Vascular-Protective Effects of High-Density Lipoprotein Include the Downregulation of the Angiotensin II Type 1 Receptor

Abstract

Abstract-There is growing evidence that a cross-talk exists between the renin-angiotensin (Ang) system and lipoproteins.We investigated the role of high-density lipoprotein (HDL) on Ang II type 1 receptor (AT1R) regulation and subsequent Ang II-mediated signaling under diabetic conditions. To investigate the effect of HDL on AT1R expression in vivo, apolipoprotein A-I gene transfer was performed 5 days after streptozotocin injection. Six weeks after apolipoprotein A-I gene transfer, the 1.9-fold (Pϭ0.001) increase of HDL cholesterol was associated with a 4.7-fold (PϽ0.05) reduction in diabetes mellitus-induced aortic AT1R expression. Concomitantly, NAD(P)H oxidase activity, Nox 4, and p22 phox mRNA expression were reduced 2.6-fold, 2.0-fold, and 1.5-fold (PϽ0.05), respectively, whereas endothelial NO synthase dimerization was increased 3.3-fold (PϽ0.005). Apolipoprotein A-I transfer improved NO bioavailability as indicated by ameliorated acetylcholine-dependent vasodilation in the streptozotocin-Ad.hapoA-I group compared with streptozotocin-induced diabetes mellitus. In vitro, HDL reduced the hyperglycemia-induced upregulation of the AT1R in human aortic endothelial cells. This was associated with a 1.3-fold and 2.2-fold decreases in reactive oxygen species and NAD(P)H oxidase activity, respectively (PϽ0.05). Finally, HDL reduced the responsiveness to Ang II, as indicated by decreased oxidative stress in the hyperglycemiaϩHDLϩAng II group compared with the hyperglycemiaϩAng II group. In conclusion, vascular-protective effects of HDL include the downregulation of the AT1R. Diabetes mellitus is associated with endothelial dysfunction, an early phase of atherosclerosis further characterized by a reduction in NO bioavailability, without significant morphological changes of the vessel wall. 1,2 Both diabetes mellitus-associated hyperglycemia and increased angiotensin (Ang) II levels 3 induce reactive oxygen species (ROS), which contribute to endothelial dysfunction, partly via oxidative degradation of NO. Recent studies have demonstrated that ROS are predominantly produced by vascular NAD(P)H oxidase (NOX), 4 whereas "uncoupled" endothelial NO synthase (eNOS; when eNOS produces O 2 ⅐Ϫ rather then NO) is another important source of ROS in diseased, including diabetic, blood vessels. 5 Numerous studies have shown that activation of the Ang II type 1 receptor (AT1R) contributes to the induction of oxidative stress and apoptosis of vascular cells, thus contributing to the initiation and progression of endothelial dysfunction. 6 Ang II not only increases NOX activity but also uncouples eNOS in diabetic mice. 7 The expression levels of the AT1R define the biological efficacy of Ang II and have been shown to be regulated by several agonists, such as Ang II, glucose, insulin, ROS, low-density lipoprotein (LDL), and many others, 8,9 including diabetes mellitus. 10,11 The importance of the enhanced vascular AT1R expression and Ang II-mediated signaling in diabetes mellitus-associated endothelial dysfunction follows from the ...

Therapeutic Potential of HDL in Cardioprotection and Tissue Repair

“…Uncoupling of eNOS is, besides NADPH oxidases (Cai 2005), an important source of reactive oxygen species in diseased, including diabetic, blood vessels (Hink et al 2001). Consistent with the demonstrated role of the angiotensin II receptor, type 1 (AT1R) in mediating increased NADPD oxidase activity, and eNOS uncoupling in diabetes (Oak and Cai 2007), the downregulation in diabetes-induced AT1R (Hodroj et al 2007;Nyby et al 2007) after apo A-I transfer (Van Linthout et al 2009) was postulated to be the predominant mediator of reduced NADPH oxidase activity and eNOS uncoupling. This hypothesis is further supported by in vitro findings showing that the HDL-mediated reduction in AT1R expression in human aortic endothelial cells was associated with a decline in hyperglycaemia-induced oxidative stress and a reduced responsiveness to angiotensin II (Van Linthout et al 2009).…”

“…Consistent with the demonstrated role of the angiotensin II receptor, type 1 (AT1R) in mediating increased NADPD oxidase activity, and eNOS uncoupling in diabetes (Oak and Cai 2007), the downregulation in diabetes-induced AT1R (Hodroj et al 2007;Nyby et al 2007) after apo A-I transfer (Van Linthout et al 2009) was postulated to be the predominant mediator of reduced NADPH oxidase activity and eNOS uncoupling. This hypothesis is further supported by in vitro findings showing that the HDL-mediated reduction in AT1R expression in human aortic endothelial cells was associated with a decline in hyperglycaemia-induced oxidative stress and a reduced responsiveness to angiotensin II (Van Linthout et al 2009). These observations underline the finding of Tolle et al (2008), who showed that HDL reduce NADPH oxidase-dependent reactive oxygen species generation via inhibition of the activation of Rac1, which is a downstream AT1R-dependent mediator of angiotensin II (Ohtsu et al 2006).…”

“…The anti-inflammatory and antifibrotic effects of HDL are supported by findings in apo A-I knockout mice, which are associated with increased inflammatory cells and collagen deposition in the lung (Wang et al 2010), and from observations with the mimetic apo A-I peptide 4F demonstrating an L-4F-mediated decreased endothelial cell matrix production (Ou et al 2005). Since cardiac NADPH oxidases play a predominant role in the development of diabetic cardiomyopathy (Guo et al 2007;Wold et al 2006) and in diabetic cardiac remodelling (Li et al 2010b), decreased cardiac NADPH oxidase activity following apo A-I transfer (Van Linthout et al 2009) may be a critical mediator of reduced cardiac oxidative stress and subsequent fibrosis in the myocardium.…”

“…In the vasculature of diabetic rats, apo A-I transfer decreased oxidative stress as indicated by the reduction in diabetes-enhanced NADPH oxidase activity and eNOS uncoupling (Van Linthout et al 2009), a phenomenon occurring when eNOS produces O 2 À rather than NO. Uncoupling of eNOS is, besides NADPH oxidases (Cai 2005), an important source of reactive oxygen species in diseased, including diabetic, blood vessels (Hink et al 2001).…”

“…supra, via paraoxonase and platelet-activating factor-acetyl hydrolase) may contribute to the downregulation of the AT1R under diabetes mellitus, which results in less NADPH oxidase activity and reactive oxygen formation and in turn may decrease AT1R expression. Concomitant with the reduced vascular oxidative stress, including decreased eNOS uncoupling, apo A-I transfer in streptozotocin rats resulted in an enhanced NO bioavailability (Chalupsky and Cai 2005) and consequently in a decrease in endothelial dysfunction (Van Linthout et al 2009), which is another hallmark of diabetic cardiomyopathy.…”