Niacin Inhibits Vascular Inflammation via the Induction of Heme Oxygenase-1

Abstract

Background— Heme oxygenase-1 (HO-1) is a cytoprotective protein whose expression is consistently associated with therapeutic benefits in a number of pathological conditions such as atherosclerotic vascular disease and inflammation. Niacin is a pleiotropic drug that slows the progression of coronary artery disease and increases serum levels of the HO-1 enzymatic product bilirubin. This study asks if the cardioprotective properties of niacin involve the induction of HO-1. Methods and Results— New Zealand White rabbits received chow or chow supplemented with 0.6% (wt/wt) niacin for 2 weeks. Acute vascular inflammation was induced in the animals by placing a nonocclusive silastic collar around the left common carotid artery. At 24 hours after collar implantation, serum bilirubin and vascular, liver, and spleen HO-1 messenger RNA levels were significantly increased. Vascular inflammation was decreased in the niacin-supplemented animals compared with control. Treatment of the animals with tin protoporphyrin-IX, a global HO inhibitor, or HO-1 small interfering RNA to knock down carotid artery HO-1 attenuated the ability of niacin to inhibit vascular inflammation. Treatment of cultured human coronary artery endothelial cells with niacin increased HO-1 expression by activating the nuclear factor-E2–related factor 2/p38 mitogen-activated protein kinase signaling pathway and inhibiting tumor necrosis factor α–induced endothelial inflammation. The antiinflammatory effects of niacin in human coronary artery endothelial cells were mimicked by bilirubin and abolished by incubation with tin protoporphyrin-IX and knock down of nuclear factor-E2–related factor 2. Conclusions— Niacin activates HO-1 in vivo and in vitro. Induction of HO-1 may be partly responsible for the vascular protective properties of niacin.