Alison C. Holloway

The fetal placental hypothalamic-pituitary-adrenal (HPA) axis, parturition and post natal health

A general characteristic of fetal endocrine maturation across different species is the enhanced activity of the fetal hypothalamic-pituitary-adrenal (HPA) axis during late gestation. Precocious activation of this axis may occur when the fetus is exposed to an adverse intra-uterine environment, such as hypoxemia. HPA development is associated with increased levels of ACTH(1-39) and adrenal corticosteroids (cortisol in sheep and human) in the fetal circulation, and increased expression of mRNA encoding corticotrophin releasing hormone (CRH) in the hypothalamus, proopiomelanocortin (POMC) in the pituitary, and key steroidogenic enzymes in the fetal adrenal. At term, increased levels of cortisol act on the placenta/trophoblast derived cells to increase expression of prostaglandin synthase Type II (PGHS-II). In human gestation, cortisol also decreases expression of 15-hydroxyprostaglandin dehydrogenase (PGDH) in chorionic trophoblast cells. Increased synthesis and decreased metabolism of prostaglandin (PG) results, during late gestation, in enhanced output of primary PG, which in turn increases the activity of 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) in the human fetal membranes. Increased chorionic 11 beta HSD-1 results in increased local generation of cortisol from cortisone, with further paracrine/autocrine stimulation of PG output. Increased fetal cortisol contributes to the maturation of organ systems required for postnatal extra-uterine survival. However, excessive levels of feto-placental glucocorticoid, derived from maternal administration of synthetic corticosteroids or sustained endogenous fetal cortisol production, results in intrauterine growth restriction. Fetal sheep, exposed to maternal betamethasone in late gestation, develop insulin resistance and exaggerated adrenal responses to HPA stimulation by 6-12 months postnatal life. Thus, the level of fetal HPA activity is crucial not only for determining gestation length, but may also predict pathophysiologic adjustments in later life.

Long-Term Consequences of Fetal and Neonatal Nicotine Exposure: A Critical Review

Cigarette smoking during pregnancy is associated with numerous obstetrical, fetal, and developmental complications, as well as an increased risk of adverse health consequences in the adult offspring. Nicotine replacement therapy (NRT) has been developed as a pharmacotherapy for smoking cessation and is considered to be a safer alternative for women to smoking during pregnancy. The safety of NRT use during pregnancy has been evaluated in a limited number of short-term human trials, but there is currently no information on the long-term effects of developmental nicotine exposure in humans. However, animal studies suggest that nicotine alone may be a key chemical responsible for many of the long-term effects associated with maternal cigarette smoking on the offspring, such as impaired fertility, type 2 diabetes, obesity, hypertension, neurobehavioral defects, and respiratory dysfunction. This review will examine the long-term effects of fetal and neonatal nicotine exposure on postnatal health.

Glucocorticoid Regulation of Human and Ovine Parturition: The Relationship Between Fetal Hypothalamic-Pituitary-Adrenal Axis Activation and Intrauterine Prostaglandin Production

Abstract Birth in many animal species and in humans is associated with activation of hypothalamic-pituitary-adrenal function in the fetus and the increased influence of glucocorticoids on trophoblast cells of the placenta and fetal membranes. We suggest that in ovine pregnancy glucocorticoids directly increase fetal placental prostaglandin production, and indirectly increase prostaglandin production by maternal uterine tissues through the stimulation of placental estradiol synthesis. The events of ovine parturition are compared with those of human parturition. In the latter, we suggest similar direct effects of glucocorticoids on prostaglandin synthesis and metabolism in fetal membranes and similar indirect effects mediated by glucocorticoid-stimulated increases in intrauterine corticotropin-releasing hormone expression.

Evaluation of the Association between Maternal Smoking, Childhood Obesity, and Metabolic Disorders: A National Toxicology Program Workshop Review

Background: An emerging literature suggests that environmental chemicals may play a role in the development of childhood obesity and metabolic disorders, especially when exposure occurs early in life. Objective: Here we assess the association between these health outcomes and exposure to maternal smoking during pregnancy as part of a broader effort to develop a research agenda to better understand the role of environmental chemicals as potential risk factors for obesity and metabolic disorders. Methods: PubMed was searched up to 8 March 2012 for epidemiological and experimental animal studies related to maternal smoking or nicotine exposure during pregnancy and childhood obesity or metabolic disorders at any age. A total of 101 studies—83 in humans and 18 in animals—were identified as the primary literature. Discussion: Current epidemiological data support a positive association between maternal smoking and increased risk of obesity or overweight in offspring. The data strongly suggest a causal relation, although the possibility that the association is attributable to unmeasured residual confounding cannot be completely ruled out. This conclusion is supported by findings from laboratory animals exposed to nicotine during development. The existing literature on human exposures does not support an association between maternal smoking during pregnancy and type 1 diabetes in offspring. Too few human studies have assessed outcomes related to type 2 diabetes or metabolic syndrome to reach conclusions based on patterns of findings. There may be a number of mechanistic pathways important for the development of aberrant metabolic outcomes following perinatal exposure to cigarette smoke, which remain largely unexplored. Conclusions: From a toxicological perspective, the linkages between maternal smoking during pregnancy and childhood overweight/obesity provide proof-of-concept of how early-life exposure to an environmental toxicant can be a risk factor for childhood obesity.

