Studies on the nature of protease-induced growth stimulation in normal and transformed BHK cells

In the presence of growth-limiting serum concentrations trypsin displays mitogenic activity on actively-growing but not quiescent BHK cells. These results suggest that BHK cells arrested in G1 (G0) are not sensitive to protease-induced growth stimulation. Previous work strongly suggested that the trypsin active-site is not directly involved in its mitogenic activity on BHK cells. Additional studies on denatured trypsin fragments further indicate that the molecular conformation and size of native trypsin may not be absolutely required for mitogenic activity. Cellular multiplication induced by the addition of fresh serum to quiescent BHK cultures is not inhibited by high concentrations of soybean trypsin inhibitor. Similar to our previous findings with trypsin, it has been further observed that plasmin is not sufficient to initiate the growth of BHK cells in soft agar. Trypsin also fails to enhance the growth of a thermosensitive polyoma-transformed BHK line in soft agar at the restrictive temperature. Finally, the growth of transformed BHK cells in soft agar does not display a requirement for plasminogen and is not inhibited by soybean trypsin inhibitor. These studies argue against the involvement of plasmin or other exogenous trypsin-like enzymes in the growth and transformation of BHK cells.