Serum and glucocorticoid-inducible kinase1 increases plasma membrane wt-CFTR in human airway epithelial cells by inhibiting its endocytic retrieval

Increased intracellular Cl- concentration mediates neutrophil extracellular traps formation in atherosclerotic cardiovascular diseases

Neutrophil extracellular traps (NETs) play crucial roles in atherosclerotic cardiovascular diseases such as acute coronary syndrome (ACS). Our preliminary study shows that oxidized low-density lipoprotein (oxLDL)-induced NET formation is accompanied by an elevated intracellular Cl- concentration ([Cl-]i) and reduced cystic fibrosis transmembrane conductance regulator (CFTR) expression in freshly isolated human blood neutrophils. Herein we investigated whether and how [Cl-]i regulated NET formation in vitro and in vivo. We showed that neutrophil [Cl-]i and NET levels were increased in global CFTR null (Cftr-/-) mice in the resting state, which was mimicked by intravenous injection of the CFTR inhibitor, CFTRinh-172, in wild-type mice. OxLDL-induced NET formation was aggravated by defective CFTR function. Clamping [Cl-]i at high levels directly triggered NET formation. Furthermore, we demonstrated that increased [Cl-]i by CFTRinh-172 or CFTR knockout increased the phosphorylation of serum- and glucocorticoid-inducible protein kinase 1 (SGK1) and generation of intracellular reactive oxygen species in neutrophils, and promoted oxLDL-induced NET formation and pro-inflammatory cytokine production. Consistently, peripheral blood samples obtained from atherosclerotic ApoE-/- mice or stable angina (SA) and ST-elevation ACS (STE-ACS) patients exhibited increased neutrophil [Cl-]i and SGK1 activity, decreased CFTR expression, and elevated NET levels. VX-661, a CFTR corrector, reduced the NET formation in the peripheral blood sample obtained from oxLDL-injected mice, ApoE-/- atherosclerotic mice or patients with STE-ACS by lowering neutrophil [Cl-]i. These results demonstrate that elevated neutrophil [Cl-]i during the development of atherosclerosis and ACS contributes to increased NET formation via Cl--sensitive SGK1 signaling, suggesting that defective CFTR function might be a novel therapeutic target for atherosclerotic cardiovascular diseases.

Platelet CFTR inhibition enhances arterial thrombosis via increasing intracellular Cl- concentration and activation of SGK1 signaling pathway

Platelet hyperactivity is essential for thrombus formation in coronary artery diseases (CAD). Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) in patients with cystic fibrosis elevates intracellular Cl- levels ([Cl-]i) and enhanced platelet hyperactivity. In this study, we explored whether alteration of [Cl-]i has a pathological role in regulating platelet hyperactivity and arterial thrombosis formation. CFTR expression was significantly decreased, while [Cl-]i was increased in platelets from CAD patients. In a FeCl3-induced mouse mesenteric arteriole thrombosis model, platelet-specific Cftr-knockout and/or pre-administration of ion channel inhibitor CFTRinh-172 increased platelet [Cl-]i, which accelerated thrombus formation, enhanced platelet aggregation and ATP release, and increased P2Y12 and PAR4 expression in platelets. Conversely, Cftr-overexpressing platelets resulted in subnormal [Cl-]i, thereby decreasing thrombosis formation. Our results showed that clamping [Cl-]i at high levels or Cftr deficiency-induced [Cl-]i increasement dramatically augmented phosphorylation (Ser422) of serum and glucocorticoid-regulated kinase (SGK1), subsequently upregulated P2Y12 and PAR4 expression via NF-κB signaling. Constitutively active mutant S422D SGK1 markedly increased P2Y12 and PAR4 expression. The specific SGK1 inhibitor GSK-650394 decreased platelet aggregation in wildtype and platelet-specific Cftr knockout mice, and platelet SGK1 phosphorylation was observed in line with increased [Cl-]i and decreased CFTR expression in CAD patients. Co-transfection of S422D SGK1 and adenovirus-induced CFTR overexpression in MEG-01 cells restored platelet activation signaling cascade. Our results suggest that [Cl-]i is a novel positive regulator of platelet activation and arterial thrombus formation via the activation of a [Cl-]i-sensitive SGK1 signaling pathway. Therefore, [Cl-]i in platelets is a novel potential biomarker for platelet hyperactivity, and CFTR may be a potential therapeutic target for platelet activation in CAD.

