Uroguanylin modulates (Na+ + K+)ATPase in a proximal tubule cell line: Interactions among the cGMP/protein kinase G, cAMP/protein kinase A, and mTOR pathways

Background: The natriuretic effect of uroguanylin (UGN) involves reduction of proximal tubule (PT) sodium reabsorption. However, the target sodium transporters as well as the molecular mechanisms involved in these processes remain poorly understood. Methods: To address the effects of UGN on PT (Na+ + K+)ATPase and the signal transduction pathways involved in this effect, we used LLC-PK1 cells. The effects of UGN were determined through ouabain-sensitive ATP hydrolysis and immunoblotting assays during different experimental conditions. Results: We observed that UGN triggers cGMP/PKG and CAMP/PKA pathways in a sequential way. The activation of PKA leads to the inhibition of mTORC2 activity, PKB phosphorylation at 5473, PKB activity and, consequently, a decrease in the mTORC1/S6K pathway. The final effects are decreased expression of the alpha 1 subunit of (Na+ + K+)ATPase and inhibition of enzyme activity. Conclusions: These results suggest that the molecular mechanism of action of UGN on sodium reabsorption in PT cells is more complex than previously thought. We propose that PKG-dependent activation of PKA leads to the inhibition of the mTORC2/PKB/mTORC1/S6K pathway, an important signaling pathway involved in the maintenance of the PT sodium pump expression and activity. General significance: The current results expand our understanding of the signal transduction pathways involved in the overall effect of UGN on renal sodium excretion. (C) 2016 Elsevier B.V. All rights reserved.