Beta-catenin signaling in the interactions of tumor cells and the tumor microenvironment

Abstract

Dynamic interactions between cancer cells and the tumor microenvironment are increasingly recognized as contributors to cancer heterogeneity and increased therapeutic resistance. Tumor hypoxia and tumor-associated macrophages (TAMs) are two characteristic elements common to many cancer types, and aberrant β-catenin signaling is frequently observed in cancers. In this study, the roles of β-catenin signaling in cancer cell response to tumor hypoxia and TAMs were investigated. In several human colorectal cancer(CRC)cell lines, hypoxic conditions simultaneously increased the expression of β-catenin and the orphan nuclear receptor Nur77. While β-catenin enhanced Nur77 mRNA expression through hypoxia-inducible factor-1αinstead of T-cell factor(TCF), Nur77 reciprocally increased β-catenin stability independent of the adenomatous polyposis coli and p53 pathways, but through Akt signaling. Overexpressed β-catenin and Nur77 synergistically stimulated migration, invasion, and epithelial-mesenchymal transition of hypoxic CRCs. These findings suggest a positive feedback circuit of β-catenin/Nur77, which could enhance the invasive growth of hypoxic CRCs. To develop a new model of spontaneous ovarian cancer metastasis, a paired cell lines with contrasting metastatic potentials were selected. When orthotopically implanted inimmunodeficient mice, the highly metastatic (HM) line spontaneously formed peritoneal metastases and ascites, thus resembling the major dissemination route of human ovarian cancer. The non-metastatic (NM) counterpart also formed primary tumor in the ovaries, but no metastasis was detected. Using label-free quantitative liquid chromatography-tandem mass spectrometry(LC-MS/MS)and pathway analysis, a prominent enrichment of β-catenin signaling was found in HM. Western blotting and reporter assays confirmed a significant increase in β-catenin expression and TCF transcription activity in HM. Moreover, β-catenin knockdown severely impaired tumor initiation and metastasis of HM in mice. These results together highlight the clinical relevance of β-catenin signaling in ovarian cancer. Previous studies on TAMs usually inferred tumor cells as homogeneous populations. However, recent data suggested the existence of cell-to-cell variation. Using live-cell imaging to track individual cells, NM and HM cells were found to react differently upon M1 macrophage co-culture. Interestingly, M1 macrophages selectively promoted survival and induced a distinct polyploid phenotype in HMs, which resulted from failed cytokinesis. A contact-dependent interaction between metadherin on HMs and CEACAM1 on macrophages could be important for inducing this phenotype. β-catenin, which was specifically activated in HM, but not NM, was responsible for the metadherin expression in HMs. On the other hand, TAM markers and cytokines that are highly enriched in ovarian cancer were also enhanced in macrophages by the co-cultured HM. These findings suggest a positive crosstalk between HM and macrophages that reinforces their respective phenotypic changes. Together, these results provide important insights in the impact of the tumor microenvironment on the signaling and phenotypes of cancer cells. Understanding these interacting processes would help to design novel microenvironment-based therapeutic strategy.