Control of differentiation in BC3H1 muscle cells

BC3H1 is a cell line that undergoes a musclelike pattern of differentiation under the appropriate conditions. We have examined the control of the synthesis of proteins characteristic of differentiated muscle in these cells as a function of their position in the cell cycle. We define two positions in the cell cycle where BC3H1 cells can remain stably quiescent. G1d is a restriction point early in the G1 portion of the cell cycle that permits the synthesis of muscle-specific proteins and is probably identical to G0. The second restriction point, G1q, occurs approximately 4 hr later in the G1 portion of the cell cycle and does not permit the synthesis of muscle-specific proteins. Movement of the cells from G1d to G1q occurs when fibroblast growth factor is added to the cells and is reversed when this growth factor is removed. Repression of the synthesis of muscle-specific proteins occurs when fibroblast growth factor is added to cells in G1d. In the case of the muscle form of creatine phosphokinase (M-CPK), the decline in the rate synthesis of this protein is a consequence of a decreased level of its mRNA. By contrast, the repression of alpha-actin synthesis, a protein synthesized only in differentiated cells, appears to be controlled at the translational level. The effect of fibroblast growth factor and other mitogens in these cells require activation of tyrosine kinase(s), but the intracellular targets of these kinases are not known. Studies by others suggest that activation of the ras oncogene can mimic the action of mitogenic polypeptides on these and other muscle cells.(ABSTRACT TRUNCATED AT 250 WORDS)