The pleiotropic actions of vitamin D

Introduction

Vitamin D can be produced in adequate amounts by moderate exposure of skin to solar ultraviolet X rays. Exposure to sun­light remains an important source of vitamin D, as many people in northern countries become deficient in circulating 25(OH)D₃ during winter, and therefore deficient in 1,25(OH)₂D₃ synthe­sized in peripheral tissues.

Vitamin D has been widely known for decades for its primary physiological role in regulating calcium homeostasis. However, accumulating evidence from epidemiological, animal, cellular, biochemical and, most recently, molecular genetic studies has revealed new actions of vitamin D.

Vitamin D can regulate the proliferation and differentiation of a wide variety of cell types, which has led to the analysis of the potential therapeutic uses of its synthetic analogues as anti- cancer agents, and as modulators of immune and nervous sys­tem function. These lines of investigation have been accelerat­ed by two recent developments: the determination of the crys­tal structure of the vitamin D receptor and the use of large- scale gene expression profiling with microarrays to identify the molecular genetic events underlying vitamin D action. Here,we will focus on the impacts of recent experimental and technolog­ical advances on the potential uses of its analogues in cancer therapy and prevention, in the treatment of autoimmune disor­ders, and as neuroprotective agents.

Moreover, we also discuss the vitamin D status in inflammatory bowel disease (IBD), infectious diseases, asthma, diabetes mellitus type I and II.

Vitamin D receptor

By 1975, the presence of the vitamin D receptor (VDR) was confirmed in the nuclei of cells incubated with radiolabelled hormonal 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. The cDNA encoding the human vitamin D receptor (VDR) was cloned in 1988, and confirmed that, similar to other steroid receptors, it is a member of the nuclear receptor family. Nuclear receptors are ligand-activated regulators of gene tran­scription with a conserved domain structure. The highly conserved DNA-binding domain (DBD) contains two zinc fin­gers that form a single structural domain containing an a-heli- cal reading head that controls specific DNA sequence recogni­tion. The VDR ligand-binding domain (LBD) not only binds lig- and but also contains a ligand-regulated C-terminal AF-2 do­main (activating function-2) that is essential for its capacity to activate transcription. Similar to several nuclear receptors, the VDR functions as a heterodimer with members of the retinoid X receptor (RXR) family of receptors. Strong interactions be­tween VDR and RXR LBDs are essential for ligand-dependent dimerization and high-affinity DNA binding. Nuclear receptors regulate target gene transcription by ligand- controlled recruitment of several accessory proteins known col­lectively as coregulators. Coregulators are essential for the his- tone modifications, chromatin remodeling and recruitment of RNA polymerase and ancillary factors necessary for initiation of transcription. Nuclear receptors regulate transcription in part by binding specific DNA sequences called hormone response elements.
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