Involvement of protein kinase C-alpha and -epsilon in extracellular Ca signalling mediated by the calcium sensing receptor

The sensing of extracellular Ca²⁺ concentration (Ca²⁺]_o) and modulation of cellular processes associated with acute or sustained changes in Ca²⁺]_o are cell-type specific and mediated by the calcium sensing receptor (CaR). Ca²⁺]_o 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 Ca²⁺]_o activated PKC-α and PKC-ε 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 Ca²⁺]_o required influx of Ca²⁺through Ni²⁺-sensitive Ca²⁺channels and phosphatidylinositol-dependent phospholipase C-β activity. Inhibition of PKC by co-expression of dominant-negative (DN) mutants of PKC-α or -ε 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 Ca²⁺]_o. Activation of ERK1/2 by high Ca²⁺]_o was not necessary for the Ca²⁺]_o-regulated secretion of parathyroid hormone (PTH) in dispersed bovine parathyroid cells. These data suggest that the CaR-mediated Ca²⁺]_o signal leading to regulated PTH secretion that requires diacylglycerol-responsive PKC isoforms is not mediated via the ERK pathway.