The antioxidant effects of isorhamnetin contribute to inhibit COX-2 expression in response to inflammation: a potential role of HO-1.

Previously, we reported that isorhamnentin, a 3'-O-methylated metabolite of quercetin, reduced inducible nitric oxide synthase (iNOS) expression and NO production. The present study further investigated the underlying mechanism of anti-inflammatory and antioxidant effects of isorhamnentin. Administration of isorhamnetin decreased the number of cyclooxygenase-2 (COX-2) positive cells in rats with carrageenan-induced paw edema. Isorhamnetin also suppressed lipopolysaccharide (LPS)-induced expression of COX-2 in cells. It is well known that LPS-induced reactive oxygen species (ROS) production leads to COX-2 induction. Isorhamnetin decreased LPS-induced ROS production and apoptosis. In addition, the basal expression of heme oxygenase-1 (HO-1) was increased by isorhamnetin treatment in agreement with the increase in nuclear translocation of NF-E2-related factor-2 (Nrf2), an essential transcription factor for the regulation of HO-1 expression. Moreover, pretreatment of tin protoporphyrin IX (SnPP), a chemical inhibitor of HO-1, reversed the ability of isothamnetin to inhibit COX-2 expression. These results demonstrate that induction of HO-1 by isorhamnetin leads to a reduction in ROS production and its antioxidant property might contribute to the inhibition of COX-2 expression in response to inflammation.