Prostaglandin E₂EP3 receptor regulates cyclooxygenase-2 expression in the kidney

Abstract

Cyclooxygenase-2 (COX-2) is constitutively expressed and highly regulated in the thick ascending limb (TAL). As COX-2 inhibitors (Coxibs) increase COX-2 expression, we tested the hypothesis that a negative feedback mechanism involving PGE(2) EP3 receptors regulates COX-2 expression in the TAL. Sprague-Dawley rats were treated with a Coxib [celecoxib (20 mg·kg(-1)·day(-1)) or rofecoxib (10 mg·kg(-1)·day(-1))], with or without sulprostone (20 μg·kg(-1)·day(-1)). Sulprostone was given using two protocols, namely, previous to Coxib treatment (prevention effect; Sulp7-Coxib5 group) and 5 days after initiation of Coxib treatment (regression effect; Coxib10-Sulp5 group). Immunohistochemical and morphometric analysis revealed that the stained area for COX-2-positive TAL cells (μm(2)/field) increased in Coxib-treated rats (Sham: 412 ± 56.3, Coxib: 794 ± 153.3). The Coxib effect was inhibited when sulprostone was used in either the prevention (285 ± 56.9) or regression (345 ± 51.1) protocols. Western blot analysis revealed a 2.1 ± 0.3-fold increase in COX-2 protein expression in the Coxib-treated group, an effect abolished by sulprostone using either the prevention (1.2 ± 0.3-fold) or regression (0.6 ± 0.4-fold vs. control, P < 0.05) protocols. Similarly, the 6.4 ± 0.6-fold increase in COX-2 mRNA abundance induced by Coxibs (P < 0.05) was inhibited by sulprostone; prevention: 0.9 ± 0.3-fold (P < 0.05) and regression: 0.6 ± 0.1 (P < 0.05). Administration of a selective EP3 receptor antagonist, L-798106, also increased the area for COX-2-stained cells, COX-2 mRNA accumulation, and protein expression in the TAL. Collectively, the data suggest that COX-2 levels are regulated by a novel negative feedback loop mediated by PGE(2) acting on its EP3 receptor in the TAL.

PGE2EP3receptor downregulates COX-2 expression in the medullary thick ascending limb induced by hypertonic NaCl

“…We hypothesize that COX-2 is under tonic suppression by a negative feedback mechanism involving EP 3 receptors, as COX-2-dependent PGE 2 synthesis by the mTAL after exposure to HS was potentiated when EP 3 receptors were inhibited. As EP 3 receptor-dependent feedback inhibition in the TAL was also observed in response to treatment with COX-2-selective inhibitors (37), the present study suggests that PGE 2 signaling via EP 3 receptors in the TAL may be part of a mechanism that regulates mTAL COX-2 expression and function in response to diverse stimuli. Since PGE 2 inhibits Na ϩ -K ϩ -Cl Ϫ cotransporter 2 activity (22), this mechanism may be teleologically important to prevent excessive loss of NaCl and/or the regulation of urinary concentrating mechanisms due to the prodigious ability of the TAL to reabsorb NaCl.…”

“…We (37) have also previously found that administration of the EP 3 receptor agonist sulprostone decreased the number of cells expressing COX-2. Upregulation of EP 3 receptors in response to HS intake is consistent with data showing that administration of a high-salt diet (4% NaCl) to rats increased EP 3 receptor mRNA levels (20) and may be part of a mechanism that regulates the COX-2/PGE 2 signaling pathway.…”

“…Expression of COX-2 in the TAL under basal conditions is less than that observed in the inner medulla (42). However, COX-2 expression is markedly increased along the mTAL by administration of the EP 3 antagonist L-798106 (37). We postulated that the suppression of COX-2 expression in the mTAL via EP 3 receptors limits the capacity of this nephron segment, which is responsible for the reabsorption of ϳ25% of filtered NaCl and Ca 2ϩ and contributes to the generation of the osmotic gradient that drives vasopressin-dependent water reabsorption by the collecting duct, to produce COX-2-derived PGE 2 in response to diverse stimuli.…”

“…PGE2 EP3 receptors were localized in renal tissue from normal and 1% NaCl-treated animals using specific anti-EP3 receptor antibody (catalog no. 101760, Cayman Chemicals) with the immunoperoxidase method (26,35,37).…”

“…We recently showed that a novel negative feedback mechanism in the mTAL, effected by PGE 2 acting via EP 3 receptors, attenuates COX-2 expression induced by COX-2 inhibitors (37). Expression of COX-2 in the TAL under basal conditions is less than that observed in the inner medulla (42).…”