Investigating cyclooxygenase-2 signaling in breast cancer-initiating cells.

Abstract

e21107 Investigating cyclooxygenase-2 signaling in breast cancer-initiating cells Background: Only a small fraction of breast tumor cells, breast cancer-initiating cells (CSC), have the ability to initiate tumor growth and metastasis. Some characteristics of breast CSCs have been described in vitro by employing non-adherent CSC-enriching culture conditions, however, little is known regarding prostanoid signaling or prostaglandin E2 (PgE2) production in CSCs. In this study we measured cyclooxygenase-2 (COX-2) and prostaglandin E2 receptor expression in CSCs and assessed the effects of COX-2 and Ep4 inhibition on in vitro CSC spheroid formation and PgE2 production. Methods: CSCs were cultured in serum-free medium using ultra low attachment plates. Cells were treated with vehicle, a COX-2 inhibitor (Celecoxib), or an EP4 inhibitor (GW627368X) at concentrations of 0.1, 1, 5, 10, 50, or 100uM for 10 days. Spheroids were quantified using a Gelcount machine. Non-linear regression was used to calculate IC50 values. Western blots were performed using anti-COX-2, anti- EP1, 2, 3 and 4 receptor antibodies. Prostaglandin E2 production was measured using a PgE2 immunoassay kit. Results: COX-2 protein expression was observed for MDA-MB-231 and SUM149 CSCs. EP2 and EP4 protein was detected for MCF-7, MDA-MB-231, and SUM149 CSCs. The Celecoxib IC50 values for MCF-7 and SUM149 CSC spheroid formation were 0.5 and 2.2 uM, respectively; GW627386X IC50 values for MCF-7 and SUM149 CSC spheroid formation were 1.2 and 0.3uM, respectively. No significant differences in PGE2 production were observed for MCF-7 CSCs compared to adherent MCF-7 cells (11.5 ± 4.4 vs. 8.7 ± 2.1 pg/mL/106 cells; p=0.88). However, PgE2 production in MDA-MB-231 CSCs was significantly higher than adherent MDA-MB-231 cells (1507.0 ± 329.0 vs. 13.8 ± 5.7 pg/mL/106 cells; p≤0.001) and SUM149 CSC PgE2 production was significantly higher than SUM149 adherent cells (4932.9 ± 501.7 vs. 194.5 ± 31.0 pg/mL/106 cells; p≤0.001). 10uM Celecoxib treatment inhibited PGE2 production in MDA-MB-231 and MDA-MB-231 CSCs. Conclusions: MDA-MB-231 and SUM149 CSCs express COX-2, EP2 and Ep4 and produce high levels of PGE2. CSC spheroid formation is significantly decreased with COX-2 and EP4 inhibition.