Inhibition of Endothelial Nitric Oxide Synthase by C-Reactive Protein: Clinical Relevance

Abstract

In 2002, 2 groups independently reported that C-reactive protein (CRP) inhibits endothelial nitric oxide synthase (eNOS) activity and bioactivity (1)(2). There was much reported debate as to whether these effects were artifacts attributable to CRP contamination with azide and endotoxin. In cells in which the endotoxin receptor Toll-like receptor 4 was knocked down by use of the small-interfering RNA technique, investigators convincingly showed that extensively dialyzed solutions of CRP inhibited eNOS, confirming that this inhibition was not an effect of endotoxin or azide(3). In a subsequent study, the molecular mechanism by which CRP inhibits eNOS was elucidated(4). This study demonstrated that CRP inhibited GTP cyclohydrolase 1 and stimulated NADPH oxidase, causing a decrease in tetrahydrobiopterin and an increase in reactive oxygen species, resulting in uncoupling of eNOS, which led to decreased eNOS activity, decreased phosphorylation of Ser1177 of eNOS, and decreased eNOS binding to Hsp90. Other investigators had shown that CRP also impairs vasoreactivity in vivo. In particular, Mineo and coworkers demonstrated in C57BL mice that intraperitoneal administration of CRP (250 μg) compared to administration of a vehicle control impaired acetylcholine-induced carotid artery vascular conductance by 50%, but no mechanistic insights were provided(5). Until the results of these 2 studies were reported, it had not been appreciated that endotoxin stimulates eNOS whereas CRP inhibits eNOS(4)(5). In 2007, Schwedler and coworkers (6) showed that subcutaneous administration of CRP for 8 weeks induced endothelial dysfunction in apolipoprotein E−/− mice, and that this effect was reversed with an inducible NOS inhibitor(6). However, these …