Bojes H K, Thurman R G
Department of Pharmacology and Curriculum in Toxicology, University of North Carolina at Chapel Hill, 27599-7365, USA.
Toxicol Appl Pharmacol. 1996 Oct;140(2):322-7. doi: 10.1006/taap.1996.0227.
It is unknown whether peroxisome proliferators decrease hepatic fatty acid oxidation via uncoupling of respiration or if they inhibit extramitochondrial fatty acyl CoA synthesis. Therefore, the purpose of this study was to examine both processes simultaneously using the isolated perfused liver, a whole cell preparation where enzymes and biochemical processes can be monitored continuously under nearly physiological conditions. Accordingly, ketone body formation (beta-hydroxybutyrate + acetoacetate) from lipid metabolism and oxygen uptake, which is increased by uncoupling agents, were monitored at the same time. 2-Bromooctanoate, a known inhibitor of acyl CoA synthetase, decreased ketone body formation in a dose-dependent manner without altering cellular respiration (half-maximal inhibition, approximately 25 microM) and concomitantly increased protein kinase C nearly fourfold also in a dose-dependent fashion. Ketogenesis was also blocked maximally 50-66% with mono(ethylhexyl)phthalate, 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio acetic acid (WY-14,643), and nafenopin, potent peroxisome proliferators and tumor promoters. These compounds also increased protein kinase C three- to fourfold without altering oxygen uptake significantly. Thus, lipid metabolism appears to be the prime target of potent peroxisome proliferators most likely on actions via acyl CoA synthetase rather than oxidative phosphorylation. In contrast, weak peroxisome proliferators and tumor promoters, di(ethylhexyl)phthalate and 2-ethylhexanol, did not affect ketogenesis, oxygen consumption, or protein kinase C at similar concentrations. Additionally, octanoate increased ketone body formation in the presence of nafenopin. Because octanoate is metabolized by mitochondrial acyl CoA synthetase independent of carnitine acyltransferase, these results indicate that nafenopin does not inhibit mitochondrial beta-oxidation. Taken together, it is concluded that potent peroxisome proliferators preferentially block ketogenesis without altering cellular respiration in the liver. This phenomenona, which occurs due to inhibition of acyl CoA synthetase, leads to an elevation of free fatty acids that stimulates protein kinase C and promotes cell proliferation.
过氧化物酶体增殖剂是通过呼吸解偶联来降低肝脏脂肪酸氧化，还是抑制线粒体外脂肪酸酰基辅酶A的合成，目前尚不清楚。因此，本研究的目的是使用离体灌注肝脏同时检测这两个过程，离体灌注肝脏是一种全细胞制剂，在接近生理条件下可以连续监测酶和生化过程。相应地，同时监测脂质代谢产生的酮体生成（β-羟基丁酸 + 乙酰乙酸）和解偶联剂增加的氧摄取。2-溴辛酸是一种已知的酰基辅酶A合成酶抑制剂，它以剂量依赖性方式降低酮体生成，而不改变细胞呼吸（半数最大抑制浓度约为25 μM），同时也以剂量依赖性方式使蛋白激酶C增加近四倍。邻苯二甲酸单（乙基己基）酯、4-氯-6-（2,3-二甲苯胺基）-2-嘧啶硫代乙酸（WY-14,643）和萘芬诺平是强效过氧化物酶体增殖剂和肿瘤促进剂，它们也能使酮体生成最大程度地减少50%-66%。这些化合物还能使蛋白激酶C增加三到四倍，而不会显著改变氧摄取。因此，脂质代谢似乎是强效过氧化物酶体增殖剂的主要作用靶点，最有可能是通过酰基辅酶A合成酶起作用，而不是氧化磷酸化。相比之下，弱效过氧化物酶体增殖剂和肿瘤促进剂邻苯二甲酸二（乙基己基）酯和2-乙基己醇在相似浓度下不影响酮体生成、氧消耗或蛋白激酶C。此外，在萘芬诺平存在的情况下，辛酸增加了酮体生成。由于辛酸是由线粒体酰基辅酶A合成酶代谢的，与肉碱酰基转移酶无关，这些结果表明萘芬诺平不抑制线粒体β-氧化。综上所述，得出的结论是，强效过氧化物酶体增殖剂优先阻断肝脏中的酮体生成，而不改变细胞呼吸。这种现象是由于酰基辅酶A合成酶受到抑制而发生的，导致游离脂肪酸升高，从而刺激蛋白激酶C并促进细胞增殖。
