Effects of troglitazone on hepatic and peripheral insulin resistance induced by growth hormone excess in rats

Insulin resistance, serum adipokines and risk of fibrosis progression in patients transplanted for hepatitis C

In the nontransplant setting diabetes mellitus is a risk factor for disease progression in patients with chronic hepatitis C virus (HCV) infection. The impact of early insulin resistance on the development of advanced fibrosis, even in the absence of clinically apparent diabetes mellitus, is not known. Our aim was to determine whether the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) can be used to identify insulin-resistant patients at risk for rapid fibrosis progression. Cohort study including patients transplanted for chronic HCV between January 1, 1995 and January 1, 2005. One hundred sixty patients were included; 25 patients (16%) were treated for diabetes mellitus and 36 patients (23%) were prediabetic, defined as HOMA-IR >2.5. Multivariate Cox regression analysis showed that insulin resistance (hazard ratio (HR) 2.07; confidence interval (CI) 1.10-3.91, p = 0.024), donor age (HR 1.33;CI 1.08-1.63, p = 0.007) and aspartate aminotransferase (HR 1.03;CI 1.01-1.05, p < 0.001) were significantly associated with a higher probability of developing advanced fibrosis, i.e. Knodell fibrosis stage 3 or 4, whereas steatosis (HR 0.94;CI 0.46-1.92, p = 0.87) and acute cellular rejection (HR 1.72;CI 0.88-3.36, p = 0.111) were not. In conclusion, posttransplant insulin resistance is strongly associated with more severe recurrence of HCV infection. HOMA-IR is an important tool for the identification of insulin resistance among patients at risk for rapid fibrosis progression after liver transplantation for HCV.

Hepatic dysfunction and insulin insensitivity in type 2 diabetes mellitus: a critical target for insulin-sensitizing agents

The liver plays an essential role in maintaining glucose homeostasis, which includes insulin-mediated processes such as hepatic glucose output (HGO) and uptake, as well as in clearance of insulin itself. In type 2 diabetes, the onset of hyperglycaemia [itself a potent inhibitor of hepatic glucose output (HGO)], alongside hyperinsulinaemia, indicates the presence of hepatic insulin insensitivity. Increased HGO is central to the onset of hyperglycaemia and highlights the need to target hepatic insulin insensitivity as a central component of glucose-lowering therapy. The mechanisms underlying the development of hepatic insulin insensitivity are not well understood, but may be influenced by factors such as fatty acid oversupply and altered adipocytokine release from dysfunctional adipose tissue and increased liver fat content. Furthermore, although the impact of insulin insensitivity as a marker of cardiovascular disease is well known, the specific role of hepatic insulin insensitivity is less clear. The pharmacological tools available to improve insulin sensitivity include the biguanides (metformin) and thiazolidinediones (rosiglitazone and pioglitazone). Data from a number of sources indicate that thiazolidinediones, in particular, can improve multiple aspects of hepatic dysfunction, including reducing HGO, insulin insensitivity and liver fat content, as well as improving other markers of liver function and the levels of mediators with potential involvement in hepatic function, including fatty acids and adipocytokines. The current review addresses this topic from the perspective of the role of the liver in maintaining glucose homeostasis, its key involvement in the pathogenesis of type 2 diabetes and the tools currently available to reduce hepatic insulin insensitivity.

Conditioned medium obtained from in vitro differentiated adipocytes and resistin induce insulin resistance in human hepatocytes

Adipocyte-derived factors might play a role in the development of hepatic insulin resistance. Resistin was identified as an adipokine linking obesity and insulin resistance. Resistin is secreted from adipocytes in rodents but in humans it was proposed to originate from macrophages and its impact for insulin resistance has remained elusive. To analyze the role of adipokines in general and resistin as a special adipokine, we cultured the human liver cell line HepG2 with adipocyte-conditioned medium (CM) containing various adipokines such as IL-6 and MCP-1, and resistin. CM and resistin both induce insulin resistance with a robust decrease in insulin-stimulated phosphorylation of Akt and GSK3. Insulin resistance could be prevented by co-treatment with troglitazone but not by co-stimulation with adiponectin. As human adipocytes do not secrete resistin, HepG2 cells were also treated with resistin added into CM. CM with resistin addition induced stronger insulin resistance than CM alone pointing to a specific role of resistin in the initiation of hepatic insulin resistance in humans.