Interleukin-15 contributes to the regulation of murine adipose tissue and human adipocytes.

An alarming global rise in the prevalence of obesity and its contribution to the development of chronic diseases is a serious health concern. Recently, obesity has been described as a chronic low‐grade inflammatory condition, influenced by both adipose tissue and immune cells suggesting proinflammatory cytokines may play a role in its etiology. Here we examined the effects of interleukin‐15 (IL‐15) on adipose tissue and its association with obesity. Over expression of IL‐15 (IL‐15tg) was associated with lean body condition whereas lack of IL‐15 (IL‐15−/−) results in significant increase in weight gain without altering appetite. Interestingly, there were no differences in proinflammatory cytokines such as IL‐6 and tumor necrosis factor‐α (TNF‐α) in serum between the three strains of mice. In addition, there were significant numbers of natural killer (NK) cells in fat tissues from IL‐15tg and B6 compared to IL‐15−/− mice. IL‐15 treatment results in significant weight loss in IL‐15−/− knockout and diet‐induced obese mice independent of food intake. Fat pad cross‐sections show decreased pad size with over expression of IL‐15 is due to adipocyte shrinkage. IL‐15 induces weight loss without altering food consumption by affecting lipid deposition in adipocytes. Treatment of differentiated human adipocytes with recombinant human IL‐15 protein resulted in decreased lipid deposition. In addition, obese patients had significantly lower serum IL‐15 levels when compared to normal weight individuals. These results clearly suggest that IL‐15 may be involved in adipose tissue regulation and linked to obesity.

Adverse Fetal and Neonatal Outcomes Associated with a Life-Long High Fat Diet: Role of Altered Development of the Placental Vasculature

Maternal obesity results in a number of obstetrical and fetal complications with both immediate and long-term consequences. The increased prevalence of obesity has resulted in increasing numbers of women of reproductive age in this high-risk group. Since many of these obese women have been subjected to hypercaloric diets from early childhood we have developed a rodent model of life-long maternal obesity to more clearly understand the mechanisms that contribute to adverse pregnancy outcomes in obese women. Female Sprague Dawley rats were fed a control diet (CON - 16% of calories from fat) or high fat diet (HF - 45% of calories from fat) from 3 to 19 weeks of age. Prior to pregnancy HF-fed dams exhibited significant increases in body fat, serum leptin and triglycerides. A subset of dams was sacrificed at gestational day 15 to evaluate fetal and placental development. The remaining animals were allowed to deliver normally. HF-fed dams exhibited a more than 3-fold increase in fetal death and decreased neonatal survival. These outcomes were associated with altered vascular development in the placenta, as well as increased hypoxia in the labyrinth. We propose that the altered placental vasculature may result in reduced oxygenation of the fetal tissues contributing to premature demise and poor neonatal survival.

Maternal nicotine exposure increases oxidative stress in the offspring.

Fetal and neonatal nicotine exposure causes beta-cell apoptosis and loss of beta-cell mass, but the underlying mechanisms are unknown. The goal of this study was to determine whether maternally derived nicotine can act via the pancreatic nicotinic acetylcholine receptor (nAChR) during fetal and neonatal development to induce oxidative stress in the pancreas. Female Wistar rats were given saline or nicotine (1 mg/kg/day) via subcutaneous injection for 2 weeks prior to mating until weaning (postnatal day 21). In male offspring, nAChR subunit mRNA expression was characterized in the developing pancreas and various oxidative stress markers were measured at weaning following saline and nicotine exposure. The nAChR subunits alpha2-alpha4, alpha6, alpha7, and beta2-beta4 were present in the pancreas during development. Fetal and neonatal exposure to nicotine significantly increased pancreatic GPx-1 and MnSOD protein expression, as well as islet ROS production. Furthermore, protein carbonyl formation was higher in nicotine-exposed offspring relative to controls, particularly within the mitochondrial fraction. There was also a nonsignificant trend toward higher serum 8-isoPG levels. These data suggest that beta-cell apoptosis in the fetal and neonatal pancreas may be the result of a direct effect of nicotine via its receptor and that this effect may be mediated through increased oxidative stress.

