Dynamic Regulation of CFTR Bicarbonate Permeability by [Cl⁻]_i and Its Role in Pancreatic Bicarbonate Secretion

Background & Aims

Pancreatic bicarbonate (HCO₃⁻) secretion is important for a healthy pancreas as well as digestive physiology. However, how human pancreatic duct cells secrete copious amounts of HCO₃⁻ has long been a puzzle. Here, we report that a dynamic increase in the cystic fibrosis transmembrane conductance regulator (CFTR) HCO₃⁻ permeability by intracellular Cl⁻ concentration ([Cl⁻]_i)-sensitive mechanisms plays a pivotal role in pancreatic HCO₃⁻ secretion.

Methods

The role of [Cl⁻]_i-sensitive kinases in CFTR-mediated HCO₃⁻ transport was examined in heterologous expression systems, PANC1 human pancreatic duct cells, and human and guinea pig pancreatic tissues using an integrated molecular and physiologic approach.

Results

In human pancreatic tissues, CFTR-positive duct cells abundantly expressed with-no-lysine (WNK1) kinase, oxidative stress-responsive kinase 1 (OSR1), and sterile 20/SPS1-related proline/alanine-rich kinase (SPAK), which are known to be activated by low [Cl⁻]_i. Interestingly, CFTR activation rapidly decreased [Cl⁻]_i in response to luminal Cl⁻ depletion in polarized PANC1 human pancreatic duct cells. Notably, the WNK1-mediated OSR1 and SPAK activation by low [Cl⁻]_i strongly increased CFTR HCO₃⁻ permeability in CFTR-transfected HEK 293T, PANC1, and guinea pig pancreatic duct cells, making CFTR primarily an HCO₃⁻ channel, which is essential for the secretion of pancreatic juice containing HCO₃⁻ at a concentration greater than 140 mmol/L. In contrast, OSR1 and SPAK activation inhibited CFTR-dependent Cl⁻/HCO₃⁻ exchange activity that may reabsorb HCO₃⁻ from the high HCO₃⁻-containing pancreatic juice.

Conclusions

These results indicate that the [Cl⁻]_i-sensitive activation of the WNK1-OSR1/SPAK pathway is the molecular switch to generate HCO₃⁻-rich fluid in the human pancreatic duct.

Keywords: CFTR, Bicarbonate, Pancreatic Secretion, WNK1/OSR1/SPAK Kinases

Abbreviations used in this paper: CFTR, cystic fibrosis transmembrane conductance regulator, [Cl⁻]_i, intracellular Cl⁻ concentration, E_rev, reversal potential, [HCO₃⁻]_i, intracellular bicarbonate concentration, IBMX, 3-isobutyl-1-methylxanthine, OSR1, oxidative stress-responsive kinase 1, P_HCO3/P_Cl, HCO₃⁻/Cl⁻ permeability ratio, pH_i, intracellular pH, SPAK, sterile 20/SPS1-related proline/alanine-rich kinase, STE20, sterile 20, WNK1, with-no-lysine kinase 1, WT, wild type