mTORC1-independent Raptor prevents hepatic steatosis by stabilizing PHLPP2

Impact of visceral adipose tissue on longevity and metabolic health: a comparative study of gene expression in perirenal and epididymal fat of Ames dwarf mice

Emerging research underscores the pivotal role of adipose tissue in regulating systemic aging processes, particularly when viewed through the lens of the endocrine hypotheses of aging. This study delves into the unique adipose characteristics in an important animal model of aging - the long-lived Ames dwarf (df/df) mice. Characterized by a Prop1df gene mutation, these mice exhibit a deficiency in growth hormone (GH), prolactin, and TSH, alongside extremely low circulating IGF-1 levels. Intriguingly, while surgical removal of visceral fat (VFR) enhances insulin sensitivity in normal mice, it paradoxically increases insulin resistance in Ames dwarfs. This suggests an altered profile of factors produced in visceral fat in the absence of GH, indicating a unique interplay between adipose tissue function and hormonal influences in these models. Our aim was to analyze the gene expression related to lipid and glucose metabolism, insulin pathways, inflammation, thermoregulation, mitochondrial biogenesis, and epigenetic regulation in the visceral (perirenal and epididymal) adipose tissue of Ames dwarf and normal mice. Our findings reveal an upregulation in the expression of key genes such as Lpl, Adrβ3, Rstn, Foxo1, Foxo3a, Irs1, Cfd, Aldh2, Il6, Tnfα, Pgc1α, Ucp2, and Ezh2 in perirenal and Akt1, Foxo3a, PI3k, Ir, Acly, Il6, Ring1a, and Ring 1b in epididymal fat in df/df mice. These results suggest that the longevity phenotype in Ames dwarfs, which is determined by peripubertal GH/IGF-1 levels, may also involve epigenetic reprogramming of adipose tissue influenced by hormonal changes. The increased expression of genes involved in metabolic regulation, tumor suppression, mitochondrial biogenesis, and insulin pathways in Ames dwarf mice highlights potentially beneficial aspects of this model, opening new avenues for understanding the molecular underpinnings of longevity and aging.

A multiprotein signaling complex sustains AKT and mTOR/S6K activity necessary for the survival of cancer cells undergoing stress

Cancer cells encounter stresses during tumor progression and metastatic spread, however, how they survive these challenges is not fully understood. We now identify a mechanism for cancer cell survival through the discovery of a multiprotein signaling complex that includes the GTPase Cdc42, the Cdc42 GEF/effector protein Dock7, AKT, mTOR and the mTORC1 regulatory partners TSC1, TSC2, and Rheb. This pro-survival signaling complex sustains the activated state of AKT by preventing its dephosphorylation at Ser473 during serum starvation, resulting in a low but critical activation of a Raptor-independent mTOR/S6K activity. We demonstrate that the Dock7 DHR1 domain, previously of unknown function, is responsible for preserving AKT phosphorylation through an interaction requiring its C2-like motif. Collectively, these findings help address long-standing questions of how Cdc42 signals mTOR activation by elucidating the unique functions of its signaling partner Dock7 as an AKT regulator necessary for resistance to anoikis and apoptosis in cancer cells.

Changes in Cells Associated with Insulin Resistance

Insulin is a polypeptide hormone synthesized and secreted by pancreatic β-cells. It plays an important role as a metabolic hormone. Insulin influences the metabolism of glucose, regulating plasma glucose levels and stimulating glucose storage in organs such as the liver, muscles and adipose tissue. It is involved in fat metabolism, increasing the storage of triglycerides and decreasing lipolysis. Ketone body metabolism also depends on insulin action, as insulin reduces ketone body concentrations and influences protein metabolism. It increases nitrogen retention, facilitates the transport of amino acids into cells and increases the synthesis of proteins. Insulin also inhibits protein breakdown and is involved in cellular growth and proliferation. On the other hand, defects in the intracellular signaling pathways of insulin may cause several disturbances in human metabolism, resulting in several chronic diseases. Insulin resistance, also known as impaired insulin sensitivity, is due to the decreased reaction of insulin signaling for glucose levels, seen when glucose use in response to an adequate concentration of insulin is impaired. Insulin resistance may cause, for example, increased plasma insulin levels. That state, called hyperinsulinemia, impairs metabolic processes and is observed in patients with type 2 diabetes mellitus and obesity. Hyperinsulinemia may increase the risk of initiation, progression and metastasis of several cancers and may cause poor cancer outcomes. Insulin resistance is a health problem worldwide; therefore, mechanisms of insulin resistance, causes and types of insulin resistance and strategies against insulin resistance are described in this review. Attention is also paid to factors that are associated with the development of insulin resistance, the main and characteristic symptoms of particular syndromes, plus other aspects of severe insulin resistance. This review mainly focuses on the description and analysis of changes in cells due to insulin resistance.

Iron drives anabolic metabolism through active histone demethylation and mTORC1

All eukaryotic cells require a minimal iron threshold to sustain anabolic metabolism. However, the mechanisms by which cells sense iron to regulate anabolic processes are unclear. Here we report a previously undescribed eukaryotic pathway for iron sensing in which molecular iron is required to sustain active histone demethylation and maintain the expression of critical components of the pro-anabolic mTORC1 pathway. Specifically, we identify the iron-binding histone-demethylase KDM3B as an intrinsic iron sensor that regulates mTORC1 activity by demethylating H3K9me2 at enhancers of a high-affinity leucine transporter, LAT3, and RPTOR. By directly suppressing leucine availability and RAPTOR levels, iron deficiency supersedes other nutrient inputs into mTORC1. This process occurs in vivo and is not an indirect effect by canonical iron-utilizing pathways. Because ancestral eukaryotes share homologues of KDMs and mTORC1 core components, this pathway probably pre-dated the emergence of the other kingdom-specific nutrient sensors for mTORC1.

Regulation of mTOR Signaling: Emerging Role of Cyclic Nucleotide-Dependent Protein Kinases and Implications for Cardiometabolic Disease

The mechanistic target of rapamycin (mTOR) kinase is a central regulator of cell growth and metabolism. It is the catalytic subunit of two distinct large protein complexes, mTOR complex 1 (mTORC1) and mTORC2. mTOR activity is subjected to tight regulation in response to external nutrition and growth factor stimulation. As an important mechanism of signaling transduction, the 'second messenger' cyclic nucleotides including cAMP and cGMP and their associated cyclic nucleotide-dependent kinases, including protein kinase A (PKA) and protein kinase G (PKG), play essential roles in mediating the intracellular action of a variety of hormones and neurotransmitters. They have also emerged as important regulators of mTOR signaling in various physiological and disease conditions. However, the mechanism by which cAMP and cGMP regulate mTOR activity is not completely understood. In this review, we will summarize the earlier work establishing the ability of cAMP to dampen mTORC1 activation in response to insulin and growth factors and then discuss our recent findings demonstrating the regulation of mTOR signaling by the PKA- and PKG-dependent signaling pathways. This signaling framework represents a new non-canonical regulation of mTOR activity that is independent of AKT and could be a novel mechanism underpinning the action of a variety of G protein-coupled receptors that are linked to the mTOR signaling network. We will further review the implications of these signaling events in the context of cardiometabolic disease, such as obesity, non-alcoholic fatty liver disease, and cardiac remodeling. The metabolic and cardiac phenotypes of mouse models with targeted deletion of Raptor and Rictor, the two essential components for mTORC1 and mTORC2, will be summarized and discussed.

miR-27b targets MAIP1 to mediate lipid accumulation in cultured human and mouse hepatic cells

Non-alcoholic liver disease (NAFLD) is a condition caused by excessive fat accumulation in the liver and developed via multiple pathways. miR-27b has been suggested to play crucial roles in the development of NAFLD, assuming via targeting genes involved in lipid catabolism and anabolism. However, other pathways regulated by miR-27b are largely unknown. Here we show that lipid accumulation was induced in miR-27b-transfected human and mouse hepatic cells and that knockdowns of three miR-27b-target genes, β-1,4-galactosyltransferase 3 (B4GALT3), matrix AAA peptidase interacting protein 1 (MAIP1) and PH domain and leucine rich repeat protein phosphatase 2 (PHLPP2), induced lipid accumulation. We also show that B4GALT3 and MAIP1 were direct targets of miR-27b and overexpression of MAIP1 ameliorated miR-27b-induced lipid accumulation. In addition, we show that hepatic Maip1 expression declined in mice fed a high-fat diet, suggesting the involvement of decreased Maip1 expression in the condition of fatty liver. Overall, we identified MAIP1/miR-27b axis as a mediator of hepatic lipid accumulation, a potential therapeutic target for NAFLD.

