“…1 a). In different epithelial tissues, ENaC activity is under the tight control by a range of hormones and local mediators [ 37 , 49 , 65 ]. Fig.…”
Section: Introductionmentioning
confidence: 99%
“…At the cellular level, ENaC regulation involves a complex interplay of extracellular factors and intracellular signal transduction pathways including a number of protein kinases [ 3 , 37 , 65 ]. Kinases are important regulators of a wide range of cellular processes and consequently may modify ENaC function by acting at many different levels.…”
Section: Introductionmentioning
confidence: 99%
“…Effects of kinases on ENaC include the phosphorylation and modification of regulatory proteins associated with ENaC or the phosphorylation of the channel itself with modulatory effects on its interaction with regulatory proteins. A prominent example is the ubiquitin ligase Nedd4-2 [ 65 , 72 , 73 ] which promotes endocytic retrieval and subsequent proteasomal degradation of ENaC. Phosphorylation of Nedd4-2 at specific sites, e.g., by serum- and glucocorticoid-induced kinase 1 (SGK1) [ 18 , 25 , 42 ], reduces its ability to bind to PY-motifs in the cytosolic C-termini of the channel resulting in reduced channel retrieval.…”
How phosphorylation of the epithelial sodium channel (ENaC) contributes to its regulation is incompletely understood. Previously, we demonstrated that in outside-out patches ENaC activation by serum- and glucocorticoid-inducible kinase isoform 1 (SGK1) was abolished by mutating a serine residue in a putative SGK1 consensus motif RXRXX(S/T) in the channel’s α-subunit (S621 in rat). Interestingly, this serine residue is followed by a highly conserved proline residue rather than by a hydrophobic amino acid thought to be required for a functional SGK1 consensus motif according to invitro data. This suggests that this serine residue is a potential phosphorylation site for the dual-specificity tyrosine phosphorylated and regulated kinase 2 (DYRK2), a prototypical proline-directed kinase. Its phosphorylation may prime a highly conserved preceding serine residue (S617 in rat) to be phosphorylated by glycogen synthase kinase 3 β (GSK3β). Therefore, we investigated the effect of DYRK2 on ENaC activity in outside-out patches of Xenopus laevis oocytes heterologously expressing rat ENaC. DYRK2 included in the pipette solution significantly increased ENaC activity. In contrast, GSK3β had an inhibitory effect. Replacing S621 in αENaC with alanine (S621A) abolished the effects of both kinases. A S617A mutation reduced the inhibitory effect of GKS3β but did not prevent ENaC activation by DYRK2. Our findings suggest that phosphorylation of S621 activates ENaC and primes S617 for subsequent phosphorylation by GSK3β resulting in channel inhibition. In proof-of-concept experiments, we demonstrated that DYRK2 can also stimulate ENaC currents in microdissected mouse distal nephron, whereas GSK3β inhibits the currents.
“…1 a). In different epithelial tissues, ENaC activity is under the tight control by a range of hormones and local mediators [ 37 , 49 , 65 ]. Fig.…”
Section: Introductionmentioning
confidence: 99%
“…At the cellular level, ENaC regulation involves a complex interplay of extracellular factors and intracellular signal transduction pathways including a number of protein kinases [ 3 , 37 , 65 ]. Kinases are important regulators of a wide range of cellular processes and consequently may modify ENaC function by acting at many different levels.…”
Section: Introductionmentioning
confidence: 99%
“…Effects of kinases on ENaC include the phosphorylation and modification of regulatory proteins associated with ENaC or the phosphorylation of the channel itself with modulatory effects on its interaction with regulatory proteins. A prominent example is the ubiquitin ligase Nedd4-2 [ 65 , 72 , 73 ] which promotes endocytic retrieval and subsequent proteasomal degradation of ENaC. Phosphorylation of Nedd4-2 at specific sites, e.g., by serum- and glucocorticoid-induced kinase 1 (SGK1) [ 18 , 25 , 42 ], reduces its ability to bind to PY-motifs in the cytosolic C-termini of the channel resulting in reduced channel retrieval.…”
