c-myc mRNA is down-regulated during myogenic differentiation by accelerated decay that depends on translation of regulatory coding elements.

Murine C2C12 myoblasts induced to differentiate into multinucleated myotubes decrease their levels of c-myc mRNA 3-10-fold through posttranscriptional mechanisms that recognize regulatory elements contained in protein-coding sequences in exons 2 and 3 of the mRNA. To determine the mechanism by which these elements mediate c-myc mRNA down-regulation, we examined the regulation of mutant MYC and human beta-globin-MYC fusion mRNAs. Regulation of mRNAs containing MYC exon 2 or 3 is abolished by insertion of an upstream termination codon indicating that regulatory function depends on their translation. Exploiting this translation dependence, we show that pharmacologic inhibition of translation with cycloheximide abolishes the down-regulation of regulated MYC and globin-MYC mRNAs and induces their levels in differentiating C2C12 cells. We exclude the possibility that this induction in mRNA levels results from cycloheximide effects on transcription or processing of parts of the RNA other than the regulatory elements, leading to the conclusion that cycloheximide induction results from mRNA stabilization. We show that the magnitude of cycloheximide induction can be used to estimate turnover rates of mRNAs whose decay is translation-dependent. By using cycloheximide inducibility to examine turnover rates of MYC and globin-MYC mRNAs, we show that the MYC exon 2 and exon 3 regulatory elements, but not MYC 3'-untranslated region or chloramphenicol acetyltransferase coding sequences, mediate accelerated mRNA decay in differentiating, but not undifferentiated, C2C12 cells. We show that these regulatory elements must be translated to confer accelerated mRNA decay and that increased turnover occurs in the cytoplasm and not in the nucleus. Finally, using cycloheximide induction to examine mRNA half-lives, we show that mRNA turnover is increased sufficiently by mechanisms targeting the exon 2 and 3 regulatory elements to account for the magnitude of c-myc mRNA down-regulation during differentiation. We conclude from these results that c-myc mRNA down-regulation during myogenic differentiation is due to translation-dependent mechanisms that target mRNAs containing myc exon 2 and 3 regulatory elements for accelerated decay.