Cystic fibrosis transmembrane conductance regulator (CFTR) is the essential chloride and bicarbonate channel in the apical membrane of epithelial cells. CFTR was also proposed earlier to conduct glutathione (GSH) out of airway epithelial cells to be enriched in the apical airway surface liquid to neutralize reactive oxygen species (ROS). Although earlier studies suggested that release of GSH by wild type (wt) CFTR may lead to an increase in cytosolic ROS, we did not detect different ROS levels in cells expressing wt-CFTR and mutant F508del-CFTR, independent of CFTR-activation or exposure to the ROS donor tert-butyl hydroperoxide. The Ca-activated phospholipid scramblase and ion channel TMEM16F (anoctamin 6, ANO6) is also expressed in airway cells. ANO6 produced outwardly rectifying Cl currents (ORCC) and scrambled plasma membrane phospholipids when activated by increase in cytosolic ROS and consecutive peroxidation of plasma membrane lipids. ANO6 activity is enhanced by CFTR, probably through translocation of signaling proteins to the plasma membrane. The present data suggest that enhanced cell death in CFTR-expressing cells is due to upregulation of ANO6-activity. In ANO6 knockout mice, the number of apoptotic cells in the intestinal epithelium was strongly reduced, supporting the role of ANO6 for cell death. Thus, ANO6 and CFTR act cooperatively on ROS-mediated cell death, which is not further augmented by cAMP-dependent stimulation. We propose that ANO6 supports cell death correlated with expression of CFTR, possibly by inducing ferroptosis.

Despite being essential for airway hydration, TMEM16A is not required for mucus (MUC5AC) production. Cell proliferation is the main driver for TMEM16A up-regulation during inflammation.

Cystic fibrosis (CF) cells display a more cancer-like phenotype vs. non-CF cells. KLF4 overexpression has been described in CF and this transcriptional factor acts as a negative regulator of wt-CFTR. KLF4 is described as exerting its effects in a cell-context-dependent fashion, but it is generally considered a major regulator of proliferation, differentiation, and wound healing, all the processes that are also altered in CF. Therefore, it is relevant to characterize the differential role of KLF4 in these processes in CF vs. non-CF cells. To this end, we used wt- and F508del-CFTR CFBE cells and their respective KLF4 knockout (KO) counterparts to evaluate processes like cell proliferation, polarization, and wound healing, as well as to compare the expression of several epithelial differentiation markers. Our data indicate no major impact of KLF4 KO in proliferation and a differential impact of KLF4 KO in transepithelial electrical resistance (TEER) acquisition and wound healing in wt- vs. F508del-CFTR cells. In parallel, we also observed a differential impact on the levels of some differentiation markers and epithelial-mesencymal transition (EMT)-associated transcription factors. In conclusion, KLF4 impacts TEER acquisition, wound healing, and the expression of differentiation markers in a way that is partially dependent on the CFTR-status of the cell.

Airway inflammation, mucus hyperproduction and epithelial remodelling are hallmarks of many chronic airway diseases, including asthma, Chronic Obstructive Pulmonary Disease and Cystic Fibrosis. While several cytokines are dysregulated in these diseases, most studies focus on the response of airways to IL-4 and IL-13, which were shown to induce mucus hyperproduction and shift the airway epithelium towards a hypersecretory phenotype.We hypothesised that other cytokines might induce the expression of chloride (Cl−) channels/transporters, regulate epithelial differentiation and mucus production. To this end, fully-differentiated human airway basal cells (BCi-NS1.1) were treated with cytokines identified as dysregulated in those diseases, namely interleukins-8, 1β, 4, 17A, 10, 22, and tumour necrosis factor-α (TNF-α).Our results show that CFTR is the main Cl− channel modulated by inflammation, in contrast to TMEM16A, whose levels only changed with IL-4. Furthermore, we identified novel roles for IL-10 and IL-22 by influencing epithelial differentiation towards ciliated cells and away from pulmonary ionocytes. Contrarily, IL-1β and IL-4 reduced the number of ciliated cells while increasing club cells. Interestingly, while IL-1β, IL-4 and IL-10 upregulated CFTR expression, IL-4 was the only cytokine that increased both its function and the number of CFTR-expressing club cells, suggesting that this cell-type may be the main contributor for CFTR function. Additionally, all cytokines assessed increased mucus production through a differential upregulation of MUC5AC and MUC5B transcript levels.Altogether, this study reveals a novel insight into differentiation resulting from the cross-talk of inflammatory mediators and airway epithelial cells, which is particularly relevant for chronic airway diseases.

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