Human alpha-Thrombin Stimulates Proliferation of Interferon-gamma Differentiated, Growth-Arrested, U937 Cells, Overcoming Differentiation-Related Changes in Expression of p21CIP1/WAF1 and Cyclin D1

In addition to its central role in blood coagulation and hemostasis, human alpha-thrombin is a growth factor for a variety of cell types. We recently demonstrated that interferon-gamma (IFNgamma)-differentiated U937 cells show increased expression of the proteolytically activated receptor for thrombin (PAR-1) relative to undifferentiated U937. In the present study we show that cell proliferation is inhibited in IFNgamma-differentiated cells relative to undifferentiated U937. Addition of thrombin to the differentiated cells, however, overcomes the inhibition and restores the cells to a highly proliferative state. Ribonuclease protection assays indicate that the IFNgamma-induced growth arrest is associated with an increased expression of the cyclin-dependent kinase inhibitor p21(CIP1/WAF1) and downregulation of cyclin D(1). Treatment of cells with thrombin downregulates p21(CIP1/WAF1) expression in these cells and upregulates cyclin D(1) mRNA expression, thus overcoming the differentiation-related effects in a coordinated manner. Treating differentiated cells with the PAR-1 activation peptide, SFLLRN, stimulates proliferation and has effects similar to those of thrombin on expression of p21(CIP1/WAF1). Thus, it appears that these thrombin stimulated proliferative effects are mediated through activation of PAR-1. These results may help explain how thrombin can overcome growth arrest in normal tissue to initiate tissue repair and why thrombin and thrombin-like enzymes may contribute to unrestricted proliferation observed in certain malignancies.