LDL cholesterol upregulates synthesis of asymmetrical dimethylarginine in human endothelial cells - Involvement of S-adenosylmethionine-dependent methyltransferases

Asymmetrical dimethylarginine (ADMA) is an endogenous nitric oxide synthase inhibitor. It is formed by protein arginine N-methyltransferases (PRMTs), which utilize S-adenosylmethionine as methyl group donor. ADMA plasma conce ntration is elevated in hypercholesterolemia, leading to endothelial dysfun ction and producing proathero-genic changes of endothelial cell function. F our different isoforms of human PRMTs have been identified. Because the rel ease of ADMA from human endothelial cells is increased in the presence of n ative or oxidized LDL cholesterol, we investigated the potential involvemen t of PRMT activity and gene expression in this effect. We found that the pr oduction of ADMA by human endothelial cells is upregulated in the presence of methionine or homocysteine and inhibited by either of the methyltransfer ase inhibitors S-adenosylhomocysteine, adenosine dialdehyde, or cycloleucin e. This effect is specific for ADMA but not symmetrical dimethylarginine. T he upregulation of ADMA release by native and oxidized LDL is abolished by S-adenosylhomocysteine and by the antioxidant pyrrollidine dithiocarbamate. Furthermore, a methyl-C-14 label is transferred from S-adenosylmethionine to ADMA but not symmetrical dimethylarginine, in human endothelial cells. T he expression of PRMTs is upregulated in the presence of native or oxidized LDL. Our data suggest that the production of ADMA by human endothelial cel ls is regulated by S-adenosylmethionine-dependent methyltransferases. This activity is upregulated by LDL cholesterol, which may be due in part to the enhanced gene expression of PRMTs. In concentrations reached by stimulatio n of methyltransferases (5 to 50 mu mol/L), ADMA significantly inhibited th e formation of N-15-nitrite from L- [gunnidino-N-15(2)]arginine. These find ings suggest a novel mechanism by which ADMA concentration is elevated in h ypercholesterolemia, leading to endothelial dysfunction and atherosclerosis .