Insulin-resistant Zucker Rats Research Articles

193-OR: Chronotherapy with the Glucokinase Activator AZD1656 Greatly Affects Its Ability to Improve Metabolism in Obese Zucker Rats

Circadian rhythms play critical roles in regulating metabolism, including daily feeding/fasting periods and oscillating expressions of certain genes. Glucokinase (Gck) , expressed in the liver and pancreas, is central for whole-body glucose homeostasis by acting as a glucose sensor and regulating metabolism according to prevailing glucose levels. Gck expression has also been shown to follow a circadian rhythm. Gck activators (GKAs) effectively reduce hyperglycemia, however, adverse events have also involved hypoglycemia, hyperlipidemia, and hepatic steatosis. Given the circadian rhythmicity and natural postprandial activation of Gck, we hypothesized that GKA treatment would benefit from being timed specifically during feeding periods. We compared the metabolic effects following differentially timed administrations of the GKA AZD1656 in the obese, insulin resistant Zucker rat. Four weeks of continuous treatment significantly improved glycemic control; however, hepatic steatosis and inflammation manifested. In contrast, a robust reduction in hepatic steatosis and inflammation on top of improved glycemia, was observed when AZD1656 administration was timed to feeding periods only. When AZD1656 was timed to fasting periods, detrimental metabolic effects were observed, including impaired glycemic control, severe liver steatosis and elevated inflammation. Mechanistically, timing AZD1656 administration to feeding periods was found to divert de novo lipogenesis-derived lipids away from hepatic storage towards VLDL synthesis and secretion, without affecting plasma triglyceride levels. In conclusion, intermittent AZD1656 dosing timed to feeding periods promotes glucose disposal when needed the most, while the daily drug holiday allows for maintained metabolic flexibility and avoids hepatic steatosis. Chronotherapeutic approaches should be considered for the development of GKAs and other drugs acting on metabolic targets. Disclosure T. Kroon: Employee; AstraZeneca. I. Alexandersson: Employee; AstraZeneca. A. Ferm: Employee; AstraZeneca. M. Petersson: Employee; AstraZeneca. S. Maurer: Employee; AstraZeneca, Merck Sharp & Dohme Corp. B. Zarrouki: Employee; AstraZeneca. K. Wallenius: Employee; AstraZeneca. N.D. Oakes: Employee; AstraZeneca. J. Boucher: Employee; AstraZeneca, Evotec International GmbH.

Insulin controls cytoskeleton reorganization and filtration barrier permeability via the PKGIα-Rac1-RhoA crosstalk in cultured rat podocytes

Podocyte foot processes are an important cellular layer of the glomerular barrier that regulates glomerular permeability. Insulin via the protein kinase G type Iα (PKGIα) signaling pathway regulates the balance between contractility and relaxation (permeability) of the podocyte barrier by regulation of the actin cytoskeleton. This mechanism was shown to be disrupted in diabetes. Rho family guanosine-5′-triphosphates (GTPases) are dynamic modulators of the actin cytoskeleton and expressed in cells that form the glomerular filtration barrier. Thus, changes in Rho GTPase activity may affect glomerular permeability to albumin. The present study showed that Rho family GTPases control podocyte migration and permeability. Moreover these processes are regulated by insulin in PKGIα-dependent manner. Modulation of the PKGI-dependent activity of Rac1 and RhoA GTPases with inhibitors or small-interfering RNA impair glomerular permeability to albumin. We also demonstrated this mechanism in obese, insulin-resistant Zucker rats. We propose that PKGIα-Rac1-RhoA crosstalk is necessary in proper organization of the podocyte cytoskeleton and consequently the stabilization of glomerular architecture and regulation of filtration barrier permeability.

1881-P: The SGLT2 Inhibitor Dapagliflozin Improves Glucose Control and Enhances Liver Fatty Acid Oxidation and Ketone Body Formation in Insulin Resistant Obese Zucker Rats

