Improved efficacy of mitochondrial disrupting agents upon inhibition of autophagy in a mouse model of BRCA1-deficient breast cancer

Breast cancer is a heterogeneous disease, and stratification of patients is fundamental to the success of treatment modalities. Breast tumors deficient in BRCA1 are mostly associated with basal-like breast cancers and targeted therapeutics for this disease subtype are still lacking. In order to address whether macroautophagy/autophagy inhibition will be effective in BRCA1-deficient mammary tumors, we generated mice with conditional deletion of an essential autophagy gene, Rb1cc1, along with Brca1 and Trp53, through utilization of the K14-Cre transgene. We found that Rb1cc1 deletion suppressed tumorigenesis in the BRCA1-deficient model when compared to wild type and heterozygous Rb1cc1 controls. However, in contrast to previous studies in the mouse mammary tumor virus (MMTV)-polyoma middle T antigen (PyMT) model, tumor growth and the distribution of histological subtypes were not affected by loss of RB1CC1. Interestingly, loss of RB1CC1 decreased mitochondrial mass and oxidative respiratory capacity of these tumor cells, along with a decrease in the phosphorylation of MTOR substrates and transcript levels of genes involved in mitochondrial biogenesis. Importantly, we observed an increased sensitivity to mitochondrial disrupting agents upon loss of RB1CC1. Consequently, our data showed that combination of an autophagy inhibitor, spautin-1, along with a mitochondrial complex I inhibitor, metformin, was more effective in limiting oxidative respiratory capacity, colony-forming ability and tumor growth. Altogether, our results indicate that inhibition of autophagy can increase the benefits of metformin treatment in BRCA1-deficient breast cancers.

乳腺癌是一种异质性疾病，对患者进行分层是各类治疗方案取得成功的核心前提。乳腺癌易感基因1（BRCA1）缺陷型乳腺肿瘤多与基底样乳腺癌相关，目前仍缺乏针对该疾病亚型的靶向治疗手段。为探究巨自噬（macroautophagy，即自噬）抑制是否对BRCA1缺陷型乳腺肿瘤有效，我们借助K14-Cre转基因（K14-Cre transgene）构建了同时条件性敲除核心自噬基因Rb1cc1、Brca1与Trp53的小鼠模型。研究发现，与野生型及杂合子Rb1cc1对照组相比，Rb1cc1敲除可抑制BRCA1缺陷模型中的肿瘤发生。然而，与此前针对小鼠乳腺肿瘤病毒（mouse mammary tumor virus, MMTV）-多瘤病毒中间T抗原（polyoma middle T antigen, PyMT）模型的研究不同，RB1CC1缺失并不会影响肿瘤生长及组织学亚型的分布。有趣的是，RB1CC1缺失会降低该类肿瘤细胞的线粒体质量与氧化呼吸能力，同时伴随雷帕霉素靶蛋白（MTOR）底物磷酸化水平下调，以及参与线粒体生物发生的基因转录水平降低。尤为重要的是，我们发现RB1CC1缺失会使肿瘤细胞对线粒体干扰剂的敏感性升高。据此，我们的研究数据显示，将自噬抑制剂spautin-1与线粒体复合物I抑制剂二甲双胍（metformin）联合使用，可更有效地抑制氧化呼吸能力、集落形成能力与肿瘤生长。综上，本研究结果表明，抑制自噬可增强二甲双胍治疗BRCA1缺陷型乳腺癌的临床获益。