Induction of RET Proto-Oncogene Expression in Neuroblastoma Cells Precedes Neuronal Differentiation and Is Not Mediated by Protein Synthesis

RET proto-oncogene products are involved in neural crest development, and constitutional RET mutations are associated with syndromes characterized by tumors of neural crest origin. To study the regulation of RET transcription during neuronal differentiation we analyzed RET expression in neuroblastoma cell lines treated with various differentiating agents. A marked increase in RET mRNA levels was observed in all the cell lines examined shortly after retinoic acid (RA) treatment and before the onset of detectable morphological changes. Upregulation of RET expression was also found in SK-N-BE cells induced to differentiate by 12- O-tetradecanoylphorbol-13-acetate, glial cell-conditioned medium, α or γ interferon, and in SH-SY-5Y cells exposed to nerve growth factor. Induction of RET expression by RA occurred in the absence of de novo protein synthesis. On the other band, cycloheximide treatment by itself caused upregulation of RET transcripts. These results indicate that the positive transcriptional regulation of RET is closely associated with early neuronal differentiation and suggest that a negative regulatory factor/s controls RET transcription in neuroblastoma cells. Finally, anti-Ret antibodies immunoprecipitated four bands with apparent molecular weights of 150, 155, 170, and 175 kDa in RA-induced SK-N-BE cells. These bands likely represent differently glycosylated forms of the two RET primary products (117 and 122 kDa) detected in tunicamycintreated cells.