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Serum contains a growth factor derived from platelets and also growth factors derived from platelet-poor plasma. Extracts of heated (100°) human platelets function synergistically with platelet-poor plasma to induce DNA synthesis in quiescent, density-inhibited BALB/c 3T3 cells. Platelet-poor plasma alone did not induce DNA synthesis. Cells exposed to platelet extracts became competent to enter the cell cycle, but the rate of entry into the S phase depended upon the concentration of platelet-poor plasma. The time required for the induction of this competent state was a function of the concentration of the platelet extract. A 2-hr exposure to 100 μg of the platelet extract at 37° caused the entire cell population to become competent to enter the S phase. At 4° or 25° the cells did not become competent to synthesize DNA. The platelet extract-induced competent state was stable for at least 13 hr after removal of the platelet extract; however, in the absence of platelet-poor plasma, these competent cells did not progress through the cell cycle. The addition of an optimal concentration of platelet-poor plasma (5%) to these competent cells initiated cell cycle traverse with a rapid, first-order entry of cells into the S phase beginning 12 hr after addition of the plasma. The addition of a suboptimal concentration of the plasma (0.25%) did not increase the rate of cell entry into the S phase. Thus, the induction of DNA synthesis in quiescent BALB/c 3T3 cells can be resolved into at least two phases, controlled by different serum components: (i ) competence, induced by the platelet-derived growth factor; and (ii ) progression of competent cells into the cell cycle, mediated by factors in platelet-poor plasma.

Induction of DNA synthesis in BALB/c 3T3 cells by serum components: Reevaluation of the commitment process