Downstream effects on human low density lipoprotein of homocysteine exported from endothelial cells in an in vitro system

Abstract

A model system is presented using human umbilical vein endothelial cells (HUVECs) to investigate the role of homocysteine (Hcy) in atherosclerosis. HUVECs are shown to export Hcy at a rate determined by the flux through the methionine/Hcy pathway. Additional methionine increases intracellular methionine, decreases intracellular folate, and increases Hcy export, whereas additional folate inhibits export. An inverse relationship exists between intracellular folate and Hcy export. Hcy export may be regulated by intracellular S -adenosyl methionine rather than by Hcy. Human LDLs exposed to HUVECs exporting Hcy undergo time-related lipid oxidation, a process inhibited by the thiol trap dithionitrobenzoate. This is likely to be related to the generation of hydroxyl radicals, which we show are associated with Hcy export. Although Hcy is the major oxidant, cysteine also contributes, as shown by the effect of glutamate. Finally, the LDL oxidized in this system showed a time-dependent increase in uptake by human macrophages, implying an upregulation of the scavenger receptor. These results suggest that continuous export of Hcy from endothelial cells contributes to the generation of extracellular hydroxyl radicals, with associated oxidative modification of LDL and incorporation into macrophages, a key step in atherosclerosis. Factors that regulate intracellular Hcy metabolism modulate these effects. -Nakano, E.,

Responses of Biomarkers of Folate and Riboflavin Status to Folate and Riboflavin Supplementation in Healthy and Colorectal Polyp Patients (The FAB2 Study)

“…This supports the idea of a ''field effect'' in people with enhanced risk of colorectal cancer (35). The observed inverse association between colon mucosal folate and plasma homocysteine reflects the importance of cellular folate metabolism to extracellular homocysteine concentrations (36), and confirms that modest folic acid supplements can elicit significant homocysteine-lowering (37). The study has revealed interactions between genotype and histology in determining response to folate and riboflavin supplementation.…”

Role of homocysteinylation of ACE in endothelial dysfunction of arteries

“…3). In support of these findings, other investigators have demonstrated that Met-loaded endothelial cells exhibit increased accumulation and enhanced extracellular transport of Hcy metabolites, factors that are associated with an upregulation of inflammatory molecules, oxidative stress, and an increased susceptibility to coronary artery disease (35,43). Furthermore, studies of minipigs on a methionine-rich caseinate-based diet (42) and in patients with prevalent cardiovascular disease (50) show that Met intake correlates positively with hypertension.…”

Serum Cystatin C Is a Determinant of Paraoxonase Activity in Hemodialyzed and Renal Transplanted Patients

“…homocysteine thiolactone. Homocysteine thiolactone forms homocysteine thiolactone-LDL-aggregates with the increased amount of LDL particles in chronic re-nal failure, which augments the phagocytotic activity of macrophages, and leads to an increased formation of foam cells in the atherosclerotic plaques [28]. The homocysteinylation of LDL particles increases their susceptibility to oxidation, induces the production of antihomocysteinyllysine antibodies, and accelerates their uptake to macrophages [29,30].…”