Differential mitogenic effects of methyl isobutyl xanthine and a tumor growth factor on G1-arrested 3T3 T proadipocytes at the predifferentiation GD state and the growth-factor deficiency GS state.

Two growth-states exist in the G1 phase of the 3T3 T proadipocyte cell cycle. GD is the arrest state at which proadipocytes must growth-arrest prior to differentiation. GS is the arrest state at which proadipocytes growth-arrest following deprivation of serum or growth factors. In an attempt to further distinguish these arrest states, we have compared the relative ability of a variety of mitogens to induce GD- and GS-arrested cells to initiate DNA synthesis. The data show that GD-arrested cells at both high and low densities can be induced to proliferate by methyl-isobutyl-xanthine (MIX), whereas high and low density GS-arrested cells are not. The data also show that a tumor growth factor can stimulate the proliferation of both high and low density GD- and GS-arrested cells, whereas other agents are poor mitogens for high density GD-arrested cells. We conclude that MIX and a tumor growth factor (TUGF) can serve as probes to study the characteristics of the GD arrest state.