Adenosine Monophosphate Kinase Regulates ABCG1-Mediated Oxysterol Efflux From Endothelial Cells and Protects Against Hypercholesterolemia-Induced Endothelial Dysfunction

Objective—Adenosine monophosphate kinase (AMPK) has been identified as a regulator of vascular function via the preservation of endothelial cell (EC) function. We examined in this study whether the beneficial effects of AMPK on ECs are dependent on its involvement in cholesterol efflux and its impact on hypercholesterolemia-induced endothelial dysfunction. Methods and Results—Using human aortic ECs and bovine aortic ECs, we show that AMPK activation upregulates ABCG1 expression independently of LXR␣ transcriptional activity but through a posttranscriptional mechanism that increases mRNA stability. Using a heterologous system and a luciferase reporter, we further identify that the 3Ј-untranslated region of the ABCG1 mRNA is responsible for the regulatory effects of AMPK activation. 5-Aminoimidazole-4-carboxamide-1-␤-D-riboside treatment promotes endothelial 7-ketocholesterol efflux and prevents 7-ketocholesterol (7-KC)–induced reactive oxygen species production in an ABCG1-dependent manner, thus preserving endothelial nitric oxide synthase activity and nitric oxide bioavailability. Notably, in vivo studies using C57BL/6J mice receiving a high-cholesterol diet revealed that the infusion of 5-aminoimidazole-4-carboxamide-1-␤-D-riboside increases vascular ABCG1 expression and improves vascular reactivity. These effects are abrogated by the AMPK antagonist compound C and by the vascular gene transfer of ABCG1 small interfering RNA. Conclusion—Our current findings uncover a novel mechanism by which AMPK protects against hypercholesterolemia-mediated endothelial dysfunction. Key Words: ABC transporter Ⅲ endothelial function Ⅲ vascular biology Ⅲ AMP-activated protein kinase E ndothelial dysfunction, which manifests as impaired endothelial nitric oxide synthase (eNOS) activity and nitric oxide (NO) bioavailability, is a key feature of early atherosclerosis. 1– 4 A growing body of evidence has now implicated increased dietary oxysterols as a potent inhibitor of endothelium-dependent vascular relaxation and a significant contributor to the initiation and development of endo-thelial dysfunction, which suggests a vital link between hypercholesterolemia and the formation of atherosclerotic lesions. 5–7 Oxysterols are oxygenated forms of cholesterol that are present at low levels in the circulatory system and accumulate in plasma and tissues in some diseases. In characteristic lesions of atherosclerosis, 7-oxygenated oxys-terols, predominantly 7-ketocholesterol (7-KC), are detectable at high levels and have been implicated in the pathology of this disease. 8,9 AMP-activated protein kinase (AMPK) is a member of a metabolite-sensing protein kinase family, contains a catalytic ␣ subunit and regulatory ␤ and ␥ subunits, and functions as a protein serine/threonine kinase. 10 The activation of AMPK attenuates anabolic processes, such as the synthesis of proteins , fatty acids, and cholesterol, but stimulates ATP-generating catabolic pathways. 11 Recent findings further suggest that AMPK is also …