Insulin Resistance In Transgenic Mice Research Articles

Dissociation of obesity and insulin resistance in transgenic mice with skeletal muscle expression of uncoupling protein 1

We evaluated the effect of skeletal muscle mitochondrial uncoupling on energy and glucose metabolism under different diets. For 3 mo, transgenic HSA-mUCP1 mice with ectopic expression of uncoupling protein 1 in skeletal muscle and wild-type littermates were fed semisynthetic diets with varying macronutrient ratios (energy % carbohydrate-protein-fat): HCLF (41:42:17), HCHF (41:16:43); LCHF (11:45:44). Body composition, energy metabolism, and insulin resistance were assessed by NMR, indirect calorimetry, and insulin tolerance test, respectively. Gene expression in different organs was determined by real-time PCR. In wild type, both high-fat diets led to an increase in body weight and fat. HSA-mUCP1 mice considerably increased body fat on HCHF but stayed lean on the other diets. Irrespective of differences in body fat content, HSA-mUCP1 mice showed higher insulin sensitivity and decreased plasma insulin and liver triglycerides. Respiratory quotient and gene expression indicated overall increased carbohydrate oxidation of HSA-mUCP1 but a preferential channeling of fatty acids into muscle rather than liver with high-fat diets. Evidence for increased lipogenesis in white fat of HSA-mUCP1 mice suggests increased energy dissipating substrate cycling. Retinol binding protein 4 expression in white fat was increased in HSA-mUCP1 mice despite increased insulin sensitivity, excluding a causal role in the development of insulin resistance. We conclude that skeletal muscle mitochondrial uncoupling does not protect from the development of obesity in all circumstances. Rather it can lead to a "healthy" obese phenotype by preserving insulin sensitivity and a high metabolic flexibility, thus protecting from the development of obesity associated disturbances of glucose homeostasis.

Increased lipid accumulation and insulin resistance in transgenic mice expressing DGAT2 in glycolytic (type II) muscle

Insulin resistance and type 2 diabetes are frequently accompanied by lipid accumulation in skeletal muscle. However, it is unknown whether primary lipid deposition in skeletal muscle is sufficient to cause insulin resistance or whether the type of muscle fiber, oxidative or glycolytic fiber, is an important determinant of lipid-mediated insulin resistance. Here we utilized transgenic mice to test the hypothesis that lipid accumulation specifically in glycolytic muscle promotes insulin resistance. Overexpression of DGAT2, which encodes an acyl-CoA:diacylglycerol acyltransferase that catalyzes triacylglycerol (TG) synthesis, in glycolytic muscle of mice increased the content of TG, ceramides, and unsaturated long-chain fatty acyl-CoAs in young adult mice. This lipid accumulation was accompanied by impaired insulin signaling and insulin-mediated glucose uptake in glycolytic muscle and impaired whole body glucose and insulin tolerance. We conclude that DGAT2-mediated lipid deposition specifically in glycolytic muscle promotes insulin resistance in this tissue and may contribute to the development of diabetes.

FOXC2 mRNA Expression and a 5' untranslated region polymorphism of the gene are associated with insulin resistance.

The human transcription factor FOXC2 has recently been shown to protect against diet-induced insulin resistance in transgenic mice. We investigated the expression of FOXC2 in fat and muscle and performed a genetic analysis in human subjects. FOXC2 mRNA levels were increased in visceral compared with subcutaneous fat from obese subjects (12 +/- 4-fold; P = 0.0001), and there was a correlation between whole-body insulin sensitivity and FOXC2 mRNA levels in visceral fat (fS-insulin R = -0.64, P = 0.01, and homeostasis model assessment of insulin resistance [HOMA-IR] R = -0.68, P = 0.007) and skeletal muscle (fS-insulin R = -0.57, P = 0.03, and HOMA-IR R = -0.55, P = 0.04). Mutation screening of the FOXC2 gene identified a common polymorphism in the 5' untranslated region (C-512T). The T allele was associated with enhanced insulin sensitivity (HOMA-IR P = 0.007) and lower plasma triglyceride levels in females (P = 0.007). Also, the higher expression of FOXC2 in visceral than in subcutaneous fat was restricted to subjects homozygous for the T allele (P = 0.03 vs. P = 0.7). Our data suggest that increased FOXC2 expression may protect against insulin resistance in human subjects and that genetic variability in the gene may influence features associated with the metabolic syndrome.

