Mosley S T, Kalinowski S S, Schafer B L, Tanaka R D
Department of Cellular Biology, Squibb Institute for Medical Research, Princeton, NJ 08543-4000.
J Lipid Res. 1989 Sep;30(9):1411-20.
Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the key enzyme that regulates cholesterol synthesis, lower serum cholesterol by increasing the activity of low density lipoprotein (LDL) receptors in the liver. In rat liver slices, the dose-response curves for inhibition of [14C]acetate incorporation into cholesterol were similar for the active acid forms of lovastatin, simvastatin, and pravastatin. The calculated IC50 values were approximately 20-50 nM for all three drugs. Interest in possible extrahepatic effects of reductase inhibitors is based on recent findings that some inhibitors of HMG-CoA reductase, lovastatin and simvastatin, can cause cataracts in dogs at high doses. To evaluate the effects of these drugs on cholesterol synthesis in the lens, we developed a facile, reproducible ex vivo assay using lenses from weanling rats explanted to tissue culture medium. [14C]Acetate incorporation into cholesterol was proportional to time and to the number of lenses in the incubation and was completely eliminated by high concentrations of inhibitors of HMG-CoA reductase. At the same time, incorporation into free fatty acids was not inhibited. In marked contrast to the liver, the dose-response curve for pravastatin in lens was shifted two orders of magnitude to the right of the curves for lovastatin acid and simvastatin acid. The calculated IC50 values were 4.5 +/- 0.7 nM, 5.2 +/- 1.5 nM, and 469 +/- 42 nM for lovastatin acid, simvastatin acid, and pravastatin, respectively. Thus, while equally active in the liver, pravastatin was 100-fold less inhibitory in the lens compared to lovastatin and simvastatin. Similar selectivity was observed with rabbit lens. Following oral dosing, ex vivo inhibition of [14C]acetate incorporation into cholesterol in rat liver was similar for lovastatin and pravastatin, but cholesterol synthesis in lens was inhibited by lovastatin by as much as 70%. This inhibition was dose-dependent and no inhibition in lens was observed with pravastatin even at very high doses. This tissue-selective inhibition of sterol synthesis by pravastatin was likely due to the inability of pravastatin to enter the intact lens since pravastatin and lovastatin acid were equally effective inhibitors of HMG-CoA reductase enzyme activity in whole lens homogenates. We conclude that pravastatin is tissue-selective with respect to lens and liver in its ability to inhibit cholesterol synthesis.
3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶是调节胆固醇合成的关键酶，HMG-CoA还原酶抑制剂通过增加肝脏中低密度脂蛋白（LDL）受体的活性来降低血清胆固醇。在大鼠肝切片中，洛伐他汀、辛伐他汀和普伐他汀的活性酸形式对抑制[14C]乙酸掺入胆固醇的剂量反应曲线相似。三种药物的计算IC50值约为20 - 50 nM。对还原酶抑制剂可能的肝外效应的关注基于最近的发现，即一些HMG-CoA还原酶抑制剂，如洛伐他汀和辛伐他汀，在高剂量时可导致犬类白内障。为了评估这些药物对晶状体中胆固醇合成的影响，我们开发了一种简便、可重复的离体试验，使用从断奶大鼠取出并植入组织培养基中的晶状体。[14C]乙酸掺入胆固醇的量与时间以及孵育中晶状体的数量成正比，并且被高浓度的HMG-CoA还原酶抑制剂完全消除。同时，掺入游离脂肪酸的过程未受抑制。与肝脏形成显著对比的是，普伐他汀在晶状体中的剂量反应曲线相对于洛伐他汀酸和辛伐他汀酸的曲线向右移动了两个数量级。洛伐他汀酸、辛伐他汀酸和普伐他汀的计算IC50值分别为4.5±0.7 nM、5.2±1.5 nM和469±42 nM。因此，虽然普伐他汀在肝脏中具有同等活性，但与洛伐他汀和辛伐他汀相比，其在晶状体中的抑制作用低100倍。在兔晶状体中也观察到了类似的选择性。口服给药后，洛伐他汀和普伐他汀对大鼠肝脏中[14C]乙酸掺入胆固醇的离体抑制作用相似，但洛伐他汀对晶状体中胆固醇合成的抑制作用高达70%。这种抑制作用呈剂量依赖性，即使在非常高的剂量下，普伐他汀也未观察到对晶状体的抑制作用。普伐他汀对甾醇合成的这种组织选择性抑制可能是由于普伐他汀无法进入完整的晶状体，因为普伐他汀和洛伐他汀酸在整个晶状体匀浆中对HMG-CoA还原酶活性的抑制效果相同。我们得出结论，普伐他汀在抑制胆固醇合成的能力方面对晶状体和肝脏具有组织选择性。
