Prevention of diabetic alterations in transgenic mice overexpressing Myc in the liver

Correlation guided Network Integration (CoNI) reveals novel genes affecting hepatic metabolism

Objective

Technological advances have brought a steady increase in the availability of various types of omics data, from genomics to metabolomics. Integrating these multi-omics data is a chance and challenge for systems biology; yet, tools to fully tap their potential remain scarce.

Methods

We present here a fully unsupervised and versatile correlation-based method – termed Correlation guided Network Integration (CoNI) – to integrate multi-omics data into a hypergraph structure that allows for the identification of effective modulators of metabolism. Our approach yields single transcripts of potential relevance that map to specific, densely connected, metabolic subgraphs or pathways.

Results

By applying our method on transcriptomics and metabolomics data from murine livers under standard Chow or high-fat diet, we identified eleven genes with potential regulatory effects on hepatic metabolism. Five candidates, including the hepatokine INHBE, were validated in human liver biopsies to correlate with diabetes-related traits such as overweight, hepatic fat content, and insulin resistance (HOMA-IR).

Conclusion

Our method's successful application to an independent omics dataset confirmed that the novel CoNI framework is a transferable, entirely data-driven, flexible, and versatile tool for multiple omics data integration and interpretation.

Comparative study of testosterone and vitamin D analogue, elocalcitol, on insulin-controlled signal transduction pathway regulation in human skeletal muscle cells

PURPOSE

Skeletal muscle (Skm) plays a key role in regulating energetic metabolism through glucose homeostasis. Several hormones such as Testosterone (T) and Vitamin D (VD) have been shown to affect energy-dependent cell trafficking by determining Insulin (I)-like effects.

AIM

To elucidate possible hormone-related differences on muscular metabolic control, we analyzed and compared the effects of T and elocalcitol (elo), a VD analogue, on the activation of energy-dependent cell trafficking, metabolism-related-signal transduction pathways and transcription of gene downstream targets.

METHODS

Human fetal skeletal muscle cells (Hfsmc) treated with T or elo were analyzed for GLUT4 localization, phosphorylation/activation status of AKT, ERK1/2, IRS-1 signaling and c-MYC protein expression.

RESULTS

T, similar to elo, induced GLUT4 protein translocation likely in lipid raft microdomains. While both T and elo induced a rapid IRS-1 phosphorylation, the following dynamic in phosphorylation/activation of AKT and ERK1/2 signaling was different. Moreover, T but not elo increased c-MYC protein expression.

CONCLUSIONS

All together, our evidence indicates that whether both T and elo are able to affect upstream I-like pathway, they differently determine downstream effects in I-dependent cascade, suggesting diverse physiological roles in mediating I-like response in human skeletal muscle.

c-MYC-Making Liver Sick: Role of c-MYC in Hepatic Cell Function, Homeostasis and Disease

Over 35 years ago, c-MYC, a highly pleiotropic transcription factor that regulates hepatic cell function, was identified. In recent years, a considerable increment in the number of publications has significantly shifted the way that the c-MYC function is perceived. Overexpression of c-MYC alters a wide range of roles including cell proliferation, growth, metabolism, DNA replication, cell cycle progression, cell adhesion and differentiation. The purpose of this review is to broaden the understanding of the general functions of c-MYC, to focus on c-MYC-driven pathogenesis in the liver, explain its mode of action under basal conditions and during disease, and discuss efforts to target c-MYC as a plausible therapy for liver disease.

Drosophila Myc: A master regulator of cellular performance

The identification of the Drosophila homolog of the human MYC oncogene has fostered a series of studies aimed to address its functions in development and cancer biology. Due to its essential roles in many fundamental biological processes it is hard to imagine a molecular mechanism in which MYC function is not required. For this reason, the easily manipulated Drosophila system has greatly helped in the dissection of the genetic and molecular pathways that regulate and are regulated by MYC function. In this review, we focus on studies of MYC in the fruitfly with particular emphasis on metabolism and cell competition, highlighting the contributions of this model system in the last decade to our understanding of MYC's complex biological nature. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

dMyc expression in the fat body affects DILP2 release and increases the expression of the fat desaturase Desat1 resulting in organismal growth

Drosophila dMyc (dMyc) is known for its role in cell-autonomous regulation of growth. Here we address its role in the fat body (FB), a metabolic tissue that functions as a sensor of circulating nutrients to control the release of Drosophila Insulin-like peptides (Dilps) from the brain influencing growth and development. Our results show that expression of dMyc in the FB affects development and animal size. Expression of dMyc, but not of CycD/cdk4 or Rheb, in the FB diminishes the ability to retain Drosophila Insulin-like peptide-2 (DILP2) in the brain during starvation, suggesting that expression of dMyc mimics the signal that remotely controls the release of Dilps into the hemolymph. dMyc also affects glucose metabolism and increases the transcription of Glucose-transporter-1 mRNA, and of Hexokinase and Pyruvate-Kinase mRNAs, key regulators of glycolysis. These animals are able to counteract the increased levels of circulating trehalose induced by a high sugar diet leading to the conclusion that dMyc activity in the FB promotes glucose disposal. dMyc expression induces cell autonomous accumulation of triglycerides, which correlates with increased levels of Fatty Acid Synthase and Acetyl CoA Carboxylase mRNAs, enzymes responsible for lipid synthesis. We also found the expression of Stearoyl-CoA desaturase, Desat1 mRNA significantly higher in FB overexpressing dMyc. Desat1 is an enzyme that is necessary for monosaturation and production of fatty acids, and its reduction affects dMyc's ability to induce fat storage and resistance to animal survival. In conclusion, here we present novel evidences for dMyc function in the Drosophila FB in controlling systemic growth. We discovered that dMyc expression triggers cell autonomous mechanisms that control glucose and lipid metabolism to favor the storage of nutrients (lipids and sugars). In addition, the regulation of Desat1 controls the synthesis of triglycerides in FB and this may affect the humoral signal that controls DILP2 release in the brain.

