Growth hormone ameliorates high glucose-induced steatosis on in vitro cultured human HepG2 hepatocytes by inhibiting de novo lipogenesis via ChREBP and FAS suppression

OBJECTIVE

Growth hormone (GH) deficiency has been associated with increased steatosis but the molecular mechanism has not been fully elucidated. We investigated the effect of GH on lipid accumulation of HepG2 cells cultured on an in vitro steatosis model and examined the potential involvement of insulin-like growth factor 1 (IGF-1) as well as lipogenic and lipolytic molecules.

METHODS

Control and steatosis conditions were induced by culturing HepG2 cells with 5.5 or 25 mmol/l glucose for 24 h, respectively. Afterward, cells were exposed to 0, 5, 10 or 20 ng/ml GH for another 24 h. Lipid content was quantified as well as mRNA and protein levels of IGF-1, carbohydrate responsive element-binding protein (ChREBP), sterol regulatory element-binding protein 1c (SREBP1c), fatty acid synthase (FAS), carnitine palmitoyltransferase 1A (CPT1A), and peroxisome proliferator-activated receptor alpha (PPAR-alpha) by qPCR and western blot, respectively. Data were analyzed by one-way ANOVA and the Games-Howell post-hoc test.

RESULTS

In the steatosis model, HepG2 hepatocytes showed a significant 2-fold increase in lipid amount as compared to control cells. IGF-1 mRNA and protein levels were significantly increased in control cells exposed to 10 ng/ml GH, whereas high glucose abolished this effect. High glucose also significantly increased both mRNA and protein of ChREBP and FAS without having effect on SREBP1c, CPT1A and PPAR-alpha. However, GH inhibited ChREBP and FAS production, even in HepG2 hepatocytes cultured under steatosis conditions.

CONCLUSIONS

Growth hormone ameliorates high glucose-induced steatosis in HepG2 cells by suppressing de novo lipogenesis via ChREBP and FAS down-regulation.

An FGF15/19-TFEB regulatory loop controls hepatic cholesterol and bile acid homeostasis

Bile acid synthesis plays a key role in regulating whole body cholesterol homeostasis. Transcriptional factor EB (TFEB) is a nutrient and stress-sensing transcriptional factor that promotes lysosomal biogenesis. Here we report a role of TFEB in regulating hepatic bile acid synthesis. We show that TFEB induces cholesterol 7α-hydroxylase (CYP7A1) in human hepatocytes and mouse livers and prevents hepatic cholesterol accumulation and hypercholesterolemia in Western diet-fed mice. Furthermore, we find that cholesterol-induced lysosomal stress feed-forward activates TFEB via promoting TFEB nuclear translocation, while bile acid-induced fibroblast growth factor 19 (FGF19), acting via mTOR/ERK signaling and TFEB phosphorylation, feedback inhibits TFEB nuclear translocation in hepatocytes. Consistently, blocking intestinal bile acid uptake by an apical sodium-bile acid transporter (ASBT) inhibitor decreases ileal FGF15, enhances hepatic TFEB nuclear localization and improves cholesterol homeostasis in Western diet-fed mice. This study has identified a TFEB-mediated gut-liver signaling axis that regulates hepatic cholesterol and bile acid homeostasis.

Partitioning of MLX-Family Transcription Factors to Lipid Droplets Regulates Metabolic Gene Expression

Lipid droplets (LDs) store lipids for energy and are central to cellular lipid homeostasis. The mechanisms coordinating lipid storage in LDs with cellular metabolism are unclear but relevant to obesity-related diseases. Here we utilized genome-wide screening to identify genes that modulate lipid storage in macrophages, a cell type involved in metabolic diseases. Among ∼550 identified screen hits is MLX, a basic helix-loop-helix leucine-zipper transcription factor that regulates metabolic processes. We show that MLX and glucose-sensing family members MLXIP/MondoA and MLXIPL/ChREBP bind LDs via C-terminal amphipathic helices. When LDs accumulate in cells, these transcription factors bind to LDs, reducing their availability for transcriptional activity and attenuating the response to glucose. Conversely, the absence of LDs results in hyperactivation of MLX target genes. Our findings uncover a paradigm for a lipid storage response in which binding of MLX transcription factors to LD surfaces adjusts the expression of metabolic genes to lipid storage levels.

