Mineralocorticoid Receptor Phosphorylation Regulates Ligand Binding and Renal Response to Volume Depletion and Hyperkalemia

Abstract

SUMMARY Nuclear receptors are transcription factors that regulate diverse cellular processes. In canonical activation, ligand availability is sufficient to produce receptor binding, entraining downstream signaling. The mineralocorticoid receptor (MR) is normally activated by aldosterone, which is produced in both volume depletion and hyperkalemia, states that require different homeostatic responses. We report phosphorylation at S843 in the MR ligand-binding domain that prevents ligand binding and activation. In kidney, MRS843-P is found exclusively in intercalated cells of the distal nephron. In volume depletion, angiotensin II and WNK4 signaling decrease MRS843-P levels whereas hyperkalemia increases MRS843-P. Dephosphorylation of MRS843-P results in aldosterone-dependent increases of the intercalated cell apical proton pump and Cl−/HCO3− exchangers, increasing Cl− reabsorption and promoting increased plasma volume while inhibiting K+ secretion. These findings reveal a mechanism regulating nuclear hormone receptor activity and implicate selective MR activation in intercalated cells in the distinct adaptive responses to volume depletion and hyperkalemia.

Angiotensin II signaling via protein kinase C phosphorylates Kelch-like 3, preventing WNK4 degradation

“…FLAG-epitopetagged human KLHL3 (23) was expressed in COS-7 cells and purified by immunoprecipitation (IP) and SDS/PAGE. After TiO 2 affinity chromatography, phosphopeptides and specific phosphorylation sites were identified by liquid chromatography (LC) and tandem mass spectrometry (MS/MS) (14). In three independent mapping experiments, we reproducibly observed four phosphorylation sites: one at the N terminus, two between the BACK (BTB and C-terminal Kelch) and Kelch domains, and one in the Kelch domain ( Fig.…”

“…Multiple lines of evidence suggest that WNK4 lies downstream of AII signaling (14,27,28), suggesting that AII regulates WNK4 levels via phosphorylation of KLHL3 S433 (23,29). Angiotensin type 1 receptor (AT1R) activation by AII induces release of the G protein alpha subunit, G q , which leads to activation of protein kinase C (PKC) (30).…”

“…AT1R is present throughout the nephron (40), and signaling increases activities of the sodium hydrogen exchanger 3 in the proximal tubule, NKCC2 in the TAL, NCC of the DCT, and ENaC in the CNT and CD (41)(42)(43)(44). AII also regulates phosphorylation and activity of MR in renal intercalated cells, allowing induction of a transcellular Cl − reabsorption pathway (14). In addition, AII signaling inhibits ROMK activity, inhibiting renal K + secretion (45).…”

“…In hyperkalemia, the lumen-negative potential promotes K + secretion via the K + channel Kir1.1 (renal outer medullary K + channel ROMK), reducing plasma K + level (12,13). Additionally, recent studies have implicated aldosterone signaling in intercalated cell transcellular Cl − flux (14). In these cells, hyperkalemia induces phosphorylation of the mineralocorticoid receptor (MR) ligand-binding domain, making it incapable of ligand binding and activation.…”

“…In these cells, hyperkalemia induces phosphorylation of the mineralocorticoid receptor (MR) ligand-binding domain, making it incapable of ligand binding and activation. AII signaling induces dephosphorylation, and activation of the MR by aldosterone then induces transcellular Cl − flux, which is required for defense of intravascular volume (14,15). Because electrogenic Cl − reabsorption and K + secretion both dissipate the lumen-negative potential produced by ENaC, maximal Cl − reabsorption inhibits K + secretion and vice versa.…”