SGK3在MYC介导的乳腺癌PI3K靶向治疗耐药中的作用和分子机制研究

The PI3K signaling pathway is frequently altered in human breast cancer. While a large number of PI3K small molecule inhibitors are entering clinical trials, drug resistance would be unavoidably a big challenge in PI3K-targeted cancer therapy. Our previous work using an inducible transgenic mouse model of breast cancer identified that c-Myc amplification/overexpression confers resistance to PI3K inhibition and that enforced ectopic expression of c-Myc renders tumor cells independent of PI3K/AKT activation (Nature Medicine, 2011). However, the underlying molecular mechanism and the corresponding clinical meaning of PI3K-independent tumor growth incurred by c-Myc overexpression remains unknown. SGK3 is a downstream effector of PI3K signaling. We recently found that mouse mammary tumors harboring high Myc expression and PIK3CA mutation grow in an SGK3-dependent manner and of particular note, these tumors revealed increased abundance of SGK3 protein. Herein we propose to use both human breast cancer cell lines and mouse models to uncover the molecular mechanisms by which c-Myc confers resistance to PI3K inhibition. Importantly, we will test the hypothesis if overexpression of c-Myc renders the growth of mammary tumor cells in an SGK3-dependent but PI3K/AKT-independent manner. In combined with humanized mouse models of breast cancer, we will evaluate the anti-tumor effects of SGK3 inhibition in combination with PI3K-directed therapy. The long-term goal of this project is to define effective treatment strategies and inform patient selection to ultimately overcome drug resistance in PI3K-directed breast cancer therapy.

乳腺癌中PI3K信号通路变异频繁，其临床靶向治疗前景广阔，但耐药性已成为治疗成功的瓶颈。我们在转基因小鼠模型中发现，MYC基因扩增与高表达会导致乳腺肿瘤对PI3K抑制剂耐药，且过表达MYC使肿瘤细胞中PI3K下游AKT活性显著减低 (Nat. Med. 2011)。我们近期发现高表达MYC的小鼠乳腺肿瘤中，PI3K下游、非AKT通路中蛋白激酶SGK3表达水平显著增高，且这些肿瘤生长依赖SGK3。由此推测：SGK3使MYC高表达的肿瘤细胞不再依赖PI3K/AKT通路，从而对PI3K抑制剂耐药。为证明此假设，本课题拟应用乳腺癌细胞系和动物模型，研究SGK3在MYC导致PI3K抑制剂耐药中的作用；从细胞信号通路、细胞周期和能量代谢途径三个方面探明其分子机制；结合人原发乳腺癌移植瘤模型和活体组织切片培养模型，探索联合靶向 SGK3与PI3K治疗乳腺癌的有效性，为克服PI3K靶向治疗耐药提供新思路。

PI3K信号传导通路的异常激活在肿瘤中频繁发生，虽然靶向PI3K的肿瘤治疗临床前景广阔，但耐药仍是影响临床治疗成功的巨大瓶颈。本项目成功建立可诱导表达PIK3CA H1047R的乳腺癌转基因小鼠模型，通过诱导和去诱导转基因表达，我们发现在带有高表达MYC基因的小鼠复发乳腺肿瘤中，SGK3表达水平显著增高而且这些肿瘤依赖SGK3存活。体外实验中，SGK3的过表达或持续激活促进肿瘤细胞生长；敲低SGK3导致癌细胞内ROS水平上调、凋亡增加，在此条件下联合ROS诱导剂会协同促进细胞凋亡。为进一步探究SGK3在乳腺癌发生与发展中的作用，我们构建了在乳腺组织中特异性表达人源SGK3的转基因小鼠模型，初期实验结果显示长期诱导表达人源SGK3转基因可导致小鼠产生乳腺肿瘤且去诱导表达转基因后导致短暂减瘤后肿瘤迅速复发。我们的研究结果提示SGK3的高表达是导致乳腺肿瘤发生和对P13K通路抑制剂耐药的潜在原因，而且靶向SGK3从而破坏肿瘤细胞内的氧化还原平衡有望克服乳腺癌对P13K抑制剂耐药。我们的最新研究还发现，作为与SGK3同一家族成员的SGK1对癌细胞中氧化还原稳态也有调控作用，并提出了以SGK1为靶点、基于氧化还原调节机制的肿瘤治疗新策略。..我们在拓展PI3K抑制剂治疗肿瘤的研究中发现：（1） 靶向抑制PI3K信号通路可导致癌细胞中DNA损伤同源重组修复缺陷，进而增强癌细胞对PARP抑制剂的敏感性。（2）泛PI3K抑制剂GDC0941导致三阴性乳腺癌细胞中肿瘤相关巨噬细胞的浸润和巨噬细胞分泌的炎症因子水平增加，通过激活NF-kB通路而对PI3K抑制剂耐药，联合使用阿司匹林有效抑制炎症反应及NF-kB的活性从而显著提高PI3K抑制剂对乳腺癌的治疗效果。（3） 致癌因子BTF3可通过调控ESR1的转录表达和ERα介导的基因转录而使ER阳性乳腺癌细胞对PI3Kp110a抑制剂BYL719耐药，进一步证明抑制BTF3可显著增强ER阳性乳腺癌对PI3K抑制剂的治疗敏感性。..本项目中，我们还积极致力于建立乳腺癌人源性肿瘤移植瘤膜型、活组织切片短期离体培养等多种癌症临床前研究模型，为探索乳腺癌的发生发展以及靶向治疗和耐药机制等方面研究提供重要研究工具。