Systems genetics of metabolism: the use of the BXD murine reference panel for multiscalar integration of traits

Metabolic homeostasis is achieved by complex molecular and cellular networks that differ significantly among individuals and are difficult to model with genetically engineered lines of mice optimized to study single gene function. Here, we systematically acquired metabolic phenotypes by using the EUMODIC EMPReSS protocols across a large panel of isogenic but diverse strains of mice (BXD type) to study the genetic control of metabolism. We generated and analyzed 140 classical phenotypes and deposited these in an open-access web service for systems genetics (www.genenetwork.org). Heritability, influence of sex, and genetic modifiers of traits were examined singly and jointly by using quantitative-trait locus (QTL) and expression QTL-mapping methods. Traits and networks were linked to loci encompassing both known variants and novel candidate genes, including alkaline phosphatase (ALPL), here linked to hypophosphatasia. The assembled and curated phenotypes provide key resources and exemplars that can be used to dissect complex metabolic traits and disorders.

Postnatal liver growth and regeneration are independent of c-myc in a mouse model of conditional hepatic c-myc deletion

BACKGROUND

The transcription factor c-myc regulates genes involved in hepatocyte growth, proliferation, metabolism, and differentiation. It has also been assigned roles in liver development and regeneration. In previous studies, we made the unexpected observation that c-Myc protein levels were similar in proliferating fetal liver and quiescent adult liver with c-Myc displaying nucleolar localization in the latter. In order to investigate the functional role of c-Myc in adult liver, we have developed a hepatocyte-specific c-myc knockout mouse, c-mycfl/fl;Alb-Cre.

RESULTS

Liver weight to body weight ratios were similar in control and c-myc deficient mice. Liver architecture was unaffected. Conditional c-myc deletion did not result in compensatory induction of other myc family members or in c-Myc's binding partner Max. Floxed c-myc did have a negative effect on Alb-Cre expression at 4 weeks of age. To explore this relationship further, we used the Rosa26 reporter line to assay Cre activity in the c-myc floxed mice. No significant difference in Alb-Cre activity was found between control and c-mycfl/fl mice. c-myc deficient mice were studied in a nonproliferative model of liver growth, fasting for 48 hr followed by a 24 hr refeeding period. Fasting resulted in a decrease in liver mass and liver protein, both of which recovered upon 24 h of refeeding in the c-mycfl/fl;Alb-Cre animals. There was also no effect of reducing c-myc on recovery of liver mass following 2/3 partial hepatectomy.

CONCLUSIONS

c-Myc appears to be dispensable for normal liver growth during the postnatal period, restoration of liver mass following partial hepatectomy and recovery from fasting.

Therapeutic potential of c-Myc inhibition in the treatment of hypertrophic cardiomyopathy

Investigating the pathophysiological importance of the molecular and mechanical development of cardiomyopathy is critical to find new and broader means of protection against this disease that is increasing in prevalence and impact. The current available treatment options for cardiomyopathy mainly focus on treating symptoms and strive to make the patient more comfortable while preventing progression of disease and sudden death. The proto-oncogene c-Myc (Myc) has been shown to be increased in many different types of heart disease, including hypertrophic cardiomyopathy, before any signs of the disease are present. As the mechanisms of action and multiple pathways of dependent actions of Myc are being dissected by many research groups, inhibition of Myc is becoming an attractive paradigm for prevention and treatment of cardiomyopathy and heart failure. Elucidating the role Myc plays in the development, propagation and perpetuation of cardiomyopathy and heart failure will one day translate into potential therapeutics for cardiomyopathy.

Inhibition of ubiquitin ligase F-box and WD repeat domain-containing 7α (Fbw7α) causes hepatosteatosis through Krüppel-like factor 5 (KLF5)/peroxisome proliferator-activated receptor γ2 (PPARγ2) pathway but not SREBP-1c protein in mice

F-box and WD repeat domain-containing 7α (Fbw7α) is the substrate recognition component of a ubiquitin ligase that controls the degradation of factors involved in cellular growth, including c-Myc, cyclin E, and c-Jun. In addition, Fbw7α degrades the nuclear form of sterol regulatory element-binding protein (SREBP)-1a, a global regulator of lipid synthesis, particularly during mitosis in cultured cells. This study investigated the in vivo role of Fbw7α in hepatic lipid metabolism. siRNA knockdown of Fbw7α in mice caused marked hepatosteatosis with the accumulation of triglycerides. However, inhibition of Fbw7α did not change the level of nuclear SREBP-1 protein or the expression of genes involved in fatty acid synthesis and oxidation. In vivo experiments on the gain and loss of Fbw7α function indicated that Fbw7α regulated the expression of peroxisome proliferator-activated receptor (PPAR) γ2 and its target genes involved in fatty acid uptake and triglyceride synthesis. These genes included fatty acid transporter Cd36, diacylglycerol acyltransferase 1 (Dgat1), and fat-specific protein 27 (Cidec). The regulation of PPARγ2 by Fbw7α was mediated, at least in part, by the direct degradation of the Krüppel-like factor 5 (KLF5) protein, upstream of PPARγ2 expression. Hepatic Fbw7α contributes to normal fatty acid and triglyceride metabolism, functions that represent novel aspects of this cell growth regulator.

