INDEPENDENT INDUCTION OF MORPHOLOGICAL TRANSFORMATION OF CHO CELLS BYRECEPTOR-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 s timulation of G protein-coupled receptors ectopically expressed in the se 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 transduct ion pathway initiates the progression from a tumorigenic to a non-tumo rigenic phenotype. Stimulation of CHO cells expressing the dopamine D1 receptor (CHOD1) with a D1 selective agonist, SKF38393, resulted in a n increase in cAMP accumulation which correlated with morphologic tran sformation. SKF38393 had no effect on intracellular calcium levels, ar guing against a requirement for phospholipase C or calcium mobilizatio n 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), respectivel y, did result in an increase in intracellular calcium and a morphology change. The time course of carbachol-stimulated calcium influx correl ated with the time course of morphological transformation, but not wit h carbachol-stimulated cAMP or inositol, 1,4,5-trisphosphate (IP3) acc umulation. Furthermore, no increase in cAMP accumulation was observed in AVP-stimulated CHOV1a cells, suggesting a cAMP-independent stimulat ion of the transformation process. Carbachol-stimulated CHO cells expr essing the m2 muscarinic receptor (CHOm2) failed to undergo a morpholo gical transformation, yet released IP3. Therefore, phospholipase C-med iated signal transduction is not sufficient for the morphological tran sformation of CHO cells. It appears that receptor-stimulated morpholog ic transformation of CHO cells can be induced via two independent sign aling pathways, mediated by adenylate cyclase or receptor-operated cal cium channels.