mTOR Regulates Mineralocorticoid Receptor Transcriptional Activity by ULK1-Dependent and -Independent Mechanisms.

The mineralocorticoid receptor (MR) is a transcription factor for genes mediating diverse, cell-specific functions, including trophic effects as well as promoting fluid/electrolyte homeostasis. It was reported that in intercalated cells, phosphorylation of the MR at serine 843 (S843) by Unc-51-like kinase (ULK1) inhibits MR activation and that phosphorylation of ULK1 by mechanistic target of rapamycin (mTOR) inactivates ULK1, and thereby prevents MR inactivation. We extended these findings with studies in M1 mouse cortical collecting duct cells stably expressing the rat MR and a reporter gene.

Serum- and glucocorticoid-induced kinase drives hepatic insulin resistance by directly inhibiting AMP-activated protein kinase.

A hallmark of type 2 diabetes (T2D) is hepatic resistance to insulin's glucose-lowering effects. The serum- and glucocorticoid-regulated family of protein kinases (SGK) is activated downstream of mechanistic target of rapamycin complex 2 (mTORC2) in response to insulin in parallel to AKT. Surprisingly, despite an identical substrate recognition motif to AKT, which drives insulin sensitivity, pathological accumulation of SGK1 drives insulin resistance.

Behavioral effects of SGK1 knockout in VTA and dopamine neurons.

Drugs of abuse cause significant neuroadaptations within the ventral tegmental area (VTA), with alterations in gene expression tied to changes in reward behavior. Serum- and glucocorticoid-inducible kinase 1 (SGK1) transcription, catalytic activity, and phosphorylation are upregulated in the VTA by chronic cocaine or morphine treatment, positioning SGK1 as a critical mediator of reward behavior. Using transgenic mouse models, we investigated the effect of SGK1 knockout in the VTA and in dopamine (DA) neurons to evaluate the necessity of protein expression for natural and drug reward behaviors.

Mitochondrial Permeability Uncouples Elevated Autophagy and Lifespan Extension.

Autophagy is required in diverse paradigms of lifespan extension, leading to the prevailing notion that autophagy is beneficial for longevity. However, why autophagy is harmful in certain contexts remains unexplained. Here, we show that mitochondrial permeability defines the impact of autophagy on aging.

SGK1/FOXO3 Signaling in Hypothalamic POMC Neurons Mediates Glucocorticoid-Increased Adiposity.

Although the central nervous system has been implicated in glucocorticoid-induced gain of fat mass, the underlying mechanisms are poorly understood. The aim of this study was to investigate the possible involvement of hypothalamic serum- and glucocorticoid-regulated kinase 1 (SGK1) in glucocorticoid-increased adiposity. It is well known that SGK1 expression is induced by acute glucocorticoid treatment, but it is interesting that we found its expression to be decreased in the arcuate nucleus of the hypothalamus, including proopiomelanocortin (POMC) neurons, following chronic dexamethasone (Dex) treatment.

Induction of the cell survival kinase Sgk1: A possible novel mechanism for α-phenyl-N-tert-butyl nitrone in experimental stroke.

Nitrones (e.g. α-phenyl-N-tert-butyl nitrone; PBN) are cerebroprotective in experimental stroke.

Aldosterone-Sensing Neurons in the NTS Exhibit State-Dependent Pacemaker Activity and Drive Sodium Appetite via Synergy with Angiotensin II Signaling.

Sodium deficiency increases angiotensin II (ATII) and aldosterone, which synergistically stimulate sodium retention and consumption. Recently, ATII-responsive neurons in the subfornical organ (SFO) and aldosterone-sensitive neurons in the nucleus of the solitary tract (NTS neurons) were shown to drive sodium appetite. Here we investigate the basis for NTS neuron activation, identify the circuit by which NTS neurons drive appetite, and uncover an interaction between the NTS circuit and ATII signaling.

Interaction of the Mineralocorticoid Receptor With RACK1 and Its Role in Aldosterone Signaling.

The mineralocorticoid receptor (MR) is a member of the steroid-thyroid hormone receptor superfamily of ligand-dependent transcription factors with diverse functions including the biological actions of aldosterone. Identification of the various transcriptional coregulators of MR is essential for understanding the complexity of MR signaling pathways under physiological and pathological conditions. We used a yeast two-hybrid system to find proteins that interact with a full-length MR and found, among other proteins, that MR interacted specifically with receptor for activated C kinase 1 (RACK1), a scaffolding protein.

Mineralocorticoid Receptor Deficiency in Macrophages Inhibits Neointimal Hyperplasia and Suppresses Macrophage Inflammation Through SGK1-AP1/NF-κB Pathways.

