Objective-To clarify the role of very low density lipoprotein (VLDL) and apolipoprotein E (apoE) in adipogenesis, we studied newly developed hyperlipidemic obese (ob/ob;apoE Ϫ/Ϫ ) mice. Because hydrolysis of VLDL is believed to be the major source of adipogenic free fatty acids, a higher plasma level of VLDL in these mice should exaggerate obesity. Methods and Results-When fed a high-fat, high-cholesterol diet, ob/ob;apoE Ϫ/Ϫ mice did not show increased body weight or an increased amount of adipose tissue in spite of increased plasma VLDL levels, whereas ob/ob mice showed an increased body weight and amount of adipose tissue, suggesting that there is a novel apoE-dependent pathway for adipogenesis. In vitro experiments using bone marrow stromal cells and 3T3-L1 cells confirmed this notion. ApoE-deficient VLDL did not induce adipogenesis, whereas normal VLDL induced adipogenesis in these cells. The incubation of apoE-deficient VLDL with recombinant human apoE restored its adipogenic activity. Tetrahydrolipstatin, a lipoprotein lipase inhibitor, did not affect the adipogenic activity of VLDL, suggesting that hydrolysis of VLDL did not play a major role in its effects. In fact, lipid components of VLDL or free fatty acids induced only partial adipogenesis. Key Words: VLDL Ⅲ apolipoprotein E Ⅲ lipoprotein lipase Ⅲ obesity Ⅲ adipogenesis F ree fatty acids (FFAs) generated by the hydrolysis of VLDL by lipoprotein lipase (LPL) have been believed to the major driving force in adipogenesis and the development of obesity. FFAs can regulate various steps of adipogenesis, including the activation of early signals, such as peroxisome proliferator-activated receptor-␥ (PPAR␥), induction of adipocyte-specific genes, and maturation to heterogeneous adipocytes containing different sizes of lipid droplets or metabolic activities. 1,2 LPL, secreted from adipocytes and other cell types, is the rate-limiting enzyme for this process. 3,4 Heparan sulfate proteoglycan, the VLDL receptor, and LDL receptor-related protein (LRP) act as cell surface anchors for LPL. [5][6][7] Deficiency of this enzyme induces severe hypertriglyceridemia in humans and mice. 8,9 Homozygous knockout mice are lipodystrophic and die of hypertriglyceridemia soon after birth. 8 However, the importance of LPL in adipogenesis has been challenged by recent investigations studying various new mouse models and humans. Adipose tissue-specific knockout of LPL resulted in normal adiposity, which was mainly due to increased de novo lipogenesis in adipocytes. 10 Crebbp heterozygous mice show lipodystrophy with a change in the expression of a series of genes related to lipid metabolism, suggesting that adipogenesis is a complex process regulated by different steps. 11 Rodents deficient in leptin or leptin receptors show both obesity and increased activity of fatty acid synthase, suggesting the importance of intrinsic lipogenesis in obesity. 12 Human LPL deficiency and apoC-II deficiency result in impaired hydrolysis of VLDL and chylomicron but are usually associated with no...

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Objective-Atherogenic remnant lipoproteins (RLPs) are known to induce foam cell formation in macrophages in vitro and in vivo. We examined the involvement of apoB48 receptor (apoB48R), a novel receptor for RLPs, in that process in vitro and its potential regulation by pitavastatin. Methods and Results-THP-1 macrophages were incubated in the presence of RLPs (20 mg cholesterol/dL, 24 hours) isolated from hypertriglyceridemic subjects. RLPs significantly increased intracellular cholesterol ester (CE) and triglyceride (TG) contents (4.8-fold and 5.8-fold, respectively) in the macrophages. Transfection of THP-1 macrophages with short interfering RNA (siRNA) against apoB48R significantly inhibited RLP-induced TG accumulation by 44%. When THP-1 macrophages were pretreated with pitavastatin (5 mol/L, 24 hours), the expression of apoB48R was significantly decreased and RLP-induced TG accumulation was reduced by 56%. ApoB48R siRNA also inhibited TG accumulation in THP-1 macrophage induced by ␤-very-low-density lipoprotein derived from apoEϪ/Ϫ mice by 58%, supporting the notion that apoB48R recognizes and takes-up RLPs in an apoE-independent manner. Key Words: remnant lipoproteins Ⅲ foam cell formation Ⅲ apoB48 receptor Ⅲ statin Ⅲ atherosclerosis C linical studies have revealed that remnant lipoproteins (RLPs), which are produced by hydrolysis of chylomicrons (CMs) and very-low-density lipoproteins (VLDL), are closely related to atherosclerosis, independent of high-density lipoprotein and low-density lipoprotein (LDL). 1,2 There is also increasing evidence that RLPs play a causative role in atherogenesis, and we recently reported that they induced monocyte-endothelial interaction and vascular smooth muscle cell proliferation. 3,4 However, the effect(s) of RLPs on cellular mechanism(s) during atherogenesis have not been fully elucidated. Conclusions-RLPsAtherogenesis involves the appearance of lipid-loaded foam cells derived from macrophages in the arterial intima. RLPs from hypertriglyceridemic VLDL and CMs cause rapid lipid accumulation, and induce foam cell formation in macrophages, whereas normal VLDL and LDL do not. 5 Thus, inhibition of macrophage foam cell formation induced by RLPs may contribute to the prevention of atherosclerosis. It has been reported that mechanisms independent of apolipoprotein (apo) E and LDL receptor family are involved in lipid accumulation in macrophages. 6 Recently, apoB48 receptor (apoB48R) was shown to be involved in the uptake of triglyceride-rich lipoproteins (TRLs) and contribute to atherogenesis, 7 although the role of apoB48R in this process remains unclear. Herein, we report for the first time to our knowledge the dominant role of apoB48R in RLP-induced foam cell formation by selective downregulation of apoB48R with short interfering RNA (siRNA).Recently, 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductase inhibitor, or statin, has been suggested to have beneficial effects for the prevention of atherosclerosis, independent of its LDL cholesterol-lowering effect. 8 We found ...

Objective-Insulin is an antiapoptotic factor of cultured vascular cells, but it is not clear whether it also exerts antiapoptotic effects on vascular cells in vivo. We studied insulin receptor signaling in the arteries of normal and diabetic rats to establish whether insulin exhibits antiapoptotic activity toward vascular smooth muscle cells in vivo as well as in vitro. Methods and Results-Western blot analysis and real-time polymerase chain reaction revealed ␣-and ␤-subunits of the insulin receptor in association with insulin receptor substrate-1 and phosphatidylinositol 3-kinase in the media of the aorta and carotid artery. The insulin receptor signaling pathway was partially activated under physiological conditions, further activated by intravenous insulin injection, and was attenuated in streptozotocin-induced diabetic rats. Lipopolysaccharide injection induced more apoptosis of vascular smooth muscle cells in diabetic rats than in control rats, whereas insulin prevented apoptosis in the aortic wall. An in vitro study suggested that the antiapoptotic effect of insulin was mediated by phosphatidylinositol 3-kinase. Conclusions-Insulin