Mechanisms underlying upregulation of intestinal apical chloride/hydroxyl radical exchange activity by LPA

Electroneutral NaCI absorption in the human ileum and colon involves the coupled operation of Na+/H+ and Cl -/OH-(HCO3-) exchangers. Disturbances in these exchangers have been implicated in various diarrheal conditions. Since diarrheal disorders are due to increased secretion or decreased absorption or both; understanding of mechanisms of NaCl absorption may aid in the development of novel therapeutics for the treatment of diarrheal disorders. In this regard, Lysophosphatidic acid (LPA), a bioactive phospholipid, has been suggested to act as an anti-diarrheal agent by inhibiting chloride secretion and stimulating sodium absorption. However, nothing is known about the potential proabsorptive effects of LPA on apical Cl-/OH- exchangers: DRA (D&barbelow;own R&barbelow;egulated in A&barbelow;denoma) and PAT-1 (P&barbelow;utative A&barbelow;nion T&barbelow;ransporter-1). Therefore, we hypothesized that LPA may stimulate C1-/OH -(HCO3-) exchange activity by modulating DRA and PAT-1 activity and expression in human intestinal epithelial cells. Our data showed that acute LPA treatment of Caco-2 cells (100 microM, 30 min) significantly stimulated Cl/OH- exchange activity via LPA2 receptor and PI3K/AKT and PKCS mediated pathways. This increase was associated with an increase in the surface levels of DRA but not PAT-1. Additionally, Cl- transport in ileal region was increased in LPA gavaged mouse. Long term LPA treatment (50 microM, 24h) of Caco-2 cells also significantly stimulated Cl-OH- exchange activity and increased DRA mRNA levels and promoter activity. LPA increased the DRA promoter activity via LPA2 receptor, PI3K/AKT dependent pathways and c-fos transcription factor. To gain a thorough understanding of LPA mediated effects on ion transport, we characterized the LPA receptor isoform expression along the length of the human and mouse intestine. Our receptor expression studies showed that mRNA levels of LPA1, 2 and 5 were higher as compared to LPA3 and 4 in both mouse and human intestine, with LPA5 mRNA expression higher in the colonic regions, whereas, LPA1 and 2 expression was higher in the small intestinal regions. Further, LPA2 was expressed predominantly on the apical membranes with little expression on basolateral domains. Conclusion: LPA stimulates Cl-/HCO3- (OH) exchange activity via both post translational and transcriptional mechanisms in intestinal epithelial cells which could underlie the potential proabsorptive effects of LPA.