Farnesoid X receptor regulates PI3K/AKT/mTOR signaling pathway, lipid metabolism, and immune response in hybrid grouper

Some diseases related to lipid metabolism increase yearly in cultured fish, and the farnesoid X receptor (FXR) is a nuclear protein that plays a key role in inflammatory responses and lipid metabolism. However, the roles of FXR in hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂) remain poorly understood. The main objective of this study was to explore the roles of hepatic FXR in triggering the immune response and the potential functions of FXR in regulating the lipid metabolism. In the present study, the full-length sequence of fxr from hybrid grouper was cloned and characterized for the first time. Upon the Vibrio parahaemolyticus stimulation, the transcriptional level of fxr was rapidly elevated in the head kidney tissue in the early stage of infection. In vivo and vitro, activation of FXR by obeticholic acid (OA) significantly increased the concentrations and mRNA levels of hepatic inflammatory cytokines. These effects were inversed when FXR was inhibited by guggulsterone (GU). Moreover, the activation of FXR to suppress the PI₃K/AKT/mTOR signaling pathway improves hepatic lipid metabolism and reduces hepatic lipid accumulation in vivo and vitro. In addition, the inhibition of FXR activated the PI₃K/AKT/mTOR pathway, decreased the lipolysis and increased the lipogenesis, and subsequently increased the lipid accumulation in fish. These results revealed the positive roles of FXR in triggering immune responses and improving lipid metabolism and accumulation in hybrid grouper.

PHLPP isoforms differentially regulate Akt isoforms and AS160 affecting neuronal insulin signaling and insulin resistance via Scribble

BACKGROUND

The aim of the present study was to determine the role of individual PHLPP isoforms in insulin signaling and insulin resistance in neuronal cells.

METHODS

PHLPP isoforms were either silenced or overexpressed individually, and the effects were observed on individual Akt isoforms, AS160 and on neuronal glucose uptake, under insulin sensitive and resistant conditions. To determine PHLPP regulation itself, we tested effect of scaffold protein, Scribble, on PHLPP isoforms and neuronal glucose uptake.

RESULTS

We observed elevated expression of both PHLPP1 and PHLPP2 in insulin resistant neuronal cells (Neuro-2A, mouse neuroblastoma; SHSY-5Y, human neuroblastoma) as well as in the whole brain lysates of high-fat-diet mediated diabetic mice. In insulin sensitive condition, PHLPP isoforms differentially affected activation of all Akt isoforms, wherein PHLPP1 regulated serine phosphorylation of Akt2 and Akt3, while PHLPP2 regulated Akt1 and Akt3. This PHLPP mediated Akt isoform specific regulation activated AS160 affecting glucose uptake. Under insulin resistant condition, a similar trend of results were observed in Akt isoforms, AS160 and glucose uptake. Over-expressed PHLPP isoforms combined with elevated endogenous expression under insulin resistant condition drastically affected downstream signaling, reducing neuronal glucose uptake. No compensation was observed amongst PHLPP isoforms under all conditions tested, indicating independent roles and pointing towards possible scaffolding interactions behind isoform specificity. Silencing of Scribble, a scaffolding protein known to interact with PHLPP, affected cellular localization of both PHLPP1 and PHLPP2, and caused increase in glucose uptake.

CONCLUSIONS

PHLPP isoforms play independent roles via Scribble in regulating Akt isoforms differentially, affecting AS160 and neuronal glucose uptake. Video abstract.

mTOR: A Potential New Target in Nonalcoholic Fatty Liver Disease

The global prevalence of nonalcoholic fatty liver disease (NAFLD) continues to rise, yet effective treatments are lacking due to the complex pathogenesis of this disease. Although recent research has provided evidence for the “multiple strikes” theory, the classic “two strikes” theory has not been overturned. Therefore, there is a crucial need to identify multiple targets in NAFLD pathogenesis for the development of diagnostic markers and targeted therapeutics. Since its discovery, the mechanistic target of rapamycin (mTOR) has been recognized as the central node of a network that regulates cell growth and development and is closely related to liver lipid metabolism and other processes. This paper will explore the mechanisms by which mTOR regulates lipid metabolism (SREBPs), insulin resistance (Foxo1, Lipin1), oxidative stress (PIG3, p53, JNK), intestinal microbiota (TLRs), autophagy, inflammation, genetic polymorphisms, and epigenetics in NAFLD. The specific influence of mTOR on NAFLD was hypothesized to be divided into micro regulation (the mechanism of mTOR’s influence on NAFLD factors) and macro mediation (the relationship between various influencing factors) to summarize the influence of mTOR on the developmental process of NAFLD, and prove the importance of mTOR as an influencing factor of NAFLD regarding multiple aspects. The effects of crosstalk between mTOR and its upstream regulators, Notch, Hedgehog, and Hippo, on the occurrence and development of NAFLD-associated hepatocellular carcinoma are also summarized. This analysis will hopefully support the development of diagnostic markers and new therapeutic targets in NAFLD.

Loss of Raptor induces Sertoli cells into an undifferentiated state in mice

In mammals, testis development is triggered by the expression of the sex-determining Y-chromosome gene SRY to commit the Sertoli cell (SC) fate at gonadal sex determination in the fetus. Several genes have been identified to be required to promote the testis pathway following SRY activation (i.e., SRY box 9 (SOX9)) in an embryo; however, it largely remains unknown about the genes and the mechanisms involved in stabilizing the testis pathway after birth and throughout adulthood. Herein, we report postnatal males with SC-specific deletion of Raptor demonstrated the absence of SC unique identity and adversely acquired granulosa cell-like characteristics, along with loss of tubular architecture and scattered distribution of SCs and germ cells. Subsequent genome-wide analysis by RNA sequencing revealed a profound decrease in the transcripts of testis genes (i.e., Sox9, Sox8, and anti-Mullerian hormone (Amh)) and, conversely, an increase in ovary genes (i.e., LIM/Homeobox gene 9 (Lhx9), Forkhead box L2 (Foxl2) and Follistatin (Fst)); these changes were further confirmed by immunofluorescence and quantitative reverse-transcription polymerase chain reaction. Importantly, co-immunofluorescence demonstrated that Raptor deficiency induced SCs dedifferentiation into a progenitor state; the Raptor-mutant gonads showed some ovarian somatic cell features, accompanied by enhanced female steroidogenesis and elevated estrogen levels, yet the zona pellucida 3 (ZP3)-positive terminally feminized oocytes were not observed. In vitro experiments with primary SCs suggested that Raptor is likely involved in the fibroblast growth factor 9 (FGF9)-induced formation of cell junctions among SCs. Our results established that Raptor is required to maintain SC identity, stabilize the male pathway, and promote testis development.

