cAMP-independent signaling regulates steroidogenesis in mouse Leydig cells in the absence of StAR phosphorylation

In the regulation of steroid biosynthesis, a process mediated by the steroidogenic acute regulatory (StAR) protein, both cAMP-dependent and -independent pathways are involved. While the cAMP-dependent regulatory events represent, by far, the most robust increase in steroid synthesis and are well established, the knowledge regarding cAMP-independent mechanisms is lacking. The present investigation was designed to elucidate the potential involvement of the latter in regulating StAR expression and steroidogenesis in mouse Leydig tumor cells (mLTC-1 cells). Treatment of mLTC-1 cells with a number of factors including insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF), fibroblast growth factor, transforming growth factor (TGF)α, interleukin-1 (IL-1), and colony-stimulating factor-1, increased the levels of StAR mRNA, StAR protein, and progesterone to varying degrees and utilized signaling pathways that are not associated with elevations in intracellular cAMP levels. Importantly, phosphorylation of StAR in response to these stimuli was undetectable, which is in marked contrast to observations with human chorionic gonadotropin (hCG), indicating factors that do not alter intracellular cAMP, regulate the steroid biosynthesis in a StAR phosphorylation-independent manner. In addition, the roles for factors involved in cross-talk between the protein kinase pathways, PKA and PKC, were demonstrated. Further characterization of signaling by one such cAMP-independent factor, TGFα, demonstrated that the mechanism, whereby it increased StAR expression and steroid synthesis, was dependent on de novo protein synthesis and mediated via activation of the EGF receptor. TGFα was also able to augment hCG-stimulated cAMP synthesis, StAR protein and StAR phosphorylation, and influence hCG binding and LH receptor mRNA expression. Furthermore, TGFα increased phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2) and cAMP-response element-binding protein (CREB), processes inhibited by the mitogen-activated protein kinase/ERK inhibitor U0126 and by expression of non-phosphorylatable CREB-M1 respectively. Inhibition of ERK activity enhanced TGFα-mediated StAR protein expression (but not its phosphorylation) and decreased progesterone synthesis, events correlated with the expression of dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 (DAX-1) and scavenger receptor class B type 1 (SR-B1). Collectively, these findings demonstrate that, in mouse Leydig cells, cAMP-independent signaling events regulate steroidogenesis in a StAR phosphorylation-independent manner.