We compared the role of tyrosine kinases in α1A-adrenergic receptor (AR) and growth factor receptor stimulation of mitogen-activated protein kinase pathways in PC12 cells. Norepinephrine (NE) (noradrenaline), epidermal growth factor (EGF) and nerve growth factor (NGF) caused different patterns of tyrosine phosphorylation in PC12 cells stably expressing α1A-ARs. NE increased tyrosine phosphorylation of focal adhesion-related kinase Pyk2 and a 70 kDa protein, probably paxillin, whereas EGF strongly stimulated tyrosine phosphorylation of the EGF receptor and cytokine-activated kinase Jak2. The EGF receptor inhibitor AG1478 inhibited activation of extracellular signal-regulated kinases (ERKs) by EGF but not by NE. EGF and NGF strongly activated tyrosine phosphorylation of Shc and caused association of Src-homology collagen (Shc) with growth-factor-receptor-bound protein 2 (Grb2); however, neither NE nor UTP caused substantial activation of the Shc/Grb2 pathway. NE, UTP, EGF and NGF all increased tyrosine phosphorylation of Src, and this was inhibited by the Src inhibitor PP2. However, PP2 inhibited ERK activation in response to NE and UTP, but not in response to EGF or NGF. PP2 also completely blocked NE-induced PC12 cell differentiation, but had no measurable effect on NGF-induced differentiation. These studies show that activation of mitogen-activated protein kinase pathways by G-protein-coupled receptors and tyrosine kinase receptors proceed through distinct molecular pathways in PC12 cells, and support an obligatory role for Src activation in mitogenic responses to α1A-ARs in these cells.

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