SPECIFICITY OF NERVE GROWTH-FACTOR SIGNALING - DIFFERENTIAL PATTERNS OF EARLY TYROSINE PHOSPHORYLATION EVENTS INDUCED BY NGF, EGF, AND BFGF

The specificity of nerve growth factor (NGF) action was examined by co mparing early tyrosine phosphorylation events induced by NGF, epiderma l growth factor (EGF), and basic fibroblast growth factor (bFGF). In P C12 cells, administration of either the differentiation factor NGF or the mitogenic factor EGF led to tyrosine phosphorylation of multiple p olypeptides in the 100-110 kDa size range associated with PI-3 kinase. However, NGF induced a more prolonged phosphorylation, relative to a transient EGF effect. In contrast, the differentiation factor bFGF fai led to induce measurable tyrosine phosphorylation of PI-3 kinase-assoc iated proteins. Similarly, NGF but not bFGF induced marked tyrosine ph osphorylation of PLC gamma, another early signaling molecule, suggesti ng that multiple pathways exist for promoting differentiation, and/or that these signaling molecules are not essential for differentiation. TrkA signaling was also compared between PC12 cells and NIH-3T3 cells heterologously expressing trkA, where receptor activation promotes mit ogenesis. In this comparison, significant differences were observed in the tyrosine phosphorylation pattern of PI-3 kinase-associated polype ptides, suggesting the existence of cell type-specific molecular inter actions influencing trkA signaling. Mechanistically, NGF stimulation o f PC12 cells resulted in a weak or possibly indirect association betwe en trkA and PI-3 kinase. Furthermore, NGF did not appear to activate o r substantially alter the overall level of PI-3 kinase activity, raisi ng the possibility that ligand-induced phosphorylation may serve inste ad to relocalize constitutively active PI-3 kinase molecules within th e cell. Taken together, data presented suggest that the temporal patte rn of induced phosphorylation, the nature of induced associations with other phosphoproteins, and cell type-specific components may all cont ribute to the generation of NGF signaling specificity. (C) 1995 Wiley- Liss, Inc.