Insulin Resistance: From Mechanisms to Therapeutic Strategies

Insulin resistance is the pivotal pathogenic component of many metabolic diseases, including type 2 diabetes mellitus, and is defined as a state of reduced responsiveness of insulin-targeting tissues to physiological levels of insulin. Although the underlying mechanism of insulin resistance is not fully understood, several credible theories have been proposed. In this review, we summarize the functions of insulin in glucose metabolism in typical metabolic tissues and describe the mechanisms proposed to underlie insulin resistance, that is, ectopic lipid accumulation in liver and skeletal muscle, endoplasmic reticulum stress, and inflammation. In addition, we suggest potential therapeutic strategies for addressing insulin resistance.

A Mammalian Target of Rapamycin-Perilipin 3 (mTORC1-Plin3) Pathway is essential to Activate Lipophagy and Protects Against Hepatosteatosis

BACKGROUND AND AIMS

NAFLD is the most common hepatic pathology in western countries and no treatment is currently available. NAFLD is characterized by the aberrant hepatocellular accumulation of fatty acids in the form of lipid droplets (LDs). Recently, it was shown that liver LD degradation occurs through a process termed lipophagy, a form of autophagy. However, the molecular mechanisms governing liver lipophagy are elusive. Here, we aimed to ascertain the key molecular players that regulate hepatic lipophagy and their importance in NAFLD.

APPROACH AND RESULTS

We analyzed the formation and degradation of LD in vitro (fibroblasts and primary mouse hepatocytes), in vivo and ex vivo (mouse and human liver slices) and focused on the role of the autophagy master regulator mammalian target of rapamycin complex (mTORC) 1 and the LD coating protein perilipin (Plin) 3 in these processes. We show that the autophagy machinery is recruited to the LD on hepatic overload of oleic acid in all experimental settings. This led to activation of lipophagy, a process that was abolished by Plin3 knockdown using RNA interference. Furthermore, Plin3 directly interacted with the autophagy proteins focal adhesion interaction protein 200 KDa and autophagy-related 16L, suggesting that Plin3 functions as a docking protein or is involved in autophagosome formation to activate lipophagy. Finally, we show that mTORC1 phosphorylated Plin3 to promote LD degradation.

CONCLUSIONS

These results reveal that mTORC1 regulates liver lipophagy through a mechanism dependent on Plin3 phosphorylation. We propose that stimulating this pathway can enhance lipophagy in hepatocytes to help protect the liver from lipid-mediated toxicity, thus offering a therapeutic strategy in NAFLD.

Divergent regulation of inflammatory cytokines by mTORC1 in THP-1-derived macrophages and intestinal epithelial Caco-2 cells

AIMS

The sustained activation of intestinal mechanistic target of rapamycin complex 1 (mTORC1) brought about by repeated mucosal insult or injury has been linked to escalation of gut inflammatory response, which may progress to damage the epithelium if not controlled. This study investigated the role of mTORC1 in the response of macrophage and enterocyte to inflammatory stimuli.

MATERIALS AND METHODS

We genetically manipulated human THP-1 monocytes and epithelial intestinal Caco-2 cells to generate stable cell lines with baseline, low or high mTORC1 kinase activity. The effects of THP-1 macrophage secretions onto Caco-2 cells were investigated by means of conditioned media transfer experiments.

KEY FINDINGS

The priming of mTORC1 for activation promoted lipopolysaccharide (LPS)-mediated THP-1 macrophage immune response as evidenced by the stimulation of inflammatory mediators (TNFα, IL-6, IL-8, IL-1β and IL-10). The treatment of THP-1 macrophages with LPS more than the manipulated level of mTORC1 activity of macrophages determined whether cytokine gene expression was induced in Caco-2 cells. LPS carry over was not responsible for the stimulation of Caco-2 cells' cytokine response. Knocking down Raptor in Caco-2 cells or treating Caco-2 cells with rapamycin enhanced Caco-2 TNFα gene expression revealing the anti-inflammatory role of a functional mTORC1 in intestinal epithelial cells exposed to macrophage-derived pro-inflammatory stimuli.

SIGNIFICANCE

Taken together, mTORC1 differentially impacts the immune responses of THP-1-derived macrophages and Caco-2 epithelial cells when placed in a pro-inflammatory microenvironment.

Emerging roles of PHLPP phosphatases in metabolism

Over the last decades, research has focused on the role of pleckstrin homology (PH) domain leucine-rich repeat protein phosphatases (PHLPPs) in regulating cellular signaling via PI3K/Akt inhibition. The PKB/Akt signaling imbalances are associated with a variety of illnesses, including various types of cancer, inflammatory response, insulin resistance, and diabetes, demonstrating the relevance of PHLPPs in the prevention of diseases. Furthermore, identification of novel substrates of PHLPPs unveils their role as a critical mediator in various cellular processes. Recently, researchers have explored the increasing complexity of signaling networks involving PHLPPs whereby relevant information of PHLPPs in metabolic diseases was obtained. In this review, we discuss the current knowledge of PHLPPs on the well-known substrates and metabolic regulation, especially in liver, pancreatic beta cell, adipose tissue, and skeletal muscle in relation with the stated diseases. Understanding the context-dependent functions of PHLPPs can lead to a promising treatment strategy for several kinds of metabolic diseases.

Dissecting the biology of mTORC1 beyond rapamycin

[Figure: see text].

mTOR Inhibition Promotes Pneumonitis Through Inducing Endothelial Contraction and Hyperpermeability

Compromised endothelial (EC) barrier function is a hallmark of inflammatory diseases. Mammalian target of rapamycin (mTOR) inhibitors, widely applied as clinical therapies, cause pneumonitis through mechanisms not yet fully understood. This study aimed to elucidate the EC mechanisms underlying the pathogenesis of pneumonitis caused by mTOR inhibition (mTORi). Mice with EC-specific deletion of mTOR complex components (Mtor, Rptor or Rictor) were administered LPS to induce pulmonary injury. Cultured EC were treated with pharmacological inhibitors, small interfering RNA or overexpression-plasmids. EC barrier function was evaluated in vivo with Evan's blue assay and in vitro by measurement of transendothelial electrical resistance and albumin flux. mTORi increased basal and TNFα-induced EC permeability, which was caused by myosin light chain (MLC) phosphorylation-dependent cell contraction. Inactivation of mTOR kinase activity by mTORi triggered PKCδ/p38/NF-κB signaling that significantly upregulated TNFα-induced MLC kinase (MLCK) expression, while Raptor promoted the phosphorylation of PKCα/MYPT1 independent of its interaction with mTOR, leading to suppression of MLC phosphatase (MLCP) activity. EC-specific deficiency in mTOR, Raptor or Rictor aggravated lung inflammation in LPS-treated mice. These findings reveal that mTORi induces PKC-dependent endothelial MLC phosphorylation, contraction and hyperpermeability that promote pneumonitis.

