Control of growth of benzo(a)pyrene-transformed 3T3 cells.

The growth controls observed in benzo[a]pyrene-transformed 3T3 cells (BP3T3) are compared with those of virus-transformed and normal 3T3 cells. Superficially, the chemically transformed BP3T3 cells have the same behavior as virus-transformed SV3T3 cells. Both grow to high cell density in culture medium with 10% serum, both form colonies in Methocel, and both are tumorigenic. Closer examination, however, has disclosed that BP3T3 cells exhibit "normal" growth controls at low serum concentrations. In contrast to the behavior of SV3T3 cells, the initiation of DNA synthesis in BP3T3 cells is still dependent on a serum factor. If BP3T3 cells are grown in medium with 0.2% serum, the cells become quiescent, with growth arrested in the Gu or G0 phase of the cell cycle. The addition of serum or the fibroblast growth factor (FGF) to such quiescent cells leads to the initiation of DNA synthesis and the resumption of growth. As with normal 3T3 cells, if the growth rate of BP3T3 cells is limited by a suboptimal concentration of serum, the growth rate of the cells is increased by the addition of FGF. Also, BP3T3 cells show density-dependent regulation of growth, if the medium contains a low concentration of serum. BP3T3 cells, therefore, have the behavior of "transformed" cells when cultured in medium with 10% serum, but behave as "normal" cells in medium with low serum. In comparison with normal 3T3 cells, the difference in growth behavior of BP3T3 cells appears to be due to a substantial decrease in the cells' requirement for a serum growth factor of the FGF type. Exploration of possible causes of this substantial decrease indicates that the primary cause is a lower rate of depletion of the serum growth factor from the culture medium by BP3T3 cells. The decrease in rate of depletion is sufficient to account for the uncontrolled growth of BP3T3 cells in medium with 10% serum. It is suggested that a decreased rate of depletion of a growth factor may contribute to tumorigenicity of cells in vivo.