Glucose uptake and glucose transporter proteins in skeletal muscle from undernourished rats

Maternal stress induced endoplasmic reticulum stress and impaired pancreatic islets’ insulin secretion via glucocorticoid receptor upregulation in adult male rat offspring

Exposure to perinatal (prenatal and/or postnatal) stress is considered as a risk factor for metabolic disorders in later life. Accordingly, this study aimed to investigate the perinatal stress effects on the pancreatic endoplasmic reticulum (ER) stress induction, insulin secretion impairment and WFS1 (wolframin ER transmembrane Glycoprotein, which is involved in ER homeostasis and insulin secretion) expression changes, in rat offspring. According to the dams’ period of exposure to variable stress, their male offspring were divided into, control (CTRL); pre-pregnancy, pregnancy, lactation stress (PPPLS); pre-pregnancy stress (PPS); pregnancy stress (PS); lactation stress (LS); pre-pregnancy, pregnancy stress (PPPS); pregnancy, lactation stress (PLS); pre-pregnancy, lactation stress (PPLS) groups. Offspring pancreases were removed for ER extraction and the assessment of ER stress biomarkers, WFS1 gene DNA methylation, and isolated islets’ insulin secretion. Glucose tolerance was also tested. In the stressed groups, maternal stress significantly increased plasma corticosterone levels. In PPS, PS, and PPPS groups, maternal stress increased Bip (Hsp70; heat shock protein family A member 4), Chop (Ddit3; DNA- damage inducible transcript3), and WFS1 protein levels in pancreatic extracted ER. Moreover, the islets’ insulin secretion and content along with glucose tolerance were impaired in these groups. In PPS, PS, LS and PPPS groups, the pancreatic glucocorticoid receptor (GR) expression increased. Maternal stress did not affect pancreatic WFS1 DNA methylation. Thus, maternal stress, during prenatal period, impaired the islets’ insulin secretion and glucose homeostasis in adult male offspring, possibly through the induction of ER stress and GR expression in the pancreas, in this regard the role of WFS1 protein alteration in pancreatic ER should also be considered.

Impact of Early Nutrition, Physical Activity and Sleep on the Fetal Programming of Disease in the Pregnancy: A Narrative Review

Early programming is the adaptation process by which nutrition and environmental factors alter development pathways during prenatal growth, inducing changes in postnatal metabolism and diseases. The aim of this narrative review, is evaluating the current knowledge in the scientific literature on the effects of nutrition, environmental factors, physical activity and sleep on development pathways. If in utero adaptations were incorrect, this would cause a mismatch between prenatal programming and adulthood. Adequate caloric intake, protein, mineral, vitamin, and long-chain fatty acids, have been noted for their relevance in the offspring brain functions and behavior. Fetus undernutrition/malnutrition causes a delay in growth and have detrimental effects on the development and subsequent functioning of the organs. Pregnancy is a particularly vulnerable period for the development of food preferences and for modifications in the emotional response. Maternal obesity increases the risk of developing perinatal complications and delivery by cesarean section and has long-term implications in the development of metabolic diseases. Physical exercise during pregnancy contributes to overall improved health post-partum. It is also interesting to highlight the relevance of sleep problems during pregnancy, which influence adequate growth and fetal development. Taking into account these considerations, we conclude that nutrition and metabolic factors during early life play a key role of health promotion and public health nutrition programs worldwide to improve the health of the offspring and the health costs of hospitalization.

