Abstract
Prostaglandin E2 (PGE2) is a bioactive lipid mediator that exerts its biological function through interaction with four different subtypes of E-Prostanoid receptor namely EP1, EP2, EP3 and EP4. It has been known that EP2 receptor is differentially over-expressed in the epithelia of inflamed human colonic mucosa. However, the significance of the differential expression in altering epithelial barrier function is not known. In this study, we used Caco-2 cells expressing EP2 receptor, either high (EP2S) or low (EP2A), as a model epithelia and determined the barrier function of these cell monolayers by measuring the trans epithelial resistance (TER). Basal TER of EP2A (but not EP2S) monolayer was significantly lower suggesting a loss of colonic epithelial barrier integrity. In comparison, the TER of wild type Caco-2 was decreased in response to an EP2 receptor specific antagonist (AH-6809) indicating an important role for EP2 receptor in the maintenance of epithelial barrier function. The decrease TER in EP2A monolayer corresponded with a significant loss of the tight junction (TJ) protein claudin-4 without affecting other major TJ proteins. Similarly, EP2 receptor antagonism/siRNA based silencing significantly decreased claudin-4 expression in EP2S cells. Surprisingly, alteration in claudin-4 was not transcriptionally regulated in EP2A cells but rather undergoes increased proteosomal degradation. Moreover, among the TER compromising cytokines examined (IL-8, IL-1β, TNF-α, IFN-γ) only IFN-γ was significantly up regulated in EP2A cells. However, IFN-γ did not significantly decreased claudin-4 expression in Caco-2 cells indicating no role for IFN-γ in degrading claudin-4. We conclude that differential down-regulation of EP2 receptor play a major role in compromising colonic epithelial barrier function by selectively increasing proteosomal degradation of claudin-4.
Highlights
Prostaglandins E2 (PGE2) is an important bioactive lipid produced by variety of different tissues including the gastrointestinal (GI) tract [1,2,3] were it modulates both physiological and pathological functions of the gut [4]
To ascertain if the differential EP2 receptor expression had any effect on epithelial monolayer resistance, we assessed basal trans epithelial resistance (TER) of the cells grown to confluence on transwell plates
It is clear that lack of EP2 receptor almost completely abrogated the TER of Caco-2 colonic epithelial monolayer through a mechanism independent of endogenous PGE2
Summary
Prostaglandins E2 (PGE2) is an important bioactive lipid produced by variety of different tissues including the gastrointestinal (GI) tract [1,2,3] were it modulates both physiological and pathological functions of the gut [4]. PGE2 signals via four different subtypes of EP receptors namely EP1, EP2, EP3 and EP4 [7]. Altered/differential expression of EP receptor subtypes has been reported in various disease conditions in the gut [9,10,11,12,13]. The role of such alterations in modulating biological functions of PGE2 is not clearly understood
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