Mechanisms involved in tumor necrosis factor-alpha induction of insulin resistance and its reversal by thiazolidinedione(s)

Insulin attenuates TNFα-induced hemopexin mRNA: An anti-inflammatory action of insulin in rat H4IIE hepatoma cells

Proinflammatory cytokines, including TNF-α and IL-6, can contribute to insulin resistance. Conversely, insulin has some actions that can be considered anti-inflammatory. Hemopexin is a Class 2 acute phase reactant and control of its transcription is predominantly regulated by IL-6, with TNF-α and IL-1β also inducing hemopexin gene expression. Thus, we asked whether insulin could inhibit the ability of TNF-α to stimulate hemopexin mRNA expression. In cultured rat hepatoma (H4IIE) cells, TNF-α significantly increased hemopexin mRNA accumulation. The TNF-α-induced increase of hemopexin mRNA was dramatically attenuated by insulin, even though TNF-α reduced peak insulin activation of ERK. Thus, even though TNF-α can contribute to insulin resistance, the residual insulin response was still able to counteract TNF-α actions.

•

The TNF-α-induced increase of hemopexin mRNA was dramatically attenuated by insulin.

•

This occurred even though TNF-α significantly decreased insulin activation of ERK.

•

This suggests an additional mechanism for the anti-inflammatory action of insulin.

•

Cytokine-induced insulin resistance does not abolish insulin’s anti-inflammatory effect.

Molecular mechanisms of lipotoxicity in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by insulin resistance, which results in elevated serum concentration of free fatty acids (FFAs). Circulating FFAs provide the substrate for triacylglycerol formation in the liver, and may also be directly cytotoxic. Hepatocyte apoptosis is a key histologic feature of NAFLD, and correlates with progressive inflammation and fibrosis. The molecular pathways leading to hepatocyte apoptosis are not fully defined; however, recent studies suggest that FFA-induced apoptosis contributes to the pathogenesis of nonalcoholic steatohepatitis. FFAs directly engage the core apoptotic machinery by activating the proapoptotic protein Bax, in a c-jun N-terminal kinase-dependent manner. FFAs also activate the lysosomal pathway of cell death and regulate death receptor gene expression. The role of ER stress and oxidative stress in the pathogenesis of nonalcoholic steatohepatitis has also been described. Understanding the molecular mediators of liver injury should promote development of mechanism-based therapeutic interventions.

Research on the protection effect of pioglitazone for non-alcoholic fatty liver disease (NAFLD) in rats

OBJECTIVE

The prevalence of non-alcoholic fatty liver disease (NAFLD) has markedly increased. Insulin resistance has been implicated in the pathogenesis of NAFLD. This study was aimed at observing the relationship between insulin resistance and NAFLD, and evaluating the role of pioglitazone (PGZ) acting as insulin-sensitizing agents in the prevention and treatment of rat fatty liver induced by high fat feeding.

METHODS

The rats were separated randomly into 6 groups: model group I were fed high fat diet for 8 weeks, PGZ prevention group were given PGZ 4 mg/(kg.d) simultaneously, while control group I were fed normal food for 8 weeks; model group II were fed high fat diet for 16 weeks, PGZ treatment group were given PGZ 4 mg/(kg.d) orally simultaneous with high fat diet for 8 weeks after high fat feeding for 8 weeks, control group II were fed normal food for 16 weeks. The rats were sacrificed after 8 weeks and 16 weeks respectively. Liver weight, body weight, serum activities of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), tumor necrosis factor alpha (TNF-alpha), fasting blood glucose (FBG), fasting plasma insulin (FINS), HOMA (homeostasis model assessment) insulin resistance index (HOMA-IR), and the liver histology of rats of all groups were assayed.

RESULTS

After 8 weeks, the liver in model group I showed typical steatosis, accompanied with mild to moderate lobular inflammatory cell infiltration, liver indexes and serum levels of ALT, AST, ALP, TNF-alpha were significantly increased (P<0.05) compared with control group I. Whereas, the degree of hepatic injury was attenuated in PGZ prevention group, liver indexes and serum levels of ALT, ALP were significantly decreased (P<0.05) compared with model group I. After 16 weeks, notable steatosis, and lobular inflammation were observed in model group II rat liver, while the degree of hepatic injury was attenuated in the PGZ treatment group. Liver index, serum levels of ALT, AST, ALP, FINS and HOMA-IR were significantly increased (P<0.05) in model group II compared with control group II. Whereas, in PGZ treatment group, serum levels of AST and FINS showed decreasing tendency, liver indexes, serum levels of ALT, ALP, TNF-alpha and HOMA-IR were significantly decreased compared with model group II.

