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Role of the NADPH oxidase DUOX1 and the proton channel HVCN1 in airway defense

Despite the continuous microbial challenge to the airways, the healthy, uninflamed
airway epithelium is able to efficiently inactivate microbes and remain uninfected owing to constitutively active defense mechanisms expressed by the epithelium. A number of antibacterial factors secreted into the airway surface liquid (ASL) have been identified previously including lysozyme, lactoferrin, lactoperoxidase, ß-defensins and several others. The expression of molecularly and functionally unrelated and independent mechanisms results in a broad spectrum and redundancy of the airway defense system.

In contrast, CF airways are prone to bacterial infections. The reason for the dysfunction of airway defense factors in CF is not well understood. Changes of airway pH and redox state in airway disease have been noted but the relation to the innate defense function of the airway epithelium is unclear. Recently, a novel defense mechanism in the airway epithelium has been identified by us and others, which is based on the NADPH oxidase isoform DUOX1 expressed in the apical membrane of ciliated cells. Currently we investigate a model of airway epithelial defense where the pH of the ASL and HCO3 secretion across CFTR determine the activity of DUOX1.

The NADPH oxidase isoform of the airways, DUOX1. Several isoforms of the phagocytic NADPH oxidase are expressed in other tissues and we have identified DUOX1 as the isoform expressed in the airway epithelium. DUOX1 is highly expressed in normal, uninflamed tissues and it is present in the apical membane of ciliated cells. As other NADPH oxidases, DUOX1 generates intracellular acid according to NADPH⋅H+→NADP+ + 2 H+ + 2 e- and the electrons (e-) are transferred along an e- transport pathway across

the membrane to form extracellular superoxide. Concomitant H+ release is necessary to prevent excessive intracellular acidification and to form H2O2 as the final release product of DUOX1 into the ASL. There is evidence that release of H2O2 into the ASL supports bacterial defense by feeding into the lactoperoxidase defense system. Thus, DUOX1 is a component of the basic airway defense shield.

The H+ channel HVCN1 of the airways supports DUOX1. H+ channels are present in the apical membrane of airway epithelial cells. A critical characteristic of H+ channels is their activation by a transmembrane H+ gradient, i.e., when the intracellular pH is more acidic than pH ASL. As a result, H+ channels are either activated by intracellular acidification (where they act as release valves for H+) or by extracellular alkalinization (where H+ channel activity contributes to the re-acidification of the ASL). The transmembrane H+ gradient is the major determinant of H+ channel activity. For comparison, this characteristic has been well studied in phagocytes where during the respiratory burst large amounts of intracellular H+ are generated by the NADPH oxidase and H+ channels are activated as a consequence. The function of the H+ channel and the NADPH oxidase are tightly linked in phagocytes and inhibition of H+ channels results in poor NADPH oxidase function.

CFTR’s HCO3 conductance maintains DUOX1 function. There is overwhelming evidence that CFTR conducts HCO3 (originally described by us, Poulsen et al, 1994), secretion of HCO3 by airway epithelia has been shown, and the major pH buffer of the ASL is HCO3. HCO3 conduction across CFTR into the ASL is the major and probably sole mechanism of the epithelium to alkalinize the ASL. Consistent with these observations, HCO3 currents, ASL pH and ASL [HCO3] are reduced in CF airway epithelia. When mucosal airway pH was measured in CF patients, an acidic mucosal pH was measured in some but not all studies, likely owing to additional non-epithelial factors that determine ASL pH in vivo (such as inflammatory cells). HCO3 flux across CFTR is driven by the pH gradient across the apical cell membrane, which is reciprocal to the driving forces for H+ flux across HVCN1. The complementary characteristics of CFTR- and HVCN1-mediated fluxes are expected to recover ASL pH from acid or alkaline loads, respectively. Driving forces for HCO3 secretion across CFTR into the ASL increase with decreasing ASL pH and, thus, CFTR activity limits acidification. On the other hand, an alkaline ASL pH both activates HVCN1 and provides a driving force for H+ secretion thus limiting ASL alkalinization. Therefore, the complementary activities of CFTR and HVCN1 on ASL pH provide a mechanism to maintain and feed-back-regulate ASL pH. Accordingly, in CF the recovery from ASL acidification is limited by the lack of CFTR function resulting in an unbalanced ASL pH that is too acidic. Acidic ASL pH in turn inhibits cellular H+ release across H+ channels, which predicts reduced DUOX1 function resulting in reduced airway defenses. This model of NADPH oxidase function in the airways links the lack of CFTR function in CF to reduced airway defense based on the collapsed transmembrane H+ gradient that inhibits H+ channels and the NADPH oxidase.

In summary, recent investigations of airway pH regulation support the notion of CFTR as an important factor in the DUOX1/HVCN1 airway defense system. In CF, it is predicted that DUOX1 works inefficiently owing to the lack of H+ release across HVCN1 that is inhibited by an overly acidic ASL pH. This suggests that proper ASL pH regulation is critical for normal airway defense.

Role of the NADPH oxidase DUOX1 during lung alveolar maturation

Oxidant production by the lung alveolar epithelium has been little investigated despite its potential roles in alveolar host defense and cell signaling. Our preliminary studies in a cultured human lung cell model show that hormone-induced type II cell differentiation is associated with increased expression of the NADPH oxidase isoform dual oxidase 1 (DUOX1) and its maturation factor DUOXA1. Similarly, DUOX protein expression was found to be upregulated in vivo during late gestation in human lung. We also found a strong correlation between expression of DUOX1 and the production of hydrogen peroxide and acid by alveolar cells. To date, DUOX1 is the only known mechanism to control the regulated release of oxidants by the alveolar epithelium. In collaboration with Phil Ballard at UCSF we currently investigate how DUOX1 and DUOXA1 are regulated during differentiation of alveolar type II cells,  and whether they contribute to host defense and cellular signaling in the newborn lung. This project has physiological significance related to the sterility of the alveolar air space and clinical relevance for understanding deficiencies in host defense and epithelial function in premature infants and patients with chronic lung diseases. 


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