MMP-1 knockdown in 231-BR cells attenuates brain metastasis. MMP-1 shRNA expressing 231-BR cells, sh1a and sh1b, and control shNTC cells were injected into the left heart of nude mice, 1.75×105 cells per mouse. Each group includes ten mice. Mice were sacrificed after 4 weeks and the number of experimental metastases scored in serial brain sections. A, Representative H&E staining of brain sections. Arrows indicate metastatic foci. B, The sh1a and sh1b cells formed fewer large metastases and total metastases compared with the control shNTC cells. The data were combined from two independent experiments, shown as the mean and SEM of metastases scored in serial sections. Asterisks indicate that sh1b cells formed significantly fewer large and total metastases compared with shNTC cells (P = 0.002, P = 0.0067 respectively, ANOVA Dunnett’s Multiple comparison test). C, Ki-67 stained sections of brain metastases formed by injection of 231-BR control cells and MMP-1 knockdown cells and D, comparisons of Ki-67 positive cells in brain metastases. Significantly fewer Ki-67 positive cells were found in brain metastases of sh1a (P = 0.022) and sh1b (P = 0.011, Student’s t-test) compared with metastases of shNTC cells.