Inhibition of prostaglandin E2 synthesis by SC-560 is independent of cyclooxygenase 1 inhibition

Prostaglandin E-2 (PGE(2)) produced by cyclooxygenase-2 (COX-2) and microsomal prostaglandin E-2 synthase-1 (mPGES-1) plays an important role in the pathophysiology of inflammation, pain, and fever. We investigated the actions of TNF alpha toward stimulation of PGE2 synthesis in primary spinal cord neurons. TNF alpha induced COX-2 and mPGES-1 expression in neurons, followed by formation of PGE(2), which was blocked by a selective COX-2 inhibitor. Surprisingly, the "selective COX-1" inhibitor SC-560 completely inhibited TNF alpha-induced PGE(2) synthesis in neurons at nanomolar concentrations. Moreover, SC-560 inhibited PGE(2) and thromboxane A(2) synthesis in human monocytes and platelets with IC50 of 1.8 nM and 2.5 nM, respectively. SC-560 treatment neither altered TNF alpha-induced COX-2 or mPGES-1 expression nor did the addition of the calcium ionophore A23187 or arachidonic acid reverse the inhibition by SC-560. Moreover, no influence of SC-560 on PGE(2) synthase activities or PGE(2) transport was seen. Most importantly, SC-560 blocked TNF alpha-induced PGE(2) synthesis in COX-1-deficient spinal cord neurons, demonstrating a COX-1-independent inhibition of PGE(2) synthesis. Although SC-560 inhibited LPS-induced PGE(2) synthesis in neurons and RAW264.7 macrophages in whole cell assays, no inhibition was observed in lysates of the same cells. Taken together our data demonstrate that SC-560 acts at least in some cell types as an unselective COX inhibitor despite its selectivity toward COX-1 under cell-free conditions.