We previously reported that the apolipoprotein (apo) B48-carrying lipoproteins obtained from apoE knockout ( ) mice, so called E？/B48 lipoproteins, transformed mouse macrophages into foam cells and enhanced the phosphorylation of eukaryotic translation initiation factor 2α (eIF-2α). Furthermore, the eIF-2α phosphorylation inhibitor, 2-aminopurine (2-AP), attenuated E？/B48 lipoprotein-induced foam cell formation. The present report studied the effect of 2-AP on atherosclerosis in mice. Our results showed that the level of food intake, bodyweight, plasma cholesterol, and triglycerides was comparable in mice treated with or without 2-AP. However, the mean size of atherosclerotic lesions in the aorta sinus as well as the surface area of the entire aorta of 2-AP-treated mice were reduced by about 55% and 39%, respectively, compared to samples from untreated control mice. In addition, the 2-AP-treated mice showed a significant decrease in glucose-regulated protein 78 (GRP78) and phosphorylated eIF-2α in their aortic samples as compared to levels in untreated control mice. These observations suggest that endoplasmic reticulum stress is a causal mechanism for the development of atherosclerosis in mice and that therapeutic strategies can be developed for using eIF-2α phosphorylation inhibitors, such as 2-AP, to prevent or treat atherosclerosis. 1. Introduction The endoplasmic reticulum (ER) fulfills multiple cellular functions. Once ER functions are perturbed by various pathological conditions, unfolded or misfolded proteins accumulate in the ER lumen, resulting in ER stress characterized by increasing ER molecular chaperones and diminishing global protein synthesis [1]. Activation of the signaling network in response to ER stress is known as unfolded protein response (UPR). There are three distinct UPR signaling pathways triggered in response to ER stress, which are mediated by (1) RNA-dependent protein kinase-like endoplasmic reticulum kinase (PERK), (2) activating transcription factor 6 (ATF6), and (3) inositol-requiring enzyme 1 (IRE1), respectively, [2, 3]. Under physiological conditions, PERK, ATF6, and IRE1 are associated with the abundant luminal chaperone Bip (also known as glucose-regulated protein 78, GRP78). This interaction keeps PERK, ATF6, and IRE1 in an inactive state. When the ER is overloaded with newly synthesized proteins or is stimulated by agents that cause unfolded proteins to accumulate, GRP78/Bip preferentially associates with the unfolded proteins, releasing PERK, ATF6, and IRE1 to activate downstream signaling molecules. In the
References
[1]
J. Groenendyk, L. B. Agellon, and M. Michalak, “Coping with endoplasmic reticulum stress in the cardiovascular system,” Annual Review of Physiology, vol. 75, pp. 49–67, 2013.
[2]
K. Zhang and R. J. Kaufman, “Protein folding in the endoplasmic reticulum and the unfolded protein response,” Handbook of Experimental Pharmacology, no. 172, pp. 69–91, 2006.
[3]
C. Xu, B. Bailly-Maitre, and J. C. Reed, “Endoplasmic reticulum stress: cell life and death decisions,” Journal of Clinical Investigation, vol. 115, no. 10, pp. 2656–2664, 2005.
[4]
C. Nathaniel Roybal, L. A. Hunsaker, O. Barbash, D. L. Vander Jagt, and S. F. Abcouwer, “The oxidative stressor arsenite activates vascular endothelial growth factor mRNA transcription by an ATF4-dependent mechanism,” Journal of Biological Chemistry, vol. 280, no. 21, pp. 20331–20339, 2005.
[5]
B. F. Teske, S. A. Wek, P. Bunpo et al., “The eIF2 kinase PERK and the integrated stress response facilitate activation of ATF6 during endoplasmic reticulum stress,” Molecular Biology of the Cell, vol. 22, no. 22, pp. 4390–4405, 2011.
[6]
B. Feng, P. M. Yaol, Y. Li et al., “The endoplasmic reticulum is the site of cholesterol-induced cytotoxicity in macrophages,” Nature Cell Biology, vol. 5, no. 9, pp. 781–792, 2003.
[7]
D. Wu, H. Yang, Y. Zhao et al., “2-Aminopurine inhibits lipid accumulation induced by apolipoprotein E-deficient lipoprotein in macrophages: potential role of eukaryotic initiation factor-2α phosphorylation in foam cell formation,” Journal of Pharmacology and Experimental Therapeutics, vol. 326, no. 2, pp. 395–405, 2008.
[8]
Y. Zhao, Z. Guo, X. Lin et al., “Apolipoprotein E-deficient lipoproteins induce foam cell formation by activation of PERK-EIF-2α signaling cascade,” Journal of Bioanalysis and Biomedicine, vol. 2, no. 5, pp. 113–120, 2010.
