Muscarinic cholinergic receptors activate both inhibitory and stimulatory growth mechanisms in NIH3T3 cells

Activation of G(q) protein-coupled receptors can either stimulate or inhibi t cell growth. Previously, these opposite effects were explained by differe nces in the cell models. Here we show that activation of m3 muscarinic acet ylcholine receptors ectopically expressed in NIH3T3 cells can cause stimula tion and inhibition of growth in the same cell. A clonal cell line was sele cted from cells that formed foci agonist dependently (3T3/m3 cells). In qui escent 3T3/m3 cells, carbachol stimulated DNA synthesis. In contrast, when 3T3/m3 cells were growing, either due to the presence of serum or after tra nsformation with oncogenic v-src, carbachol inhibited growth. This inhibiti on was not due to reduction of extracellular signal-regulated kinase activi ty because carbachol induced extracellular signal-regulated kinase phosphor ylation in both quiescent and growing 3T3/m3 cells, Investigating the cell cycle mechanisms involved in grow th inhibition, we found that carbachol tr eatment decreased cyclin D1 levels, increased p21(cip1) expression, and led to hypophosphorylation of the retinoblastoma gene product (Rb), Proteasome inhibitors blocked the carbachol-induced degradation of cyclin D1, Effects on p21(cip1) were blocked by a protein kinase C inhibitor. Thus, m3 muscar inic acetylcholine receptors couple to both growth-stimulatory and -inhibit ory signaling pathways in NIH3T3 cells, and the observed effects of recepto r activation depend on the context of cellular growth.