Department of Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94158, USA.
Mol Pharmacol. 2010 Apr;77(4):575-92. doi: 10.1124/mol.109.061259. Epub 2010 Jan 13.
We have reported previously that the hepatic heme-regulated inhibitor (HRI)-eukaryotic initiation factor 2 alpha (eIF2 alpha) kinase is activated in acute heme-deficient states, resulting in translational shut-off of global hepatic protein synthesis, including phenobarbital (PB)-mediated induction of CYP2B enzymes in rats. These findings revealed that heme regulates hepatic CYP2B synthesis at the translational level via HRI. As a proof of concept, we have now employed a genetic HRI-knockout (KO) mouse hepatocyte model. In HRI-KO hepatocytes, PB-mediated CYP2B protein induction is no longer regulated by hepatic heme availability and proceeds undeterred even after acute hepatic heme depletion. It is noteworthy that genetic ablation of HRI led to a small albeit significant elevation of basal hepatic endoplasmic reticulum (ER) stress as revealed by the activation of ER stress-inducible RNA-dependent protein kinase-like ER-integral (PERK) eIF2 alpha-kinase, and induction of hepatic protein ubiquitination and ER chaperones Grp78 and Grp94. Such ER stress was further augmented after PB-mediated hepatic protein induction. These findings suggest that HRI normally modulates the basal hepatic ER stress tone. Furthermore, because HRI exists in both human and rat liver in its heme-sensitive form and is inducible by cytochrome P450 inducers such as PB, these findings are clinically relevant to acute heme-deficient states, such as the acute hepatic porphyrias. Activation of this exquisitely sensitive heme sensor would normally protect cells by safeguarding cellular energy and nutrients during acute heme deficiency. However, similar HRI activation in genetically predisposed persons could lead to global translational arrest of physiologically relevant enzymes and proteins, resulting in the severe and often fatal clinical symptoms of the acute hepatic porphyrias.
我们之前曾报道过，在急性血红素缺乏状态下，肝脏血红素调节抑制剂（HRI）-真核起始因子 2α（eIF2α）激酶被激活，导致肝脏蛋白质合成的全局翻译关闭，包括大鼠中苯巴比妥（PB）介导的 CYP2B 酶诱导。这些发现表明，血红素通过 HRI 调节肝脏 CYP2B 合成的翻译水平。作为概念验证，我们现在使用了遗传 HRI 敲除（KO）小鼠肝细胞模型。在 HRI-KO 肝细胞中，PB 介导的 CYP2B 蛋白诱导不再受肝脏血红素可用性的调节，并且即使在急性肝血红素耗竭后也不受阻碍地进行。值得注意的是，HRI 的遗传缺失导致肝脏内质网（ER）应激的基础水平略有但显著升高，这表现为 ER 应激诱导的 RNA 依赖性蛋白激酶样 ER 整合（PERK）eIF2α-激酶的激活，以及肝蛋白质泛素化和 ER 伴侣 Grp78 和 Grp94 的诱导。在 PB 介导的肝蛋白质诱导后，这种 ER 应激进一步增加。这些发现表明 HRI 通常调节基础肝脏 ER 应激音。此外，由于 HRI 以其血红素敏感形式存在于人和大鼠肝脏中，并可被细胞色素 P450 诱导剂如 PB 诱导，这些发现与急性血红素缺乏状态（如急性肝卟啉症）具有临床相关性。这种极其敏感的血红素传感器的激活通常通过在急性血红素缺乏期间保护细胞的能量和营养物质来保护细胞。然而，在遗传易感性个体中，类似的 HRI 激活可能导致与生理相关的酶和蛋白质的全局翻译停止，从而导致急性肝卟啉症的严重且通常致命的临床症状。