The impact of miR-34a on protein output in hepatocellular carcinoma HepG2 cells

MicroRNAs are small non-coding RNA molecules that play essential roles in biological processes ranging from cell cycle to cell migration and invasion. Accumulating evidence suggests that miR-34a, as a key mediator of p53 tumor suppression, is aberrantly expressed in human cancers. In the present study, we aimed to explore the precise biological role of miR-34a and the global protein changes in HCC cell line HepG2 cells transiently transfected with miR-34a. Transfection of miR-34a into HepG2 cells caused suppression of cell proliferation, inhibition of cell migration and invasion. It also induced an accumulation of HepG2 cells in G1 phase. Among 116 protein spots with differential expression separated by 2-DE method, 34 proteins were successfully identified by MALDI-TOF/TOF analysis. Of these, 15 downregulated proteins may be downstream targets of miR-34a. Bioinformatics analysis produced a protein-protein interaction network, which revealed that the p53 signaling pathway and cell cycle pathway were two major hubs containing most of the proteins regulated by miR-34a. Cytoskeletal proteins such as LMNA, GFAP, MACF1, ALDH2, and LOC100129335 are potential targets of miR-34a. In conclusion, abrogation of miR-34a function could cause downstream molecules to switch on or off, leading to HCC development.

c-Myc is required for the CHREBP-dependent activation of glucose-responsive genes

Glucose regulates programs of gene expression that orchestrate changes in cellular phenotype in several metabolically active tissues. Carbohydrate response element-binding protein (ChREBP) and its binding partner, Mlx, mediate glucose-regulated gene expression by binding to carbohydrate response elements on target genes, such as the prototypical glucose-responsive gene, liver-type pyruvate kinase (Pklr). c-Myc is also required for the glucose response of the Pklr gene, although the relationship between c-Myc and ChREBP has not been defined. Here we describe the molecular events of the glucose-mediated activation of Pklr and determine the effects of decreasing the activity or abundance of c-Myc on this process. Time-course chromatin immunoprecipitation revealed a set of transcription factors [hepatocyte nuclear factor (HNF)1alpha, HNF4alpha, and RNA polymerase II (Pol II)] constitutively resident on the Pklr promoter, with a relative enrichment of acetylated histones 3 and 4 in the same region of the gene. Glucose did not affect HNF1alpha binding or the acetylation of histones H3 or H4. By contrast, glucose promoted the recruitment of ChREBP and c-Myc and increased the occupancy of HNF4alpha and RNA Pol II, which were coincident with the glucose-mediated increase in transcription as determined by a nuclear run-on assay. Depletion of c-Myc activity using a small molecule inhibitor (10058-F4/1RH) abolished the glucose-mediated recruitment of HNF4alpha, ChREBP, and RNA Pol II, without affecting basal gene expression, histone acetylation, and HNF1alpha or basal HNF4alpha occupancy. The activation and recruitment of ChREBP to several glucose-responsive genes were blocked by 1RH, indicating a general necessity for c-Myc in this process.

Insulin-independent promotion of chemically induced hepatocellular tumor development in genetically diabetic mice

Diabetes mellitus has been proposed as an epidemiological risk factor for human liver cancer development. One reasonable possibility is that this is attributable to hyperinsulinemia compensatory for obesity-related insulin resistance. However, diabetes mellitus is a complex disease with multiple abnormal conditions essentially caused by hyperglycemia. Therefore, it is not evident whether hyperinsulinemia is prerequisite for the elevated cancer risk. To gain a clue to answer this question, we characterized chemically induced hepatocarcinogenesis in diabetic model mice genetically deficient for insulin. Akita inbred mice originating from the C57BL/6 strain carry a heterozygous germline mutation of the insulin II gene and suffer from inherited insulin deficiency and diabetes in an autosomal dominant manner. They were mated with normal C3H/HeJ mice with high sensitivity to liver carcinogenesis and the resultant F(1) littermates, which were either normal or insulin deficient, were exposed to diethylnitrosamine and induced hepatocellular tumors were evaluated for number, size, proliferative activity, and apoptosis. Unexpectedly, both mean and total volumes of hepatocellular tumors in the insulin-deficient animals were more than twofold larger than those in the normal controls, with no significant difference in tumor number. The tumors in insulin-deficient mice showed a significantly lower frequency of apoptosis but no alteration in cell proliferation. In conclusion, our results indicate that insulin-independent liver tumor promotion occurred in diabetic mice. Clearly, insulin-independent mechanisms for the human case also deserve consideration.

3,5-diiodo-l-thyronine, by modulating mitochondrial functions, reverses hepatic fat accumulation in rats fed a high-fat diet

BACKGROUND/AIMS

Mitochondrial dysfunction is central to the physiopathology of steatosis and/or non-alcoholic fatty liver disease. In this study on rats we investigated whether 3,5-diiodo-l-thyronine (T2), a biologically active iodothyronine, acting at mitochondrial level is able to reverse hepatic steatosis after its induction through a high-fat diet.

METHODS

Hepatic steatosis was induced by long-term high-fat feeding of rats for six weeks which were then fed the same high-fat diet for the next 4 weeks and were simultaneously treated or not treated with T2. Histological analyses were performed on liver sections (by staining with Sudan black B). In liver mitochondria fatty acid oxidation rate, mitochondrial efficiency (by measuring proton conductance) and mitochondrial oxidative stress (by measuring H(2)O(2) release, aconitase and SOD activity) were detected.

RESULTS

Stained sections showed that T2 treatment reduced hepatic fatty accumulation induced by a high-fat diet. At the mitochondrial level, the fatty acid oxidation rate and carnitine palmitoyl transferase activity were enhanced by T2 treatment. Moreover, by stimulating mitochondrial uncoupling, T2 caused less efficient utilization of fatty acid substrates and ameliorated mitochondrial oxidative stress.

CONCLUSION

These data demonstrate that T2, by activating mitochondrial processes, markedly reverses hepatic steatosis in vivo.

