Proteases stimulate proliferation of human fibroblasts.

Incubation of primary human fibroblasts in serum-free medium with small concentrations of thrombin, trypsin or plasmin resulted in manyfold increase in total DNA synthesis and in the number of 3H-thymidine labelled cells. Rise in the frequency of mitoses indicates that the proteases stimulated also cell division. Because proteases induced only a fraction of cells to proliferate increase in the total number of cells remained moderate. Calf, horse and rabbit serum inhibited the growth stimulating effect of trypsin but chicken, dog and monkey serum were permissive. Specific inhibitors of proteases prevented the stimulation of cell proliferation suggesting strongly that proteases act in virtue of their enzymatic activity.