Defective extracellular calcium (Ca o)-sensing receptor (CaR)-mediated stimulation of a Ca 2+-activated potassium channel in glioblastoma cells transfected with a dominant negative CaR

Abstract

Glioblastoma cells exhibit several forms of sensitivity to extracellular calcium (Ca o) that might be conferred by the Ca o-sensing receptor (CaR) that is intimately involved in the maintenance of Ca o homeostasis by various cell types. This receptor is expressed in human glioblastoma cell line, U87, and here we show that CaR activators stimulate a Ca 2+-activated potassium (K +) channel (CAKC) with a conductance of 140 pS. The responses to CaR activators, however, were blunted in U87 cells transfected with a CaR bearing an inactivating mutation (R185Q) that has previously been shown to exert a dominant negative (DN) action on the wild type receptor. Raising Ca o from 0.75 to 2.0 mM or addition of a polycationic CaR agonist, each activated CAKC in nontransfected wild type and empty vector-transfected U87 cells, while they had little or no effect on channel activity in cells expressing the DN CaR (DN-CaR cells). In nontransfected wild type and empty vector-transfected cells, the specific ‘calcimimetic’ CaR activator, NPS R-467, stimulated the channel, while its less active stereoisomer, NPS S-467, did not. In DN-CaR cells, in contrast, NPS R-467, had no effect on channel activity, suggesting defective coupling of the CaR to this ion channel. CaR-mediated stimulation of these K + channels could lead to membrane repolarization and related changes in cellular function under normal conditions. Since the R185Q mutation in the CaR produces a more severe phenotype in humans than most inactivating mutations of this receptor, some of its clinical consequences could potentially result from abnormal CaR-dependent channel functioning.