Molecular mechanisms governing offspring metabolic programming in rodent models of in utero stress

The results of different human epidemiological datasets provided the impetus to introduce the now commonly accepted theory coined as 'developmental programming', whereby the presence of a stressor during gestation predisposes the growing fetus to develop diseases, such as metabolic dysfunction in later postnatal life. However, in a clinical setting, human lifespan and inaccessibility to tissue for analysis are major limitations to study the molecular mechanisms governing developmental programming. Subsequently, studies using animal models have proved indispensable to the identification of key molecular pathways and epigenetic mechanisms that are dysregulated in metabolic organs of the fetus and adult programmed due to an adverse gestational environment. Rodents such as mice and rats are the most used experimental animals in the study of developmental programming. This review summarises the molecular pathways and epigenetic mechanisms influencing alterations in metabolic tissues of rodent offspring exposed to in utero stress and subsequently programmed for metabolic dysfunction. By comparing molecular mechanisms in a variety of rodent models of in utero stress, we hope to summarise common themes and pathways governing later metabolic dysfunction in the offspring whilst identifying reasons for incongruencies between models so to inform future work. With the continued use and refinement of such models of developmental programming, the scientific community may gain the knowledge required for the targeted treatment of metabolic diseases that have intrauterine origins.

Sex-specific programming of adult insulin resistance in guinea pigs by variable perinatal growth induced by spontaneous variation in litter size

Intrauterine growth restriction (IUGR) and subsequent neonatal catch-up growth are implicated in programming of insulin resistance later in life. Spontaneous IUGR in the guinea pig, due to natural variation in litter size, produces offspring with asymmetric IUGR and neonatal catch-up growth. We hypothesized that spontaneous IUGR and/or accelerated neonatal growth would impair insulin sensitivity in adult guinea pigs. Insulin sensitivity of glucose metabolism was determined by hyperinsulinemic-euglycemic clamp (HEC) in 38 (21 male, 17 female) young adult guinea pigs from litters of two-to-four pups. A subset (10 male, 8 female) were infused with d-[3-³H]glucose before and during the HEC to determine rates of basal and insulin-stimulated glucose utilization, storage, glycolysis, and endogenous glucose production. n males, the insulin sensitivity of whole body glucose uptake ( r = 0.657, P = 0.002) and glucose utilization ( r = 0.884, P = 0.004) correlated positively and independently with birth weight, but not with neonatal fractional growth rate (FGR_10-28). In females, the insulin sensitivity of whole body and partitioned glucose metabolism was not related to birth weight, but that of endogenous glucose production correlated negatively and independently with FGR_10-28 ( r = -0.815, P = 0.025). Thus, perinatal growth programs insulin sensitivity of glucose metabolism in the young adult guinea pig and in a sex-specific manner; impaired insulin sensitivity, including glucose utilization, occurs after IUGR in males and impaired hepatic insulin sensitivity after rapid neonatal growth in females.

Intra-uterine undernutrition amplifies age-associated glucose intolerance in pigs via altered DNA methylation at muscle GLUT4 promoter

The present study aimed to investigate the effect of maternal malnutrition on offspring glucose tolerance and the epigenetic mechanisms involved. In total, twelve primiparous Landrace×Yorkshire gilts were fed rations providing either 100 % (control (CON)) or 75 % (undernutrition (UN)) nutritional requirements according to the National Research Council recommendations, throughout gestation. Muscle samples of offspring were collected at birth (dpn1), weaning (dpn28) and adulthood (dpn189). Compared with CON pigs, UN pigs showed lower serum glucose concentrations at birth, but showed higher serum glucose and insulin concentrations as well as increased area under the blood glucose curve during intravenous glucose tolerance test at dpn189 (P<0·05). Compared with CON pigs, GLUT-4 gene and protein expressions were decreased at dpn1 and dpn189 in the muscle of UN pigs, which was accompanied by increased methylation at the GLUT4 promoter (P<0·05). These alterations in methylation concurred with increased mRNA levels of DNA methyltransferase (DNMT) 1 at dpn1 and dpn28, DNMT3a at dpn189 and DNMT3b at dpn1 in UN pigs compared with CON pigs (P<0·05). Interestingly, although the average methylation levels at the muscle GLUT4 promoter were decreased at dpn189 compared with dpn1 in pigs exposed to a poor maternal diet (P<0·05), the methylation differences in individual CpG sites were more pronounced with age. Our results indicate that in utero undernutrition persists to silence muscle GLUT4 likely through DNA methylation during the ageing process, which may lead to the amplification of age-associated glucose intolerance.

