The effect of polymyxin B on steroidogenesis from adrenocortical cells

The action of the cyclic peptide polymyxin B (a well known inhibitor of protein kinase C) on adrenal steroid synthesis was examined with Y-1 adrenal tumor cells. Polymyxin B produces a biphasic effect on the stimulation of steroid synthesis by 2 nM ACTH in these cells, with inhibition at low concentrations (less than 10 microM) and a return to control levels at high concentrations (greater than 100 microM). Polymyxin B does not inhibit the stimulatory effect of Bu2cAMP on steroidogenesis. Inhibition of the steroidogenic response to ACTH by a fixed concentration (20 microM) of polymyxin B is overcome by high concentrations of ACTH. Polymyxin B causes a concentration-dependent stimulation of steroid synthesis by Y-1 cells and, over the same concentration range, increases the production of cAMP by these cells. Polymyxin B also partially inhibits the increased production of the cyclic nucleotide produced by ACTH. In addition, polymyxin B inhibits binding of [125I](Phe2,Nle4)ACTH-(1-38) to Y-1 cells. Polymyxin B, like ACTH, promotes rounding of Y-1 cells and partially inhibits rounding produced by ACTH. These effects of polymyxin B are specific to the extent that polymyxins E1 and E2 do not exert similar effects. The actions of polymyxin B are not confined to transformed cells, since responses similar to those seen with Y-1 cells were also observed with cultured rat fasciculata cells. On the other hand, the effect of polymyxin B is specific for adrenal cells, since the cyclic peptide does not influence the steroidogenic response of rat Leydig cells to LH. It is concluded that polymyxin B is a partial agonist of ACTH which is likely to prove useful in studying the molecular basis of the interaction between ACTH and its adrenal receptor.