Developmental regulation of intermediate filament and actin mRNAs during myogenesis is disrupted by oncogenic ras genes

Terminal differentiation of skeletal myoblasts is accompanied by down-regulation of vimentin and β-, γ-actins and up-regulation of desmin and sarcomeric α-actin(s). To investigate whether the normal decline in expression of vimentin and β-, γ-actins was coupled to withdrawal of proliferating myoblasts from the cell cycle or was a direct consequence of terminal differentiation, expression of the mRNAs encoding the actin microfilaments and intermediate filaments was examined in differentiation-defective C2 myoblasts bearing oncogenic ras genes. When transferred to mitogen-deficient medium, myoblasts transfected with the valine-12 allele of the human Harvey (H)-ras gene ceased dividing but failed to appropriately down-regulate vimentin and β-, γ-actin mRNAs. On the contrary, the level of vimentin mRNA expression was increased about 2.5-fold. Conversely, α-sarcomeric actin and desmin mRNAs continued to be expressed at basal levels in ras-transfected myoblasts that had withdrawn from the cell cycle. The ability of the ras oncogene to interfere with developmental regulation of actin and intermediate filament mRNAs was dependent upon mutational activation and was not observed in myoblasts transfected with proto-oncogenic ras alleles. These results demonstrate that, in addition to preventing up-regulation of muscle-specific gene products during myogenesis, the oncogenic forms of ras interfere with the mechanism(s) responsible for down-regulation of genes whose expression declines during myoblast fusion.