Postnatal exposure to a high-carbohydrate diet interferes epigenetically with thyroid hormone receptor induction of the adult male rat skeletal muscle glucose transporter isoform 4 expression

Early life nutritional intervention causes adult-onset insulin resistance and obesity in rats. Thyroid hormone receptor (TR), in turn, transcriptionally enhances skeletal muscle Glut4 expression. We tested the hypothesis that reduced circulating thyroid-stimulating hormone and T4 concentrations encountered in postnatal (PN4-PN24) high-carbohydrate (HC) milk formula-fed versus the mother-fed controls (MF) would epigenetically interfere with TR induction of adult (100 days) male rat skeletal muscle Glut4 expression, thereby providing a molecular mechanism mediating insulin resistance. We observed increased DNA methylation of the CpG island with enhanced recruitment of Dnmt3a, Dnmt3b and MeCP2 in the glut4 promoter region along with reduced acetylation of histone (H)2A.Z and H4 particularly at the H4.lysine (K)16 residue, which was predominantly mediated by histone deacetylase 4 (HDAC4). This was followed by enhanced recruitment of heterochromatin protein 1β to the glut4 promoter with increased Suv39H1 methylase concentrations. These changes reduced TR binding of the T3 response element of the glut4 gene (TREs; -473 to -450 bp) detected qualitatively in vivo (electromobility shift assay) and quantified ex vivo (chromatin immunoprecipitation). In addition, the recruitment of steroid receptor coactivator and CREB-binding protein to the glut4 promoter-protein complex was reduced. Co-immunoprecipitation experiments confirmed the interaction between TR and CBP to be reduced and HDAC4 to be enhanced in HC versus MF groups. These molecular changes were associated with diminished skeletal muscle Glut4 mRNA and protein concentrations. We conclude that early postnatal exposure to HC diet epigenetically reduced TR induction of adult male skeletal muscle Glut4 expression, uncovering novel molecular mechanisms contributing to adult insulin resistance and obesity.

Caloric restriction can improve learning ability in C57/BL mice via regulation of the insulin-PI3K/Akt signaling pathway

To identify the molecular mechanism underlying improved spatial learning ability of C57/BL mice on a caloric restricted (CR) diet. Seven-week-old male C57/BL mice were randomly divided into three groups: normal control group (NC group, n = 10), high energy group (n = 10), and low energy group (CR group, n = 10). Body mass and levels of blood glucose were measured every 2 weeks over the course of 30 weeks. After 30 weeks, metabolic parameters, serum total cholesterol, and insulin-like growth factor 1 (IGF-1) were measured, and learning and memory ability of animals were tested using the Morris water maze. The expression of insulin signaling pathway-related proteins in the brain tissues also were tested for molecular mechanism. When compared with the NC group, body weight, and levels of serum glucose decreased in the CR group and increased in the high energy group at all time points tested. Average escape latency and swimming distance were lower in the CR group as compared to the control group after 30 weeks. The serum cholesterol level of the high energy group was significantly higher than that of the control group. The expressions of IGF-1, IR, IRS-1, PI3K, Akt/PKB, and p-CREB protein in the CR group were significantly lower and the expressions of PI3K and Akt/PKB protein in the high energy group were significantly lower than those of the control group at post 30 weeks treatment. Our findings demonstrate that the low energy diet may improve hippocampus-dependent spatial learning ability in C57/BL mice, possibly through a regulatory mechanism of the insulin-PI3K/Akt signaling pathway.

