The stromal-derived factor (SDF)-1/CXCR-4 pathway has been implicated to play an important role in skeletal metastases of several cancer types including prostate cancer (CaP). High expression levels of the chemokine receptor CXCR-4 correlates with the presence of metastatic disease in CaP patients. Bisphosphonates, including zoledronic acid (ZA), have been demonstrated to reduce skeletal tumor burden and prevent metastasis to bone. Bisphosphonates function primarily by initiating biochemical processes that result in apoptosis of osteoclasts; however, the mechanisms of their anti-tumor effects are not well defined. Accordingly, we first determined the effect of ZA on proliferation of LNCaP, C4-2B, and PC3 CaP cell lines. ZA inhibited CaP cell proliferation in a dose-dependent manner in all of these cell lines. Furthermore, ZA inhibited expression of CXCR-4 mRNA and protein levels, and ZA inhibited CXCR4 promoter activity. ZA also inhibited in vitro invasiveness of the CaP cells. Neutralizing antibody for CXCR-4 abolished CaP cell invasion. NFκB has been shown to promote breast cancer cell migration and metastasis by inducing CXCR-4 and activate the CXCR-4 promoter. Thus, we investigated whether ZA inhibits CXCR-4-mediated CaP invasion through inhibition of NFκB signaling. Inhibition of NFκB activity by overexpression of IκBα in CaP cells reduced CXCR-4 expression and reduced SDF-1-induced in vitro invasion by 65% that is consistent with the possibility that NFκB promotes CaP invasion through up-regulation of CXCR-4. Transfection of CaP cells with CXCR-4 cDNA partially (by 70%) restored the overexpression of IκBα-mediated reduction of SDF-1-induced invasion. These results demonstrate that NFκB contributes to CXCR-4-mediated CaP cell invasion. To determine if ZA mediates inhibition of CXCR-4-dependent invasion we evaluated ZA’s effect on protein expression of NFκB subunits, NFκB DNA binding activity and effects on NFκB binding to CXCR-4 promoter. ZA had no effect on any of these parameters. Taken together, we concluded that ZA inhibits CXCR-4-dependent prostate cancer cell invasion through an NFκB independent pathway in vitro.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]