Liver Reptin/RUVBL2 controls glucose and lipid metabolism with opposite actions on mTORC1 and mTORC2 signalling

Targeting mTOR Signaling in Type 2 Diabetes Mellitus and Diabetes Complications

The mechanistic target of rapamycin (mTOR) is a pivotal regulator of cell metabolism and growth. In the form of two different multi-protein complexes, mTORC1 and mTORC2, mTOR integrates cellular energy, nutrient and hormonal signals to regulate cellular metabolic homeostasis. In type 2 diabetes mellitus (T2DM) aberrant mTOR signaling underlies its pathological conditions and end-organ complications. Substantial evidence suggests that two mTOR-mediated signaling schemes, mTORC1-p70S6 kinase 1 (S6K1) and mTORC2-protein kinase B (AKT), play a critical role in insulin sensitivity and that their dysfunction contributes to development of T2DM. This review summaries our current understanding of the role of mTOR signaling in T2DM and its associated complications, as well as the potential use of mTOR inhibitors in treatment of T2DM.

The Role of Ophiopogonin D in Atherosclerosis: Impact on Lipid Metabolism and Gut Microbiota

Gut microbiota has been proven to play an important role in many metabolic diseases and cardiovascular disease, particularly atherosclerosis. Ophiopogonin D (OPD), one of the effective compounds in Ophiopogon japonicus, is considered beneficial to metabolic syndrome and cardiovascular diseases. In this study, we have illuminated the effect of OPD in ApoE knockout (ApoE[Formula: see text] mice on the development of atherosclerosis and gut microbiota. To investigate the potential ability of OPD to alleviate atherosclerosis, 24 eight-week-old male ApoE[Formula: see text] mice (C57BL/6 background) were fed a high-fat diet (HFD) for 12 weeks, and 8 male C57BL/6 mice were fed a normal diet, serving as the control group. ApoE[Formula: see text] mice were randomly divided into the model group, OPD group, and simvastatin group ([Formula: see text]= 8). After treatment for 12 consecutive weeks, the results showed that OPD treatment significantly decreased the plaque formation and levels of serum lipid compared with those in the model group. In addition, OPD improved oral glucose tolerance and insulin resistance as well as reducing hepatocyte steatosis. Further analysis revealed that OPD might attenuate atherosclerosis through inhibiting mTOR phosphorylation and the consequent lipid metabolism signaling pathways mediated by SREBP1 and SCD1 in vivo and in vitro. Furthermore, OPD treatment led to significant structural changes in gut microbiota and fecal metabolites in HFD-fed mice and reduced the relative abundance of Erysipelotrichaceae genera associated with cholesterol metabolism. Collectively, these findings illustrate that OPD could significantly protect against atherosclerosis, which might be associated with the moderation of lipid metabolism and alterations in gut microbiota composition and fecal metabolites.

RUVBL1/2 Complex Regulates Pro-Inflammatory Responses in Macrophages via Regulating Histone H3K4 Trimethylation

Macrophages play an important role in the host defense mechanism. In response to infection, macrophages activate a genetic program of pro-inflammatory response to kill any invading pathogen, and initiate an adaptive immune response. We have identified RUVBL2 - an ATP-binding protein belonging to the AAA+ (ATPase associated with diverse cellular activities) superfamily of ATPases - as a novel regulator in pro-inflammatory response of macrophages. Gene knockdown of Ruvbl2, or pharmacological inhibition of RUVBL1/2 activity, compromises type-2 nitric oxide synthase (Nos2) gene expression, nitric oxide production and anti-bacterial activity of mouse macrophages in response to lipopolysaccharides (LPS). RUVBL1/2 inhibitor similarly inhibits pro-inflammatory response in human monocytes, suggesting functional conservation of RUVBL1/2 in humans. Transcriptome analysis further revealed that major LPS-induced pro-inflammatory pathways in macrophages are regulated in a RUVBL1/2-dependent manner. Furthermore, RUVBL1/2 inhibition significantly reduced the level of histone H3K4me3 at the promoter region of Nos2 and Il6, two prototypical pro-inflammatory genes, and diminished the recruitment of NF-kappaB to the corresponding enhancers. Our study reveals RUVBL1/2 as an integral component of macrophage pro-inflammatory responses through epigenetic regulations, and the therapeutic potentials of RUVBL1/2 inhibitors in the treatment of diseases caused by aberrant activation of pro-inflammatory pathways.

Reptin/RUVBL2 is required for hepatocyte proliferation in vivo, liver regeneration and homeostasis

Previous studies have shown that Reptin is overexpressed in hepatocellular carcinoma and that it is necessary for in vitro proliferation and cell survival. However, its pathophysiological role in vivo remains unknown. We aimed to study the role of Reptin in hepatocyte proliferation after regeneration using a liver Reptin knock-out model (Reptin^LKO ). Interestingly, hepatocyte proliferation is strongly impaired in Reptin^LKO mice 36 h after partial hepatectomy, associated with a decrease of cyclin-A expression and mTORC1 and MAPK signalling, leading to an impaired liver regeneration. Moreover, in the Reptin^LKO model, we have observed a progressive loss of Reptin invalidation associated with an atypical liver regeneration. Hypertrophic and proliferative hepatocytes gradually replace Reptin^KO hypotrophic hepatocytes. To conclude, our results show that Reptin is required for hepatocyte proliferation in vivo and liver regeneration and that it plays a crucial role in hepatocyte survival and liver homeostasis.

