CMTM6

*仅供医学专业人士阅读参考坚固的堡垒，往往都是从内部被攻破的，抗癌战场上也是同样的道理，如果用免疫治疗强攻收效不好，不妨就去找找癌细胞的弱点，不能力取的就靠智取嘛。而癌细胞内调控PD-L1表达的机制[1]，就是一个很好的“智取”目标。近日，浙江大学夏宏光、王东睿团队在Nature Communications期刊发表的最新研究成果显示，热休克蛋白70（Hsc70）可通过与CMTM6的竞争性结合，在癌细胞内促进PD-L1蛋白经内体-溶酶体途径降解，从而减少经循环回到细胞表面的PD-L1，更有利于PD-1/L1抑制剂等免疫治疗药物起效[2]。论文核心机制总结既往对PD-L1降解机制的探索显示，PD-L1的降解要么走蛋白酶体途径，要么就走内体-溶酶体途径，任选其一进行抑制，再配合PD-1/L1抑制剂就可实现协同增效[3]，但癌细胞也有反制手段，比如2017年登上《自然》的一项研究就显示，CMTM6能够结合并保护PD-L1蛋白不被蛋白酶体途径降解，由此稳定癌细胞的PD-L1表达、抑制抗肿瘤免疫[4]。而本次研究的主角Hsc70，打通的就是另一边的内体-溶酶体途径，不过研究者们的实验是从PD-L1的内化（internalization）和回收（recycling）开始的，这是PD-L1抑制剂治疗后癌细胞常见的情况，被回收的PD-L1蛋白虽然有一部分会经内体-溶酶体途径被降解，但大多数仍然能够再次回到细胞膜上，继续与PD-1结合来抑制抗肿瘤免疫。研究者们分析了经PD-L1抑制剂处理24小时的乳腺癌细胞，发现细胞膜上PD-L1的表达水平先降后升，其中的“后升”就是PD-L1内化和回收的结果；借助免疫共沉淀和质谱分析技术，研究者们初步筛选出能与PD-L1蛋白发生相互作用，可能介导了PD-L1内化和回收的蛋白，再结合患者预后资料分析后，Hsc70就是唯一满足所有条件的研究对象了。接下来的实验显示，过表达Hsc70就可以在不改变PD-L1转录水平的状况下，显著降低癌细胞内PD-L1蛋白水平和细胞膜PD-L1表达，而只有抑制溶酶体功能会使PD-L1表达水平回升，抑制蛋白酶体和自噬（也可经溶酶体降解PD-L1）都无相同效果，说明Hsc70促进PD-L1蛋白的降解，确实要依赖内体-溶酶体途径，与初步猜想一致。Hsc70可通过内体-溶酶体途径促进PD-L1蛋白的降解当然了，Hsc70也不只是个纯纯的“中间商”，它本身的两重功能才是促进PD-L1蛋白降解的关键：首先，Hsc70是介导内体微自噬（endosomal microautophagy, eMI）的关键伴侣蛋白，这正是PD-L1蛋白在溶酶体内被降解的主要途径；其次，Hsc70与奇点糕前面提到的CMTM6在PD-L1蛋白上的结合区域重叠，意味着二者存在竞争性结合关系，敲除CMTM6能显著增强Hsc70与PD-L1蛋白的结合和随后的促PD-L1降解。在荷瘤小鼠上的实验也明确证实，过表达Hsc70的癌细胞虽然能形成肿瘤，但肿瘤生长速度会被免疫系统大幅抑制，肿瘤内浸润的CD8+T细胞明显较Hsc70野生型肿瘤更多，而癌细胞PD-L1表达水平因过表达Hsc70大幅下降，是CD8+T细胞前来抑癌的必要条件。小鼠实验证实过表达Hsc70的抑癌作用研究者们还结合此前的探索成果，找到了可使癌细胞过表达Hsc70的药物——Hsp90抑制剂（AUY-922），它能起到与基因层面过表达Hsc70相似的效果，与PD-1/L1抑制剂乃至CTLA-4抑制剂实现协同增效，成为潜在的免疫联合治疗策略。此外研究者们还提出，未来还可继续探索基于Hsc70介导eMI，来调控其它疾病中关键蛋白质降解的治疗策略。参考文献：[1]Lin X, Kang K, Chen P, et al. Regulatory mechanisms of PD-1/PD-L1 in cancers[J]. Molecular Cancer, 2024, 23: 108.[2]Xu X, Xie T, Zhou M, et al. Hsc70 promotes anti-tumor immunity by targeting PD-L1 for lysosomal degradation[J]. Nature Communications, 2024, 15: 4237.[3]Gou Q, Dong C, Xu H, et al. PD-L1 degradation pathway and immunotherapy for cancer[J]. Cell Death & Disease, 2020, 11: 955.[4]Burr M L, Sparbier C E, Chan Y C, et al. CMTM6 maintains the expression of PD-L1 and regulates anti-tumour immunity[J]. Nature, 2017, 549(7670): 101-105.本文作者丨谭硕

Scientists identify a new and surprising function of a membrane protein on the surface of cancer cells: It supports and stabilizes an important 'co-stimulatory' factor that enhances the activation of T cells, thus improving the immune response against the tumor. Scientists from the German Cancer Research Center (DKFZ) identify a new and surprising function of a membrane protein on the surface of cancer cells: It supports and stabilizes an important "co-stimulatory" factor that enhances the activation of T cells, thus improving the immune response against the tumor. The study was conducted in collaboration with researchers from the Netherlands Cancer Institute. Many types of cancer can be successfully treated with immune checkpoint inhibitors (ICI). This treatment is often simply referred to as "immunotherapy." The blockade of inhibitory immune checkpoint proteins such as PD-L1 