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) are effective treatments for improving glucose control and reducing cardiovascular events in patients with T2D. In the present study we examined the metabolic effects of chronic treatment of the SGLT2i dapagliflozin in obese insulin resistant Zucker rats. Three different studies were performed with the aims to assess: 1) liver CoA intermediates, 2) whole-body FA metabolism using a constant infusion of 3H-palmitate and 3) tissue-specific rates of FA utilization and storage using 3H-labelled bromo-palmitate and 14C-labelled palmitate, respectively. Four weeks treatment with dapagliflozin (1 mg/kg/d p.o.) markedly increased glucosuria, reduced HbA1c and fasting plasma glucose and insulin levels and increased pancreatic insulin content compared to vehicle treated rats. Liver acetyl-CoA levels were increased while the malonyl-CoA/acetyl-CoA ratio was decreased following dapagliflozin treatment, suggestive of an increased liver FA oxidation. Treatment with dapagliflozin also increased liver HMG-CoA levels, an intermediate in ketone body formation, indicating that the acetyl-CoA is diverted to ketone body formation. This was supported by increased fasting plasma levels of the ketone body β-hydroxybutyrate in the dapagliflozin-treated rats compared to vehicle. Dapagliflozin treatment resulted in increased plasma FA levels and rates of whole-body FA oxidation. The increased whole-body FA oxidation was subsequently found to be driven by an increase in liver FA utilization. In conclusion, dapagliflozin improves glucose homeostasis and enhances FA utilization in insulin resistant obese animals. These results suggest that dapagliflozin-induced glucose loss in urine signals a fasting state to the liver, which leads to an increased FA oxidation and ketone body production. Disclosure T. Kroon: Employee; Self; AstraZeneca. T. Hagstedt: None. A. Kjellstedt: None. A. Lindblom: Employee; Self; AstraZeneca. L. Löfgren: Employee; Self; AstraZeneca. J. Boucher: Employee; Self; AstraZeneca. M. Sörhede Winzell: Employee; Self; AstraZeneca. D. Linden: Employee; Self; AstraZeneca. K. Wallenius: Employee; Self; AstraZeneca. Stock/Shareholder; Self; AstraZeneca.

PKCδ is activated in the liver of obese Zucker rats and mediates diet-induced whole body insulin resistance and hepatocyte cellular insulin resistance

Insulin resistance can arise when pathological levels of free fatty acids (FFAs) and proinflammatory cytokines disrupt insulin signaling. Protein kinase C delta (PKCδ) is a FFA- and a proinflammatory cytokine-regulated protein kinase that is associated with inhibition of insulin signaling and action. To gain insight into the role of PKCδ in insulin resistance, PKCδ activation was studied in a genetic model of obesity-linked insulin resistance. PKCδ was found to be activated in the liver of obese insulin-resistant Zucker rats and in isolated cultured hepatocytes. PKCδ was further studied in PKCδ-null mice and their wild-type littermates fed a high-fat or control diet for 10 weeks. PKCδ-null mice on a high-fat diet had improved insulin sensitivity and hepatic insulin signaling compared to wild-type littermates. Additionally, the deleterious effect of a high-fat diet on glucose tolerance in wild-type mice was completely blocked in PKCδ-null mice. To directly test the role of PKCδ in cellular insulin resistance, primary hepatocytes from the high-fat diet mice were isolated and stimulated with insulin. Primary hepatocytes from PKCδ-null mice had improved insulin-stimulated Akt and FOXO phosphorylation compared to hepatocytes from wild-type littermates. Consistent with this result, tumor necrosis factor alpha-mediated inhibition of insulin signaling was blocked in PKCδ knockdown primary hepatocytes. These results indicate that PKCδ plays a role in insulin resistance and is consistent with the hypothesis that PKCδ is a negative regulator of insulin signaling and thus may be a therapeutic target for the treatment of type 2 diabetes.

Fatty acid transport proteins chronically relocate to the transverse-tubules in muscle from obese Zucker rats but are resistant to further insulin-induced translocation

ObjectivesRecently, we have demonstrated that FA transport proteins are located within the t-tubule fraction of rodent muscle, and that insulin stimulation causes their translocation to this membrane fraction. Chronic relocation of the FA transport protein FAT/CD36 to the sarcolemma is observed in obese rodents and humans, and correlates with intramuscular lipid accumulation and insulin resistance. It is not known whether in an obese, insulin resistant state FA transporters also chronically relocate to the t-tubules. Furthermore, it is not known whether the insulin-stimulated translocation of the various FA transport proteins to the t-tubules is impaired in insulin resistance. MethodsSarcolemmal and t-tubule membrane fractions were isolated via differential centrifugation from muscles of lean and obese female Zucker rats during basal or insulin stimulated conditions. FA transport proteins were measured via western blot on both membrane fractions. ResultsOur results demonstrate that in muscle from insulin resistant Zucker rats, FAT/CD36, FABPpm and FATP1 are all increased on the t-tubules in the basal state (+72%, +120%, and +69%, respectively), potentially contributing to the accumulation of intramuscular lipids. Insulin failed to increase the content of the FA transport proteins on either the t-tubule or sarcolemma above the elevated basal levels, analogous to the well characterized impairment of insulin-stimulated GLUT4 translocation to both membrane domains in obesity. ConclusionFA transport proteins chronically relocate to the t-tubule domain in insulin resistant muscle, potentially contributing to lipid accumulation. Further translocation of the FA transport proteins to this domain during insulin stimulation, however, is impaired.

