene regulation by 1,25-(OH)2D3 was generally more transient. Treatment of cells with 1,25-(OH)2D3 and the cytochrome P450 inhibitor ketoconazole produced profiles of regulation essentially identical to those observed with EB1089 alone, indicating that the more sustained regulation by EB1089 was due to its resistance to inactivation by induced 24-hydroxylase activity. This suggests that differences in action of the two compounds arise more from their relative sensitivities to metabolism than from differing effects on VDR function.

Cancer chemoprevention using natural vitamin D and synthetic analogs

Substantial epidemiologic data support a role for vitamin D in cancer prevention. However, dose-limiting hypercalcemic effects have proved a major obstacle to the development of natural vitamin D as a cancer chemopreventive. Structure-activity studies have sought to disassociate the toxicities and chemopreventive activities of vitamin D, and a number of synthetic deltanoids (vitamin D analogs) have shown considerable promise in this regard. Several such compounds have chemopreventive efficacy in preclinical studies, as does natural vitamin D. Data supporting further development of agents of this class include in vitro and in vivo evidence of antiproliferative, proapoptotic, prodifferentiating and antiangiogenic activities. Ongoing studies are aimed at further defining the molecular mechanisms through which vitamin D and synthetic deltanoids affect gene expression and cellular fate. Additional efforts are focused on establishing the chemopreventive index (efficacy vs toxicity) of each synthetic deltanoid.