Kog1/Raptor mediates metabolic rewiring during nutrient limitation by controlling SNF1/AMPK activity

In changing environments, cells modulate resource budgeting through distinct metabolic routes to control growth. Accordingly, the TORC1 and SNF1/AMPK pathways operate contrastingly in nutrient replete or limited environments to maintain homeostasis. The functions of TORC1 under glucose and amino acid limitation are relatively unknown. We identified a modified form of the yeast TORC1 component Kog1/Raptor, which exhibits delayed growth exclusively during glucose and amino acid limitations. Using this, we found a necessary function for Kog1 in these conditions where TORC1 kinase activity is undetectable. Metabolic flux and transcriptome analysis revealed that Kog1 controls SNF1-dependent carbon flux apportioning between glutamate/amino acid biosynthesis and gluconeogenesis. Kog1 regulates SNF1/AMPK activity and outputs and mediates a rapamycin-independent activation of the SNF1 targets Mig1 and Cat8. This enables effective glucose derepression, gluconeogenesis activation, and carbon allocation through different pathways. Therefore, Kog1 centrally regulates metabolic homeostasis and carbon utilization during nutrient limitation by managing SNF1 activity.

MicroRNA-302a promotes neointimal formation following carotid artery injury in mice by targeting PHLPP2 thus increasing Akt signaling

The excessive proliferation and migration of smooth muscle cells (SMCs) play an important role in restenosis following percutaneous coronary interventions. MicroRNAs are able to target various genes and involved in the regulation of diverse cellular processes including cell growth and proliferation. In this study we investigated whether and how MicroRNAs regulated vascular SMC proliferation and vascular remodeling following carotid artery injury in mice. We showed that carotid artery injury-induced neointimal formation was remarkably ameliorated in microRNA (miR)-302 heterozygous mice and SMC-specific miR-302 knockout mice. In contrast, delivery of miR-302a adenovirus to the injured carotid artery enhanced neointimal formation. Upregulation of miR-302a enhanced the proliferation and migration of mouse aorta SMC (MASMC) in vitro by promoting cell cycle transition, whereas miR-302a inhibition caused the opposite results. Moreover, miR-302a promoted Akt activation by corporately decreasing Akt expression and increasing Akt phosphorylation in MASMCs. Application of the Akt inhibitor GSK690693 (5 μmol/L) counteracted the functions of miR-302a in promoting MASMC proliferation and migration. We further revealed that miR-302a directly targeted at the 3' untranslated region of PH domain and leucine rich repeat protein phosphatase 2 (PHLPP2) and negatively regulated PHLPP2 expression. Restoration of PHLPP2 abrogated the effects of miR-302a on Akt activation and MASMC motility. Furthermore, knockdown of PHLPP2 largely abolished the inhibition of neointimal formation that was observed in miR-302 heterozygous mice. Our data demonstrate that miR-302a exacerbates SMC proliferation and restenosis through increasing Akt signaling by targeting PHLPP2.

Adipocyte PHLPP2 inhibition prevents obesity-induced fatty liver

Increased adiposity confers risk for systemic insulin resistance and type 2 diabetes (T2D), but mechanisms underlying this pathogenic inter-organ crosstalk are incompletely understood. We find PHLPP2 (PH domain and leucine rich repeat protein phosphatase 2), recently identified as the Akt Ser473 phosphatase, to be increased in adipocytes from obese mice. To identify the functional consequence of increased adipocyte PHLPP2 in obese mice, we generated adipocyte-specific PHLPP2 knockout (A-PHLPP2) mice. A-PHLPP2 mice show normal adiposity and glucose metabolism when fed a normal chow diet, but reduced adiposity and improved whole-body glucose tolerance as compared to Cre- controls with high-fat diet (HFD) feeding. Notably, HFD-fed A-PHLPP2 mice show increased HSL phosphorylation, leading to increased lipolysis in vitro and in vivo. Mobilized adipocyte fatty acids are oxidized, leading to increased peroxisome proliferator-activated receptor alpha (PPARα)-dependent adiponectin secretion, which in turn increases hepatic fatty acid oxidation to ameliorate obesity-induced fatty liver. Consistently, adipose PHLPP2 expression is negatively correlated with serum adiponectin levels in obese humans. Overall, these data implicate an adipocyte PHLPP2-HSL-PPARα signaling axis to regulate systemic glucose and lipid homeostasis, and suggest that excess adipocyte PHLPP2 explains decreased adiponectin secretion and downstream metabolic consequence in obesity.

Obesity can be associated with an increased risk of metabolic complications. Here, the authors show that adipocyte-specific ablation of the phosphatase PHLPP2 improves glucose homeostasis in high-fat diet fed obese mice, and that this may be due at least in part to PHLPP2 dephosphorylation of HSL.

A Web of Science-based scientometric analysis about mammalian target of rapamycin signaling pathway in kidney disease from 1986 to 2020

Background

The mammalian target of rapamycin (mTOR) signaling pathway is vital for the regulation of cell metabolism, growth and proliferation in the kidney. This study aims to show current research focuses and predict future trends about mTOR pathway in kidney disease by the methods of scientometric analysis.

Methods

We referred to publications from the Web of Science^TM Core Collection (WoSCC) Database. Carrot2, VOSviewer and CiteSpace programs were applied to evaluate the distribution and contribution of authors, institutes and countries/regions of extensive bibliographic metadata, show current research focuses and predict future trends in kidney disease's area.

Results

Until July 10, 2020, there are 2,585 manuscripts about mTOR signaling pathway in kidney disease in total and every manuscript is cited 27.39 times on average. The big name of course is the United States. Research hot spots include "diabetic nephropathy", "kidney transplantation", "autosomal dominant polycystic kidney disease", "tuberous sclerosis complex", "renal cell carcinoma" and "autophagy". Seven key clusters are detected, including "kidney transplantation", "autosomal dominant polycystic kidney disease", "renal transplantation", "renal cell carcinoma", "hamartin", "autophagy" and "tuberous sclerosis complex".

Conclusions

Diabetic nephropathy, kidney transplantation, autosomal dominant polycystic kidney disease, tuberous sclerosis complex, renal cell carcinoma and autophagy are future research hot spots by utilizing scientometric analysis. In the future, it is necessary to research these fields.

Mechanistic Target of Rapamycin Signaling Activation Antagonizes Autophagy To Facilitate Zika Virus Replication

Zika virus (ZIKV), a mosquito-transmitted flavivirus, is linked to microcephaly and other neurological defects in neonates and Guillain-Barré syndrome in adults. The molecular mechanisms regulating ZIKV infection and pathogenic outcomes are incompletely understood. Signaling by the mechanistic (mammalian) target of rapamycin (mTOR) kinase is important for cell survival and proliferation, and viruses are known to hijack this pathway for their replication. Here, we show that in human neuronal precursors and glial cells in culture, ZIKV infection activates both mTOR complex 1 (mTORC1) and mTORC2. Inhibition of mTOR kinase by Torin1 or rapamycin results in reduction in ZIKV protein expression and progeny production. Depletion of Raptor, the defining subunit of mTORC1, by small interfering RNA (siRNA) negatively affects ZIKV protein expression and viral replication. Although depletion of Rictor, the unique subunit of mTORC2, or the mTOR kinase itself also inhibits the viral processes, the extent of inhibition is less pronounced. Autophagy is transiently induced early by ZIKV infection, and impairment of autophagosome elongation by the class III phosphatidylinositol 3-kinase (PI3K) inhibitor 3-methyladenine (3-MA) enhances viral protein accumulation and progeny production. mTOR phosphorylates and inactivates ULK1 (S757) at later stages of ZIKV infection, suggesting a link between autophagy inhibition and mTOR activation by ZIKV. Accordingly, inhibition of ULK1 (by MRT68921) or autophagy (by 3-MA) reversed the effects of mTOR inhibition, leading to increased levels of ZIKV protein expression and progeny production. Our results demonstrate that ZIKV replication requires the activation of both mTORC1 and mTORC2, which negatively regulates autophagy to facilitate ZIKV replication.IMPORTANCE The re-emergence of Zika virus (ZIKV) and its association with neurological complications necessitates studies on the molecular mechanisms that regulate ZIKV pathogenesis. The mTOR signaling cascade is tightly regulated and central to normal neuronal development and survival. Disruption of mTOR signaling can result in neurological abnormalities. In the studies reported here, we demonstrate for the first time that ZIKV infection results in activation of both mTORC1 and mTORC2 to promote virus replication. Although autophagy is activated early in infection to counter virus replication, it is subsequently suppressed by mTOR. These results reveal critical roles of mTOR signaling and autophagy in ZIKV infection and point to a possible mechanism underlying ZIKV-induced pathogenesis. Elucidating the role of mTOR signaling in ZIKV infection will provide insights into the mechanisms of ZIKV-induced neurological complications and potential targets for therapeutic approaches.