CONCLUSION

Insulin resistance plays a role in the pathogenesis of NAFLD in rats. Pioglitazone can attenuate insulin resistance and biochemical and histological injury in high fat-induced fatty liver in rats.

Proteome of H-411E (liver) cells exposed to insulin and tumor necrosis factor-alpha: analysis of proteins involved in insulin resistance

Insulin resistance may be modeled in H-411E liver cells in tissue culture with the use of the cytokine tumor necrosis factor-alpha (TNF-alpha) and insulin. This tissue-culture model nicely mimics IR in human type 2 diabetes mellitus. After incubation of liver cells in tissue culture with INS alone, TNF-alpha alone, and TNF-alpha plus insulin, as well as a control sample, liver-cell extracts were separated on 2D polyacrylamide-gel electrophoresis on the basis of isoelectric point and molecular weight. We analyzed the gel images with the use of PD Quest software (Bio-Rad Laboratories, Hercules, Calif) to identify differentially expressed protein spots (ie, up or down with insulin vs down or up with TNF-alpha plus insulin). In separate experiments, phosphorus-32 incorporation/autoradiography and phosphoprotein staining were used to characterize treatment-induced phosphorylations. Affected protein spots were identified with the use of peptide fingerprinting and matrix-assisted laser desorption ionization time of flight mass spectrometry. The first series of experiments identified 6 differentially expressed proteins: eukaryotic translation initiation factor-3, subunit 2, regulator of G-protein signaling-5, superoxide dismutase, protein disulfide isomerase A6, proteasome subunit-alpha type 3, and regucalcin. In addition, we observed changes in the phosphorylation of protein disulfide isomerase A6. A second series of experiments identified 7 additional proteins with significantly altered differential expression: cell-division protein kinase-4, kinogen heavy chain, carbonic anhydrase-7, E 3 ubiquitin protein ligase, URE-B1; Rab GDP dissociation inhibitor-beta, Rab GDP dissociation inhibitor-beta2, and MAWDBP. It can be seen that differentially expressed proteins, affected by treatment with insulin or with TNF-alpha plus insulin, include regulators of translation, protein degradation, cellular Ca ++ , G-proteins, and free-radical production. Although one cannot detail the mechanism or mechanisms of TNF-alpha induced IR from this data alone, it is easy to relate all of these proteins to a role in insulin signal transduction and, hence, insulin resistance.

A role for sphingolipids in producing the common features of type 2 diabetes, metabolic syndrome X, and Cushing's syndrome

Metabolic syndrome X and type 2 diabetes share many metabolic and morphological similarities with Cushing's syndrome, a rare disorder caused by systemic glucocorticoid excess. Pathologies frequently associated with these diseases include insulin resistance, atherosclerosis, susceptibility to infection, poor wound healing, and hypertension. The similarity of the clinical profiles associated with these disorders suggests the influence of a common molecular mechanism for disease onset. Interestingly, numerous studies identify ceramides and other sphingolipids as potential contributors to these sequelae. Herein we review studies demonstrating that aberrant ceramide accumulation contributes to the development of the deleterious clinical manifestations associated with these diseases.

Rosiglitazone ameliorates insulin resistance in brown adipocytes of Wistar rats by impairing TNF-alpha induction of p38 and p42/p44 mitogen-activated protein kinases

AIMS/HYPOTHESIS

TNF-alpha caused insulin resistance on glucose uptake and on insulin signalling in fetal brown adipocytes. Since treatment with TNF-alpha activates stress kinases, including c-jun NH2 terminal kinase (JNK), and p42/p44 and p38 mitogen-activated protein kinases (MAPK), we explored the contribution of these pathways to insulin resistance by TNF-alpha. Rosiglitazone is used to treat Type 2 diabetes as it improves insulin sensitivity in vivo. However, its ability to ameliorate TNF-alpha-induced insulin resistance in brown adipocytes remains to be explored.