Increased renal glucose‐6‐phosphatase gene expression and activity in mice lacking insulin receptors in the renal proximal tubule cells

Enhanced gluconeogenesis hasbeen demonstrated in renal proximal tubules of insulin resistant Zucker rats. We have demonstrated reduced insulin receptor expression in renal‐tubular‐epithelial cells of insulin‐resistant rats. Since insulin has been shown to inhibit gluconeogenesis, we tested whether reduced insulin receptor in the proximal tubule affects renal gluconeogenesis. We analyzed gluconeogenesis components in the kidney tissues from mice with targeted deletion of insulin receptor from the proximal tubule (n=7/genotype). For glucose‐6‐ phosphatase (G6Pase) activity, male mice were fasted for 4 hours prior to study followed by intraperitoneal injection of insulin (0.5 U/kg·bw) in 300 μl saline plus 300 μl 25% dextrose in saline in order to amplify the difference between KO and WT (wild type) littermates. After 20 minutes, mice were euthanized and kidneys were removed for G6Pase activity and mRNA levels. Real time RT‐PCR revealed 2.5 fold higher expression of G6Pase mRNA in KO relative to WT littermates (p=0.02, n=7/genotype). In addition, renal G6Pase activity was trending higher (20%) in KO relative to WT; G6Pase activities (in μmolPi/min/mg of protein at 37°C) were 70.1 ± 7.4 in WT versus 84.5 ± 6.3 in KO). Furthermore, the KO mice were mildly hyperglycemic in the fasting state. These results support a physiologic role for the insulin receptor in renal proximal tubule to affect renal gluconeogenesis; and suggest that downregulation of the insulin receptor in renal proximal tubule in the insulin‐resistant state may further contribute to hyperglycemia through enhanced gluconeogenesis. Department of Biotechnology (Government of India) grant to S.T. (BT/HRD/35/02/17/2008).

Toxicity of cytotoxic agents to granulocyte–macrophage progenitors is increased in obese Zucker and non-obese but insulin resistant Goto-Kakizaki rats

Increased risk of anticancer chemotherapy in seriously obese patients is known. Obesity may be among factors that predict treatment-related toxicity during chemotherapy. We investigated whether functional changes in granulopoiesis may also contribute to increased myelotoxicity in addition to the known alterations of pharmacokinetic parameters in obesity. Hemopoiesis – as measured by cellularity, frequency of granulocyte–macrophage progenitors (CFU-GM) and total CFU-GM content of the femoral bone marrow – did not differ in obese, insulin resistant Zucker rats compared with Wistar rats. Nevertheless increased sensitivity of their CFU-GM progenitor cells to cytotoxic drugs was found by culturing them in vitro in the presence of carboplatin, doxorubicin and 5-fluorouracil. All drugs were more toxic on CFU-GM progenitor cells of insulin resistant Zucker rats than on CFU-GM cells of the control strain. This might be based on metabolic disorders, at least in part, because we could demonstrate a similar increase in toxicity of the studied anticancer drugs to the CFU-GM progenitors originated from the non-obese but insulin resistant Goto-Kakizaki rats in the same dose ranges. After in vivo administration of rosiglitazone, an insulin sensitizer, the anticancer drug sensitivity of CFU-GM progenitors of Goto-Kakizaki rats was decreased concurrently with improvement of insulin resistance. Although the increased treatment-related myelotoxicity and mortality are well-known among obese patients with malignant diseases, only the altered half lives, volumes of distribution and clearances of cytotoxic drugs are thought to be the underlying reasons. According to our knowledge the results presented here, are the first observations about an impaired granulopoiesis in obese animals.

Effects of High Glucose on Vascular Endothelial Growth Factor Synthesis and Secretion in Aortic Vascular Smooth Muscle Cells from Obese and Lean Zucker Rats

