Craig W Y, Cooper A D
Department of Medicine, Stanford University School of Medicine, CA 94305.
J Lipid Res. 1988 Mar;29(3):299-308.
The regulation of lipoprotein secretion in the cell line HepG2 was studied. HepG2 cells were preincubated with chylomicron remnants (triglyceride- and cholesterol-rich) or with beta very low density lipoproteins (beta-VLDL) (cholesterol-rich). The medium was removed and the cells were incubated for and additional 24 hr in a lipoprotein-free medium that contained either [2-3H]glycerol or DL-[2-3H]mevalonate. Cells and media were harvested, and lipoproteins were separated and fractionated. The mass and radioactivity of the lipids in cells and in the lipoproteins were measured. The activities of cellular acyl-CoA:cholesterol acyltransferase (ACAT) and 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase were also determined. Preincubation with chylomicron remnants induced an increase in cellular triglyceride and stimulated both HMG-CoA reductase and ACAT. Preincubation with beta-VLDL induced an increase in cellular free and esterified cholesterol, inhibited HMG-CoA reductase and stimulated ACAT. Although the absolute amount of VLDL is small, chylomicron remnants induced large relative increases in the amount of triglyceride and phospholipid secreted in VLDL and decreases in the amount of triglyceride secreted in low density (LDL) and high density (HDL) lipoproteins as well as a decrease in the amount of phospholipid secreted in HDL. In contrast, preincubation with beta-VLDL did not affect triglyceride secretion, but markedly stimulated the amount of phospholipid secreted in HDL. Comparison of the mass of glycerolipid actually secreted with that calculated from the cellular specific activity suggested that glycerolipids are secreted from single, rapidly equilibrating pools. Cholesterol and cholesteryl ester secretion were affected differently. Preincubation with chylomicron remnants increased the amount of free cholesterol secreted in both VLDL and LDL, but did not alter cholesteryl ester secretion. Preincubation with beta-VLDL increased free cholesterol secretion in all lipoprotein fractions and increased cholesteryl ester secretion in VLDL and LDL, but not HDL. Comparison of isotope and mass data suggested that the cholesteryl ester secreted came primarily from a preformed, rather than an newly synthesized, pool. In summary, these data provide insight to the mechanism whereby a liver cell regulates the deposition of exogenous lipid.
研究了细胞系HepG2中脂蛋白分泌的调节机制。将HepG2细胞与乳糜微粒残粒（富含甘油三酯和胆固醇）或β极低密度脂蛋白（β-VLDL，富含胆固醇）进行预孵育。去除培养基后，将细胞在含有[2-³H]甘油或DL-[2-³H]甲羟戊酸的无脂蛋白培养基中再孵育24小时。收获细胞和培养基，分离并分级脂蛋白。测量细胞和脂蛋白中脂质的质量和放射性。还测定了细胞酰基辅酶A：胆固醇酰基转移酶（ACAT）和3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶的活性。与乳糜微粒残粒预孵育可导致细胞甘油三酯增加，并刺激HMG-CoA还原酶和ACAT。与β-VLDL预孵育可导致细胞游离胆固醇和酯化胆固醇增加，抑制HMG-CoA还原酶并刺激ACAT。尽管极低密度脂蛋白的绝对量很小，但乳糜微粒残粒可导致极低密度脂蛋白中甘油三酯和磷脂分泌量的相对大幅增加，低密度（LDL）和高密度（HDL）脂蛋白中甘油三酯分泌量减少，以及高密度脂蛋白中磷脂分泌量减少。相比之下，与β-VLDL预孵育不影响甘油三酯分泌，但显著刺激高密度脂蛋白中磷脂的分泌量。将实际分泌的甘油脂质量与根据细胞比活性计算得出的量进行比较表明，甘油脂是从单一的、快速平衡的池中分泌出来的。胆固醇和胆固醇酯的分泌受到不同的影响。与乳糜微粒残粒预孵育可增加极低密度脂蛋白和低密度脂蛋白中游离胆固醇的分泌量，但不改变胆固醇酯的分泌。与β-VLDL预孵育可增加所有脂蛋白组分中游离胆固醇的分泌量，并增加极低密度脂蛋白和低密度脂蛋白中胆固醇酯的分泌量，但不增加高密度脂蛋白中的分泌量。同位素和质量数据的比较表明，分泌的胆固醇酯主要来自预先形成的池，而不是新合成的池。总之，这些数据为肝细胞调节外源性脂质沉积的机制提供了见解。
