The PI3K-AKT kinase signaling pathway is frequently deregulated in human cancers, particularly breast cancer, where amplification and somatic mutations of PIK3CA occur with high frequency in patients. Numerous small-molecule inhibitors targeting both PI3K and AKT are under clinical evaluation, but dose-limiting toxicities and the emergence of resistance limit therapeutic efficacy. Various resistance mechanisms to PI3K inhibitors have been identified, including de novo mutations, feedback activation of AKT, or cross-talk pathways. We found a previously unknown resistance mechanism to PI3K pathway inhibition that results in AKT rebound activation. In a subset of triple-negative breast cancer cell lines, treatment with a PI3K inhibitor or depletion of PIK3CA expression ultimately promoted AKT reactivation in a manner dependent on the E3 ubiquitin ligase Skp2, the kinases IGF-1R (insulin-like growth factor 1 receptor) and PDK-1 (phosphoinositide-dependent kinase-1), and the cell growth and metabolism-regulating complex mTORC2 (mechanistic target of rapamycin complex 2), but was independent of PI3K activity or PIP₃ production. Resistance to PI3K inhibitors correlated with the increased abundance of Skp2, ubiquitylation of AKT, cell proliferation in culture, and xenograft tumor growth in mice. These findings reveal a ubiquitin signaling feedback mechanism by which PI3K inhibitor resistance may emerge in aggressive breast cancer cells.

PI3K-AKT激酶信号传导途径在人类癌症中尤其是乳腺癌中经常失控，在乳腺癌中，PIK3CA的扩增和体细胞突变在患者中频繁发生。针对PI3K和AKT的许多小分子抑制剂正在临床评估中，但是剂量限制的毒性和耐药性的出现限制了治疗效果。已经确定了对PI3K抑制剂的各种抗性机制，包括从头突变，AKT的反馈激活或串扰途径。我们发现以前未知的PI3K途径抑制的抵抗机制，导致AKT反弹激活。在三阴性乳腺癌细胞系的子集中，用PI3K抑制剂治疗或耗尽PIK3CA表达最终以依赖于E3泛素连接酶Skp2，激酶IGF-1R（胰岛素样生长因子1受体）和PDK-1（磷酸肌醇依赖性激酶-1）以及细胞生长和代谢的方式促进了AKT的再激活。调节复合物mTORC2（雷帕霉素复合物2的机械靶标），但独立于PI3K活性或PIP _3的产生。对PI3K抑制剂的抗性与Skp2丰度增加，AKT泛素化，培养中的细胞增殖以及小鼠异种移植肿瘤的生长相关。这些发现揭示了泛素信号反馈机制，通过这种机制PI3K抑制剂耐药性可能出现在侵袭性乳腺癌细胞中。