Hepcidin and 1,25(OH)2D3 effectively restore Ca2+ transport in ß-thalassemic mice: reciprocal phenomenon of Fe2+ and Ca2+ absorption.

Previously, ß-thalassemia, an inherited anemic disorder with iron overload caused by loss-of-function mutation of ß-globin gene, has been reported to induce osteopenia and impaired whole body calcium metabolism, but the pathogenesis of aberrant calcium homeostasis remains elusive. Herein, we investigated how ß-thalassemia impaired intestinal calcium absorption and whether it could be restored by administration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] or hepcidin, the latter of which was the liver-derived antagonist of intestinal iron absorption. The results showed that, in hemizygous ß-globin knockout (BKO) mice, the duodenal calcium transport was lower than that in wild-type littermates, and severity was especially pronounced in female mice. Both active and passive duodenal calcium fluxes in BKO mice were found to be less than those in normal mice. This impaired calcium transport could be restored by 7-day 1,25(OH)2D3 treatment. The 1,25(OH)2D3-induced calcium transport was diminished by inhibitors of calcium transporters, e.g., L-type calcium channel, NCX1, and PMCA1b, as well as vesicular transport inhibitors. Interestingly, the duodenal calcium transport exhibited an inverse correlation with transepithelial iron transport, which was markedly enhanced in thalassemic mice. Thus, 3-day subcutaneous hepcidin injection and acute direct hepcidin exposure in the Ussing chamber were capable of restoring the thalassemia-associated impairment of calcium transport; however, the positive effect of hepcidin on calcium transport was completely blocked by proteasome inhibitors MG132 and bortezomib. In conclusion, both 1,25(OH)2D3 and hepcidin could be used to alleviate the ß-thalassemia-associated impairment of calcium absorption. Therefore, our study has shed light on the development of a treatment strategy to rescue calcium dysregulation in ß-thalassemia.

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 HCO3 - 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 HCO3 - efflux through CFTR often required the intracellular H+/HCO3 - production and/or the Na+-dependent basolateral HCO3 - uptake, the intracellular pH (pHi) balance might be disturbed, especially as a consequence of increased endogenous H+ and HCO3 - production. However, measurement of pHi by a pH-sensitive dye suggested that the PTH-exposed Caco-2 cells were able to maintain normal pH despite robust HCO3 - transport. In addition, although the plasma membrane Na+/K+-ATPase (NKA) is normally essential for basolateral HCO3 - 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 pHi or NKA abundance on cell membrane.