Diabetic atherosclerosis in APOE*4 mice: synergy between lipoprotein metabolism and vascular inflammation

Abstract

More than 10% of the United States adult population is estimated to have diabetes ( 1 ), and the leading cause of increased mortality in patients with diabetes is enhanced atherosclerosis ( 2, 3 ). A common cluster of harmful changes to lipoprotein metabolism, including increased plasma VLDL and LDL cholesterol, occurs in type-2 diabetic patients and is characterized by the presence of small, dense LDL particles, low HDL , increased triglycerides, and postprandial lipemia ( 4, 5 ). Serum lipid and lipoprotein levels in patients who have well-controlled type-1 diabetes are generally not higher than those in people without diabetes. However, hypertriglyceridemia and reduced HDL are common in poorly controlled patients, and plasma apoB100 levels, which indicate the number of LDL particles, correlate with glycemic control ( 5 ). This underscores the importance of diabetes-induced changes in lipoprotein composition and distribution as a cardiovascular disease (CVD) risk ( 4, 5 ), but how these relatively small changes in lipid metabolism that occur in the general human population lead to increased diabetic complications of atherosclerosis is not completely understood. Current animal models for atherosclerosis have not been able to address this issue satisfactorily, as they rely on gross lipid abnormalities, such as the lack of LDLR or apoE which cause severe hypercholesterolemia. Although modest increases in atherosclerosis have been observed when these hypercholesterolemic mice are made Abstract Diabetes is a major risk factor for cardiovascular disease. To examine how diabetes interacts with a mildly compromised lipid metabolism, we introduced the diabetogenic Ins2 C96Y/+ (Akita) mutation into mice expressing human apoE4 (E4) combined with either an overexpressing human LDL receptor gene (hLDLR) or the wild-type mouse gene. The hLDLR allele caused 2-fold reductions in plasma HDLcholesterol, plasma apoA1, and hepatic triglyceride secretion. Diabetes increased plasma total cholesterol 1.3-fold and increased apoB48 secretion 3-fold, while reducing triglyceride secretion 2-fold. Consequently, diabetic E4 mice with hLDLR secrete increased numbers of small, cholesterol-enriched, apoB48-containing VLDL, although they have near normal plasma cholesterol (<120 mg/dl). Small foam cell lesions were present in the aortic roots of all diabetic E4 mice with hLDLR that we analyzed at six months of age. None were present in nondiabetic mice or in diabetic mice without hLDLR. Aortic expression of genes affecting leukocyte recruitment and adhesion was enhanced by diabetes. ApoA1 levels, but not diabetes, were strongly correlated with the ability of plasma to effl ux cholesterol from macrophages. We conclude that the diabetes-induced proinfl ammatory changes in the vasculature and the hLDLR-mediated cholesterol accumulation in macrophages synergistically trigger atherosclerosis in mice with human apoE4, although neither alone is suffi cient.

2013 Russell Ross Memorial Lecture in Vascular Biology

“…20,24 Increased formation of atherosclerotic lesions in the setting of diabetes, without increased plasma lipid levels in diabetic mice, was also later observed in STZ-diabetic Ldlr +/− mice, STZ-diabetic chow-fed Apoe −/− mice, and more recently in a model in which Akita mice (which have a mutation in the Ins2 gene, rendering insulin non-functional) were crossed with mice carrying the human ApoE4 and LDLR genes (Table 1). 25–27 Together, these different mouse models consistently show that diabetes accelerates formation of atherosclerotic lesions by promoting macrophage accumulation in susceptible arteries (Figure 1). …”

Apolipoprotein E–low density lipoprotein receptor interaction affects spatial memory retention and brain ApoE levels in an isoform-dependent manner

“…Of particular interest is the class of cholesterol lowering drugs designed to inhibit PCSK9, an enzyme which enhances the degradation of the LDLR. Given the harmful effects of increasing LDLR expression in the presence of apoE4 in a mouse model of CVD (Altenburg et al, 2007, Johnson et al, 2013, Malloy et al, 2004), as well as the E4-associated effects on spatial memory observed in this study, additional caution may be needed when considering E4+ patients.…”

“…In mice, the introduction of human LDLR results in apoE isoform-dependent differences in diet-induced atherosclerotic plaque development (Malloy et al, 2004). In addition, the development of atherosclerosis is accelerated in mice when human LDLR and E4 were combined in the background of diabetes (Johnson et al, 2013). It has been proposed that these harmful effects might originate from a reduction in the available pool of functional E4 due to “trapping” by the LDLR (Altenburg et al, 2008, Malloy et al, 2004), but regardless of the mechanism, the interaction between LDLR and E4 appears to be an important disease modifying mechanism in the setting of CVD.…”

“…However, whether this strategy is beneficial in the presence of all apoE isoforms remains unclear. Our previous work in the context of CVD suggests that lipoprotein metabolism in the periphery – and possibly in the brain as well – may be negatively affected by this strategy in the presence of apoE4 (Altenburg et al, 2007, Malloy et al, 2004, Johnson et al, 2013). …”

“…For example, the disparities in CVD risk usually associated with APOE genotype disappear in patients who lack functioning LDLR, suggesting that the diseases-modifying effects of the apoE isoforms depend on the presence of a fully functioning LDLR protein (Vermissen et al, 2011). In mice, introduction of human LDLR results in higher plasma cholesterol and increased cardiovascular complications in the presence of E4 (Altenburg et al, 2007, Malloy et al, 2004, Johnson et al, 2013). This effect does not occur in mice expressing E3, and in the presence of E2 actually lowers cholesterol and improves cardiovascular outcomes (Knouff et al, 2001).…”