Apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) is a multifunctional protein that plays a central role in the cellular response to DNA damage and redox regulation against oxidative stress. APE1/Ref-1 is essential for cellular survival and embryonic lethal in knockout mouse models. Heterozygous APE1/Ref-1 mice showed impaired endothelium-dependent vasorelaxation, reduced vascular NO levels, and are hypertensive. APE1/Ref-1 reduces intracellular reactive oxygen species production by negatively regulating the activity of the NADPH oxidase. APE1/Ref-1 is predominantly localized in the nucleus; however, its subcellular localization is dynamically regulated. Recently, it was shown that APE1/Ref-1 is secreted in response to hyperacetylation at specific lysine residues. We investigated the functions of extracellular APE1/Ref-1 with respect to leading anti-inflammatory signaling in TNF-α-stimulated endothelial cells in response to acetylation. Trichostatin A (TSA), an inhibitor of histone deacetylase, considerably suppressed vascular cell adhesion molecule-1 (VCAM-1) in TNF-α-stimulated endothelial cells. During TSA-mediated acetylation in culture, a time-dependent increase in secreted APE1/Ref-1 was confirmed. Recombinant human APE1/Ref-1 with reducing activity induced a conformational change in TNFR1 by thiol-disulfide exchange. Following treatment with the neutralizing anti-APE1/Ref-1 antibody, inflammatory signals via the binding of TNF-α to TNFR1 were remarkably recovered. Furthermore, rhAPE1/Ref-1 inhibited IL-1β-induced VCAM-1 expression in endothelial cells, and it inhibited iNOS or COX-2 expression in lipopolysaccharide-stimulated RAW 264.7 macrophage cells. These results strongly indicate that anti-inflammatory effects of secreted APE1/Ref-1 and its property of secreted APE1/Ref-1 may be useful as a therapeutic biomolecule in cardiovascular disease.