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Keywords:

  • animal models;
  • epigenetic modifications;
  • insulin signalling;
  • intrauterine growth restriction;
  • low birth weight;
  • type 2 diabetes

Abstract.

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

It is widely accepted that an association exists between the intrauterine environment in which a fetus grows and develops and the subsequent development of type 2 diabetes. Any disturbance in maternal ability to provide nutrients and oxygen to the fetus can lead to fetal intrauterine growth restriction (IUGR). Here we will review IUGR in rodent models, in which maternal metabolism has been experimentally manipulated to investigate the molecular basis of the relationship between IUGR and development of type 2 diabetes in later life, and the identification of the molecular derangements in specific metabolically - sensitive organs/tissues.


Introduction

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

Epidemiological studies in man have revealed links between impaired intrauterine growth and susceptibility to adult chronic diseases such as type 2 diabetes mellitus. Low birth weight has been associated with glucose intolerance, insulin resistance, hyperlipidaemia and type 2 diabetes [1]. Furthermore, it has been demonstrated that patterns of intrauterine growth can be linked to specific adult disease, for example a thin infant with a low ponderal index is more likely to develop type 2 diabetes as an adult than a symmetrically small baby [2]. It has been proposed that various fetal growth patterns and intrauterine growth restriction (IUGR) arise as a result of an early stimulus or insult that occurs during a critical period of development and affects the development and organization of key tissues and organ systems. The effects of the environmental challenges depend on their severity, duration, the gestational age at onset and may also be influenced by the sex of the fetus. The alterations in fetal growth and development that occur in response to the intrauterine environment are meant to give the fetus optimal chance of survival in the postnatal life; however, they may have adverse outcomes for the long-term health of an individual.

Despite the substantial epidemiological evidence linking early growth restriction with the subsequent development of diabetes, the molecular mechanisms underlying this association are not known. Various animal models have been developed to study the molecular basis of the relationship between IUGR and the development of type 2 diabetes in later life (Fig. 1). As the genetic background in animal models can be controlled, it is possible to experimentally study the effects and influences of the environment during either gestation or early postnatal life on long-term health. The majority of models are based on the work with rodents as they offer significant advantage in terms of relatively short gestation and lifespan, although larger models such as sheep and pigs have also been used. The most commonly used animal models for studying the impairment of insulin signalling and secretion include nutritional interventions such as caloric or protein restriction, induction of uteroplacental insufficiency, over-exposure of the fetus to glucocorticoids and experimentally induced maternal diabetes, for example, using streptozotocin. We will concentrate on rodent models of IUGR in this review.

image

Figure 1.  Intrauterine programming of type 2 diabetes.

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Nutritional models of IUGR

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

Fetal response and adaptations to undernutrition

In order to adapt to a changing intrauterine environment the fetus must be able to ‘sense’ it. It has been suggested recently that the placenta might act as a nutrient sensor with the mammalian target of rapamycin kinase (mTOR) playing a major part in the nutrient sensing pathway [3]. The fetus responds and adapts to an inadequate amount of nutrients (such as glucose, amino acids and oxygen) using a number of strategies in order to maximize its chances of postnatal survival. The immediate response to undernutrition is catabolic consumption of substrates to provide energy [4]. In response to prolonged undernutrition the fetus changes its metabolic rate and alters the production of hormones and the sensitivity of tissues to them, for example there is a decrease in concentrations of fetal insulin and insulin-like growth factor-I (IGF-I) [5, 6]. This leads to reduction in the rate of fetal growth and IUGR. As blood flow is redistributed to protect key organs, especially the brain, the growth and development of other tissues such as muscle, kidneys and the endocrine pancreas is affected [7]. Disproportion in the organ and tissue size takes place if the insult causing the decrease in growth rate occurs at the critical time of development of a particular organ.

Maternal calorie restriction

Reduction in maternal total food intake to 50% of ad-lib during pregnancy and lactation in rats results in the birth of offspring with low birth weight who remain smaller throughout adulthood. The same food regime limited to pregnancy only also leads to low birth weight pups being born, but body weight of these pups increases above the control values postnatally [8]. Fifty per cent restriction of energy intake during the last week of pregnancy leads to the birth of microsomic pups, which have impaired beta cell development and reduced plasma glucose and insulin concentrations [9, 10]. When maternal calorie restriction is continued during suckling, the offspring display a minor, although significant, permanent reduction in beta cell number and mass alongside altered insulin response and sensitivity at a young age [11]. Beta cell proliferation and insulin content are normal at 3 weeks of age [12]. The reduction in beta cell mass is still apparent in adult life even when rats are fed a normal diet after weaning. At the age of 8 months, these offspring have 40% lower pancreatic insulin content and increased non-fasting plasma glucose concentrations. The impaired development of the endocrine pancreas eventually leads to fasting hypoinsulinaemia, hyperglycaemia and impaired glucose tolerance [9].

