Synthesis of labile, serum-dependent protein in early G 1 controls animal cell growth

Abstract

A model is presented 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 [Swiss mouse fibroblast 3T3 cells] whose ability to synthesize total protein was inhibited with cycloheximide (by up to 70%) grow at rates approximately proportional to their rates of protein synthesis. Virtually the entire elongation of the cell cycle occurs in the part of the G1 phase that depends on a high concentration of serum in the medium. This part of the cycle was suggested to lie prior to the restriction point, i.e., the point beyond the main regulatory processes of G1. The remainder of the cycle, from restriction point to mitosis, is markedly insensitive to these concentrations of cycloheximide and to growth regulation. The specific lengthening of that part of the cycle involved in growth regulation is quantitatively accounted for 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 .apprx. 2 h) 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 accounts for greater variations of G1 durations as the growth of cell populations is made slower. It predicts 2 sorts of quiescence: 1 of cells slowly traversing G1, in slightly suboptimal conditions; the other of cells that enter G0 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.