Neonatal dexamethasone treatment leads to alterations in cell signaling cascades controlling hepatic and cardiac function in adulthood

Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty

For at least 50 years, the prevailing view has been that the adenylate cyclase (AC)/cyclic AMP (cAMP)/protein kinase A pathway is the predominant signal mediating the hepatic glucose‐mobilizing actions of glucagon. A wealth of evidence, however, supports the alternative, that the operative signal most of the time is the phospholipase C (PLC)/inositol‐phosphate (IP3)/calcium/calmodulin pathway. The evidence can be summarized as follows: (1) The consensus threshold glucagon concentration for activating AC ex vivo is 100 pM, but the statistical hepatic portal plasma glucagon concentration range, measured by RIA, is between 28 and 60 pM; (2) Within that physiological concentration range, glucagon stimulates the PLC/IP3 pathway and robustly increases glucose output without affecting the AC/cAMP pathway; (3) Activation of a latent, amplified AC/cAMP pathway at concentrations below 60 pM is very unlikely; and (4) Activation of the PLC/IP3 pathway at physiological concentrations produces intracellular effects that are similar to those produced by activation of the AC/cAMP pathway at concentrations above 100 pM, including elevated intracellular calcium and altered activities and expressions of key enzymes involved in glycogenolysis, gluconeogenesis, and glycogen synthesis. Under metabolically stressful conditions, as in the early neonate or exercising adult, plasma glucagon concentrations often exceed 100 pM, recruiting the AC/cAMP pathway and enhancing the activation of PLC/IP3 pathway to boost glucose output, adaptively meeting the elevated systemic glucose demand. Whether the AC/cAMP pathway is consistently activated in starvation or diabetes is not clear. Because the importance of glucagon in the pathogenesis of diabetes is becoming increasingly evident, it is even more urgent now to resolve lingering uncertainties and definitively establish glucagon’s true mechanism of glycemia regulation in health and disease.

Diazinon exposure activated transcriptional factors CCAAT-enhancer-binding proteins α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ) and induced adipogenesis in 3T3-L1 preadipocytes

Environmental chemical exposure could be a contributor to the increasing obesity epidemic. Diazinon, an organophosphate insecticide, has been widely used in the agriculture, and exposure of the general population to diazinon has been reported. Diazinon has been known to induce neurotoxic effects mainly through the inhibition of acetylcholinesterase (AChE). However, its association with dysregulation of adipogenesis has been poorly investigated. The current study aimed to examine the mechanism of diazinon's effect on adipogenesis using the 3T3-L1 preadipocytes combined with a single-cell-based high-content analysis. The results showed that diazinon induced lipid droplet accumulation in a dose-dependent manner. The dynamic changes of adipogenic regulatory proteins and genes were examined at the three stages of adipogenesis (induction, differentiation, and maturation) in 3T3-L1 cells treated with various doses of diazinon (0, 1, 10, 100 μM) using real-time quantitative RT-PCR and Western Blot respectively. Diazinon significantly induced protein expression of transcriptional factors CCAAT-enhancer-binding proteins α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ), their downstream proteins, fatty acid synthase (FASN), acetyl CoA carboxylase (ACC), fatty acid-binding protein 4 (FABP4), lipoprotein lipase (LPL), adiponectin and perilipin in dose and time-dependent manners. Similarly, the adipogenic genes were significantly induced in a dose and time-dependent manner compared to the relative controls. The current study demonstrates that diazinon promotes lipid accumulation and activates the adipogenic signaling pathway in the in vitro model.

Effects of neonatal dexamethasone administration on cardiac recovery ability under ischemia-reperfusion in 24-wk-old rats

BACKGROUND

Evaluations of stress-induced cardiac functional alterations in adults after neonatal glucocorticoid (GC) treatment have been limited. In the present study, we evaluated adult cardiac functional recovery during postischemic reperfusion and measured cardiac gene expression involved energy metabolism in rats neonatally treated with dexamethasone (DEX).

METHOD

Male Wistar rats were injected DEX in first 3 d after birth and controls were received saline (SAL). At 24 wk of age, insulin tolerance tests were performed, plasma lipid levels were measured, and left ventricular function and myocardial infarct size were evaluated. Expressions of genes involved in cardiac energy metabolism were measured by quantitative real-time polymerase chain reaction (PCR) and western blot.

RESULTS

In 24-wk-old rats, neonatal DEX administration caused dyslipidemia, impaired cardiac recovery function and increased size of infarction, decreased cardiac expression of glucose transporter 4(GLUT4), peroxisome proliferative-activated receptor gamma coactivator 1α (PGC-1α) and ratios of phospho-forkhead box O1/forkhead box O1 (p-FoxO1/FoxO1) and phospho AMP-activated protein kinase/AMP-activated protein kinase (p-AMPK/AMPK) but increased pyruvate dehydrogenase kinase isoenzyme 4 (PDK4) expression compared with controls.

CONCLUSION

Neonatal DEX administration impairs cardiac functional recovery during reperfusion following ischemia in 24-wk-old rats. Reduced cardiac glucose utilization may contribute to the long-term detrimental effects caused by neonatal DEX treatment.

