Abstract A097: Inhibition of the Na+/H+ exchanger (NHE1) increases susceptibility to paclitaxel in invasive breast cancer cells

Abstract

Abstract The pH gradient in normal cells is tightly controlled by the activity of various pH regulatory membrane proteins including the Na+/H+ exchanger, NHE1. NHE1 becomes constitutively active in a neoplastic milieu, dysregulating pH homeostasis and altering the survival, differentiation, and proliferation of cells, thus causing them to become tumorigenic. In breast cancer cells, cytoplasmic alkalinization as a result of NHE1 hyper-activity results in an acidic tumor microenvironment that facilitates aggressive cellular proliferation, migration, and invasion leading to tumor metastasis. The pathophysiological role of NHE1 in tumor progression with regards to ion flux is well known, however, the manipulation of pH in and around tumor cells has only recently been considered as a strategy in augmenting cancer therapy. We studied the effect of NHE1 inhibitors EMD87580 ((2-methyl-4,5-di-(methylsulfonyl)-benzoyl)-guanidine) and HMA (5-(N, N-hexamethylene)-amiloride), either alone or in combination with paclitaxel (Taxol), on NHE1 exchanger activity, cell viability, proliferation, migration, and invasive potential, in a highly invasive breast cancer cell line MDA MB 231. We found that cells treated with EMD or HMA in combination with paclitaxel at low doses (0.1nM to 1nM), were significantly more susceptible to cell death than cells treated with paclitaxel or inhibitors alone. While no discernible differences between treatments were observed in cell proliferation rates, a significant reduction in the rate of migration and invasion was observed in cells treated with paclitaxel and either EMD or HMA. To highlight the importance of NHE1 function, we also generated an MDA-MB-231NHE1- knockout cell line for comparison with the wild-type cells that endogenously express NHE1. The NHE1 knockout cell line showed no demonstrable Na+/H+ exchange activity, and much lower rates of proliferation, migration and invasion compared to wild-type cells. Our results demonstrate, for the first time, that inhibition of NHE1 potentially increases the susceptibility of invasive breast cancer cells to paclitaxel-mediated cell death at doses much lower than the established IC50 of paclitaxel in MDA MB 231 cells. Since pH regulation appears to play an integral role in the switch from a normal to a neoplastic microenvironment, these data lend further credence to the importance of NHE1 as a potential target in breast cancer chemotherapy. Supported by the Canadian Breast Cancer Foundation. Citation Format: Schammim Ray Amith, Jodi Marie Wilkinson, Larry Fliegel. Inhibition of the Na+/H+ exchanger (NHE1) increases susceptibility to paclitaxel in invasive breast cancer cells. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A097.