Induction of hepatic cyclooxygenase-2 by hyperhomocysteinemia via nuclear factor-κB activation

Abstract

Wu N, Siow YL, O K. Induction of hepatic cyclooxygenase-2 by hyperhomocysteinemia via nuclear factor-B activation. Am J Physiol Regul Integr Comp Physiol 297: R1086 -R1094, 2009. First published August 5, 2009 doi:10.1152/ajpregu.00293.2009.-Hyperhomocysteinemia, an elevation of blood homocysteine (Hcy), is a metabolic disorder associated with dysfunction of multiple organs. Apart from endothelial dysfunction, Hcy can cause hepatic lipid accumulation and liver injury. However, the mechanism responsible for Hcy-induced liver injury is poorly understood. The aim of this study was to investigate the regulation of cyclooxygenase-2 (COX-2), a proinflammatory factor, expression in the liver during the initial phase of hyperhomocysteinemia. Sprague-Dawley rats were fed a high-methionine diet for 1 or 4 wk. Serum and liver concentrations of Hcy were significantly elevated after 1 or 4 wk of dietary treatment. COX-2 mRNA and protein levels were significantly elevated in the liver of hyperhomocysteinemic rats. The induction of COX-2 expression was more prominent in 1-wk hyperhomocysteinemic rats than that in the 4-wk group. EMSA revealed an activation of NF-B in the same liver tissue in which COX-2 was induced. Administration of a NF-B inhibitor to hyperhomocysteinemic rats effectively abolished hepatic COX-2 expression, inhibited the formation of inflammatory foci, and improved liver function. Further investigation revealed that oxidative stress due to increased superoxide generation was responsible for increased phosphorylation and degradation of IB␣ leading to NF-B activation in the liver. Administration of 4-hydroxy-tetramethyl-piperidine-1-oxyl, an SOD mimetic, to hyperhomocysteinemic rats not only inhibited NF-B activation but also prevented hepatic COX-2 induction and improved liver function. These results suggest that hyperhomocysteinemia-induced COX-2 expression is mediated via NF-B activation. Increased oxidative stress and inflammatory response may contribute to liver injury associated with hyperhomocysteinemia.homocysteine; oxidative stress; inflammatory response; liver injury HYPERHOMOCYSTEINEMIA IS A metabolic disorder characterized by an elevation of total plasma homocysteine (Hcy) levels (26). Epidemiological studies indicate that hyperhomocysteinemia is associated with dysfunction of multiple systems, including cardiovascular and cerebrovascular disease, osteoporosis, glomerulosclerosis, and fatty liver (15,23,33,41,42,50). Hcy is an intermediate amino acid produced in cells through metabolism of methionine to cysteine. The cellular homeostasis of Hcy is tightly regulated and any perturbation of Hcy metabolism causes an increase in its cellular concentrations, leading to hyperhomocysteinemia. Moderately fatty liver (lipid accumulation) has been observed in the autopsy of pediatric patients with hyperhomocysteinemia (25). Hcy, at elevated levels, causes abnormal lipid metabolism, endoplasmic reticulum stress, and oxidative stress in hyperhomocysteinemic rodents (45,46,49). It is increasingly recognized t...

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