Increased leptin response and inhibition of apoptosis in thymocytes of young rats offspring from protein deprived dams during lactation

We investigated the consequences of mild maternal malnutrition in rat dams, in terms of thymocyte responses and the putative role of leptin. The young progeny of dams submitted to protein deprivation (PD) during lactation showed at 30 days of age lower body and thymus weights, significant alterations in CD4/CD8-defined T cell subsets without modifications in total thymocyte number as well as in proliferative response. Despite, the rats from PD group did not present alterations in leptin circulating levels, the expression of leptin receptor ObRb was enhanced in their thymocytes. This change was accompanied by an increase in leptin signaling response of thymocytes from PD rats, with an increase in JAK2 and STAT3 phosphorylation after leptin stimulation. Thymocytes from PD rats also presented a decreased rate of spontaneous apoptosis when compared to controls. Accordingly, higher expression of anti-apoptotic protein Bcl-2, and lower of pro-apoptotic protein Bax, with no change of pro-apoptotic Bad, and higher pro-caspase 3 content were detected in PD thymocytes. Moreover, thymocytes from PD group exhibited a constitutive higher nuclear content of p65 NF-kB associated to a lower IkB content in the cytoplasm. Finally, although there was no change in ob gene expression in PD thymocytes, a higher mRNA expression for the Ob gene was observed in the thymic microenvironment from PD animals. Taken together, the results show that mild maternal protein deprivation during lactation affects thymic homeostasis, enhancing leptin activity, which in turn protects thymocytes from apoptosis in the young progeny, with possible consequences upon the immune response of these animals in adult life.

Large litters rearing changes brain expression of GLUT3 and acetylcholinesterase activity in adult rats

Effects of malnutrition in the brain are more pronounced during the period of growth spurt, corresponding to the suckling in rodents. Neuronal glucose transporter GLUT3 expression and acetylcholinesterase activity were studied in the brain of adult young rats (84 days old) suckled in litters formed by 6 (control group) or 12 pups (malnourished group). In the adult rats, brain weight, blood glucose levels and GLUT3 expression were decreased in malnourished group (5%, 18%, 58%, respectively, P<0.001, Student's t test) compared to the control. Increased activity of acetylcholinesterase was found in cerebral cortex homogenates and a significant interaction (P=0.019, ANOVA two-way, Tukey's test) was found between nutritional state and homogenate fraction. In summary, malnutrition during suckling period decreased GLUT3 expression and increased acetylcholinesterase activity in the rat brain that could contribute to possible cognitive deficits and changes of brain metabolic activity.

Early undernutrition induces glucagon resistance and insulin hypersensitivity in the liver of suckling rats

Developing brains are vulnerable to nutritional insults. Early undernutrition alters their structure and neurochemistry, inducing long-term pathological effects whose causal pathways are not well defined. During suckling, the brain uses glucose and ketone bodies as substrates. Milk is a high-fat low-carbohydrate diet, and the liver must maintain high rates of gluconeogenesis and ketogenesis to address the needs of these substrates. Insulin and glucagon play major roles in this adaptation: throughout suckling, their blood concentrations are low and high, respectively, and the liver maintains low insulin sensitivity and increased glucagon responsiveness. We propose that disturbances in the endocrine profile and available plasma substrates along with undernutrition-related changes in brain cortex capacity for ketone utilization may cause further alterations in some brain functions. We explored this hypothesis in 10-day-old suckling rats whose mothers were severely food restricted from the 14th day of gestation. We measured the plasma/serum concentrations of glucose, ketone body, insulin and glucagon, and hepatic insulin and glucagon responses. Undernutrition led to hypoglycemia and hyperketonemia to 84% (P < 0.001) and 144% (P < 0.001) of control values, respectively. Liver responsiveness to insulin and glucagon became increased and reduced, respectively; intraperitoneal glucagon reduced liver glycogen by 90% (P < 0.01) in control and by 35% (P < 0.05) in restricted. Cortical enzymes of ketone utilization remained unchanged, but their carrier proteins were altered: monocarboxylate transporter (MCT) 1 increased: 73 ± 14, controls; 169 ± 20, undernourished (P < 0.01; densitometric units); MCT2 decreased: 103 ± 3, controls; 37 ± 4, undernourished (P < 0.001; densitometric units). All of these changes, coinciding with the brain growth spurt, may cause some harmful effects associated with early undernutrition.

