Effect of a tumour promoter on myogenesis

Abstract

EVIDENCE that viruses and chemical carcinogens alter the ongoing synthetic programme of cells has been presented^1,2. Chondroblasts and melanoblasts transformed with a temperature-sensitive Rous sarcoma virus cease synthesising the chondroblast-unique sulphated proteoglycan and melanin, respectively³. Shifted to non-permissive temperature, these cells re-initiate the synthesis of the chondroblast-unique sulphated proteoglycan and melanin. Similar results have been obtained using chick myogenic cells^4,5. Normal presumptive myoblasts undergo a limited number of replications, yielding postmitotic, mononucleated myoblasts. Postmitotic myoblasts initiate the synthesis of muscle-specific myosin heavy and light chains and fuse into myotubes^6,7. In contrast, ts-viral transformed myogenic cells continue to replicate, do not initiate the synthesis of muscle-specific myosins and do not fuse. 24–48 h after shifting to non-permissive temperature, many transformed myogenic cells withdraw irreversibly from the cell cycle, initiate the synthesis of muscle-specific (unpublished data) myosins and fuse. Apparently when the transformation factor is expressed the cells do not withdraw from the cell cycle, and as a consequence of being forced to cycle, do not terminally differentiate. To probe the mechanisms that might be common to the generation of terminally differentiated cells from normal precursor cells, and to the generation of malignant cells from normal precursor cells, we studied the effects of phorbol-12-myristate-13-acetate (PMA) on myogenesis. Of the purified phorbol esters, PMA is the most potent as a tumour-promoting agent and as a mitogen^8–12. We report here that PMA mimics some of the effects of the Rous sarcoma tumour agent on myogenesis.