Introduction:

We have previously reported a dysfunction of the major electroneutral sodium absorptive transporter, the Na⁺/H⁺ exchanger NHE3, in colonocytes of patients with ulcerative colitis and in several mouse models for inflammatory bowel disease, despite apparently normal NHE3 expression and localization to the brush border membrane.

Objective:

In this study, we searched for a potential causal relationship between Interleukin-1ß (IL-1ß)-induced PDZK1 downregulation and the functional defect in NHE3 transport.

Methods:

PDZK1 and NHE3 mRNA and protein expression was measured in IL-1ß-treated stably NHE3-expressing Caco2bbe cells. pH-fluorimetry using the pH sensitive dye BCECF was performed to measure acid activated NHE3 activity.

Results:

Treatment of Caco-2bbe cells stably expressing human NHE3 with IL-1ß lead to a significant decrease in PDZK1 protein to approx. 50% of control value, but no change in NHE3 protein expression. However, acid activated NHE3 transport rates were significantly decreased after IL-1ß treatment in these cells. When PDZK1 expression was decreased by lentiviral shRNA knockdown in NHE3-transfected Caco2bbe cells to approx 80% of control value, and NHE3 transport activity assessed, an even stronger decrease in acid-activated NHE3 activity was observed than in IL-1ß treated cells. This demonstrates a causal relationship of PDZK1 downregulation and NHE3 dysfunction, independent of inflammation.

Conclusion:

A marked decrease in the PDZ-adaptor protein, PDZK1, was observed in the IL-1ß treated Caco-2bbe cells, along with a decrease in NHE3 transport function but no change in its expression. We therefore conclude that during intestinal inflammation, cytokine mediated PDZK1 down regulation is likely an important causative factor for inflammation-associated NHE3 dysfunction and diarrhea.