Abstract C37: Differential expression of carbonic anhydrases and proton transporters in co-cultured models of the breast cancer microenvironment
Authors: Chen Zhijuan , Frost Susan C. 
Published: October 2016

Extracellular acidification (pHe) within the tumor microenvironment is a well-accepted hallmark for aggressive tumor behavior. Hypoxia is thought to play a key role in the acidic tumor microenvironment. Hypoxia triggers a shift to glycolytic metabolism and leads to an excess of acidic products, including lactic acid, protons and carbon dioxide. To counteract the acidosis, cells activate the pH regulatory machinery which has two major transport arms: the lactate and proton export arm and bicarbonate import arm. These two arms operate through several types of transmembrane proton transporters (TPTs). These transporters include the monocarboxylate transporters (MCTs), the vacuolar ATPase (v-ATPase), the sodium/proton exchangers (NHE-1), and the sodium/bicarbonate transporters (NBC). In addition, it is hypothesized that membrane-bound isoforms of the carbonic anhydrase family (CAIX and CAXII) may balance pHe to the advantage of cancer cells by improved buffering utilizing the CO2/bicarbonate system.

Within the tumor microenvironment, there are both malignant cells and adjacent stromal cells, including fibroblasts, endothelial cells, adipocytes, pericytes, and inflammatory macrophages. Communication and interactions between tumor cells and stromal cells may contribute to the pH regulation of the tumor microenvironment. A recent study by Fiaschi et al. (2013) showed that cultured, healthy human prostate fibroblasts could be converted to the phenotype of cancer-associated fibroblasts (CAFs) by incubation with conditioned medium from cultured prostate cancer cells. This conversion induced the upregulation of CAIX. This might suggest that CAFs play a role in pH regulation in the tumor microenvironment.

To determine if breast cancer fibroblasts respond in like, we co-cultured MDA-MB-231 (triple-negative, CAIX-positive) and T47D (ER-positive, CAXII-positive) breast cancer cells with human, mammary-specific, cancer-associated fibroblasts using the transwell system. In this setting, we were unable to demonstrate that either CAIX or CAXII expression changed in the CAFs. We also explored the expression of the TPTs. Interestingly, triple negative and ER-positive breast cancer cells showed differential expression of these family members. In the co-culture system, vATPase expression increased in CAFs co-cultured with the MDA cells where reduced expression was observed. Knockdown of CAIX in the MDA cells did not alter this difference. Yet loss of CAIX in the MDA cells reduced their expression of NHE-1. We observed no expression difference in CAFs co-cultured with T47D cells. Knockdown of CAXII in T47D cells enhanced NHE-1 expression but reduced MCT4 expression. These preliminary data suggest that there are complicated interactions between CAFs and breast cancer cells not related to CAIX or CAXII expression. Yet, loss of expression of CAIX or CAXII did change the expression of TPTs in the cancer cells, suggesting the existence of compensatory mechanisms.

Citation Format: Zhijuan Chen, Susan C. Frost. Differential expression of carbonic anhydrases and proton transporters in co-cultured models of the breast cancer microenvironment. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr C37.