Abstract
Cyclooxygenase-2 (COX-2)-dependent prostaglandin E 2 (PGE 2) synthesis correlates with the onset of proteinuria and increased glomerular capillary pressure ( P gc) glomerular disease models. We previously showed that an in vitro surrogate for P gc (cyclical mechanical stretch) upregulates the expression of both COX-2 and the PGE 2 responsive E-Prostanoid receptor, EP 4 in cultured mouse podocytes. In the present study we further delineate the signaling pathways regulating podocyte COX-2 induction. Time course experiments carried out in conditionally-immortalized mouse podocytes revealed that PGE 2 transiently increased phosphorylated p38 MAPK levels at 10 min, and induced COX-2 protein expression at 4 h. siRNA-mediated knockdown of EP 4 receptor expression, unlike treatment with the EP 1 receptor antagonist SC 19220, completely abrogated PGE 2-induced p38 phosphorylation and COX-2 upregulation suggesting the involvement of the EP 4 receptor subtype. PGE 2-induced COX-2 induction was abrogated by inhibition of either p38 MAPK or AMP activated protein kinase (AMPK), and was mimicked by AICAR, a selective AMPK activator, and by the cAMP-elevating agents, forskolin (FSK) and IBMX. Surprisingly, neither PGE 2 nor FSK/IBMX-dependent p38 activation and COX-2 expression were blocked by PKA inhibitors or mimicked by 8-cPT-cAMP a selective EPAC activator, but were instead abrogated by Compound C, suggesting the involvement of AMPK. These results indicate that in addition to mechanical stretch, PGE 2 initiates a positive feedback loop in podocytes that drives p38 MAPK activity and COX-2 expression through a cAMP/AMPK-dependent, but PKA-independent signaling cascade. This PGE 2-induced signaling network activated by increased P gc could be detrimental to podocyte health and glomerular filtration barrier integrity.
Published Version
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