The obligatory action of protein tyrosine phosphatases in ACTH-stimulated steroidogenesis is exerted at the level of StAR protein.

A key regulatory step in the steroidogenic hormones signaling pathway is the synthesis of steroidogenic acute regulatory protein (StAR). This protein facilitates the delivery of cholesterol to the inner mitochondrial membrane, the rate-limiting step in steroidogenesis. ACTH and LH pathway also includes tyrosine dephosphorylation processes. Indeed, our previous studies have demonstrated that both hormones increase protein tyrosine phosphatase (PTP) activity by a PKA-dependent mechanism and that the action of PTPs is required for the stimulation of steroid biosynthesis in adrenal and Leydig cells. In order to test the putative relationship between PTP activity and StAR protein induction in adrenocortical cells, in the present study we evaluated steroid production and StAR protein level in Y1 adrenocortical cells under PTP inhibition. Phenylarsine oxide (PAO), a powerful cell permeable PTP inhibitor, reduced ACTH-stimulated steroidogenesis in a concentration-dependent fashion. A concentration of 2.5 microM of this compound inhibited steroid synthesis in a 56% (ACTH = 318 +/- 30, ACTH + PAO = 145 +/- 18 ng progesterone/mL, P < 0.001) and also abrogated StAR protein induction. Phenylarsine oxide reduced the protein level after 60 min and this effect still remained at 120 min. A second PTP inhibitor, benzyl phosphonic acid, acting by a different mechanism, reproduced PAO effects on both steroidogenesis and StAR protein. Taken together, these results indicate that PTP activity participates in StAR protein induction and led us to attribute to the PKA-mediated PTP activation in steroidogenic systems a functional role, as mediator of StAR protein induction.