Petras S F, Lindsey S, Harwood H J
Department of Metabolic Diseases, Pfizer Central Research, Pfizer Inc., Eastern Point Road, Groton, CT 06340, USA.
J Lipid Res. 1999 Jan;40(1):24-38.
The activity of HMG-CoA reductase (HMGR) is tightly regulated, in part through post-transcriptional mechanisms that are mediated by nonsterol products of mevalonate metabolism. Previous reports have suggested that these mediators are derived from farnesyl pyrophosphate (FPP). Recent studies have implicated FPP hydrolysis products (e.g., farnesol), the squalene synthetase (SQS) reaction products presqualene pyrophosphate (PSQPP) and squalene, or their metabolites. To distinguish among these possible mediators, we evaluated the ability of HMGR and SQS inhibitors to induce compensatory increases in HMGR activity in cultured IM-9 cells. Mevinolin (HMGR inhibitor) produced predicted increases in HMGR activity that were related to the degree of cholesterolgenesis inhibition (e.g., 4-fold, 9-fold, and 17-fold increases relative to 50%, 76%, and 90% inhibition, respectively). By contrast, a variety of structurally distinct reversible, competitive, first half-reaction SQS inhibitors all reduced cholesterolgenesis by up to 90% with no appreciable increases in HMGR activity. These observations strongly suggest that nonsterol-mediated post-transcriptional mechanisms regulating HMGR activity remain intact after SQS first half-reaction inhibition, indicating that nonsterol regulator production is independent of SQS action and ruling out PSQPP, squalene and their metabolites as possible mediators. Unexpectedly, the SQS mechanism-based irreversible inactivator, zaragozic acid A (ZGA) exhibited the greatest degree of HMGR modulation, producing 5-fold, 11-fold, and 40-fold increases in HMGR activity at concentrations that produced 25%, 50%, and 75% cholesterolgenesis inhibition, respectively. The markedly greater magnitude of HMGR stimulation by ZGA versus mevinolin at similar levels of cholesterolgenesis inhibition suggests that ZGA may directly interfere with the production or action of the nonsterol regulator.
3-羟基-3-甲基戊二酰辅酶A还原酶（HMGR）的活性受到严格调控，部分是通过甲羟戊酸代谢的非甾醇产物介导的转录后机制实现的。先前的报道表明，这些介质来源于法尼基焦磷酸（FPP）。最近的研究表明，FPP水解产物（如法尼醇）、角鲨烯合酶（SQS）反应产物前角鲨烯焦磷酸（PSQPP）和角鲨烯或其代谢产物也与之有关。为了区分这些可能的介质，我们评估了HMGR和SQS抑制剂在培养的IM-9细胞中诱导HMGR活性代偿性增加的能力。美伐他汀（HMGR抑制剂）使HMGR活性出现了预期的增加，这与胆固醇生成抑制程度相关（例如，相对于50%、76%和90%的抑制率，分别增加了4倍、9倍和17倍）。相比之下，多种结构不同的可逆性、竞争性、上半场反应SQS抑制剂均使胆固醇生成降低了90%，而HMGR活性没有明显增加。这些观察结果强烈表明，在SQS上半场反应受到抑制后，调节HMGR活性的非甾醇介导的转录后机制仍然完整，这表明非甾醇调节剂的产生独立于SQS的作用，并排除了PSQPP、角鲨烯及其代谢产物作为可能的介质。出乎意料的是，基于SQS机制的不可逆失活剂扎拉戈昔酸A（ZGA）表现出最大程度的HMGR调节作用，在分别产生25%、50%和75%胆固醇生成抑制的浓度下，使HMGR活性分别增加了5倍、11倍和40倍。在相似的胆固醇生成抑制水平下，ZGA对HMGR的刺激程度明显大于美伐他汀，这表明ZGA可能直接干扰非甾醇调节剂的产生或作用。