Dams subjected to much more severe food restriction of 30% of ad-lib intake give birth to IUGR offspring who develop hyperphagia and obesity in adult life accompanied by hyperinsulinism, hyperleptinaemia and hypertension [13].

Maternal low protein

The low-protein model is one of the most extensively studied models of early growth restriction and is relevant to groups of people who lack adequate protein intake either for economical or cultural reasons. The model, which was initially set up by Snoeck and colleagues, involves feeding dams a low (5–8%) protein (LP) diet during pregnancy and lactation in comparison with the control group fed an isocaloric 20% protein diet. The regime does not affect conception rates or litter sizes. However, the pups born to LP mothers have a reduced birth weight of approximately 15% [14, 15]. Despite weaning the offspring onto a control diet fed ad libitum, permanent growth restriction occurs if maternal protein restriction is continued during lactation [14]. In young adult life (6 weeks to 3 months) these offspring have improved glucose tolerance and reduced insulin concentrations, which indicate improved insulin sensitivity [16–18]. However, IUGR offspring undergo greater age-dependent loss of glucose tolerance than controls and the process occurs in a sex-dependent manner. The male LP offspring have impaired glucose tolerance by 15 months of age and frank diabetes with insulin resistance by 17 months [19], whilst female LP offspring only develop hyperglycaemia and impaired glucose tolerance at a much older age of 21 months [15].

The effect of the LP diet limited to pregnancy or lactation only on body weight glucose homeostasis and longevity has also been investigated. Mice that were IUGR by maternal protein restriction and then were suckled by control mothers (‘recuperated animals’) underwent ‘catch up’ growth so their body weights caught up with those of controls by week 4 and subsequently exceeded them. The recuperated animals remained heavier than controls throughout life. In contrast, mice born to control mothers but cross-fostered to LP mothers during lactation (‘postnatal low protein’) gained less weight than controls [20]. Both recuperated and postnatal low-protein animals were significantly more glucose tolerant than controls at 6 weeks of age, and there was an increased expression of insulin receptor in adipocyte membranes [18]. The detrimental effects of ‘catch up’ growth in humans have been associated with development of obesity, chronic diseases including diabetes and reduction in longevity [21, 22].

A large number of studies have investigated the long-term effects of maternal protein restriction on the structure and function of organs and insulin-sensitive tissues. Many studies have focused on the effects of the diet on the development of the endocrine pancreas. It has been observed that neonates of protein-restricted dams have impaired pancreatic development as their pancreatic islet vascularization, beta cell mass, islet size and insulin content were reduced [23] due to reduced beta cell proliferation and increased apoptosis [10, 24]. This was associated with a reduction in expression of a transcription factor pancreatic duodenal homeobox-1 (Pdx-1), which regulates an array of genes in the developing and mature pancreas, and its disruption in both mice and humans not only leads to the absence of insulin production but also causes the arrest of the early pancreatic development [25, 26]. Additionally, the expression of insulin-like growth factor-II (IGF-II) was also reduced in fetal and neonatal islet cells of growth-restricted rats [27]. IGF-II acts as beta cell mitogen and prevents apoptosis [28]. Reduced pancreatic expression of IGF-II may contribute to the lower beta cell proliferation rate and increased apoptosis in low-protein offspring. Interestingly, taurine supplementation of the low-protein diet given to the pregnant rat partially prevents beta cell mass reduction by normalizing the proliferation and apoptosis rates in conjunction with restoration of IGF-II expression [24, 29]. Taurine supplementation also normalized the vascularization of the fetal endocrine pancreas [30].

A number of studies have addressed the question as to whether the structural changes were associated with any changes in insulin secretion. There were no differences in basal secretion. However, a reduction in insulin secretion in response to arginine and leucine stimulation was observed in the islets of 21.5-day-old fetuses of the LP mother [31]. A decreased insulin secretory response to glucose and amino acids was also present in the islets from 3-month-old offspring and was associated with a decrease in the activity of mitochondrial glycerophosphate dehydrogenase [32].

Maternal protein restriction has also been shown to have long-term effects on both structure and function of the liver. LP offspring have fewer but larger hepatic lobules at 3 months of age [33]. The ability of glucagon to stimulate hepatic glucose output is impaired in ex vivo liver perfusions from 3-month LP offspring and is related to reduced expression of glucagon receptors [34]. This is accompanied by increased gluconeogenic phosphoenolpyruvate carboxykinase activity (PEPCK) [35] and reduced glycolytic glucokinase levels [36, 37]. These changes in hepatic enzyme activity were still present at 11 months of age even when the animals were fed a normal laboratory chow diet [36]. Interestingly, the decrease in hepatic glucokinase activity has also been reported in subjects with type 2 diabetes [38]. The response to insulin is also altered in the livers of LP offspring as there is a loss of initial insulin suppression of hepatic glucose output despite increased expression of insulin receptor [34]. A similar anomalous response to insulin has been reported in humans with type 2 diabetes [39]. There was a decrease in both circulating IGF-I concentration and the production of IGF-I in hepatocytes derived from LP fetuses [40]. It has been suggested that a maternal low-protein diet alters ‘set point’ of hepatic endocrine control.