Objective: Restenosis after percutaneous coronary intervention remains to be a serious medical problem. Although mineralocorticoid receptor (MR) has been implicated as a potential target for treating restenosis, the cellular and molecular mechanisms are largely unknown. This study aims to explore the functions of macrophage MR in neointimal hyperplasia and to delineate the molecular mechanisms.

Hepatic serum- and glucocorticoid-regulated protein kinase 1 (SGK1) regulates insulin sensitivity in mice via extracellular-signal-regulated kinase 1/2 (ERK1/2).

Insulin resistance is a major hallmark of metabolic syndromes, including Type 2 diabetes. Although numerous functions of SGK1 (serum- and glucocorticoid-regulated kinase 1) have been identified, a direct effect of SGK1 on insulin sensitivity has not been previously reported. In the present study, we generated liver-specific SGK1-knockout mice and found that these mice developed glucose intolerance and insulin resistance.

The AGC kinase SGK1 regulates TH1 and TH2 differentiation downstream of the mTORC2 complex.

SGK1 is an AGC kinase that regulates the expression of membrane sodium channels in renal tubular cells in a manner dependent on the metabolic checkpoint kinase complex mTORC2. We hypothesized that SGK1 might represent an additional mTORC2-dependent regulator of the differentiation and function of T cells. Here we found that after activation by mTORC2, SGK1 promoted T helper type 2 (TH2) differentiation by negatively regulating degradation of the transcription factor JunB mediated by the E3 ligase Nedd4-2.

Functionally distinct PI 3-kinase pathways regulate myelination in the peripheral nervous system.

The PI 3-kinase (PI 3-K) signaling pathway is essential for Schwann cell myelination. Here we have characterized PI 3-K effectors activated during myelination by probing myelinating cultures and developing nerves with an antibody that recognizes phosphorylated substrates for this pathway. We identified a discrete number of phospho-proteins including the S6 ribosomal protein (S6rp), which is down-regulated at the onset of myelination, and N-myc downstream-regulated gene-1 (NDRG1), which is up-regulated strikingly with myelination.

Inducible kidney-specific Sgk1 knockout mice show a salt-losing phenotype.

The expression of the serum- and glucocorticoid-regulated kinase 1 (Sgk1) is induced by mineralocorticoids and, in turn, upregulates the renal epithelial Na(+) channel (ENaC). Total inactivation of Sgk1 has been associated with transient urinary Na(+) wasting with a low-Na(+) diet, while the aldosterone-mediated ENaC channel activation was unchanged in the collecting duct. Since Sgk1 is ubiquitously expressed, we aimed to study the role of renal Sgk1 and generated an inducible kidney-specific knockout (KO) mouse.

Serum- and glucocorticoid-induced kinase 3 in recycling endosomes mediates acute activation of Na+/H+ exchanger NHE3 by glucocorticoids.

Na(+)/H(+) exchanger 3 (NHE3) is the major Na(+) transporter in the intestine. Serum- and glucocorticoid-induced kinase (SGK) 1 interacts with NHE regulatory factor 2 (NHERF2) and mediates activation of NHE3 by dexamethasone (Dex) in cultured epithelial cells. In this study, we compared short-term regulation of NHE3 by Dex in SGK1-null and NHERF2-null mice.

Molecular signature of mineralocorticoid receptor signaling in cardiomyocytes: from cultured cells to mouse heart.

Excess mineralocorticoid signaling is deleterious for cardiovascular functions, as demonstrated by the beneficial effects of mineralocorticoid receptor (MR) antagonism on morbidity and mortality in patients with heart failure. However, the understanding of signaling pathways after MR activation in the heart remains limited. We performed transcriptomic analyses in the heart of double-transgenic mice with conditional, cardiomyocyte-specific, overexpression of the MR (MRcardio mice) or the glucocorticoid receptor (GR; GRcardio mice).

Serum- and glucocorticoid-inducible kinase 1 (SGK1) regulates adipocyte differentiation via forkhead box O1.

The serum and glucocorticoid-inducible kinase 1 (SGK1) is an inducible kinase the physiological function of which has been characterized primarily in the kidney. Here we show that SGK1 is expressed in white adipose tissue and that its levels are induced in the conversion of preadipocytes into fat cells. Adipocyte differentiation is significantly diminished via small interfering RNA inhibition of endogenous SGK1 expression, whereas ectopic expression of SGK1 in mesenchymal precursor cells promotes adipogenesis.

Regulation of epithelial sodium transport by promyelocytic leukemia zinc finger protein.