[9]
R. W. Mahley, Y. Huang, and S. C. Rall Jr., “Pathogenesis of type III hyperlipoproteinemia (dysbetalipoproteinemia): questions, quandaries, and paradoxes,” Journal of Lipid Research, vol. 40, no. 11, pp. 1933–1949, 1999.
[10]
J. A. Piedrahita, S. H. Zhang, J. R. Hagaman, P. M. Oliver, and N. Maeda, “Generation of mice carrying a mutant apolipoprotein E gene inactivated by gene targeting in embryonic stem cells,” Proceedings of the National Academy of Sciences of the United States of America, vol. 89, no. 10, pp. 4471–4475, 1992.
[11]
Z. Guo, F. Mitchell-Raymundo, H. Yang et al., “Dietary restriction reduces atherosclerosis and oxidative stress in the aorta of apolipoprotein E-deficient mice,” Mechanisms of Ageing and Development, vol. 123, no. 8, pp. 1121–1131, 2002.
[12]
J. Zhou, ？. Lhoták, B. A. Hilditch, and R. C. Austin, “Activation of the unfolded protein response occurs at all stages of atherosclerotic lesion development in apolipoprotein E-deficient mice,” Circulation, vol. 111, no. 14, pp. 1814–1821, 2005.
[13]
J. Zhou, G. H. Werstuck, ？. Lhoták et al., “Association of multiple cellular stress pathways with accelerated atherosclerosis in hyperhomocysteinemic apolipoprotein E-deficient mice,” Circulation, vol. 110, no. 2, pp. 207–213, 2004.
[14]
A. H. Hasty, M. F. Linton, L. L. Swift, and S. Fazio, “Determination of the lower threshold of apolipoprotein E resulting in remnant lipoprotein clearance,” Journal of Lipid Research, vol. 40, no. 8, pp. 1529–1538, 1999.
[15]
L. Zhou, H. Yang, X. Lin, E. U. Okoro, and Z. Guo, “Cholecystokinin elevates mouse plasma lipids,” PLoS ONE, vol. 7, no. 12, Article ID e51011, 2012.
[16]
X. Lin, H. Yang, L. Zhou, and Z. Guo, “Nrf2-dependent induction of NQO1 in mouse aortic endothelial cells overexpressing catalase,” Free Radical Biology and Medicine, vol. 51, no. 1, pp. 97–106, 2011.
[17]
H. C. McGill, “Overview,” in Atherosclerosis and Coronary Artery Disease, V. Fuster, R. Ross, and E. J. Topol, Eds., pp. 25–41, Lippincott-Raven Publishers, Philadelphia, Pa, USA, 1999.
[18]
J. Lewerenz and P. Maher, “Basal levels of eIF2α phosphorylation determine cellular antioxidant status by regulating ATF4 and xCT expression,” Journal of Biological Chemistry, vol. 284, no. 2, pp. 1106–1115, 2009.
[19]
R. C. Wek, H.-Y. Jiang, and T. G. Anthony, “Coping with stress: EIF2 kinases and translational control,” Biochemical Society Transactions, vol. 34, no. 1, pp. 7–11, 2006.
[20]
D. Wu, H. Yang, W. Xiang et al., “Heterozygous mutation of ataxia-telangiectasia mutated gene aggravates hypercholesterolemia in apoE-deficient mice,” Journal of Lipid Research, vol. 46, no. 7, pp. 1380–1387, 2005.
[21]
M. Schr？der and R. J. Kaufman, “ER stress and the unfolded protein response,” Mutation Research, vol. 569, no. 1-2, pp. 29–63, 2005.
[22]
L. Zhao and S. L. Ackerman, “Endoplasmic reticulum stress in health and disease,” Current Opinion in Cell Biology, vol. 18, no. 4, pp. 444–452, 2006.
[23]
D. Wu, C. Sharan, H. Yang et al., “Apolipoprotein E-deficient lipoproteins induce foam cell formation by downregulation of lysosomal hydrolases in macrophages,” Journal of Lipid Research, vol. 48, no. 12, pp. 2571–2578, 2007.
[24]
J. O'Neil, G. Hoppe, and H. F. Hoff, “Phospholipids in oxidized low density lipoproteins perturb the ability of macrophages to degrade internalized macromolecules and reduce intracellular cathepsin B activity,” Atherosclerosis, vol. 169, no. 2, pp. 215–224, 2003.
[25]
O. Zschenker, T. Illies, and D. Ameis, “Overexpression of lysosomal acid lipase and other proteins in atherosclerosis,” Journal of Biochemistry, vol. 140, no. 1, pp. 23–38, 2006.
[26]
H. Du, S. Schiavi, N. Wan, M. Levine, D. P. Witte, and G. A. Grabowski, “Reduction of atherosclerotic plaques by lysosomal acid lipase supplementation,” Arteriosclerosis, Thrombosis, and Vascular Biology, vol. 24, no. 1, pp. 147–154, 2004.
