Inhibition of the NA(+)/H(+) exchanger reduces rat hepatic stellate cell activity and liver fibrosis: an in vitro and in vivo study

The changing face of the Na+/H+ exchanger, NHE1: structure, regulation, and cellular actions

The NHE family of ion exchangers includes six isoforms (NHE1-NHE6) that function in an electroneutral exchange of intracellular H(+) for extracellular Na(+). This review focuses on the only ubiquitously expressed isoform, NHE1, which is localized at the plasma membrane where it plays a critical role in intracellular pH (pHi) and cell volume homeostasis. All NHE isoforms share a similar topology: an N-terminus of 12 transmembrane (TM) alpha-helices that collectively function in ion exchange, and a C-terminal cytoplasmic regulatory domain that modulates transport activity by the TM domain. Extracellular signals, mediated by diverse classes of cell-surface receptors, regulate NHE1 activity through distinct signaling networks that converge to directly modify the C-terminal regulatory domain. Modifications in the C-terminus, including phosphorylation and the binding of regulatory proteins, control transport activity by altering the affinity of the TM domain for intracellular H(+). Recently, it was determined that NHE1 also functions as a membrane anchor for the actin-based cytoskeleton, independently of its role in ion translocation. Through its effects on pHi homeostasis, cell volume, and the actin cortical network, NHE1 regulates a number of cell behaviors, including adhesion, shape determination, migration, and proliferation.

Intracellular pH regulation and Na+/H+ exchange activity in human hepatic stellate cells: effect of platelet-derived growth factor, insulin-like growth factor 1 and insulin

BACKGROUND/AIMS

The Na+/H+ exchanger is involved in rat hepatic stellate cell (HSC) proliferation induced by platelet-derived growth factor (PDGF). We therefore evaluated in human HSC: (1) the mechanisms of intracellular pH regulation; (2) the relationship between Na+/H+ exchange activation and cell proliferation induced by PDGF, insulin-like growth factor 1 (IGF-1) and insulin.

METHODS/RESULTS

pH(i) regulation was mainly dependent on the activity of the Na+/H+ exchanger, which was evaluated by measuring pH(i) recovery from an acute acid load. PDGF (25 ng/ml) gradually increased the activity of the Na+/H+ exchanger which peaked at 18 h and remained stable until the 24th h. IGF-1 (10 nmol/l), but not insulin (100 nmol/l), slightly but significantly increased the activity of the Na+/H+ exchanger. Amiloride (100 micromol/l) and 20 micromol/l 5-N-ethyl-N-isopropyl-amiloride completely inhibited HSC proliferation (evaluated by measurement of bromodeoxyuridine incorporation) induced by PDGF and IGF-1, but did not affect proliferation of HSC induced by insulin. Finally, IGF-1 did not modify the activity of the Na+/Ca2+ exchanger.

CONCLUSIONS

The Na+/H+ exchanger is involved in HSC proliferation induced by PDGF and IGF-1, whereas the proliferative effect of insulin is mediated by intracellular pathways which are Na+/H+ exchange-independent.