The M2 muscarinic receptors are essential for signaling in the heart left ventricle during restraint stress in mice

We hypothesized that muscarinic receptors (MRs) in the heart have a role in stress responses and thus investigated changes in MR signaling (gene expression, number of receptors, adenylyl cyclase (AC), phospholipase C (PLC), protein kinase A and C (PKA and PKC) and nitric oxide synthase [NOS]) in the left ventricle, together with telemetric measurement of heart rate (HR) in mice (wild type [WT] and M2 knockout [KO]) during and after one (1R) or seven sessions (7R) of restraint stress (seven mice per group). Stress decreased M2 MR mRNA and cell surface MR in the left ventricle in WT mice. In KO mice, 1R, but not 7R, decreased surface MR. Similarly, AC activity was decreased in WT mice after 1R and 7R, whereas in KO mice, there was no change. PLC activity was also decreased after 1R in WT and KO mice. This is in accord with the concept that cAMP is a key player in HR regulation. No change was found with stress in NOS activity. Amount of AC and PKA protein was not changed, but was altered for PKC isoenzymes (PKCα, β, γ, η and ϵ (increased) in KO mice, and PKCι (increased) in WT mice). KO mice were more susceptible to stress as shown by inability to compensate HR during 120 min following repeated stress. The results imply that not only M2 but also M3 are involved in stress signaling and in allostasis. We conclude that for a normal stress response, the expression of M2 MR to mediate vagal responses is essential.

Expression of adrenoceptor subtypes in preterm piglet heart is different to term heart

Preterm delivery increases the risk of inadequate systemic blood flow and hypotension, and many preterm infants fail to respond to conventional inotrope treatments. If the profile of cardiac adrenoceptor subtypes in the preterm neonate is different to that at term this may contribute to these clinical problems. This study measured mRNA expression of β1, β2, α1A, α2A and α2B-adrenoceptor subtypes by real time PCR in term (113d), preterm (91d) and preterm piglets (91d) exposed to maternal glucocorticoid treatment. Abundance of β-adrenoceptor binding sites in the left ventricle was measured using saturation binding assays. Relative abundance of β1-adrenoceptor mRNA in untreated preterm hearts was ∼50% of term abundance in both left and right ventricles (P<0.001). Trends in receptor binding site density measurements supported this observation (P = 0.07). Glucocorticoid exposure increased β1-adrenoceptor mRNA levels in the right ventricle of preterm hearts (P = 0.008) but did not alter expression in the left ventricle (P>0.1). Relative abundance of α1A-adrenoceptor mRNA was the same in preterm and term piglet hearts (P = >0.1) but was reduced by maternal glucocorticoid treatment (P<0.01); α2A-adrenoceptor mRNA abundance was higher in untreated and glucocorticoid exposed preterm piglet hearts than in term piglets (P<0.001). There was no difference between male and female piglets in mRNA abundance of any of the genes studied. In conclusion, there is reduced mRNA abundance of β1-adrenoceptors in the preterm pig heart. If this lower expression of β-adrenoceptors occurs in human preterm infants, it could explain their poor cardiovascular function and their frequent failure to respond to commonly used inotropes.

Non-genomic effect of glucocorticoids on cardiovascular system

Glucocorticoids (GCs) are essential steroid hormones for homeostasis, development, metabolism, and cognition and possess anti-inflammatory and immunosuppressive actions. Since glucocorticoid receptor II (GR) is nearly ubiquitous, chronic activation or depletion of GCs leads to dysfunction of diverse organs, including the heart and blood vessels, resulting predominantly from changes in gene expression. Most studies, therefore, have focused on the genomic effects of GC to understand its related pathophysiological manifestations. The nongenomic effects of GCs clearly differ from well-known genomic effects, with the former responding within several minutes without the need for protein synthesis. There is increasing evidence that the nongenomic actions of GCs influence various physiological functions. To develop a GC-mediated therapeutic target for the treatment of cardiovascular disease, understanding the genomic and nongenomic effects of GC on the cardiovascular system is needed. This article reviews our current understanding of the underlying mechanisms of GCs on cardiovascular diseases and stress, as well as how nongenomic GC signaling contributes to these conditions. We suggest that manipulation of GC action based on both GC and GR metabolism, mitochondrial impact, and the action of serum- and glucocorticoid-dependent kinase 1 may provide new information with which to treat cardiovascular diseases.

Role of environmental chemicals in diabetes and obesity: a National Toxicology Program workshop review

BACKGROUND

There has been increasing interest in the concept that exposures to environmental chemicals may be contributing factors to the epidemics of diabetes and obesity. On 11-13 January 2011, the National Institute of Environmental Health Sciences (NIEHS) Division of the National Toxicology Program (NTP) organized a workshop to evaluate the current state of the science on these topics of increasing public health concern.

