Density‐dependent inhibition of growth: fractionation of inhibitory diffusible factor(s) released by dense cultures of 3T3 cells

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Abstract
Different data suggested that the density-dependent inhibition of growth in normal 3T3 cells is due to the release of inhibitory factor(s) into the medium. The present results provide more evidence supporting this hypothesis. From conditioned medium of dense cultures of 3T3 cells, a thermolabile inhibitory diffusible factor (IDF) with a MW of .apprx. 40,000 was fractionated. This factor decreased the [14C]-inosine incorporation into purine nucleotides and into nucleic acids of stimulated 3T3 cells. DNA synthesis, determined by [14C]-thymidine incorporation into nucleic acids, and protein synthesis (determined by the protein content of the cultures maintained in the presence or absence of IDF), were also decreased. The possibility the IDF is involved in density-dependent inhibition of growth is discussed.