Activation of the insulin receptor (IR) by insulin and a synthetic peptide has different effects on gene expression in IR-transfected L6 myoblasts

Single-chain peptides have been recently produced that display either mimetic or antagonistic properties against the insulin and IGF-1 (insulin-like growth factor 1) receptors. We have shown previously that the insulin mimetic peptide S597 leads to significant differences in receptor activation and initiation of downstream signalling cascades despite similar binding affinity and in vivo hypoglycaemic potency. It is still unclear how two ligands can initiate different signalling responses through the IR (insulin receptor). To investigate further how the activation of the IR by insulin and S597 differentially activates post-receptor signalling, we studied the gene expression profile in response to IR activation by either insulin or S597 using microarray technology. We found striking differences between the patterns induced by these two ligands. Most remarkable was that almost half of the genes differentially regulated by insulin and S597 were involved in cell proliferation and growth. Insulin either selectively regulated the expression of these genes or was a more potent regulator. Furthermore, we found that half of the differentially regulated genes interact with the genes involved with the MAPK (mitogen-activated protein kinase) pathway. These findings support our signalling results obtained previously and confirm that the main difference between S597 and insulin stimulation resides in the activation of the MAPK pathway. In conclusion, we show that insulin and S597 acting via the same receptor differentially affect gene expression in cells, resulting in a different mitogenicity of the two ligands, a finding which has critical therapeutic implications.

Nutritional control of protein biosynthetic capacity by insulin via Myc in Drosophila

Animals use the insulin/TOR signaling pathway to mediate their response to fluctuations in nutrient availability. Energy and amino acids are monitored at the single-cell level via the TOR branch of the pathway and systemically via insulin signaling to regulate cellular growth and metabolism. Using a combination of genetics, expression profiling, and chromatin immunoprecipitation, we examine nutritional control of gene expression and identify the transcription factor Myc as an important mediator of TOR-dependent regulation of ribosome biogenesis. We also identify myc as a direct target of FOXO and provide genetic evidence that Myc has a key role in mediating the effects of TOR and FOXO on growth and metabolism. FOXO and TOR also converge to regulate protein synthesis, acting via 4E-BP and Lk6, regulators of the translation factor eIF4E. This study uncovers a network of convergent regulation of protein biosynthesis by the FOXO and TOR branches of the nutrient-sensing pathway.

Co-transfection of GK and mhPINS genes into HepG2 cells confers glucose-stimulated insulin secretion

BACKGROUND

The purpose of this study was to construct an 'artificial beta cell' that can exhibit physiologic glucose-stimulated insulin secretion for the treatment of type 1 diabetes.

METHODS

Retroviral vector containing the glucokinase (GK) gene and mutated human proinsulin (mhPINS) gene was constructed. HepG2 cells were first infected with recombinant retrovirus carrying the GK and mhPINS genes, then selectively cultured with G418 to obtain the positive clones. GK and mhPINS gene transcription and expression were identified by radioimmunity, Western blot and RT-PCR techniques. Finally, the dose-response effect of glucose on insulin secretion from those HepG2 cells that expressed both GK and mhPINS genes was tested with HepG2 cells that only expressed the mhPINS gene as a control.

RESULTS

HepG2 cells with transferred GK and mhPINS genes were selectively cultured with G418 and the positive clones were obtained in 3 weeks. Four clones with GK and mhPINS gene expression were selected from 20 positive clones by radioimmunity and Western blot. We picked up one clone with a strong GK and mhPINS gene expression and named it clone Beta. In clone Beta, differences in insulin secretion at 0.5 and 0.75 mmol/L glucose concentrations were not significant (P>0.05) and differences in insulin secretion at 2.0, 3.0, 4.0, 5.0 and 6.0 mmol/L glucose concentrations were not significant (P>0.05), while there were significant differences in insulin secretion at other glucose concentrations(P<0.05). The artificial beta cell, clone Beta, obtained a glucose-stimulated insulin secretion with maximal insulin secretion at 1.75-2.00 mmol/L glucose concentrations.

DISCUSSION

An artificial beta cell that exhibits glucose-stimulated insulin secretion can be constructed successfully.

Somatostatin receptor subtype-2-deficient mice with diet-induced obesity have hyperglycemia, nonfasting hyperglucagonemia, and decreased hepatic glycogen deposition

Hypersecretion of glucagon contributes to abnormally increased hepatic glucose output in type 2 diabetes. Somatostatin (SST) inhibits murine glucagon secretion from isolated pancreatic islets via somatostatin receptor subtype-2 (sst2). Here, we characterize the role of sst2 in controlling glucose homeostasis in mice with diet-induced obesity. Sst2-deficient (sst2(-/-)) and control mice were fed high-fat diet for 14 wk, and the parameters of glucose homeostasis were monitored. Hepatic glycogen and lipid contents were quantified enzymatically and visualized histomorphologically. Enzymes regulating glycogen and lipid synthesis and breakdown were measured by real-time PCR and/or Western blot. Gluconeogenesis and glycogenolysis were determined from isolated primary hepatocytes and glucagon or insulin secretion from isolated pancreatic islets. Nonfasting glucose, glucagon, and fasting nonesterified fatty acids of sst2(-/-) mice were increased. Inhibition of glucagon secretion from sst2-deficient pancreatic islets by glucose or somatostatin was impaired. Insulin less potently reduced blood glucose concentration in sst2-deficient mice as compared with wild-type mice. Sst2-deficient mice had decreased nonfasting hepatic glycogen and lipid content. The activity/expression of enzymes controlling hepatic glycogen synthesis of sst2(-/-) mice was decreased, whereas enzymes facilitating glycogenolysis and lipolysis were increased. Somatostatin and an sst2-selective agonist decreased glucagon-induced glycogenolysis, without influencing de novo glucose production using cultured primary hepatocytes. This study demonstrates that ablation of sst2 leads to hyperglucagonemia. Increased glucagon concentration is associated with impaired glucose control in sst2(-/-) mice, resulting from decreased hepatic glucose storage, increased glycogen breakdown, and reduced lipid accumulation. Sst2 may constitute a therapeutic target to lower hyperglucagonemia in type 2 diabetes.

