Inhibition of the diclofenac-induced cyclooxygenase-2 activity by paracetamol in cultured macrophages is not related to the intracellular lipid hydroperoxide tone.

ABSTRACT

Paracetamol, a weak inhibitor of cyclooxygenase COX-1 and COX-2 activities, has been reported to inhibit the activity of COX-2 induced by diclofenac in J774.2 macrophage cell line. The lack of inhibition of COX-2 by paracetamol in inflamed tissues and thereby the lack of anti-inflammatory activity has been attributed to high lipid hydroperoxide (LHP) tone. In this study, we demonstrate that the inhibition of the diclofenac-induced COX-2 activity in J774.2 cells by paracetamol is not related to the intracellular LHP tone as paracetamol inhibited this activity in the absence and presence of T-butyl hydroperoxide, which is an LHP donor, to the same extents. In fact, treatment of the cells with diclofenac resulted in an increase in the LHP tone. Stimulation of the cells with lipopolysaccharide (LPS) results in the induction of a COX-2 activity, which was not inhibited by paracetamol. This represents the classical induction pathway for COX-2. LPS stimulation did not alter the LHP tone. These results suggest that the enzymatic activity of the diclofenac-induced COX-2 protein does not depend on the supply of hydroperoxides to its peroxidase active site.