Strong and Persistent Activation of Inositol Lipid Breakdown Induces Early Mitogenic Events but Not Go to S Phase Progression in Hamster Fibroblasts COMPARISON OF THROMBIN AND CARBACHOL ACTION IN CELLS EXPRESSING Ml MUSCARINIC

In order to evaluate the role of phosphoinositide turnover in growth factor action, we expressed human Ml muscarinic acetylcholine (Hml) receptors in Chinese hamster lung fibroblasts (CCL39 cell line). In the transfected cells (39Ml-81 clone), but not in wild type fibroblasts, the muscarinic agonist carbachol induced a release of inositol phosphates as strong as (Ythrombin, a very potent growth factor and activator of phosphoinositide-specific phospholipase C (PLC) in this cell system. In contrast to thrombin, carbacholstimulated PLC activity was not inhibited by pertussis toxin treatment of cells. At concentrations that elicited a comparable initial rate of inositol phosphate release (10 nM for thrombin and 0.1 mM for carbachol), both agents gave rise to an identical calcium signal and equally stimulated Na+/H’ exchange and the transcription of the early genes c-jun, c-fos, and c-myc. Surprisingly, however, carbachol is not a mitogen for 39Ml-81 cells, and even if tested in association with insulin or fibroblast growth factor, its effects on cell proliferation remained weak when compared with thrombin. Also, the muscarinic agonist did not stimulate soft agar colony forming capacity and did not prevent growth arrest in Go upon serum deprivation of cycling 39Ml-81 cells. The failure of carbachol to induce cell proliferation could not be attributed to rapid and complete desensitization of Hml receptors nor to the activation of inhibitory pathways like adenylyl cyclase stimulation. We conclude that strong and persistent activation of phosphoinositide turnover elicits early biochemical events generally associated with mitogenesis, but is not sufficient to stimulate or maintain continuous cell proliferation. On the basis of our results, we postulate that thrombin mitogenesis depends critically on signaling events different from phosphoinositide turnover, possibly the stimulation of a receptor tyrosine kinase or a Gi protein-activated tyrosine kinase.