KetogenicHMGCS2Is a c-Myc Target Gene Expressed in Differentiated Cells of Human Colonic Epithelium and Down-Regulated in Colon Cancer

HMGCS2, the gene that regulates ketone body production, is expressed in liver and several extrahepatic tissues, such as the colon. In CaCo-2 colonic epithelial cells, the expression of this gene increases with cell differentiation. Accordingly, immunohistochemistry with specific antibodies shows that HMGCS2 is expressed mainly in differentiated cells of human colonic epithelium. Here, we used a chromatin immunoprecipitation assay to study the molecular mechanism responsible for this expression pattern. The assay revealed that HMGCS2 is a direct target of c-Myc, which represses HMGCS2 transcriptional activity. c-Myc transrepression is mediated by blockade of the transactivating activity of Miz-1, which occurs mainly through a Sp1-binding site in the proximal promoter of the gene. Accordingly, the expression of human HMGCS2 is down-regulated in 90% of Myc-dependent colon and rectum tumors. HMGCS2 protein expression is down-regulated preferentially in moderately and poorly differentiated carcinomas. In addition, it is also down-regulated in 80% of small intestine Myc-independent tumors. Based on these findings, we propose that ketogenesis is an undesirable metabolic characteristic of the proliferating cell, which is down-regulated through c-Myc-mediated repression of the key metabolic gene HMGCS2.

Mitochondrial dysfunction in NASH: causes, consequences and possible means to prevent it

Calorie-enriched diet and lack of exercise are causing a worldwide surge of obesity, insulin resistance and lipid accretion in liver (i.e. hepatic steatosis), which can lead to steatohepatitis. Steatosis and nonalcoholic steatohepatitis (NASH) can also be induced by drugs such as amiodarone, tamoxifen and some antiretroviral drugs, including stavudine and zidovudine. There is accumulating evidence that mitochondrial dysfunction (more particularly respiratory chain deficiency) plays a key role in the physiopathology of NASH whatever its initial cause. In contrast, the mitochondrial beta-oxidation of fatty acids can be either increased (as in insulin resistance-associated NASH) or decreased (as in drug-induced NASH). However, in both circumstances, generation of reactive oxygen species (ROS) by the damaged respiratory chain can be augmented. ROS generation in an environment enriched in lipids in turn induces lipid peroxidation which releases highly reactive aldehydic derivatives (e.g. malondialdehyde) that have diverse detrimental effects on hepatocytes and other hepatic cells. In hepatocytes, ROS, reactive nitrogen species and lipid peroxidation products further impair the respiratory chain, either directly or indirectly through oxidative damage to the mitochondrial genome. This consequently leads to the generation of more ROS and a vicious cycle occurs. Mitochondrial dysfunction can also lead to apoptosis or necrosis depending on the energy status of the cell. ROS and lipid peroxidation products also increase the generation of several cytokines (TNF-alpha, TGF-beta, Fas ligand) playing a key role in cell death, inflammation and fibrosis. Recent investigations have shown that some genetic polymorphisms can significantly increase the risk of steatohepatitis and that several drugs can prevent or even reverse NASH. Interestingly, most of these drugs could exert their beneficial effects by improving directly or indirectly mitochondrial function in liver. Finding a drug, which could fully prevent oxidative stress and mitochondrial dysfunction in NASH is a major challenge for the next decade.

TFE3 transcriptionally activates hepatic IRS-2, participates in insulin signaling and ameliorates diabetes

Using an expression cloning strategy, we have identified TFE3, a basic helix-loop-helix protein, as a transactivator of metabolic genes that are regulated through an E-box in their promoters. Adenovirus-mediated expression of TFE3 in hepatocytes in culture and in vivo strongly activated expression of IRS-2 and Akt and enhanced phosphorylation of insulin-signaling kinases such as Akt, glycogen synthase kinase 3beta and p70S6 kinase. TFE3 also induced hexokinase II (HK2) and insulin-induced gene 1 (INSIG1). These changes led to metabolic consequences, such as activation of glycogen and protein synthesis, but not lipogenesis, in liver. Collectively, plasma glucose levels were markedly reduced both in normal mice and in different mouse models of diabetes, including streptozotocin-treated, db/db and KK mice. Promoter analyses showed that IRS2, HK2 and INSIG1 are direct targets of TFE3. Activation of insulin signals in both insulin depletion and resistance suggests that TFE3 could be a therapeutic target for diabetes.

A mutant Stat5b with weaker DNA binding affinity defines a key defective pathway in nonobese diabetic mice

A number of cytokines that finely regulate immune response have been implicated in the pathogenesis or protection of type 1 diabetes and other autoimmune diseases. It is, therefore, of pivotal importance to examine a family of proteins that serve as signal transducers and activators of transcription (STATs), which regulate the transcription of a variety of cytokines. We report here a defective gene (Stat5b) located on chromosome 11 within a previously mapped T1D susceptibility interval (Idd4) in the nonobese diabetic (NOD) mice. Our sequencing analysis revealed a unique mutation C1462A that results in a leucine to methionine (L327M) in Stat5b of NOD mice. Leu(327), the first residue in the DNA binding domain of STAT proteins, is conserved in all identified mammalian STAT proteins. Homology modeling predicted that the mutant Stat5b has a weaker DNA binding, which was confirmed by DNA-protein binding assays. The inapt transcriptional regulation ability of the mutated Stat5b is proved by decreased levels of RNA of Stat5b-regulated genes (IL-2Rbeta and Pim1). Consequently, IL-2Rbeta and Pim1 proteins were shown by Western blotting to have lower levels in NOD compared with normal B6 mice. These proteins have been implicated in immune regulation, apoptosis, activation-induced cell death, and control of autoimmunity. Therefore, the Stat5b pathway is a key molecular defect in NOD mice.

