Signaling through Ras is essential for ret oncogene-induced cell differentiation in PC12 cells

Specific germline mutations of the receptor tyrosine kinase, Ret, predispos e to multiple endocrine neoplasia types 2A and 2B and familial medullary th yroid carcinoma. The mechanisms by which different Ret isoforms (Ret-2A and Ret-2B) cause distinct neoplastic diseases remain largely unknown. On the other hand, forced expression of these mutated versions of Ret induces the rat pheochromocytoma cell line, PC12, to differentiate. Here we used an ind ucible vector encoding a dominant-negative Ras (Ras p21(N17)) to investigat e the contributions of the Ras pathway to the phenotype induced in PC12 cel ls by the expression of either Ret-2A or Ret-2B mutants. We show that the R et-induced molecular and morphological changes are both mediated by Ras-dep endent pathways. However, even though inhibition of Ras activity was suffic ient to revert Ret-induced differentiation, the kinetics of morphological r eversion of the Ret-2B- was more rapid than the Ret-SA-transfected cells, F urther, we show that in Ret-transfected cells the suc1-associated neurotrop hic factor-induced tyrosine phosphorylation target, SNT, is chronically pho sphorylated in tyrosine residues, and associates with the Sos substrate. Th ese results indicate the activation of the Res cascade as an essential path way triggered by the chronic active Ret mutants in PC12 cells, Moreover, ou r data indicate SNT as a substrate for both Ret mutants, which might mediat e the activation of this cascade.