Intestinal Na+ Loss and Volume Depletion in JAK3-Deficient Mice

Background/Aims: The Janus kinase 3 JAK3 participates in the signaling of immune cells. Lack of JAK3 triggers inflammatory bowel disease, which in turn has been shown to affect intestinal activity of the epithelial Na+ channel ENaC and thus colonic sodium absorption. At least in theory, inflammatory bowel disease in JAK3-deficient mice could lead to intestinal salt loss compromizing extracellular volume maintenance and blood pressure regulation. The present study thus explored whether JAK3 deficiency impacts on colonic ENaC activity, fecal Na+ exretion, blood pressure and extracellular fluid volume regulation. Methods: Experiments were performed in gene-targeted mice lacking functional JAK3 (jak3-/-) and in wild type mice (jak3+/+). Colonic ENaC activity was estimated from amiloride-sensitive current in Ussing chamber experiments, fecal, serum and urinary Na+ concentration by flame photometry, blood pressure by the tail cuff method and serum aldosterone levels by immunoassay. Results: The amiloride (50 µM)-induced deflection of the transepithelial potential difference was significantly lower and fecal Na+ excretion significantly higher in jak3-/- mice than in jak3+/+ mice. Moreover, systolic arterial blood pressure was significantly lower and serum aldosterone concentration significantly higher in jak3-/- mice than in jak3+/+ mice. Both, absolute and fractional renal Na+ excretion were significantly lower in jak3-/- mice than in jak3+/+ mice. Conclusions: JAK3 deficiency leads to impairment of colonic ENaC activity with intestinal Na+ loss, decrease of blood pressure, increased aldosterone release and subsequent stimulation of renal tubular Na+ reabsorption

Upregulation of Peptide Transporters PEPT1 and PEPT2 by Janus Kinase JAK2

Background/Aims: Janus-activated kinase-2 JAK2 participates in the signaling of several hormones including growth hormone, fosters tumor growth and modifies the activity of several Na+ coupled nutrient transporters. Peptide uptake into intestinal and tumor cells is accomplished by electrogenic peptide transporters PEPT1 and PEPT2. The present study thus explored whether JAK2 contributes to the regulation of PEPT1 and PEPT2 activity. Methods: cRNA encoding either PEPT1 or PEPT2 was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild type JAK2, constitutively active V617FJAK2 or inactive K882EJAK2. The current created by the dipeptide glycine-glycine (Igly-gly) was determined by dual electrode voltage clamp and taken as measure for electrogenic peptide transport. Results: No appreciable Igly-gly was observed in water injected oocytes. In PEPT1 or PEPT2 expressing oocytes Igly-gly was significantly increased by additional coexpression of JAK2. As shown in PEPT1 expressing oocytes, Igly-gly without significantly modifying the concentration required for halfmaximal Igly-gly (KM). Following disruption of carrier insertion with brefeldin A (5 µM) Igly-gly declined similarly fast in Xenopus oocytes expressing PEPT1 with JAK2 and in Xenopus oocytes expressing PEPT1 alone. In oocytes expressing both, PEPT1 and V617FJAK2, Igly-gly was gradually decreased by JAK2 inhibitor AG490 (40 µM). According to Ussing chamber experiments pharmacological JAK2 inhibition similarly decreased Igly-gly in mouse intestine. Conclusion: Regulation of the peptide transporters PEPT and PEPT2 does involve the Janus-activated kinase-2 JAK2