ROLE OF TYROSINE KINASE-ACTIVITY OF EPIDERMAL GROWTH-FACTOR RECEPTOR IN THE LYSOPHOSPHATIDIC ACID-STIMULATED MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY

Recent evidence indicates that the epidermal growth factor (EGF) recep tor mediates a branch of lysophosphatidic acid (LPA)-induced signal tr ansduction pathways that activate mitogen-activated protein (MAP) kina se. However, it is unclear whether the intrinsic tyrosine kinase activ ity of EGF receptor is involved, We previously showed that reactive ox ygen species (ROS) were involved in the LPA-stimulated MAP kinase path way. Here, we identify tyrosine phosphorylation of EGF receptor as an LPA signaling step that requires ROS. To evaluate the role of the tyro sine kinase activity of EGF receptor in the LPA-stimulated MAP kinase pathway, we examined the effects of are EGF receptor-specific tyrosine kinase inhibitor, PD158780. PD158780 potently inhibited the LPA-stimu lated MAP kinase kinase 1/2 (MKK1/2) activation and EGF receptor tyros ine phosphorylation in HeLa cells, while it had no detectable effect o n c-Src kinase activity, PD158780 also inhibited LPA-induced MKK1/2 ac tivation and DNA synthesis in NIH 3T3 cells. Furthermore, we compared LPA-stimulated MKK1/2 and MAP kinase activation, transcriptional activ ity of the c-fos promoter, and DNA synthesis in B82L cells, which lack endogenous EGF receptor, and B82L cells expressing kinase-defective o r wild-type human EGF receptor. Results obtained from analysis of thes e cell Lines suggest that the EGF receptor tyrosine kinase contributes to the LPA-stimulated MAP kinase activation, c-fos transcription, and mitogenesis.