THROMBIN POTENTLY STIMULATES CYTOKINE PRODUCTION IN HUMAN VASCULAR SMOOTH-MUSCLE CELLS BUT NOT IN MONONUCLEAR PHAGOCYTES

Thrombosis frequently occurs during atherogenesis and in response to v ascular injury. Accumulating evidence supports a role for inflammation in the same situation. The present study therefore sought links betwe en thrombosis and inflammation by determining whether thrombin, which is present in active form at sites of thrombosis, can elicit inflammat ory functions of human monocytes and vascular smooth muscle cells (SMC s), two major constituents of advanced atheroma. Human alpha-thrombin (EC(50), approximate to 500 pmol/L) potently induced interleukin (IL)- 6 release from SMCs. The tethered-ligand thrombin receptor appeared to mediate this effect. Furthermore, alpha-thrombin also rapidly increas ed levels of mRNA encoding IL-6 and monocyte chemotactic protein-1 (MC P-1) in SMCs. In contrast, only alpha-thrombin concentrations of great er than or equal to 100 nmol/L could stimulate release of IL-6 or tumo r necrosis factor-alpha (TNF alpha) in peripheral blood monocytes or m onocyte-derived macrophages. Lipid loading of macrophages did not augm ent thrombin responsiveness. Likewise, only alpha-thrombin concentrati ons of greater than or equal to 100 nmol/L increased levels of IL-6, I L-1 beta, MCP-1, or TNF alpha mRNA in monocytes. Differential response s of SMCs and monocytes to thrombin extended to early agonist-mediated increases in [Ca2+](i). SMCs and endothelial cells, but not monocytes , contained abundant mRNA encoding the thrombin receptor and displayed cell surface thrombin receptor expression detected with a novel monoc lonal antibody. Thus, the level of thrombin receptors appeared to acco unt for the differential thrombin susceptibility of SMCs and monocytes . These data suggest that SMCs may be more sensitive than monocytes/ma crophages to thrombin activation in human atheroma. Cytokines produced by thrombin-activated SMCs may contribute to ongoing inflammation in atheroma complicated by thrombosis or subjected to angioplasty.