Hirano T, Furukawa S, Kurokawa M, Ebara T, Dixon J L, Nagano S
First Department of Internal Medicine, Showa University School of Medicine, Tokyo, Japan.
Kidney Int. 1995 Feb;47(2):421-31. doi: 10.1038/ki.1995.55.
It is generally accepted that hepatic secretion of apoprotein (apo) B-containing lipoproteins is substantially increased in nephrosis. To elucidate the mechanisms for the oversecretion of apo B, we investigated the effect of a various concentration of albumin on apo B kinetics in the absence or presence of oleate in Hep G2 cells. Hep G2 cells were labeled with [3H]-leucine in leucine-free medium containing 0, 1.5, 3.0 or 4.5% BSA for 180 minutes, and the secreted radiolabeled apo B, apo A1 and albumin were isolated by immunoprecipitation and counted. The secretions of apo B and albumin were suppressed by BSA (bovine serum albumin) in a dose-dependent manner, but the secretion of apo A1 was not suppressed significantly. Oleate (0.4 mM) increased the rate of apo B secretion by 2.5-fold when oleate was bound to 1.5% BSA, but at higher concentrations of BSA (3.0 or 4.5%), apo B secretion was less responsive to oleate. A pulse-chase study indicated that early apo B degradation was significantly suppressed in cells incubated with lower concentrations of BSA (0 or 1.5% BSA), thereby rapidly stimulating apo B secretion. Oleate (0.4 mM) potently inhibited apo B degradation when oleate was bound to 1.5% BSA, whereas the inhibition was not observed when oleate was bound to 4.5% BSA. Intracellular albumin synthesis was stimulated in BSA-free medium, but intracellular decay of albumin was essentially unaffected by concentration of BSA. Similar to BSA, a higher concentration of dextran (3.0 or 4.5%) reduced apo B secretion, and this was the result of increased early apo B degradation in the cells. These results indicate that reduced albumin suppresses intracellular apo B degradation, and the inhibition of apo B degradation by oleate is manifested only at a low concentration of albumin. Therefore, the present study suggests that free fatty acids bound to low concentration of albumin in the circulating plasma play an important role on hepatic oversecretion of apo B-containing lipoprotein in hypoalbuminemic state, such as nephrotic syndrome.
一般认为，肾病时含载脂蛋白（apo）B的脂蛋白的肝脏分泌会大幅增加。为阐明apo B分泌过多的机制，我们研究了在有无油酸存在的情况下，不同浓度白蛋白对Hep G2细胞中apo B动力学的影响。Hep G2细胞在含0、1.5、3.0或4.5%牛血清白蛋白（BSA）的无亮氨酸培养基中用[3H] -亮氨酸标记180分钟，通过免疫沉淀分离分泌的放射性标记的apo B、apo A1和白蛋白并计数。apo B和白蛋白的分泌被BSA（牛血清白蛋白）以剂量依赖方式抑制，但apo A1的分泌未被显著抑制。当油酸与1.5% BSA结合时，油酸（0.4 mM）使apo B分泌速率增加2.5倍，但在较高浓度的BSA（3.0或4.5%）时，apo B分泌对油酸的反应较小。脉冲追踪研究表明，在较低浓度BSA（0或1.5% BSA）孵育的细胞中，早期apo B降解被显著抑制，从而迅速刺激apo B分泌。当油酸与1.5% BSA结合时，油酸（0.4 mM）有力地抑制apo B降解，而当油酸与4.5% BSA结合时未观察到这种抑制作用。在无BSA培养基中细胞内白蛋白合成受到刺激，但白蛋白的细胞内降解基本不受BSA浓度影响。与BSA类似，较高浓度的右旋糖酐（3.0或4.5%）降低apo B分泌，这是细胞内早期apo B降解增加的结果。这些结果表明，白蛋白减少会抑制细胞内apo B降解，并且油酸对apo B降解的抑制仅在低浓度白蛋白时表现出来。因此，本研究表明，在循环血浆中与低浓度白蛋白结合的游离脂肪酸在低白蛋白血症状态（如肾病综合征）下肝脏含apo B脂蛋白的分泌过多中起重要作用。
