BIPHASIC REGULATION BY N-6,2'-O-DIBUTYRYL ADENOSINE 3',5'-CYCLIC-MONOPHOSPHATE (DBCAMP) OF STEROID 21-HYDROXYLASE ACTIVITY IN RAT HEPATOCYTES

Steroid 21-hydroxylase activity has been identified in many tissues, i ncluding liver. But it is possible that the enzyme found in the liver is different from adrenal 21-hydroxylase. In the adrenal cortex, stero id 21-hydroxylase activity is increased by corticotropin (ACTH); the e ffect of ACTH is mediated by cyclic AMP (cAMP), and presumably involve s a cAMP-dependent protein kinase (PKA). It is not yet clear, however, how extra-adrenal steroid 21-hydroxylase activity is regulated. In th e present study, we examined the effect of N-6,2'-O-dibutyryl adenosin e 3',5'-cyclic monophosphate (dbcAMP), forskolin, N-[2-(methylamino)et hyl]5-isoquinolinesulfonamide (H-8) and 12-O-tetradecanoylphorbol-13-a cetate (TPA) on steroid 21-hydroxylase activity in primary cultures of rat hepatocytes to determine the nature of regulation of extra-adrena l steroid 21-hydroxylase activity. Steroid 21-hydroxylase activity in hepatocytes incubated with 10(-11) M dbcAMP for 24 h was 1.6 times hig her than that in control hepatocytes untreated with dbcAMP. On the oth er hand, steroid 21-hydroxylase activity decreased by 20 and 50% when the cells were incubated with 10(-5) and 10(-3) M dbcAMP, respectively . The stimulatory effect of 10(-11) M dbcAMP was not blocked by 10(-5) M H-8 (PKA inhibitor), but the inhibitory effect of 10(-5) or 10(-3) M cAMP was. TPA did not alter the activity of steroid 21-hydroxylase. These findings indicate that the steroid 21-hydroxylase in rat liver i s regulated by mechanisms different from those in the adrenal glands.