DOXORUBICIN REPRESSES THE FUNCTION OF THE MYOGENIC HELIX-LOOP-HELIX TRANSCRIPTION FACTOR MYOD - INVOLVEMENT OF ID GENE INDUCTION

We have shown recently that Doxorubicin (Hox, Adriamycin), a potent br oad spectrum chemotherapeutic agent with a major side effect of cardio myopathy, completely prevents myoblast fusion and accumulation of musc le-specific transcripts in the mouse C2 skeletal muscle cell line. Her e we use mouse embryonic fibroblast 10T1/2 cells to demonstrate that H ox represses muscle-specific gene expression by interfering with MyoD activity. As assayed by transient cotransfection, Hox inhibits the abi lity of MyoD to trans-activate muscle-specific reporter genes. A stabl e cell system was developed in which MyoD is constitutively expressed in 10T1/2 cells (M10 cells). Dox-treated M10 cells express MyoD from a long terminal repeat-driven vector but fail to activate endogenous My oD and myogenin loci. Dox did not effect E2A gene transcript levels, b ut Id mRNA levels are significantly increased in Dox-treated M10 cells . Interestingly, overexpression of E2-5, which forms inactive heterodi mers with Id, can overcome the Dox-induced suppression of the trans ac tivation function of MyoD in 10T1/2 cells, Furthermore, we demonstrate that the 5'-flanking region of the Id2 gene mediates its Dox-inducibl e transcriptional expression. These findings support a model in which Dox inhibits muscle specific gene expression by interfering with the f unction of MyoD protein through, at least in part, induction of Id gen e expression. The implications of our results for the molecular mechan isms underlying the myofibrillar loss observed in Dox-induced cardiomy opathy are discussed.