Anti-inflammatory effects of Na⁺/H⁺ exchanger inhibitors

Abstract

In macrophages, Na⁺/H⁺ exchangers (NHEs) are activated by various stimuli and regulate the functions of macrophages. The NHE inhibitors amiloride, 5-(N,N-dimethyl)-amiloride (DMA) and 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) inhibited the lipopolysaccharide (LPS)-induced production of prostaglandin (PG) E₂ in the mouse macrophage-like cell line RAW 264. They inhibited both the LPS-induced release of arachidonic acid from membrane phospholipids at 4 h and the LPS-induced increase in the level of cyclooxygenase (COX)-2 protein at 6 h, but did not directly inhibit the COX activity. The vacuolar-type (H⁺)-ATPase (V-ATPase) inhibitor, bafilomycin A1, which activates NHE via reducing intracellular pH, also increased the level of COX-2 protein. The bafilomycin A1-induced expression of COX-2 was inhibited by the NHE inhibitors and by the Na⁺/Ca²⁺ exchanger (NCX) inhibitor SN-6, indicating that the activation of NHE leads to COX-2 expression via, in part, functional coupling with NCX. In an air pouch-type LPS-induced inflammation model in mice, amiloride and EIPA, as well as the COX inhibitor indomethacin, significantly reduced the level of PGE₂ in the pouch fluid collected at 8 h and the vascular permeability during 4 to 8 h. The accumulation of leukocytes in the pouch fluid collected at 8 h was significantly inhibited by amiloride and EIPA but not by indomethacin. Thus, NHE inhibitors showed more effective anti-inflammatory activity than indomethacin. Therefore, NHEs might be a novel target for developing anti-inflammatory drugs that inhibit the expression of COX-2 in the activated inflammatory cells.