Human placental growth hormone causes severe insulin resistance in transgenic mice

OBJECTIVE: The insulin resistance of pregnancy is considered to be mediated by human placental lactogen, but the metabolic effects of human placental growth hormone have not been well defined. Our aim was to evaluate the effect of placental growth hormone on insulin sensitivity in vivo using transgenic mice that overexpress the human placental growth hormone gene. STUDY DESIGN: Glucose and insulin tolerance tests were performed on 5 transgenic mice that overexpressed the human placental growth hormone variant gene and 6 normal littermate controls. The body composition of the mice was assessed by dual-energy radiograph absorptiometry, and free fatty acid levels were measured as a marker of lipolysis. RESULTS: The human placental growth hormone levels in the transgenic mice were comparable to those attained in the third trimester of pregnancy. These mice were nearly twice as heavy as the control mice, and their body composition differed by a significant increase in bone density and a small decrease in percentage of body fat. Fasting insulin levels in the transgenic mice that overexpressed placental growth hormone were approximately 4-fold higher than the control mice (1.57 ± 0.22 ng/mL vs 0.38 ± 0.07 ng/mL; P <.001) and 7 times higher 30 minutes after glucose stimulation (4.17 ± 0.54 ng/mL vs 0.62 ± 0.10 ng/mL; P <.0001) with no significant difference in either fasting or postchallenge glucose levels. Insulin sensitivity was markedly decreased in the transgenic mice, as demonstrated by an insignificant decline in glucose levels after insulin injection compared with the control mice, which demonstrated more than a 65% reduction in glucose levels (P <.001). CONCLUSION: Human placental growth hormone causes insulin resistance as manifested by fasting and postprandial hyperinsulinemia and minimal glucose lowering in response to insulin injection. Human placental growth hormone is a highly likely candidate to mediate the insulin resistance of pregnancy. (Am J Obstet Gynecol 2002;186:512-7.)

Protection from obesity and insulin resistance in mice overexpressing human apolipoprotein C1.

Apolipoprotein (APO) C1 is a 6.6-kDa protein present in plasma and associated with lipoproteins. Using hyperinsulinemic-euglycemic clamp tests, we previously found that in APOC1 transgenic mice, the whole-body insulin-mediated glucose uptake is increased concomitant with a decreased fatty acid uptake. These latter results are confirmed in the present study, showing that APOC1 transgenic mice exhibit a 50% reduction in the uptake of the fatty acid analog 15-(p-iodophenyl)-3-(R,S)-methyl pentadecanoic acid in white adipose tissue stores. We next investigated whether APOC1 overexpression can modulate the initiation and/or development of obesity and insulin resistance. When crossbred on the genetically obese ob/ob background, APOC1 transgenic mice were fully protected from the development of obesity compared with ob/ob only mice, as reflected by a strong reduction in body weight (21 +/- 4 vs. 44 +/- 7 g), total adipose tissue stores (15 +/- 3 vs. 25 +/- 3% body wt), and average adipocyte size (7,689 +/- 624 vs. 15,295 +/- 1,289 microm(2)). Although less pronounced, APOC1 overexpression also reduced body weight on a wild-type background, solely due to a reduction in adipose tissue. Furthermore, despite elevated plasma free fatty acid and triglyceride levels, APOC1 overexpression significantly improved insulin sensitivity in ob/ob mice, as demonstrated by a strong reduction in plasma glucose and insulin levels, as well as a better performance in the glucose tolerance test. In conclusion, a marked reduction in the uptake of fatty acids into adipocytes may underlie the protection from obesity and insulin resistance in transgenic mice overexpressing human APOC1.

Mechanism of hexosamine-induced insulin resistance in transgenic mice overexpressing glutamine:fructose-6-phosphate amidotransferase: decreased glucose transporter GLUT4 translocation and reversal by treatment with thiazolidinedione.

