Olwin B B, Hauschka S D
Department of Biochemistry, University of Wisconsin, Madison 53706.
J Cell Biol. 1988 Aug;107(2):761-9. doi: 10.1083/jcb.107.2.761.
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.
骨骼肌分化的一个特征是增殖细胞转变为不可逆的有丝分裂后细胞群体。这种发育变化可在体外通过剥夺培养物中的特定促有丝分裂因子，如成纤维细胞生长因子（FGF）来诱导。对几种成年小鼠肌肉细胞系中的细胞表面FGF受体（FGFR）以及小鼠MM14细胞中的表皮生长因子受体（EGFR）进行分析，发现受体丢失与有丝分裂后表型的获得之间存在相关性。静止的MM14细胞、缺乏促有丝分裂因子且分化缺陷的MM14细胞以及分化的BC3H1肌肉细胞（一种在分化时不能进入有丝分裂后状态的细胞系）保留了它们的细胞表面FGFR。这些结果表明，FGFR的丢失与肌肉细胞增殖的可逆停止或生化分化均无关，因此，进一步支持了受体丢失与有丝分裂后表型获得之间的相关性。对生长因子受体丢失的动力学以及MM14细胞进入有丝分裂后表型的动力学进行比较，结果显示，在去除FGF后12小时，FGFR从大约700个受体/细胞短暂上升至最多约2000个受体/细胞，此时同时有超过95%的细胞进入有丝分裂后状态。然后，在去除FGF后24小时，FGFR水平降至检测不到的水平。在FGFR增加然后消失的这段时间内，其对FGF的亲和力没有变化。生长因子受体的短暂增加似乎是由于配体介导的内化作用减弱，因为当培养物被剥夺FGF时，EGFR会立即下降（Lim, R. W., and S. D. Hauschka. 1984. J. Cell Biol. 98:739 - 747），当细胞在含有EGF和FGF的培养基中生长，然后切换到不含这些添加因子的培养基中时，EGFR也表现出类似的短暂上升。这些结果表明，某些生长因子受体的丢失是骨骼肌分化过程中获得的一种特定表型，但它们并未解决FGFR数量的调节是否是有丝分裂后表型起始的原因。在讨论中提出了一个通用模型。
