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Rapid phosphorylation of the L‐myc protein induced by phorbol ester tumor promoters and serum.

We have examined post‐translational modification of the L‐myc protein using polyclonal and monoclonal antibodies against a peptide well conserved in the predicted amino acid sequences of the c‐myc, N‐myc and L‐myc genes. These antibodies precipitate three polypeptides of Mr 60‐66,000 from [35S]methionine or [32P]orthophosphate‐labelled human small cell lung cancer cell lines expressing amplified L‐myc genes, but not the other myc genes. Treatment of the L‐myc immunoprecipitates with alkaline phosphatase prior to electrophoresis converts the three methionine‐labelled polypeptides into a single band migrating at Mr 59,000, and efficiently removes radioactivity from the 32P‐labelled L‐myc protein, suggesting that, in contrast to the c‐myc and N‐myc proteins, the L‐myc polypeptide heterogeneity is due to differential phosphorylation of a common precursor. When the phorbol ester 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA) or serum is added to cultures of U‐1690 cells the Mr 66,000 polypeptide is rapidly enriched while the Mr 60,000 form is decreased in the L‐myc immunoprecipitates. This effect is correlated with the ability of phorbol ester and diacylglycerol analogues to activate protein kinase C. The TPA‐induced phosphorylation of the L‐myc protein occurs in a protein synthesis‐independent manner as it is not inhibited by cycloheximide or anisomycin. These data indicate that the phosphorylation of the L‐myc nuclear oncoprotein is modulated in response to TPA via a rapid signal transduction system involving protein kinase C. This mechanism could play an important role in the response of lung cells to e.g. bombesin‐related growth factors.