forms the basis of this treatment. But a concomitant lack of stimulatory signaling can render the therapies useless. This is considered one of the reasons why many cancer patients do not benefit from immune checkpoint inhibitors. T cells are the key players in the immune defense against tumors. Their activation is tightly controlled by a variety of inhibitory and also stimulatory immune checkpoints. However, tumor cells often sabotage this system by manipulating the expression of checkpoint proteins to escape destruction by the immune system. Among the activity-promoting co-stimulatory proteins is CD58. When it binds to its receptor on an immune cell, the cell receives a stimulatory signal. If the binding of CD58 to its receptor is blocked, the immune response against many cancers is impaired. "It is intriguing to observe that many cancer cells inherently express CD58, a molecule that essentially contradicts their own survival when they come under immune attack. We therefore wanted to understand what controls the expression of CD58," says Chong Sun, an immunologist at the German Cancer Research Center. With their current study, the researchers found that the membrane protein CMTM6 interacts and positively regulates the expression of CD58. The surprising thing is that CMTM6 simultaneously interacts with PD-L1, the important inhibitory immune checkpoint molecule that most current ICI therapies target. CMTM6 protects PD-L1 from degradation and likewise also stabilizes CD58 via this mechanism. This mechanism potentially fine-tunes immune response. Moreover, using a model of TCR-T cell treatment in the culture dish, the researchers showed that CMTM6 loss from tumor cells impairs T cell activation. They also found that the influence of CMTM6 on CD58 plays an important role in the antigen-specific interaction of T cells with tumor cells and also affects the response to PD-L1 blockade. "It is fascinating that CMTM6 controls two important players in our immune system, CD58 and PD-L1, even though they have opposing functions. And what is more interesting is that when we dive into the analysis of tumor samples from patients who received ICI therapies, it appears that CD58 might just take a leading role in shaping the response, in most cases," explains Beiping Miao, one of the first authors. Using mice to which human leukemia cells had been grafted, the team demonstrated that loss of CMTM6 protects cancer cells from CAR-T cell therapy. Additionally, in human cancer cells from tumor biopsies, widespread expression of CMTM6 and CD58 was observed, with higher expression of CMTM6 or CD58 significantly correlated with better response to immunotherapies. "Our findings highlight the importance of CMTM6 and CD58 expression in cancer cells during an immune response against tumors. Our next step is to explore the possibility of adjusting their expression in laboratory experiments. Our goal is thereby finding a way to improve cancer immunotherapies," said Sun.