Color-Coded Imaging of Breast Cancer Metastatic Niche Formation in Nude Mice

We report here a color-coded imaging model in which metastatic niches in the lung and liver of breast cancer can be identified. The transgenic green fluorescent protein (GFP)-expressing nude mouse was used as the host. The GFP nude mouse expresses GFP in all organs. However, GFP expression is dim in the liver parenchymal cells. Mouse mammary tumor cells (MMT 060562) (MMT), expressing red fluorescent protein (RFP), were injected in the tail vein of GFP nude mice to produce experimental lung metastasis and in the spleen of GFP nude mice to establish a liver metastasis model. Niche formation in the lung and liver metastasis was observed using very high resolution imaging systems. In the lung, GFP host-mouse cells accumulated around as few as a single MMT-RFP cell. In addition, GFP host cells were observed to form circle-shaped niches in the lung even without RFP cancer cells, which was possibly a niche in which future metastasis could be formed. In the liver, as with the lung, GFP host cells could form circle-shaped niches. Liver and lung metastases were removed surgically and cultured in vitro. MMT-RFP cells and GFP host cells resembling cancer-associated fibroblasts (CAFs) were observed interacting, suggesting that CAFs could serve as a metastatic niche.