Kaptein A, de Wit E C, Princen H M
Gaubius Laboratory IVVO-TNO, Institute of Ageing and Vascular Research, Leiden, The Netherlands.
Biochim Biophys Acta. 1994 Aug 4;1213(3):349-56. doi: 10.1016/0005-2760(94)00064-6.
We have shown previously that sodium butyrate induces a 2-fold increase in the secretion of apo B-100 by HepG2 cells. The apo B-100 mRNA level was not changed in butyrate-treated cells, indicating regulation at the translational or co- or posttranslational level (Biochem. J. (1991) 278, 557-564). In this paper, the mechanism by which butyrate increases apo B-100 secretion was further investigated. Pulse-chase analysis showed that in control incubations only 18 +/- 4% of the total amount of labelled apo B-100, present intracellularly after a 10 min pulse period, was secreted after a 90 min chase period, indicating that the major part of newly synthesized apo B-100 is degraded intracellularly. After addition of butyrate the secreted amount increased to 32 +/- 6% of the total synthesized amount. Treatment of HepG2 cells with butyrate resulted in an enhanced intracellular concentration of triacylglycerols (+30%), with no or only a marginal effect on the cellular content of cholesterol and cholesteryl esters. Secretion of triacylglycerols (+90%) and cholesteryl esters (+78%), but not of cholesterol, was increased to the same extent as apo B-100 secretion (+102%). The total mass of triacylglycerols, i.e., the sum of triacylglycerols present intracellularly and secreted by HepG2 cells, was significantly increased upon incubation with butyrate (+32%), whereas the total mass of cholesteryl esters was not affected. Butyrate did not affect the buoyant density of apo B-100-containing lipoproteins secreted by HepG2 cells. These results suggest that an increased availability of triacylglycerols, formed after the addition of butyrate regulates the amount of apo B-100 degraded intracellularly and consequently apo B-100 secretion.
我们之前已经表明，丁酸钠可使HepG2细胞分泌的载脂蛋白B-100增加两倍。在丁酸钠处理的细胞中，载脂蛋白B-100的mRNA水平没有变化，这表明是在翻译或共翻译或翻译后水平进行调控（《生物化学杂志》（1991年）278卷，557 - 564页）。在本文中，我们进一步研究了丁酸钠增加载脂蛋白B-100分泌的机制。脉冲追踪分析表明，在对照孵育中，经过10分钟脉冲期后细胞内存在的标记载脂蛋白B-100总量中，只有18±4%在90分钟追踪期后被分泌，这表明新合成的载脂蛋白B-100的主要部分在细胞内被降解。添加丁酸钠后，分泌量增加到总合成量的32±6%。用丁酸钠处理HepG2细胞导致细胞内三酰甘油浓度升高（+30%），对胆固醇和胆固醇酯的细胞含量没有影响或只有轻微影响。三酰甘油（+90%）和胆固醇酯（+78%）的分泌增加程度与载脂蛋白B-100分泌（+102%）相同，但胆固醇的分泌没有增加。与丁酸钠孵育后，三酰甘油的总质量，即细胞内存在的三酰甘油与HepG2细胞分泌的三酰甘油之和，显著增加（+32%），而胆固醇酯的总质量没有受到影响。丁酸钠不影响HepG2细胞分泌的含载脂蛋白B-100的脂蛋白的浮力密度。这些结果表明，添加丁酸钠后形成的三酰甘油可用性增加，调节了细胞内降解的载脂蛋白B-100的量，从而调节了载脂蛋白B-100的分泌。
