Regulation of the Na+/H+ exchanger in fibroblasts overexpressing the Na+/Ca2+ exchanger

Down-regulation of Na+/H+ exchanger 1 by Toll-like receptor stimulation in macrophages

The role of Na⁺/H⁺ exchanger 1 (NHE1) in various cell types, including inflammatory cells, has been extensively studied. However, regulation of NHE1 protein level in activated inflammatory cells is yet to be characterized. In this study, we investigated whether Toll-like receptor (TLR) ligands can regulate NHE1 protein level in the mouse macrophage-like RAW 264 cell line. We found that lipopolysaccharide (LPS), a TLR4 ligand, lowered NHE1 level and activity in RAW 264 cells and in primary murine macrophages. Other TLR ligands, such as zymosan A and poly(I:C), also displayed reduced NHE1 level. LPS promoted NHE1 ubiquitination and reduced the expression of calcineurin homologous protein 1 (CHP1), a regulator of NHE1 activity and stability. These responses were inhibited by c-Jun N-terminal kinase (JNK) inhibitor SP600125 and dexamethasone. A proteasome inhibitor, but not caspase-3 or lysosomal inhibitors, blocked the LPS-induced NHE1 down-regulation. These results suggested that LPS promotes the degranulation of NHE1 mediated by the ubiquitin-proteasome system and CHP1 downregulation resulting from activation of JNK.

Response of alkalinization or acidification by phytohemagglutinin is dependent on the activity of protein kinase C in human peripheral T Cells

The increase of intracellular free calcium concentration ([Ca(2+)](i)) and protein kinase C (PKC) activity are two major early mitogenic signals to initiate proliferation of human T cells. However, a rapid change in intracellular pH (pH(i)), acidification or alkalinization during the activation, is also associated after these two signals. The aim of this study was to define whether the change in pH(i) is affected by calcium and protein kinase C (PKC), in phytohemagglutinin (PHA)-stimulated T cells. T cells were isolated from human peripheral blood. The [Ca(2+)](i) and the pH(i) were measured using, respectively, the fluorescent dyes, Fura-2, and BCECF. In addition, down-regulation of PKC activity by PMA (1 microM, 18 h) was confirmed in these cells using a protein kinase assay. The results indicated that, (1) alkalinization was induced by PHA or PMA in T cells; the results of alkalinization was PKC-dependent and Ca(2+)-independent, (2) in PKC down-regulated T cells, PHA induced acidification; this effect was enhanced by pre-treating the cells with the Na(+)/H(+) exchange inhibitor, 5-(N,N-dimethyl)-amiloride, (DMA, 10 microM, 20 min), (3) the acidification was dependent on the Ca(2+) influx and blocked by removal of extracellular calcium or the addition of the inorganic channel blocker, Ni(2+), and (4) Thapsigargin (TG), a Ca(2+)-ATPase inhibitor, confirmed that acidification by the Ca(2+) influx occurred in T cells in which PKC was not down-regulated. These findings indicate two mechanisms, alkalinization by PKC and acidification by Ca(2+) influx, exist in regulating pH(i) in T cells. This is the first report that PHA stimulates the acidification by Ca(2+) influx but not alkalinization in T cells after down-regulation of PKC. In conclusion, the activity of PKC in T cells determines the response in alkalinization or acidification by PHA.

Cardiac Na + -Ca 2+ Exchange

Abstract —The Na + -Ca 2+ exchanger (NCX) is one of the essential regulators of Ca 2+ homeostasis in cardiomyocytes and thus an important modulator of the cardiac contractile function. The purpose of this review is to survey recent advances in cardiac NCX research, with particular emphasis on molecular and pharmacological aspects. The NCX function is thought to be regulated by a variety of cellular factors. However, data obtained by use of different experimental systems often appear to be in conflict. Where possible, we endeavor to provide a rational interpretation of such data. We also provide a summary of current work relating to the structure and function of the cardiac NCX. Recent molecular studies of the NCX protein are beginning to shed light on structural features of the ion translocation pathway in the NCX membrane domain, which seems likely to be formed, at least partly, by the phylogenetically conserved α-1 and α-2 repeat structures and their neighboring membrane-spanning segments. Finally, we discuss new classes of NCX inhibitors with improved selectivity. One of these, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), appears to exhibit unique selectivity for Ca 2+ -influx–mode NCX activity. Data obtained with these inhibitors should provide a basis for designing more selective and clinically useful drugs targeting NCX.