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Focuses on the ectopic expression of p107 in rat fibroblasts which attenuated the stimulation of messenger RNA translation and global protein synthesis by serum growth factors. Factor that may restore the defect in translation elicited by p107; Overview of Rat1 fibroblast cell lines that overexpress p107; Effect of Akt1 on p107 expression.

The Trk/Nerve Growth Factor receptor mediates the rapid activation of a number of intracellular signaling proteins, including phosphatidylinositol 3-kinase (PI 3-kinase). Here, we describe a novel, NGF-inducible system that we used to specifically address the signaling potential of endogenous PI 3-kinase in NGF-mediated neuronal survival and differentiation processes. This system utilizes a Trk receptor mutant (Trkdef) lacking sequences Y490, Y785 and KFG important for the activation of the major Trk targets; SHC, PLC-γl, Ras, PI 3-kinase and SNT. Trkdef was kinase active but defective for NGF-induced responses when stably expressed in PC12nnr5 cells (which lack detectable levels of TrkA and are non-responsive to NGF). The PI 3-kinase consensus binding site, YxxM (YVPM), was introduced into the insert region within the kinase domain of Trkdef. NGF-stimulated tyrosine phosphorylation of the Trkdef+PI 3-kinase addback receptor, resulted in the direct association and selective activation of PI 3-kinase in vitro and the production of PI(3,4)P2 and PI(3,4,5)P3 in vivo (comparable to wild-type). PC12nnr5 cells stably expressing Trkdef+PI 3-kinase, initiated neurite outgrowth but failed to stably extend and maintain these neurites in response to NGF as compared to PC12 parental cells, or PC12nnr5 cells overexpressing wild-type Trk. However, Trkdef+PI 3-kinase was fully competent in mediating NGF-induced survival processes. We propose that while endogenous PI 3-kinase can contribute in part to neurite initiation processes, its selective activation and subsequent signaling to downstream effectors such as Akt, functions mainly to promote cell survival in the PC12 system. [ABSTRACT FROM AUTHOR]

The TrkA receptor protein tyrosine kinase is involved in signalling PC12 cell differentiation and cessation of cell division in response to nerve growth factor (NGF). To assess the importance of adaptor proteins and Ras in NGF control of phosphoinositide 3-OH kinase (PI 3-kinase), specific receptor mutations in Trk have been employed. We show that phosphorylation of tyrosine 490, but not 785, of Trk is essential for activation of both Ras and PI 3-kinase in vivo, correlating with tyrosine phosphorylation of Shc and binding of Shc to the adaptor Grb2 and the Ras exchange factor Sos. A mutant receptor that lacks Y490 and Y785, but contains an introduced YxxM motif which binds the regulatory domain of PI 3-kinase, is unable to activate Ras despite causing increased PI 3-kinase activity. This indicates clearly that activation of PI 3-kinase by itself is not sufficient to cause activation of Ras, arguing against a model in which PI 3-kinase acts upstream of Ras. The Shc site of Trk is thus crucial for the activation of Ras and PI 3-kinase. [ABSTRACT FROM AUTHOR]