Modulation of 1,25-Dihydroxyvitamin D3 Signaling: Implications for Aging and Neuropsychiatric Disorders

The significance of hormonal vitamin D in the numerous facets of health stresses the importance of elucidating the molecular mechanism(s) associated with 1,25D-VDR signaling modulators (e.g., resveratrol and sirtuin-1). Resveratrol (Res), a natural antioxidant, is a potent activator of NAD-dependent deacetylase sirtuin-1 (SIRT-1), an enzyme associated with longevity in animal models. This present study employed mammalian 2-hybrid (M2H) and vitamin D responsive element (VDRE)-based transcriptional assays to investigate the potential effects of Res and SIRT-1 on VDR signal transduction. Results from VDRE-based assays indicate that Res and SIRT-1 potentiate 1,25D-VDR activity via cell-and-promoter-specific pathways. In addition, 1,25D displacement experiments revealed an increase in VDR-bound radiolabeled 1,25D in the presence of Res, suggesting that Res may potentiate VDR transactivation by stimulating 1,25D binding. M2H assays in HEK293 cells were then utilized to assess levels of interaction between VDR and VDR comodulators, including RXR, SRC-1, and DRIP-205. Both Res and SIRT-1 increased the ability of VDR to associate with RXR; however, SRC-1 and DRIP-205 interactions were not enhanced. The activity of a novel, non-acetylatable VDR mutant, K413R, was probed revealing that K413R possesses amplified transactivation capacity over wild-type VDR. A SIRT-1 inhibitor, EX-527, was used to suppress endogenous SIRT-1, resulting in significantly decreased VDR transactivation. Finally, qPCR results in HEK293 cells revealed that the 1,25D-mediated induction of CYP24A1, an endogenous VDR target gene, was enhanced (85%) by SIRT-1 while Res increased CYP24A1 expression by 294%. The combination of 1,25D, SIRT-1, and Res amplified CYP24A1 expression by 326% over 1,25D, although this effect did not reach statistical significance when compared to the Res only treated group. We conclude that acetylation of VDR comprises a negative feedback loop that attenuates 1,25D-VDR signaling. This loop is suppressed by resveratrol/SIRT-1-catalyzed deacetylation of VDR, restoring VDR activity. The two compounds, 1,25-dihydroxyvitamin D (1,25D, vitamin D) and 5-hydroxytryptamine (5-HT, serotonin), have been proposed to play a significant role in abnormal social behavior associated with psychological conditions including autism spectrum disorders (ASDs) and depression; however, the mechanism underlying these associations has yet to be elucidated. Deficiencies in 1,25D or 5-HT have been linked to the increased incidence of ASDs. Thus, examining the modulation of genes involved in 5-HT biosynthesis, reuptake, and degradation is fundamental in linking low 1,25D levels to the increased incidence of psychiatric disorders. We propose that 1,25D regulates tryptophan hydroxylase-2 (TPH2), the initial and rate-limiting enzyme in the biosynthetic pathway of 5-HT. In order to evaluate the regulation of TPH2 in neuronal cells, three formulations of media were examined to optimize the cell culture conditions necessary for growth and morphology of embryonic rat medullary raphe (B14) serotonergic neurons. Next, quantitative real time-PCR (qPCR) was utilized to examine TPH2 expression in cultured human glioblastoma (U-87) cells and rat serotonergic neurons (B-14). Human TPH2 mRNA in U-87 cells was induced dose-dependently resulting in a 2.4-fold increase at 10 nM 1,25D. Strikingly, TPH2 mRNA in B-14 cells was observed to be 26- to 86-fold upregulated at 10 nM 1,25D; however, 1 nM and 100 nM 1,25D elicited significantly smaller inductions (8-fold and 1.2-fold, respectively).