Multiple Drug Resistance-associated Protein 4 Exports the Cyclooxygenase Product Prostaglandin E2 as a Possible Mechanism to Enhance Metastasis of Basal-like Breast Cancer

Abstract

Breast cancer, like many epithelial malignancies, is frequently characterized by dysregulated levels of the enzyme cyclooxygenase 2 (COX-2). Elevated COX-2 and its major lipid product, prostaglandin E2 (PGE2), are associated with a poor prognosis in breast cancer. Targeting this inflammatory pathway with NSAIDs or selective COX-2 inhibitors has proven effective in slowing tumor progression, but has also been associated with an increased risk of adverse cardiovascular events. Multiple drug resistance-associated protein 4 (MRP4) is responsible for the active export of PGE2 from cells where this ligand binds four cognate G-protein coupled receptors (EP1-EP4) and activates multiple tumor-promoting pathways. PGE2-mediated activation of EP4 has been implicated in increased metastatic potential and support of the breast cancer stem-like phenotype. We investigated the expression and role of MRP4 in breast cancer as a possible mechanism to achieve elevated PGE2 levels in the tumor microenvironment. By evaluating publicly available breast cancer gene expression datasets and a panel of breast cancer cell lines, we report that MRP4 is expressed in breast cancer and, for the first time, that elevated expression of MRP4 is found in breast cancer subtypes (basal, HER2-enriched) with the worst prognoses. MRP4 exports PGE2 and this activity is reduced by suppressing MRP4 expression or activity through genetic or pharmacologic means. Mice bearing breast cancer xenografts with reduced expression of MRP4 developed fewer spontaneous metastases than mice bearing control tumors. The contribution of MRP4 to the metastatic cascade may be at an early step as there was no difference in trapping or colonization of cells expressing different MRP4 levels. Other members of the COX-2 pathway are implicated in support of the breast cancer stem-like cell (BCSLC) phenotype, but MRP4 expression was reduced in BCSLCs. Furthermore, altered expression of MRP4 did not significantly affect phenotypic markers of BCSLCs. In summary, elevated MRP4 is expressed in aggressive subtypes of breast cancer, is partially responsible for the level of PGE2 in the tumor microenvironment, and enhances spontaneous metastasis in an animal model. Therefore, further investigation of MRP4 as a potential therapeutic target is indicated.