Maternal protein restriction during early lactation induces GLUT4 translocation and mTOR/Akt activation in adipocytes of adult rats

Epidemiological and experimental studies have demonstrated that early postnatal nutrition has been associated with long-term effects on glucose homeostasis in adulthood. Recently, our group demonstrated that undernutrition during early lactation affects the expression and activation of key proteins of the insulin signaling cascade in rat skeletal muscle during postnatal development. To elucidate the molecular mechanisms by which undernutrition during early life leads to changes in insulin sensitivity in peripheral tissues, we investigated the insulin signaling in adipose tissue. Adipocytes were isolated from epididymal fat pads of adult male rats that were the offspring of dams fed either a normal or a protein-free diet during the first 10 days of lactation. The cells were incubated with 100 nM insulin before the assays for immunoblotting analysis, 2-deoxyglucose uptake, immunocytochemistry for GLUT4, and/or actin filaments. Following insulin stimulation, adipocytes isolated from undernourished rats presented reduced tyrosine phosphorylation of IR and IRS-1 and increased basal phosphorylation of IRS-2, Akt, and mTOR compared with controls. Basal glucose uptake was increased in adipocytes from the undernourished group, and the treatment with LY294002 induced only a partial inhibition both in basal and in insulin-stimulated glucose uptake, suggesting an involvement of phosphoinositide 3-kinase activity. These alterations were accompanied by higher GLUT4 content in the plasma membrane and alterations in the actin cytoskeleton dynamics. These data suggest that early postnatal undernutrition impairs insulin sensitivity in adulthood by promoting changes in critical steps of insulin signaling in adipose tissue, which may contribute to permanent changes in glucose homeostasis.

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 1alpha, 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.

In vivo insulin signaling through PI3-kinase is impaired in skeletal muscle of adult rat offspring exposed to ethanol in utero

It is now known that prenatal ethanol (EtOH) exposure is associated with impaired glucose tolerance and insulin resistance in rat offspring, but the underlying mechanism(s) is not known. To test the hypothesis that in vivo insulin signaling through phosphatidylinositol 3 (PI3)-kinase is reduced in skeletal muscle of adult rat offspring exposed to EtOH in utero, we gave insulin intravenously to these rats and probed steps in the PI3-kinase insulin signaling pathway. After insulin treatment, EtOH-exposed rats had decreased tyrosine phosphorylation of the insulin receptor β-subunit and of insulin receptor substrate-1 (IRS-1), as well as reduced IRS-1-associated PI3-kinase in the gastrocnemius muscle compared with control rats. There was no significant difference in basal or insulin-stimulated Akt activity between EtOH-exposed rats and controls. Insulin-stimulated PKC isoform ζ phosphorylation and membrane association were reduced in EtOH-exposed rats compared with controls. Muscle insulin binding and peptide contents of insulin receptor, IRS-1, p85 subunit of PI3-kinase, Akt/PKB, and atypical PKC isoform ζ were not different between EtOH-exposed rats and controls. Thus insulin resistance in rat offspring exposed to EtOH in utero may be explained, at least in part, by impaired insulin signaling through the PI3-kinase pathway in skeletal muscle.

Maternal food restriction enhances insulin-induced GLUT-4 translocation and insulin signaling pathway in skeletal muscle from suckling rats

Restriction of protein calories during stages of immaturity has a major influence on glucose metabolism and increases the risk of type 2 diabetes in adulthood. However, it is known that reduction of food intake alleviates insulin resistance. We previously demonstrated an improved insulin-induced glucose uptake in skeletal muscle of chronically undernourished adult rats. The purpose of this work was to investigate whether this condition is present during suckling, a period characterized by physiological insulin resistance as well as elucidate some of the underlying mechanisms. With this aim, 10-d-old pups from food-restricted dams were studied. We showed that undernourished suckling rats are glucose normotolerants, despite their depressed insulin secretion capacity. The content of the main glucose transporters in muscle, GLUT-4 and GLUT-1, was not affected by undernutrition, but fractionation studies showed an improved insulin-stimulated GLUT-4 translocation. p38MAPK protein, implicated in up-regulation of intrinsic activity of translocated GLUT-4, was increased. These changes suggest an improved insulin-induced glucose uptake associated with undernutrition. Insulin receptor content as well as that of both regulatory and catalytic phosphoinositol 3-kinase subunits was increased by food restriction. Insulin receptor substrate-1-associated phosphoinositol 3-kinase activity after insulin was enhanced in undernourished rats, as was phospho-glycogen synthase kinase-3, in line with insulin hypersensitivity. Surprisingly, protein tyrosine phosphatase-1B association with insulin receptor was also increased by undernutrition. These adaptations to a condition of severely limited nutritional resources might result in changes in the development of key tissues and be detrimental later in life, when a correct amount of nutrients is available, as the thrifty phenotype hypothesis predicts.

