SIGNAL-TRANSDUCTION PATHWAYS ACTIVATED BY RET ONCOPROTEINS IN PC12 PHEOCHROMOCYTOMA CELLS

Gene alterations in the ret proto-oncogene, which encodes a receptor t yrosine kinase, have been found to associate with several human diseas es. In this study, we showed that induction of the vgf promoter activi ty is a good molecular indicator for RET activation in PC12 cells, a r at pheochromocytoma cell line. We demonstrated that all forms of RET o ncoprotein, including RET chimeric oncoproteins found in human papilla ry thyroid carcinomas (RET/PTC) as well as RET oncoproteins found in p atients with multiple endocrine neoplasia type 2A and 2B (2A/RET and 2 B/RET) can induce vgf promoter activity in PC12 cells. In contrast, a RET mutant found in a patient with Hirschsprung's disease, as well as a RET/PTC1 mutant with deletion of the dimerization domain, failed to induce vgf promoter activity in PC12 cells. We further determined that the signaling events mediated by phosphorylated Tyr(294) and phosphor ylated Tyr(451) binding sites are essential for RET/PTC1 to induce vgf promoter activity in PC12 cells. We also showed that RET/PTC1, 2A/RET , and 2B/RET induce ELK-, cAMP-responsive element binding protein (CRE B), or JUN-mediated gene expression in PC12 cells, and these three sig naling events are mediated by phosphorylated Tyr(294) and phosphorylat ed Tyr(451) binding sites in RET/PTC1.