Type 1 diabetes is characterized by insulin deficiency, type 2 by both insulin deficiency and insulin resistance: in both conditions, hyperglycaemia is accompanied by an increased cardiovascular risk, due to increased atherosclerotic plaque formation/instabilization and impaired collateral vessel formation. An important factor in these phenomena is the Vascular Endothelial Growth Factor (VEGF), a molecule produced also by Vascular Smooth Muscle Cells (VSMC). We aimed at evaluating the role of high glucose on VEGF-A164 synthesis and secretion in VSMC from lean insulin-sensitive and obese insulin-resistant Zucker rats (LZR and OZR). In cultured aortic VSMC from LZR and OZR incubated for 24 h with d-glucose (5.5, 15 and 25 mM) or with the osmotic controls l-glucose and mannitol, we measured VEGF-A164 synthesis (western, blotting) and secretion (western blotting and ELISA). We observed that: (i) d-glucose dose-dependently increases VEGF-A164 synthesis and secretion in VSMC from LZR and OZR (n = 6, ANOVA p = 0.002–0.0001); (ii) all the effects of 15 and 25 mM d-glucose are attenuated in VSMC from OZR vs. LZR (p = 0.0001); (iii) l-glucose and mannitol reproduce the VEGF-A164 modulation induced by d-glucose in VSMC from both LZR and OZR. Thus, glucose increases via an osmotic mechanism VEGF synthesis and secretion in VSMC, an effect attenuated in the presence of insulin resistance.

AMPK Enhances Insulin-Stimulated GLUT4 Regulation via Lowering Membrane Cholesterol

AMP-activated protein kinase (AMPK) enhances glucose transporter GLUT4 regulation. AMPK also suppresses energy-consuming pathways such as cholesterol synthesis. Interestingly, recent in vitro and in vivo data suggest that excess membrane cholesterol impairs GLUT4 regulation. Therefore, this study tested whether a beneficial, GLUT4-regulatory aspect of AMPK stimulation involved cholesterol lowering. Using L6 myotubes stably expressing an exofacial myc-epitope-tagged-GLUT4, AMPK stimulation by 5-aminoimidazole-4-carboxamide-1-β-d-ribonucleoside (AICAR; 45 min, 1 mm) or 2,4-dinitrophenol (DNP; 30 min, 200 μm) increased cell surface GLUT4myc labeling by approximately ≈ 25% (P < 0.05). Insulin (20 min, 100 nm) also increased GLUT4myc labeling by about 50% (P < 0.05), which was further enhanced (≈ 25%, P < 0.05) by AICAR or DNP. Consistent with AMPK-mediated suppression of cholesterol synthesis, AICAR and DNP decreased membrane cholesterol by 20-25% (P < 0.05). Whereas AMPK knockdown prevented the enhanced basal and insulin-stimulated GLUT4myc labeling by AICAR and DNP, cholesterol replenishment only blocked the AMPK-associated enhancement in insulin action. Cells cultured in a hyperinsulinemic milieu, resembling conditions in vivo that promote the progression/worsening of insulin resistance, displayed an increase in membrane cholesterol. This occurred concomitantly with a loss of cortical filamentous actin (F-actin) and defects in GLUT4 regulation by insulin. These derangements were prevented by AMPK stimulation. Examination of skeletal muscle from insulin-resistant Zucker rats revealed a similar elevation in membrane cholesterol and loss of F-actin. Lowering cholesterol to control levels restored F-actin structure and insulin sensitivity. In conclusion, these data suggest a novel aspect of GLUT4 regulation by AMPK involves membrane cholesterol lowering. Moreover, this AMPK-mediated process protected against hyperinsulinemia-induced insulin resistance.

Deposition of docosahexaenoic acid (DHA) is limited in forebrain of young obese fa/ fa Zucker rats fed a diet high in α-linolenic acid but devoid of DHA

Docosahexaenoic acid (DHA) is required for neurotransmitter synthesis and learning. Conversion of α-linolenic acid (ALA) to DHA is considered adequate to support brain function in youth, but it is unknown if brain DHA can be maintained in insulin resistant states. This study investigated brain fatty acid and desaturase activities in young insulin resistant Zucker rats on diets with and without DHA. Male fa/fa and lean rats were fed diets enriched with flaxseed (FXO, ALA: 35.5% fatty acids), menhaden (MO, DHA: 9.2%) or safflower oil (SO, linoleic acid: 54.1%) for 9 weeks, n=8 per diet per genotype. Compared to lean, the 15 week old fa/fa rats were obese (56% heavier) and insulin-resistant (>18-fold in homeostasis model assessment of insulin resistance). The forebrain of fa/fa rats had higher palmitoleic (16:1n-7) and dihomo-γ-linolenic (20:3n-6) acids, and higher Δ9, Δ6 but lower Δ5 (all P≤.006) desaturase indices than lean. The Δ9 and Δ6 desaturase indices positively, while the Δ5 negatively (all P≤.01) correlated with insulin resistance. The Δ9 desaturase index positively correlated with adiposity index. The percentage of forebrain DHA of fa/fa rats was lower ( P=.011) than lean rats when fed FXO diet while there was no difference ( P>.05) between fa/fa and lean rats fed MO or SO diet. Thus, the alterations in the fatty acid and desaturase indices in the brain were consistent inhibited forebrain synthesis of DHA in the fa/fa rats. ALA may not have potential to effectively serve as a precursor for synthesizing DHA for youth forebrain during insulin resistance since Δ5 desaturase activity is limited.

