G proteins in rat liver proliferation during cholestasis.

Abstract

Liver proliferation appears to be dually regulated, in part by cyclic AMP levels. Here we studied the alterations in the stimulatory action of cholera toxin and other agents on the adenylyl cyclase system, as well as the status of Gs and Gi protein subunits during the liver proliferation that follows bile duct ligation in rats. The stimulatory effects of glucagon and vasoactive intestinal peptide (which act through membrane receptors) or guanosine 5'-[beta gamma-imido]triphosphate (which interacts with G proteins) and forskolin (which directly activates the adenylyl cyclase catalytic subunit) on liver adenylyl cyclase activity were blunted in cholestasis. The results indicated an impairment in the stimulatory interaction between the alpha s subunit of Gs protein and the adenylyl cyclase catalytic subunit. Indeed, we observed an important decrease in the stimulation of adenylyl cyclase activity by cholera toxin in cholestasis that was accompanied by a reduced extent of [32P]ADP ribosylation of alpha s protein catalyzed by cholera toxin, as revealed by the poor labeling of the 42,000 Da band in liver membranes from cholestatic rats. However, there was no change in the amount of alpha s or beta proteins as measured with immunoblotting techniques. Experiments on [32P]ADP ribosylation of alpha i subunits of Gi proteins indicated an impairment in liver membranes from cholestatic rats, whereas Western blotting for the detection of alpha i subunits showed decreased alpha i3 and increased alpha i2 levels in this condition. Further efforts are needed to better understand the molecular mechanisms underlying the relationship between the observed divergent expression of Gs and Gi proteins and liver cell proliferation in the cholestatic liver.