Protein restriction during pregnancy or the early postnatal period results in a reduction in skeletal muscle mass [14]. Despite this young adult LP offspring have an improved sensitivity to insulin in skeletal muscle in terms of its ability to stimulate glucose uptake. The increase in insulin sensitivity is associated with a twofold increase in insulin receptor and this, at least partially, contributes to better glucose tolerance observed at this age [41]. By 15 months of age insulin-stimulated glucose uptake was reduced in skeletal muscle and it has been suggested that the defect causing impaired insulin signalling lies downstream of the insulin receptor. This has been supported by the finding that the expression of zeta-isoform of protein kinase C (PKC) was reduced in the muscle of LP rat offspring. It should be added that there was no detectable change in expression of either the insulin receptor or glucose transporter 4 (GLUT 4) at that age [42]. Similar impaired insulin signalling has also been shown in the muscle of low birth weight young men with p85 alpha regulatory subunit of phosphatidyl inositol-3 (PI3-kinase), p110 beta regulatory subunit of PI3-kinase, Glut 4 and PKC zeta all being reduced [43].

Protein restriction also affects adipose tissue. Epididymal adipocytes isolated from 6-week-old males exposed to a maternal LP diet during pregnancy or lactation were smaller than controls. When the LP diet was given during both pregnancy and lactation an even larger reduction was observed. At this time-point there was an increase in the expression of the insulin receptor in offspring exposed to LP diet during gestation and/or lactation [18]. At 3 months of age an elevated basal and insulin-stimulated glucose uptake was present in offspring exposed to maternal LP diet during pregnancy and lactation [44]. At this time-point these animals had an increased expression of insulin receptor in epididymal and intra-abdominal but not subcutaneous adipocytes [44, 45]. Basal glucose uptake was higher in adipocytes from epididymal, intra-abdominal and subcutaneous fat pads but insulin-stimulated glucose uptake was smaller in the low-protein group in all depots compared to controls. The magnitude of isoproterenol-stimulated lipolysis was greater in epididymal and intra-abdominal adipocytes than in subcutaneous adipocytes in offspring exposed to maternal LP diet during pregnancy and lactation. However, epididymal and intra-abdominal adipocytes of these animals were resistant to the anti-lipolytic action of insulin [44]. These findings imply that in this animal model changes in adipose tissue associated with IUGR are depot-specific, being enhanced in the more metabolically active intra-abdominal and epididymal tissues. As with muscle, age has detrimental effects on insulin-stimulated glucose uptake and the anti-lipolytic action of insulin as both were reduced in adipocytes isolated from 15-month-old male offspring. At this age, both muscle and adipocyte insulin receptor expression was similar to that of controls, suggesting that the molecular alterations that lead to insulin resistance must occur downstream of the insulin receptor [46]. This has been confirmed as reduced expression of the p110 beta subunit of PI3-kinase at 3 and 15 months and reduced activity of insulin-stimulated protein kinase B were found in adipocytes of IUGR animals [45, 46]. These data also suggest that PI3-kinase might be required to mediate the antilipolytic action of insulin. Reduced levels of p110 beta, p85 alpha, IRS1 and GLUT4 were recently found in the subcutaneous adipose tissue of young men with low birth weight [47]. The findings that low birth weight is associated with reduced expression of insulin signalling proteins in muscle and adipose tissue is of significance as these differences precede the development of diabetes and may help predict disease risk.

Uteroplacental insufficiency

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

Uteroplacental insufficiency in humans is caused by such factors as maternal smoking, pre-eclampsia and abnormalities of uteroplacental development. It is the most common cause of IUGR in western societies. As the supply of oxygen and critical nutrients such as glucose and branched chain amino acids is reduced, the infant is born with a low ponderal index and asymmetrical IUGR. This can be mimicked in animal models. Wigglesworth pioneered a model of uteroplacental insufficiency by developing the methodology for unilateral and bilateral artery ligation in pregnant rats at either 18 or 19 days gestation that causes asymmetrical IUGR [48]. IUGR rat fetuses show critical features of a metabolic profile characteristic of IUGR human fetuses such as hypoxia, hypoglycaemia, acidosis, and reduced concentrations of insulin and IGF-I [49]. The effects brought about by placental insufficiency normalize within days after birth in the human and within 48–72 h after the surgery in the rat [50]. Following bilateral uterine ligation, IUGR rats develop diabetes with a phenotype similar to that observed in humans with type 2 diabetes, namely progressive dysfunction in insulin secretion and insulin action.

