| Abstract: | It has been well documented that trypsin or thrombin initiate proliferation of quiescent secondary chick embryo cells. However, there has been less certainty about the ability of these proteases to initiate division of quiescent mammalian cells. Accordingly, we studied the conditions under which quiescent chick embryo (CE), mouse embryo (ME), and human diploid foreskin (HF) cells respond to trypsin or thrombin Extended culture of these cell strains in serum-free medium increased the initiation of cell division by both proteases. Under these conditions, CE cell number was increased 90% over controls by trypsin and 70% by thrombin. In contrast, quiescent ME and HF cells both responded better to thrombin than trypsin, giving maximal increases, respectively, of 70 and 40% over controls with thrombin and 22 and 14% with trypsin. Calf serum inhibited the initiation of these cell strains, particularly the ME cells, by both trypsin and thrombin. This inhibition of initiation could be attributed to decreased proteolytic activity in the case of trypsin, but not thrombin In contrast to the cell strains tested, quiescent cultures of the 3T3 cell line showed no detectable increase in cell number with trypsin or thrombin in the absence of serum, and only a slight increase in cell number with thrombin in the presence of serum. However, in the presence of plasma, 3T3 cell number increased up to 20% with thrombin Initiation of cell division by proteases has been reported and confirmed mostly for early passage cell strains rather than cell lines. Our experiments with CE cells indicate that this is possibly the result of a rapid decline in protease responsiveness upon serial subculture. With these cells we found a decline in response first to trypsin, then thrombin, and finally serum Throughout these studies, we compared the ability of trypsin and thrombin to initiate cell division under various conditions. We found several differences between the two proteases, indicating that they initiate cell division by somewhat different mechanisms. |
