Both hypoxanthine and adenosine stimulate DNA synthesis independently in serum-starved L cells treated with platelet protein.

The effect of human platelet factors and purine derivatives on DNA synthesis has been investigated in mouse fibroblast-like L cells whose growth was arrested by serum starvation. When such cells were exposed to diluted platelet extract (e.g., 35 micrograms of protein per ml), a stimulatory effect on net DNA synthesis was observed. This effect was almost abolished by dialysis of the extract. The stimulation was, however, recovered by supplementing the diluted and dialyzed extract with hypoxanthine or adenosine. Similar phenomena were observed in pulse-labeling experiments performed with [3H]thymidine. In this case, however, there was a marginal stimulatory effect of adenosine or hypoxanthine alone. When the cells were treated with saturating concentrations of pure platelet-derived growth factor (PDGF), a stimulatory effect on pulse labeling was again obtained by the simultaneous presence of hypoxanthine or adenosine. In serum-starved cells of a mutant line of L cells deficient in hypoxanthine phosphoribosyltransferase, there was, however, no stimulatory effect on pulse labeling by hypoxanthine when it was added alone or together with either PDGF or diluted dialyzed platelet extract. It is suggested that the stimulation of DNA synthesis by the purine derivatives in the presence of a certain type of platelet proteins, probably involving PDGF, may be explained by their function as precursors for a purine ribonucleotide pool that is specifically related to DNA synthesis. Treatment of serum-starved L cells with high concentrations of dialyzed platelet extract (e.g., 240 micrograms of protein per ml) showed that platelets contain an additional type of factor that may substitute for the requirement of adenosine or hypoxanthine for DNA synthesis to take place. It is suggested that the effect of this type of factor may be to activate the catabolic activity of the purine salvage pathway.