Redox Regulation of Dihydrofolate Reductase: Friend or Troublemaker?

xidative stress is a hallmark of cardiovascular diseases 1 and a major contributor to vascular dysfunction. 2 On the basis on recent concepts, vascular oxidative stress is caused mainly by infiltrating inflammatory cells such as monocytes/ macrophages or leucocytes, 3,4 producing so-called kindling radicals that lead to the activation of secondary, vascular enzymatic sources of reactive oxygen species (mainly super-oxide). A prominent example is the uncoupled nitric oxide (NO) synthase, which means that an NO-producing antiath-erosclerotic enzyme is getting switched to a superoxide-producing proatherosclerotic enzyme. 2 Molecular mechanisms causing endothelial NO synthase (eNOS) uncoupling or dys-function include phosphorylation at Thr495 and Tyr657, 6,7 S-glutathionylation of cysteines in the reductase domain, 8 oxidative depletion of tetrahydrobiopterin (BH 4), 9 oxidative disruption of the zinc-sulfur dimer-binding site, 10 and redox regulation of asymmetrical dimethylarginine formation and break down. In theory, eNOS function is also regulated by S-nitros(yl)ation in a negative feedback fashion. 12 Another enzyme that contains several redox switches is the soluble guanylyl cyclase. It was reported that its activity is getting inhibited on oxidation by superoxide and peroxynitrite, 13,14 by nitros(yl)ation at different cysteine residues 15 as well as oxidation of its heme moiety. Previous reports have shown that the enzyme responsible for de novo synthesis of the eNOS cofactor BH 4 , the GTP-cyclohydolase-1, is redox-regulated at its expression level via increased proteasomal degradation under oxidative stress conditions. 17 The 26S proteasome responsible for this degradation has been shown for getting activated by nitra-tion of specific tyrosine residues in the setting of hypergly-cemia 17 and experimental hypertension, 18 only leaving the salvage pathway for restoration of BH 4 levels via reduction of dihydrobiopterin (BH 2) by dihydrofolate reductase (DHFR). 19 Taking into account that BH 4 plays an essential role for eNOS function, its oxidative depletion or simultaneous inhibition of its de novo synthesis will ultimately lead to eNOS dysfunction/uncoupling, despite the futile counter-regulatory upregulation of DHFR expression and activity. Even more complicating the story is the observation that BH 4 per se prevents oxidative inhibition of soluble guanylyl cyclase activity. 20 In their article in this ATVB issue, Cai et al 21 report on a novel redox regulatory mechanism of DHFR. S-nitros(yl) ation of DHFR by eNOS-derived NO prevents the degradation of the enzyme by the 26S proteasome. Knockdown of eNOS in primary endothelial cells and NO scavenging by PTIO