METHODS

We used fetal rat primary brown adipocytes cultured with TNF-alpha, with or without stress kinase inhibitors or rosiglitazone, and further stimulated with insulin. Then, we measured glucose uptake and GLUT4 translocation. To determine the insulin signalling cascade, we submitted cells to lysis, immunoprecipitation and immunoblotting.

RESULTS

Exposure to TNF-alpha for 24 h impairs insulin stimulation of the phosphatidylinositol (PI) 3-kinase activity associated with IRS-2 and Akt activity. Pretreatment with PD98059 or PD169316, which inhibit p42/p44MAPK and p38MAPK respectively, restored insulin signalling and insulin-induced glucose uptake in the presence of TNF-alpha. However, in the presence of SP600125, an inhibitor of JNK, TNF-alpha still produced insulin resistance. Rosiglitazone ameliorated insulin resistance by TNF-alpha in brown adipocytes, restoring completely insulin-stimulated glucose uptake and insulin-induced GLUT4 translocation to plasma membrane in parallel to the insulin signalling cascade IRS-2/PI 3-kinase/Akt.

CONCLUSIONS/INTERPRETATION

Rosiglitazone treatment impaired TNF-alpha activation of p38 and p42/p44MAPK, restoring insulin signalling and leading to normalisation of glucose uptake.

Type 2 diabetes, cardiovascular risk, and the link to insulin resistance

BACKGROUND

Patients with type 2 diabetes mellitus frequently have coexistent dyslipidemia, hypertension, and obesity, and are at risk for microvascular and macrovascular disease complications such as myocardial infarction, stroke, retinopathy, and microalbuminuria. To optimize cardiovascular health outcomes for patients with type 2 diabetes, strategies to reduce the risks of microvascular and macrovascular disease are needed in clinical practice.

OBJECTIVE

This article provides an overview of the cardiovascular risk profile of patients with type 2 diabetes and discusses the cardiovascular consequences of use of the thiazolidinediones (insulin-sensitizing agents) in the treatment of type 2 diabetes.

METHODS

A literature search of MEDLINE/PubMed was performed to identify relevant articles published from 1966 to April 2003. Search terms used were diabetes, cardiovascular disease, atherosclerosis, dyslipidemia, obesity, hypertension, blood pressure, hyperglycemia, inflammation, C-reactive protein, fibrinolysis, plasminogen activator inhibitor type-1, microalbuminuria, thiazolidinediones, safety, hepatotoxicity, and edema. Bibliographies within the identified articles were also evaluated for additional relevant articles and information.

RESULTS

Recommendations for cardiovascular risk reduction through preventive and therapeutic strategies that target the symptoms of insulin resistance may reduce the microvascular and macrovascular sequelae of diabetes and ameliorate the impact of other components of the metabolic syndrome, including hypertension, hyperglycemia, and obesity. In this regard, thiazolidinediones are promising therapies.

CONCLUSIONS

Early data suggest that, in addition to reducing hyperglycemia, pioglitazone and rosiglitazone effect changes in the dyslipidemic profile, hemodynamics, vascular inflammation, and endothelial functioning of patients with type 2 diabetes. Additional research is needed to further distinguish the cardiovascular benefits of these drugs.

Inflammation in end-stage renal disease: sources, consequences, and therapy

Cardiovascular disease (CVD) remains the main cause of morbidity and mortality in patients with end-stage renal disease (ESRD). Although traditional risk factors are common in ESRD patients, they alone may not be sufficient to account for the high prevalence of CVD in this condition. Recent evidence demonstrates that chronic inflammation, a nontraditional risk factor which is commonly observed in ESRD patients, may cause malnutrition and progressive atherosclerotic CVD by several pathogenetic mechanisms. The causes of inflammation in ESRD are multifactorial and, while it may reflect underlying CVD, an acute-phase reaction may also be a direct cause of vascular injury by several pathogenetic mechanisms. Available data suggest that proinflammatory cytokines play a central role in the genesis of both malnutrition and CVD in ESRD. Thus it could be speculated that suppression of the vicious cycle of malnutrition, inflammation, and atherosclerosis (MIA syndrome) would improve survival in dialysis patients. Recent evidence has demonstrated strong associations between inflammation and both increased oxidative stress and endothelial dysfunction in ESRD patients. As there is not yet any recognized, or even proposed, treatment for ESRD patients with chronic inflammation, it would be of obvious interest to study the long-term effect of various anti-inflammatory treatment strategies on the nutritional and cardiovascular status as well as outcome in these patients.