Overexpression of c-myc in the liver prevents obesity and insulin resistance

Alterations in hepatic glucose metabolism play a key role in the development of the hyperglycemia observed in type 2 diabetes. Because the transcription factor c-Myc induces hepatic glucose uptake and utilization and blocks gluconeogenesis, we examined whether hepatic overexpression of c-myc counteracts the insulin resistance induced by a high-fat diet. After 3 months on this diet, control mice became obese, hyperglycemic, and hyperinsulinemic, indicating that they had developed insulin resistance. In contrast, transgenic mice remained lean and showed improved glucose disposal and normal levels of blood glucose and insulin, indicating that they had developed neither obesity nor insulin resistance. These findings were concomitant with normalization of hepatic glucokinase and pyruvate kinase gene expression and enzyme activity, which led to normalization of intrahepatic glucose-6-phosphate and glycogen content. In the liver of control mice fed a high-fat diet, the expression of genes encoding proteins that control energy metabolism, such as sterol receptor element binding protein 1-c, peroxisome proliferator activated receptor alpha, and uncoupling protein-2, was altered. In contrast, in the liver of transgenic mice fed a high-fat diet, the expression of these genes was normal. These results suggest that c-myc overexpression counteracted the obesity and insulin resistance induced by a high-fat diet by modulating the expression of genes that regulate hepatic metabolism.

c-Myc is required for the glucose-mediated induction of metabolic enzyme genes

Glucose exerts powerful effects on hepatocyte gene transcription by mechanisms that are incompletely understood. c-Myc regulates hepatic glucose metabolism by increasing glycolytic enzyme gene transcription while concomitantly decreasing gluconeogenic and ketogenic enzyme gene expression. However, the molecular mechanisms by which c-Myc exerts these effects is not known. In this study, the glucose-mediated induction of L-type pyruvate kinase and glucose-6-phosphatase mRNA levels was diminished by maneuvers involving recombinant adenoviral vectors that interfere with (i) c-Myc protein levels by antisense expression or (ii) c-Myc function through a dominant-negative Max protein. These results were obtained using both HL1C rat hepatoma cells and primary rat hepatocytes. Furthermore, a decrease in c-Myc abundance reduced glucose production in HL1C cells, presumably by decreasing glucose-6-phosphatase activity. The repression of hormone-activated phosphoenolpyruvate carboxykinase gene transcription by glucose was not affected by a reduction in c-Myc levels. The basal mRNA levels for L-pyruvate kinase and glucose-6-phosphatase were not altered to any significant degree by adenoviral treatment. Furthermore, adenoviral overexpression of the c-Myc protein induced glucose-6-phosphatase mRNA in the absence of glucose stimulation. We conclude that multiple mechanisms exist to communicate the glucose-derived signal and that c-Myc has a key role in the hepatic glucose signaling pathway.

Overexpression of c-myc in diabetic mice restores altered expression of the transcription factor genes that regulate liver metabolism

Overexpression of the c-Myc transcription factor in liver induces glucose uptake and utilization. Here we examined the effects of c- myc overexpression on the expression of hepatocyte-specific transcription factor genes which regulate the expression of genes controlling hepatic metabolism. At 4 months after streptozotocin (STZ) treatment, most diabetic control mice were highly hyperglycaemic and died, whereas in STZ-treated transgenic mice hyperglycaemia was markedly lower, the serum levels of beta-hydroxybutyrate, triacylglycerols and non-esterified fatty acids were normal, and they had greater viability in the absence of insulin. Furthermore, long-term STZ-treated transgenic mice showed similar glucose utilization and storage to healthy controls. This was consistent with the expression of glycolytic genes becoming normalized. In addition, restoration of gene expression of the transcription factor, sterol receptor element binding protein 1c, was observed in the livers of these transgenic mice. Further, in STZ-treated transgenic mice the expression of genes involved in the control of gluconeogenesis (phosphoenolpyruvate carbokykinase), ketogenesis (3-hydroxy-3-methylglutaryl-CoA synthase) and energy metabolism (uncoupling protein 2) had returned to normal. These findings were correlated with decreased expression of genes encoding the transcription factors hepatocyte nuclear factor 3gamma, peroxisome proliferator-activated receptor alpha and retinoid X receptor. These results indicate that c- myc overexpression may counteract diabetic changes by controlling hepatic glucose metabolism, both directly by altering the expression of metabolic genes and through the expression of key transcription factor genes.

Glucose-regulated glucose uptake by transplanted muscle cells expressing glucokinase counteracts diabetic hyperglycemia

Type 1 diabetic patients depend on insulin replacement therapy. However, chronic hyperglycemia due to failure to maintain proper glycemic control leads to microvascular, macrovascular, and neurological complications. Increased glucose disposal by tissues engineered to overexpress key regulatory genes in glucose transport or phosphorylation can reduce diabetic hyperglycemia. Here we report that differentiated myoblast cells expressing the glucose-phosphorylating enzyme glucokinase (GK) showed a glucose-dependent increase in glucose uptake and utilization in vitro. Transplantation of GK-expressing myotubes into healthy mice did not alter blood glucose levels and recipient mice maintained normoglycemia. After streptozotocin treatment, mice transplanted with GK-expressing myotubes counteracted hyperglycemia, polydipsia, and polyphagia, whereas mice transplanted with control myotubes developed diabetes. Similarly, diabetic mice transplanted with control myotubes remained hyperglycemic. In contrast, transplantation of GK-expressing myotubes into diabetic mice lowered hyperglycemia. These results suggest that the use of genetically engineered muscle cells to express glucokinase may provide a glucose-regulated approach to reduce diabetic hyperglycemia.