Regulation of PGC-1α expression by a GSK-3β-TFEB signaling axis in skeletal muscle

OBJECTIVE

In muscle cells, the peroxisome proliferator-activated receptor γ co-activator 1 (PGC-1) signaling network, which has been shown to be disturbed in the skeletal muscle in several chronic diseases, tightly controls mitochondrial biogenesis and oxidative substrate metabolism. Previously, we showed that inactivation of glycogen synthase kinase (GSK)-3β potently increased Pgc-1α abundance and oxidative metabolism in skeletal muscle cells. The current study aims to unravel the molecular mechanism driving the increase in Pgc-1α mediated by GSK-3β inactivation.

METHODS

GSK-3β was inactivated genetically or pharmacologically in C2C12 myotubes and the requirement of transcription factors known to be involved in Pgc-1α transcription for increases in Pgc-1α abundance mediated by inactivation of GSK-3β was examined.

RESULTS

Enhanced PGC-1α promoter activation after GSK-3β inhibition suggested a transcriptionally-controlled mechanism. While myocyte enhancer factor (MEF)2 transcriptional activity was unaltered, GSK-3β inactivation increased the abundance and activity of the transcription factors estrogen-related receptor (ERR)α and ERRγ. Pharmacological inhibition or knock-down of ERRα and ERRγ however failed to prevent increases in Pgc-1α mRNA mediated by GSK-3β inactivation. Interestingly, GSK-3β inactivation activated transcription factor EB (TFEB), evidenced by decreased phosphorylation and enhanced nuclear localization of the TFEB protein. Moreover, knock-down of TFEB completely prevented increases in Pgc-1α gene expression, PGC-1α promoter activity and PGC-1α protein abundance induced by GSK-3β inactivation. Furthermore, mutation of a specific TFEB binding site on the PGC-1α promoter blocked promoter activation upon inhibition of GSK-3β.

CONCLUSIONS

In skeletal muscle, GSK-3β inactivation causes dephosphorylation and nuclear translocation of TFEB resulting in TFEB-dependent induction of Pgc-1α expression.

A selective high affinity MYC-binding compound inhibits MYC:MAX interaction and MYC-dependent tumor cell proliferation

MYC is a key player in tumor development, but unfortunately no specific MYC-targeting drugs are clinically available. MYC is strictly dependent on heterodimerization with MAX for transcription activation. Aiming at targeting this interaction, we identified MYCMI-6 in a cell-based protein interaction screen for small inhibitory molecules. MYCMI-6 exhibits strong selective inhibition of MYC:MAX interaction in cells and in vitro at single-digit micromolar concentrations, as validated by split Gaussia luciferase, in situ proximity ligation, microscale thermophoresis and surface plasmon resonance (SPR) assays. Further, MYCMI-6 blocks MYC-driven transcription and binds selectively to the MYC bHLHZip domain with a KD of 1.6 ± 0.5 μM as demonstrated by SPR. MYCMI-6 inhibits tumor cell growth in a MYC-dependent manner with IC50 concentrations as low as 0.5 μM, while sparing normal cells. The response to MYCMI-6 correlates with MYC expression based on data from 60 human tumor cell lines and is abrogated by MYC depletion. Further, it inhibits MYC:MAX interaction, reduces proliferation and induces massive apoptosis in tumor tissue from a MYC-driven xenograft tumor model without severe side effects. Since MYCMI-6 does not affect MYC expression, it is a unique molecular tool to specifically target MYC:MAX pharmacologically and it has good potential for drug development.