Pathological Consequences of Hepatic mTORC1 Dysregulation

The mammalian target of rapamycin complex 1 (mTORC1) is a central regulator of metabolism that integrates environmental inputs, including nutrients, growth factors, and stress signals. mTORC1 activation upregulates anabolism of diverse macromolecules, such as proteins, lipids, and nucleic acids, while downregulating autolysosomal catabolism. mTORC1 dysregulation is often found in various diseases, including cancer, cardiovascular and neurodegenerative diseases, as well as metabolic syndromes involving obesity and type II diabetes. As an essential metabolic organ, the liver requires proper regulation of mTORC1 for maintaining homeostasis and preventing pathologies. For instance, aberrant hyper- or hypoactivation of mTORC1 disrupts hepatocellular homeostasis and damages the structural and functional integrity of the tissue, leading to prominent liver injury and the development of hepatocellular carcinogenesis. Proper regulation of mTORC1 during liver diseases may be beneficial for restoring liver function and ameliorating the detrimental consequences of liver failure.

Mechanisms of Fibrosis Development in Nonalcoholic Steatohepatitis

Nonalcoholic fatty liver disease is the most prevalent liver disease worldwide, affecting 20%-25% of the adult population. In 25% of patients, nonalcoholic fatty liver disease progresses to nonalcoholic steatohepatitis (NASH), which increases the risk for the development of cirrhosis, liver failure, and hepatocellular carcinoma. In patients with NASH, liver fibrosis is the main determinant of mortality. Here, we review how interactions between different liver cells culminate in fibrosis development in NASH, focusing on triggers and consequences of hepatocyte-macrophage-hepatic stellate cell (HSC) crosstalk. We discuss pathways through which stressed and dead hepatocytes instigate the profibrogenic crosstalk with HSC and macrophages, including the reactivation of developmental pathways such as TAZ, Notch, and hedgehog; how clearance of dead cells in NASH via efferocytosis may affect inflammation and fibrogenesis; and insights into HSC and macrophage heterogeneity revealed by single-cell RNA sequencing. Finally, we summarize options to therapeutically interrupt this profibrogenic hepatocyte-macrophage-HSC network in NASH.

Targeting of Secretory Proteins as a Therapeutic Strategy for Treatment of Nonalcoholic Steatohepatitis (NASH)

Nonalcoholic steatohepatitis (NASH) is defined as a progressive form of nonalcoholic fatty liver disease (NAFLD) and is a common chronic liver disease that causes significant worldwide morbidity and mortality, and has no approved pharmacotherapy. Nevertheless, growing understanding of the molecular mechanisms underlying the development and progression of NASH has suggested multiple potential therapeutic targets and strategies to treat this disease. Here, we review this progress, with emphasis on the functional role of secretory proteins in the development and progression of NASH, in addition to the change of expression of various secretory proteins in mouse NASH models and human NASH subjects. We also highlight secretory protein-based therapeutic approaches that influence obesity-associated insulin resistance, liver steatosis, inflammation, and fibrosis, as well as the gut–liver and adipose–liver axes in the treatment of NASH.

IRF-1 mediates the suppressive effects of mTOR inhibition on arterial endothelium

AIMS

Mammalian target of rapamycin (mTOR) inhibitors used in drug-eluting stents (DES) to control restenosis have been found to delay endothelialization and increase incidence of late-stent thrombosis through mechanisms not completely understood. We revealed that mTOR inhibition (mTORi) upregulated the expression of cell growth suppressor IRF-1 in primary human arterial endothelial cells (HAEC). This study aimed to examine how mTOR-regulated IRF-1 expression contributes to the suppressive effect of mTORi on arterial endothelial proliferation.

METHODS AND RESULTS

Western blotting, quantitative PCR, and a dual-luciferase reporter assay indicated that mTOR inhibitors rapamycin and torin 1 upregulated IRF-1 expression and increased its transcriptional activity. IRF-1 in turn contributed to the suppressive effect of mTORi by mediating HAEC apoptosis and cell cycle arrest in part through upregulation of caspase 1 and downregulation of cyclin D3, as revealed by CCK-8 assay, Annexin V binding assay, measurement of activated caspase 3, BrdU incorporation assay, and matrigel tube formation assay. In a mouse model of femoral artery wire injury, administration of rapamycin inhibited EC recovery, an effect alleviated by EC deficiency of IRF-1. Chromatin immunoprecipitation assay with HAEC and rescue expression of wild type or dominant-negative IRF-1 in EC isolated from Irf1^-/- mice confirmed transcriptional regulation of IRF-1 on the expression of CASP1 and CCND3. Furthermore, mTORi activated multiple PKC members, among which PKCζ was responsible for the growth-inhibitory effect on HAEC. Activated PKCζ increased IRF1 transcription through JAK/STAT-1 and NF-κB signaling. Finally, overexpression of wild type or mutant raptor incapable of binding mTOR indicated that mTOR-free raptor contributed to PKCζ activation in mTOR-inhibited HAEC.

CONCLUSIONS

The study reveals an IRF-1-mediated mechanism that contributes to the suppressive effects of mTORi on HAEC proliferation. Further study may facilitate the development of effective strategies to reduce the side effects of DES used in coronary interventions.

Scientometric analysis of mTOR signaling pathway in liver disease

Background

The mTOR pathway is vital for homeostasis, metabolism, cancer transplantation and regeneration in the liver. The aim of this study is to use a bibliometric method to reveal current research hotspots and promising future trends in mTOR signaling in liver diseases.

Methods

Publications were searched and downloaded from the Web of Science Core Collection (WOSCC) Database. CiteSpace, Carrot2, and VOSviewer programs were utilized to analyze the contribution of various countries/regions, institutes, and authors; and to reveal research hotspots and promising future trends in this research area.

Results

Until May 21, 2019, a total of 2,232 papers regarding mTOR signaling pathway in liver disease were included, and each paper was cited 23.21 times on average. The most active country was the USA. 5 landmark articles with centrality and burstiness were determined by co-citation analysis. Research hotspots included "liver transplantation" "hepatic stellate cell proliferation" "NAFLD" "therapy of HCC". Moreover, six key clusters were discovered during the procedure of "clustering", including "liver transplantation" "protein synthesis" "mTOR inhibitor" "following early cyclosporine withdrawal" "srebp-1 activation", and "hepatocellular cancer".

Conclusions

Various scientific methods were applied to reveal scientific productivity, collaboration, and research hotspots in the mTOR signaling pathway in liver disease. Liver transplantation, hepatic stellate cell proliferation, non-alcoholic fatty liver disease (NAFLD), therapy of hepatocellular carcinoma (HCC), cell growth and autophagy, are research hotspots and are likely to be promising in the next few years. Further studies in this field are needed.

