The Antiangiogenic Agent SU5416 Down-Regulates Phorbol Ester-Mediated Induction of Cyclooxygenase 2 Expression by Inhibiting Nicotinamide Adenine Dinucleotide Phosphate Oxidase Activity

Increased expression of cyclooxygenase (COX) 2 and the production of PGs appear to provide a survival advantage to transformed cells through the inhibition of apoptosis, increased attachment to extracellular matrix, increased invasiveness and the stimulation of angiogenesis. The purpose of this study was to determine whether an angiogenic antagonist, SU5416, could inhibit endogenous and phorbol 12-myristate 13-acetate (PMA)-mediated induction of COX-2 expression. SU5416 (5 μM) inhibited endogenous as well as PMA-mediated induction of COX-2 expression when analyzed by immunoblot and Northern blot analysis. However, COX-1 expression remained unchanged under similar conditions. PMA is a potent inducer of reactive oxygen species that can play an important role during the induction of COX-2 expression. Our results demonstrated that PMA-mediated induction of COX-2 expression was found to be dependent on NADPH oxidase activity. An inhibitor of NADPH oxidase (diphenyleneiodonium chloride) blocked the PMA-mediated induction of COX-2 expression. The oxidase complex exhibited a temporal pattern of activation after exposure to PMA in which maximum activation was observed at 30 min after the addition of PMA. Activation of NADPH oxidase was also inhibited by SU5416, whereas an inhibitor of epidermal growth factor receptor signaling was unable to prevent the PMA-mediated induction of NADPH oxidase activity. When we blocked the PMA-mediated production of reactive oxygen species by blocking NADPH oxidase with SU5416, COX-2 expression and PGE₂ synthesis were also inhibited. Our results suggest that inhibition of NADPH oxidase activity, blocking of COX-2 expression, and PGE₂ synthesis may represent novel targets for SU5416.