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

Abstract

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)a, 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, TGFa, 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. TGFa was also able to augment hCG-stimulated cAMP synthesis, StAR protein and StAR phosphorylation, and influence hCG binding and LH receptor mRNA expression. Furthermore, TGFa 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 TGFa-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, cAMPindependent signaling events regulate steroidogenesis in a StAR phosphorylation-independent manner.

Increased extracellular signal regulated kinases phosphorylation in the adrenal gland in response to chronic ACTH treatment

“…In fact, when PD98059 was injected in a chronic manner, corticosterone production was impaired, implicating ERKs in the chronic steroidogenic process. These results lead us to think that ERKs role may be related to the increased transcription of steroidogenic enzymes as proposed earlier in adrenal (Gyles et al 2001) and non-adrenal cells (Martinat et al 2005, Manna et al 2006. Our group is now performing further studies using ERKs inhibitors in vivo to confirm this point.…”

Soluble epoxide hydrolase and epoxyeicosatrienoic acids modulate two distinct analgesic pathways

“…An important requirement for the interaction between EETs, TSPO activity, and StARD1 expression may be the presence of elevated cAMP because expression and phosphorylation of StARD1 is greatly enhanced upon gonadotropic hormone stimulation, which increases intracellular cAMP levels (42,43). Separately, the maintenance of hyperalgesia in inflammatory and neuropathic pain states is known to be largely regulated by the activation of the cAMP signaling pathway (44)(45)(46).…”

The Role of Mitochondria in Syncytiotrophoblast Cells: Bioenergetics and Steroidogenesis

“…5). Although other PKA-dependent and -independent signaling pathways have been reported, the responses generated are generally less potent than those mediated by PKA/cAMP (Manna et al, 2006).…”

“…Nevertheless, independent of the factors involved modulating the intracellular concentration of cAMP, the likely participation of different hormones and the trophic factors and cytokines that could have endocrine, paracrine, autocrine or intracrine effect in the regulation of steroidogenesis through the signaling pathway mediated by PKA is still unknown; since, as it has been said, there are other pathways that can participate in the regulation of P4 synthesis (Manna et al, 2006;Stocco et al, 2005) (Fig. 6).…”

“…Other factors also increased in a variable way the production of P4; nevertheless, it has been suggested that its main effect would be predominantly as a regulator and just in a few cases as a stimulant of steroidogenesis, although through the activation of cAMP-dependent pathways and PKA activation (Manna et al, 2006). Although the signaling pathways associated with P4 synthesis in human placenta are unknown, one of the best studied mechanism is the one mediated by PKA activation.…”