Involvement of protein kinase C-alpha and -epsilon in extracellular Ca(2+) signalling mediated by the calcium sensing receptor.

The sensing of extracellular Ca(2+) concentration (Ca(2+)) and modulation of cellular processes associated with acute or sustained changes in Ca(2+) are cell-type specific and mediated by the calcium sensing receptor (CaR). Ca(2+) signalling requires protein kinase C (PKC), but the identity and role of PKC isoforms in CaR-mediated responses remain unclear. Here we show that high Ca(2+) activated PKC-alpha and PKC- in parathyroid cells and in human embryonic kidney (HEK293) cells overexpressing the CaR (HEK-CaR) and that this response correlated with the CaR-dependent activation of mitogen-activated protein kinases ERK1/2. Activation of ERK1/2 by acute high Ca(2+) required influx of Ca(2+)through Ni(2+)-sensitive Ca(2+)channels and phosphatidylinositol-dependent phospholipase C-beta activity. Inhibition of PKC by co-expression of dominant-negative (DN) mutants of PKC-alpha or - with the CaR attenuated sustained ERK1/2 activation. Overexpression of a PKC phosphorylation site (T888A) mutant CaR in HEK293 cells showed that this site was important for ERK1/2 activation at high Ca(2+). Activation of ERK1/2 by high Ca(2+) was not necessary for the Ca(2+)-regulated secretion of parathyroid hormone (PTH) in dispersed bovine parathyroid cells. These data suggest that the CaR-mediated Ca(2+) signal leading to regulated PTH secretion that requires diacylglycerol-responsive PKC isoforms is not mediated via the ERK pathway.