Silke Tobias, Alice Habermeier, Daniel Siuda, Gisela Reifenberg, Ning Xia, Ellen I Closs, Ulrich Förstermann, Huige Li. Dexamethasone, tetrahydrobiopterin and uncoupling of endothelial nitric oxide synthase[J]. Journal of Geriatric Cardiology, 2015, 12(5): 528-539. DOI: 10.11909/j.issn.1671-5411.2015.05.013
Citation: Silke Tobias, Alice Habermeier, Daniel Siuda, Gisela Reifenberg, Ning Xia, Ellen I Closs, Ulrich Förstermann, Huige Li. Dexamethasone, tetrahydrobiopterin and uncoupling of endothelial nitric oxide synthase[J]. Journal of Geriatric Cardiology, 2015, 12(5): 528-539. DOI: 10.11909/j.issn.1671-5411.2015.05.013

Dexamethasone, tetrahydrobiopterin and uncoupling of endothelial nitric oxide synthase

  • Objective To find out whether dexamethasone induces an uncoupling of the endothelial nitric oxide synthase (eNOS). Methods & Results A major cause of eNOS uncoupling is a deficiency of its cofactor tetrahydrobiopterin (BH4). Treatment of human EA.hy 926 endothelial cells with dexamethasone decreased mRNA and protein expression of both BH4-synthesizing enzymes: GTP cyclohydrolase I and dihydrofolate reductase. Consistently, a concentration- and time-dependent reduction of BH4, dihydrobiopterin (BH2) as well as BH4: BH2 ratio was observed in dexamethasone-treated cells. Surprisingly, no evidence for eNOS uncoupling was found. We then analyzed the expression and phosphorylation of the eNOS enzyme. Dexamethasone treatment led to a down-regulation of eNOS protein and a reduction of eNOS phosphorylation at serine 1177. A reduction of eNOS expression may lead to a relatively normal BH4: eNOS molar ratio in dexamethasone-treated cells. Because the B H4-eNOS stoichiometry rather than the absolute B H4 amount is the key determinant of eNOS functionality (i.e., coupled or uncoupled), the down-regulation of eNOS may represent an explanation for the absence of eNOS uncoupling. Phosphorylation of eNOS at serine 1177 is needed for both the NO-producing activity of the coupled eNOS and the superoxide-producing activity of the uncoupled eNOS. Thus, a reduction of serine 1177 phosphorylation may render a potentially uncoupled eNOS hardly detectable. Conclusions Although dexamethasone reduces BH4 levels in endothelial cells, eNOS uncoupling is not evident. The reduction of NO production in dexamethasone-treated endothelial cells is mainly attributable to reduced eNOS expression and decreased eNOS phosphorylation at serine 1177.
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