NHE-1: a promising target for novel anti-cancer therapeutics.

Among the many factors involved in the maintenance of homeostatic growth is the tight regulation of cellular pH. Intracellular pH of normal cells is maintained within a physiological range thanks to the activity of a number of pH regulators that respond to the acidbase shifts associated with normal cellular metabolic processes. Interestingly, there is a preponderance of evidence that dysregulation of intracellular pH is associated with processes that favor cell transformation such as cell cycle progression, enhanced proliferation, insensitivity to growth inhibitory stimuli, resistance to apoptosis, genomic instability and angiogenesis. Among the strategies employed by the cells to regulate intracellular pH, the Na⁺/H⁺ exchanger 1 (NHE1) protein from the Na⁺/H⁺ exchanger (NHE) family has been directly associated with cellular transformation, invasion and metastasis. These observations have heightened the interest in NHE1 as a promising novel drug target for more effective and selective anti-cancer therapeutics. Here we present a review of the basic biology of this remarkable protein and present evidence to support targeting NHE1 as a potential anti-cancer strategy.

Repression of NHE1 expression by PPARgamma activation is a potential new approach for specific inhibition of the growth of tumor cells in vitro and in vivo.

Ligand-induced activation of peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits proliferation in cancer cells in vitro and in vivo; however, the downstream targets remain undefined. We report the identification of a peroxisome proliferator response element in the promoter region of the Na(+)/H(+) transporter gene NHE1, the overexpression of which has been associated with carcinogenesis. Exposure of breast cancer cells expressing high levels of PPARgamma to its natural and synthetic agonists resulted in downregulation of NHE1 transcription as well as protein expression. Furthermore, the inhibitory effect of activated PPARgamma on tumor colony-forming ability was abrogated on overexpression of NHE1, whereas small interfering RNA-mediated gene silencing of NHE1 significantly increased the sensitivity of cancer cells to growth-inhibitory stimuli. Finally, histopathologic analysis of breast cancer biopsies obtained from patients with type II diabetes treated with the synthetic agonist rosiglitazone showed significant repression of NHE1 in the tumor tissue. These data provide evidence for tumor-selective downregulation of NHE1 by activated PPARgamma in vitro and in pathologic specimens from breast cancer patients and could have potential implications for the judicious use of low doses of PPARgamma ligands in combination chemotherapy regimens for an effective therapeutic response.