Effects of growth hormone and pioglitazone in viscerally obese adults with impaired glucose tolerance: a factorial clinical trial

OBJECTIVE

Recombinant human growth hormone (GH) and pioglitazone (PIO) in abdominally obese adults with impaired glucose tolerance were evaluated under the hypothesis that the combination attenuates GH-induced increases in glucose concentrations, reduces visceral adipose tissue (VAT), and improves insulin sensitivity over time.

DESIGN

Randomized, double-blind, placebo-controlled, 2 x 2 factorial design.

SETTING

Veterans Affairs Palo Alto Health Care System, Palo Alto, California, United States.

PARTICIPANTS

62 abdominally obese adults aged 40-75 with impaired glucose tolerance.

INTERVENTIONS

GH (8 microg/kg/d, or placebo) and pioglitazone (30 mg/d, or placebo) for 40 wk.

OUTCOME MEASURES

Baseline and after 40 wk of treatment, VAT content was quantified by CT scan, glucose tolerance was assessed using a 75-g oral glucose tolerance test, and insulin sensitivity was measured using steady-state plasma glucose levels obtained during insulin suppression test.

RESULTS

BASELINE: body mass index (BMI), plasma glucose, and visceral fat content were similar. 40 wk: visceral fat area declined 23.9 +/- 7.4 cm(2) in GH group, mean difference from placebo: -28.1 cm(2) (95% CI -49.9 to -6.3 cm(2); p = 0.02). Insulin resistance declined 52 +/- 11.8 mg/dl with PIO, mean difference from placebo of -58.8 mg/dl (95% CI -99.7 to -18.0 mg/dl; p = 0.01). VAT and SSPG declined with GH and PIO combined, mean differences from placebo of -31.4 cm(2) (95% CI -56.5 cm(2) to -6.3 cm(2); p = 0.02) and -55.3 mg/dl (95% CI -103.9 to -6.7 mg/dl; p = 0.02), respectively. Fasting plasma glucose increased transiently in GH group. No significant changes in BMI were observed.

CONCLUSIONS

Addition of PIO to GH attenuated the short-term diabetogenic effect of GH; the drug combination reduced VAT and insulin resistance over time. GH plus PIO may have added benefit on body composition and insulin sensitivity in the metabolic syndrome.

The novel hypoglycemic agent YM440 improves hepatic insulin resistance in obese Zucker fatty rats

The novel hypoglycemic agent YM440 ((Z)-1,4-bis{4-[(3,5-dioxo-1,2,4-oxadiazolidin-2-yl)methyl] phenoxy}but-2-ene) is a ligand of the peroxisome proliferator-activated receptor (PPAR) gamma. YM440 has unique pharmacological profiles both in vitro and in vivo, but, it is not clear whether the compound has a significant effect on hepatic or peripheral insulin response throughout the body. The aim of this study is to examine the effects of YM440 on hepatic and peripheral insulin resistance in Zucker fatty (ZF) rats using the euglycemic-hyperinsulinaemic clamp technique. Treatment of ZF rats with YM440 (300 mg/kg per day) for 2 weeks significantly decreased plasma concentrations of glucose and insulin without inducing obesity. YM440 caused a 2-fold increase in the glucose infusion rate during euglycemic clamping compared with the vehicle control. YM440 also decreased the percent change in hepatic glucose production rate caused by intravenous insulin infusion in ZF rats. YM440 had no significant effect on the glucose disposal rate. These results indicate that YM440 ameliorates hepatic, but not peripheral insulin resistance in ZF rats. These findings strongly suggest that the main target organ of YM440 is the liver, unlike other PPARgamma agonist.

Regulation of glucose transport and insulin signaling by troglitazone or metformin in adipose tissue of type 2 diabetic subjects

Type 2 diabetic subjects failing glyburide therapy were randomized to receive additional therapy with either metformin (2,550 mg/day) or troglitazone (600 mg/day) for 3-4 months. Biopsies of subcutaneous abdominal adipose tissue were obtained before and after therapy. Glycemic control was similar with both treatments. Metformin treatment increased insulin-stimulated whole-body glucose disposal rates by 20% (P < 0.05); the response to troglitazone was greater (44% increase, P < 0.01 vs. baseline, P < 0.05 vs. metformin). Troglitazone-treated subjects displayed a tendency toward weight gain (5 +/- 2 kg, P < 0.05), increased adipocyte size, and increased serum leptin levels. Metformin-treated subjects were weight-stable, with unchanged leptin levels and reduced adipocyte size (to 84 +/- 4% of control, P < 0.005). Glucose transport in isolated adipocytes from metformin-treated subjects was unaltered from pretreatment. Glucose transport in both the absence (321 +/- 134% of pre-Rx, P < 0.05) and presence of insulin (418 +/- 161%, P < 0.05) was elevated after troglitazone treatment. Metformin treatment had no effect on adipocyte content of GLUT1 or GLUT4 proteins. After troglitazone treatment, GLUT4 protein expression was increased twofold (202 +/- 42%, P < 0.05). Insulin-stimulated serine phosphorylation of Akt was augmented after troglitazone (170 +/- 34% of pre-Rx response, P < 0.05) treatment and unchanged by metformin. We conclude that the ability of troglitazone to upregulate adipocyte glucose transport, GLUT4 expression, and insulin signaling can contribute to its greater effect on whole-body glucose disposal.

