M1 muscarinic receptors block caspase activation by phosphoinositide 3-kinase- and MAPK/ERK-independent pathways

When PC12 cells are deprived of trophic support they undergo apoptosis. We have previously shown that survival of trophic factor-deprived PC12M1 cells can be promoted by activation of the G protein-coupled muscarinic receptor s, The mechanism whereby muscarinic receptors inhibit apoptosis is poorly u nderstood. In the present study we investigated this mechanism by examining the effect of muscarinic receptor activation on the serum deprivation-indu ced activity of key players in apoptosis, the caspases, in PC12M1 cells, Th e results showed that m1 muscarinic activation inhibits caspase activity in duced by serum deprivation. This effect appeared to be caused by the preven tion of activation of caspases such as caspase-2 and caspase-3, and not by the inhibition of existing activity. Muscarinic receptor activation also st imulated the mitogen-activated protein kinase/extracellular signaling-regul ated kinase (MAPK/ERK) and phosphoinositide (PI) 3-kinase signaling pathway s. The PI 3-kinase pathway inhibitors wortmannin and LY294002, as well as t he MAPK/ERK pathway PD98059 inhibitor, did not however suppress the inhibit ory effect of the muscarinic receptors on caspase activity. The results the refore suggested that the muscarinic survival effect is mediated by a pathw ay that leads to caspase inhibition by MAPK/ERK- and PI 3-kinase-independen t signaling cascades.