Synthesis of labile, serum-dependent protein in early G1 controls animal cell growth

We present a model to account for several major observations on growth control of animal cells in culture. This model is tested by means of kinetic experiments which show that exponentially growing animal cells whose ability to synthesize total protein has been inhibited with cycloheximide (by up to 70%) grow at rates approximately proportional to their rates of protein synthesis. However, virtually the entire elongation of the cell cycle occurs in the part of the G(1) phase that depends on a high concentration of serum in the medium. This part of the cycle has earlier been suggested to lie prior to the restriction point-i.e., the point beyond the main regulatory processes of G(1). The remainder of the cycle, from restriction point to mitosis, is markedly insensitive to these concentrations of cycloheximide as well as to growth regulation. We quantitatively account for the specific lengthening of that part of the cycle involved in growth regulation by assuming that cells must accumulate a specific protein in a critical amount before they can proceed beyond the restriction point. The lability of this protein (half-life about 2 hr) makes its accumulation unusually sensitive to inhibition of total protein synthesis by cycloheximide. Its production appears to depend on growth factors provided by serum. The model can also account for greater variations of G(1) durations as the growth of cell populations is made slower. It also predicts two sorts of quiescence: one of cells slowly traversing G(1), in slightly suboptimal conditions; the other of cells that enter G(0) under inadequate conditions. Transformation of different sorts could create cells with altered variables for initiation, synthesis, or inactivation of the regulatory protein or could altogether eliminate the need for the protein.