Counteraction of type 1 diabetic alterations by engineering skeletal muscle to produce insulin: insights from transgenic mice

Insulin replacement therapy in type 1 diabetes is imperfect because proper glycemic control is not always achieved. Most patients develop microvascular, macrovascular, and neurological complications, which increase with the degree of hyperglycemia. Engineered muscle cells continuously secreting basal levels of insulin might be used to improve the efficacy of insulin treatment. Here we examined the control of glucose homeostasis in healthy and diabetic transgenic mice constitutively expressing mature human insulin in skeletal muscle. Fed transgenic mice were normoglycemic and normoinsulinemic and, after an intraperitoneal glucose tolerance test, showed increased glucose disposal. When treated with streptozotocin (STZ), transgenic mice showed increased insulinemia and reduced hyperglycemia when fed and normoglycemia and normoinsulinemia when fasted. Injection of low doses of soluble insulin restored normoglycemia in fed STZ-treated transgenic mice, while STZ-treated controls remained highly hyperglycemic, indicating that diabetic transgenic mice were more sensitive to the hypoglycemic effects of insulin. Furthermore, STZ-treated transgenic mice presented normalization of both skeletal muscle and liver glucose metabolism. These results indicate that skeletal muscle may be a key target tissue for insulin production and suggest that muscle cells secreting basal levels of insulin, in conjunction with insulin therapy, may permit tight regulation of glycemia.

Structural analysis of alpha-enolase. Mapping the functional domains involved in down-regulation of the c-myc protooncogene

Myc-binding protein-1 (MBP-1) is a 37-kDa protein with sequence homology to the 3' portion of the alpha-enolase gene. alpha-Enolase is a 48-kDa protein, which plays a critical role in the glycolytic pathway. MBP-1 binds to the c-myc P2 promoter and down-regulates c-myc expression. We have investigated the role of alpha-enolase in regulation of the c-myc protooncogene. RNase protection assay shows that alpha-enolase is transcribed into a single RNA species in HeLa cells. A start codon, 400 base pairs downstream of the alpha-enolase ATG, corresponds to the MBP-1 ATG, suggesting that MBP-1 is an alternative translation initiation product of the alpha-enolase RNA. Domain mapping was performed using constructs containing truncations of the alpha-enolase gene. In vitro binding to the c-myc gene was abolished after deletion of the N-terminal portion of alpha-enolase. In order to determine the relationship between DNA binding activity and transcription inhibition, we performed co-transfection assays in HeLa cells. These studies confirmed that an N-terminal deletion of alpha-enolase is unable to down-regulate c-myc promoter activity. Our data suggest that alpha-enolase plays an important role in regulation of c-myc promoter activity in the form of an alternative translation product MBP-1, which is distinct from its role as a glycolytic enzyme.

Gender- and androgen-related influence on the expression of proto-oncogene and apoptotic factor mRNAs in lacrimal glands of autoimmune and non-autoimmune mice

Our previous studies have shown that the mRNA levels of c-myb, c-myc, bcl-2 and p53 are higher, and partial Fas antigen (i.e. exons 1-2) lower, in lacrimal tissues of female, as compared to male, MRL/lpr mice, which are a model of Sjögren's syndrome. We have also found that this gender-related difference in bcl-2 and c-myb expression appears to be due to the influence of androgens. To extend these findings, we sought to determine: first, whether these gender- and/or hormone-associated variations in mRNA content are unique to MRL/lpr mice, or are also present in lacrimal glands of other murine strains, including autoimmune NZB/NZW F1 (F1) and non-obese diabetic (NOD), as well as non-autoimmune C3H/HeJ (C3H) and BALB/c, mice; and second, whether the levels of these apoptotic factor mRNAs are altered in lacrimal tissues of mice (i.e. testicular feminized (Tfm) with dysfunctional androgen receptors, as compared to glandular amounts in their 'normal' controls (i.e. Tabby). Lacrimal tissues were obtained from adult mice, which were either untreated or treated with placebo or testosterone for 21 days. Glands were processed for the analysis of proto-oncogene mRNAs by RT-PCR (at exponential phase of amplification) and data were standardized to the corresponding levels of beta-actin mRNA. Our results demonstrate that Fas antigen, Fas ligand, c-myb, c-myc, bcl-2, Bax and p53 mRNAs are present in lacrimal tissues of F1, NOD, C3H, BALB/c, Tabby and Tfm mice. The relative levels of Fas antigen mRNA are consistently higher in glands of males, whereas amounts of bcl-2 mRNA are greater in tissues of F1, C3H and BALB/c females. Testosterone administration induced a significant increase in the lacrimal gland content of Bax mRNA, but a striking decrease in the lacrimal tissue level of bcl-2 mRNA in F1 and C3H mice. Lacrimal glands of Tfm mice contained elevated amounts of bcl-2 mRNA, as compared to values in tissues of their Tabby controls. In summary, our findings show that fundamental gender-related differences exist in the expression of genes associated with programmed cell death in lacrimal glands of autoimmune and normal mice. In addition, some of these differences may be due, at least in part, to the effect of androgens.

High-dose biotin, an inducer of glucokinase expression, may synergize with chromium picolinate to enable a definitive nutritional therapy for type II diabetes

Glucokinase (GK), expressed in hepatocyte and pancreatic beta cells, has a central regulatory role in glucose metabolism. Efficient GK activity is required for normal glucose-stimulated insulin secretion, postprandial hepatic glucose uptake, and the appropriate suppression of hepatic glucose output and gluconeogenesis by elevated plasma glucose. Hepatic GK activity is subnormal in diabetes, and GK may also be decreased in the beta cells of type II diabetics. In supraphysiological concentrations, biotin promotes the transcription and translation of the GK gene in hepatocytes; this effect appears to be mediated by activation of soluble guanylate cyclase. More recent evidence indicates that biotin likewise increases GK activity in islet cells. On the other hand, high-dose biotin suppresses hepatocyte transcription of phosphoenolpyruvate carboxykinase, the rate-limiting enzyme for gluconeogenesis. Administration of high-dose biotin has improved glycemic control in several diabetic animals models, and a recent Japanese clinical study concludes that biotin (3 mg t.i.d. orally) can substantially lower fasting glucose in type II diabetics, without side-effects. The recently demonstrated utility of chromium picolinate in type II diabetes appears to reflect improved peripheral insulin sensitivity--a parameter which is unlikely to be directly influenced by biotin. Thus, the joint administration of supranutritional doses of biotin and chromium picolinate is likely to combat insulin resistance, improve beta-cell function, enhance postprandial glucose uptake by both liver and skeletal muscle, and inhibit excessive hepatic glucose production. Conceivably, this safe, convenient, nutritional regimen will constitute a definitive therapy for many type II diabetics, and may likewise be useful in the prevention and management of gestational diabetes. Biotin should also aid glycemic control in type I patients.

