title: Density-dependent arrest of DNA replication is accompanied by decreased levels of c-myc mRNA in myogenic but not in differentiation-defective myoblasts.
abstract: Myoblasts from primary rat cultures and established mouse (Cl10) and rat (L6, Ama 420) cell lines were examined for c-oncogene expression during exponential growth and under conditions which allowed myogenic differentiation. The abundance of c-Ki-ras transcripts in mRNA from confluent, quiescent cultures was reduced to 15-40% of that in mRNA from exponentially growing cells. This reduction was found both in primary myoblast cultures, myoblast lines that formed myotubes (L6 and Cl10) and in a differentiation defective subline (Ama 420). The level of c-myc transcripts was lowered when myogenic rat L6 myoblasts reached a high cell density, stopped DNA synthesis and formed myotubes. At the same cell density, growth arrested myoblasts of differentiation defective Ama 420 cells maintained a high level of c-myc expression. This shows that DNA replication and c-myc expression are independently regulated. All myoblast lines also showed expression of c-abl during exponential growth phase. Reduced expression was seen in differentiated L6 and Cl10 cultures. No expression was detected when mRNA from multiplying and differentiating myoblasts cultures were probed for c-myb, c-erbA, c-erbB, c-mos, c-fes, and c-src. The observations are consistent with a role for c-Ki-ras in myoblast proliferation and suggest that a reduction in c-myc expression may be a necessary prerequisite for terminal myogenic differentiation.
mesh_terms: D000818:Animals; D002452:Cell Count; D002455:Cell Division; D002459:Cell Fusion; D002460:Cell Line; D002471:Cell Transformation, Neoplastic / Q000378:metabolism* / Q000473:pathology; D003402:Creatine Kinase / Q000378:metabolism; D004261:DNA Replication; D051379:Mice; D009132:Muscles / Q000378:metabolism / Q000473:pathology / Q000502:physiology*; D009857:Oncogenes; D012334:RNA, Neoplasm / Q000378:metabolism*; D051381:Rats; D014158:Transcription, Genetic
chemical_list: D012334:RNA, Neoplasm; D003402:Creatine Kinase