A novel labelling strategy enables spatial resolution of metastatic niche cellular composition and uncovers lung stem cell activity proximal to breast cancer

Cancer cell behaviour is strongly influenced by the surrounding cellular environment, making the characterization of the local tumour microenvironment (or niche) a fundamental question in tumour biology. To date, a direct investigation of the early cellular changes induced by metastatic cells within the surrounding tissue is difficult to achieve, especially at early micro-metastatic stages and for low frequency niche populations. Here we present the strategy whereby metastatic cancer cells release a cell-penetrating fluorescent protein that is efficiently taken up by neighbouring cells, allowing spatial identification of the local metastatic cellular environment within the whole tissue. Notably, this strategy can be used to follow metastatic niches from early micro-metastasis to late macro-metastasis, allowing temporal resolution. Moreover, the presence of low represented niche cells can be detected and characterized among the bulk tissue. To highlight its potential, we have used this niche-labelling strategy to study the lung metastatic environment of breast cancer cells. We uncover the presence of lung parenchymal cells within the metastatic niche where lung epithelial cells show stem cell-like features with expression of lung progenitor markers, multi-lineage differentiation potential and self-renewal activity. Moreover, lung epithelial cells can be directly perturbed by cancer cells in ex vivo co-culture assays and support their growth. In summary, here we describe a novel labelling system that enables spatial resolution of the metastatic microenvironment and provide evidence that the tissue cellular environment surrounding metastatic growth is characterized by undifferentiated features. The data highlight the significant potential of this method as a platform for new discoveries.

肿瘤细胞的行为受周围细胞微环境的强烈调控，因此对局部肿瘤微环境（tumour microenvironment，又称niche）的表征研究是肿瘤生物学领域的核心基础问题。截至目前，直接探究转移细胞在周围组织中诱导的早期细胞变化仍极具挑战，尤其是在早期微转移阶段，以及针对低丰度的龛细胞群时。本研究提出一种研究策略：转移癌细胞可释放一种可穿透细胞的荧光蛋白（cell-penetrating fluorescent protein），该蛋白可被邻近细胞高效摄取，从而实现对完整组织内局部转移细胞微环境的空间定位识别。值得注意的是，该策略可用于追踪从早期微转移至晚期大转移阶段的转移龛，从而实现时序维度的解析。此外，该方法可在整块组织中检测并表征低丰度的龛细胞群。为验证该策略的应用潜力，我们利用这一龛标记策略探究了乳腺癌细胞的肺转移微环境。本研究发现转移龛内存在肺实质细胞，其中肺上皮细胞表现出干细胞样特性：表达肺祖细胞标志物、具备多系分化潜能与自我更新能力。此外，在体外共培养实验中，肺上皮细胞可直接被癌细胞调控，并可促进癌细胞的增殖。综上，本研究报道了一种可实现转移微环境空间分辨率分析的新型标记系统，并证实转移灶周围的组织细胞微环境以未分化特性为主要特征。本研究数据表明，该方法作为全新的研究发现平台，具备极高的应用潜力。