MondoA Is an Essential Glucose-Responsive Transcription Factor in Human Pancreatic β-Cells

Although the mechanisms by which glucose regulates insulin secretion from pancreatic β-cells are now well described, the way glucose modulates gene expression in such cells needs more understanding. Here, we demonstrate that MondoA, but not its paralog carbohydrate-responsive element-binding protein, is the predominant glucose-responsive transcription factor in human pancreatic β-EndoC-βH1 cells and in human islets. In high-glucose conditions, MondoA shuttles to the nucleus where it is required for the induction of the glucose-responsive genes arrestin domain-containing protein 4 (ARRDC4) and thioredoxin interacting protein (TXNIP), the latter being a protein strongly linked to β-cell dysfunction and diabetes. Importantly, increasing cAMP signaling in human β-cells, using forskolin or the glucagon-like peptide 1 mimetic Exendin-4, inhibits the shuttling of MondoA and potently inhibits TXNIP and ARRDC4 expression. Furthermore, we demonstrate that silencing MondoA expression improves glucose uptake in EndoC-βH1 cells. These results highlight MondoA as a novel target in β-cells that coordinates transcriptional response to elevated glucose levels.

A Druggable TCF4- and BRD4-Dependent Transcriptional Network Sustains Malignancy in Blastic Plasmacytoid Dendritic Cell Neoplasm

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive and largely incurable hematologic malignancy originating from plasmacytoid dendritic cells (pDCs). Using RNAi screening, we identified the E-box transcription factor TCF4 as a master regulator of the BPDCN oncogenic program. TCF4 served as a faithful diagnostic marker of BPDCN, and its downregulation caused the loss of the BPDCN-specific gene expression program and apoptosis. High-throughput drug screening revealed that bromodomain and extra-terminal domain inhibitors (BETis) induced BPDCN apoptosis, which was attributable to disruption of a BPDCN-specific transcriptional network controlled by TCF4-dependent super-enhancers. BETis retarded the growth of BPDCN xenografts, supporting their clinical evaluation in this recalcitrant malignancy.

Novel role of miR-29a in pancreatic cancer autophagy and its therapeutic potential

Pancreatic Ductal Adenocarcinoma (PDAC) is a highly lethal malignancy that responds poorly to current therapeutic modalities. In an effort to develop novel therapeutic strategies, we found downregulation of miR-29 in pancreatic cancer cells, and overexpression of miR-29a sensitized chemotherapeutic resistant pancreatic cancer cells to gemcitabine, reduced cancer cell viability, and increased cytotoxicity. Furthermore, miR-29a blocked autophagy flux, as evidenced by an accumulation of autophagosomes and autophagy markers, LC3B and p62, and a decrease in autophagosome-lysosome fusion. In addition, miR-29a decreased the expression of autophagy proteins, TFEB and ATG9A, which are critical for lysosomal function and autophagosome trafficking respectively. Knockdown of TFEB or ATG9A inhibited autophagy similar to miR-29a overexpression. Finally, miR-29a reduced cancer cell migration, invasion, and anchorage independent growth. Collectively, our findings indicate that miR-29a functions as a potent autophagy inhibitor, sensitizes cancer cells to gemcitabine, and decreases their invasive potential. Our data provides evidence for the use of miR-29a as a novel therapeutic agent to target PDAC.

Induction of the ChREBPβ Isoform Is Essential for Glucose-Stimulated β-Cell Proliferation

Carbohydrate-responsive element-binding protein (ChREBP) is a glucose-sensing transcription factor required for glucose-stimulated proliferation of pancreatic β-cells in rodents and humans. The full-length isoform (ChREBPα) has a low glucose inhibitory domain (LID) that restrains the transactivation domain when glucose catabolism is minimal. A novel isoform of ChREBP (ChREBPβ) was recently described that lacks the LID domain and is therefore constitutively and more potently active. ChREBPβ has not been described in β-cells nor has its role in glucose-stimulated proliferation been determined. We found that ChREBPβ is highly expressed in response to glucose, particularly with prolonged culture in hyperglycemic conditions. In addition, small interfering RNAs that knocked down ChREBPβ transcripts without affecting ChREBPα expression or activity decreased glucose-stimulated expression of carbohydrate response element-containing genes and glucose-stimulated proliferation in INS-1 cells and in isolated rat islets. Quantitative chromatin immunoprecipitation, electrophoretic mobility shift assays, and luciferase reporter assays were used to demonstrate that ChREBP binds to a newly identified powerful carbohydrate response element in β-cells and hepatocytes, distinct from that in differentiated 3T3-L1 adipocytes. We conclude that ChREBPβ contributes to glucose-stimulated gene expression and proliferation in β-cells, with recruitment of ChREBPα to tissue-specific elements of the ChREBPβ isoform promoter.

