Vitamin D (VD) is essential for bone health, and VD or its analogues are widely used in clinics to ameliorate bone loss. The targets and mode of VD anti-osteoporotic actions appear to be different from those of other classes of drugs modulating bone remodeling. VD exerts its biological activities through the nuclear VD receptor (VDR)-mediated transcriptional regulation of target mRNA and non-coding RNA genes. VD-induced gene regulation involves epigenetic modifications of chromatin conformation at the target loci as well as reconfiguration of higher-order chromosomal organization through VDR-mediated recruitment of various regulatory factors. Enhancer RNAs (eRNA), a class of non-coding enhancer-derived RNAs, have recently emerged as VDR target gene candidates that act through reorganization of chromatin looping to induce enhancer-promoter interaction in activation of mRNA-encoding genes. This review outlines the molecular mechanisms of VD actions mediated by the VDR and suggests novel function of eRNAs in VDR transactivation.
Bone Remodeling ; Chromatin ; Epigenomics ; Metabolism ; Receptors, Calcitriol ; RNA ; RNA, Messenger ; RNA, Untranslated ; Transcriptional Activation ; Vitamin D ; Vitamins

Vitamin D (VD) exerts a wide variety of biological actions in addition to its well-known roles in calcium homeostasis. Nutritional VD deficiency induces rachitic abnormalities in growing children and osteomalacia in adults, and it has been proposed to underlie the onset and development of multiple non-communicable chronic diseases. Therefore, the administration of VD or synthetic VD analogues represents a promising therapeutic strategy; indeed, VD and a VD agonist have shown clinical promise in mitigating osteoporosis and symptoms of insufficient calcium intake. However, even though high doses of VD analogues have shown pre-clinical efficacy against several diseases, including cancers, they have not yet had wide-spread clinical success. This difference may be due to limitation of clinical doses in light of the inherent calcemic action of VD. An approach to overcome this problem involves the development of VD analogues with lower calcemic activity, which could be administered in high doses to attenuate the onset and progress of disease. In a similar strategy, selective estrogen receptor modulators have had success as anti-osteoporosis drugs, and they have shown benefit for other estrogen target organs by serving as partial antagonists or agonists of estrogen receptor α. It is thus conceivable to generate synthetic partial antagonists or agonists for the VD receptor (VDR) that would exert beneficial effects on bone and other VD target organs. In this review, we discuss the molecular basis of the development of such synthetic VDR ligands from the viewpoint of roles of VDR in gene regulation.