Law Nathan C, Donaubauer Elyse M, Zeleznik Anthony J, Hunzicker-Dunn Mary
School of Molecular Biosciences, College of Veterinary Medicine, Washington State University, Pullman, Washington 99164.
Department of Obstetrics, Gynecology and Reproductive Sciences, Magee Women's Research Institute, University of Pittsburgh, Pittsburgh, Pennsylvania 15213.
Endocrinology. 2017 Jul 1;158(7):2043-2051. doi: 10.1210/en.2017-00163.
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.
蛋白激酶A（PKA）最近被证明可通过激活促进颗粒细胞（GC）分化的信号通路来模拟促卵泡激素（FSH）的作用，这些信号通路包括磷脂酰肌醇3激酶（PI3K）和丝裂原活化蛋白激酶/细胞外信号调节激酶（MAPK/ERK）。我们试图阐明丝氨酸/苏氨酸激酶PKA与通常由受体酪氨酸激酶（RTK）激活的PI3K/AKT和MAPK/ERK通路相交的机制。我们的结果表明，对于这两条通路，RTK在没有FSH的情况下是活跃的，但向下游通路发出信号以启动转录反应需要FSH刺激的PKA激活。对于这两条通路，PKA通过调节蛋白磷酸酶（PP）的活性来启动信号传导。对于PI3K/AKT通路，PKA激活与胰岛素样生长因子1受体（IGF-1R）和胰岛素受体底物1（IRS1）复合的丝氨酸/苏氨酸PP1，使IRS1上的丝氨酸残基去磷酸化，从而允许IGF-1R对IRS1进行磷酸化以激活PI3K。用FSH和外源性IGF-1处理GC可启动协同的IRS1酪氨酸磷酸化并导致基因激活。PKA激活PI3K的机制在排卵前的GC、MCF7乳腺癌细胞和FRTL甲状腺细胞中是保守的。对于MAPK/ERK通路，PKA促进MAPK磷酸酶（MKP）双特异性磷酸酶（DUSP）MKP3/DUSP6的失活，以允许MEK磷酸化的ERK在表皮生长因子受体下游积累。因此，对于调节GC中基因表达的两条主要信号通路，FSH通过PKA通过调节PP的活性与经典的RTK调节信号相交。
