Mammary Cells with Active Wnt Signaling Resist ErbB2-Induced Tumorigenesis

Aberrant activation of Wnt signaling is frequent in human malignancies. In normal epithelial tissues, including the breast, Wnt signaling is active only in a subset of cells, but it is unknown whether this subset of Wnt signaling-active cells is at increased risk of carcinogenesis. We created transgenic mice (TOP-tva) in which the synthetic Wnt-responsive promoter TOP controlled the gene encoding TVA, which confers susceptibility to infection by the retroviral vector RCAS. Thus, only cells in which Wnt signaling is active will express tva and be targeted by RCAS. Surprisingly, we found that RCAS-mediated delivery of cDNA encoding a constitutively activated version of ErbB2 (HER2/Neu) into the small number of TVA+ mammary epithelial cells in TOP-tva mice failed to induce tumor, while the same virus readily induced mammary tumors after it was delivered into a comparable number of cells in our previously reported mouse line MMTV-tva, whose tva is broadly expressed in mammary epithelium. Furthermore, we could not even detect any early lesions or infected cells in TOP-tva mice at the time of necropsy. Therefore, we conclude that the Wnt pathway-active cell subset in the normal mammary epithelium does not evolve into tumors following ErbB2 activation–rather, they apparently die due to apoptosis, an anticancer “barrier” that we have reported to be erected in some mammary cells followed ErbB2 activation. In accord with these mouse model data, we found that unlike the basal subtype, ErbB2+ human breast cancers rarely involve aberrant activation of Wnt signaling. This is the first report of a defined sub-population of mammalian cells that is “protected” from tumorigenesis by a potent oncogene, and provides direct in vivo evidence that mammary epithelial cells are not equal in their response to oncogene-initiated transformation.Wnt信号通路（Wnt signaling）的异常激活在人类恶性肿瘤中极为常见。在包括乳腺在内的正常上皮组织中，Wnt信号通路仅在部分细胞亚群中处于激活状态，但目前尚不明确这类Wnt信号通路激活的细胞亚群是否具有更高的癌变（carcinogenesis）风险。我们构建了转基因小鼠（TOP-tva），其中人工合成的Wnt响应启动子TOP可调控编码TVA的基因的表达——TVA可使细胞对逆转录病毒载体RCAS（retroviral vector RCAS）的感染产生易感特性。因此，仅Wnt信号通路激活的细胞会表达TVA并被RCAS靶向感染。令人意外的是，我们将编码组成型激活型ErbB2（HER2/Neu）的互补DNA（cDNA）通过RCAS载体递送至TOP-tva小鼠体内少量的TVA阳性乳腺上皮细胞中，并未诱导肿瘤生成；而在我们此前报道的MMTV-tva小鼠品系中，将相同病毒递送至数量相当的细胞时，该病毒可高效诱导乳腺肿瘤——该品系的TVA在乳腺上皮中广泛表达。此外，在尸检时，我们甚至无法在TOP-tva小鼠体内检测到任何早期病变或被RCAS感染的细胞。因此我们得出结论：正常乳腺上皮中具有Wnt通路活性的细胞亚群在ErbB2激活后并不会演变为肿瘤；相反，这类细胞显然会因细胞凋亡（apoptosis）而死亡——这是一种抗癌“屏障”，我们此前曾报道过ErbB2激活后部分乳腺细胞会建立此类屏障。与这些小鼠模型的数据相符，我们发现与基底亚型乳腺癌不同，ErbB2阳性的人类乳腺癌极少伴随Wnt信号通路的异常激活。本研究首次报道了一类可被强效致癌基因“保护”免于肿瘤发生的哺乳动物细胞明确亚群，并为乳腺上皮细胞在致癌基因诱导的细胞转化过程中存在应答差异提供了直接的体内（in vivo）证据。