Hexosamines have been hypothesized to mediate aspects of glucose sensing and toxic effects of hyperglycemia. For example, insulin resistance results when the rate-limiting enzyme for hexosamine synthesis, glutamine:fructose-6-phosphate amidotransferase (GFA), is overexpressed in muscle and adipose tissue of transgenic mice. The glucose infusion rates required to maintain euglycemia at insulin infusion rates of 0.5, 2, 15, and 20 mU/kg x min were 39-90% lower in such transgenic mice, compared with their control littermates (P < or = 0.01). No differences were observed in hepatic glucose output, serum insulin levels, or muscle ATP levels. Uptake of 2-deoxyglucose, measured under conditions of hyperinsulinemia, was significantly lower in transgenic hindlimb muscle, compared with controls (85.9 +/- 17.8 vs. 166.8 +/- 15.1 pmol deoxyglucose/g x min). The decrease in glucose uptake by transgenic muscle was associated with a disruption in the translocation of the insulin-stimulated glucose transporter GLUT4. Fractionation of muscle membranes on a discontinuous sucrose gradient revealed that insulin stimulation of control muscle led to a 28.8% increase in GLUT4 content in the 25% fraction and a 61.2% decrease in the 35% fraction. In transgenic muscle, the insulin-stimulated shifts in GLUT4 distribution were inhibited by over 70%. Treatment of the transgenic animals with the thiazolidinedione troglitazone completely reversed the defect in glucose disposal without changing GFA activity or the levels of uridine 5'-diphosphate-N-acetylglucosamine. Overexpression of GFA in skeletal muscle thus leads to defects in glucose transport similar to those seen in type 2 diabetes. These data support the hypothesis that excess glucose metabolism through the hexosamine pathway may be responsible for the diminished insulin sensitivity and defective glucose uptake that are seen with hyperglycemia.

Mechanism of Hexosamine-Induced Insulin Resistance in Transgenic Mice Overexpressing Glutamine:Fructose-6-Phosphate Amidotransferase: Decreased Glucose Transporter GLUT4 Translocation and Reversal by Treatment with Thiazolidinedione

Hexosamines have been hypothesized to mediate aspects of glucose sensing and toxic effects of hyperglycemia. For example, insulin resistance results when the rate-limiting enzyme for hexosamine synthesis, glutamine:fructose-6-phosphate amidotransferase (GFA), is overexpressed in muscle and adipose tissue of transgenic mice. The glucose infusion rates required to maintain euglycemia at insulin infusion rates of 0.5, 2, 15, and 20 mU/kg·min were 39–90% lower in such transgenic mice, compared with their control littermates (P ≤ 0.01). No differences were observed in hepatic glucose output, serum insulin levels, or muscle ATP levels. Uptake of 2-deoxyglucose, measured under conditions of hyperinsulinemia, was significantly lower in transgenic hindlimb muscle, compared with controls (85.9 ± 17.8 vs. 166.8 ± 15.1 pmol deoxyglucose/g·min). The decrease in glucose uptake by transgenic muscle was associated with a disruption in the translocation of the insulin-stimulated glucose transporter GLUT4. Fractionation of muscle membranes on a discontinuous sucrose gradient revealed that insulin stimulation of control muscle led to a 28.8% increase in GLUT4 content in the 25% fraction and a 61.2% decrease in the 35% fraction. In transgenic muscle, the insulin-stimulated shifts in GLUT4 distribution were inhibited by over 70%. Treatment of the transgenic animals with the thiazolidinedione troglitazone completely reversed the defect in glucose disposal without changing GFA activity or the levels of uridine 5′-diphosphate-N-acetylglucosamine. Overexpression of GFA in skeletal muscle thus leads to defects in glucose transport similar to those seen in type 2 diabetes. These data support the hypothesis that excess glucose metabolism through the hexosamine pathway may be responsible for the diminished insulin sensitivity and defective glucose uptake that are seen with hyperglycemia.

Insulin signaling and glucose transport in insulin resistant skeletal muscle. Special reference to GLUT4 transgenic and GLUT4 knockout mice.

Glucose homeostasis is impaired in patients with non-insulin dependent diabetes mellitus (NIDDM) and this defect in due in part, to defects in glucose transport in skeletal muscle. Intense interest is now focused on whether reduced insulin-mediated glucose transport in muscle from NIDDM patients results from alterations in the insulin signal transduction pathway or from alterations in traffic and/or translocation of GLUT4 to the plasma membrane. Recently, potential targets for impaired traffic/translocation of GLUT4 have been reported to include defective phosphorylation of IRS-1 and reduced PI-3 kinase activity. In addition to insulin signaling defects, impaired glucose transport may result from a defect(s) in the activation or functional capacity of GLUT4. Because GLUT4 is dysregulated in skeletal muscle from NIDDM patients, it is an attractive target for gene therapy. Overexpression of GLUT4 in muscle results in increased glucose uptake and metabolism, and protects against the development of insulin resistance in transgenic mice. Genetic ablation of GLUT4 results in impaired insulin tolerance and defects in glucose metabolism in skeletal muscle. Because impaired muscle glucose transport leads to reduced whole body glucose uptake and hyperglycemia, understanding the molecular regulation of glucose transport in skeletal muscle is necessary to develop effective strategies to prevent or reduce the incidence of NIDDM.