Nongastric H,K-ATPase: structure and functional properties

Pump Proton and Laryngeal H+/K+ ATPases

Purpose

The presence of extra-gastric H⁺/K⁺ ATPases may explain the clinically significant effect of proton pump inhibitor (PPI) pharmacotherapy in patients with chronic laryngitis related to laryngopharyngeal reflux disease (LPRD) but without gastroesophageal reflux disease (GERD) symptoms. Given the need for a better understanding of GERD and LPRD, we review the various proton pumps with respect to their classification, function, and distribution. We then consider the potential role of the laryngeal H⁺/K⁺ ATPase pump in LPRD.

Methods

We searched databases of PubMed, EMBASE, and Web of Science to achieve related published before September 15, 2020.

Results

There were only seven English-literatures meeting inclusive criteria about laryngeal H⁺/K⁺ ATPases. Some studies provide convincing evidence of a laryngeal H⁺/K⁺ ATPase in normal laryngeal tissues but also suggest the potential role of the proton pump in the abnormal mucus secretion frequently seen in patients with chronic laryngitis.

Conclusion

A laryngeal H⁺/K⁺ ATPase expresses in normal laryngeal tissues. These findings question the current understanding of GERD and LPRD.

Proton pump inhibitors decrease eotaxin-3/CCL26 expression in patients with chronic rhinosinusitis with nasal polyps: Possible role of the nongastric H,K-ATPase

BACKGROUND

Chronic rhinosinusitis with nasal polyps (CRSwNP) is often characterized by tissue eosinophilia that is associated with poor prognosis. Recent findings that proton pump inhibitors (PPIs) directly modulate the expression of eotaxin-3, an eosinophil chemoattractant, in patients with eosinophilic diseases suggest therapeutic potential for PPIs in those with CRSwNP.

OBJECTIVE

We assessed the effect of type 2 mediators, particularly IL-13 and eotaxin-3, on tissue eosinophilia and disease severity in patients with chronic rhinosinusitis (CRS). Further investigation focused on PPI suppression of eotaxin-3 expression in vivo and in vitro, with exploration of underlying mechanisms.

METHODS

Type 2 mediator levels in nasal tissues and secretions were measured by using a multiplex immunoassay. Eotaxin-3 and other chemokines expressed in IL-13-stimulated human sinonasal epithelial cells (HNECs) and BEAS-2B cells with or without PPIs were assessed by using ELISA, Western blotting, real-time PCR, and intracellular pH imaging.

RESULTS

Nasal tissues and secretions from patients with CRSwNP had increased IL-13, eotaxin-2, and eotaxin-3 levels, and these were positively correlated with tissue eosinophil cationic protein levels and radiographic scores in patients with CRS (P < .05). IL-13 stimulation of HNECs and BEAS-2B cells dominantly induced eotaxin-3 expression, which was significantly inhibited by PPIs (P < .05). Patients with CRS taking PPIs also showed lower in vivo eotaxin-3 levels compared with those without PPIs (P < .05). Using intracellular pH imaging and altering extracellular K⁺, we found that IL-13 enhanced H⁺,K⁺-exchange, which was blocked by PPIs and the mechanistically unrelated H,K-ATPase inhibitor, SCH-28080. Furthermore, knockdown of ATP12A (gene for the nongastric H,K-ATPase) significantly attenuated IL-13-induced eotaxin-3 expression in HNECs. PPIs also had effects on accelerating IL-13-induced eotaxin-3 mRNA decay.

CONCLUSION

Our results demonstrated that PPIs reduce IL-13-induced eotaxin-3 expression by airway epithelial cells. Furthermore, mechanistic studies suggest that the nongastric H,K-ATPase is necessary for IL-13-mediated epithelial responses, and its inhibitors, including PPIs, might be of therapeutic value in patients with CRSwNP by reducing epithelial production of eotaxin-3.

Ouabain attenuates cardiotoxicity induced by other cardiac steroids

BACKGROUND AND PURPOSE

All cardiac steroids have a similar structure, bind to and inhibit the ubiquitous transmembrane protein Na(+), K(+)-ATPase and increase the force of contraction of heart muscle. However, there are diverse biological responses to different cardiac steroids both at the cellular and at the molecular level. Moreover, we have recently shown that ouabain inhibits digoxin- and bufalin-induced changes in membrane traffic. The present study was designed to test the hypothesis that ouabain also has an inhibitory effect on cardiotoxicity induced by other cardiac steroids.

