CXCR2-dependent senescence restrains hPTTG1-induced metastasis. (A) Left column: Images of murine lungs 12 weeks after orthotopic implantation of MCF-7Mock cells, MCF-7Mock cells stably infected with shCXCR2-1, and MCF-7hPTTG1 cells infected with shGFP or shCXCR2-1. Right panel: H&E staining of lungs from mice bearing the indicated tumors; arrows indicate metastatic foci. Magnification, ×200. Scale bars, 100 μm. (B) Lung metastases in mice bearing representative tumors were observed by performing H&E analysis of paraffin-embedded lung sections. The numbers of metastatic nodules in the H&E stains were microscopically counted. Quantitative results of the lung metastasis of representative cells (n = 6 per group) are presented as the mean ± SEM. **P < 0.01; ***P < 0.0001.