Signalling of the M3-muscarinic receptor to the anti-apoptotic pathway

The process of programmed cell death (or apoptosis) occurs widely in tissue maintenance and embryonic development, and is under tight regulatory control. It is now clear that one of the important regulators of apoptosis are G-protein-coupled receptors. In the present study, we investigate the regulatory mechanism employed by the G(q/11)-coupled M-3-muscarinic receptor in mediating an anti-apoptotic response. Using a CHO (Chinese-hamster ovary) cell model, we demonstrate that the M-3-muscarinic receptor antiapoptotic response is independent of calcium/phospholipase C signalling. This response can, however, be inhibited by the transcriptional inhibitor actinomycin D at a concentration that inhibits the rapid increase in gene transcription mediated by M-3-muscarinic receptor stimulation. Furthermore, apoptosis in CHO cells induced by the DNA-damaging agent, etoposide, is associated with a fall in the levels of the anti-apoptotic Bcl-2 protein. This fall in Bcl-2 protein concentration can be attenuated by M-3-muscarinic receptor stimulation. We conclude, therefore, that the M-3-muscarinic receptor signals to the anti-apoptotic pathway via a mechanism that is independent of calcium/phospholipase C signalling, but in a manner that involves both gene transcription and the up-regulation of Bcl-2 protein.