Control by Fibroblast Growth Factor of Differentiation in the BCgH

The regulation of creatine phosphokinase (CPK) expression by polypeptide growth factors has been examined in the clonal mouse muscle BC3H1 cell line. After arrest of cell growth by exposure to low concentrations of serum, BC3H1 cells accumulate high levels of muscle-specific proteins including CPK. The induction of this enzyme is reversible in the presence of high concentrations of fetal calf serum, which cause quiescent, differentiated cells to reenter the cell cycle. Under these conditions, the rate of CPK synthesis is drastically reduced. We show in the present communication that either pituitary-derived fibroblast growth factor (FGF) or brain-derived FGF are as effective as serum in repressing the synthesis of CPK when added to quiescent, differentiated cells. The decrease in the rate of synthesis of CPK occurs within 22 h after the addition of pituitary FGF to the cells. Pituitary FGF had very little effect, if any, on the rate CPK degradation. The overall rate of protein synthesis and the pattern of synthesis of the major polypeptides made by these cells was not altered by the addition of FGF. Although pituitary FGF was mitogenic for BC3H1 cells, the rate of cell growth was not absolutely correlated with the extent of repression of CPK. Brain-derived FGF fully repressed CPK induction under conditions where it showed no significant mitogenic activity. These results show that the expression of a muscle-specific protein, CPK, can be controlled by a single defined polypeptide growth factor in fully differentiated cultures, and that initiation of cell division is not required for their regulation to take place. The relationship between cell growth and differentiation has been a subject of extensive investigation in a number of systems. Muscle cells in culture have been a preferred system for the investigation of this relationship (3, 11, 19, 26, 28). After depletion of mitogens from the culture medium (16), muscle cells cease exponential growth and undergo differentiation (13, 21). Morphological differentiation is usually accompanied by fusion of mononucleated myoblasts into multinucleated myotubes and by the expression of a number of muscle-specific proteins including myosin and other components of the contractile apparatus as well as creatine phosphokinase (CPK)' and the acetylcholine receptor (3, 18, 19, 28). In examining the relationship between growth and induction, a number of investigations have demonstrated the usefulness of causing quiescent, differentiated myoblasts under ' Abbreviations used in this paper: B-CPK, brain isozyme of CPK; CPK, creatine phosphokinase; DME, Dulbecco's modified Eagle's medium; FCS, fetal calf serum; FGF, fibroblast growth factor; LSM, low-serum medium; M-CPK, muscle isozyme of CPK. 1540 nonfusing conditions to reenter the cell cycle in response to a mitogenic stimulation (3-5, 22-25). A cell line particularly suitable for these studies is the muscle-like cell line, BC3H 1. After growth is arrested at moderate cell densities by exposure to low levels of fetal calf serum (FCS) the cells differentiate as defined by the expression of high concentrations of CPK as well as other muscle-specific proteins including the acetylcholine receptor; however, unlike myoblasts, these cells do not fuse (20, 30). Differentiation in this system has been shown to be reversible. That is, nongrowing cells that are induced to synthesize muscle-specific proteins will reinitiate cell growth in response to high levels of serum with a concomitant loss of the differentiated phenotype (23-25). This repression 2 appears to involve both transcriptional (23, 24) and posttrans2 Induction and repression are used to indicate situations in which the cells synthesize or fail to synthesize muscle-specific proteins. No particular mechanism of regulation is implied by these terms. Furthermore, repression in this context should not be confused with the use of the term to represent a temporal delay of differentiation. THE JOURNAL OF CELL BIOLOGY • VOLUME 100 MAY 1985 1540-1547 © The Rockefeller University Press • 0021-9525/85/05/1540/08 $1.00 on M ay 7, 2017 D ow nladed fom Published May 1, 1985