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p38 belongs to a family of mitogen-activated protein kinases, which transfer extracellular signals into intracellular responses. p38 is also frequently detected in clinical breast cancer specimens, but its role as a prognostic factor is not known. Of the various p38 isoforms, p38 β has been shown to mediate the in vitro invasiveness of breast cancer cells through up-regulation of urokinase plasminogen activator (uPA). We studied the role of p38 β in breast cancer bone metastases, using dominant negative blockade approach. Human MDA-MB-231 breast cancer clones stably expressing dominant negative p38 β (p38/AF) exhibited decreased basal MMP-9 activity. TGF- β 1-induced MMP-9 activity was also blunted in these clones, as compared with controls in which TGF- β 1 up-regulated MMP-9 activity. Consistent with these findings, SB202190, a specific p38 inhibitor, also inhibited TGF-β1-induced MMP-9 activity in parental cells. The p38/AF clones exhibited also reduced uPA production after growth on vitronectin and decreased cell motility, as compared with controls. VEGF production levels in all the studied clones were similar. The p38/AF clone, which had similar in vitro growth rate as the control pcDNA3 clone, formed significantly less bone metastases in a mouse model, as compared with the control clone. In conclusion, inhibition of the p38 β pathway results in decreased MMP-9 activity, impaired uPA expression and decreased motility, all of which may contribute to the decreased formation of bone metastasis. [ABSTRACT FROM AUTHOR]