Reciprocal regulation of NADPH oxidases and the cyclooxygenase-2 pathway

The objective of this work was to analyze the possible association between cyclooxygenase-2 (COX-2) and NADPH oxidases (NOX) in liver cells, in response to various proinflammatory and toxic insults. First, we observed that treatment of Chang liver (CHL) cells with various COX-2 inducers increased reactive oxygen species (ROS) production concomitant with GSH depletion, phorbol 12-myristate 13-acetate (PMA) being the most effective treatment. Moreover, early changes in the oxidative status induced by PMA were inhibited by glutathione ethyl ester, which also impeded COX-2 induction. In fact, CHL cells expressed NOX1 and NOX4, although only NOX4 expression was up-regulated in the presence of PMA. Knock-down experiments suggested that PMA initiated a pathway in which NOX1 activation controlled COX-2 expression and activity, which, in turn, induced NOX4 expression by activation of the prostaglandin receptor EP4. In addition, CHL cells overexpressing COX-2 showed higher NOX4 expression and ROS content, which were decreased in the presence of the COX-2 inhibitor DFU. Interestingly, we found that addition of prostaglandin E(2) (PGE(2)) also induced NOX4 expression and ROS production, which might promote cell adhesion. Finally, we determined that NOX4 induction by PGE(2) was dependent on ERK1/2 signaling. Taken together, these results indicate that NOX proteins and COX-2 are reciprocally regulated in liver cells.