SIGNALING OF THE RET RECEPTOR TYROSINE KINASE THROUGH THE C-JUN NH2-TERMINAL PROTEIN-KINASES (JNKS) - EVIDENCE FOR A DIVERGENCE OF THE ERKSAND JNKS PATHWAYS INDUCED BY RET

The RET proto-oncogene encodes a functional receptor tyrosine kinase ( Ret) for the Glial cell line Derived Neurotrophic Factor (GDNF). RET i s involved in several neoplastic and non-neoplastic human diseases. On cogenic activation of RET is detected in human papillary thyroid tumou rs and in multiple endocrine neoplasia type 2 syndromes. Inactivating mutations of RET have been associated to the congenital megacolon, i.e . Hirschprung's disease. In order to identify pathways that are releva nt for Ret signalling to the nucleus, we have investigated its ability to induce the c-Jun NH2-terminal protein kinases (JNK). Here we show that triggering the endogenous Pet, expressed in PC12 cells, induces J NK activity; moreover, Ret is able to activate JNK either when transie ntly transfected in COS-1 cells or when stably expressed in NIH3T3 fib roblasts or in PC Cl 3 epithelial thyroid cells. JNK activation is dep endent on the Pet kinase function, as a kinase-deficient RET mutant, a ssociated with Hirschsprung's disease, fails to activate JNK. The path way leading to the activation of JNK by RET is clearly divergent from that leading to the activation of ERK: substitution of the tyrosine 10 62 of Pet, the Shc binding site, for phenylalanine abrogates ERK but n ot JNK activation. Experiments conducted with dominant negative mutant s or with negative regulators demonstrate that JNK activation by Pet i s mediated by Rho/Rac related small GTPases and, particularly, by Cdc4 2.