One characteristic of skeletal muscle differentiation is the conversion of proliferating cells to a population that is irreversibly postmitotic. This developmental change can be induced in vitro by depriving the cultures of specific mitogens such as fibroblast growth factor (FGF). Analysis of cell surface FGF receptor (FGFR) in several adult mouse muscle cell lines and epidermal growth factor receptor (EGFR) in mouse MM14 cells reveals a correlation between receptor loss and the acquisition of a postmitotic phenotype. Quiescent MM14 cells, mitogen-depleted, differentiation-defective MM14 cells, and differentiated BC3H1 muscle cells (a line that fails to become postmitotic upon differentiation) retained their cell surface FGFR. These results indicate that FGFR loss is not associated with either reversible cessation of muscle cell proliferation or biochemical differentiation and thus, further support a correlation between receptor loss and acquisition of a postmitotic phenotype. Comparison of the kinetics for growth factor receptor loss and for commitment of MM14 cells to a postmitotic phenotype reveals that FGFR rises transiently from approximately 700 receptors/cell to a maximum of approximately 2,000 receptors/cell 12 h after FGF removal, when at the same time, greater than 95% of the cells are postmitotic. FGFR levels then decline to undetectable levels by 24 h after FGF removal. During the interval in which FGFR increases and then disappears there is no change in its affinity for FGF. The transient increase in growth factor receptors appears to be due to a decrease in ligand-mediated internalization because EGFR, which undergoes an immediate decline when cultures are deprived of FGF (Lim, R. W., and S. D. Hauschka. 1984. J. Cell Biol. 98:739-747), exhibits a similar transient rise when cultures are grown in media containing both EGF and FGF before switching the cells to media without these added factors. These results indicate that the loss of certain growth factor receptors is a specific phenotype acquired during skeletal muscle differentiation, but they do not resolve whether regulation of FGFR number is causal for initiation of the postmitotic phenotype. A general model is presented in the discussion.

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