Regulation of the vitamin D receptor by dexamethasone
Hidalgo, Alejandro A.
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Calcitriol, the active metabolite of vitamin D plays an important role in calcium homeostasis. More recently, it has been described that calcitriol has a role in inducing cell proliferation, differentiation and apoptosis. Because of this discovery, calcitriol has been proposed as a therapy for cancer treatment and prevention. Pre-clinical evidences using in vitro and in vivo models of cancer indicate that calcitriol has potent anti-tumor effects, alone or in combination with cytotoxic or anti-mitotic drugs. One of the most interesting combinations is dexamethasone (Dex) and calcitriol. The synthetic glucocorticoid dexamethasone is a potent anti-inflammatory. Dex has anti-proliferative effects and is used to treat lymphoma and myeloma. In addition Dex is used as a hormone replacement treatment in patients treated with hormone ablation therapy. Another effect of Dex is that it decreases the absorption of calcium from the intestine; therefore it can be used to treat hypercalcaemia. Because a limiting factor in the treatment with calcitriol can be the induction of hypercalcaemia, the combination therapy of calcitriol with Dex is promising. The first section of this work, describes the history of vitamin D, the discovery of its metabolites, how it was defined as a hormone derived from cholesterol and the evidence that support the rationale to study vitamin D and cancer. The regulation of the vitamin D receptor signaling axis and how it cross-talks with the glucocorticoid receptor signaling axis is discussed, before presenting evidence that demonstrates how Dex increases the vitamin D receptor and calcitriol-mediated anti-proliferative effects. These observations lead to establishing the hypothesis that "Glucocorticoids increase calcitriol effects by increasing the expression of the vitamin D receptor." To test this hypothesis the effects of Dex in VDR-mediated effects were studied, including VDR-mediated transcription and growth inhibition. To further study how Dex modulates calcitriol effects, the synthesis of the VDR was studied in the presence of calcitriol, Dex and the combination. The ability of Dex to induce VDR expression was tested in vitro by cloning a region 5,2 kb upstream of Vdr gene that contains two putative glucocorticoids response elements (GRE), finding that the most distal GRE is transcriptionally active. The ability of Dex to induce transcription was confirmed using the chromatin immunoprecipitation assay that defines the most distal GRE as the active binding site for GR in this regulatory region of the Vdr gene. In summary, the Vdr gene was defined as a genomic target of glucocorticoids. This work ends discussing the nature of the new GRE described and how it compares to other active GREs and the classic consensus sequence. We also discuss possible feedback regulation between the calcitriol/VDR and the glucocorticoid/GR signaling pathways. Finally, some reflections are made regarding the methods of choice when looking at new regulatory sequences in the post-genomic era.