Diabetogenic activity of 20 kDa human growth hormone (20K-hGH) and 22K-hGH in rats

To compare the diabetogenic activity of 20 kDa human growth hormone (20K-hGH) with that of 22K-hGH, we evaluated insulin sensitivity with a euglycaemic clamp in rats. The glucose infusion rate (GIR) in euglycaemic clamp studies was measured as an indicator of insulin sensitivity. [(14)C]glucose and 2-[(3)H] deoxy- D -glucose injection were used to calculate the rate of glucose utilization (R(d)), the hepatic glucose output (HGO), and the glucose metabolic index (R(g)'). Both 20K- and 22K-hGH were infused at equivalent rates (1.0 (mg/kg)/day). A 24 h infusion of 20K-hGH had no significant effects on the GIR, R(d), HGO and R(g)(')except for in gastrocnemius muscle. In contrast, 22K-hGH significantly lowered the GIR compared with the control (P< 0.001) and 20K-hGH groups (P< 0.01). The infusion of 22K-hGH also reduced R(d)compared with the controls and the 20K-hGH rats by 46.6% (P< 0.001) and 39.6% (P< 0.05) respectively, while no differences were observed in the HGO. Moreover, 22K-hGH inhibited glucose uptake, which was estimated from the insulin-stimulated R(g)' in some tissues. These results suggest that 22K-hGH inhibits the uptake and use of glucose in various tissues, which then leads to insulin resistance. In conclusion, the diabetogenicity of 20K-hGH is much weaker than that of 22K-hGH, and the reduced insulin-antagonizing action of 20K-hGH could have important clinical benefits.

Differential effects of troglitazone and D-chiroinositol on glucosamine-induced insulin resistance in vivo in rats

Troglitazone and D-chiroinositol have been shown to exert antidiabetic effects by either potentiating or mimicking insulin action. We studied whether pretreatment with these compounds can prevent the deleterious effects of glucosamine on insulin action that may play an important role in hyperglycemia-induced insulin resistance. Normal Wistar rats were pretreated with troglitazone (100 mg/kg/d), D-chiroinositol (100 mg/kg/d), or placebo (saline) for 7 days. Glucosamine (50 micromol/kg/min) was then infused for 210 minutes, and a euglycemic glucose clamp was performed during the last 120 minutes. Pretreatment with troglitazone or D-chiroinositol had no effect on fasting plasma glucose or insulin or basal hepatic glucose output (HGO). Under the euglycemic-hyperinsulinemic (956+/-93 pmol/L) clamp condition, HGO in glucosamine-infused placebo-treated rats was not suppressed, but instead was increased over the basal level, indicative of hepatic insulin resistance. In contrast, HGO failed to increase during glucosamine infusion in rats pretreated with troglitazone but was not normally suppressed. This may indicate a partial improvement in the hepatic insulin resistance. D-Chiroinositol pretreatment had no effect on the glucosamine-induced increase in HGO. The glucose disposal rate (GDR) was 25% lower in rats infused with glucosamine versus saline-infused rats (25.5+/-2.5 v 34.1+/-2.0 mg/kg/min), indicative of peripheral insulin resistance. Pretreatment with D-chiroinositol (34.5+/-2.3 mg/kg/min) prevented the glucosamine-induced decrease in the GDR, indicating an improvement in peripheral insulin resistance. Troglitazone (25.2+/-3.3 mg/kg/min) was without effect. In conclusion, (1) in normal control rats, glucosamine infusion induced hepatic and peripheral insulin resistance; (2) D-chiroinositol, but not troglitazone, pretreatment prevented glucosamine-induced peripheral insulin resistance; and (3) troglitazone, but not D-chiroinositol, partially blocked the glucosamine-induced hepatic insulin resistance. D-Chiroinositol may provide a novel pharmacological approach to hexosamine-induced peripheral insulin resistance.