Loss of Serum Glucocorticoid-Inducible Kinase 1 SGK1 Worsens Malabsorption and Diarrhea in Microvillus Inclusion Disease (MVID)

Microvillus inclusion disease (MVID), a lethal congenital diarrheal disease, results from loss of function mutations in the apical actin motor myosin VB (MYO5B). How loss of MYO5B leads to both malabsorption and fluid secretion is not well understood. Serum glucocorticoid-inducible kinase 1 (SGK1) regulates intestinal carbohydrate and ion transporters including cystic fibrosis transmembrane conductance regulator (CFTR). We hypothesized that loss of SGK1 could reduce CFTR fluid secretion and MVID diarrhea. Using CRISPR-Cas9 approaches, we generated R26^CreER;MYO5B^f/f conditional single knockout (cMYO5BKO) and R26^CreER;MYO5B^f/f;SGK1^f/f double knockout (cSGK1/MYO5B-DKO) mice. Tamoxifen-treated cMYO5BKO mice resulted in characteristic features of human MVID including severe diarrhea, microvillus inclusions (MIs) in enterocytes, defective apical traffic, and depolarization of transporters. However, apical CFTR distribution was preserved in crypts and depolarized in villus enterocytes, and CFTR high expresser (CHE) cells were observed. cMYO5BKO mice displayed increased phosphorylation of SGK1, PDK1, and the PDK1 target PKCι in the intestine. Surprisingly, tamoxifen-treated cSGK1/MYO5B-DKO mice displayed more severe diarrhea than cMYO5BKO, with preservation of apical CFTR and CHE cells, greater fecal glucose and reduced SGLT1 and GLUT2 in the intestine. We conclude that loss of SGK1 worsens carbohydrate malabsorption and diarrhea in MVID.

SGK1 is essential for meiotic resumption in mammalian oocytes

In mammalian females, oocytes are stored in the ovary and meiosis is arrested at the diplotene stage of prophase I. When females reach puberty oocytes are selectively recruited in cycles to grow, overcome the meiotic arrest, complete the first meiotic division and become mature (ready for fertilization). At a molecular level, the master regulator of prophase I arrest and meiotic resumption is the maturation-promoting factor (MPF) complex, formed by the active form of cyclin dependent kinase 1 (CDK1) and Cyclin B1. However, we still do not have complete information regarding the factors implicated in MPF activation. In this study we document that out of three mammalian serum-glucocorticoid kinase proteins (SGK1, SGK2, SGK3), mouse oocytes express only SGK1 with a phosphorylated (active) form dominantly localized in the nucleoplasm. Further, suppression of SGK1 activity in oocytes results in decreased CDK1 activation via the phosphatase cell division cycle 25B (CDC25B), consequently delaying or inhibiting nuclear envelope breakdown. Expression of exogenous constitutively active CDK1 can rescue the phenotype induced by SGK1 inhibition. These findings bring new insights into the molecular pathways acting upstream of MPF and a better understanding of meiotic resumption control by presenting a new key player SGK1 in mammalian oocytes.

Glucocorticoids and serum- and glucocorticoid-inducible kinase 1 are potent regulators of CFTR in the native intestine: implications for stress-induced diarrhea

