Bone metastasis is one of the most severe cancer complications. To analyze the mechanism of bone metastasis, we established highly invasive cell lines from the human breast cancer cell line MDA-MB-231 using an in vitro sequential selection system. The cell lines, MDA-231-S10 and MDA-231-S5, were more invasive and more motile than the parental cell line. Moreover, MDA-231-S10 metastasized to bone more often when inoculated into the arterial circulation of nude mice. MDA-231-S10-bearing nude mice had a significantly poorer prognosis, and their bony metastatic tumors grew more rapidly than those of the mice bearing the parental cell line (MDA-231-P). Given that a high expression of matrix metalloproteinase (MMP) is reported to be associated with cancer invasiveness, we examined MMP expression. Our results showed that the expression of MMP-3, -5, -7, -9, -13 and -14 was decreased on Multiplex real-time quantitative RT-PCR analysis in the two new cell lines. The zymographic analysis showed no MMP-2 activity and a decreased MMP-9 activity in MDA-231-S10. However, the expression of MMP-1 in MDA-231-S10 was increased. We therefore concluded that MMP-1 plays a crucial role in breast cancer bone metastasis. Furthermore, our MDA-231-derived cell lines are useful analytical models of MMP-1- associated breast cancer bone metastasis.

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