THROMBIN RECEPTOR EXPRESSION AND RESPONSIVENESS OF HUMAN MONOCYTIC CELLS TO THROMBIN IS LINKED TO INTERFERON-INDUCED CELLULAR-DIFFERENTIATION

Human thrombin has been shown to stimulate monocyte chemstaxis, phagoc ytosis, and interleukin (IL8) production, but the mechanisms responsib le for stimulation are not well defined. In some cells, thrombin stimu lation of proliferation appears to require both cleavage of the proteo lytically activated receptor for thrombin (PAR1) and activation of a n onproteolytically activated thrombin receptor (N-PAR), while in others activation of either receptor alone may be sufficient for stimulation . We, therefore, have initiated studies to address thrombin receptor e xpression and cell responsiveness to thrombin in interferon gamma (IFN gamma)-differentiated and nondifferentiated U937 monocytic cells. Nor thern blot analysis shows that PAR1 expression is upregulated upon dif ferentiation. Experiments with biotinylated and I-125-thrombin show th at specific thrombin binding is dramatically increased by differentiat ion although it is not clear if this binding is to PAR1 or to a separa te binding component such as N-PAR which is present on fibroblasts and other cells. Addition of thrombin at concentrations of 1-10 mu g/ml ( 30-300 nM, concentrations where specific thrombin binding is observed) stimulates proliferation of IFN gamma-differentiated U937 cells but n ot of undifferentiated U937 cells. Thrombin also stimulates interleuki n-6 (IL6) production in IFN gamma-differentiated U937 cells. Moreover, thrombin induces high levels of IL6, interleukin-1 beta (IL-1 beta), and tumor necrosis factor-alpha (TNF alpha) production by peripheral b lood mononuclear cells (PBMC) and monocytes. These results show that d ifferentiated U937 cells and mature PBMC are responsive to thrombin wh ereas nondifferentiated U937 are not. Further, this responsiveness app ears to correlate with expression of PAR1 and to a dramatic increase i n specific thrombin binding. That thrombin stimulates cytokine product ion and proliferation in populations of differentiated monocytes sugge sts that thrombin may be an important regulator of inflammation and wo und healing. J. Cell. Physiol. 177:76-84, 1998. (C) 1998 Wiley-Liss, I nc.