Natural Epigenetic Modulators of Vitamin D Receptor

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Abstract

Vitamin D plays an important role in every tissue due to its differentiating properties and the control of calcium homeostasis. The reversion of the epigenetic repression of the vitamin D receptor (VDR) could lead to an increased sensitivity of the cells to the beneficial activity of the hormone and could be exploited in many vitamin D-resistant diseases. In this study we analyzed the effects of three natural epigenetic modulators: sulforaphane, curcumin, and the products of the fermentative activity of probiotics. Sulforaphane and curcumin are inhibitors of the DNA methyltransferases (DNMT) and of the histone deacetylases (HDAC); it has been demonstrated that sulforaphane and curcumin increase VDR expression in intestinal epithelial cells and in a human liver cancer cell line, respectively. The anti-inflammatory properties associated with the probiotic administration in vivo can be linked to the increased activity of intestinal VDR. Butyrate, an inhibitor of HDAC and a known modulator of VDR expression, is the candidate byproduct of fermentation by gut microbiome that could mediate the enhanced expression of VDR triggered by probiotics in vivo. Many other natural compounds wait to be investigated and recognized as epigenetic modulators of VDR, thus opening promising therapeutic avenues for many diseases by natural means.

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Identification of Nrf2-regulated genes induced by the chemopreventive agent sulforaphane by oligonucleotide microarray.

Linh Mai, Sorachai Srisuma, Thomas W. Kensler … (2002)

Electrophiles formed during metabolic activation of chemical carcinogens and reactive oxygen species generated from endogenous and exogenous sources play a significant role in carcinogenesis. Cancer chemoprevention by induction of phase 2 proteins to counteract the insults of these reactive intermediates has gained considerable attention. Nuclear factor E2 p45-related factor 2 (Nrf2), a bZIP transcription factor, plays a central role in the regulation (basal and or inducible expression) of phase 2 genes by binding to the "antioxidant response element" in their promoters. Identification of novel Nrf2-regulated genes is likely to provide insight into cellular defense systems against the toxicities of electrophiles and oxidants and may define effective targets for achieving cancer chemoprevention. Sulforaphane is a promising chemopreventive agent that exerts its effect by strong induction of phase 2 enzymes via activation of Nrf2. In the present study, a transcriptional profile of small intestine of wild-type (nrf2 +/+) and knock out (nrf2 -/-) mice treated with vehicle or sulforaphane (9 micromol/day for 1 week, p.o.) was generated using the Murine Genome U74Av2 oligonucleotide array (representing approximately 6000 well-characterized genes and nearly 6000 expressed sequence tags). Comparative analysis of gene expression changes between different treatment groups of wild-type and nrf2-deficient mice facilitated identification of numerous genes regulated by Nrf2 including previously reported Nrf2-regulated genes such as NAD(P)H:quinone reductase (NQO1), glutathione S-transferase (GST), gamma-glutamylcysteine synthetase (GCS), UDP-glucuronosyltransferases (UGT),epoxide hydrolase, as well as a number of new genes. Also identified were genes encoding for cellular NADPH regenerating enzymes (glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and malic enzyme), various xenobiotic metabolizing enzymes, antioxidants (glutathione peroxidase, glutathione reductase, ferritin, and haptaglobin), and biosynthetic enzymes of the glutathione and glucuronidation conjugation pathways. The data were validated by Northern blot analysis and enzyme assays of selected genes. This investigation expands the horizon of Nrf2-regulated genes, highlights the cross-talk between various metabolic pathways, and divulges the pivotal role played by Nrf2 in regulating cellular defenses against carcinogens and other toxins.

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Effects of dietary curcumin on glutathione S-transferase and malondialdehyde-DNA adducts in rat liver and colon mucosa: relationship with drug levels.

Frances M. Williams, A. Gescher, C Leuratti … (2001)

Curcumin prevents colon cancer in rodent models. It inhibits lipid peroxidation and cyclooxygenase-2 (COX-2) expression and induces glutathione S-transferase (GST) enzymes. We tested the hypothesis that 14 days of dietary curcumin (2%) affects biomarkers relevant to cancer chemoprevention in the rat. Levels of inducible COX-2, as reflected by prostaglandin E(2) production by blood leukocytes, were measured ex vivo. Total GST activity and adducts of malondialdehyde with DNA (M(1)G), which reflect endogenous lipid peroxidation, were measured in colon mucosa, liver, and blood leukocytes. Curcumin and its metabolites were analyzed by high-performance liquid chromatography in plasma, and its pharmacokinetics were compared following a diet containing 2% curcumin versus intragastric (i.g.) administration of curcumin suspended in an amphiphilic solvent. The curcumin diet did not alter any of the markers in the blood but increased hepatic GST by 16% and decreased colon M(1)G levels by 36% when compared with controls. Administration of carbon tetrachloride during the treatment period increased colon M(1)G levels, and this increase was prevented by dietary curcumin. Dietary curcumin yielded low drug levels in the plasma, between 0 and 12 nM, whereas tissue concentrations of curcumin in liver and colon mucosa were 0.1--0.9 nmol/g and 0.2--1.8 micromol/g, respectively. In comparison with dietary administration, suspended curcumin given i.g. resulted in more curcumin in the plasma but much less in the colon mucosa. The results show that curcumin mixed with the diet achieves drug levels in the colon and liver sufficient to explain the pharmacological activities observed and suggest that this mode of administration may be preferable for the chemoprevention of colon cancer.

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Up-regulation of the vitamin D receptor in response to 1,25-dihydroxyvitamin D3 results from ligand-induced stabilization.

A M Uhland, Nathan Ross, J Prahl … (1992)

Several studies have shown that the 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) receptor protein levels increase in response to 1,25-(OH)2D3. We have studied the mechanism of this regulation in both mouse fibroblasts and rat intestinal epithelial cells. Cell extracts and total RNA were prepared at varying times after addition of 1,25-(OH)2D3. The 1,25-(OH)2D3 receptor protein levels, measured using an immunoradiometric assay, rose significantly 2-3 h posttreatment and had risen 3-fold at 8 h. Concurrently, the 1,25-(OH)2D3 receptor mRNA content, measured using a ribonuclease protection assay, was not altered by 1,25-(OH)2D3 during this time. In cycloheximide-blocked cells, the administration of 1,25-(OH)2D3 markedly reduced the degradation rate of previously formed receptor. The 1,25-(OH)2D3 receptor protein half-life was determined as 4 h in the absence of 1,25-(OH)2D3 and increased to at least 8 h in the presence of 1,25-(OH)2D3. We also measured the 1,25-(OH)2D3 receptor mRNA levels in the duodena and kidney of vitamin D-deficient rats after a single 150-pmol injection of 1,25-(OH)2D3. Again, we found that 1,25-(OH)2D3 receptor mRNA levels were not changed in these tissues after 1,25-(OH)2D3 treatment. Therefore, the elevation of the 1,25-(OH)2D3 receptor protein following 1,25-(OH)2D3 administration is apparently the result of increased receptor protein lifetime and not increased transcription.