Insulin production by engineered muscle cells

Type 1 diabetic patients depend dramatically on insulin replacement therapy, which involves the administration of intermediate- or long-acting insulin, together with short-acting insulin to mimic physiological insulin profiles. However, the delayed-action preparations available are not generally able to produce smooth background levels of insulin. Muscle cells were tested for long-term delivery of active human insulin as an approach to achieve a constant basal level of insulin. Thus, C2C12 mouse myoblast cells were stably transfected with a chimeric gene obtained by linking the myosin-light chain 1 (MLC1) promoter to the human proinsulin gene, containing genetically engineered furin endoprotease cleavage sites (MLC1/Insm). When differentiated, C2C12Insm myotube cells expressed high levels of insulin mRNA and protein, whereas no insulin was detected in myoblast cells. HPLC fractionation of culture medium and cell extracts from differentiated C2C12Insm cells revealed that about 90% of the proinsulin was processed to mature insulin. In addition, these cells released significant levels (about 100 microU/10(6) cells/hr) of mature insulin to the medium. The hormone was biologically active since it increased glucose consumption and utilization by the differentiated C2C12Insm cells and was able to block the expression of the endogenous phosphoenolpyruvate carboxykinase (PEPCK) gene in FTO-2B rat hepatoma cells. Furthermore, when C2C12Insm myoblast cells were transplanted into diabetic mice an increase in insulinemia and a decrease in hyperglycemia were observed. Thus, our results suggest that the use of engineered myotube cells continuously secreting a defined level of insulin might be a useful approach to improve the efficacy of insulin injection treatment.

Nutrient and hormonal regulation of pyruvate kinase gene expression

Mammalian pyruvate kinase (PK), a key glycolytic enzyme, has two genes named PKL and PKM, which produce the L- and R-type isoenzymes by means of alternative promoters, and the M1-and M2-types by mutually exclusive alternative splicing respectively. The expression of these genes is tissue-specific and under developmental, dietary and hormonal control. The L-type isoenzyme (L-PK) gene contains multiple regulatory elements necessary for regulation in the 5' flanking region, up to position -170. Both L-II and L-III elements are required for stimulation of L-PK gene transcription by carbohydrates such as glucose and fructose, although the L-III element is itself responsive to carbohydrates. The L-II element is also responsible for the gene regulation by polyunsaturated fatty acids. Nuclear factor-1 proteins and hepatocyte nuclear factor 4, which bind to the L-II element, may also be involved in carbohydrate and polyunsaturated fatty acid regulation of the L-PK gene respectively. However, the L-III-element-binding protein that is involved in carbohydrate regulation remains to be clarified, although involvement by an upstream stimulating factor has been proposed. Available evidence suggests that the carbohydrate signalling pathway to the L-PK gene includes a glucose metabolite, possibly glucose 6-phosphate or xylulose 5-phosphate, as well as phosphorylation and dephosphorylation mechanisms. In addition, at least five regulatory elements have been identified in the 5' flanking region of the PKM gene up to position -279. Sp1-family proteins bind to two proximal elements, but the binding of proteins to other elements have not yet been clarified. Glucose may stimulate the transcription of the PKM gene via hexosamine derivatives. Sp1 may be involved in this regulation via its dephosphorylation, although the carbohydrate response element has not been determined precisely in the PKM gene. Thus glucose stimulates transcription of the PKM gene by the mechanism which is probably different from the L-PK gene.

Transgenic mice in the analysis of metabolic regulation

In normal animals, the extracellular concentration of glucose is maintained within a very narrow range by the matching of glucose flux into and out of the extracellular space through the tightly coordinated secretion of insulin and glucagon. Functional alterations in beta-cells, liver, or skeletal muscle and adipose tissue may disrupt glucose homeostasis and lead to the development of non-insulin-dependent diabetes mellitus (type 2 diabetes). This review outlines the contribution of these organs and tissues to the control of glucose homeostasis. We discuss new insights obtained through studies of transgenic mice that overexpress or show decreased expression of putative key genes in the regulation of pancreatic beta-cell function, in the control of hepatic glucose uptake and output, and in the regulation of glucose uptake and utilization by skeletal muscle and adipose tissue.

Regulated production of mature insulin by non-beta-cells

Rat hepatoma cells were engineered to express, in a regulated manner, mature human insulin as an approach to the development of artificial beta-cells for insulin-dependent diabetes mellitus (IDDM) gene therapy. A chimeric gene obtained by linking a 2.4-kb fragment of the P-enolpyruvate carboxykinase (PEPCK) gene promoter to a human proinsulin gene (PEPCK/Insm), containing genetically engineered furin endoprotease cleavage sites, was stably transfected into FTO-2B rat hepatoma cells. The FTOInsm cells expressed high levels of insulin mRNA and protein after Northern blot or immunocytochemical analysis. High-performance liquid chromatography (HPLC) fractionation of culture medium and cell extracts revealed that about 90% of the proinsulin was processed to mature insulin. Insulin secretion was very fast, and 15 min after induction with dibutyryl cyclic AMP (Bt2cAMP) plus dexamethasone significant amounts of the hormone were released. Moreover, during the first hour, the rise in insulin concentration in the medium was 10-fold that detected in nontreated FTOInsm cells. Insulin produced by FTOInsm cells was biologically active because it blocked endogenous PEPCK gene expression and induced glucose uptake and lactate production. Thus, our results showed that genetically engineered FTOInsm hepatoma cells synthesized, processed, and secreted active insulin. The implantation of encapsulated engineered FTOInsm cells might provide a safe and practical therapeutic approach for IDDM treatment.