Differential mechanisms of inositol phosphate generation at the receptors for bombesin and platelet-derived growth factor.

We investigated the mechanism(s) whereby activation of a growth-factor receptor typically endowed with tyrosine kinase activity, such as the platelet-derived growth factor (PDGF) receptor, triggers phosphoinositide hydrolysis. In Swiss 3T3 cells permeabilized with streptolysin O, an analogue of GTP, guanosine 5'-[gamma-thio]triphosphate, was found to potentiate the coupling of the bombesin receptor to phospholipase C. In contrast, the activation of the enzyme by PDGF occurred in a GTP-independent manner. Moreover, the inactive analogue of GTP, guanosine 5'-[beta-thio]diphosphate, significantly inhibited the bombesin-induced InsP3 generation, whereas it did not decrease the same effect when stimulated by PDGF.