How Protein Kinase A Activates Canonical Tyrosine Kinase Signaling Pathways To Promote Granulosa Cell Differentiation

Abstract

Protein kinase A (PKA) has recently been shown to mimic the actions of follicle-stimulating hormone (FSH) by activating signaling pathways that promote granulosa cell (GC) differentiation, such as phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK). We sought to elucidate the mechanism by which PKA, a Ser/Thr kinase, intersected the PI3K/AKT and MAPK/ERK pathways that are canonically activated by receptor tyrosine kinases (RTKs). Our results show that for both of these pathways, the RTK is active in the absence of FSH yet signaling down the pathways to commence transcriptional responses requires FSH-stimulated PKA activation. For both pathways, PKA initiates signaling by regulating the activity of a protein phosphatase (PP). For the PI3K/AKT pathway, PKA activates the Ser/Thr PP1 complexed with the insulinlike growth factor 1 receptor (IGF-1R) and insulin receptor substrate 1 (IRS1) to dephosphorylate Ser residues on IRS1, authorizing phosphorylation of IRS1 by the IGF-1R to activate PI3K. Treatment of GCs with FSH and exogenous IGF-1 initiates synergistic IRS1 Tyr phosphorylation and resulting gene activation. The mechanism by which PKA activates PI3K is conserved in preovulatory GCs, MCF7 breast cancer cells, and FRTL thyroid cells. For the MAPK/ERK pathway, PKA promotes inactivation of the MAPK phosphatase (MKP) dual specificity phosphatase (DUSP) MKP3/DUSP6 to permit MEK-phosphorylated ERK to accumulate downstream of the epidermal growth factor receptor. Thus, for the two central signaling pathways that regulate gene expression in GCs, FSH via PKA intersects canonical RTK-regulated signaling by modulating the activity of PPs.