On the interpretation of the shortening of the G1-phase by overexpression of cyclins in mammalian cells.

Increasing the concentration of cyclins in mammalian cells leads to a shortening of the G1-phase of the division cycle. This observation has been interpreted as indicating that these cyclins act during, and are rate limiting for, passage through the G1-phase. Here it is argued that it is not possible to interpret experiments involving cyclin overexpression-induced changes in the lengths of individual cell cycle phases without considering changes in the overall cellular growth rate. A rigorous reanalysis of these experiments demonstrates that the results are consistent with the proposal that the shortening of the G1-phase is merely due to an increase in the rate of mass synthesis in all phases of the cell cycle. Increased cyclin concentrations leads to a faster rate of mass synthesis and a concomitant shortening of the G1-phase. Cyclins can also affect the length of the S- and G2-phases, which leads to the observed shortening of the G1-phase. Thus, the experiments on cyclin overexpression and their effect on G1-phase length cannot be used to support the proposal that cyclins act specifically during the G1-phase of the division cycle.