Defects in liver and muscle glycogen metabolism in neonatal and adult New Zealand obese mice

The Role of Fatty Acid Signaling in Islet Beta-Cell Adaptation to Normal Pregnancy

BACKGROUND

Maintenance of a normal fetal nutrient supply requires major adaptations in maternal metabolic physiology, including of the islet beta-cell. The role of lipid signaling processes in the mechanisms of islet beta-cell adaptation to pregnancy has been minimally investigated.

OBJECTIVE

To determine the effects of pregnancy on islet fatty acid (FA) metabolic partitioning and FA augmentation of glucose-stimulated insulin secretion (GSIS).

METHODS

Age matched virgin, early pregnant (gestational day-11, G11) and late pregnant (G19) Sprague-Dawley rats were studied. Fasted and fed state biochemistry, oral glucose tolerance tests (OGTT), and fasted and post-OGTT liver glycogen, were determined to assess in vivo metabolic characteristics. In isolated islets, FA (BSA-bound palmitate 0.25 mmol/l) augmentation of GSIS, FA partitioning into esterification and oxidation processes using metabolic tracer techniques, lipolysis by glycerol release, triacylglycerols (TG) content, and the expression of key beta-cell genes were determined.

RESULTS

Plasma glucose in pregnancy was lower, including during the OGTT (glucose area under the curve 0-120 min (AUC_0-120); 655±24 versus 849±13 mmol.l^-1.min; G19 vs virgin; P<0.0001), with plasma insulin concentrations equivalent to those of virgin rats (insulin AUC_0-120; 97±7 versus 83±7 ng.ml^-1.min; G19 vs virgin; not significant). Liver glycogen was depleted in fasted G19 rats with full recovery after oral glucose. Serum TG increased during pregnancy (4.4±0.4, 6.7±0.5; 17.1±1.5 mmol/l; virgin, G11, G19, P<0.0001), and islet TG content decreased (147±42, 172±27, 73±13 ng/µg protein; virgin, G11, G19; P<0.01). GSIS in isolated islets was increased in G19 compared to virgin rats, and this effect was augmented in the presence of FA. FA esterification into phospholipids, monoacylglycerols and TG were increased, whereas FA oxidation was reduced, in islets of pregnant compared to virgin rats, with variable effects on lipolysis dependent on gestational age. Expression of Ppargc1a, a key regulator of mitochondrial metabolism, was reduced by 51% in G11 and 64% in G19 pregnant rat islets compared to virgin rat islets (P<0.001).

CONCLUSION

A lowered set-point for islet and hepatic glucose homeostasis in the pregnant rat has been confirmed. Islet adaptation to pregnancy includes increased FA esterification, reduced FA oxidation, and enhanced FA augmentation of glucose-stimulated insulin secretion.

Insulin-like growth factor 2 and the insulin receptor, but not insulin, regulate fetal hepatic glycogen synthesis

Whether insulin or IGFs regulate glycogen synthesis in the fetal liver remains to be determined. In this study, we used several knockout mouse strains, including those lacking Pdx-1 (pancreatic duodenal homeobox-1), Insr (insulin receptor), and Igf2 (IGF-II) to determine the role of these genes in the regulation of fetal hepatic glycogen synthesis. Our data show that insulin deficiency does not alter hepatic glycogen stores, whereas Insr and Igf2 deficiency do. We found that both insulin receptor isoforms (IR-A and IR-B) are present in the fetal liver, and their expression is gestationally regulated. IR-B is highly expressed in the fetal liver; nonetheless, the percentage of hepatic IR-A isoform, which binds Igf2, was significantly higher in the fetus than the adult. In vitro experiments demonstrate that Igf2 increases phosphorylation of hepatic Insr, insulin receptor substrate-2, and Akt proteins and also the activity of glycogen synthase. Igf2 ultimately increased glycogen synthesis in fetal hepatocytes. This increase could be blocked by the phosphoinositide 3-kinase inhibitor LY294008. Taken together, we propose Igf2 as a major regulator of fetal hepatic glycogen metabolism, the insulin receptor as its target receptor, and phosphoinositide 3-kinase as the signaling pathway leading to glycogen formation in the fetal liver.

Muscle glycogen content in type 2 diabetes mellitus

Muscle contains the largest reservoir of glycogen (Glyc), a depot that is closely regulated and with influence on insulin sensitivity. The current study examines muscle Glyc in type 2 diabetes mellitus (T2DM) and obesity and with respect to muscle fiber type, intramyocellular lipid content (IMCL), and mitochondrial function (oxidative enzyme activity; OX-Enz). There is increasing interest in the relation of IMCL and mitochondrial dysfunction with insulin resistance (IR), yet the association with muscle Glyc has not been examined with regard to these parameters. Using a quantitative histological approach specific to muscle fiber types, we assessed muscle Glyc, IMCL, and OX-Enz in vastus lateralis obtained by percutaneous biopsy in lean nondiabetic (L; n = 16), obese nondiabetic (Ob; n = 15), and T2DM volunteers (n = 14). Insulin sensitivity was estimated using homeostasis model assessment (HOMA)-IR. Muscle Glyc was reduced in T2DM, a deficit evident for type IIa fibers, yet minor in types I and IIb fibers. Low Glyc in T2DM correlated with fasting hyperglycemia. Also, in T2DM and Ob, there was significantly higher IMCL and lower OX-Enz in all fiber types. The IMCL-to-OX-Enz ratio, especially for type I fibers, correlated strongly with IR. Similarly, a Glyc-to-OX-Enz ratio correlated with IR, particularly for type IIb fibers. This ratio tended to be higher in Ob and T2DM. In summary, there is decreased muscle Glyc in T2DM yet a disproportional Glyc-to-OX-Enz relationship that is related to IR, although not as robustly as the IMCL-to-OX-Enz ratio.

