Rab25调节EGFR细胞内化引起肿瘤放疗抵抗的机制研究

Radiotherapy is the first line treatment for nasopharyngeal carcinoma (NPC). Due to variation in radiation sensitivity, however, some patients develop tolerance to radiotherapy. Existence of radio-resistant cells within tumor has been suggested as the main cause for radiation resistance. These cells develop a strong metastatic and anti-anoikis potential by undergoing epithelial-mesenchymal transition. It has been reported that activation of downstream signaling pathways mediated by EGFR caused anoikis resistance. Located on circulating endosomes, the small GTPase Rab25 expressed in a variety of tumors, however, little is known about its biological function and effector molecules. We previously found that Rab25 was highly and specifically expressed in radio-resistant NPC cells and radio-resistant tumor tissues of mice, and knockdown of Rab25 was associated with down-regulation of EGFR and increase in anoikis in radio-resistant NPC cells. Thus, we hypothesize that Rab25 regulates anti-anoikis ability of cells through EGFR signaling pathway by affecting either the vesicle formation or transportation, through which EGFR can be recycled back onto the cytoplasmic membrane to maintain EGFR signaling leading to radio-resistance. The present study will investigate the roles of Rab25 in endosome formation and its interactive partners during the process, define the pathways controlling the cellular trafficking of EGFR-endosomes and anoikis, and unveil the novel mechanisms of radio-resistance via establishing a Rab25-mediated endosome recycling.

肿瘤组织中存在放疗不敏感细胞被认为是放疗抗拒产生的原因之一。这些细胞可通过EMT获得极强的抗失巢凋亡能力和转移潜能。EGFR介导的信号通路活化是肿瘤细胞抵抗失巢凋亡主要机制之一。循环囊泡蛋白小 GTPase Rab25的高表达与多种肿瘤的发生发展密切相关。但其互作分子及生物学功能的研究有限。我们的前期研究发现Rab25在放疗抵抗细胞和放疗抵抗小鼠瘤体组织中高表达，且与细胞抗失巢凋亡能力密切相关。Rab25可调节EGFR在细胞内的含量。我们推测Rab25作为特异表达于循环囊泡上的Rab蛋白，通过改变EGFR内化和的胞内运输维持细胞中的EGFR水平，使得细胞规避失巢凋亡，获得放射抵抗性。本项目将利用已建立的放疗抵抗细胞和动物模型，聚焦Rab25在细胞中通过囊泡运输系统对EGFR的转运、细胞定位等过程的影响，探讨EGFR内化改变对失巢凋亡相关信号网络的调节，进而阐明鼻咽癌放疗抵抗的一种新机制。

肿瘤获得性放射抵抗仍然是肿瘤放射治疗的主要限制因素。尽管Rab25（一种关键的受体循环蛋白）和EGFR都被报道在具有侵袭性表型的肿瘤中增强，但目前尚不清楚这两种蛋白在肿瘤放疗获得性抵抗方面发挥的作用。本课题在一系列抗辐射的人癌细胞株、体内抗辐射的异种移植瘤和对RT反应不良的鼻咽癌患者中均检测Rab25表达升高，并且与肺腺癌患者的预后不良相关。鼻咽癌放疗抵抗细胞系和肺腺癌耐药细胞系中EGFR活性增强，且受Rab25表达调控。进一步研究表明，在辐照哦抵抗细胞中，Rab25可与EGFR结合，将已发生内化的EGFR重新转运到细胞表面，减少EGFR的降解反应。Rab25基因表达抑制后，细胞表现出协同的放射敏感性和降低的侵袭性表型，阻断Rab25介导的EGFR循环是消除抗辐射癌细胞的有效途径。