Article

Metabolic consequences of folate-induced reduction of hyperhomocysteinemia in uremia

Institute of Biochemistry of Macromolecules, Department of Pediatrics, School of Medicine and Surgery, Second University of Naples, Italy.
Journal of the American Society of Nephrology (Impact Factor: 9.47). 01/1998; 8(12):1899-905.
Source: PubMed

ABSTRACT Plasma homocysteine, a well-recognized risk factor for cardiovascular disease, is elevated in uremic patients on hemodialysis. The authors have recently demonstrated that one consequence is the reduction in red cell membrane protein methylation levels, caused by a rise of intracellular adenosylhomocysteine, a potent inhibitor of methyltransferases. Protein methylation is involved in a repair mechanism of damaged membrane proteins, and an impairment in methylation leads to the accumulation of altered proteins. Therapy with folates, cofactors in the transformation of homocysteine to methionine, is effective in lowering plasma homocysteine. This article details a study on the metabolic effects of oral methyltetrahydrofolate, the active form of folic acid, on 14 uremic hemodialysis patients. Two months of therapy led to a significant reduction of plasma homocysteine levels, with a proportional response to pre-folate levels. In five of 13 patients with homocysteine levels above 20 microM, plasma homocysteine level was reduced to less than 15 microM. After treatment, levels of adenosylmethionine, the methyl donor in transmethylations, had significantly increased; levels of adenosylhomocysteine had increased to a smaller extent. Therefore, the ratio between the two compounds, an excellent indicator of the presence and the degree of methylation inhibition, was significantly ameliorated. Methionine plasma levels increased after treatment in all patients and were correlated with posttreatment adenosylmethionine levels. It was concluded that treatment with methyltetrahydrofolate brings the plasma homocysteine concentration back to an "acceptable" level, and the metabolic consequences are in the direction of an increase in the normal flow of transmethylations, as monitored by an increase in the [adenosylmethionine]/[adenosylhomocysteine] ratio.

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In Übereinstimmung mit diesen Ergebnissen zeigte sich in der FACS-Analyse propidiumjodidgefärbter Zellen ein Zellzyklusarrest in der G0/ G1-Phase des Zellzyklus. Diese Effekte waren weder durch eine gesteigerte Apoptoserate noch durch eine erhöhte Toxizität dieser Substanz bedingt. Weitergehende Untersuchungen zeigten, dass die Applikation von c3Ado über die Hemmung der SAH-hydrolase zu einer verminderten Aktivität der ICMT führt. Dies hemmt den letzten Schritt der posttranslationalen Modifikationen der Ras-Proteine, die Carboxylmethylierung. Die Translokalisation von Ras an die Plasmamembran und dessen nachfolgende Aktivierung ist somit nicht mehr möglich. Folge ist eine Hemmung der Ras-abhängigen Signaltransduktionswege. Entsprechend dieser Ergebnisse zeigten weitere Western Blot Analysen eine dosisabhängige Abnahme der FCS-induzierten ERK und Akt-Phosphorylierung. Durch die Überexpression einer konstitutiv aktiven Ras-Mutante konnte der Effekt von c3Ado auf die Proliferation glatter Gefäßmuskelzellen aufgehoben werden. Für die in vivo Versuche wurde die A. femoralis von C57BL/6 Mäusen dilatiert. Anschließend erhielten sie eine atherogene Diät ohne oder mit Zusatz von c3Ado. Die Fütterung mit 150 µg c3Ado verhinderte die durch Dilatation induzierte Ras-Aktivierung sowie die Phosphorylierung von Akt und ERK. Des Weiteren führte die orale Aufnahme von c3Ado über 21 Tage nach Dilatation zu einer signifikanten Reduktion der Anzahl proliferierender Zellen in Neointima und Media sowie der Fläche der Neointima im Vergleich zur Kontrollgruppe. Auch in vivo führte die Applikation von c3Ado nicht zu einer Steigerung der Apoptoserate. Die hier vorgelegten Ergebnisse zeigen, dass c3Ado in die Ras-Methylierung und Aktivierung eingreift und somit die mitogene Aktivierung von ERK und Akt hemmt. Dies resultiert in einer Hemmung des Zellzykluseintritts, einer verminderten Proliferation glatter Gefäßmuskelzellen und somit auch in einer reduzierten Neointimabildung nach Angioplastie. Somit könnte die Hemmung der SAH-hydrolase durch c3Ado einen neuen Ansatz zur Verhinderung vaskuloproliferativer Erkrankungen darstellen. 3-Deazaadenosine (c3Ado) is a potent inhibitor of S-adenosylhomocysteine (SAH)-hydrolase which regulates cellular methyltransferase activity. In the present study we sought to determine c3Ado’s effect on vascular smooth muscle cell (VSMC) function and neointima formation in vivo. C3Ado dose-dependently prevented the proliferation and migration of human coronary VSMC in vitro. This was accompanied by an increased expression of the cyclin-dependend kinase inhibitors p21WAF1/Cip1, p27Kip1, a decreased expression of G1/S-phase cyclins and a lack of retinoblastoma protein hyperphosphorylation. 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