A subset of dysregulated metabolic and survival genes is associated with severity of hepatic steatosis in obese Zucker rats

We aimed to characterize the primary abnormalities associated with fat accumulation and vulnerability to hepatocellular injury of obesity-related fatty liver. We performed functional analyses and comparative transcriptomics of isolated primary hepatocytes from livers of obese insulin-resistant Zucker rats (comprising mild to severe hepatic steatosis) and age-matched lean littermates, searching for novel genes linked to chronic hepatic steatosis. Of the tested genome, 1.6% was identified as steatosis linked. Overexpressed genes were mainly dedicated to primary metabolism (100%), signaling, and defense/acute phase (approximately 70%); detoxification, steroid, and sulfur metabolism (approximately 65%) as well as cell growth/proliferation and protein synthesis/transformation (approximately 70%) genes were downregulated. The overexpression of key genes involved in de novo lipogenesis, fatty acid and glycerolipid import and synthesis, as well as acetyl-CoA and cofactor provision was paralleled by enhanced hepatic lipogenesis and production of large triacylglycerol-rich VLDL. Greatest changes in gene expression were seen in those encoding the lipogenic malic enzyme (up to 7-fold increased) and cell-to-cell interacting cadherin 17 (up to 8-fold decreased). Among validated genes, fatty acid synthase, stearoyl-CoA desaturase 1, fatty acid translocase/Cd36, malic enzyme, cholesterol-7 alpha hydroxylase, cadherin 17, and peroxisome proliferator-activated receptor alpha significantly correlated with severity of hepatic steatosis. In conclusion, dysregulated expression of metabolic and survival genes accompany hepatic steatosis in obese insulin-resistant rats and may render steatotic hepatocytes more vulnerable to cell injury in progressive nonalcoholic fatty liver disease.

Ability of GHTD-amide and analogs to enhance insulin activity through zinc chelation and dispersal of insulin oligomers

GHTD-amide is a tetrapeptide originally isolated from human urine that has hypoglycemic activity. Insulin occurs in secretory granules of beta cells as zinc-stabilized hexamers and must disperse to monomeric form in order to bind to its receptor. The aim of this study was to identify whether GHTD-amide and an analog called ISF402 (VHTD-amide) reduce blood glucose through enhancement of insulin activity by dispersing oligomers of insulin. Peptides containing the HTD-amide sequence and a free α-amino group were optimal at binding Zn 2+ and adopting secondary structure in the presence of Zn 2+. Binding was concentration dependent and resulted in a 1:1 Zn:peptide complex. In vitro the tetrapeptides dispersed hexameric insulin to dimers and monomers. GHTD-amide and ISF402 potentiated the activity of hexameric insulin when co-injected into insulin resistant Zucker rats. Injection of peptides with insulin caused reductions in blood glucose and C-peptide significantly larger than achieved with insulin alone, and serum insulin time profiles were also altered consistent with a reduced clearance or enhanced dispersal of the injected insulin. Insulin potentiation by ISF402 was reduced when lispro insulin, which does not form zinc-stabilized hexamers, was used in place of hexameric zinc insulin. In conclusion, GHTD-amide and ISF402 are zinc binding peptides that disperse hexameric insulin in vitro, and potentiate the activity of hexameric insulin more so than monomeric lispro insulin. These results suggest that dispersal of hexameric insulin through chelation of Zn 2+ contributes to the hypoglycemic activity of these tetrapeptides.

N-methyl-D-aspartate increases cortical blood flow in part by evoking cortical spreading depression in Wistar, and insulin resistant Zucker rats

