Transcriptional and posttranscriptional control of c-myc during myogenesis: its mRNA remains inducible in differentiated cells and does not suppress the differentiated phenotype.

Abstract

It is widely accepted that the cellular oncogene c-myc plays an important role in the control of cell proliferation and that its expression diminishes in differentiated cells. We examined whether there is a correlation between c-myc expression and cell proliferation or differentiation by using a subclone of a rat skeletal muscle cell line L6E9. Myoblasts irreversibly withdraw from the cell cycle, fuse to form multinucleated myotubes, and express muscle-specific genes (terminal differentiation). Muscle-specific genes can also be expressed in the absence of fusion (biochemical differentiation). Such mononucleated but biochemically differentiated cells can be stimulated to reenter the cell cycle. c-myc was induced by insulin, insulin-like growth factor, or serum factors in GO-arrested cells, whereas induction by protein synthesis inhibitors or superinduction by protein synthesis inhibitors in combination with serum factors occurred in all physiological states tested. We found that c-myc expression was reduced in biochemically and terminally differentiated cells as well as in quiescent undifferentiated cells but that it remained inducible by growth factors in all three physiological states. Results of nuclear runoff transcription assays suggested that the induction of c-myc mRNA by growth factors and its deinduction in these physiological states were regulated mainly at the transcriptional level. In contrast, induction and superinduction of c-myc mRNA by protein synthesis inhibitors alone and in combination with growth factors, respectively, were regulated posttranscriptionally mainly by stabilization of c-myc mRNA. Moreover, c-myc and muscle-specific genes could be simultaneously transcribed in both biochemically and terminally differentiated cells. These results indicate that irreversible repression of c-myc is not required for terminal myogenic differentiation and that its expression is insufficient by itself to suppress the differentiated phenotype.

Autonomous expression of c-myc in BC3H1 cells partially inhibits but does not prevent myogenic differentiation.

“…However, it is conjectural whether the normal decline of this growth factor-inducible protein is a cause or, instead, a consequence of differentiation. For example, reinduction of c-myc was not sufficient to inactivate muscle-specific gene expression in L6E9 myotubes that were terminally differentiated (10). Furthermore, contrasting reports have questioned whether mitogen-inducible cellular oncogenes are either obligatory or sufficient to account for the changes in cell phenotype evoked by altered growth factor levels in a variety of other systems (4,8,23,27).…”

Elevated Expression of Proto‐Oncogenes during Interleukin‐5‐Induced Growth and Differentiation of Murine B Lineage Cells

“…In other types of cultured cells in which terminal differentiation can be induced, the level of c-myc mRNA decreases markedly (2,6,14,31). These phenomena suggested that c-myc plays an important role in the control of cell proliferation.…”

Dissociated expression of c-myc and a fos-related competence gene during cardiac myogenesis.

“…Our observations should be interpreted cautiously, however, in view of recent investigations that failed to confirm either a direct relationship between c-myc induction and DNA synthesis or, alternatively, between c-myc down regulation and the induction or maintenance of the differentiated state (6,9,12). Therefore, it is critical to test more directly the possible functional role for c-myc and other putative regulatory elements that might influence cell prolif- (15) and containing 5 ,ug of insulin per ml, 5 ,ug of transferrin per ml, 10 ,uM hydrocortisone, 1 …”

“…For myogenic cell lines, whether differentiation or continued division occurs depends in part upon ambient growth factor levels in the culture medium (24, 31, 34, 41; cf. references 12,23,32). Mutational analysis has suggested that quiescence, but not irreversible withdrawal from the cell cycle, may be obligatory for the induction of muscle-specific genes (40).…”