Following bilateral uterine ligation there was 10–15% reduction in the birth weight of animals [50–52]. In this model the rate of catch-up growth after birth depends on the severity of IUGR and the level of postnatal nutrition. Bilateral uterine ligation at day 17 of gestation results in a very slow rate of catch-up growth (of up to about 100 days) in both male and female offspring [53]. The same operation at day 19 of gestation and cross-fostering to nonoperated mothers after birth leads to accelerated catch-up growth so the IUGR pup weights are similar to that of controls by 7 weeks of age and as their weight continues to accelerate, it surpasses that of controls and by 26 weeks of age the animals are obese [51].

There appears to be conflicting evidence as to whether the signs of pancreatic impairment are present in newborn IUGR animals following uterine artery ligation. Styrud et al. recently reported a 35–40% reduction in beta cell mass and insulin content in newborn IUGR offspring [52] whilst in the model developed by Simmons et al., beta cell mass, islet size and pancreatic weight were not different than those of controls at 1 week of age [51]. Another study reported that there was a reduction in percentage of insulin containing beta cells after birth [50]. These discrepancies are probably caused by the variations in the protocols used as one group carried out the ligation on gestational day 16 [52], the other on day 19 [51]. There is consensus however that there is an age-dependent decline in the beta cell mass such that by 15 weeks of age the relative beta cell mass is 50% that of controls and by 26 weeks one-third of controls [51, 52].

Following bilateral uterine ligation, IUGR rats were glucose intolerant and insulin resistant at an early age (1 week of age). By 7–10 weeks of age they developed mild fasting hyperglycaemia and hyperinsulinaemia. The onset of hyperglycaemia and a reduction in beta cell mass were preceded by the impairment in the first-phase insulin secretion in response to glucose [51]. Similar findings were observed in humans predestined to develop diabetes [54, 55]. As the arginine-stimulated insulin release was similar in IUGR and control rats, it was concluded that the loss of glucose-stimulated insulin response is caused by an intrinsic defect in the β-cell induced by uteroplacental insufficiency. This defect could be related to impaired glucose sensing or defective oxidative phosphorylation of glucose. Eventually, progressive impairment of glucose tolerance in the IUGR rat led to overt diabetes in young adulthood (by 6 months of age) as the beta cells failed to compensate for secretory defects and insulin resistance [51]. It should be mentioned, that in healthy human subjects, elevated levels of glucose lead to an increase in insulin secretion per β-cell and induce an expansion in β-cell mass [56]. The latter did not occur in the IUGR rat; furthermore, total pancreatic β-cell mass progressively declined, resulting in a further deterioration in glucose homeostasis [51].

When the offspring of intrauterine artery-ligated dams themselves became pregnant they developed gestational diabetes. Their offspring were macrosomic at birth and remained significantly heavier than controls throughout life, even when cross-fostered to normal mothers. They were insulin resistant from an early age and their glucose homeostasis progressively deteriorated. Defects in insulin secretion were detectable at 5 weeks of age and, like in their mothers, were glucose specific as arginine-stimulated insulin secretion was preserved. Adult rats displayed basal hyperglycaemia, insulinaemia and impaired glucose tolerance, and by 3–6 months of age overt diabetes, that was associated with insulin resistance. There was a permanent decrease in protein levels of GLUT 1 in neonatal muscle and GLUT 4 in adult muscle [57], which could be an adaptive mechanism to protect the fetus from toxic effects of high cellular glucose concentrations. This therefore demonstrates how the detrimental effects of IUGR can be transmitted to the second generation.

It has been suggested recently that altered hepatic glucose metabolism in IUGR rats may contribute to the onset of fasting hyperglycaemia before the development of obesity and diabetes [58]. IUGR rats had increased basal hepatic glucose production (HGP) independently of fatty acid levels and impaired insulin suppression of HGP, which are indicative of hepatic insulin resistance. Concurrent with these changes protein expression of insulin-stimulated IRS-1 and Akt-2 phosphorylation was decreased, whilst mRNA levels of PEPCK and glucose-6-phosphate (G6Pase) were significantly increased [58]. This indicates the failure of Akt signalling to induce insulin-mediated inhibition of gluconeogenic enzyme gene expression. Increased levels of the gluconeogenic enzymes at birth and at 21 days of age rat were also reported in another study, but this time they were associated with an increased mRNA expression of peroxisome proliferator-activated receptor-γ coactivator-1 (PGC-1) [59]. PGC-1 is a transcriptional co-activator of nuclear receptors that controls hepatic mRNA levels of the key gluconeogenic enzymes including PEPCK and G6Pase. Additionally, it has been reported that gene expression and function of key mitochondrial hepatic fatty acid metabolizing enzymes is altered in newborn, juvenile and adult male, but not female, IUGR rats. The changes in adult male offspring were associated with an increase in both hepatic malonyl-CoA and serum triglyceride levels, markers of altered hepatic fatty acid metabolism [60]. These findings demonstrate a potential mechanism that links the altered in utero environment of uteroplacental insufficiency and subsequent dyslipidaemia and insulin resistance in adulthood.

