Anne Poder Andersen:
pH-regulatory proteins as potential targets in breast cancer

Date: 29-04-2016    Supervisor: Stine Falsig Pedersen

Tumor cells exhibit increased glycolytic metabolism, and combined with their high proliferative and anabolic rates, this results in increased intracellular acid production compared to normal cells. As maintenance of a slightly alkaline intracellular pH (pHi) is a prerequisite for cellular survival and proliferation, tumor cells must initiate strategies to circumvent intracellular acid loading. The main facilitators of acid extrusion in tumor cells include the pH-regulatory ion transporters Na+/H+ exchanger NHE1, electroneutral Na+-HCO3 - cotransporter NBCn1 and the lactate-H+ cotransporters MCT1 and -4. In conjunction with the poor diffusion in tumors, upregulated acid extrusion leads to acidification of the extracellular environment. Combined with hypoxia and contributions from the associated activated stromal cells this results in a tumor microenvironment that is fundamentally different from that of normal tissues. The focus of the present PhD study is on understanding the mechanisms through which pH-regulatory transporters are regulated by the breast tumor microenvironment, and how these transporters in turn favor cancer progression.

In Paper I, we summarized the recent knowledge on the dynamic interactions of pHregulatory transporters with tumor cell metabolism and the tumor microenvironment, and discussed how this interplay can be exploited clinically. In Paper II we employed the 3D spheroid model to mimic essential aspects of the tumor microenvironment and show that NHE1, NBCn1, MCT1 and - 4 exhibit distinct spatial organization during 3D growth of MCF-7 and MDA-MB-231 breast cancer cells. By pharmacological inhibition and stable shRNA-mediated knockdown, we addressed the specific contributions of the transporters to spheroid growth and show that the specific transporters contribute to breast cancer spheroid growth in a cell-type dependent manner, with MCT1 and NBCn1 playing particular important roles in MCF-7 cells and NHE1 in MDA-MB-231 cells.

In Papers III-IV we employed mouse models to study the functional relevance and the relative roles of NHE1, NBCn1 and MCT4 in breast cancer development in the more complex in vivo setting. In Paper III, we found that chemical induction of breast cancer in NBCn1 knockout (KO) mice is delayed and tumor growth rates reduced, compared to those in wildtype (WT) mice, demonstrating a causal link between NBCn1 and breast cancer development. In Paper IV we show that NHE1, NBCn1 or MCT4 all play important roles for in vivo tumor growth of MDA-MB-231 breast cancer cell xenografts. Knockdown of either of the three transporters reduced tumor xenograft growth rate, while knockdown of NBCn1 or MCT4 additionally prolonged tumor latency. Collectively, this work demonstrates important functional roles for the pH-regulatory ion transporters NHE1, NBCn1, MCT1 and -4 in breast cancer and highlights their potential as novel targets in breast cancer treatment.