CFTR-mediated anion secretion in parathyroid hormone-treated Caco-2 cells is associated with PKA and PI3K phosphorylation but not intracellular pH changes or Na/K-ATPase abundance.

Parathyroid hormone (PTH) has previously been shown to enhance the transepithelial secretion of Cl and HCO across the intestinal epithelia including Caco-2 monolayer, but the underlying cellular mechanisms are not completely understood. Herein, we identified the major signaling pathways that possibly mediated the PTH action to its known target anion channel, i.e., cystic fibrosis transmembrane conductance regulator anion channel (CFTR). Specifically, PTH was able to induce phosphorylation of protein kinase A and phosphoinositide 3-kinase. Since the apical HCO efflux through CFTR often required the intracellular H/HCO production and/or the Na-dependent basolateral HCO uptake, the intracellular pH (pH) balance might be disturbed, especially as a consequence of increased endogenous H and HCO production. However, measurement of pH by a pH-sensitive dye suggested that the PTH-exposed Caco-2 cells were able to maintain normal pH despite robust HCO transport. In addition, although the plasma membrane Na/K-ATPase (NKA) is normally essential for basolateral HCO uptake and other transporters (e.g., NHE1), PTH did not induce insertion of new NKA molecules into the basolateral membrane as determined by membrane protein biotinylation technique. Thus, together with our previous data, we concluded that the PTH action on Caco-2 cells is dependent on PKA and PI3K with no detectable change in pH or NKA abundance on cell membrane.