Sherin U. Devaskar

Liver-specific deletion of negative regulator Pten results in fatty liver and insulin hypersensitivity [corrected].

In the liver, insulin controls both lipid and glucose metabolism through its cell surface receptor and intracellular mediators such as phosphatidylinositol 3-kinase and serine-threonine kinase AKT. The insulin signaling pathway is further modulated by protein tyrosine phosphatase or lipid phosphatase. Here, we investigated the function of phosphatase and tension homologue deleted on chromosome 10 (PTEN), a negative regulator of the phosphatidylinositol 3-kinase/AKT pathway, by targeted deletion of Pten in murine liver. Deletion of Pten in the liver resulted in increased fatty acid synthesis, accompanied by hepatomegaly and fatty liver phenotype. Interestingly, Pten liver-specific deletion causes enhanced liver insulin action with improved systemic glucose tolerance. Thus, deletion of Pten in the liver may provide a valuable model that permits the study of the metabolic actions of insulin signaling in the liver, and PTEN may be a promising target for therapeutic intervention for type 2 diabetes.

Fetal Origins of Adult Disease

Dr. David Barker first popularized the concept of fetal origins of adult disease (FOAD). Since its inception, FOAD has received considerable attention. The FOAD hypothesis holds that events during early development have a profound impact on ones risk for development of future adult disease. Low birth weight, a surrogate marker of poor fetal growth and nutrition, is linked to coronary artery disease, hypertension, obesity, and insulin resistance. Clues originally arose from large 20th century, European birth registries. Today, large, diverse human cohorts and various animal models have extensively replicated these original observations. This review focuses on the pathogenesis related to FOAD and examines Dr. David Barkers landmark studies, along with additional human and animal model data. Implications of the FOAD extend beyond the low birth weight population and include babies exposed to stress, both nutritional and nonnutritional, during different critical periods of development, which ultimately result in a disease state. By understanding FOAD, health care professionals and policy makers will make this issue a high health care priority and implement preventive measures and treatment for those at higher risk for chronic diseases.

Histone Code Modifications Repress Glucose Transporter 4 Expression in the Intrauterine Growth-restricted Offspring

We examined transcriptional and epigenetic mechanism(s) behind diminished skeletal muscle GLUT4 mRNA in intrauterine growth-restricted (IUGR) female rat offspring. An increase in MEF2D (inhibitor) with a decline in MEF2A (activator) and MyoD (co-activator) binding to the glut4 promoter in IUGR versus control was observed. The functional role of MEF2/MyoD-binding sites and neighboring three CpG clusters in glut4 gene transcription was confirmed in C2C12 muscle cells. No differential methylation of these three and other CpG clusters in the glut4 promoter occurred. DNA methyltransferase 1 (DNMT1) in postnatal, DNMT3a, and DNMT3b in adult was differentially recruited with increased MeCP2 (methyl CpG-binding protein) concentrations to bind the IUGR glut4 gene. Covalent modifications of the histone (H) code consisted of H3.K14 de-acetylation by recruitment of histone deacetylase (HDAC) 1 and enhanced association of HDAC4 enzymes. This set the stage for Suv39H1 methylase-mediated di-methylation of H3.K9 and increased recruitment of heterochromatin protein 1α, which partially inactivates postnatal and adult IUGR glut4 gene transcription. Further increased interactions in the adult IUGR between DNMT3a/DNMT3b and HDAC1 and MEF2D and HDAC1/HDAC4 and decreased association between MyoD and MEF2A existed. We conclude that epigenetic mechanisms consisting of histone code modifications repress skeletal muscle glut4 transcription in the postnatal period and persist in the adult female IUGR offspring.

Insulin Hypersensitivity and Resistance to Streptozotocin-Induced Diabetes in Mice Lacking PTEN in Adipose Tissue

ABSTRACT In adipose tissue, insulin controls glucose and lipid metabolism through the intracellular mediators phosphatidylinositol 3-kinase and serine-threonine kinase AKT. Phosphatase and a tensin homolog deleted from chromosome 10 (PTEN), a negative regulator of the phosphatidylinositol 3-kinase/AKT pathway, is hypothesized to inhibit the metabolic effects of insulin. Here we report the generation of mice lacking PTEN in adipose tissue. Loss of Pten results in improved systemic glucose tolerance and insulin sensitivity, associated with decreased fasting insulin levels, increased recruitment of the glucose transporter isoform 4 to the cell surface in adipose tissue, and decreased serum resistin levels. Mutant animals also exhibit increased insulin signaling and AMP kinase activity in the liver. Pten mutant mice are resistant to developing streptozotocin-induced diabetes. Adipose-specific Pten deletion, however, does not alter adiposity or plasma fatty acids. Our results demonstrate that in vivo PTEN is a potent negative regulator of insulin signaling and insulin sensitivity in adipose tissue. Furthermore, PTEN may be a promising target for nutritional and/or pharmacological interventions aimed at reversing insulin resistance.