GLUT4 expression and subcellular localization in the intrauterine growth-restricted adult rat female offspring

Intrauterine growth restriction (IUGR) leads to obesity, glucose intolerance, and type 2 diabetes mellitus in the adult. To determine the mechanism(s) behind this "metabolic imprinting" phenomenon, we examined the effect of total calorie restriction during mid- to late gestation modified by postnatal ad libitum access to nutrients (CM/SP) or nutrient restriction (SM/SP) vs. postnatal nutrient restriction alone (SM/CP) on skeletal muscle and white adipose tissue (WAT) insulin-responsive glucose transporter isoform (GLUT4) expression and insulin-responsive translocation. A decline in skeletal muscle GLUT4 expression and protein concentrations was noted only in the SM/SP and SM/CP groups. In contrast, WAT demonstrated no change in GLUT4 expression and protein concentrations in all experimental groups. The altered in utero hormonal/metabolic milieu was associated with a compensatory adaptation that persisted in the adult and consisted of an increase in the skeletal muscle basal plasma membrane-associated GLUT4 concentrations. This perturbation led to no further exogenous insulin-induced GLUT4 translocation, thereby disabling the insulin responsiveness of the skeletal muscle but retaining it in WAT. These changes, which present at birth, collectively maximize basal glucose transport to the compromised skeletal muscle with a relative resistance to exogenous/postprandial insulin. Preservation of insulin responsiveness in WAT may serve as a sink that absorbs postprandial nutrients that can no longer efficiently access skeletal muscle. We speculate that, in utero, GLUT4 aberrations may predict type 2 diabetes mellitus, whereas postnatal nutrient intake may predict obesity, thereby explaining the heterogeneous phenotype of the IUGR adult offspring.

Stearoyl-CoA desaturase 1 deficiency elevates insulin-signaling components and down-regulates protein-tyrosine phosphatase 1B in muscle

We have shown previously that mice with a targeted disruption in the stearoyl-CoA desaturase 1 gene (SCD1-/-) have increased insulin sensitivity compared with control mice. Here we show that the SCD1-/- mice have increased insulin signaling in muscle. The basal tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2 are elevated. The tyrosine phosphorylation of insulin-like growth factor-1 receptor was similar between SCD1+/+ and SCD1-/- mice. The association of insulin receptor substrates 1 and 2 with alphap85 subunit of phosphatidylinositol 3-kinase as well as the phosphorylation of Akt-Ser-473 and Akt-Thr-308 are also elevated in the SCD1-/- mice. Interestingly, the mRNA levels, protein mass, and activity of the protein-tyrosine phosphatase-1B implicated in the attenuation of the insulin signal are reduced in the SCD1-/- mice, whereas the levels of the leukocyte antigen-related protein phosphatase are similar between two groups of mice. The content of glucose transporter 4 in the plasma membrane and basal as well as insulin-mediated glucose uptake are increased in the SCD1-/- mice. In addition, the muscle glycogen content and the activities of glycogen synthase and phosphorylase are increased in the SCD1-/- mice. We hypothesize that loss of SCD1 function induces increased insulin signaling at least in part by a reduction in the expression of protein-tyrosine phosphatase 1B. SCD1 could be a therapeutic target in the treatment of diabetes.