Mitogenic signalling in murine 3T3 cells: the cyclic AMP pathway.

Treatment of isolated hepatocyte plasma membranes with cholera toxin caused the ADP-ribosylation of a variety of proteins, including a-G, [ 1x1. The ribosylation o f one of these, a 25 kDa protein is attenuated by the addition of insulin [ 181. This led us to suggest that a cholera toxin-sensitive G-protein may be involved in mediating certain of insulin's actions. In support of this, we have shown that insulin appears to stimulate a GTPase activity in human platelets that is modified by cholera toxin [ 191. However, further resolution of such a species demands a suitable reconstitution system. Insulin also appears to affect the ability of cholera toxin t o activate adenylate cyclasc in intact hepatocytes. Thus, although treatment of intact hepatocytes with cholera toxin caused the ADP-ribosylation o f a-G,, and thus the constitutive activation of adenylate cyclase, if insulin was also added then the ability of cholera toxin to activate adenylatc cyclase was dramatically reduced [20]. This was not due to insulin affecting the ability of a-G, to be ADP-ribosylated by cholera toxin, but rather appeared to be an attenuating action exerted either at the function of adenylate cyclase itself o r the functioning of ADP-ribosylated a-G,.