PHLPP2 as a novel metastatic and prognostic biomarker in non-small cell lung cancer patients

Background

PH domain and leucine‐rich repeat protein phosphatase 2 (PHLPP2) has been reported to be a potent tumor suppressor in many human cancers. However, PHLPP2 has not been fully researched as a putative clinical prognostic biomarker of lung cancer.

Methods

The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases including data on 1383 non‐small cell lung cancer (NSCLC) patients were used to determine PHLPP2 expression. PHLPP2 expression was then examined by immunohistochemistry, and its clinical significance analyzed in 134 NSCLC patients, including 73 patients with adenocarcinoma and 81 with squamous cell carcinoma.

Results

We found PHLPP2 expression to be less pronounced in NSCLC tissue samples than that in nontumoral lung tissues according to data taken from TCGA and GEO datasets; this outcome was further validated by immunohistochemistry assay. The low PHLPP2 expression level was found to be associated with the presence of lymph node metastasis (P = 0.003). Importantly, PHLPP2 was found to be an independent indicator of prognosis for overall (hazard ratio [HR] = 0.520, 95% confidence interval [Cl] = 0.327–0.827; P = 0.006) and disease‐free survival (HR = 0.489, 95% Cl = 0.308–0.775; P = 0.002) in patients with surgically‐resected NSCLC by multivariate analysis.

Conclusion

Taken together, our findings show that PHLPP2 is a robust clinical marker for NSCLC survival and could serve as a potential therapeutic target.

Triplet Male Lambs Are More Susceptible than Twins to Dietary Soybean Oil-Induced Fatty Liver

BACKGROUND

Litter size affects fetal development but its relation to diet-induced fatty liver later in life is unknown.

OBJECTIVES

This aim of this study was to test the hypothesis that litter size influences postweaning fatty liver development in response to soybean oil-supplemented diet.

METHODS

Weanling twin (TW) or triplet (TP) male lambs (n = 16) were fed a control diet or 2% soybean oil-supplemented diet (SO) for 90 d. Liver tissue morphology, biochemical parameters, and lipid metabolic enzymes were determined. Hepatic gene expression was analyzed by RNA sequencing (n = 3), followed by enrichment analysis according to Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes. Differentially expressed genes involved in lipid metabolism were further verified by quantitative reverse transcriptase-polymerase chain reaction (n = 4). All data were analyzed by a 2-factor ANOVA, apart from differentially expressed genes, which were identified by the Benjamini-Hochberg approach (q value ≤0.05).

RESULTS

SO increased liver triglyceride (by 55%) and nonesterified fatty acid (by 54%) concentrations in TPs (P ≤ 0.05) but not in TWs (P > 0.05). SO also induced a 2.3- and 2.1-fold increase in the liver steatosis score of TPs and TWs, respectively (P ≤ 0.05). Moreover, SO reduced the activity of lipolytic enzymes including hepatic lipase and total lipase in TPs by 47% and 25%, respectively (P ≤ 0.05). In contrast, activities of lipogenic enzymes, including malic enzyme and acetyl coenzyme A carboxylase, were significantly higher in TPs (P ≤ 0.05). Moreover, TPs had higher expression of lipogenic genes, such as FASN (by 45%) and APOB (by 72%), and lower expression of lipolytic genes, such as PRKAA2 (by 28%) and CPT1A (by 43%), compared with TWs (P ≤ 0.05).

CONCLUSIONS

TPs have a gene expression profile that is more susceptible to SO-induced fatty liver than that of TWs, which indicates that insufficient maternal nutrient supply at fetal and neonatal stages may increase the risk of nonalcoholic fatty liver disease.

miR-96 and autophagy are involved in the beneficial effect of grape seed proanthocyanidins against high-fat-diet-induced dyslipidemia in mice

We aimed to investigate the possible signaling pathways underlying the regulation of grape seed proanthocyanidins extracts (GSPE) on lipid metabolism. One hundred male C57BL/6 mice were divided into four groups: control group (normal diet), GSPE group (normal diet + GSPE), high-fat diet group (HFD), and high-fat diet plus GSPE (200 mg/kg/day) group (HFD + GSPE). Mice received the diets for 180 days. Body weight and serum lipid levels were measured. Autophagic flux characteristics, such as accumulation of lipids, mitochondria, and autophagosomes in the liver, were detected using transmission electron microscopy. Expression profile of microRNAs (miRNAs) in the liver was determined using RNA microarray and quantitative real time polymerase chain reaction (qRt-PCR). GSPE significantly decreased the weight gain, serum levels of triglycerides, total cholesterol, and low-density lipoprotein cholesterol but increased high-density lipoprotein cholesterol in the HFD mice. Autophagic flux was significantly increased by HFD but decreased by GSPE treatment. GSPE significantly attenuated HFD-induced miR-96 upregulation, which in turn reduced the expressions of miR-96 downstream molecules, FOXO1, mTOR, p-mTOR, and LC3A/B. These results suggested that the miR-96 is involved in the protective effect of GSPE against HFD-induced dyslipidemia. Possible mechanisms might be through mTOR and FOXO1, which facilitate autophagic flux for clearance of lipid accumulation.

Hepatocyte Notch activation induces liver fibrosis in nonalcoholic steatohepatitis

Fibrosis is the major determinant of morbidity and mortality in patients with nonalcoholic steatohepatitis (NASH) but has no approved pharmacotherapy in part because of incomplete understanding of its pathogenic mechanisms. Here, we report that hepatocyte Notch activity tracks with disease severity and treatment response in patients with NASH and is similarly increased in a mouse model of diet-induced NASH and liver fibrosis. Hepatocyte-specific Notch loss-of-function mouse models showed attenuated NASH-associated liver fibrosis, demonstrating causality to obesity-induced liver pathology. Conversely, forced activation of hepatocyte Notch induced fibrosis in both chow- and NASH diet-fed mice by increasing Sox9-dependent Osteopontin (Opn) expression and secretion from hepatocytes, which activate resident hepatic stellate cells. In a cross-sectional study, we found that OPN explains the positive correlation between liver Notch activity and fibrosis stage in patients. Further, we developed a Notch inhibitor [Nicastrin antisense oligonucleotide (Ncst ASO)] that reduced fibrosis in NASH diet-fed mice. In summary, these studies demonstrate the pathological role and therapeutic accessibility of the maladaptive hepatocyte Notch response in NASH-associated liver fibrosis.

DNA methylation and FoxO3a regulate PHLPP1 expression in chondrocytes

The protein phosphatase Phlpp1 is an essential enzyme for proper chondrocyte function. Altered Phlpp1 levels are associated with cancer and degenerative diseases such as osteoarthritis. While much is known about the post-transcriptional mechanisms controlling Phlpp1 levels, transcriptional regulation of the Phlpp1 gene locus is underexplored. We previously showed that CpG methylation of the PHLPP1 promoter is lower in osteoarthritic cartilage than in normal cartilage, and indirectly correlates with gene expression. Here we further defined the effects of DNA methylation on PHLPP1 promoter activity in chondrocytes. We cloned a 1791 bp fragment of the PHLPP1 promoter (-1589:+202) and found that the first 500 bp were required for maximal promoter activity. General methylation of CpG sites within this fragment significantly blunts transcriptional activity, whereas site-specific methyltransferases HhaI or HpaII decrease transcriptional activation by approximately 50%. We located putative FoxO consensus sites within the PHLPP1 promoter region. Inhibition of DNA methylation by incorporation of 5-azacytidine increases Phlpp1 mRNA levels, but FoxO inhibition abolishes this induction. To determine which FoxO transcription factor mediates Phlpp1 expression, we performed overexpression and siRNA-mediated knock down experiments. Overexpression of FoxO3a, but not FoxO1, increases Phlpp1 levels. Likewise, siRNAs targeting FoxO3a, but not FoxO1, diminished Phlpp1 levels. Last, FoxO inhibition increases glycosaminoglycan staining of cultured chondrocytes and leads to concomitant increases in FGF18 and HAS2 expression. Together, these data demonstrate that CpG methylation and FoxO3a regulate PHLPP1 expression.