Oncogenic H-Ras up-regulates acid β-hexosaminidase by a mechanism dependent on the autophagy regulator TFEB

The expression of constitutively active H-RasV12 oncogene has been described to induce proliferative arrest and premature senescence in many cell models. There are a number of studies indicating an association between senescence and lysosomal enzyme alterations, e.g. lysosomal β-galactosidase is the most widely used biomarker to detect senescence in cultured cells and we previously reported that H-RasV12 up-regulates lysosomal glycohydrolases enzymatic activity in human fibroblasts. Here we investigated the molecular mechanisms underlying lysosomal glycohydrolase β-hexosaminidase up-regulation in human fibroblasts expressing the constitutively active H-RasV12. We demonstrated that H-Ras activation increases β-hexosaminidase expression and secretion by a Raf/extracellular signal-regulated protein kinase dependent pathway, through a mechanism that relies on the activity of the transcription factor EB (TFEB). Because of the pivotal role of TFEB in the regulation of lysosomal system biogenesis and function, our results suggest that this could be a general mechanism to enhance lysosomal enzymes activity during oncogene-induced senescence.

Role of glucose-6-phosphate and xylulose-5-phosphate in the regulation of glucose-stimulated gene expression in the pancreatic β cell line, INS-1E

Whether glucose-6-phosphate (G6P) or xylulose-5-phosphate (X5P) is the signaling molecule for carbohydrate response element binding protein (ChREBP) transactivation has been controversial. In this study, we tested the role of G6P and X5P in the regulation of ChREBP transactivation in the pancreatic β cell line, INS-1E. In contrast to glucose, which can be converted into both G6P and X5P, 2DG is only converted into 2DG6P. The potency of 2-deoxy-glucose (2DG) to induce Chrebp target mRNA was weaker and less persistent than that of glucose. Moreover, the results from siRNA knockdown of ChREBP, a reporter assay involving the pGL3 promoter with carbohydrate response element (ChoRE), and a ChIP assay with an anti-ChREBP antibody revealed that 2DG does not increase ChREBP transactivity in INS-1E cells. In accordance with these results, transfection of siRNA against Chrebp tended to reduce glucose-stimulated, but not 2DG-stimulated, expression of ChREBP target genes. Conversely, the expression of xylulokinase (Xylb), which converts xylitol to X5P, was much lower than in primary hepatocytes. In INS-1E cells infected by adenovirus bearing Xylb cDNA, xylitol increased expression of ChREBP target genes, although with a weaker potency than glucose. Finally, X5P partly induced ChREBP transactivity in INS-1E cells overexpressing Xylb cDNA. In conclusion, G6P and X5P can activate ChREBP transactivity, but their potencies to induce ChREBP transactivity were much lower than that of glucose, suggesting that other factors such as fructose 2,6-bisphosphate may be needed for full activation of glucose-induced gene expression.

Activation and repression of glucose-stimulated ChREBP requires the concerted action of multiple domains within the MondoA conserved region

Carbohydrate response element-binding protein (ChREBP) is a glucose-dependent transcription factor that stimulates the expression of glycolytic and lipogenic genes in mammals. Glucose regulation of ChREBP has been mapped to its conserved NH(2)-terminal region of 300 amino acids, designated the MondoA conserved region (MCR). Within the MCR, five domains (MCR1-5) have a particularly high level of conservation and are likely to be important for glucose regulation. We carried out a large-scale deletion and substitution mutational analysis of the MCR domain of ChREBP. This analysis revealed that MCRs 1-4 function in a concerted fashion to repress ChREBP activity in basal (nonstimulatory) conditions. Deletion of the entire MCR1-4 segment or the combination of four specific point mutations located across this region leads to a highly active, glucose-independent form of ChREBP. However, deletion of any individual MCR domain and the majority of point mutations throughout MCR1-4 rendered ChREBP inactive. These observations suggest that the MCR1-4 region interacts with an additional coregulatory factor required for activation. This possibility is supported by the observation that the MCR1-4 region can compete for activity with wild-type ChREBP in stimulatory conditions. In contrast, mutations in the MCR5 domain result in increased activity, suggesting that this domain may be the target of intramolecular repression in basal conditions. Thus, the MCR domains act in a complex and coordinated manner to regulate ChREBP activity in response to glucose.

