Time dependence of the methylation of myelin basic protein from bovine brain; evidence for protein-methylarginine demethylation

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 concentration is elevated in hypercholesterolemia, leading to endothelial dysfunction and producing proatherogenic changes of endothelial cell function. Four different isoforms of human PRMTs have been identified. Because the release of ADMA from human endothelial cells is increased in the presence of native or oxidized LDL cholesterol, we investigated the potential involvement of PRMT activity and gene expression in this effect. We found that the production of ADMA by human endothelial cells is upregulated in the presence of methionine or homocysteine and inhibited by either of the methyltransferase inhibitors S-adenosylhomocysteine, adenosine dialdehyde, or cycloleucine. This effect is specific for ADMA but not symmetrical dimethylarginine. The upregulation of ADMA release by native and oxidized LDL is abolished by S-adenosylhomocysteine and by the antioxidant pyrrollidine dithiocarbamate. Furthermore, a methyl-(14)C label is transferred from S-adenosylmethionine to ADMA but not symmetrical dimethylarginine, in human endothelial cells. The expression of PRMTs is upregulated in the presence of native or oxidized LDL. Our data suggest that the production of ADMA by human endothelial cells 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 stimulation of methyltransferases (5 to 50 micromol/L), ADMA significantly inhibited the formation of (15)N-nitrite from L-[guanidino-(15)N(2)]arginine. These findings suggest a novel mechanism by which ADMA concentration is elevated in hypercholesterolemia, leading to endothelial dysfunction and atherosclerosis.

RNA and protein interactions modulated by protein arginine methylation

This review summarizes the current status of protein arginine N-methylation reactions. These covalent modifications of proteins are now recognized in a number of eukaryotic proteins and their functional significance is beginning to be understood. Genes that encode those methyltransferases specific for catalyzing the formation of asymmetric dimethylarginine have been identified. The enzyme modifies a number of generally nuclear or nucleolar proteins that interact with nucleic acids, particularly RNA. Postulated roles for these reactions include signal transduction, nuclear transport, or a direct modulation of nucleic acid interactions. A second methyltransferase activity that symmetrically dimethylates an arginine residue in myelin basic protein, a major component of the axon sheath, has also been characterized. However, a gene encoding this activity has not been identified to date and the cellular function for this methylation reaction has not been clearly established. From the analysis of the sequences surrounding known arginine methylation sites, we have determined consensus methyl-accepting sequences that may be useful in identifying novel substrates for these enzymes and may shed further light on their physiological role.