Effects of homocysteine on endothelial nitric oxide production

Abstract

Hyperhomocysteinemia (HHCy) is an independent and graded cardiovascular risk factor. HHCy is prevalent in patients with chronic renal failure, contributing to the increased mortality rate. Controversy exists as to the effects of HHCy on nitric oxide (NO) production: it has been shown that HHCy both increases and suppresses it. We addressed this problem by using amperometric electrochemical NO detection with a porphyrinic microelectrode to study responses of endothelial cells incubated with homocysteine (Hcy) to the stimulation with bradykinin, calcium ionophore, or L-arginine. Twenty-four-hour preincubation with Hcy (10, 20, and 50 microM) resulted in a gradual decline in responsiveness of endothelial cells to the above stimuli. Hcy did not affect the expression of endothelial nitric oxide synthase (eNOS), but it stimulated formation of superoxide anions, as judged by fluorescence of dichlorofluorescein, and peroxynitrite, as detected by using immunoprecipitation and immunoblotting of proteins modified by tyrosine nitration. Hcy did not directly affect the ability of recombinant eNOS to generate NO, but oxidation of sulfhydryl groups in eNOS reduced its NO-generating activity. Addition of 5-methyltetrahydrofolate restored NO responses to all agonists tested but affected neither the expression of the enzyme nor formation of nitrotyrosine-modified proteins. In addition, a scavenger of peroxynitrite or a cell-permeant superoxide dismutase mimetic reversed the Hcy-induced suppression of NO production by endothelial cells. In conclusion, electrochemical detection of NO release from cultured endothelial cells demonstrated that concentrations of Hcy >20 microM produce a significant indirect suppression of eNOS activity without any discernible effects on its expression. Folates, superoxide ions, and peroxynitrite scavengers restore the NO-generating activity to eNOS, collectively suggesting that cellular redox state plays an important role in HCy-suppressed NO-generating function of this enzyme.

Endothelial Dysfunction

“…Folic acid supplementation was able to reduce homocysteine levels and improve endothelial dysfunction in children with chronic renal failure (72). Cellular (73), animal (71), and human studies (22) suggest that homocysteine reduces NO bioavailability by oxidative excess. There is now also evidence that homocysteine may cause ADMA accumulation by inhibition of DDAH (74).…”

“…Thus, decrease of homocysteine levels in hyperhomocysteinemia by supplementation with folic acid can improve endothelial dysfunction (72,73). LArginine (137) and tetrahydrobiopterin, as well as tetrahydrobiopterin mimetics (138), may improve endothelial function via increased NO bioavailability.…”

Cardiac Allograft Vasculopathy: A Review of Risk Factors and Pathogenesis

“…Hyperhomocysteinemia [141][142][143][144][145][146][147] and high C-Reactive protein (CRP) [148][149][150][151][152] levels are few of the novel risk factors that have been reported to contribute to the pathogenesis of the CAV. High levels of homocysteine can reduce the endothelial nitric oxide production and cause endothelial dysfunction [144,146].…”

Role of homocysteinylation of ACE in endothelial dysfunction of arteries

“…Excessive accumulation of Hcy and HCTL can result in cellular dysfunction by covalently binding to proteins at cysteine (S-homocysteinylation) or at lysine (N-homocysteinylation) moieties, thereby causing alterations in protein function and/or aggregation of protein (21,45). Previous studies have characterized several proteins that are targeted by Hcy (48), including NOS (41,57), dimethylarginine dimethylaminohydrolase (46), LDL lipoproteins (9), and metallothioneins (1). In all cases, homocysteinylation modified their capacity to regulate metabolic pathways.…”

“…Not only does Hcy itself have prooxidant properties, but also accumulating evidence from clinical and basic science studies indicate that the primary pathogenesis of diseases associated with HHcy is a concomitant generation of superoxide anion, which inactivates NO (7,27,28,51,56,58). To date, several studies have suggested the existence of an interaction between Hcy and angiotensin II (ANG II) signaling as a pathway that contributes significantly to the potential oxidative stress in the vasculature (2,31).…”