How phosphorylation of the epithelial sodium channel (ENaC) contributes to its regulation is incompletely understood. Previously, we demonstrated that in outside-out patches ENaC activation by serum- and glucocorticoid-inducible kinase isoform 1 (SGK1) was abolished by mutating a serine residue in a putative SGK1 consensus motif RXRXX(S/T) in the channel’s α-subunit (S621 in rat). Interestingly, this serine residue is followed by a highly conserved proline residue rather than by a hydrophobic amino acid thought to be required for a functional SGK1 consensus motif according to invitro data. This suggests that this serine residue is a potential phosphorylation site for the dual-specificity tyrosine phosphorylated and regulated kinase 2 (DYRK2), a prototypical proline-directed kinase. Its phosphorylation may prime a highly conserved preceding serine residue (S617 in rat) to be phosphorylated by glycogen synthase kinase 3 β (GSK3β). Therefore, we investigated the effect of DYRK2 on ENaC activity in outside-out patches of Xenopus laevis oocytes heterologously expressing rat ENaC. DYRK2 included in the pipette solution significantly increased ENaC activity. In contrast, GSK3β had an inhibitory effect. Replacing S621 in αENaC with alanine (S621A) abolished the effects of both kinases. A S617A mutation reduced the inhibitory effect of GKS3β but did not prevent ENaC activation by DYRK2. Our findings suggest that phosphorylation of S621 activates ENaC and primes S617 for subsequent phosphorylation by GSK3β resulting in channel inhibition. In proof-of-concept experiments, we demonstrated that DYRK2 can also stimulate ENaC currents in microdissected mouse distal nephron, whereas GSK3β inhibits the currents.
“…S G K 1 -d e p e n d e n t e x p r e s s i o n o f C T G F a s s i s t s i n mineralocorticoid-stimulated cardiac fibrosis and skin aging (13,42). Overexpression of SGK1 has been observed in various fibrotic tissues, including pulmonary fibrosis, diabetic renal fibrosis, liver cirrhosis, and hypertensive cardiac fibrosis, and SGK1 inhibitors can significantly reduce the degree of fibrosis in various tissues, suggesting that SGK1 has an important role in the development of inflammation and fibrosis (43)(44)(45)(46)(47)(48).…”
Patients with severe chronic graft-versus-host disease (cGVHD) always experience debilitating tissue injury and have poorer quality of life and shorter survival time. The early stage of cGVHD is characterized by inflammation, which eventually leads to extensive tissue fibrosis in various organs, such as skin and lung, eventually inducing scleroderma-like changes and bronchiolitis obliterans syndrome. Here we review the functions of serum/glucocorticoid regulated kinase 1 (SGK1), a hub molecule in multiple signal transduction pathways and cell phosphorylation cascades, which has important roles in cell proliferation and ion channel regulation, and its relevance in cGVHD. SGK1 phosphorylates the ubiquitin ligase, NEDD4, and induces Th cells to differentiate into Th17 and Th2 phenotypes, hinders Treg development, and promotes inflammatory fibrosis. Phosphorylation of NEDD4 by SGK1 also leads to up-regulation of the transcription factor SMAD2/3, thereby amplifying the fibrosis-promoting effect of TGF-β. SGK1 also up-regulates the inflammatory transcription factor, nuclear factor-κB (NF-κB), which in turn stimulates the expression of multiple inflammatory mediators, including connective tissue growth factor. Overexpression of SGK1 has been observed in various fibrotic diseases, including pulmonary fibrosis, diabetic renal fibrosis, liver cirrhosis, hypertensive cardiac fibrosis, peritoneal fibrosis, and Crohn’s disease. In addition, SGK1 inhibitors can attenuate, or even reverse, the effect of fibrosis, and may be used to treat inflammatory conditions and/or fibrotic diseases, such as cGVHD, in the future.
“…Furthermore, ENaC mediates sodium absorption across the epithelia of the colon, sweat ducts, reproductive tract, and lung. ENaC is a constitutively active ion channel and its expression, membrane abundance, and open probability (P O ) are controlled by multiple intracellular and extracellular mediators and mechanisms [ 9 ]. Aberrant ENaC regulation is associated with severe human diseases, including hypertension, cystic fibrosis, pulmonary edema, pseudohypoaldosteronism type 1, and nephrotic syndrome [ 9 ].…”
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