Nguyen L B, Shefer S, Salen G, Tint G S, Ruiz F, Bullock J
Department of Medicine/Division of Gastroenterology and Liver Center, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Newark, NJ 07103, USA.
J Lipid Res. 2001 Feb;42(2):195-200.
The effects of feeding cholesterol, sitosterol, and lovastatin on cholesterol absorption, biosynthesis, esterification, and LDL receptor function were examined in the rat jejunal mucosa. Cholesterol absorption was measured by the dual-isotope plasma ratio method; the rate-limiting enzyme of cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, was measured as total and expressed enzyme activities (in the absence and presence of a phosphatase inhibitor, NaF, respectively); mucosal total and esterified cholesterol concentrations were determined by gas-liquid chromatography; LDL receptor function was assayed as receptor-mediated binding of (125)I-labeled LDL to mucosal membranes. Feeding 2% sitosterol or 0.04% lovastatin for 1 week significantly (P < 0.01) decreased the amounts of cholesterol absorbed per day (-85% and -63%, respectively). In contrast, feeding 2% cholesterol for 1 week increased the amounts of absorbed cholesterol 27-fold, even though the percent absorption significantly decreased. With all three treatments, there was a coordinate regulation of total HMG-CoA reductase activity and receptor-mediated LDL binding. Cholesterol feeding downregulated both total jejunal HMG-CoA reductase activity (P < 0.05) and receptor-mediated LDL binding (P < 0.01), whereas lovastatin- and sitosterol-supplemented diets significantly upregulated both of these parameters. In the control, cholesterol-fed, and sitosterol-fed animals, about half of the total jejunal HMG-CoA reductase activity was expressed (in functional dephosphorylated form). However, in the lovastatin-treated rats with 4-fold stimulation of HMG-CoA reductase, only 23% of the total enzyme activity was expressed. Changes in total HMG-CoA reductase activity and receptor-mediated LDL binding in all tested groups occurred with no change in total concentrations of mucosal cholesterol, and only cholesterol-fed animals had increased mucosal esterified cholesterol concentrations. Thus, in response to various fluxes of dietary or newly formed cholesterol, HMG-CoA reductase and receptor-mediated LDL binding are coordinately regulated to maintain constant cellular cholesterol concentrations in the jejunum.
在大鼠空肠黏膜中研究了喂食胆固醇、植物甾醇和洛伐他汀对胆固醇吸收、生物合成、酯化及低密度脂蛋白（LDL）受体功能的影响。采用双同位素血浆比率法测定胆固醇吸收；胆固醇生物合成的限速酶3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶，分别测定其总活性和表达的酶活性（分别在不存在和存在磷酸酶抑制剂氟化钠的情况下）；通过气液色谱法测定黏膜中总胆固醇和酯化胆固醇的浓度；通过（125）I标记的LDL与黏膜膜的受体介导结合来测定LDL受体功能。喂食2%植物甾醇或0.04%洛伐他汀1周显著（P<0.01）降低了每日吸收的胆固醇量（分别降低85%和63%）。相反，喂食2%胆固醇1周使吸收的胆固醇量增加了27倍，尽管吸收百分比显著降低。在所有三种处理中，总HMG-CoA还原酶活性和受体介导的LDL结合存在协同调节。喂食胆固醇下调了空肠中总HMG-CoA还原酶活性（P<0.05）和受体介导的LDL结合（P<0.01），而补充洛伐他汀和植物甾醇的饮食显著上调了这两个参数。在对照组、喂食胆固醇组和喂食植物甾醇组动物中，空肠中总HMG-CoA还原酶活性约有一半以表达形式（功能性去磷酸化形式）存在。然而，在洛伐他汀处理的大鼠中，HMG-CoA还原酶受到4倍刺激，仅23%的总酶活性以表达形式存在。所有测试组中总HMG-CoA还原酶活性和受体介导的LDL结合的变化发生时，黏膜胆固醇的总浓度没有变化，只有喂食胆固醇的动物黏膜酯化胆固醇浓度增加。因此，为响应饮食或新合成胆固醇的各种通量，HMG-CoA还原酶和受体介导的LDL结合受到协同调节，以维持空肠中细胞胆固醇浓度恒定。