Effects of betaine on non-alcoholic liver disease

The increasing prevalence of non-alcoholic fatty liver disease (NAFLD) poses a growing challenge in terms of its prevention and treatment. The 'multiple hits' hypothesis of multiple insults, such as dietary fat intake, de novo lipogenesis, insulin resistance, oxidative stress, mitochondrial dysfunction, gut dysbiosis and hepatic inflammation, can provide a more accurate explanation of the pathogenesis of NAFLD. Betaine plays important roles in regulating the genes associated with NAFLD through anti-inflammatory effects, increased free fatty oxidation, anti-lipogenic effects and improved insulin resistance and mitochondrial function; however, the mechanism of betaine remains elusive.

Hepatic DNAJB9 Drives Anabolic Biasing to Reduce Steatosis and Obesity

Nutrients stimulate the anabolic synthesis of proteins and lipids, but selective insulin resistance in obesity biases the anabolic program toward lipogenesis. Here, we report the identification of a DNAJB9-driven program that favors protein synthesis and energy production over lipid accumulation. We show there are two pools of DNAJB9 cochaperone. DNAJB9 in the ER lumen promotes the degradation of the lipogenic transcription factor SREBP1c through ERAD, whereas its counterpart on the ER membrane promotes the assembly of mTORC2 in the cytosol and stimulates the synthesis of proteins and ATP. The expression of Dnajb9 is induced by nutrients and downregulated in the obese mouse liver. Restoration of hepatic DNAJB9 expression effectively improves insulin sensitivity, restores protein synthesis, and suppresses food intake, accompanied by reduced hepatic steatosis and adiposity in multiple mouse models of obesity. Therefore, targeting the anabolic balance may provide a unique opportunity to tackle obesity and diabetes.

iTRAQ- and LC-MS/MS-based quantitative proteomics reveals Pqlc2 as a potential regulator of hepatic glucose metabolism and insulin signalling pathway during fasting

Glucose homeostasis is tightly controlled by balance between glucose production and uptake in liver tissue upon energy shortage condition. Altered glucose homeostasis contributes to the pathophysiology of metabolic disorders including diabetes and obesity. Here, we aimed to analyse the change of proteomic profile upon prolonged fasting in mice with isobaric tag for relative and absolute quantification (iTRAQ) labelling followed by liquid chromatography-mass spectrometry (LC/MS) technology. Adult male mice were fed or fasted for 16 hours and liver tissues were collected for iTRAQ labelling followed by LC/MS analysis. A total of 322 differentially expressed proteins were identified, including 189 upregulated and 133 downregulated proteins. Bioinformatics analyses, including Gene Ontology analysis (GO), Kyoto encyclopaedia of genes and genomes analysis (KEGG) and protein-protein interaction analysis (PPI) were conducted to understand biological process, cell component, and molecular function of the 322 differentially expressed proteins. Among 322 hepatic proteins differentially expressed between fasting and fed mice, we validated three upregulated proteins (Pqlc2, Ehhadh and Apoa4) and two downregulated proteins (Uba52 and Rpl37) by western-blotting analysis. In cultured HepG2 hepatocellular cells, we found that depletion of Pqlc2 by siRNA-mediated knockdown impaired the insulin-induced glucose uptake, inhibited GLUT2 mRNA level and suppressed the insulin-induced Akt phosphorylation. By contrast, knockdown of Pqlc2 did not affect the cAMP/dexamethasone-induced gluconeogenesis. In conclusion, our study provides important information on protein profile change during prolonged fasting with iTRAQ- and LC-MS/MS-based quantitative proteomics, and identifies Pqlc2 as a potential regulator of hepatic glucose metabolism and insulin signalling pathway in this process.

PPARγ enhanced Adiponectin polymerization and trafficking by promoting RUVBL2 expression during adipogenic differentiation

Adipocyte differentiation is an essential part of adipose tissue development, and is closely related to obesity and obesity-related diseases. In this study, we found that the expression of PPARγ, RUVBL2 and Adiponectin were concurrently obviously increased in the 5th-7th day of 3T3-L1 cell differentiation. PPARγ overexpression or the PPARγ activator facilitated Adiponectin trafficking and secretion and upregulated RUVBL2 expression as well as AS160 phosphorylation during adipogenic differentiation of 3T3-L1 cells. Consistently RUVBL2 overexpression also enhanced the polymerization and secretion of Adiponectin, in contrast, RUVBL2 knockdown reduced Adiponectin secretion. Further, PPARγ significantly enhanced RUVBL2 promoter activity and transcription. The progressive deletions and mutations of RUVBL2 promoter for PPARγ binding sites suggested that the PPARγ binding motif situated at -804/-781 bp is an essential component required for RUVBL2 promoter activity. Chromatin immunoprecipitation (ChIP) assays determined that PPARγ can directly interact with the RUVBL2 promoter DNA. Taken together, these data suggest that PPARγ promotes the expression, polymerization and secretion of Adiponectin by activating RUVBL2 transcriptionally, which accelerates 3T3-L1 cell differentiation.