Insulin resistance of gluconeogenic pathways in neonatal rats after prenatal ethanol exposure

Alcohol exposure during pregnancy is associated with fetal growth restriction and programs the offspring to insulin resistance later in life. The underlying mechanisms are still uncertain, but a dysregulation of gluconeogenesis and adipose hormones may be contributory. Newborn rats from dams that had been given ethanol (EtOH) or water (controls) during pregnancy were studied. Adiponectin mRNA was determined in subcutaneous fat by RT-PCR, and serum adiponectin was measured by RIA. Subsets of rats were killed before and after intraperitoneal administration of insulin, to determine, by RT-PCR, the hepatic expression of gluconeogenic enzymes and that of the transcription factor peroxisome proliferator-activated receptor-coactivator (PGC)-1, which promotes gluconeogenesis. EtOH offspring had delayed hypoglycemic response to insulin but normal adiponectin mRNA and serum levels compared with controls. The inhibitory response of the gluconeogenic enzyme phospho enol- pyruvate carboxykinase (PEPCK) and PGC-1 mRNAs to insulin was blunted in EtOH offspring compared with controls. The data suggest that intrauterine EtOH exposure causes insulin resistance of genes for PGC-1 and PEPCK early in life.

Physiological roles of glycogen synthase kinase-3: potential as a therapeutic target for diabetes and other disorders

Glycogen synthase kinase-3 (GSK-3) has perplexed signal transduction researchers since its detection in skeletal muscle 25 years ago. The enzyme confounds most of the rules normally associated with protein kinases in that it exhibits significant activity, even in resting, unstimulated cells. However, the protein is highly regulated and potently inactivated in response to signals such as insulin and polypeptide growth factors. The enzyme also displays a distinct and unusual preference for substrates that have been previously phosphorylated by other protein kinases which provides obvious opportunities for cross-talk. Its substrates are diverse and are predominantly regulatory molecules. The molecular cloning of the kinase revealed it to be encoded by two related but distinct genes. Moreover, the mammalian proteins showed remarkable similarity to a fruitfly protein isolated on the basis of its role in cell fate determination. From these humble beginnings, study of the enzyme has accrued further surprises such as its inhibition by lithium, its regulation by serine and tyrosine phosphorylation and its implication in several human disorders including Alzheimers disease, bipolar disorder, cancer and diabetes. Most recently, small molecule inhibitors of GSK-3 have been developed and assessed for therapeutic potential in several of models of pathophysiology. The question is whether modulation of such an "involved" enzyme could lead to selective restoration of defects without multiple unwanted side effects. This review summarizes current knowledge of GSK-3 with respect to its known functions, together with an assessment of its real-life potential as a drug target for chronic conditions such as type 2 diabetes.

The role of glycogen synthase kinase-3 in insulin resistance and type 2 diabetes

Glycogen synthase kinase-3 (GSK-3) is a ubiquitous cytosolic serine/threonine protein kinase that has been implicated in multiple receptor-mediated intracellular processes. Its unique feature, which distinguishes it from other protein kinases, is that it is constitutively active in resting conditions and acts as a suppressor of signalling pathways. The fact that the function of two key targets of insulin action, glycogen synthase and insulin receptor substrate-1, are suppressed by GSK-3, as well as the fact that GSK-3 activity is higher in diabetic tissues, makes it a promising drug discovery target for insulin resistance and Type 2 diabetes. Thus, the development of GSK-3 inhibitors has received attention as an attempt to control both the spread of the disease and its severity.

Regulation of hepatic glycogen in the insulin-like growth factor II-deficient mouse

Insulin-like growth factor II (IGF-II), a polypeptide hormone with structural homologies to insulin-like growth factor I (IGF-I) and insulin, regulates the metabolism and growth of many tissues. In this study, we examined the role of IGF-II in hepatic glycogen metabolism in normal and growth-retarded IGF-II-deficient (knockout) mice. Liver glycogen content was significantly lower in the IGF-II knockout than in control livers during embryonic day 18 and postnatal day 0. Biochemical results were verified histologically using a glycogen-specific stain. The enzymatic activity of glycogen synthase, the rate-limiting enzyme for glycogen synthesis, was significantly lower in livers of knockout mice than in livers from wild-type controls on embryonic day 18 and postnatal day 0. The levels of glycogen synthase messenger RNA were not different between the two groups at any age studied, indicating that IGF-II acts posttranscriptionally. Hepatic glycogen content, measured in newborns after food withdrawal, was significantly lower in knockout mice compared with that in wild-type mice after 0, 3, and 6 h of fasting. Blood glucose was significantly lower in knockouts vs. wild-type newborn mice before fasting and was similar in both genotypes after 6 h of fasting. Consistent with this, only 23% of IGF-II knockout newborn mice survived fasting for 12 h, whereas 93% of wild-type mice survived this treatment. These results indicate that IGF-II is required for the regulation of glycogen metabolism of the mouse in the perinatal period, possibly via stimulation of glycogen synthase activity. IGF-II, via perinatal regulation of glycogen synthesis, may regulate fetal growth as well as play an important role in the transition from fetal to postnatal life by protecting the neonate against hypoglycemia during periods of fasting.