Loss of Protooncogene c-Myc Function Impedes G1 Phase Progression Both before and after the Restriction Point

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Abstract
C-myc is an important protooncogene whose misregulation is believed to causally affect the development of numerous human cancers. c-myc null rat fibroblasts are viable but display a severe (two- to threefold) retardation of proliferation. The rates of RNA and protein synthesis are reduced by approximately the same factor, whereas cell size remains unaffected. We have performed a detailed kinetic cell cycle analysis of c-myc −/− cells by using several labeling and synchronization methods. The majority of cells (>90%) in asynchronous, exponential phase c-myc −/−cultures cycle continuously with uniformly elongated cell cycles. Cell cycle elongation is due to a major lengthening of G1 phase (four- to fivefold) and a more limited lengthening of G2phase (twofold), whereas S phase duration is largely unaffected. Progression from mitosis to the G1 restriction point and the subsequent progression from the restriction point into S phase are both drastically delayed. These results are best explained by a model in which c-Myc directly affects cell growth (accumulation of mass) and cell proliferation (the cell cycle machinery) by independent pathways.