Mechanism of action of vitamin C in sepsis: ascorbate modulates redox signaling in endothelium

Abstract

Circulating levels of vitamin C (ascorbate) are low in patients with sepsis. Parenteral administration of ascorbate raises plasma and tissue concentrations of the vitamin and may decrease morbidity. In animal models of sepsis, intravenous ascorbate injection increases survival and protects several microvascular functions, namely, capillary blood flow, microvascular permeability barrier, and arteriolar responsiveness to vasoconstrictors and vasodilators. The effects of parenteral ascorbate on microvascular function are both rapid and persistent. Ascorbate quickly accumulates in microvascular endothelial cells, scavenges reactive oxygen species, and acts through tetrahydrobiopterin to stimulate nitric oxide production by endothelial nitric oxide synthase. A major reason for the long duration of the improvement in microvascular function is that cells retain high levels of ascorbate, which alter redox-sensitive signaling pathways to diminish septic induction of NADPH oxidase and inducible nitric oxide synthase. These observations are consistent with the hypothesis that microvascular function in sepsis may be improved by parenteral administration of ascorbate as an adjuvant therapy.

Fig. 1

Intracellular ascorbate modulates the effects of septic insult on microvascular endothelial cell function. The largest rectangle represents a microvascular endothelial cell, in which arrows with solid lines indicate stimulation and those with dotted lines indicate inhibition. Septic insult increases NADPH oxidase (Nox) and inducible nitric oxide synthase (iNOS) activities, which elevate reactive oxygen species (ROS) and peroxynitrite (ONOO^–) levels. ROS and ONOO^– impair capillary blood flow, capillary barrier function, and arteriolar responsiveness to vasoconstrictors and vasodilators. Ascorbate (Asc) and dehydroascorbic acid (DHA) enter the cell through sodium-dependent vitamin C transporter 2 (SVCT2) and glucose transporter 1 (GLUT1), respectively, and DHA becomes reduced to Asc. Intracellular Asc rapidly scavenges ROS and ONOO^–, while stimulating tetrahydrobiopterin (BH4)-dependent endothelial nitric oxide synthase (eNOS), to increase the local concentration of nitric oxide (NO). Asc also decreases Nox activity, prevents induction of the enzyme's p47phox subunit, and blocks induction of iNOS.

Fig. 2

Intracellular ascorbate (Asc) modulates redox-sensitive signaling pathways in microvascular endothelial cells during sepsis. The largest rectangle represents a microvascular endothelial cell, in which arrows with solid lines indicate stimulation and those with dotted lines indicate inhibition. Lipopolysaccharide (LPS) and cytokines such as interferon-gamma (IFNgamma) stimulate signaling pathways that trigger the expression of NADPH oxidase (Nox) subunit p47phox, inducible nitric oxide synthase (iNOS), and other inflammatory mediators. Hypoxia increases hypoxia-inducible factor 1 (HIF-1), by inhibiting HIF-1 prolyl-hydroxylase (PHD), and thereby induces the expression of sepsis-associated genes. Asc inhibits the activation of Jak2-Stat1-IRF1 pathway and increases the activity of PHD. Asc further regulates local nitric oxide (NO) concentration by scavenging superoxide (${\mathrm{O}}_2^{\cdot -}$) and peroxynitrite (ONOO^–). IKK; NFκB. Ascorbate does not affect the IKB kinase (IKK) and nuclear factor-κB that also mediate iNos induction in microvascular endothelial cells exposed to LPS and IFN gamma.