Inhibitors of the proteasome stimulate the epithelial sodium channel (ENaC) through SGK1 and mimic the effect of aldosterone
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The epithelial sodium channel (ENaC) marks the tightly regulated, rate-limiting step of sodium re-absorption in the aldosterone-sensitive distal nephron (ASDN). Stimulation of ENaC activity by aldosterone involves the serum and glucocorticoid-induced kinase 1 (SGK1) and is mediated via complex mechanisms including inhibition of channel retrieval. Retrieved channels may be recycled or degraded, e.g. by the proteasomal pathway. The aim of the present study was to investigate whether inhibitors of the proteasome affect ENaC activity and surface expression, and to explore a possible involvement of SGK1. Short circuit current (I SC) measurements were performed on confluent mCCDcl1 murine cortical collecting duct cells to investigate the effect of two distinct proteasomal inhibitors, MG132 and bortezomib, on amiloride-sensitive ENaC-mediated I SC. Both inhibitors robustly stimulated amiloride-sensitive I SC. The time course and magnitude of the stimulatory effect of the proteasomal inhibitors on I SC were similar to those of aldosterone. Both, MG132 and aldosterone, significantly increased the abundance of β-ENaC at the cell surface. SGK1 activity was assessed by monitoring the phosphorylation of a downstream target, NDRG1, and was found to be increased by MG132. Importantly, inhibiting SGK1 activity prevented not only the stimulatory effect of aldosterone but also that of proteasomal inhibition. In conclusion, these data suggest that ENaC stimulation following proteasomal inhibition is due to an accumulation of active SGK1 resulting in increased expression of ENaC at the cell surface. Thus, inhibition of the proteasome mimics SGK1-dependent stimulation of ENaC by aldosterone.
KeywordsRenal collecting duct cells ENaC Aldosterone SGK1 Proteasome MG132
Epithelial sodium channel
Serum and glucocorticoid-induced kinase 1
n-myc downstream-regulated gene 1
Neural precursor cell expressed developmentally downregulated gene 4-2
We are grateful to Bernard Rossier (Department of Pharmacology and Toxicology, University of Lausanne, CH-1005 Lausanne, Switzerland) for providing us with the mCCDcl1 cells. We thank Christina Lang, Lorenz Reeh, Ralf and Jessica Rinke and Celine Grüninger for their expert technical assistance.
This work was supported by The Alexander von Humboldt Foundation (3.3-GRO/1143730 STP, Morag K Mansley) and Bayerische Forschungsstiftung (PDOK-74-10, Christoph Korbmacher).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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