Aldosterone is the principal regulator of Na homeostasis, and thereby blood pressure. One of the main targets of aldosterone is the epithelial Na channel (ENaC) located in the apical membrane of target cells. Previous studies identified several genes involved in the regulation of ENaC such as SGK1; however, SGK1 knockout mice have only a mild salt-losing phenotype, indicating that further genes must be involved in the action of aldosterone.

Epithelial Na+ channel activation and processing in mice lacking SGK1.

Amiloride-sensitive Na(+) channel activity was examined in the cortical collecting ducts of a mouse line (SGK1(-/-)) deficient in the serum- and glucocorticoid-dependent protein kinase SGK1. This activity was correlated with changes in renal Na handling and in the maturation of epithelial Na(+) channel (ENaC) protein. Neither SGK1(-/-) mice nor paired SGK1(+/+) animals expressed detectable channel activity, measured as amiloride-sensitive whole-cell current (I(Na)), under control conditions with standard chow.

Melanophilin, a novel aldosterone-induced gene in mouse cortical collecting duct cells.

The molecular mechanisms of aldosterone-regulated Na+ transport are not entirely clear. The goal of this study was to identify aldosterone-induced genes potentially involved in the trafficking of the epithelial Na+ channel (ENaC). We report that the transcript levels of melanophilin (MLPH), a protein involved in vesicular trafficking in melanocytes, are rapidly increased by aldosterone in cortical collecting duct (CCD) cells.

Steroid-mediated regulation of the epithelial sodium channel subunits in mammary epithelial cells.

The epithelial sodium channel (ENaC) is a key mediator of sodium transport in epithelia; however, little is known about ENaC expression in mammary epithelia. Using real-time PCR, we demonstrated the expression of the ENaC subunit mRNAs in mouse and human mammary cell lines and in vivo mouse mammary tissue. We determined the effects of glucocorticoids, progesterone, and prolactin on ENaC expression in four mammary cell lines.

Early aldosterone-regulated genes in cardiomyocytes: clues to cardiac remodeling?

Recent clinical studies demonstrated beneficial effects of mineralocorticoid receptor (MR) antagonists in patients with heart failure and other cardiovascular diseases. However, the underlying molecular mechanisms are poorly understood, and the genes that mediate direct effects of aldosterone in the cardiovascular system are yet to be identified. The goal of this study was to identify genes that are directly regulated by aldosterone in cardiomyocytes and thus potentially play a role in initiating MR-mediated effects in the heart.

Subcellular location of serum- and glucocorticoid-induced kinase-1 in renal and mammary epithelial cells.

Serum- and glucocorticoid-induced kinase-1 (SGK1) is involved in aldosterone-induced Na(+) reabsorption by increasing epithelial Na(+) channel (ENaC) activity in cortical collecting duct (CCD) cells, but its exact mechanisms of action are unknown. Although several potential targets such as Nedd4-2 have been described in expression systems, endogenous substrates mediating SGK1's physiological effects remain to be identified. In addition, subcellular localization studies of SGK1 have provided controversial results.

Novel mouse strain with Cre recombinase in 11beta-hydroxysteroid dehydrogenase-2-expressing cells.

Here we describe the generation and characterization of a mouse strain that expresses an improved Cre (iCre) recombinase (48) under the control of the endogenous 11beta-hydroxysteroid dehydrogenase type 2 (11HSD2) promoter. Progeny of 11HSD2/iCre and ROSA26 reporter mice were used to determine the pattern of iCre expression by measuring the activity of the LacZ gene product beta-galactosidase in a panel of tissues. On Cre recombinase activity, intense beta-galactosidase activity (X-gal staining) was observed in the classic mineralocorticoid target segments of the kidney, as well as in the colon, and both female and male reproductive organs.

The kidney-specific WNK1 isoform is induced by aldosterone and stimulates epithelial sodium channel-mediated Na+ transport.

WNK1 belongs to a unique family of Ser/Thr kinases that have been implicated in the control of blood pressure. Intronic deletions in the WNK1 gene result in its overexpression and lead to pseudohypoaldosteronism type II, a disease with salt-sensitive hypertension and hyperkalemia. How overexpression of WNK1 leads to Na(+) retention and hypertension is not entirely clear.

Gene regulation of ENaC subunits by serum- and glucocorticoid-inducible kinase-1.

Aldosterone is a key regulator of epithelial Na+ channels (ENaC) in renal cortical collecting ducts (CCD). The goal of this study was to examine whether serum- and glucocorticoid-inducible kinase-1 (SGK1), an aldosterone-induced gene, is vital to the delayed effect of aldosterone by increasing the gene expression of ENaC subunits. To test this hypothesis, we compared the levels of ENaC mRNA in mouse CCD cells that stably express either full-length (FL)-SGK1 or a kinase-dead dominant negative (K127M)-SGK1.