Mnt takes control as key regulator of the myc/max/mxd network

Myc is the most frequently deregulated oncogene in human tumors. The protein belongs to the Myc/Max/Mxd network of transcriptional regulators important for cell growth, proliferation, differentiation, and apoptosis. The ratio between Mnt/Max and c-Myc/Max on the 5'-CACGTG-3' E-box sequence at shared target genes is of great importance for cell cycle progression and arrest. Serum stimulation of quiescent cells results in phosphorylation of Mnt and disruption of the critical Mnt-mSin3-HDAC1 interaction. This in turn leads to increased expression of the Myc/Mnt target gene cyclin D2. It is therefore possible that Myc function relies on its ability to overcome transcriptional repression by Mnt and that relief of Mnt-mediated transcriptional repression is of greater importance for regulation of target genes than the sole activation by Myc. In addition, Mnt has many features of a tumor suppressor and may thus be nonfunctional or inactivated in human tumors. In summary, accumulating evidence supports the model of Mnt as the key regulator of the network in vivo.

Regulation of lipogenesis via BHLHB2/DEC1 and ChREBP feedback looping

BHLHB2/DEC1 is a transcription factor implicated in cell proliferation, apoptosis, and metabolism, and is also known to play an important role in the regulation of the mammalian circadian rhythm. However, its precise role in metabolism remains unclear. We investigated the link between BHLHB2 and ChREBP, a glucose-activated transcription factor involved in the regulation of lipogenesis. Glucose stimulation and overexpression of dominant active ChREBP induced Bhlhb2 mRNA expression in rat hepatocytes. Deletion studies showed that ChoRE (-160 to -143bp) in the mouse Bhlhb2 promoter region is functional in vivo. Overexpression of BHLHB2 inhibited glucose and ChREBP-mediated induction of rat Fasn and liver pyruvate kinase (Lpk) mRNA. ChIP assay demonstrated that BHLHB2 bound to ChoRE in the Fasn, Lpk, and Bhlhb2 promoter regions in vivo. In conclusion, BHLHB2 and ChREBP constitute a novel feedback loop involved in the regulation of lipogenesis.

Efficacy, pharmacokinetics, tisssue distribution, and metabolism of the Myc-Max disruptor, 10058-F4 [Z,E]-5-[4-ethylbenzylidine]-2-thioxothiazolidin-4-one, in mice

OBJECTIVES

c-Myc is commonly activated in many human tumors and is functionally important in cellular proliferation, differentiation, apoptosis and cell cycle progression. The activity of c-Myc requires noncovalent interaction with its client protein Max. In vitro studies indicate the thioxothiazolidinone, 10058-F4, inhibits c-Myc/Max dimerization. In this study, we report the efficacy, pharmacokinetics and metabolism of this novel protein-protein disruptor in mice.

METHODS

SCID mice bearing DU145 or PC-3 human prostate cancer xenografts were treated with either 20 or 30 mg/kg 10058-F4 on a qdx5 schedule for 2 weeks for efficacy studies. For pharmacokinetics and metabolism studies, mice bearing PC-3 or DU145 xenografts were treated with 20 mg/kg of 10058-F4 i.v. Plasma and tissues were collected 5-1440 min after dosing. The concentration of 10058-F4 in plasma and tissues was determined by HPLC, and metabolites were characterized by LC-MS/MS.

RESULTS

Following a single iv dose, peak plasma 10058-F4 concentrations of approximately 300 muM were seen at 5 min and declined to below the detection limit at 360 min. Plasma concentration versus time data were best approximated by a two-compartment, open, linear model. The highest tissue concentrations of 10058-F4 were found in fat, lung, liver, and kidney. Peak tumor concentrations of 10058-F4 were at least tenfold lower than peak plasma concentrations. Eight metabolites of 10058-F4 were identified in plasma, liver, and kidney. The terminal half-life of 10058-F4 was approximately 1 h, and the volume of distribution was >200 ml/kg. No significant inhibition of tumor growth was seen after i.v. treatment of mice with either 20 or 30 mg/kg 10058-F4.