The Mammalian Glucose Transporters

We have described the properties of glucose transporters expressed in several mammalian tissues and have summarized some of the adaptations that take place involving these molecules in various normal and abnormal states. With the exception of a few cell types, such as adipocytes and skeletal muscle, glucose transport is not a rate-limiting step in cellular glucose metabolism, and other substrates may be equally important for cellular metabolism. Nevertheless, an understanding of the mechanisms behind the regulation of glucose transport in individual tissues may facilitate an understanding of in vivo glucose utilization and clearance processes as they relate to normal and disease states. Although adult studies provide an impetus toward a mechanistic approach in preventing and treating various disease states involving derangements in glucose homeostasis, there remains a need for similar studies in the fetus and newborn. These developmental studies should help unravel the fetal/neonatal responses to normal and abnormal hormonal and substrate milieu.

Will the original glucose transporter isoform please stand up

Monosaccharides enter cells by slow translipid bilayer diffusion by rapid, protein-mediated, cation-dependent cotransport and by rapid, protein-mediated equilibrative transport. This review addresses protein-mediated, equilibrative glucose transport catalyzed by GLUT1, the first equilibrative glucose transporter to be identified, purified, and cloned. GLUT1 is a polytopic, membrane-spanning protein that is one of 13 members of the human equilibrative glucose transport protein family. We review GLUT1 catalytic and ligand-binding properties and interpret these behaviors in the context of several putative mechanisms for protein-mediated transport. We conclude that no single model satisfactorily explains GLUT1 behavior. We then review GLUT1 topology, subunit architecture, and oligomeric structure and examine a new model for sugar transport that combines structural and kinetic analyses to satisfactorily reproduce GLUT1 behavior in human erythrocytes. We next review GLUT1 cell biology and the transcriptional and posttranscriptional regulation of GLUT1 expression in the context of development and in response to glucose perturbations and hypoxia in blood-tissue barriers. Emphasis is placed on transgenic GLUT1 overexpression and null mutant model systems, the latter serving as surrogates for the human GLUT1 deficiency syndrome. Finally, we review the role of GLUT1 in the absence or deficiency of a related isoform, GLUT3, toward establishing the physiological significance of coordination between these two isoforms.

Insulin II gene expression in rat central nervous system.

Controversy persists concerning the origin of insulin in the central nervous system. While there has been convincing evidence in vitro to demonstrate the presence of neuronal insulin mRNA, conventional assays have failed to detect the same in whole brain preparations. Here we employed RNAse-protection and sensitive reverse transcription-polymerase chain reaction (RT-PCR) assays in attempts to detect insulin I and II mRNAs in rat brains obtained from different developmental stages. The RNAse-protection assay did not detect insulin I or insulin II transcripts in fetal (13 to 21 day gestation) or adult brains. RT-PCR, while detecting low amounts of insulin I transcripts in other extrapancreatic tissues such as the rat yolk sac and fetal liver previously shown to express insulin II, failed to detect insulin I in brain at any age examined. Insulin II mRNA was detected by RT-PCR in fetal, neonatal and adult rat brains, just as in yolk sac, fetal and adult livers. We conclude that while the duplicated insulin I gene is not expressed, the ancestral insulin II gene is expressed in fetal, neonatal and adult rat brains. Our observations support the concept of de novo brain insulin II synthesis beyond the pre-pancreatic stage of embryonic development.