Skeletal muscle metabolism is also altered in this model of IUGR. Glycogen content and insulin-stimulated 2-deoxyglucose uptake were significantly decreased in muscle from IUGR rats. This coincided with defects in mitochondria that were attributed to chronic reduction in the supply of ATP available from oxidative phosphorylation. It was suggested that impaired ATP synthesis in muscle compromised energy-dependent GLUT4 recruitment to the cell surface, glucose transport and glycogen synthesis [61]. Increased mRNA levels of PGC-1 have also been reported in the skeletal muscle of young IUGR rats but these changes were skeletal muscle type dependent, i.e. the levels of PGC-1 mRNA were increased in perinatal hind limb skeletal muscle and juvenile extensor digitorum longus, but were decreased in juvenile soleus. There was also an effect of gender as PGC-1 expression in postnatal muscle was higher in male IUGR rats [62]. Furthermore, the gene expression and function of mitochondrial β-oxidation enzymes was altered in isolated skeletal muscle from 21-day-old IUGR rats as there was an increase in mRNA levels of carnitine palmitoyltransferase 1 (CPT1), uncoupling protein-3 (UCP-3) and trifunctional protein of β-oxidation (HADH). Interestingly, skeletal muscle triglycerides were significantly increased in IUGR skeletal muscle despite increased CPT1 and HADH gene expression [63]. This observation combined with the finding that expression of acetyl-CoA carboxylase, the rate-limiting enzyme of hepatic fatty acid synthesis, was increased in the liver of the 21-day-old IUGR pre-weaning rat pups [60] led to the suggestion that at that age an equilibrium has been reached between increased hepatic synthesis and increased skeletal muscle mitochondrial lipid oxidation, which subsequently resulted in only a modest increase in plasma triglycerides [63].

Maternal glucocorticoid exposure

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

In humans, glucocorticoids are administered during pregnancy primarily in the management of women at risk of preterm delivery to advance fetal maturation and reduce neonatal morbidity and mortality [64]. Fetal overexposure to maternal glucocorticoids in utero in both humans and animals leads to altered fetal and placental growth that results in the birth of IUGR offspring [65]. The reduction in the birth weight is most apparent when glucocorticoids are administered in the latter stages of pregnancy [66].

Glucocorticoid action within cells is regulated by the expression of the glucocorticoid receptor (GR) and two isoforms of 11beta-hydroxysteroid dehydrogenase (11β-HSD), 1 and 2. 11β-HSD-1 acts mainly as a reductase, catalysing the conversion of 11-dehydrocorticosterone to corticosterone in rats (cortisone to bioactive cortisol in humans) and amplifying glucocorticoid action at its receptor, whilst 11β-HSD-2 acts as dehydrogenase, which catalyses the conversion of physiologically active glucocorticoids: corticosterone and cortisol to their inactive 11-keto metabolite forms: 11-dehydrocorticosterone and cortisone. Fetal glucocorticoid levels are much lower than maternal levels and this is attributed to the high expression of 11β-HSD-2 in the placenta [67]. However, the barrier that 11β-HSD-2 forms to maternal glucocorticoids is not complete and there is about 10–20% passage of active maternal glucocorticoids to the fetus [68].

Prenatal overexposure to exogenous or endogenous glucocorticoids in the last third of gestation, prenatal stress and gestational inhibition of 11β-HSD not only cause IUGR but also lead to the development of hypercorticosteronaemia, hypertension, hyperglycaemia, hyperinsulinaemia, glucose intolerance, insulin resistance and overactive hypothalamic-pituitary- adrenal (HPA) axis in the adult rat offspring [66, 69, 70]. Early exposure to maternal dexamethasone (dex), a poor substrate of 11β-HSD-2, or postpartum treatment do not programme hyperglycaemia nor hyperinsulinaemia in the offspring, so the critical period for glucocorticoid programming of glucose–insulin homeostasis is very narrow [66]. The offspring of dexamethasone-treated dams remain lighter than controls until about 6 months of age, despite some early catch-up growth [71].

Maternal exposure to dexamethasone during the last week of gestation also reduces the insulin content of the pancreatic beta cells of the offspring. The mechanisms by which glucocorticoids modulate pancreatic development are not clear but an in vitro study using organ cultures of mouse embryonic pancreas has shown that dexamethasone downregulates the beta cell transcription factor Pdx-1 and enhances the transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta), critical factors in the induction and suppression respectively of insulin expression [72].

