| Growth factors and their respective receptor tyrosine kinases play critical roles in precisely regulating numerous cellular functions including, DNA synthesis and cell division, cellular growth and differentiation, cell survival, cell adhesion and motility, and metabolism. Although many tyrosine kinase receptor signal transduction pathways involve the recruitment and activation of identical signalling molecules, each growth factor is able to elicit a highly specific biological response. This study demonstrates several differences within the signal transduction pathways of nerve growth factor (NGF), insulin-like growth factor-I (IGF-I), and insulin, and their tyrosine kinase receptors, which may account for each ligand-receptor pair's ability to regulate unique cellular functions.;First, I have characterized the trafficking and processing events of NGF, IGF-I, and insulin following receptor-mediated endocytosis and have found differences in their receptors' endocytic rate constants, in the kinetics of ligand-receptor dissociation, in the kinetics and efficiency of ligand degradation, and in their abilities to undergo retroendocytosis. Given that internalized receptors can continue to participate in signal transduction, these observed differences may provide a mechanism by which specificity is achieved.;Secondly, I investigated mechanisms underlying NGF's and IGF-I's abilities to elicit opposing effects on cell proliferation and found distinct differences in their abilities to stimulate MAP kinase and SNT phosphorylation. I demonstrate that the time-course of MAP kinase activation plays a critical role in regulating cell cycle progression and that activation of this protein is differentially regulated by NGF and IGF-I.;Thirdly, I elucidated the roles of Ras, PLC-$gamma$1, Shc, and Crk, and their molecular domains in NGF stimulated neurite outgrowth of PC12 cells. I demonstrated that inhibition of Ras function prevents NGF mediated differentiation. Various degrees of inhibition were also achieved when the function of endogenous PLC-$gamma$1, Crk, and Shc were impaired. Inhibition of the PLC-$gamma$1 and the Crk pathway resulted in additive inhibitory effects, demonstrating redundancy in the NGF signalling pathway.;Taken together, this work demonstrates clear differences in the trafficking and processing events following receptor-mediated endocytosis of NGF, IGF-I, and insulin and in their signal transduction pathways and, thus, provides potential mechanisms by which specificity can be achieved. |
