Cell proliferation inhibited by MyoD1 independently of myogenic differentiation

Abstract

CELL growth and differentiation are usually mutually exclusive¹. Transformation of myoblasts by retroviruses containing the myc oncogene inhibits differentiation, preventing cells from withdrawing from the cell cycle^4,5. If cell-cycle withdrawal is a prerequisite for myoblast differentiation, it is probably an early event in terminal cell differentiation, but this has not yet been established⁶. MyoD1 regulates myogenesis⁷. It is expressed only in skeletal muscle⁸, but can convert other cells to muscle cells^9,10. The MyoD1 protein, a nuclear phosphoprotein in part similar to the myc family of proteins, is a DNA-binding protein binding to the enhancer sequences of the muscle-specific creatine phosphokinase gene¹¹. Thus, introduction of MyoD1 into cells provides a simple approach to study the effect of induction of differentiation on cell growth. In cultured NIH 3T3 cells, inhibition of cell proliferation occurs within 18 hours, and expression of myosin starts after 72 hours. Furthermore, injection of MyoD1 into quiescent NIH3T3 cells inhibits the serum-induced G0–S phase transition of the cell cycle. In CV1 cells, which are not induced to differentiate by MyoD1, DNA synthesis is also inhibited. Expression of MyoD1 can thus inhibit cell proliferation independently of induction of differentiation. Deletion of the myc-like domain in the MyoD1 gene¹² eliminates the inhibition of DNA synthesis, but substitution of the basic domain with the analogous domain from the E12 transcription factor¹³ inhibits growth yet fails to induce differentiation. Inhibition of DNA synthesis, therefore, seems to be controlled separately from myogenic differentiation.