Glucocorticoids regulate the expression of key hepatic metabolic enzymes, including PEPCK and phosphoenolpyruvate carboxykinase 2 (PCK2). Prenatal overexposure to dexamethasone in the third week of gestation, but not earlier, results in permanent upregulation of hepatic GR mRNA, PCK2 mRNA and PEPCK mRNA expression and activity, which contributes to the glucose intolerance seen in the offspring [66]. The increase in the expression of gluconeogenic PCK2 is accompanied by the increase in hepatocyte nuclear factor 4 alpha (HNF4α) mRNA in the liver, one of the transcription factors that regulate PCK2. The increase in HNF4a is associated with a premature switch from fetal to adult promoter predominance. Therefore, HNF4α might mediate PCK2 overexpression and subsequent lifelong hyperglycaemia in the rats exposed in utero to dexamethasone [73]. The increase in the hepatic GR mRNA expression alongside the increased plasma glucose responses to exogenous corticosterone implicate increased tissue sensitivity to glucocorticoids [66]. As neither hypercorticosteronaemia nor hyperinsulinaemia are sufficient to cause the changes in GR expression, it has been postulated that these changes may be primary in determining the programmed insulin-resistant phenotype [74].

The potential link between other insulin-sensitive tissues and permanent hyperglycaemia seen in this model of IUGR has been also investigated. GR expression is programmed in skeletal muscle by prenatal exposure to glucocorticoids in a fibre type-specific manner. GR mRNA was decreased in soleus muscle (predominantly type I fibres) but not in extensor digitorum longus (predominantly type II fibres). Although glycogen storage was reduced in quadriceps of rats exposed in utero to dexamethasone, there was no decrease in glucose uptake into either quadriceps or soleus muscle in these animals. Fat deposition was reduced in these animals. Leptin mRNA and plasma leptin in visceral fat pad were unaltered, despite the reduced fat pad size and leanness of the animals. GR mRNA was upregulated in the visceral fat, but not in subcutaneous fat. Prenatal treatment did not change uptake of glucose into adipose tissue. In the absence of altered glucose uptake in muscle and adipose tissue, hepatic glucose output is most likely to be responsible for the programmed hyperglycaemia [71, 74].

Interestingly, the effects of prenatal dexamethasone exposure – lower birth weight, impaired glucose tolerance and elevated PEPCK – are also present in the second-generation offspring (third generation) of dexamethasone-treated dams [75].

Gestational diabetes

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

Gestational diabetes or hyperglycaemia occurring temporarily during pregnancy not only is associated with an increased risk of diabetes in the mother but also with increased risk of diabetes in the offspring [76]. The most common pharmaceutical model for studying the effects of gestational diabetes on the health of the offspring in rodents and sheep is maternal administration of streptozotocin (STZ), a chemical which destroys pancreatic beta cells. STZ-induced diabetes is dose dependent: high doses cause severe maternal hyperglycaemia, which leads to fetal hyperglycaemia and birth of offspring with IUGR, whereas low doses induce mild gestational diabetes associated with hypertrophy of the endocrine pancreas and hyperplasia of beta cells as well as fetal hyperinsulinaemia and macrosomia. Mild maternal diabetes results in impaired glucose tolerance in the offspring [77]. The U-shaped relationship between birth weight and subsequent development of diabetes seen in this animal model is also true for the human populations.

Severe maternal diabetes causes severe fetal hyperglycaemia. Hyperstimulation of fetal pancreatic islets leads to degranulation and disorganization of fetal beta cells [78] so they are unable to secrete insulin in vivo and in vitro [79] and only arginine can induce a sustained monophasic insulin secretory response [80]. Beta cell exhaustion results in fetal hypoinsulinaemia, which in conjunction with a reduced number of insulin receptors on target cells [81] causes a reduction in fetal glucose uptake [82]. As a result, fetal protein mass is decreased and protein synthesis is lower than in controls [83].

Intrauterine growth-restricted pups born to severely diabetic mothers remain small up to adulthood [78]. If the offspring are cross-fostered by a normoglycaemic rat, the pancreatic mass in these animals increases, and the pancreas is hypertrophic with a high number of very small islets – a consequence of beta cell neogenesis rather than cell replication. By 3 months of age the beta cell mass is normal and insulin concentrations are either elevated or normal [84]. There is a decreased sensitivity to insulin in the liver and other tissues [85], and peripheral glucose uptake is specifically reduced in skeletal muscle [86]. The insulin resistance in the offspring of STZ-induced diabetes can be partly, but not completely, restored by normalizing maternal glycaemia with islet transplantation in the course of pregnancy [87].

The offspring of severely diabetic rats, similar to the perinatally protein-restricted offspring, develop signs of glucose intolerance themselves during pregnancy, and have higher glucose and lower insulin concentrations than in normal pregnant rats. Their fetuses are macrosomic, hyperglycaemic and hyperinsulinaemic with pancreatic islets displaying hyperplasia and beta cell degranulation [85]. In adulthood these second-generation offspring develop impaired glucose tolerance [88].

