Independent induction of morphological transformation of CHO cells by receptor-activated cyclic AMP synthesis or by receptor-operated calcium influx

Morphological transformation of Chinese hamster ovary (CHO) cells can be induced by exogenous addition of cyclic AMP (cAMP) or through the stimulation of G protein-coupled receptors ectopically expressed in these cells. The morphological transformation has been shown to represent a phenotypic suppres sion of CHO cell tumorigenic potential. Studies were undertaken to determine which receptor-activated signal transduction pathway initiates the progression from a tumorigenic to a non-tumorigenic phenotype. Stimulation of CHO cells expressing the dopamine D1 receptor (CHOD1) with a D1 selective agonist, SKF38393, resulted in an increase in cAMP accumulation which correlated with morphologic transformation. SKF38393 had no effect on intracellular calcium levels, arguing against a requirement for phospholipase C or calcium mobilization in the D1-stimulated morphology change. Tn contrast, stimulation of muscarinic m5 (CHOm5) or vasopressin V1a (CHOV1a) receptors expressed in CHO cells with carbachol or arginine vasopressin (AVP), respectively, did result in an increase in intracellular calcium and a morphology change. The time course of carbachol-stimulated calcium influx correlated with the time course of morphological transformation, but not with carbachol-stimulated cAMP or inositol, 1,4,5-trisphosphate (IP3) accumulation. Furthermore, no increase in cAMP accumulation was observed in AVP-stimulated CHOV1a cells, suggesting a cAMP-independent stimulation of the transformation process. Carbachol-stimulated CHO cells expressing the m2 muscarinic receptor (CHOm2) failed to undergo a morphological transformation, yet released IP3. Therefore, phospholipase C-mediated signal transduction is not sufficient for the morphological transformation of CHO cells. It appears that receptor-stimulated morphologic transformation of CHO cells can be induced via two independent signaling pathways, mediated by adenylate cyclase or receptor-operated calcium channels.