Regulation of the Na+/H+ Exchanger (NHE1) in Breast Cancer Metastasis

Abstract

The pH gradient in normal cells is tightly controlled by the activity of various pH-regulatory membrane proteins including the isoform protein of the Na þ /H þ exchanger (NHE1). NHE1 is constitutively active in a neoplastic microenvironment, dysregulating pH homeostasis and altering the survival, differentiation, and proliferation of cancer cells, thereby causing them to become tumorigenic. Cytoplasmic alkalinization in breast cancer cells occurs as a result of increased NHE1 activity and, while much is known about the pathophysiologic role of NHE1 in tumor progression with regard to ion flux, the regulation of its activity on a molecular level is only recently becoming evident. The membrane domain of NHE1 is sufficient for ion exchange. However, its activity is regulated through the phosphorylation of key amino acids in the cytosolic domain as well as by its interaction with other intracellular proteins and lipids. Here, we review the importance of these regulatory sites and what role they may play in the disrupted functionality of NHE1 in breast cancer metastasis. Cancer Res; 73(4); 1259-64. Ó2013 AACR.

Defining the Na+/H+ exchanger NHE1 interactome in triple-negative breast cancer cells

“…We validated several putative intracellular protein binding partners of NHE1, both as determined by the above MS analysis and previous studies (reviewed in [3]), by co- was associated with NHE1 in greater amounts than the monomeric form. Also, α-enolase is mainly associated with NHE1 in stimulated cells.…”

“…PI3K, ERK1/2, p38MAPK), or by binding to a variety of cytosolic proteins and lipids (e.g. ezrin-radixin-moesin, 14-3-3, calmodulin, phosphatidylinositol 4,5-biphosphate (PIP2), carbonic anhydrase; reviewed in [3,4]). This activation is due to an increased affinity of NHE1's allosteric proton-binding site for intracellular H + ions [4].…”

Na + /H + exchanger-mediated hydrogen ion extrusion as a carcinogenic signal in triple-negative breast cancer etiopathogenesis and prospects for its inhibition in therapeutics

“…NHE1 can also be activated by the binding of intracellular signal proteins and lipids to the Cterminal regulatory domain (43,48,51). While the transmembrane domain of NHEs are highly conserved across the different isoforms, the cytosolic domain exhibits considerable variability.…”

“…Several different serine/threonine kinases can activate NHE1 by phosphorylation, in addition to the exchanger's activation by its intracellular binding partners (extensively reviewed in (48,51,53). Examples of kinase-mediated activators of NHE1 include: extracellular signal regulated kinases ERK1/2 which are important in the activation of NHE1 in response to intracellular acidosis, and for which NHE1 serves as a signal scaffold (54,55); AKT (protein kinase B) which activates NHE1 in response to insulin and platelet-derived growth factor in fibroblasts (56), but inhibits NHE1 activity in cardiomyocytes (57); -Raf which phosphorylates NHE1 at amino acid threonine 653 (58); and p90 ribosomal S6 kinase (p90 RSK ) which phosphorylates NHE1 at serine 703, activating the exchanger in response to serum (59).…”

KR-33028, a potent inhibitor of the Na+/H+ exchanger NHE1, suppresses metastatic potential of triple-negative breast cancer cells

“…NHE1 is an integral membrane protein 815 amino acids in length that is ubiquitously expressed in mammalian cells [3]. It comprises an N-terminal transmembrane domain through which ion exchange occurs, and a cytosolic C-terminal domain that regulates exchanger activity via phosphorylation by kinases and association with lipid and protein binding partners [4,5]. NHE1 plays a critical role in cancer cells, becoming hyperactive in cells undergoing neoplastic transformation.…”