Abstract 2257: Multiple drug resistance-associated protein 4 (MRP4) may contribute to breast cancer metastasis by exporting the COX-2 product PGE2
Authors: Kochel Tyler J. , Kundu Namita , Ma Xinrong , Reader Jocelyn , Fulton Amy 
Published: August 2015

Abstract
Cyclooxygenase-2 (COX-2) and its enzymatic product, prostaglandin E2 (PGE2), are elevated in breast cancer and are associated with a poor prognosis and increased metastatic potential. PGE2 initiates various signaling pathways upon binding to each of four cognate EP receptors. We have previously shown that PGE2 signaling through the EP4 receptor increases metastatic potential and supports the survival of breast cancer stem-like cells. Multiple drug resistance-associated protein 4 (MRP4) is responsible for the active export of PGE2 from cells, while the prostaglandin transporter (PGT) imports PGE2 for 15-hydroxyprostaglandin dehydrogenase (15-PGDH)-mediated degradation. The role of neither MRP4 nor PGT has been investigated in breast cancer progression. The purpose of this study is to elucidate the role of MRP4 in PGE2 signaling in breast cancer progression. We hypothesize that elevated expression of MRP4 would cause increased PGE2 export and receptor-mediated signaling and, therefore, enhance metastatic potential, tumor progression, and support breast cancer stem-like cells. We examined MRP4 gene expression data from multiple breast cancer datasets using Oncomine. We found higher expression of ABCC4 in breast cancer versus normal breast. MRP4 was elevated in invasive (IDC) versus localized (DCIS) lesions and elevated in the majority of basal-type breast cancer. To investigate the role of MRP4 further, we examined MRP4 mRNA and protein expression in cell lines representing several molecular subtypes and metastatic capacities. Normal mammary epithelium (MCF10A), luminal (MCF7, T47D), basal (MDA-MB-231, MDA-MB-468, MDA-MB-436, BT549), and Her2-enriched (SKBR3) cell lines were evaluated. As in the primary sample data, MRP4 mRNA and protein expression are elevated in basal and Her2 enriched cell lines (231, 436, BT549, SKBR3) while expression of PGT mRNA and protein is decreased in these cells when compared to cells with lower metastatic potential. This inverse relationship between MRP4 and PGT should lead to higher concentrations of extracellular PGE2 in the tumor microenvironment. We evaluated MRP4 activity by measuring PGE2 export from cells via enzyme immunoassay or resistance to the cytotoxic compound 6-mercaptopurine (6-MP), two substrates of MRP4. Pharmacologic inhibition of MRP4 with MK571 (MRP antagonist) results in decreased efflux of PGE2 and increased sensitivity to 6-MP as expected. Likewise, genetic suppression by RNAi results in lower levels of PGE2 exported from cells and increased sensitivity to 6-MP, confirming altered MRP4 activity. Conversely, ectopic MRP4 overexpression increases PGE2 export. These data support the hypothesis that MRP4 is a critical member of the PGE2 signaling pathway that leads to high extracellular PGE2 and increased PGE2 signaling, implicating MRP4 as a possible therapeutic target in this oncogenic pathway.
Citation Format: Tyler J. Kochel, Namita Kundu, Xinrong Ma, Jocelyn Reader, Amy Fulton. Multiple drug resistance-associated protein 4 (MRP4) may contribute to breast cancer metastasis by exporting the COX-2 product PGE2. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2257. doi:10.1158/1538-7445.AM2015-2257