Extracellular calcium-sensing receptor activation induces vitamin D receptor levels in proximal kidney HK-2G cells by a mechanism that requires phosphorylation of p38alpha MAPK.

In hypocalcaemia, elevated parathyroid hormone transitorily down-regulates the kidney vitamin D receptor, which returns to normal levels with the rise in serum extracellular calcium Ca(2+). In this study, we investigated the mechanism that underlies VDR increase in kidney in association with elevated Ca(2+). Examination of MAP kinase signals in a proximal tubule human kidney (HK-2G) epithelial cell line showed that treatment of Ca(2+) in the culture medium elevated phosphorylation of both ERK and p38 MAPKs. Blockade of p38 phosphorylation with SB203580 or SB202190 in turn abolished Ca(2+)-mediated VDR protein increase, while treatment with PD98059 and U0126, specifically blocked ERK phosphorylation, but had no effect on VDR stimulation by Ca(2+). Furthermore, SB203580 treatment potently repressed Ca(2+)-mediated activation of VDR promoter. We also demonstrate that si-RNA knock down of p38alpha completely diminished high Ca(2+)-mediated VDR induction. Direct CaSR involvement was demonstrated by using an si-RNA of CaSR that impeded Ca(2+)-mediated induction of VDR. In conclusion, a high extracellular Ca(2+) concentration in the physiological range is capable of directly increasing renal proximal VDR expression, and the induction mechanism requires activation of the CaSR and signal mediation by the p38alpha MAP kinase pathway.