EXPERIMENTAL APPROACH

The hypothesis was tested in isolated heart muscle preparations and in an in vivo model of cardiotoxicity in guinea pigs.

KEY RESULTS

Ouabain at a low dose attenuated the toxicity induced by bufalin and digoxin in heart muscle preparations. In addition, ouabain at the low dose (91 ng.kg(-1).h(-1)), but not at a higher dose (182 ng.kg(-1).h(-1)), delayed the development of digoxin-induced (500 microg.kg(-1).h(-1)) cardiotoxicity in anaesthetized guinea pigs, as manifested by delayed arrhythmia and terminal ventricular fibrillation, as well as a reduced heart rate. In addition, as observed with ouabain, the phosphoinositide 3-kinase inhibitor wortmannin (100 microg.kg(-1).h(-1)) delayed the digoxin-induced arrhythmia in anaesthetized guinea pigs.

CONCLUSIONS AND IMPLICATIONS

The present study demonstrates the inhibitory effect, probably through signal transduction pathways, of ouabain on digoxin- and bufalin-induced cardiotoxicity in guinea pigs. Further understanding of this phenomenon could be beneficial for increasing the therapeutic window for cardiac steroids in the treatment of chronic heart failure.

Role of endosomal Na+-K+-ATPase and cardiac steroids in the regulation of endocytosis

Plasma membrane Na(+)-K(+)-ATPase, which drives potassium into and sodium out of the cell, has important roles in numerous physiological processes. Cardiac steroids (CS), such as ouabain and bufalin, specifically interact with the pump and affect ionic homeostasis, signal transduction, and endocytosed membrane traffic. CS-like compounds are present in mammalian tissues, synthesized in the adrenal gland, and considered to be new family of steroid hormones. In this study, the mechanism of Na(+)-K(+)-ATPase involvement in the regulation of endocytosis is explored. We show that the effects of various CS on changes in endosomal pH are mediated by the pump and correspond to their effects on endosomal membrane traffic. In addition, it was found that CS-induced changes in endocytosed membrane traffic were dependent on alterations in [Na(+)] and [H(+)] in the endosome. Furthermore, we show that various CS differentially regulate endosomal pH and membrane traffic. The results suggest that these differences are due to specific binding characteristics. Based on our observations, we propose that Na(+)-K(+)-ATPase is a key player in the regulation of endosomal pH and endocytosed membrane traffic. Furthermore, our results raise the possibility that CS-like hormones regulate differentially intracellular membrane traffic.

Identification of the β-subunit for nongastric H-K-ATPase in rat anterior prostate

The structural organization of nongastric H-K-ATPase, unlike that of closely related Na-K-ATPase and gastric H-K-ATPase, is not well characterized. Recently, we demonstrated that nongastric H-K-ATPase α-subunit (αng) is expressed in apical membranes of rodent prostate. Its highest level, as well as relative abundance, with respect to α1-isoform of Na-K-ATPase, was observed in anterior lobe. Here, we aimed to determine the subunit composition of nongastric H-K-ATPase through the detailed analysis of the expression of all known X-K-ATPase β-subunits in rat anterior prostate (AP). RT-PCR detects transcripts of β-subunits of Na-K-ATPase only. Measurement of absolute protein content of these three β-subunit isoforms, with the use of quantitative Western blotting of AP membrane proteins, indicates that the abundance order is β1> β3≫ β2. Immunohistochemical experiments demonstrate that β1is present predominantly in apical membranes, coinciding with αng, whereas β3is localized in the basolateral compartment, coinciding with α1. This is the first direct demonstration of the αng-β1colocalization in situ indicating that, in rat AP, αngassociates only with β1. The existence of αng-β1complex has been confirmed by immunoprecipitation experiments. These results indicate that β1-isoform functions as the authentic subunit of Na-K-ATPase and nongastric H-K-ATPase. Putatively, the intracellular polarization of X-K-ATPase isoforms depends on interaction with other proteins.