IGFBP-1 hyperphosphorylation in IUGR: Role of mTOR and CK2

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Intrauterine growth restriction (IUGR) increases the risk for perinatal complications and predisposes for adult disease. Insulin-like growth factor I (IGF-I) is a key regulator of fetal growth, and IGF-I bioavailability is modulated by binding to IGF binding protein-1 (IGFBP-1). We and others have provided evidence that phosphorylation of IGFBP-1 markedly increases its affinity to bind IGF-I and that IGFBP-1 hyperphosphorylation may constitute an important mechanism by which growth is reduced in IUGR. However, the molecular mechanisms causing IGFBP-1 phosphorylation in IUGR are largely unknown. The central hypothesis in this mechanistic proposal is that inhibition of mTOR signaling and activation of protein kinase CK2 in the fetal liver constitutes a key molecular link between nutrient deprivation and increased IGFBP-1 secretion and phosphorylation in vitro and in IUGR in vivo. Our hypothesis has been formulated based on our compelling preliminary data including the demonstration that (1) mTOR inhibition induces marked IGFBP-1 phosphorylation and silencing of protein kinase CK2 abolishes IGFBP-1 hyperphosphorylation induced by mTOR inhibition in HepG2 cells and (2) mTOR activity is decreased whereas CK2 expression and IGFBP-1 phosphorylation are increased in the liver of IUGR baboons. To test our hypothesis, we will study HepG2 cells, primary fetal hepatocytes from baboons, and blood and liver tissue of control and IUGR baboon fetuses in two specific aims: In Aim 1 we will use gene silencing approaches and pharmacological inhibitors in HepG2 cells and fetal baboon primary hepatocytes to mechanistically link mTOR signalling to regulation of protein kinase CK2 and IGFBP-1 phosphorylation. In addition, we will test the hypothesis that the increased IGFBP-1 phosphorylation due to amino acid deprivation is mediated by inhibition of mTOR and activation of CK2. In Aim 2 we will use a well-established baboon model of IUGR involving maternal nutrient restriction. We will determine the activity of mTOR and CK2, IGFBP-1 expression and phosphorylation in fetal liver samples and IGFBP-1 serum concentrations and phosphorylation in control and IUGR baboon fetuses. Significance: This work has the potential to identify a novel molecular mechanism underlying the development of IUGR. Furthermore, we will utilize a highly relevant non-human primate model that provides unique access to fetal samples not available through human studies. Innovation: This work will provide an innovative mechanistic link between mTOR and IGF-I signaling, two critical pathways in the regulation of cell growth. The combination of mechanistic approaches in cultured HepG2 and primary fetal baboon hepatocytes with studies of liver tissue from a baboon IUGR model is translational and innovative.