Acidosis regulation of epithelial barrier function and immune signaling

Abstract

A common but often underappreciated aspect of the inflammatory microenvironment is extracellular acidification. As with other chronic inflammatory diseases, tissue acidification has been observed in inflammatory bowel disease (IBD) patients. Colonic pH as low as 4 have been reported in IBD patients. In contrast, the pH of a healthy colon is between a pH of 7.0‐7.4. Recently, we identified an adaptive tissue response in intestinal epithelial cells (IEC) which buffered extracellular acidification during neutrophil transepithelial migration, suggesting pH homeostasis is an important aspect of acute inflammation. Extensions of these studies identified a previously undescribed CREB‐dependent pathway for sensing extracellular pH and a unique acidosis gene signature in IEC. The full impact of acidosis on IEC function remains unclear. To better understand the impact of chronic extracellular acidification on IEC function, we investigated changes in IEC immune signaling and barrier function in the setting of acute and chronic exposure to acidosis. We initially examined barrier function in T84 IEC cultured under acidic conditions and observed a significant decrease in barrier function, as determined by transepithelial electrical resistance (TER) and paracellular flux, in cells exposed to acidosis in a pH range of 6.0 – 7.4. Interestingly, following acute acidification, treatment with low pH for less than 8hr, barrier function recovered within 16hr. However, following chronic acidification, treatment with low pH beyond 16hr resulted in an appreciable barrier dysfunction for several days. We extended these studies to examine expression and localization of tight‐junction proteins in IEC exposed to acidosis and observed a time and dose‐dependent decrease in ZO‐1 and E‐cadherin expression, with almost complete loss of expression after 72hr at a pH of 6.0. Likewise, expression and localization of claudins 1, 2, and 4 decreased over time. In parallel to a loss of barrier function, IEC exposed to chronic acidosis revealed a prominent increase in the expression of inflammatory markers including several chemokines (IL‐8, CCL20, TNF) as well as anti‐microbial peptides (mucins and beta‐defensins). Thus, chronic extracellular acidosis induces the loss of IEC tight junction function, while inducing the expression of several pro‐inflammatory chemokines and the production of various anti‐microbial peptides. These results identify extracellular acidification as significant signaling mechanism which induces a loss of epithelial features and the acquisition of a more immune‐like phenotype. Furthering our understandings of the mechanisms involved in acidosis induced phenotype switching in IEC has the potential to uncover novel therapeutic drug targets.