PGE₂ potentiates tonicity-induced COX-2 expression in renal medullary cells in a positive feedback loop involving EP2-cAMP-PKA signaling

Abstract

(COX-2)-derived PGE2 is critical for the integrity and function of renal medullary cells during antidiuresis. The present study extended our previous finding that tonicity-induced COX-2 expression is further stimulated by the major COX-2 product PGE2 and investigated the underlying signaling pathways and the functional relevance of this phenomenon. Hyperosmolality stimulated COX-2 expression and activity in Madin-Darby canine kidney (MDCK) cells, a response that was further increased by PGE2-cAMP signaling, suggesting the existence of a positive feedback loop. This effect was diminished by AH-6809, an EP2 antagonist, and by the PKA inhibitor H-89, but not by AH-23848, an EP4 antagonist. The effect of PGE2 was mimicked by forskolin and dibutyryl-cAMP, suggesting that the stimulatory effect of PGE2 on COX-2 is mediated by a cAMP-PKAdependent mechanism. Accordingly, cAMP-responsive element (CRE)-driven reporter activity paralleled the effects of PGE2, AH-6809, AH-23848, H-89, forskolin, and dibutyryl-cAMP on COX-2 expression. In addition, the stimulatory effect of PGE2 on tonicity-induced COX-2 expression was blunted in cells transfected with dominantnegative CRE binding (CREB) protein, as was the case in a COX-2 promoter reporter construct in which a putative CRE was deleted. Furthermore, PGE2 resulted in PKA-dependent phosphorylation of the pro-apoptotic protein Bad at Ser155, a mechanism that is known to inactivate Bad, which coincided with reduced caspase-3 activity during osmotic stress. Conversely, pharmacological interruption of the PGE 2-EP2-cAMP-PKA pathway abolished Ser155 phosphorylation of Bad and blunted the protective effect of PGE 2 on cell survival during osmotic stress. These observations indicate the existence of a positive feedback loop of PGE 2 on COX-2 expression during osmotic stress, an effect that apparently is mediated by EP2-cAMP-PKA signaling, and that contributes to cell survival under hypertonic conditions. cyooxygenase-2; prostaglandin E 2; osmoadaptation; apoptosis; survival factor THE INTERSTITIAL ENVIRONMENT of the renal medulla is characterized by extremely high interstitial osmolalities and low oxygen tension (34). Nevertheless, medulla-resident cells not only have the ability to survive in this hostile environment but also to function normally. This feature is of integral importance for maintaining the kidneys' function in regulating systemic electrolyte and fluid homeostasis.A characteristic feature of renal medullary cells is their high capacity for production of prostaglandins (PG) (4). PGs are autacoids acting close to their site of formation and contribute essentially to the regulation of tubular solute and water reabsorption and adjusting medullary blood flow to metabolic requirements (12,32,34). Moreover, PGs promote the adaptation of renal medullary cells to high extracellular osmolalities by virtue of enhancing the expression of osmoprotective genes (29,35,37). The formation of PGs depends on the sequential action of cytosolic phospholipase A 2 (cPLA 2 ), which releases ...

AMPK Potentiates Hypertonicity-induced Apoptosis by Suppressing NFκB/COX-2 in Medullary Interstitial Cells

“…PGI 2 promotes RMIC viability via activation of its nuclear receptor PPAR␤/␦, 38 whereas PGE 2 enhances cell survival in parallel with increased expression of osmoprotective genes 39 and potentiates tonicityinduced COX-2 expression in a positive feedback loop involving EP 2 -cAMP-PKA signaling. 40 Our finding that AMPK activation inhibited the NFB-COX-2-PG survival pathway demonstrates that AMPK activity represents a key determinant in the fate of RMICs exposed to hypertonic stress. This conclusion was further supported by the adenoviral overexpression of COX-2 attenuating AICAR-induced RMIC apoptosis under the hyperosmotic condition.…”

Prostaglandin E2EP3 receptor regulates cyclooxygenase-2 expression in the kidney

“…COX-2 also is subject to feedback regulation by several mechanisms acting at both the transcriptional and posttranscriptional levels. Additionally, increases in cAMP induced via activation of EP2 were part of a positive feedback loop in renal Madin-Darby canine kidney cells (45).…”

Control of tight junctional sealing: roles of epidermal growth factor and prostaglandin E2

“…S5), in a similar way as observed with the inhibition of cPLA 2 and COX. Moreover, p38 ERK mediates cPLA 2 activation in MDCK cells under hypertonic stress (32), which causes the release of TGF-␣, activation of EGFR, ERK1/2, p38 ERK, cPLA 2 , upregulation of COX-2, and synthesis of PGE 2 , which is essential in cell survival (32,58). Also, p38 ERK is a negative regulator of TJ, because it is essential in the disruption of TJ induced by transforming growth factor-␤3 in Sertoli cells or group B coxsackievirus-3 (CVB3) in human umbilical vein endothelial cells (27,41).…”