CYTOSOLIC PHOSPHOLIPASE A(2) IS COUPLED TO MUSCARINIC RECEPTORS IN THE HUMAN ASTROCYTOMA CELL-LINE 1321N1 - CHARACTERIZATION OF THE TRANSDUCING MECHANISM

The cholinergic agonist carbachol induced the release of arachidonic a cid in the 1321N1 astrocytoma cell line, and this was blocked by atrop ine, suggesting the involvement of muscarinic receptors. To assess the mechanisms of signalling involved in the response to carbachol, a set of compounds characterized by eliciting responses through different m echanisms was tested. A combination of 4 beta-phorbol 12 beta-myristat e 13 alpha-acetate and thapsigargin, an inhibitor of endomembrane Ca2- ATPase that induces a prolonged elevation of cytosolic Ca2+ concentr ation, induced an optimal response, suggesting at first glance that bo th protein kinase C (PKC) and Ca2+ mobilization were involved in the r esponse. This was consistent with the observation that carbachol elici ted Ca2+ mobilization and PKC-dependent phosphorylation of cytosolic p hospholipase A(2) (cPLA(2); phosphatide sn-2-acylhydrolase, EC 3.1.1.4 ) as measured by a decrease in electrophoretic mobility. Nevertheless, the release of arachidonate induced by carbachol was unaltered in med ia containing decreased concentrations of Ca2+ or in the presence of n eomycin, a potent inhibitor of phospholipase C which blocks phosphoino sitide turnover and Ca2+ mobilization. Guanosine 5'-[gamma-thio]tripho sphate added to the cell-free homogenate induced both [H-3]arachidonat e release and cPLA(2) translocation to the cell membrane fraction in t he absence df Ca2+ thus suggesting the existence of an alternative mec hanism of cPLA(2) translocation dependent on G-proteins and independen t of Ca2+ mobilization. From the combination of experiments utilizing biochemical and immunological tools the involvement of cPLA(2) was asc ertained. In summary, these data indicate the existence in the astrocy toma cell line 1321N1 of a pathway involving the cPLA(2) which couples the release of arachidonate to the occupancy of receptors for a neuro transmitter, requires PKC activity and G-proteins and might operate in the absence of Ca2+ mobilization.