Event Abstract Back to Event N-methyl-D-aspartate increases cortical blood flow in part by evoking cortical spreading depression in Wistar, and insulin resistant Zucker rats Laura Lenti1, 2*, Ferenc Domoki1, 2, David W. Busija2 and Ferenc Bari1 1 Department of Physiology, University of Szeged, Hungary 2 Department of Physiology & Pharmacology, Wake Forest University Health Sciences, United States NMDA dilates pial arterioles via evoking cortical spreading depression (CSD) in mice; however, a dose-dependent action was described in piglets. The mechanism of the cerebral vascular response in rats is unclear. The purpose of our study was to critically examine the effect of NMDA on the cortical blood flow (CoBF) in this model. Further, we sought to determine whether this microvascular response is reduced in the Zucker obese rats, since insulin resistance (IR) impairs vascular reactivity of large cerebral arteries. Anesthetized, ventilated Wistar (8 weeks old), Zucker obese and lean (12 weeks old, n=8-8) rats were equipped with closed cranial window. NMDA (10-4 10-3 M) was applied to the parietal cortex, while CoBF (using Laser-Doppler flowmetry) and cortical DC potential were monitored. NMDA evoked CSD at both concentrations in all experimental groups. After a rapid, transient hyperemia, sustained increases in CoBF were observed. The latency and peak amplitude of the CoBF response to CSD was dose-related: the higher dose resulted in shorter latency (100±5* vs. 146±11 s, all data from Wistars, *p<0.05) and larger amplitude (237±45* vs. 168±34, % of baseline). The sustained CoBF elevation after the CSD was also dose-dependent: 51±10* vs. 19±4% (average of 2-6th minutes after the peak values). CoBF responses were similar in lean and obese rats. In conclusion, the cerebrovascular response to NMDA in rats is partially due to CSDs. The unchanged CoBF response to NMDA/CSD in Zucker obese rats indicates that the microvascular response to this stimulus remains intact in IR. Supported by OTKA (K63401 and IN69967) Conference: 12th Meeting of the Hungarian Neuroscience Society, Budapest, Hungary, 22 Jan - 24 Jan, 2009. Presentation Type: Poster Presentation Topic: Pathophysiology and neurology - degenerative disorders Citation: Lenti L, Domoki F, Busija DW and Bari F (2009). N-methyl-D-aspartate increases cortical blood flow in part by evoking cortical spreading depression in Wistar, and insulin resistant Zucker rats. Front. Syst. Neurosci. Conference Abstract: 12th Meeting of the Hungarian Neuroscience Society. doi: 10.3389/conf.neuro.01.2009.04.162 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 05 Mar 2009; Published Online: 05 Mar 2009. * Correspondence: Laura Lenti, Department of Physiology, University of Szeged, Szeged, Hungary, lauralenti@yahoo.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Laura Lenti Ferenc Domoki David W Busija Ferenc Bari Google Laura Lenti Ferenc Domoki David W Busija Ferenc Bari Google Scholar Laura Lenti Ferenc Domoki David W Busija Ferenc Bari PubMed Laura Lenti Ferenc Domoki David W Busija Ferenc Bari Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Fatty liver and insulin resistance in obese Zucker rats: No role for mitochondrial dysfunction

The relationship between insulin resistance and mitochondrial function is of increasing interest. Studies looking for such interactions are usually made in muscle and only a few studies have been done in liver, which is known to be a crucial partner in whole body insulin action. Recent studies have revealed a similar mechanism to that of muscle for fat-induced insulin resistance in liver. However, the exact mechanism of lipid metabolites accumulation in liver leading to insulin resistance is far from being elucidated. One of the hypothetical mechanisms for liver steatosis development is an impairment of mitochondrial function. We examined mitochondrial function in fatty liver and insulin resistance state using isolated mitochondria from obese Zucker rats. We determined the relationship between ATP synthesis and oxygen consumption as well as the relationship between mitochondrial membrane potential and oxygen consumption. In order to evaluate the quantity of mitochondria and the oxidative capacity we measured citrate synthase and cytochrome c oxidase activities. Results showed that despite significant fatty liver and hyperinsulinemia, isolated liver mitochondria from obese Zucker rats display no difference in oxygen consumption, ATP synthesis, and membrane potential compared with lean Zucker rats. There was no difference in citrate synthase and cytochrome c oxidase activities between obese and lean Zucker rats in isolated mitochondria as well as in liver homogenate, indicating a similar relative amount of hepatic mitochondria and a similar oxidative capacity. Adiponectin, which is involved in bioenergetic homeostasis, was increased two-fold in obese Zucker rats despite insulin resistance. In conclusion, isolated liver mitochondria from lean and obese insulin-resistant Zucker rats showed strictly the same mitochondrial function. It remains to be elucidated whether adiponectin increase is involved in these results.

