Apoptosis results in cell shrinkage and intracellular acidification, processes opposed by the ubiquitously expressed NHE1 Na ؉ /H ؉ exchanger. In addition to mediating Na ؉ /H ؉ transport, NHE1 interacts with ezrin/radixin/moesin (ERM), which tethers NHE1 to cortical actin cytoskeleton to regulate cell shape, adhesion, motility, and resistance to apoptosis. We hypothesize that apoptotic stress activates NHE1-dependent Na ؉ /H ؉ exchange, and NHE1-ERM interaction is required for cell survival signaling. Apoptotic stimuli induced NHE1-regulated Na ؉ /H ؉ transport, as demonstrated by ethyl-Nisopropyl-amiloride-inhibitable, intracellular alkalinization. Ectopic NHE1, but not NHE3, expression rescued NHE1-null cells from apoptosis induced by staurosporine or N-ethylmaleimide-stimulated KCl efflux. When cells were subjected to apoptotic stress, NHE1 and phosphorylated ERM physically associated within the cytoskeleton-enriched fraction, resulting in activation of the pro-survival kinase, Akt. NHE1-associated Akt activity and cell survival were inhibited in cells expressing ERM binding-deficient NHE1, dominant negative ezrin constructs, or ezrin mutants with defective binding to phosphoinositide 3-kinase, an upstream regulator of Akt. We conclude that NHE1 promotes cell survival by dual mechanisms: by defending cell volume and pH i through Na ؉ /H ؉ exchange and by functioning as a scaffold for recruitment of a signalplex that includes ERM, phosphoinositide 3-kinase, and Akt.

Renal tubular epithelial cell (RTC) apoptosis causes tubular atrophy, a hallmark of renal disease progression. Apoptosis is generally characterized by reduced cell volume and cytosolic pH, but epithelial cells are relatively resistant to shrinkage due to regulatory volume increase, which is mediated by Na+/H+ exchanger (NHE) 1. We investigated whether RTC apoptosis requires caspase cleavage of NHE1. Staurosporine- and hypertonic NaCl-induced RTC apoptosis was associated with cell shrinkage and diminished cytosolic pH, and apoptosis was potentiated by amiloride analogs, suggesting NHE1 activity opposes apoptosis. NHE1-deficient fibroblasts demonstrated increased susceptibility to apoptosis, which was reversed by NHE1 reconstitution. NHE1 expression was markedly decreased in apoptotic RTC due to degradation, and preincubation with peptide caspase antagonists restored NHE1 expression, indicating that NHE1 is degraded by caspases. Recombinant caspase-3 cleaved the in vitro-translated NHE1 cytoplasmic domain into five distinct peptides, identical in molecular weight to NHE1 degradation products derived from staurosporine-stimulated RTC lysates. In vivo, NHE1 loss-of-function C57BL/6.SJL- swe/swemice with adriamycin-induced nephropathy demonstrated increased RTC apoptosis compared with adriamycin-treated wild-type controls, thereby implicating NHE1 inactivation as a potential mechanism of tubular atrophy. We conclude that NHE1 activity is critical for RTC survival after injury and that caspase cleavage of RTC NHE1 may promote apoptosis and tubular atrophy by preventing compensatory intracellular volume and pH regulation.

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