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PGE₂ INDUCES IL-8 GENE TRANSCRIPTION BY ACTIVATING EP4/PKA/CREB PATHWAY IN HUMAN COLONIC EPITHELIAL CELLS

I Dey, K Chadee
Department of Microbiology and Infectious Diseases, University of Calgary, Calgary, Alberta

Even though PGE₂ and IL-8 are simultaneously over expressed during inflammation, it is unclear how IL-8 synthesis is regulated by PGE₂. IL-8 is a potent neutrophil chemo-attractant and activator that can initiate or exacerbate tissue injury if activated. PGE₂ signals through EP receptors to stimulate and exert cAMP-dependent cellular functions. To determine which EP receptor subtype(s) are involved in regulating IL-8, we knocked out or over expressed EP2 and EP4 receptors separately in stably transfected Caco2 cells in vitro. PGE₂ stimulated cAMP and IL-8 production by ~500% and ~1700% respectively only in cells over-expressing EP4 receptors. Similar results were obtained with 1-OH-PGE1, an EP2/4 receptor specific agonist, whereas, butaprost an EP2 receptor specific and sulprostone an EP1/3 receptor agonist had no effect. Competitive radio ligand binding assays showed that EP4 receptor have the highest affinity for PGE₂. PGE₂ induced IL-8 production mediated through EP4 receptor was dependent on phosphorylation of ERK-1/2 but not on activation of p-38 and JNK. Moreover, PGE₂ induced IL-8 synthesis was mediated via PKA/CREB pathway but not via PI3K/Akt/CREB pathway. Interestingly, when PGE₂ couples through the EP2 receptor it down regulated IL-8 gene expression through induction of inducible cAMP early repressor (ICER). Taken together, this study reveals that PGE₂ preferentially couples through the EP4 receptor to stimulate cAMP-dependent IL-8 production, whereas coupling of PGE₂ to the EP2 receptor down-regulates IL-8 synthesis in colonic epithelial cells. These dual and opposite functions of PGE2 as both anti and pro-inflammatory molecule to suppress or induce IL-8 synthesis could explain the potency of PGE₂ as a suppressor or inducer of IL-8 during different phases of inflammation.
This work was partially supported by the Canadian Institute for Health Research and the Canadian Association of Gastroenterology-Axcan Pharma-CIHR Research and Fellowship Award

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