IntroductionIn murine breast cancer models, the two interferon-gamma (IFN-γ) inducible chemokines and CXC-chemokine receptor 3 (CXCR3) receptor ligands, monokine induced by γ-interferon (CXCL9) and interferon-γ-inducible protein-10 (CXCL10) impair tumor growth and metastasis formation through recruitment of natural killer (NK) cells and tumor-suppressive T lymphocytes. In human breast cancer, CXCL9 mRNA overexpression correlates with the number of tumor infiltrating lymphocytes and predicts response to different chemotherapeutic regimens. Raising the intratumoral CXCR3 ligand concentration is therefore a possible way to enhance immune intervention in breast cancer. Little is known, however, about expression levels and regulation of these chemokines in human breast cancer. Since the inhibition of cyclooxygenases (COX) has been shown to reduce tumor growth and incidence of metastases in a lymphocytic and IFN-γ dependent manner, we argued that COX isoenzymes are a pharmacologic target to increase intratumoral CXCR3 ligand concentration in human breast cancer.MethodsCXCL9 was visualized in breast cancer specimens by immunohistochemistry, expression levels of CXCL9 and cyclooxygenases were determined by ELISA and western blotting, respectively. For regulation studies, Michigan Cancer Foundation-7 (MCF-7) and M.D. Anderson - Metastatic Breast 231 (MDA-MB 231) breast cancer cells were stimulated with IFN-γ with or without prostaglandin E2 (PGE2) or COX inhibitors (indomethacin, acetylsalicylic acid (ASA), celecoxib). CXCR3 ligand release from cells was measured by ELISA.ResultsWithin the tumor microenvironment, cancer cells are the major source of CXCL9. PGE2 impairs IFN-γ mediated CXCL9 and CXCL10 release from MCF-7 and MDA-MB 231 cells, and inhibition of endogenous cyclooxygenases by indomethacin or ASA correspondingly increases this secretion. Otherwise, high concentrations of the Cyclooxygenase-2 (COX-2) specific antagonist celecoxib have opposite effects and impair CXCL9 and CXCL10 release. In human breast cancer tissue specimens there is an inverse correlation between COX-2 overexpression and CXCL9 concentration, suggesting that the observed in vitro effects are of importance in vivo as well.ConclusionsSuppressing endogenous PGE2 synthesis by cyclooxygenase inhibition increases CXCL9 and CXCL10 release from breast cancer cells and is therefore a pharmacologic candidate to enhance intratumoral immune infiltration. Yet, to this end the unselective COX inhibitors ASA and indomethacin seem preferable to celecoxib that at higher concentrations reduces CXCR3 ligand release most probably due to COX independent mechanisms.

Cathepsin B (CTSB) is a lysosomal cysteine protease that has been linked to the progression of breast cancer, for example by activation of other proteases and tumor-promoting cytokines, thereby supporting tumor invasion and metastasis. Previously, it was shown that CTSB cleaves and inactivates C-X-C motif chemokine receptor 3 (CXCR3) chemokines. As CXCR3 ligands have been demonstrated to induce proteases in cancer cells, the present study hypothesized that they may also affect CTSB in breast cancer cells. The results demonstrated that the human breast cancer tumor cell lines MCF-7 and MDA-MB-231 express the CXCR3 splice variants A and B and CTSB. Upon binding to CXCR3, the two chemokine ligands C-X-C motif chemokine ligand (CXCL) 9 and CXCL10 trigger upregulation of CTSB in these breast cancer cells, whereas the CXCR3-B-specific ligand CXCL4 has no such effect, suggesting the involvement of CXCR3-A in the regulation of CTSB. In early-stage human breast cancer specimens (n=81), overexpression of CXCR3 is associated with statistically significant poorer overall survival, independent of lymph node status, tumor size and nuclear grading (hazard ratio=1.99; 95% confidence interval=1.00–3.97; P=0.050). In conclusion, the data from the current study propose a so far unknown mechanism by which breast cancer cells may exploit tumor-suppressive chemokines to enhance their invasiveness and reduce immune cell infiltration by the degradation of these chemokines. This mechanism may support the established unfavorable prognostic feature of CXCR3 expression in breast cancer.

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45 Background: In murine cancer models, the two IFN-γ inducible chemokines CXCL9 and CXCL10, those bind to the common receptor CXCR3, recruit NK cells and tumor-suppressive lymphocytes into the tumor site and impair tumor growth and metastatic spread. In human breast cancer (BC), we and others have shown that high levels of CXCL9 mRNA correlate with favorable prognosis and the number of infiltrating lymphocytes. Raising the intratumoral level of CXCR3 ligands might therefore be a feasible way to enhance the infiltration by tumor-suppressive immune cells and to improve immune intervention in breast cancer. Inhibition of cyclooxygenases (COX) has been shown to inhibit tumor growth and metastases formation in a lymphocytic and IFN-γ dependent manner. We therefore tested whether COX inhibition induces CXCR3 ligand secretion from breast cancer cells. Methods: Human MCF-7 and MDA-MB 231 BC cells were stimulated with IFN-γ with or without prostaglandin E2 (PGE2) or COX inhibitors (indomethacin, aspirin, celecoxib). CXCL9 and CXCL10 release was measured by ELISA. COX-1 and COX-2 expression was measured in 45 BC samples and correlated with intratumoral CXCR3 ligand concentration. Results: Prostaglandin E2 inhibits CXCL10 and CXCL9 release from breast cancer cells. Aspirin and indomethacin enhance the INF-γ mediated secretion of these CXCR3 ligands by inhibition of endogenous cyclooxygenases. Celecoxib has this effect only at low concentrations, at higher concentrations is shows PGE2 agonistic effects. In human breast cancer samples, COX-2 overexpression inversely correlates with CXCR3 ligand concentration, which shows that the mechanism of PGE2 induced CXCL9/CXCL10 suppression might also be relevant in vivo. Conclusions: Suppressing endogenous PGE2 by cyclooxygenase inhibition increases CXCL9 and CXCL10 release from breast cancer cells and is therefore a feasible way to enhance the infiltration of breast tumors by tumor-suppressive lymphocytes. However, our results show that unselective COX inhibitors might be more suitable than the COX-2 specific celecoxib. Clinical trials are now warranted to clarify the mechanisms and therapeutic efficacy of COX inhibition in breast cancer.