Fetal Hypothalamic-Pituitary Adrenal (HPA) Development and Activation as a Determinant of the Timing of Birth, and of Postnatal Disease

Birth in most animal species is triggered by the fetus through activation of the fetal hypothalamic-pituitary-adrenal (HPA) axis. Preterm birth, may be associated with precocious activation of fetal HPA function, reflecting the fetal response to an adverse intrauterine environment. There is a progressive and concurrent increase of ACTH1–39 and cortisol (F) in the circculation of fetal sheep during the last 15–20 days of pregnancy (term, day 145–150) associated with increased expression of hypothalamic CRH pituitary POMC and adrenal ACTH receptor and steroidogenic enzymes, particularly P450 C17. Similar changes occur with fetal hypoxemia. Negative feedback is ameliorated by decreased pituitary and hypothalamic glucocorticoid receptor, increased CBG, and altered fetal pituitary 11B-hydroxysteroid dehydrogenase type 1. Repeated fetal hypoxemia, diminishes the fetal-pituitary ACTH response, but increases fetal adrenal responsiveness. Fetuses exposed to maternal glucocorticoid in late gestation are growth restricted with altered postnatal HPA responsiveness and glycemic responses that reproduce the insulin resistance of type 2 diabetes. We conclude that the level of fetal HPA activity is crucial not only for determining gestation length, but also predicts pathophysiologic adjustment in later life.

Fetal and neonatal nicotine exposure and postnatal glucose homeostasis: identifying critical windows of exposure.

Fetal and lactational exposure to nicotine at concentrations comparable with those in women who smoke causes impaired glucose tolerance in male offspring in postnatal life. It remains unknown whether there are critical windows of susceptibility to nicotine exposure. Female nulliparous Wistar rats were given saline vehicle or nicotine bitartrate (1 mg/kg per day) prior to pregnancy, which was then: A) discontinued during pregnancy and lactation; B) continued until parturition; C) continued until weaning; and D) discontinued during pregnancy and restarted from lactation until weaning. At 26 weeks of age, offspring in each group were challenged with an oral glucose load. Beta-cell mass, apoptosis, and proliferation were measured at birth, and at 4 and 26 weeks of age. The animals in group C (exposed to nicotine throughout pregnancy and lactation) had reduced beta-cell mass from birth through 26 weeks of age and impaired glucose homeostasis at 26 weeks of age. beta-Cell mass was also reduced at birth and at 4 weeks of age in animals exposed to nicotine during pregnancy alone (group B). However, enhanced proliferation following weaning led to recovery of this defect to 98% of control levels by week 26. The response to the glucose load in groups A, B, and D did not differ from controls. Continued exposure to nicotine from conception through lactation results in permanent beta-cell loss and subsequent impaired glucose tolerance. This model of type 2 diabetes requires that nicotine exposure occurs both in utero and during lactation.

Increased Pancreatic Beta-Cell Apoptosis following Fetal and Neonatal Exposure to Nicotine Is Mediated via the Mitochondria

In Canada, nicotine replacement therapy is recommended as a safe smoking cessation aid for pregnant women. However, we have shown in an animal model that fetal and neonatal nicotine exposure causes increased beta-cell apoptosis and loss of beta-cell mass, which leads to the development of postnatal dysglycemia and obesity. The goal of this study was to determine whether the observed beta-cell apoptosis is mediated via the mitochondrial and/or death receptor pathway. Female Wistar rats were given saline (control) or nicotine bitartrate (1 mg/kg/day) via sc injection for 2 weeks prior to mating until weaning (postnatal day 21). At weaning, pancreas tissue was collected for Western blotting, electron microscopy (EM), and immunohistochemistry. Key markers of each apoptotic pathway were examined in whole pancreas homogenates and mitochondrial/cytosolic pancreas fractions. In the death receptor pathway, Fas and soluble Fas ligand (FasL) protein were significantly increased in the nicotine-exposed offspring compared to control animals; there was no difference in the ratio of inactive/active caspase-8 or membrane-bound FasL expression. In the mitochondrial pathway, there was a significant increase in the ratio of Bcl2/Bax, Bax translocation to the mitochondria, cytochrome c release to the cytosol, and the ratio of active/inactive caspase-3 in nicotine-exposed offspring relative to control animals. Furthermore, increased mitochondrial swelling was observed by EM in the pancreatic beta cells of nicotine-exposed offspring. Taken together, these data suggest that beta-cell apoptosis following developmental nicotine exposure is mediated via the mitochondria.