Nongenomic glucocorticoid (GC) and serum- and glucocorticoid-inducible kinase 1 (SGK1) signaling regulate ion transport, but CFTR has not been investigated in the intestine. We examined GC, SGK1, and phosphatidylinositol 3-kinase (PI3K) kinase signaling of CFTR ion transport in native intestine and the role of GCs on mRNA, protein, surface expression, and cyclic guanosine monophosphate (cGMP)-elicited diarrhea. Rats were treated with dexamethasone (DEXA; 2 mg/kg ip) or DMSO for 1, 4, and 24 h. Cyclic adenosine monophosphate (cAMP)-activated ion transport was examined in the presence or absence of SGK1 and PI3K inhibitors. Phosphorylation of SGK1, phosphoinositide-dependent kinase 1, and Akt kinases was confirmed by immunoblots using phosphor-specific antibodies. Tissue lysates were analyzed by mass spectrometry. CFTR and SGK1 mRNA were measured by quantitative PCR. Changes in total and surface CFTR protein were determined. The role of GC in cGMP-activated CFTR ion transport was examined. GC synergistically increased CFTR ion transport by SGK1 and PI3K signaling and increased CFTR protein without altering SGK1 or CFTR mRNA. GC induced highest levels of CFTR protein at 4 h that were associated with marked increase in surface CFTR, phosphorylation of the ubiquitin ligase neural precursor cell expressed developmentally downregulated 4-like (Nedd4-2), and 14-3-3ε, supporting their roles in surface retention and stability. Coimmunoprecipitation of CFTR, Nedd4-2, and 14-3-3ε indicated that assembly of this complex is a likely effector of the SGK and Akt pathways. Mass spectrometry identified phosphorylated peptides in relevant proteins. GC-SGK1 potently regulates CFTR in the intestine and is implicated in diarrheal disease.NEW & NOTEWORTHY This is the first study to examine the mechanisms of glucocorticoid, serum- and glucocorticoid-inducible kinase 1, and nongenomic kinase signaling of CFTR in the native intestine. We identified unique and druggable intestine-specific factors of the pathway that are targets for treating stress-induced diarrhea.

Signaling Cascade Involved in Rapid Stimulation of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) by Dexamethasone

Impairment of mucociliary clearance with reduced airway fluid secretion leads to chronically inflamed airways. Cystic fibrosis transmembrane conductance regulator (CFTR) is crucially involved in airway fluid secretion and dexamethasone (dexa) has previously been shown to elevate CFTR activity in airway epithelial cells. However, the pathway by which dexa increases CFTR activity is largely unknown. We aimed to determine whether the increase of CFTR activity by dexa is achieved by non-genomic signaling and hypothesized that the phosphoinositide 3-kinase (PI3K) pathway is involved in CFTR stimulation. Primary rat airway epithelial cells and human bronchial submucosal gland-derived Calu-3 cells were analyzed in Ussing chambers and kinase activation was determined by Western blots. Results demonstrated a critical involvement of PI3K and protein kinase B (AKT) signaling in the dexa-induced increase of CFTR activity, while serum and glucocorticoid dependent kinase 1 (SGK1) activity was not essential. We further demonstrated a reduced neural precursor cell expressed, developmentally downregulated 4-like (NEDD4L) ubiquitin E3 ligase activity induced by dexa, possibly responsible for the elevated CFTR activity. Finally, increases of CFTR activity by dexa were demonstrated within 30 min accompanied by rapid activation of AKT. In conclusion, dexa induces a rapid stimulation of CFTR activity which depends on PI3K/AKT signaling in airway epithelial cells. Glucocorticoids might thus represent, in addition to their immunomodulatory actions, a therapeutic strategy to rapidly increase airway fluid secretion.

Effects of Pseudomonas aeruginosa on CFTR chloride secretion and the host immune response

In the healthy lung the opportunistic pathogen, Pseudomonas aeruginosa, is rapidly eliminated by mucociliary clearance, a process that is dependent on the activity of the CFTR anion channel that, in concert with a number of other transport proteins, regulates the volume and composition of the periciliary surface liquid. This fluid layer is essential to enable cilia to clear pathogens from the lungs. However, in cystic fibrosis (CF), mutations in the CFTR gene reduce Cl^- and [Formula: see text] secretion, thereby decreasing periciliary surface liquid volume and mucociliary clearance of bacteria. In CF this leads to persistent infection with the opportunistic pathogen, P. aeruginosa, which is the cause of reduced lung function and death in ~95% of CF patients. Others and we have conducted studies to elucidate the effects of P. aeruginosa on wild-type and Phe508del-CFTR Cl^- secretion as well as on the host immune response. These studies have demonstrated that Cif (CFTR inhibitory factor), a virulence factor secreted by P. aeruginosa, is associated with reduced lung function in CF and induces the ubiquitination and degradation of wt-CFTR as well as TAP1, which plays a key role in viral and bacterial antigen presentation. Cif also enhances the degradation of Phe508del-CFTR that has been rescued by ORKAMBI, a drug approved for CF patients homozygous for the Phe508del-CFTR mutation, thereby reducing drug efficacy. This review is based on the Hans Ussing Distinguished Lecture at the 2016 Experimental Biology Meeting given by the author.

