CXCR2/p21-dependent senescence inhibits hPTTG1-induced tumor growth. (A) In vitro growth curves were determined in MCF-7Mockcells, MCF-7hPTTG1 cells, MCF-7CXCR2 cells, MCF-7Mock cells infected with shCXCR2-1, and MCF-7hPTTG1 cells infected with shGFP, shCXCR2-1, shCXCR2-2, shp21-1, or shp21-2. (B) The colorimetric ELISA assay for BrdU incorporation was performed in representative cells. The results are presented as the mean ± SEM (n = 3). **P < 0.01. (C) MCF-7Mock cells, MCF-7Mock cells infected with shCXCR2-1, MCF-7hPTTG1 cells, and MCF-7hPTTG1 cells infected with shGFP, shCXCR2-1, or shCXCR2-2 were orthotopically injected into the fourth mammary fat pads of SCID mice. Tumor growth was measured weekly. The experiments were terminated after 12 weeks because of the tumor burden. Representative tumors are shown in (D); n = 6 per group per time point. Each time point is presented as the mean ± SEM. **P < 0.01. (D) Tumor weights were measured 12 weeks after orthotopic implantation of representative cells. The representative tumors are shown in the upper panel; n = 6 per group. The results are presented as the mean ± SEM. *P < 0.05. (E) Representative tumors were embedded in paraffin and sectioned for immunohistologic analysis of representative proteins. Original magnification, ×400. Scale bars, 25 μm.