Alternative Approaches To Target the COX-2 Pathway To Reduce Breast Cancer Metastasis.

Abstract Background: The cyclooxygenase-2 (COX-2) enzyme is highly expressed in breast cancer and is associated with a poorer prognosis. Past efforts have focused on targeting the COX-2 enzyme with pharmacologic COX-2 inhibitors including celecoxib and rofecoxib. Recent safety concerns have led us to test the hypothesis that targeting downstream members of the COX-2 pathway will be more efficacious and safer. The principle COX-2 product in tumors is prostaglandin E2 (PGE2) which mediates cellular effects by binding to four distinct G-protein-coupled receptors designated EP1, EP2, EP3 and EP4.Materials and Methods: Using pharmacologic and genetic approaches and a preclinical model of metastatic, triple-negative (ER, PR, Her-2 negative) breast cancer, we have examined the role of EP4 and EP1 in malignant behavior. We determined the effect of antagonizing either EP1 or EP4 with the pharmacologic antagonists AH23848, ONO-AE3-208 or SC19220 on metastatic ability of line 66.1 or line 410.4 mammary tumor cells injected into syngeneic Balb/cByJ female mice and compared efficacy of receptor antagonists to either a COX-2 inhibitor (celecoxib) or a dual COX-1/COX-2 inhibitor (indomethacin). We also reduced the expression of either receptor by tranducing tumor cells with specific shRNAs and determined the effect of EP gene silencing on metastatic potential. Using mice with specific deletions of immune effector cells, we also examined the role of host immunity in controlling tumor metastasis.Results: Inhibition of EP4 receptor signaling with the selective EP4 antagonists AH23848 or ONO-AE3-208 limits breast cancer metastasis to the same degree as achieved with a COX inhibitor. Likewise, gene silencing of EP4 with a vector expressing EP4 siRNA also limits metastatic capacity. The therapeutic effect of EP4 blockade is compromised in mice lacking functional Natural Killer cells and NK-mediated lysis of mammary tumor cells is enhanced in the presence of EP4 antagonists. Conversely, blocking EP1 with either the pharmacologic antagonist SC19220 or by gene-silencing promotes tumor metastasis. The expansion of tumors implanted into the mammary fat pad was not affected by modulating EP expression levels, indicating that EP1 functions as a suppressor of metastasis. Interestingly, breast tumors from African-American women express significantly less EP1 than tumors from Caucasian women, suggesting that EP1 could contribute to breast cancer disparities. Ongoing studies are evaluating the relationship of EP expression to long term survival.Discussion: These data support the hypothesis that therapeutic targeting of EP4 may be an alternative approach to the use of COX inhibitors to limit metastatic disease. The mechanism of metastasis control is likely to involve activation of Natural Killer cells. Shifting the balance towards activation of the metastasis suppressor EP1 could be an additional means to reduce breast cancer metastasis. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6140.