Decreased Lipoprotein Clearance Is Responsible for Increased Cholesterol in LDL Receptor Knockout Mice With Streptozotocin-Induced Diabetes

Abstract

OBJECTIVE—Patients with diabetes often have dyslipidemia and increased postprandial lipidmia. Induction of diabetes in LDL receptor (Ldlr^−/−) knockout mice also leads to marked dyslipidemia. The reasons for this are unclear.

RESEARCH DESIGN AND METHODS—We placed Ldlr^−/− and heterozygous LDL receptor knockout (Ldlr^+/−) mice on a high-cholesterol (0.15%) diet, induced diabetes with streptozotocin (STZ), and assessed reasons for differences in plasma cholesterol.

RESULTS—STZ-induced diabetic Ldlr^−/− mice had plasma cholesterol levels more than double those of nondiabetic controls. Fast-performance liquid chromatography and ultracentrifugation showed an increase in both VLDL and LDL. Plasma VLDL became more cholesterol enriched, and both VLDL and LDL had a greater content of apolipoprotein (apo)E. In LDL the ratio of apoB48 to apoB100 was increased. ApoB production, assessed using [³⁵S]methionine labeling in Triton WR1339–treated mice, was not increased in fasting STZ-induced diabetic mice. Similarly, postprandial lipoprotein production was not increased. Reduction of cholesterol in the diet to normalize the amount of cholesterol intake by the control and STZ-induced diabetic animals reduced plasma cholesterol levels in STZ-induced diabetic mice, but plasma cholesterol was still markedly elevated compared with nondiabetic controls. LDL from STZ-induced diabetic mice was cleared from the plasma and trapped more rapidly by livers of control mice. STZ treatment reduced liver expression of the proteoglycan sulfation enzyme, heparan sulfate N-deacetylase/N-sulfotrasferase-1, an effect that was reproduced in cultured hepatocytyes by a high glucose–containing medium.

CONCLUSIONS—STZ-induced diabetic, cholesterol-fed mice developed hyperlipidemia due to a non-LDL receptor defect in clearance of circulating apoB-containing lipoproteins.