A TRANSIENT INCREASE OF SNON TRANSCRIPT BY GROWTH ARREST UPON SERUM DEPRIVATION AND CELL-TO-CELL CONTACT

To analyze the possible involvement of c-ski and c-sno during the cour se of in vitro myogenesis, expression of their transcripts during diff erentiation of a murine muscle cell line (C2C12) was monitored by comp etitive reverse transcription-polymerase chain reaction (RT-PCR). The transcripts of c-snoN mere temporarily increased 25-fold above basal l evel at 12 h prior to the onset of transcription of muscle-specific ge ne, e.g. myogenin and muscle creatine kinase, whereas c-ski was expres sed invariably, The transient increase of c-snoN was blocked when myog enesis was interrupted by the presence of fetal calf serum in culture medium, probably due to growth factors being included; basic fibroblas t growth factor (b-FGF) blocked the transient increase whereas epiderm al growth factor (EGF) did not, consistent with the inhibitory effect of b-FGF and no effect of EGF on myotube formation of C2C12. In fibrob lastic C3H10T1/2 cells, snoN exhibited a similar transient increase of transcript when growth arrested under the same conditions as for in v itro myogenesis, indicating that the expression of snoN is not suffici ent to induce the onset of muscle differentiation and an unknown facto r involved in myogenic cells is necessary, The transient increase of s noN transcript may represent a common entrance step of cells into the GO phase where muscle differentiation is substantiated, considering th at it was observed upon growth arrest of fibroblastic C3H10T1/2 cells and prior to the elevation of MCK in C2C12 but undetected when entry i nto G0 was blocked by b-FGF.