Liver haploinsufficiency of RuvBL1 causes hepatic insulin resistance and enhances hepatocellular carcinoma progression

RuvBL1 is an AAA+ ATPase whose expression in hepatocellular carcinoma (HCC) correlates with a poor prognosis. In vitro models suggest that targeting RuvBL1 could be an effective strategy against HCC. However, the role of RuvBL1 in the onset and progression of HCC remains unknown. To address this question, we developed a RuvBL1^hep+/- mouse model and evaluated the outcome of DEN-induced liver carcinogenesis up to 12 months of progression. We found that RuvBL1 haploinsufficiency initially delayed the onset of liver cancer, due to a reduced hepatocyte turnover in RuvBL1^hep+/- mice. However, RuvBL1^hep+/- mice eventually developed HCC nodules that, with aging, grew larger than in the control mice. Moreover, RuvBL1^hep+/- mice developed hepatic insulin resistance and impaired glucose homeostasis. We could determine that RuvBL1 regulates insulin signaling through the Akt/mTOR pathway in liver physiology in vivo as well as in normal hepatocytic and HCC cells in vitro. Whole transcriptome analysis of mice livers confirmed the major role of RuvBL1 in the regulation of hepatic glucose metabolism. Finally, RuvBL1 expression was found significantly correlated to glucose metabolism and mTOR signaling by bioinformatic analysis of human HCC sample from the publicly available TGCA database. These data uncover a role of RuvBL1 at the intersection of liver metabolism, hepatocyte proliferation and HCC development, providing a molecular rationale for its overexpression in liver cancer.

Multilevel regulation of RUVBL2 expression predicts poor prognosis in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is the second-most lethal cancer worldwide with a complex pathogenesis. RuvB-like 2 (RUVBL2) was previously found to contribute to hepatocarcinogenesis. However, its expression, regulation and clinical significance have not been systematically evaluated in a large number of clinical samples.

Methods

Here, we performed a comprehensive analysis of RUVBL2 based on multiple datasets from 371 liver cancer patients of The Cancer Genome Atlas (TCGA) and on immunohistochemical staining in 153 subjects. In addition, the aberrant signaling pathways caused by RUVBL2 overexpression were investigated.

Results

We demonstrated that promoter hypomethylation, copy number gain, MYC amplification and CTNNB1 mutation were all responsible for RUVBL2 overexpression in HCC. High levels of RUVBL2 mRNA were associated with shorter recurrence-free survival time (RFS) but not overall survival time (OS). Furthermore, RUVBL2 protein was overexpressed in the nucleus and cytoplasm of HCC samples. Univariate and multivariate survival analyses showed that strong nuclear and cytoplasmic staining of RUVBL2 independently predicted worse OS and RFS with a 2.03-fold and a 1.71-fold increase in the hazard ratio, respectively. High levels of RUVBL2 promoted carcinogenesis through the heat shock protein 90 (HSP90)-Cell Division Cycle 37 (CDC37), AKT serine/threonine kinase (AKT) and mitogen-activated protein kinase (ERK/MAPK) pathways.

Conclusion

The deregulation of RUVBL2 in HCC is influenced at the genomic, epigenetic and transcriptional levels. Our findings highlight the potential roles of RUVBL2 as a promising prognostic marker as well as a therapeutic target for HCC.

PGC1A regulates the IRS1:IRS2 ratio during fasting to influence hepatic metabolism downstream of insulin

Precise modulation of hepatic glucose metabolism is crucial during the fasting and feeding cycle and is controlled by the actions of circulating insulin and glucagon. The insulin-signaling pathway requires insulin receptor substrate 1 (IRS1) and IRS2, which are found to be dysregulated in diabetes and obesity. The peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1A) is a fasting-induced transcriptional coactivator. In nonalcoholic fatty liver disease and in patients with type 2 diabetes, low hepatic PGC1A levels are associated with insulin resistance. However, how PGC1A activity impacts the hepatic insulin-signaling pathway is still unclear. We used gain- and loss-of-function models in mouse primary hepatocytes and measured hepatocyte insulin response by gene and protein expression and ex vivo glucose production. We found that the PGC1A level determines the relative ratio of IRS1 and IRS2 in hepatocytes, impacting insulin receptor signaling via protein kinase B/AKT (AKT). PGC1A drove the expression of IRS2 downstream of glucagon signaling while simultaneously reducing IRS1 expression. We illustrate that glucagon- or PGC1A-induced IRS2 expression was dependent on cAMP Response Element Binding Protein activity and that this was essential for suppression of hepatocyte gluconeogenesis in response to insulin in vitro. We also show that increased hepatic PGC1A improves glucose homeostasis in vivo, revealing a counterregulatory role for PGC1A in repressing uncontrolled glucose production in response to insulin signaling. These data highlight a mechanism by which PGC1A plays dual roles in the control of gluconeogenesis during the fasting-to-fed transition through regulated balance between IRS1 and IRS2 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.