Time-dependent physiological regulation of rodent and ovine placental glucose transporter (GLUT-1) protein

To examine the in vivo and in vitro time-dependent effects of glucose on placental glucose transporter (GLUT-1) protein levels, we employed Western blot analysis using placenta from the short-term streptozotocin-induced diabetic pregnancy (STZ-D), uterine artery ligation-intrauterine growth restriction (IUGR) rat models, pregnant sheep exposed to chronic maternal glucose and insulin infusions, and the HRP.1 rat trophoblastic cell line exposed to differing concentrations of glucose. In the rat, 6 days of STZ-D with maternal and fetal hyperglycemia caused no substantive change, whereas 72 h of IUGR with fetal hypoglycemia and ischemic hypoxia resulted in a 50% decline in placental GLUT-1 levels (P < 0.05). In late-gestation ewes, maternal and fetal hyperglycemia caused an initial threefold increase at 48 h (P < 0.05), with a persistent decline between 10 to 21 days, whereas maternal and fetal hypoglycemia led to a 30-50% decline in placental GLUT-1 levels (P < 0.05). Studies in vitro demonstrated no effect of 0 mM, whereas 100 mM glucose caused a 60% decline (P < 0.05; 48 h) in HRP.1 GLUT-1 levels compared with 5 mM of glucose. The added effect of hypoxia on 0 and 100 mM glucose concentrations appeared to increase GLUT-1 concentrations compared with normoxic cells (P < 0.05; 100 mM at 18 h). We conclude that abnormal glucose concentrations alter rodent and ovine placental GLUT-1 levels in a time- and concentration-dependent manner; hypoxia may upregulate this effect. The changes in placental GLUT-1 concentrations may contribute toward the process of altered maternoplacentofetal transport of glucose, thereby regulating placental and fetal growth.

Broviac catheterization in low birth weight infants: incidence and treatment of associated complications

Fifty-two Broviac catheters were inserted in 40 preterm and eight term infants for 1733 days of catheter use. Thirty-six (69%) catheters were associated with complications of infection and/or thrombosis, a complication rate of 1/48 catheter days. The patients who developed complications were of a significantly lower gestational age and had a lower mean birth weight when compared with those who developed no complications. The incidence of catheter-related sepsis was 69% in the very low birth weight infants and only 20% in the infants with birth weights over 1500 g. Eighteen of the 26 catheter-associated infections were treated with antibiotics without catheter removal. Successful resolution of the infections with retention of the catheter occurred in 14 of the 18 episodes. Infections with Staphylococcus aureus constituted three of four treatment failures. Urokinase infusion was successful in causing thrombolysis in eight of the nine cases. Broviac catheters in neonates, and especially in preterm infants under 1500 g, are associated with a high incidence of complications. Our experience indicates that some complications can be selectively managed without sacrificing the venous access.

Developmental regulation of insulin in the mammalian central nervous system

We delineated the ontogeny of rabbit brain insulin concentrations to define the regulatory role of development on this hormone in the central nervous system. Employing a sensitive ELISA, we observed higher concentrations in the late gestation fetal brain (approximately 80-90 ng/g) and early neonatal brain (approximately 195 ng/g) in comparison to the adult (approximately 32 ng/g; P less than 0.01). Further, we characterized this hormone to determine the identity of insulin (or an insulin-like substance) in brain. Employing porcine/bovine or rabbit insulin as standards, we observed that brain insulin mimicked authentic insulin in its migration on SDS-polyacrylamide and native gel electrophoresis, immunogenicity on Western blot analysis, and its elution profile on immunoaffinity column chromatographic, and high performance liquid chromatographic separation. We then examined the developmental effects on circulating and cerebrospinal fluid (CSF) radioimmunoassayable insulin levels. No statistically significant differences (ANOVA) existed through development in either the serum or CSF insulin levels. Employing multiple regression analysis, no correlation was evident between brain and either serum or CSF insulin concentration. A search for insulin mRNA by Northern blot analysis yielded minute amounts of atypical large sized transcripts. We conclude that the insulin peptide in the central nervous system closely resembles (or is identical to) circulating insulin in many properties and that there is a developmental increase in brain insulin concentrations, the maximal peak occuring in the late gestation fetus and early neonate. Insulin concentrations in brain demonstrate no conventional relationship to either the serum or CSF insulin levels, suggesting an additional source of peptide, which contributes beyond that which is available via the circulation. The amounts of insulin present within the central nervous system are minute (difficult to detect) but in the range (10-100 ng) where the hormone can interact with either insulin or insulin-like growth factor I (IGF-I) receptors that are abundantly present on developing brain cells, thereby executing the biological function of the hormone.