Insights into the mechanisms of inflammatory acidification within the mucosa

Abstract

Tissue acidification is commonly associated with a number of inflammatory states. The primary mechanism of extracellular acidification is thought to be increased accumulation of lactic acid resulting from enhanced glycolysis. Like other chronic inflammatory diseases, tissue acidification has been observed in inflammatory bowel disease patients, with reports of colonic pH as low as 4. We have recently demonstrated that neutrophil (PMN) transepithelial migration rapidly acidifies the microenvironment resulting in inflammatory acidification in the mucosa. Very little is known with regard to the mechanism(s) involved in this acidification or the impact of such acidification on the intestinal epithelial cell (IEC) function. Initial observations showed that physical interaction between IEC and PMN was required for inflammatory acidification; however, when transmigration was blocked using anti‐CD11b antibodies, we observed significant extracellular acidification in the absence of PMN transepithelial migration. This observation suggests that binding of the CD11b receptor plays a crucial role promoting extracellular acidification. Expanding upon these observations, we analyzed extracellular lactate secretion. We observed that lactic acid was released by IEC 4 to 5 hours after the addition of PMN and in quantities not high enough to account for the extracellular acidification observed. Additionally, the concentration of lactic acid released in the anti‐CD11b treated cells was similar to control IEC in the absence of PMN. When combined, these data indicate lactate is not a significant mediator of extracellular acidification following PMN transmigration. Given the large number of PMNs present and chronic tissue acidification observed during active inflammation, we investigated the impact of extracellular acidification on the regulation of epithelial derived pro‐inflammatory markers. When T84 IEC were cultured in pH 6.0 media for 3 or 6 hours we observed increases in pro‐inflammatory markers, including IL‐8. To better understand how extracellular acidification alters IEC gene regulation, we utilized a transcription factor array to examine changes in transcription factor expression under acidic conditions. This screen identified CREB as one of the major transcription factors controlling responses to acidification. Further examination revealed that CREB phosphorylation occurs rapidly, is cAMP independent, and requires MSK1. Inhibition of MSK1 prevents the induction of IL‐8 and other acidification associated gene transcription. Theses result identified extracellular acidification as a significant signaling mechanism during tissue inflammation. Targeted regulation of MSK1 may serve as a new therapeutic approach to limit pro‐inflammatory signaling during mucosal inflammation.Support or Funding InformationR01 DK095491, NIH