A role for src tyrosine kinase in regulating adrenal aldosterone production

Adrenal aldosterone synthesis is influenced by a variety of factors. The major physiological regulators of aldosterone production are angiotensin II (Ang II) and potassium (K⁺). Ang II stimulates aldosterone production through the activation of multiple intracellular signaling pathways. It has recently been demonstrated that Ang II activates src tyrosine kinases in vascular smooth muscles cells. The src family of tyrosine kinases are widely distributed non-receptor kinases that influence several signal transduction pathways. In the present study we evaluated the effect of a selective src family inhibitor, PP2, on aldosterone production using a human adrenocortical carcinoma-derived (H295R) cell line. Treatments for 6 or 48 h with PP2 (0·3 μM-10 μM) inhibited basal, Ang II, K⁺ and dibutyryladenosine cyclic monophosphate (db-cAMP) stimulation of aldosterone production in a concentration-dependent manner. PP2 did not affect cell viability at any of the concentrations tested. Moreover, time course studies using PP2 (10 υM) for 6, 12, 24, and 48 h revealed a time-dependent inhibition of aldosterone production. Inhibition by PP2 (0·3-10 μM) was also observed for the metabolism of 22R-hydroxycholesterol (22R-OHChol) to aldosterone in H295R cells. Since 22R-OHChol is a substrate for cytochrome P450 side-chain cleavage enzyme (CYP11A) that does not require steroidogenic acute regulatory (StAR) protein for transport to the inner mitochondrial membrane, these results suggest that PP2 inhibition occurred beyond the rate-limiting step in aldosterone synthesis. Genistein, a non-specific tyrosine kinase inhibitor also blocked aldosterone production, but the inhibition was the result of a non-specific effect on 3β-hydroxysteroid dehydrogenase (3βHSD). In contrast, PP2 did not appear to act as a direct inhibitor of 3βHSD activity. To further investigate the site of PP2 action, we examined its effect on H295R cell metabolism of [¹⁴C]progesterone using thin layer chromatography. PP2 treatment for 48 h caused an increase in the conversion of progesterone to 17α-hydroxyprogesterone. To determine if this apparent increase in 17α-hydroxylase activity was due to increased transcript, we examined the effect of PP2 on CYP17 mRNA. PP2 treatment caused an increase in CYP17 mRNA without an effect on 3βHSD mRNA levels. Inhibition of protein synthesis with cycloheximide increased basal levels of CYP17 mRNA levels and blocked the induction observed by PP2. This suggests that new protein synthesis is a necessary part of PP2 induction of CYP17. Taken together these data suggest that the src tyrosine kinase inhibitor, PP2, is a potent inhibitor of aldosterone production. One mechanism for the inhibition is through an induction of CYP17 mRNA and enzyme activity. Src tyrosine kinases, therefore, may be involved with the promotion of a glomerulosa phenotype through the inhibition of CYP17 expression.