N‐methyl‐D‐aspartate (NMDA) increases cortical blood flow in part by evoking cortical spreading depression (CSD) in insulin resistant Zucker rats

Insulin resistance (IR) leads to selective impairment of responsiveness of larger cerebral arteries but effects on total cortical blood flow (CoBF) have not been examined. The purpose of our study was to test whether NMDA‐induced cerebral vasodilation is altered in the Zucker obese rat, a genetic model of IR. Anesthetized, ventilated, 12 week old male Zucker obese (n=8) and lean (n=8) rats were equipped with closed cranial windows. NMDA (10‐4, 10‐3 M) was applied to the parietal cortex, while CoBF (Laser‐Doppler flowmetry) and cortical DC potential were monitored. NMDA evoked CSD often at 10‐4 and always at 10‐3 M concentration. The amplitude of the response was similar in the two groups at all doses. After the transient CSD‐induced hyperemia, sustained increases in CoBF occurred with both 10‐4–10‐3 M NMDA at 6–10 minutes and there were no differences between obese and lean rats. Increases in CoBF during application of 10‐4 and 10‐3 M NMDA (% of baseline, 6–10th min of NMDA, *p&lt;0.05 compared to vehicle) were 12±6 and 40±9*% in lean and 19±8% and 40±8*% in obese rats. Thus, similar to mice, the total cerebral vascular response to NMDA is due to direct effects by individual neurons as well as evoked CSDs. The retention of normal increases in CoBF in obese rats to NMDA/CSD indicates microvascular responses to some stimuli are intact in IR animals. Supported by the NIH (HL30260, HL65380, HL77731) and by the OTKA (K68976 and K63401).

Antidiabetogenic Effects of Chromium Mitigate Hyperinsulinemia-Induced Cellular Insulin Resistance via Correction of Plasma Membrane Cholesterol Imbalance

Previously, we found that a loss of plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate (PIP2)-regulated filamentous actin (F-actin) structure contributes to insulin-induced insulin resistance. Interestingly, we also demonstrated that chromium picolinate (CrPic), a dietary supplement thought to improve glycemic status in insulin-resistant individuals, augments insulin-regulated glucose transport in insulin-sensitive 3T3-L1 adipocytes by lowering PM cholesterol. Here, to gain mechanistic understanding of these separate observations, we tested the prediction that CrPic would protect against insulin-induced insulin resistance by improving PM features important in cytoskeletal structure and insulin sensitivity. We found that insulin-induced insulin-resistant adipocytes display elevated PM cholesterol with a reciprocal decrease in PM PIP2. This lipid imbalance and insulin resistance was corrected by the cholesterol-lowering action of CrPic. The PM lipid imbalance did not impair insulin signaling, nor did CrPic amplify insulin signal transduction. In contrast, PM analyses corroborated cholesterol and PIP2 interactions influencing cytoskeletal structure. Because extensive in vitro study documents an essential role for cytoskeletal capacity in insulin-regulated glucose transport, we next evaluated intact skeletal muscle from obese, insulin-resistant Zucker (fa/fa) rats. Because insulin resistance in these animals likely involves multiple mechanisms, findings that cholesterol-lowering restored F-actin cytoskeletal structure and insulin sensitivity to that witnessed in lean control muscle were striking. Also, experiments using methyl-beta-cyclodextrin to shuttle cholesterol into or out of membranes respectively recapitulated the insulin-induced insulin-resistance and protective effects of CrPic on membrane/cytoskeletal interactions and insulin sensitivity. These data predict a PM cholesterol basis for hyperinsulinemia-associated insulin resistance and importantly highlight the reversible nature of this abnormality.

Reduced Expression of Insulin Receptors in the Kidneys of Insulin-Resistant Rats

Insulin resistance is accompanied by hyperinsulinemia and activation of the renin-angiotensin system, both of which are associated with hypertension. Because the kidney plays a major role in the regulation of blood pressure, we studied the regulation of insulin receptor expression in the kidney during states of insulin resistance. Using two rat models of insulin resistance, Western blot analysis demonstrated a significant reduction in the expression of insulin receptor subunits in the kidney compared to lean control rats. Treatment of insulin resistance in Zucker rats with the insulin-sensitizing drug rosiglitazone partially restored renal insulin receptor levels. Conversely, treatment with the angiotensin II type 1 receptor (AT1) antagonist candesartan increased renal insulin receptor expression compared to untreated rats. Streptozotocin-induced hyperglycemia, which results from hypoinsulinemia, reduced expression of renal insulin receptors. Hyperinsulinemia induced by insulin infusion, however, did not produce a similar effect. In conclusion, insulin receptors are downregulated in the kidneys of insulin resistant rats, possibly mediated by hyperglycemia and angiotensin II.