Transgeneration effect

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

It has been shown in several studies in humans that the intrauterine exposure to maternal diabetes increases risk of the development of the disease in children. One long-term complication of maternal diabetes is the presence of impaired glucose tolerance in the offspring [89]. Children exposed in utero to mild maternal hyperglycaemia have an increased early insulin response after oral glucose and increased HbA(1c), which indicate a reduction in beta cell function [90].

A growing number of studies in recent years have shown that the consequences of an altered intrauterine environment can be passed transgenerationally from mother (F0) to daughter (F1) and to the F2 progeny. Maternal calorie restriction not only leads to decreased beta cell mass in the first generation of offspring but it also impairs the subsequent beta cell adaptation to pregnancy. Pancreatic islet mass normally increases in well-nourished pregnant rats but in IUGR rats (F1) it remains at nonpregnant levels throughout pregnancy [11]. The impaired beta cell activity and deficient islet mass in these animals contribute to the development of gestational diabetes, which is primarily caused by insulin deficiency. The fetuses from IUGR-born female rats also show a decrease in the beta cell mass, insulin content and islet number at day 20 of gestation, which is associated with decreased expansion of the epithelial population expressing Pdx-1 [91]. Similar mode of transmissions to the second generation have been observed in the low protein and maternal dexamethasone models [92] (Fig. 2).

image

Figure 2.  Transgenerational effects of maternal malnutrition and maternal diabetes.

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Pregnant offspring of mildly and severely diabetic mothers have adequate insulin levels and insulin resistance in early pregnancy is comparable to that of controls. However, gestational diabetes develops as a result of deficiency in pregnancy-induced adaptations of glucose metabolism such as inadequate increase in pancreatic beta cell mass and impairment in beta cell basal activity (Fig. 2). Interestingly, the diabetogenic tendency in this model is transmitted to the next generation via the maternal line only [93]. In contrast, in the maternal dexamethasone model, diabetogenic effect can be transmitted either by the maternal or paternal line [75].

Common cellular and molecular mechanisms

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

Endocrine mechanisms of intrauterine programming

As the phenotypic outcomes of the different intrauterine insults mentioned above have been remarkably similar, the existence of common mechanism(s) underlying the association between IUGR and subsequent development of diabetes in adult life has been investigated. Fowden hypothesized that adult diseases including diabetes arise in utero, at least partially, as a result of changes in the development of key endocrine axes, namely the HPA axis during suboptimal intrauterine conditions associated with impaired growth [6]. It is proposed that suboptimal intrauterine conditions lead to changes in fetal endocrine environment, which then adapt the development of the fetus and slow the rate of its growth to decrease the nutrient requirements in the postnatal life. At the same time the sensitivity of the peripheral tissues to metabolic hormones such as insulin and glucocorticoids is increased in order to ensure survival in a nutrient-deficient postnatal life. If postnatal nutrition is greater than predicted prenatally, enhanced postnatal growth (‘catch up’ growth) and fat deposition will follow, which subsequently will increase the risk of developing insulin resistance, particularly in faster growing males. Additionally, the enhanced sensitivity of the postnatal HPA axis to stress will cause acceleration in metabolic dysfunction in a nutrient-plenty postnatal life, particularly as the deterioration in secretion and function of anabolic hormones takes place with age.

Mitochondrial dysfunction and oxidative stress

Another hypothesis has been recently proposed stating that ‘oxidative stress’, particularly in the pancreas, might be a common mechanism via which an adverse intrauterine milieu impacts on the development of the fetus and subsequent development of type 2 diabetes [94–97]. It has been observed that uteroplacental insufficiency leads to oxidative stress in the fetus [98–100]. It has also been shown that mitochondrial DNA content is reduced in the pancreas, liver and skeletal muscle of male offspring born to dams fed a low-protein diet during both gestation and lactation. This was associated with reduced expression of mitochondrial DNA-encoded genes [101]. Impaired antioxidant defence mechanisms and increased free radical production through xanthine oxidase activation, which is the main free radical-producing enzyme, has been recently reported in cord plasma and placenta of neonates born to mothers with gestational diabetes [102]. As beta cells have a high oxidative energy requirement and very low expression of antioxidant enzymes, they are especially vulnerable to attacks by reactive oxygen species (ROS) [103, 104]. The overabundance of ROS initiates many oxidative reactions that cause oxidative damage in the mitochondria and in cellular proteins, lipids and nucleic acids. Increased ROS have been linked to impaired glucose-stimulated insulin secretion [105, 106], decreased expression of key genes regulating beta cell function and proliferation [107, 108] and induction of cell death [109, 110]. The increase in ROS production may be caused by mitochondrial dysfunction. As mitochondrial DNA point mutations accumulate with age in IUGR islets, there is a decrease in mitochondrial DNA content and reduction in the expression of mitochondria-encoded genes [96, 101]. This leads to further increases in ROS production, progressive deterioration in mitochondrial function and corresponding decline in beta cell function. Mitochondrial dysfunction is not just limited to the pancreatic beta cells in IUGR animals. Impairment of mitochondrial oxidative phosphorylation in the liver contributes to increased HGP by suppressing pyruvate oxidation and increasing gluconeogenesis [111]. Mitochondrial dysfunction in skeletal muscle leads to a chronic reduction in the supply of ATP available from oxidative phosphorylation which compromises energy-dependent GLUT4 recruitment to the cell surface, glucose transport and glycogen synthesis, hence contributing to insulin resistance and hyperglycaemia of type 2 diabetes [60, 94].