Involvement of serum glucocorticoid-regulated kinase 1 in reproductive success

Reproductive processes, in particular events that concern pregnancy, are fine-tuned to produce offspring. Reproductive success is of prime importance for the survival of every species. The highly conserved and ubiquitously expressed serum glucocorticoid-regulated kinase 1 (SGK1) was first implicated in infertility as a regulator of a Na⁺ channel. In this review, we emphasize the prominent role of SGK1 during early pregnancy: 1) balancing uterine luminal fluid secretion and reabsorption to aid blastocyst adhesion and to import nutrients and energy; 2) transducing signals from the blastocyst to the receptive endometrium; 3) inducing multiple genes that are involved in uterine receptivity and trophoblast invasion; 4) regulating cell differentiation and antioxidant defenses at the fetomaternal interface; and 5) contributing to the proliferation and survival of decidual stromal cells. Accordingly, SGK1 coordinates many cellular processes that are crucial to reproductive activities. Aberrant expression or function of SGK1 results in implantation failure and early pregnancy loss. Further investigation of the molecular mechanisms of the function of SGK1 might provide novel diagnostic tools and interventions for reproductive complications.-Lou, Y., Hu, M., Mao, L., Zheng, Y., Jin, F. Involvement of serum glucocorticoid-regulated kinase 1 in reproductive success.

Evaluation of a systems biology approach to identify pharmacological correctors of the mutant CFTR chloride channel

BACKGROUND

Mistrafficking of CFTR protein caused by F508del, the most frequent mutation in cystic fibrosis (CF), can be corrected by cell incubation at low temperature, an effect that may be mediated by altered expression of proteostasis genes.

METHODS

To identify small molecules mimicking low temperature, we compared gene expression profiles of cells kept at 27°C with those previously generated from more than 1300 compounds. The resulting candidates were tested with a functional assay on a bronchial epithelial cell line.

RESULTS

We found that anti-inflammatory glucocorticoids, such as mometasone, budesonide, and fluticasone, increased mutant CFTR function. However, this activity was not confirmed in primary bronchial epithelial cells. Actually, glucocorticoids enhanced Na(+) absorption, an effect that could further impair mucociliary clearance in CF airways.

CONCLUSIONS

Our results suggest that rescue of F508del-CFTR by low temperature cannot be easily mimicked by small molecules and that compounds with closer transcriptional and functional effects need to be found.

Serum- and Glucocorticoid-Inducible Kinase-1 (SGK-1) Plays a Role in Membrane Trafficking in Caenorhabditis elegans

The mammalian serum- and glucocorticoid-inducible kinase SGK1 regulates the endocytosis of ion channels. Here we report that in C. elegans sgk-1 null mutants, GFP-tagged MIG-14/Wntless, the sorting receptor of Wnt, failed to localize to the basolateral membrane of intestinal cells; instead, it was mis-sorted to lysosomes. This effect can be explained in part by altered sphingolipid levels, because reducing glucosylceramide biosynthesis restored the localization of MIG-14::GFP. Membrane traffic was not perturbed in general, as no obvious morphological defects were detected for early endosomes, the Golgi apparatus, and the endoplasmic reticulum (ER) in sgk-1 null animals. The recycling of MIG-14/Wntless through the Golgi might be partially responsible for the observed phenotype because the subcellular distribution of two plasma membrane cargoes that do not recycle through the trans-Golgi network (TGN) was affected to a lesser degree. Consistently, knockdown of the ArfGEF gbf-1 altered the distribution of SGK-1 at the basolateral membrane of intestinal cells. In addition, we found that sgk-1(RNAi) induced unfolded protein response in the ER, suggesting at least an indirect role of SGK-1 early in the secretory pathway. We propose that SGK-1 function is required for lipid homeostasis and that it acts at different intracellular trafficking steps.

