Department of Pharmacology, College of Medicine, Chungnam National University, Daejeon, 35015, South Korea.
Department of Medical Science, Metabolic Syndrome and Cell Signaling Laboratory, Institute for Cancer Research, College of Medicine, Chungnam National University, Daejeon, 35015, South Korea.
Cell Death Dis. 2022 Jun 10;13(6):543. doi: 10.1038/s41419-022-04993-5.
The selective autophagy of damaged mitochondria is called mitophagy. Mitochondrial dysfunction, mitophagy, and apoptosis have been suggested to be interrelated in various human lung carcinomas. Leucine zipper EF-hand-containing transmembrane protein-1 (LETM1) was cloned in an attempt to identify candidate genes for Wolf-Hirschhorn syndrome. LETM1 plays a role in mitochondrial morphology, ion homeostasis, and cell viability. LETM1 has also been shown to be overexpressed in different human cancer tissues, including lung cancer. In the current study, we have provided clear evidence that LETM1 acts as an anchoring protein for the mitochondria-associated ER membrane (MAM). Fragmented mitochondria have been found in lung cancer cells with LETM1 overexpression. In addition, a reduction of mitochondrial membrane potential and significant accumulation of microtubule-associated protein 1 A/1B-light chain 3 punctate, which localizes with Red-Mito, was found in LETM1-overexpressed cells, suggesting that mitophagy is upregulated in these cells. Interestingly, glucose-regulated protein 78 kDa (GRP78; an ER chaperon protein) and glucose-regulated protein 75 kDa (GRP75) were posited to interact with LETM1 in the immunoprecipitated LETM1 of H460 cells. This interaction was enhanced in cells treated with carbonyl cyanide m-chlorophenylhydrazone, a chemical mitophagy inducer. Treatment of cells with honokiol (a GRP78 inhibitor) blocked LETM1-mediated mitophagy, and CRISPR/Cas9-mediated GRP75 knockout inhibited LETM1-induced autophagy. Thus, GRP78 interacts with LETM1. Taken together, these observations support the notion that the complex formation of LETM1/GRP75/GRP78 might be an important step in MAM formation and mitophagy, thus regulating mitochondrial quality control in lung cancer.
受损线粒体的选择性自噬称为线粒体自噬。线粒体功能障碍、线粒体自噬和细胞凋亡被认为与各种人类肺癌有关。Leucine zipper EF-hand-containing transmembrane protein-1（LETM1）是在试图鉴定 Wolf-Hirschhorn 综合征候选基因时克隆的。LETM1 在线粒体形态、离子稳态和细胞活力中发挥作用。已经表明 LETM1 在不同的人类癌症组织中过度表达，包括肺癌。在本研究中，我们提供了明确的证据表明 LETM1 作为线粒体相关内质网膜（MAM）的锚定蛋白发挥作用。在 LETM1 过表达的肺癌细胞中发现了碎片化的线粒体。此外，在 LETM1 过表达的细胞中发现线粒体膜电位降低，微管相关蛋白 1A/1B-轻链 3 点状明显积累，该点状与 Red-Mito 定位，表明这些细胞中的线粒体自噬上调。有趣的是，葡萄糖调节蛋白 78kDa（GRP78；内质网伴侣蛋白）和葡萄糖调节蛋白 75kDa（GRP75）被假定在内质网 GRP75 的 LETM1 免疫沉淀中与 LETM1 相互作用。这种相互作用在用羰基氰化物 m-氯苯腙（一种化学线粒体自噬诱导剂）处理的细胞中增强。用 honokiol（GRP78 抑制剂）处理细胞阻断 LETM1 介导的线粒体自噬，而 CRISPR/Cas9 介导的 GRP75 敲除抑制 LETM1 诱导的自噬。因此，GRP78 与 LETM1 相互作用。总之，这些观察结果支持 LETM1/GRP75/GRP78 复合物的形成可能是 MAM 形成和线粒体自噬的重要步骤，从而调节肺癌中线粒体的质量控制。
