Anion secretion by a model epithelium: more lessons from Calu‐3

Abstract

Anion transport drives fluid into the airways and is essential for humidifying inspired air and supplying surface liquid for mucociliary transport. Despite the importance of airway secretion in diseases such as cystic fibrosis, the cellular mechanisms remain poorly understood, in part due to the small size and complicated structure of the submucosal glands that produce most of the fluid. The Calu-3 human lung adenocarcinoma cell line has become a popular model for studying airway secretion because it can be cultured as a flat sheet, expresses the cystic fibrosis transmembrane conductance regulator and several acinar cell markers, forms polarized monolayers with tight junctions, has robust cAMP-stimulated anion transport, and responds to secretagogues that regulate the glands in vivo. However, some properties of Calu-3 cells are less consistent with those of native tissue. In particular, Calu-3 monolayers do not secrete chloride when stimulated by forskolin under short-circuit conditions. Bicarbonate ions are thought to carry the short-circuit current (I(sc)) and the drive secretion of alkaline fluid, in contrast to the neutral pH secretions that are produced by submucosal glands. Calu-3 cells also have abnormal chromosomes and characteristics of both serous and mucus cells. In this article, we discuss Calu-3 as a model in light of our ongoing studies, which suggest that Calu-3 monolayers resemble submucosal glands more closely than was previously thought. For example, we find that net HCO(3)(-) flux fully accounts for I(sc) as previously suggested but Cl(-) is the main anion transported under physiological conditions. A novel, HCO(3)(-) -dependent mechanism of Cl(-) transport is emerging which may explain secretion by Calu-3 and perhaps other epithelial cells.

Physiology of Epithelial Chloride and Fluid Secretion

“…Carbonic anhydrase was proposed to catalyze cellular HCO 3 synthesis, and because CO 2 hydration would generate carbonic acid and acidload the cell, H extrusion, for example, by amiloride-sensitive Na/H exchange, was proposed to maintain intracellular pH and thus transepithelial Cl secretion. Basolateral Cl uptake by anion exchangers has been suggested in various epithelia including equine trachea (Tessier et al 1990) and the Calu-3 cell line (Cuthbert et al 2003;Krouse et al 2004;Shan et al 2011). …”

Role of anion exchangers in Cl− and HCO3− secretion by the human airway epithelial cell line Calu-3

“…The Calu-3 cell line was derived from a human lung carcinoma and has proved to be a useful experimental model for studying CFTR-dependent HCO 3 Ϫ secretion (54). Its phenotype resembles that of serous cells of the submucosal glands, it expresses abundant CFTR (15,57), and it has been shown to secrete a moderately HCO 3 Ϫ -rich fluid in vitro (11,21,55).…”

The cytokines interleukin-1β and tumor necrosis factor-α stimulate CFTR-mediated fluid secretion by swine airway submucosal glands

“…The contradictions may result from differences in the transport machinery expressed in Calu-3 compared with the native tissue. Calu-3 cells lack chromosomes 1, 13, 15, and 17, and is triploid for some other unidentified chromosomes; thus some genes may be missing or have altered expression (25). In any case, it is clear that cAMP/PKA is the major signaling pathway mediating cytokine-stimulated secretion in swine tracheal glands.…”