Patterns of long-term steroidogenesis stimulation by ACTH and phorbol ester.

Adrenocorticotropic hormone (ACTH) triggers well-defined responses in Y-1 cells. Among them is steroidogenesis stimulation. We have previously shown that phorbol 12-myristate 13-acetate (PMA), an activator of the calcium- and phospholipid-dependent protein kinase (PKC) is able to mimic all the responses triggered by ACTH in these cells, including steroidogenesis stimulation. Short (2 h) treatment with PMA leads to only 20-30% of the maximal steroidogenesis stimulation obtained with ACTH. However, the steroid secretion in the 2 h that follows the short-term (2 h) PMA treatment reaches the same levels as observed with ACTH, i.e., a 12- to 15-fold increase. We also show that this effect is restricted to cells treated with PMA for up to 4 h, while treatment for longer periods of time causes a reduction of the steroid biosynthesis rate, an effect that is not observed in cells treated with ACTH or N6,2'-0-dibutyryladenosine 3',5'-cyclic monophosphate (dcAMP). These results suggest that activation of PKC can elicit the first phase of ACTH steroidogenesis stimulation, but not the second one, which strictly depends on activation of cAMP-dependent protein kinase.

Patterns of long-term steroidogenesis stimulation by ACTH and phorbol ester

Adrenocorticotropic hormone (ACTH) triggers well-defined responses in Y-1 cells. Among them is steroidogenesis stimulation. We have previously shown that phorbol 12-myristate 13-acetate (PMA), an activator of the calcium- and phospholipid-dependent protein kinase (PKC) is able to mimic all the responses triggered by ACTH in these cells, including steroidogenesis stimulation. Short (2 h) treatment with PMA leads to only 20-30 per cent of the maximal steroidogenesis stimulation obtained with ACTH. However, the steroid secretion in the 2 h that follows the short-term (2 h) PMA treatment reaches the same levels as observed with ACTH, i.e., a 12- to 15-fold increase. We also show that this effect is restricted to cells treated with PMA for up to 4 h, while treatment for longer periods of time causes a reduction of the steroid biosynthesis rate, an effect that is not observed in cells treated with ACTH or N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (dcAMP). These results suggest that activation of PKC can elicit the first phase of ACTH steroidogenesis stimulation, but not the second one, which strictly depends on activation of cAMP-dependent protein kinase.

Relevance of c-fos proto-oncogene induction for the steroidogenic response to ACTH, dcAMP and phorbol ester in adrenocortical cells.

We previously reported that ACTH, but not dibutyryl cAMP, rapidly induces the c-fos proto-oncogene in Y-1 adrenocortical cells. Here we show that PMA induces c-fos with similar kinetics when compared with ACTH (0.5-1 h peak) but reaches only 60% of the maximal ACTH induction and dcAMP is a weak c-fos inducer (15% of ACTH). However, combination of PMA and dcAMP has a synergistic effect leading to maximal c-fos induction. c-fos expression may play a role in the RNA synthesis-dependent corticosteroidogenesis response and/or growth regulation by ACTH. We also show that, in contrast to dcAMP, PMA is a poor steroidogenesis stimulator (15 to 17% of maximum ACTH-stimulated level), its activity being completely dependent on RNA synthesis. Combination of dcAMP and PMA yields an additive steroidogenesis stimulation, an effect that is also dependent on RNA synthesis. Although no strict correlation was found between c-fos induction and early steroidogenesis stimulation, particularly with respect to cAMP derivatives, the results suggest that a PKC pathway is likely to cooperate with the classical cAMP-PKA pathway in adrenal cells' RNA-dependent steroidogenesis.