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Publicação
Disturbed protein arginine methylation in hyperhomocysteinemia
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Resumo(s)
Homocysteine (Hcy) is a metabolite of the essential amino acid methionine. Elevation of Hcy in plasma, or hyperhomocysteinemia (HHcy), is associated with cardiovascular and neurological diseases by mechanisms still ill-defined. Cellular hypomethylation secondary to build-up of S-adenosylhomocysteine (AdoHcy), the Hcy precursor, may be involved in the pathophysiology of HHcy. AdoHcy is the product of most methylation reactions and, simultaneously, a competitive inhibitor of those reactions.Our previous investigation has consistently shown that excess AdoHcy can suppress DNA methylation. The working hypothesis for this thesis postulates that, in addition to modulating DNA methylation, AdoHcy may also decrease protein arginine methylation in HHcy. This widespread post-translational modification is emerging as an important player in cell homeostasis.To test our working hypothesis, we used a cell model in which AdoHcy accumulation was achieved by pharmacological inhibition of AdoHcy production, and two murine models in which the Hcy metabolism was manipulated to promote different patterns of accumulation of Hcy and AdoHcy. Our results documented suppression of global protein arginine methylation induced by AdoHcy accumulation in all studied models. In the cell model, we observed hypomethylation of both DNA and arginine residues in proteins, the effect on proteins being more pronounced that on DNA. In the animal models, decreased arginine methylation was observed in heart and brain of hyperhomocysteinemic rats (moderate HHcy induced by diet), and in liver and brain of mice with severe HHcy due to a genetic defect in Hcy metabolism. Additionally, we were able to document the ability of AdoHcy to decrease histone arginine methylation, a key modulator of gene expression, in the animal model of severe HHcy. Taken together, these studies show that protein arginine hypomethylation may be involved in the mechanistic etiology of disease states associated with HHcy.We also determined the effect of folate (a nutritional player in Hcy metabolism) treatment and methionine availability on the levels of AdoHcy and protein arginine methylation in our cell model. In fact, this information may be particularly relevant within the context of several recent clinical trials in which folate supplementation reduced Hcy levels, but not cardiovascular disease risk. Our results suggest that Hcy-lowering with folate may be effective in reducing intracellular AdoHcy levels, and therefore in augmenting cellular methylation capacity, only if used in combination with methionine restriction.Lastly, we focused on one of the most extensive methylation reactions in mammals, the reaction catalyzed by guanidinoacetate methyltransferase. By documenting increased levels of guanidinoacetate in liver and brain of mice with severe HHcy when compared to controls, we reinforce the overall AdoHcy inhibitory effect on methyltransferases activity.Taken together, our results provide new insights on the pathophysiology of HHcy and lay ground for further studies.
Descrição
Tese de doutoramento, Farmácia (Bioquímica), Universidade de Lisboa, Faculdade de Farmácia, 2013
Palavras-chave
Teses de doutoramento - 2013