Nutrition and epigenetic programming

There is growing evidence that the intrauterine environment can programme adult disease susceptibility by altering the epigenetic state of the fetal genome, hence affecting the phenotype without changing the DNA sequence. All biological methylation reactions are dependent on dietary methyl donors such as methionine and choline, and on cofactors, which include folic acid, vitamin B12 and pyridoxal phosphate, which are required for the re-establishment of the patterns of cytosine methylation after implantation and for the maintenance of these patterns during many cycles of cellular proliferation. Inadequate supply of amino acids and micronutrients, a key factor in IUGR, may affect methylation and histone modifications. The preimplantation embryo is particularly sensitive to epigenetic changes as shown when folate supplementation of the maternal diet at conception in the agouti mouse model led to increased DNA methylation of the agouti gene and increased longevity of the offspring [112]. It has been shown that uteroplacental insufficiency affects key hepatic one-carbon metabolizing enzymes as mRNA expression of cystathionine-beta-synthase (CBS) is increased whilst expression of methionine adenosyltransferase (MAT) is decreased. Hypoxia and ROS production can decrease MAT activity, whilst both glucose and insulin affect CBS. In addition, levels of S-adenosylhomocysteine (SAH) – a critical hepatic metabolite of one-carbon metabolism – were also elevated. These changes induce DNA hypomethylation and histone acetylation thereby altering chromatin dynamics and lead to persistent changes in hepatic gene expression [113]. It has also been suggested that folate deficiency may underlie all these changes as folate deficiency increases hepatic SAH levels, causes DNA hypomethylation and can alter expression of genes such as MeCP2 [113, 114]. In contrast, inadequate supply of alpha-threonine has been suggested to underlie the epigenetic changes caused by maternal protein restriction. The endogenous methylation of DNA was elevated in the livers of fetuses from dams fed low-protein diet and increased further when the diet was supplemented with threonine. Threonine deficiency has been speculated to lead to changes in methionine metabolism, increasing homeocysteine production, thereby causing hypermethylation of fetal hepatic DNA [115]. It has been also reported that glucocorticoids can affect DNA methylation but the relevance of this process to the aetiology of diabetes remains to be investigated [116]. An adverse intrauterine milieu and the accumulation of DNA methylation errors over time could lead to premature ‘epigenetic aging’, thereby contributing to increased susceptibility to diabetes in adult life [117].

Epigenetic modifications of imprinted genes

The intrauterine and early postnatal environment may also induce epigenetic modifications of imprinted genes. The media used to culture early mouse embryos can alter fetal DNA methylation and allelic expression of growth-related imprinted genes; however, it remains to be seen whether such in vitro nutritional effects persist beyond fetal development [118]. A post-weaning diet deficient in methyl donors and cofactors has been shown to affect the allelic imprinting of Igf2, which is paternally expressed [117]. Although the parental imprinting is erased during extensive demethylation of the preimplantation mammalian embryo's genome [119], recent evidence suggests that some epigenetic modifications may not be completely erased, potentially contributing to intergenerational inheritance of the epigenetic state [120, 121]. This could explain how adverse consequences of altered intrauterine environments can be passed transgenerationally from mother to daughter and further generations.

Conclusions

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References

There is no doubt that adverse intrauterine environment leads to altered fetal growth and development, inducing lasting consequences for the glucose metabolism in the offspring. In humans, the intrauterine environment varies with each pregnancy, so fetal adaptations to early environment are variable as well as their lasting consequences for the health of an individual. The use of various animal models allows investigation of the effects of different maternal treatments on the health of the offspring. Persistent deterioration in glucose tolerance seen in the IUGR offspring coupled with stressed glucose metabolism during pregnancy can exert effects that affect glucose homeostasis of further generations. Findings from different experimental animal models provide an insight into the factors and mechanisms underlying the association between early environment and type 2 diabetes.

References

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Nutritional models of IUGR
  5. Uteroplacental insufficiency
  6. Maternal glucocorticoid exposure
  7. Gestational diabetes
  8. Transgeneration effect
  9. Common cellular and molecular mechanisms
  10. Conclusions
  11. Conflict of interest statement
  12. References
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