Apolipoprotein E4 exaggerates diabetic dyslipidemia and atherosclerosis in mice lacking the LDL receptor

OBJECTIVE

We investigated the differential roles of apolipoprotein E (apoE) isoforms in modulating diabetic dyslipidemia—a potential cause of the increased cardiovascular disease risk of patients with diabetes.

RESEARCH DESIGN AND METHODS

Diabetes was induced using streptozotocin (STZ) in human apoE3 (E3) or human apoE4 (E4) mice deficient in the LDL receptor (LDLR^−/−).

RESULTS

Diabetic E3LDLR^−/− and E4LDLR^−/− mice have indistinguishable levels of plasma glucose and insulin. Despite this, diabetes increased VLDL triglycerides and LDL cholesterol in E4LDLR^−/− mice twice as much as in E3LDLR^−/− mice. Diabetic E4LDLR^−/− mice had similar lipoprotein fractional catabolic rates compared with diabetic E3LDLR^−/− mice but had larger hepatic fat stores and increased VLDL secretion. Diabetic E4LDLR^−/− mice demonstrated a decreased reliance on lipid as an energy source based on indirect calorimetry. Lower phosphorylated acetyl-CoA carboxylase content and higher gene expression of fatty acid synthase in the liver indicated reduced fatty acid oxidation and increased fatty acid synthesis. E4LDLR^−/− primary hepatocytes cultured in high glucose accumulated more intracellular lipid than E3LDLR^−/− hepatocytes concomitant with a 60% reduction in fatty acid oxidation. Finally, the exaggerated dyslipidemia in diabetic E4LDLR^−/− mice was accompanied by a dramatic increase in atherosclerosis.

CONCLUSIONS

ApoE4 causes severe dyslipidemia and atherosclerosis independent of its interaction with LDLR in a model of STZ-induced diabetes. ApoE4-expressing livers have reduced fatty acid oxidation, which contributes to the accumulation of tissue and plasma lipids.Cardiovascular disease (CVD) caused by a worsening of atherosclerosis is an important complication of diabetes and is the leading cause of mortality among patients with diabetes (1). Patients with poorly managed type 1 diabetes or type 2 diabetes commonly have elevated VLDL triglycerides (TGs), a reduction of HDL cholesterol, and smaller, dense LDL. This common cluster of harmful changes in lipid metabolism is referred to as diabetic dyslipidemia (2).Apolipoprotein E (apoE) is a small circulating protein associated predominantly with VLDL and HDL. It is the primary ligand for several lipoprotein receptors, making it a crucial component in the clearance of lipid from the circulation and a major determinant of plasma cholesterol and CVD risk (3). In humans, the APOE gene is polymorphic, resulting in production of three common isoforms: apoE2, apoE3, and apoE4. The apoE4 isoform is carried by more than a quarter (28%) of the U.S. population and is associated with higher LDL cholesterol and an increased risk of CVD (3). In addition to its well-established role in CVD, recent findings have implicated a role for apoE in glucose metabolism. Epidemiological studies have suggested that in certain populations, APOE genotype may influence plasma glucose and insulin levels (4,5), postprandial glucose response (6), the development of metabolic syndrome (7,8), and a myriad of diabetes complications (9). In addition, apoE4 carriers with diabetes have been shown to have increased carotid atherosclerosis (10), and elderly apoE4 carriers with diabetes have an increased risk of CVD-associated death (11).Increases in VLDL TGs; decreases in HDL; the accumulation of smaller, more dense LDL; slower clearance of postprandial chylomicrons; and a decrease in LDL receptor (LDLR) expression are all noted phenotypes associated with both type 1 and type 2 diabetes (2). All of these components of diabetic dyslipidemia are areas of normal lipid metabolism in which apoE has previously been shown to play a direct role. The major receptor through which apoE mediates lipoprotein clearance is the LDLR, and the apoE isoforms exhibit differential LDLR binding affinities (12). Therefore, we sought whether apoE isoforms retain differential roles in diabetic dyslipidemia and atherosclerosis in the absence of the LDLR by using a mouse model of diabetes induced by streptozotocin (STZ).In this study, we show that dyslipidemia and atherosclerosis are greatly exaggerated in diabetic LDLR^−/− mice expressing human apoE4 (E4LDLR^−/−) compared with those with human apoE3 (E3LDLR^−/−), despite a similar degree of hyperglycemia. This E4-specific aggravation of diabetic dyslipidemia is central to the liver and is associated with a reduction in hepatic lipid oxidation, an accumulation of liver TGs, and increased rates of VLDL secretion.