Glucocorticoids Distinctively Modulate the CFTR Channel with Possible Implications in Lung Development and Transition into Extrauterine Life

During fetal development, the lung is filled with fluid that is secreted by an active Cl- transport promoting lung growth. The basolateral Na⁺,K⁺,2Cl^- cotransporter (NKCC1) participates in Cl^- secretion. The apical Cl^- channels responsible for secretion are unknown but studies suggest an involvement of the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is developmentally regulated with a high expression in early fetal development and a decline in late gestation. Perinatal lung transition is triggered by hormones that stimulate alveolar Na⁺ channels resulting in fluid absorption. Little is known on how hormones affect pulmonary Cl^- channels. Since the rise of fetal cortisol levels correlates with the decrease in fetal CFTR expression, a causal relation may be assumed. The aim of this study was to analyze the influence of glucocorticoids on pulmonary Cl^- channels. Alveolar cells from fetal and adult rats, A549 cells, bronchial Calu-3 and 16HBE14o- cells, and primary rat airway cells were studied with real-time quantitative PCR and Ussing chambers. In fetal and adult alveolar cells, glucocorticoids strongly reduced Cftr expression and channel activity, which was prevented by mifepristone. In bronchial and primary airway cells CFTR mRNA expression was also reduced, whereas channel activity was increased which was prevented by LY-294002 in Calu-3 cells. Therefore, glucocorticoids strongly reduce CFTR expression while their effect on CFTR activity depends on the physiological function of the cells. Another apical Cl^- channel, anoctamin 1 showed a glucocorticoid-induced reduction of mRNA expression in alveolar cells and an increase in bronchial cells. Furthermore, voltage-gated chloride channel 5 and anoctamine 6 mRNA expression were increased in alveolar cells. NKCC1 expression was reduced by glucocorticoids in alveolar and bronchial cells alike. The results demonstrate that glucocorticoids differentially modulate pulmonary Cl^- channels and are likely causing the decline of CFTR during late gestation in preparation for perinatal lung transition.

Regulation of hormone-sensitive renal phosphate transport

Phosphate is essential for growth and maintenance of the skeleton and for generating high-energy phosphate compounds. Evolutionary adaptation to high dietary phosphorous in humans and other terrestrial vertebrates involves regulated mechanisms assuring the efficient renal elimination of excess phosphate. These mechanisms prominently include PTH, FGF23, and Vitamin D, which directly and indirectly regulate phosphate transport. Disordered phosphate homeostasis is associated with pathologies ranging from kidney stones to kidney failure. Chronic kidney disease results in hyperphosphatemia, an elevated calcium×phosphate product with considerable morbidity and mortality, mostly associated with adverse cardiovascular events. This chapter highlights recent findings and insights regarding the hormonal regulation of renal phosphate transport along with imbalances of phosphate balance due to acquired or inherited diseases states.

Serum- and glucocorticoid-induced protein kinase 1 (SGK1) increases the cystic fibrosis transmembrane conductance regulator (CFTR) in airway epithelial cells by phosphorylating Shank2E protein

The glucocorticoid dexamethasone increases cystic fibrosis transmembrane conductance regulator (CFTR) abundance in human airway epithelial cells by a mechanism that requires serum- and glucocorticoid-induced protein kinase 1 (SGK1) activity. The goal of this study was to determine whether SGK1 increases CFTR abundance by phosphorylating Shank2E, a PDZ domain protein that contains two SGK1 phosphorylation consensus sites. We found that SGK1 phosphorylates Shank2E as well as a peptide containing the first SGK1 consensus motif of Shank2E. The dexamethasone-induced increase in CFTR abundance was diminished by overexpression of a dominant-negative Shank2E in which the SGK1 phosphorylation sites had been mutated. siRNA-mediated reduction of Shank2E also reduced the dexamethasone-induced increase in CFTR abundance. Taken together, these data demonstrate that the glucocorticoid-induced increase in CFTR abundance requires phosphorylation of Shank2E at an SGK1 consensus site.