CONCLUSION

The lack of significant antitumor activity of 10058-F4 in tumor-bearing mice may have resulted from its rapid metabolism and low concentration in tumors.

Disruption of the MYC transcriptional function by a small-molecule antagonist of MYC/MAX dimerization

Inhibition of MYC/MAX dimerization by a small-molecule antagonist (IIA6B17) has been shown to interfere with MYC-induced transformation of chick embryo fibroblasts, suggesting that the functional inhibitors of the MYC family of oncoproteins have potential as therapeutic agents. In the present study, a functional MYC reporter gene assay has been developed, using a luciferase gene construct under the control of the ornithine decarboxylase (ODC) gene promoter. This luciferase gene construct has been stably transfected into the MYCN amplified neuroblastoma cell line (NGP) and MYCC-overexpressed neuroepithelioma cell line (NB100). After exposure of the cell lines to IIA6B17 for 24 h, a significant reduction of luciferase activity was only observed in the NB100 cells, with IC50 values of approximately 28+/-9 microM, indicating that IIA6B17 has cell line-specific activity which may be selective for individual members of the MYC family.

Inflammatory disease and lymphomagenesis caused by deletion of the Myc antagonist Mnt in T cells

Mnt is a Max-interacting protein that can antagonize the activities of Myc oncoproteins in cultured cells. Mnt null mice die soon after birth, but conditional deletion of Mnt in breast epithelium leads to tumor formation. These and related data suggest that Mnt functions as a tumor suppressor. Here we show that conditional deletion of Mnt in T cells leads to tumor formation but also causes inflammatory disease. Deletion of Mnt caused increased apoptosis of thymic T cells and interfered with T-cell development yet led to spleen, liver, and lymph node enlargement. The proportion of T cells in the spleen and lymph nodes was reduced, and the numbers of cells in non-T-cell immune cell populations were elevated. The disruption of immune homeostasis is linked to a strong skewing toward production of T-helper 1 (Th1) cytokines and enhanced proliferation of activated Mnt-deficient CD4+ T cells. Consistent with Th1 polarization in vivo, extensive intestinal inflammation and liver necrosis developed. Finally, most mice lacking Mnt in T cells ultimately succumbed to T-cell lymphoma. These results strengthen the argument that Mnt functions as a tumor suppressor and reveal a critical and surprising role for Mnt in the regulation of T-cell development and in T-cell-dependent immune homeostasis.

Mnt transcriptional repressor is functionally regulated during cell cycle progression

The Myc/Max/Mad network of transcription factors regulates cell proliferation, differentiation, and transformation. Similar to other proteins of the network, Mnt forms heterodimers with Max and binds CACGTG E-Box elements. Transcriptional repression by Mnt is mediated through association with mSin3, and deletion of the mSin3-interacting domain (SID) converts Mnt to a transcriptional activator. Mnt is coexpressed with Myc in proliferating cells and has been suggested to be a modulator of Myc function. We report that Mnt is expressed both in growth-arrested and proliferating mouse fibroblasts and is phosphorylated when resting cells are induced to re-enter the cell cycle. Importantly, the interaction between Mnt and mSin3 is disrupted upon serum stimulation resulting in decreased Mnt-associated HDAC activity. Furthermore, we demonstrate that Mnt binds and recruits mSin3 to the Myc target gene cyclin D2 in quiescent mouse fibroblasts. Interference with Mnt expression by RNAi resulted in upregulation of cyclin D2 expression in growth-arrested fibroblasts, supporting the view that Mnt represses cyclin D2 transcription in quiescent cells. Our data suggest a model in which phosphorylation of Mnt at cell cycle entry results in disruption of Mnt-mSin3-HDAC1 interaction, which allows induction of Myc target genes by release of Mnt-mediated transcriptional repression.