Adrenomedullin Inhibits Adipogenesis Under Transcriptional Control of Insulin

We generated preadipocyte cell lines impaired in adrenomedullin production through integration of an adrenomedullin small interfering RNA expression vector. The reduction of adrenomedullin synthesis strongly accelerated adipose differentiation. These results were bolstered when overexpression of active adrenomedullin peptide led to delayed differentiation. Therefore, we propose that adrenomedullin is an antiadipogenic factor. Moreover, we checked whether insulin, a proadipogenic factor, regulates expression of adrenomedullin. We observed that insulin had an inhibitory effect on adrenomedullin expression in isolated human adipocyte cells. This response was dose dependent and was reversed by resistin, a new anti-insulin agent. We quantified circulating adrenomedullin in healthy obese patients and observed a threefold increase of adrenomedullin compared with lean patients. Furthermore, adrenomedullin plasma levels are negatively correlated to plasma insulin levels in these obese patients. The insulin inhibitory response was also observed in vivo in Sprague-Dawley rats but not in the insulin-resistant Zucker rat, suggesting that adrenomedullin expression is upregulated in insulin-resistant adipose cells. Using adrenomedullin promoter-luciferase reporter gene constructs, we have shown that the adrenomedullin response to insulin is mediated by insulin-responsive elements. These findings provide new insight into fat mass development and the relationship between obesity and elevated circulating adrenomedullin levels in diabetic patients.

LKB1-AMPK signaling in muscle from obese insulin-resistant Zucker rats and effects of training

AMPK is a key regulator of fat and carbohydrate metabolism. It has been postulated that defects in AMPK signaling could be responsible for some of the metabolic abnormalities of type 2 diabetes. In this study, we examined whether insulin-resistant obese Zucker rats have abnormalities in the AMPK pathway. We compared AMPK and ACC phosphorylation and the protein content of the upstream AMPK kinase LKB1 and the AMPK-regulated transcriptional coactivator PPARgamma coactivator-1 (PGC-1) in gastrocnemius of sedentary obese Zucker rats and sedentary lean Zucker rats. We also examined whether 7 wk of exercise training on a treadmill reversed abnormalities in the AMPK pathway in obese Zucker rats. In the obese rats, AMPK phosphorylation was reduced by 45% compared with lean rats. Protein expression of the AMPK kinase LKB1 was also reduced in the muscle from obese rats by 43%. In obese rats, phosphorylation of ACC and protein expression of PGC-1alpha, two AMPK-regulated proteins, tended to be reduced by 50 (P = 0.07) and 35% (P = 0.1), respectively. There were no differences in AMPKalpha1, -alpha2, -beta1, -beta2, and -gamma3 protein content between lean and obese rats. Training caused a 1.5-fold increase in AMPKalpha1 protein content in the obese rats, although there was no effect of training on AMPK phosphorylation and the other AMPK isoforms. Furthermore, training also significantly increased LKB1 and PGC-1alpha protein content 2.8- and 2.5-fold, respectively, in the obese rats. LKB1 protein strongly correlated with hexokinase II activity (r = 0.75, P = 0.001), citrate synthase activity (r = 0.54, P = 0.02), and PGC-1alpha protein content (r = 0.81, P < 0.001). In summary, obese insulin-resistant rodents have abnormalities in the LKB1-AMPK-PGC-1 pathway in muscle, and these abnormalities can be restored by training.

Apolipoprotein AV does not contribute to hypertriglyceridaemia or triglyceride lowering by dietary fish oil and rosiglitazone in obese Zucker rats

Apolipoprotein AV (apoAV) is a recently discovered apolipoprotein with a triglyceride-lowering effect in genetically modified mice. Transcription of the human gene encoding apoAV (APOA5) is suppressed by insulin and stimulated by fibrates. Our goal was to study the expression of Apoa5, in comparison with Apoa4 and Apoc3, in hypertriglyceridaemic, obese and insulin-resistant Zucker rats receiving the insulin sensitiser rosiglitazone and/or a fish oil diet to lower triglycerides. Hepatic Apoa5, Apoa4 and Apo3 mRNA and liver and plasma apoAV were measured in lean and obese Zucker rats receiving rosiglitazone while on a coconut oil or fish oil diet. Basal hepatic Apoa5 expression was similar in obese and lean Zucker rats. Unexpectedly, obese Zucker rats tended to have higher plasma apoAV levels despite their hypertriglyceridaemic state. Both rosiglitazone and the fish oil diet significantly increased Apoa5 mRNA, by about 70%, but tended to lower liver and plasma apoAV. Rosiglitazone had no effect on Apoa5 mRNA in cultured rat hepatocytes. No intact PPAR (peroxisome proliferator-activated receptor) response element was identified in the rat Apoa5 promoter. Our data indicate that apoAV does not contribute to the hypertriglyceridaemia of obese Zucker rats or to the hypolipidaemic effect of rosiglitazone or a fish oil diet. The divergent changes of Apoa5 mRNA and apoAV levels suggest co- or post-translational regulation. The increase in Apoa5 mRNA induced by rosiglitazone is not directly mediated by peroxisome proliferator-activated receptor gamma.