Regulation of CFTR channels by HCO₃⁻-sensitive soluble adenylyl cyclase in human airway epithelial cells

Abstract

CFTR channels conduct HCO(3)(-) in addition to Cl(-) in airway epithelial cells. A defective HCO(3)(-)-transporting function of CFTR may underlie the pathogenesis of cystic fibrosis. In the present study, we have investigated whether a HCO(3)(-)-sensitive soluble adenylyl cyclase (sAC) is functionally coupled with CFTR and thus forms an autoregulatory mechanism for HCO(3)(-) transport in human airway epithelial Calu-3 cells. A reverse transcriptase-polymerase chain reaction showed that transcripts of both full-length and truncated sACs are present in Calu-3 cells. Truncated sAC protein is the predominant, if not the only, isoform expressed in Calu-3 cells. HCO(3)(-) stimulated a modest increase in cAMP production, and the increase was sensitive to 2-hydroxyestradiol (2-HE), a sAC inhibitor, but not to SQ22,536, a blocker of conventional transmembrane adenylyl cyclases. These results suggest that sAC is functional in Calu-3 cells. Adding 25 mM HCO(3)(-) to the bath stimulated CFTR-mediated whole cell currents in the absence, but not in the presence, of 2-HE. In cell-attached membrane patches, 25 or 50 mM HCO(3)(-) in the bath markedly increased the product of channel number and open probability of CFTR, and this activation was attenuated by 2-HE. These findings demonstrate that sAC signaling pathway is involved in the regulation of CFTR function in human airway epithelium and thereby provides a link between the level of intracellular HCO(3)(-)/CO(2) and the modulation of HCO(3)(-)-conductive CFTR function by cAMP/PKA.

CFTR mediates bicarbonate-dependent activation of miR-125b in preimplantation embryo development

“…The conserved wide distribution of CFTR and sAC in various cell types suggests that the presently demonstrated CFTR-HCO 3 − -dependent signaling pathway may have implications in other cellular processes far beyond embryo development. Of interest, CFTR expression and its channel activity are also found to be regulated by various environmental factors, such as hypoxia [57], ion concentrations [58,59] and mechanical stimulation [60], suggesting its potential in sensing a wide spectrum of environmental changes and converting these changes into epigenetic changes, such as miRNA alteration, in the cell under various physiological and pathological conditions.…”

Decreased Soluble Adenylyl Cyclase Activity in Cystic Fibrosis Is Related to Defective Apical Bicarbonate Exchange and Affects Ciliary Beat Frequency Regulation

“…The importance of sAC for CBF and possibly other cellular functions is highlighted by the recent finding that chloride and bicarbonate conductance through CFTR is increased during states of low intracellular pH (42), allowing sAC activation to counteract the negative CBF effect of low pH even more. The fact that CFTR conductance is also increased by sAC itself, at least in airway epithelial cell lines (22), makes this rescue system a powerful, self-enhancing circuit. Bicarbonate comes from two major sources in the airways.…”

“…Electrolyte conductance through CFTR is usually associated with chloride transport, but bidirectional conductance of bicarbonate through CFTR has also been demonstrated and is thought to be important in the airway and in pancreatic ducts (14, 19 -21). Regulation of HCO 3 Ϫ transport through CFTR has been related to sAC in Calu3 cells, an airway epithelial cell line (22), and in corneal endothelium (23), but there are no reports of the influence of diminished bicarbonate transport through CFTR on the activity of sAC in primary airway epithelial cells, especially as it relates to components of mucociliary transport, especially CBF. Here we investigate the role of CFTR-dependent HCO 3 Ϫ transport on the activity of sAC and its influence on CBF regulation in airway epithelial cells.…”

Soluble Adenylyl Cyclase Is Not Required for Axon Guidance to Netrin-1

“…Outside of this role, sAC may function as a bicarbonate sensor in tissues that respond to or regulate bicarbonate levels, such as kidney, choroid plexus, small intestine, lungs, and leukocytes (Chen et al, 2000;Geng et al, 2005;Han et al, 2005;Wang et al, 2005;Schmid et al, 2007). This expression, however, appears to mediate nonessential functions as sAC-null female mice are viable, fertile, and exhibit no obvious abnormal phenotype (Esposito et al, 2004).…”