Role of mTOR in Glucose and Lipid Metabolism

The mammalian target of rapamycin, mTOR is the master regulator of a cell’s growth and metabolic state in response to nutrients, growth factors and many extracellular cues. Its dysregulation leads to a number of metabolic pathological conditions, including obesity and type 2 diabetes. Here, we review recent findings on the role of mTOR in major metabolic organs, such as adipose tissues, liver, muscle, pancreas and brain. And their potentials as the mTOR related pharmacological targets will be also discussed.

mTOR coordinates transcriptional programs and mitochondrial metabolism of activated Treg subsets to protect tissue homeostasis

Regulatory T (Treg) cells derived from the thymus (tTreg) and periphery (pTreg) have central and distinct functions in immunosuppression, but mechanisms for the generation and activation of Treg subsets in vivo are unclear. Here, we show that mechanistic target of rapamycin (mTOR) unexpectedly supports the homeostasis and functional activation of tTreg and pTreg cells. mTOR signaling is crucial for programming activated Treg-cell function to protect immune tolerance and tissue homeostasis. Treg-specific deletion of mTOR drives spontaneous effector T-cell activation and inflammation in barrier tissues and is associated with reduction in both thymic-derived effector Treg (eTreg) and pTreg cells. Mechanistically, mTOR functions downstream of antigenic signals to drive IRF4 expression and mitochondrial metabolism, and accordingly, deletion of mitochondrial transcription factor A (Tfam) severely impairs Treg-cell suppressive function and eTreg-cell generation. Collectively, our results show that mTOR coordinates transcriptional and metabolic programs in activated Treg subsets to mediate tissue homeostasis.

Hamartin regulates cessation of mouse nephrogenesis independently of Mtor

Nephrogenesis concludes by the 36th week of gestation in humans and by the third day of postnatal life in mice. Extending the nephrogenic period may reduce the onset of adult renal and cardiovascular disease associated with low nephron numbers. We conditionally deleted either Mtor or Tsc1 (coding for hamartin, an inhibitor of Mtor) in renal progenitor cells. Loss of one Mtor allele caused a reduction in nephron numbers; complete deletion led to severe paucity of glomeruli in the kidney resulting in early death after birth. By contrast, loss of one Tsc1 allele from renal progenitors resulted in a 25% increase in nephron endowment with no adverse effects. Increased progenitor engraftment rates ex vivo relative to controls correlated with prolonged nephrogenesis through the fourth postnatal day. Complete loss of both Tsc1 alleles in renal progenitors led to a lethal tubular lesion. The hamartin phenotypes are not dependent on the inhibitory effect of TSC on the Mtor complex but are dependent on Raptor.

γ-Secretase Inhibition Lowers Plasma Triglyceride-Rich Lipoproteins by Stabilizing the LDL Receptor

Excess plasma triglycerides (TGs) are a key component of obesity-induced metabolic syndrome. We have shown that γ-secretase inhibitor (GSI) treatment improves glucose tolerance due to inhibition of hepatic Notch signaling but found additional Notch-independent reduction of plasma TG-rich lipoproteins (TRLs) in GSI-treated, as well as hepatocyte-specific, γ-secretase knockout (L-Ncst) mice, which suggested a primary effect on hepatocyte TRL uptake. Indeed, we found increased VLDL and LDL particle uptake in L-Ncst hepatocytes and Ncst-deficient hepatoma cells, in part through reduced γ-secretase-mediated low-density lipoprotein receptor (LDLR) cleavage and degradation. To exploit this novel finding, we generated a liver-selective Nicastrin ASO, which recapitulated glucose and lipid improvements of L-Ncst mice, with increased levels of hepatocyte LDLR. Collectively, these results identify the role of hepatic γ-secretase to regulate LDLR and suggest that liver-specific GSIs may simultaneously improve multiple aspects of the metabolic syndrome.

Role of miR-195 in cigarette smoke-induced chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is regarded as a persistent respiratory symptom, mainly caused by cigarette smoking. Recent data have suggested that some miRNAs are involved in the pathogenesis of COPD. Here, we found that miR-195 was significantly upregulated in the lung tissues of patients with COPD compared to in never smokers. miR-195 expression was also upregulated in cigarette smoke (CS)-exposed mice. Lentivirus-mediated knockdown of miR-195 alleviated CS-induced lung pathological changes and reduced inflammatory cell infiltration as well as production of interleukin-6 and tumor necrosis factor-α in bronchoalveolar lavage fluid. Mechanically, a positive correlation was found between miR-195 and phosphorylation of Akt in lung tissues of COPD patients. PHLPP2 was confirmed as a direct downstream target of miR-195 and negative regulator of miR-195 expression. Inhibition of PHLPP2 enhanced Akt phosphorylation and increased interleukin-6 and tumor necrosis factor-α production in BEAS-2B cells, resembling the effects of miR-195 overexpression. Collectively, our data indicate that miR-195 has a pathogenetic role in CS-induced COPD and regulates Akt signaling by suppressing PHLPP2 expression. miR-195 may be an effective therapeutic target in COPD.

Degradation of PHLPP2 by KCTD17, via a Glucagon-Dependent Pathway, Promotes Hepatic Steatosis

BACKGROUND & AIMS

Obesity-induced nonalcoholic fatty liver disease (NAFLD) develops, in part, via excess insulin-stimulated hepatic de novo lipogenesis, which increases, paradoxically, in patients with obesity-induced insulin resistance. Pleckstrin homology domain leucine-rich repeat protein phosphatase 2 (PHLPP2) terminates insulin signaling by dephosphorylating Akt; levels of PHLPP2 are reduced in livers from obese mice. We investigated whether loss of hepatic PHLPP2 is sufficient to induce fatty liver in mice, mechanisms of PHLPP2 degradation in fatty liver, and expression of genes that regulate PHLPP2 in livers of patients with NAFLD.

METHODS

C57BL/6J mice (controls), obese db/db mice, and mice with liver-specific deletion of PHLPP2 (L-PHLPP2) fed either normal chow or high-fat diet (HFD) were analyzed for metabolic phenotypes, including glucose tolerance and hepatic steatosis. PHLPP2-deficient primary hepatocytes or CRISPR/Cas9-mediated PHLPP2-knockout hepatoma cells were analyzed for insulin signaling and gene expression. We performed mass spectrometry analyses of liver tissues from C57BL/6J mice transduced with Ad-HA-Flag-PHLPP2 to identify posttranslational modifications to PHLPP2 and proteins that interact with PHLPP2. We measured levels of mRNAs by quantitative reverse transcription polymerase chain reaction in liver biopsies from patients with varying degrees of hepatic steatosis.

