Abrogation of nerve growth factor-induced terminal differentiation by ret oncogene involves perturbation of nuclear translocation of ERK

Oncogenic variants of the receptor tyrosine kinase, Ret, cause formation of tumors of neuroendocrine derivation in the multiple endocrine neoplasia ty pe 2 and, thus, likely interfere with antiproliferative and/or differentiat ive extracellular signals. Here we took advantage of two rat pheochromocyto ma-derived cell lines (PC12/MEN2A and PC12/MEN2B) to investigate whether Be t-induced nerve growth factor (NGF) unresponsiveness might involve impairme nt of ERK signaling. In fact, these cells, stably transfected with distinct forms of the active ret oncogene, fail to block proliferation, even upon N GF stimulation. In these cells we show the presence of both chronic ERKs ac tivity and high expression levels of MKP-3, an ERK-specific phosphatase, De spite the presence of MKP-3, ERK activity can be further stimulated by NGF, but it fails to translocate into the nucleus and consequently to induce im mediate-early gene transcription. Because of the presence of MKP-3, our res ults suggest the existence of a negative regulatory feedback acting on ERKs as a mechanism responsible for the abrogation of NGF-induced terminal diff erentiation. Indeed, MKP-3 seems to be implicated in the persistence of ERK s in cell cytoplasm, This interpretation is further supported by the observ ation that in ret-transfected cells, forced expression of an active form of MEK-1 may overcome this block; it restores transcription from the c-fos pr omoter, induces translocation of ERKs into the nucleus, and inhibits cell p roliferation.