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 G s and G i 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′‐[βγ‐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 α s subunit of G s 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 [ 32 P]ADP ribosylation of α 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 α s or β proteins as measured with immunoblotting techniques. Experiments on [ 32 P]ADP ribosylation of α i subunits of G i proteins indicated an impairment in liver membranes from cholestatic rats, whereas Western blotting for the detection of α i subunits showed decreased α i3 and increased α i2 levels in this condition. Further efforts are needed to better understand the molecular mechanisms underlying the relationship between the observed divergent expression of G s and G i proteins and liver cell proliferation in the cholestatic liver. (Hepatology 1994;20:1041–1047).