RESULTS

PHLPP2-knockout hepatoma cells and hepatocytes from L-PHLPP2 mice showed normal initiation of insulin signaling, but prolonged insulin action. Chow-fed L-PHLPP2 mice had normal glucose tolerance but hepatic steatosis. In HFD-fed C57BL/6J or db/db obese mice, endogenous PHLPP2 was degraded by glucagon and PKA-dependent phosphorylation of PHLPP2 (at Ser1119 and Ser1210), which led to PHLPP2 binding to potassium channel tetramerization domain containing 17 (KCTD17), a substrate-adaptor for Cul3-RING ubiquitin ligases. Levels of KCTD17 mRNA were increased in livers of HFD-fed C57BL/6J or db/db obese mice and in liver biopsies patients with NAFLD, compared with liver tissues from healthy control mice or patients without steatosis. Knockdown of KCTD17 with small hairpin RNA in primary hepatocytes increased PHLPP2 protein but not Phlpp2 mRNA, indicating that KCTD17 mediates PHLPP2 degradation. KCTD17 knockdown in obese mice prevented PHLPP2 degradation and decreased expression of lipogenic genes.

CONCLUSIONS

In mouse models of obesity, we found that PHLPP2 degradation induced lipogenesis without affecting gluconeogenesis. KCTD17, which is up-regulated in liver tissues of obese mice and patients with NAFLD, binds to phosphorylated PHLPP2 to target it for ubiquitin-mediated degradation; this increases expression of genes that regulate lipogenesis to promote hepatic steatosis. Inhibitors of this pathway might be developed for treatment of patients with NAFLD.

MTORC1 Regulates both General Autophagy and Mitophagy Induction after Oxidative Phosphorylation Uncoupling

Mechanistic target of rapamycin complex 1 (MTORC1) is a critical negative regulator of general autophagy. We hypothesized that MTORC1 may specifically regulate autophagic clearance of damaged mitochondria. To test this, we used cells lacking tuberous sclerosis complex 2 (TSC2^-/- cells), which show constitutive MTORC1 activation. TSC2^-/- cells show MTORC1-dependent impaired autophagic flux after chemical uncoupling of mitochondria, increased mitochondrial-protein aging, and accumulation of p62/SQSTM1-positive mitochondria. Mitochondrial autophagy (mitophagy) was also deficient in cells lacking TSC2, associated with altered expression of PTEN-induced putative kinase 1 (PINK1) and PARK2 translocation to uncoupled mitochondria, all of which were recovered by MTORC1 inhibition or expression of constitutively active forkhead box protein O1 (FoxO1). These data prove the necessity of intact MTORC1 signaling to regulate two synergistic processes required for clearance of damaged mitochondria: (i) general autophagy initiation and (ii) PINK1/PARK2-mediated selective targeting of uncoupled mitochondria to the autophagic machinery.

PHLPPing through history: a decade in the life of PHLPP phosphatases

In the decade since their discovery, the PH domain leucine-rich repeat protein phosphatases (PHLPP) have emerged as critical regulators of cellular homeostasis, and their dysregulation is associated with various pathophysiologies, ranging from cancer to degenerative diseases, such as diabetes and heart disease. The two PHLPP isozymes, PHLPP1 and PHLPP2, were identified in a search for phosphatases that dephosphorylate Akt, and thus suppress growth factor signaling. However, given that there are over 200 000 phosphorylated residues in a single cell, and fewer than 50 Ser/Thr protein phosphatases, it is not surprising that PHLPP has many other cellular functions yet to be discovered, including a recently identified role in regulating the epigenome. Both PHLPP1 and PHLPP2 are commonly deleted in human cancers, supporting a tumor suppressive role. Conversely, the levels of one isozyme, PHLPP1, are elevated in diabetes. Thus, mechanisms to correctly control PHLPP activity in cells are critical for normal cellular homeostasis. This review summarizes the known functions of PHLPP and its role in disease.

PHLPP: a putative cellular target during insulin resistance and type 2 diabetes

Progressive research in the past decade converges to the impact of PHLPP in regulating the cellular metabolism through PI3K/AKT inhibition. Aberrations in PKB/AKT signaling coordinates with impaired insulin secretion and insulin resistance, identified during T2D, obesity and cardiovascular disorders which brings in the relevance of PHLPPs in the metabolic paradigm. In this review, we discuss the impact of PHLPP isoforms in insulin signaling and its associated cellular events including mitochondrial dysfunction, DNA damage, autophagy and cell death. The article highlights the plausible molecular targets that share the role during insulin-resistant states, whose understanding can be extended into treatment responses to facilitate targeted drug discovery for T2D and allied metabolic syndromes.

Essential Role of mTORC1 in Self-Renewal of Murine Alveolar Macrophages

Alveolar macrophages (AMϕ) have the capacity of local self-renewal through adult life; however, mechanisms that regulate AMϕ self-renewal remain poorly understood. We found that myeloid-specific deletion of Raptor, an essential component of the mammalian/mechanistic target of rapamycin complex (mTORC)1, resulted in a marked decrease of this population of cells accompanying altered phenotypic features and impaired phagocytosis activity. We demonstrated further that Raptor/mTORC1 deficiency did not affect AMϕ development, but compromised its proliferative activity at cell cycle entry in the steady-state as well as in the context of repopulation in irradiation chimeras. Mechanically, mTORC1 confers AMϕ optimal responsiveness to GM-CSF-induced proliferation. Thus, our results demonstrate an essential role of mTORC1 for AMϕ homeostasis by regulating proliferative renewal.

Nonalcoholic fatty liver disease: cause or consequence of type 2 diabetes?

Growing epidemiological evidence suggests that nonalcoholic fatty liver disease (NAFLD) is an early predictor of and determinant for the development of type 2 diabetes and other features of the metabolic syndrome. This finding may have important clinical implications for the diagnosis, prevention and treatment of type 2 diabetes and its chronic complications. However, given the complex and bi-directional relationships between NAFLD, insulin resistance and chronic hyperglycaemia, it is extremely difficult to distinguish whether NAFLD is a cause or a consequence of insulin resistance and type 2 diabetes. Indeed, at the molecular level, hepatic lipogenesis and hepatic glucose production depend on differentially regulated branches of the insulin signalling pathway. Furthermore, genetic studies suggest that excess hepatic fat is associated with progressive liver disease, but does not always increase the risk of incident type 2 diabetes. Here, we will briefly review the epidemiological, pathophysiological and molecular evidence linking NAFLD to the development of type 2 diabetes. We will also discuss some recent genetic and therapeutic advances that seem to challenge a causal role of NAFLD in the pathogenesis type 2 diabetes, and propose a working hypothesis to explain this apparent conundrum. In conclusion, progressive liver disease and type 2 diabetes are divergent though inter-related consequences of insulin resistance and the metabolic syndrome.

Functional characterization of AMP-activated protein kinase signaling in tumorigenesis

AMP-activated protein kinase (AMPK) is a ubiquitously expressed metabolic sensor among various species. Specifically, cellular AMPK is phosphorylated and activated under certain stressful conditions, such as energy deprivation, in turn to activate diversified downstream substrates to modulate the adaptive changes and maintain metabolic homeostasis. Recently, emerging evidences have implicated the potential roles of AMPK signaling in tumor initiation and progression. Nevertheless, a comprehensive description on such topic is still in scarcity, especially in combination of its biochemical features with mouse modeling results to elucidate the physiological role of AMPK signaling in tumorigenesis. Hence, we performed this thorough review by summarizing the tumorigenic role of each component along the AMPK signaling, comprising of both its upstream and downstream effectors. Moreover, their functional interplay with the AMPK heterotrimer and exclusive efficacies in carcinogenesis were chiefly explained among genetically altered mice models. Importantly, the pharmaceutical investigations of AMPK relevant medications have also been highlighted. In summary, in this review, we not only elucidate the potential functions of AMPK signaling pathway in governing tumorigenesis, but also potentiate the future targeted strategy aiming for better treatment of aberrant metabolism-associated diseases, including cancer.