Cianflone K M, Yasruel Z, Rodriguez M A, Vas D, Sniderman A D
McGill Unit, Royal Victoria Hospital, Montreal, Quebec, Canada.
J Lipid Res. 1990 Nov;31(11):2045-55.
Secretion of hepatic apoB lipoproteins removes excess triglyceride from the liver. However, the mechanism by which synthesis of apoB, which occurs on the rough endoplasmic reticulum, is coordinated with synthesis of triglyceride, which takes place in the smooth endoplasmic reticulum, is not known. To examine this question, we have manipulated intracellular synthesis of triglyceride and cholesteryl ester in HepG2 cells and determined the impact of these maneuvers on apoB secretion. Since cholesteryl ester is the only major lipid class synthesized in the rough endoplasmic reticulum, our hypothesis was that, in response to a fatty acid challenge, synthesis of cholesteryl ester rather than synthesis of triglyceride would be the immediate trigger to apoB secretion. Oleate complexed to bovine serum albumin caused intracellular triglyceride synthesis to increase 6-fold and cholesteryl ester synthesis to increase almost 3-fold, while apoB secretion into the medium increased by 2.5-fold (P less than 0.0125) at all time points between 4 and 24 h. Addition of acylation stimulating protein to the medium further stimulated both triglyceride and cholesteryl ester synthesis (58% and 108%, respectively) above oleate alone and this resulted in a 50% increase in apoB secretion (P less than 0.0025). By contrast, both progesterone and 2-bromooctanoate inhibited triglyceride and cholesteryl ester synthesis and these effects were associated with reduced apoB secretion. Lovastatin inhibited cholesteryl ester synthesis (45%, P less than 0.0025); however, at the doses used, triglyceride formation was unaffected. Under these circumstances, apoB secretion was reduced by 25% (P less than 0.05). Similarly, 58-035 (an inhibitor of acyl CoA:cholesterol acyltransferase) on the one hand reduced cholesteryl ester synthesis markedly (59%, P less than 0.005), but on the other increased triglyceride synthesis though not statistically significantly (65%, P NS), and again this resulted in decreased apoB secretion (25%, P less than 0.005). Control experiments established that changes in low density lipoprotein catabolism did not contribute importantly to the quantity of apoB in the medium. Taken together, the data indicate that, at least in HepG2 cells, there are parallel changes in cholesteryl ester synthesis and apoB secretion and suggest that it is cholesteryl ester synthesis, not triglyceride synthesis, that is the immediate regulator of apoB secretion when these cells are exposed to an increased influx of fatty acids. However, alternative or additional regulatory mechanisms, such as, for example, a role for acylation of apoB, are not excluded by these studies.
肝脏载脂蛋白B脂蛋白的分泌可清除肝脏中多余的甘油三酯。然而，在内质网上进行的载脂蛋白B合成与在内质网中进行的甘油三酯合成是如何协调的，目前尚不清楚。为了研究这个问题，我们在HepG2细胞中操纵了甘油三酯和胆固醇酯的细胞内合成，并确定了这些操作对载脂蛋白B分泌的影响。由于胆固醇酯是在内质网上合成的唯一主要脂质类别，我们的假设是，在脂肪酸刺激下，胆固醇酯的合成而非甘油三酯的合成将是载脂蛋白B分泌的直接触发因素。与牛血清白蛋白结合的油酸使细胞内甘油三酯合成增加6倍，胆固醇酯合成增加近3倍，而在4至24小时之间的所有时间点，培养基中载脂蛋白B的分泌增加了2.5倍（P小于0.0125）。向培养基中添加酰化刺激蛋白进一步刺激了甘油三酯和胆固醇酯的合成（分别比单独使用油酸增加了58%和108%），这导致载脂蛋白B分泌增加了50%（P小于0.0025）。相比之下，孕酮和2-溴辛酸都抑制了甘油三酯和胆固醇酯的合成，这些作用与载脂蛋白B分泌减少有关。洛伐他汀抑制胆固醇酯合成（45%，P小于0.0025）；然而，在所使用的剂量下，甘油三酯的形成不受影响。在这种情况下，载脂蛋白B分泌减少了25%（P小于0.05）。同样，58-035（一种酰基辅酶A：胆固醇酰基转移酶抑制剂）一方面显著降低了胆固醇酯的合成（59%，P小于0.005），但另一方面增加了甘油三酯的合成，尽管没有统计学意义（65%，P无显著性差异），这再次导致载脂蛋白B分泌减少（25%，P小于0.005）。对照实验表明，低密度脂蛋白分解代谢的变化对培养基中载脂蛋白B的量没有重要贡献。综上所述，数据表明，至少在HepG2细胞中，胆固醇酯合成和载脂蛋白B分泌存在平行变化，这表明当这些细胞暴露于增加的脂肪酸流入时，胆固醇酯合成而非甘油三酯合成是载脂蛋白B分泌的直接调节因子。然而，这些研究并未排除其他调节机制，例如载脂蛋白B酰化的作用。
