Metastasis is a complex event in cancer progression and causes most deaths from cancer. Repeated transplantation of metastatic cancer cells derived from transplanted murine organs can be used to select the population of highly metastatic cancer cells; this method is called as in vivo selection. The in vivo selection method and highly metastatic cancer cell lines have contributed to reveal the molecular mechanisms of cancer metastasis. Here, we present an overview of the methodology for the in vivo selection method. Recent comparative analysis of the transplantation methods for metastasis have revealed the divergence of metastasis gene signatures. Even cancer cells that metastasize to the same organ show various metastatic cascades and gene expression patterns by changing the transplantation method for the in vivo selection. These findings suggest that the selection of metastasis models for the study of metastasis gene signatures has the potential to influence research results. The study of novel gene signatures that are identified from novel highly metastatic cell lines and patient-derived xenografts (PDXs) will be helpful for understanding the novel mechanisms of metastasis.

HER2 is overexpressed in 25–30% of breast cancers, and approximately 30% of HER2-positive breast cancers metastasize to the brain. Although the incidence of brain metastasis in HER2-positive breast cancer is high, previous studies have been mainly based on cell lines of the triple-negative subtype, and the molecular mechanisms of brain metastasis in HER2-positive breast cancer are unclear. In the present study, we performed intracranial injection using nine HER2-positive breast cancer cell lines to evaluate their proliferative activity in brain tissue. Our results show that UACC-893 and MDA-MB-453 cells rapidly proliferated in the brain parenchyma, while the other seven cell lines moderately or slowly proliferated. Among these nine cell lines, the proliferative activity in brain tissue was not correlated with either the HER2 level or the HER2 phosphorylation status. To extract signature genes associated with brain colonization, we conducted microarray analysis and found that these two cell lines shared 138 gene expression patterns. Moreover, some of these genes were correlated with poor prognosis in HER2-positive breast cancer patients. Our findings might be helpful for further studying brain metastasis in HER2-positive breast cancer.

Breast cancer is a type of cancer with relatively good prognosis; however, once it metastasizes to the brain, it deteriorates clinical outcomes. Previously, we compared the brain-topical proliferative activity of multiple breast cancer cell lines by intracranial injection and identified UACC-893 cells as highly proliferative in the brain. In this study, we performed RNA-seq analysis of brains harboring UACC-893 tumors and explored the genes characteristically expressed in tumor cells colonizing the brain and in their surrounding brain tissue. Enrichment analysis suggested that immune-related signals may be involved in the brain-topical proliferative activity of tumor cells. In addition, ligand-coding genes were extracted among those characteristically expressed in the intracranial tumor or in the brain containing it. This result was validated by quantitative real-time PCR analysis. NicheNet analysis using RNA-seq data suggested the interaction between tumor cells and the brain microenvironment, which suggested possible communications via the ligand-receptor axis. In summary, this study highlights the genes and signals characteristic of highly proliferative tumor cells and the affected brain tissue influencing tumor colonization.