OBJECTIVE

The main objective of the workshop was to develop recommendations for a research agenda after completing a critical analysis of the literature for humans and experimental animals exposed to certain environmental chemicals. The environmental exposures considered at the workshop were arsenic, persistent organic pollutants, maternal smoking/nicotine, organotins, phthalates, bisphenol A, and pesticides. High-throughput screening data from Toxicology in the 21st Century (Tox21) were also considered as a way to evaluate potential cellular pathways and generate -hypotheses for testing which and how certain chemicals might perturb biological processes related to diabetes and obesity.

CONCLUSIONS

Overall, the review of the existing literature identified linkages between several of the environmental exposures and type 2 diabetes. There was also support for the "developmental obesogen" hypothesis, which suggests that chemical exposures may increase the risk of obesity by altering the differentiation of adipocytes or the development of neural circuits that regulate feeding behavior. The effects may be most apparent when the developmental exposure is combined with consumption of a high-calorie, high-carbohydrate, or high-fat diet later in life. Research on environmental chemical exposures and type 1 diabetes was very limited. This lack of research was considered a critical data gap. In this workshop review, we outline the major themes that emerged from the workshop and discuss activities that NIEHS/NTP is undertaking to address research recommendations. This review also serves as an introduction to an upcoming series of articles that review the literature regarding specific exposures and outcomes in more detail.

Developmental neurotoxicity targeting hepatic and cardiac sympathetic innervation: effects of organophosphates are distinct from those of glucocorticoids

Early-life exposure to organophosphate pesticides leads to subsequent hyperresponsiveness of β-adrenergic receptor-mediated cell signaling that regulates hepatic gluconeogenesis, culminating in metabolic abnormalities resembling prediabetes. In the current study, we evaluated the effects of chlorpyrifos or parathion on presynaptic sympathetic innervation to determine whether the postsynaptic signaling effects are accompanied by defects in neuronal input. We administered either chlorpyrifos or parathion to newborn rats using exposure paradigms known to elicit the later metabolic changes but found no alterations in either hepatic or cardiac norepinephrine levels in adolescence or adulthood. However, shifting chlorpyrifos exposure to the prenatal period did evoke changes: exposure early in gestation produced subsequent elevations in norepinephrine, whereas later gestational exposure produced significant deficits. We also distinguished the organophosphate effects from those of the glucocorticoid, dexamethasone, a known endocrine disruptor that leads to later-life metabolic and cardiovascular disruption. Postnatal exposure to dexamethasone elicited deficits in peripheral norepinephrine levels but prenatal exposure did not. Our results indicate that early-life exposure to organophosphates leads to subsequent abnormalities of peripheral sympathetic innervation through mechanisms entirely distinct from those of glucocorticoids, ruling out the possibility that the organophosphate effects are secondary to stress or disruption of the HPA axis. Further, the effects on innervation were separable from those on postsynaptic signaling, differing in critical period as well as tissue- and sex-selectivity. Organophosphate targeting of both presynaptic and postsynaptic β-adrenergic sites, each with different critical periods of vulnerability, thus sets the stage for compounding of hepatic and cardiac functional abnormalities.

Does early-life exposure to organophosphate insecticides lead to prediabetes and obesity?

Human exposures to organophosphate insecticides are ubiquitous. Although regarded as neurotoxicants, increasing evidence points toward lasting metabolic disruption from early-life organophosphate exposures. We gave neonatal rats chlorpyrifos, diazinon or parathion in doses devoid of any acute signs of toxicity, straddling the threshold for barely-detectable cholinesterase inhibition. Organophosphate exposure during a critical developmental window altered the trajectory of hepatic adenylyl cyclase/cyclic AMP signaling, culminating in hyperresponsiveness to gluconeogenic stimuli. Consequently, the animals developed metabolic dysfunction resembling prediabetes. When the organophosphate-exposed animals consumed a high fat diet in adulthood, metabolic defects were exacerbated and animals gained excess weight compared to unexposed rats on the same diet. At the same time, the high fat diet ameliorated many of the central synaptic defects caused by organophosphate exposure, pointing to nonpharmacologic therapeutic interventions to offset neurodevelopmental abnormalities, as well as toward fostering dietary choices favoring high fat intake. These studies show how common insecticides may contribute to the increased worldwide incidence of obesity and diabetes.

Staging perspectives in neurodevelopmental aspects of neuropsychiatry: agents, phases and ages at expression

Neurodevelopmental risk factors have assumed a critical role in prevailing notions concerning the etiopathogenesis of neuropsychiatric disorders. Staging, diagnostic elements at which phase of disease is determined, provides a means of conceptualizing the degree and extent of factors affecting brain development trajectories, but is concurrently specified through the particular interactions of genes and environment unique to each individual case. For present purposes, staging perspectives in neurodevelopmental aspects of the disease processes are considered from conditions giving rise to neurodevelopmental staging in affective states, adolescence, dopamine disease states, and autism spectrum disorders. Three major aspects influencing the eventual course of individual developmental trajectories appear to possess an essential determinant influence upon outcome: (i) the type of agent that interferes with brain development, whether chemical, immune system activating or absent (anoxia/hypoxia), (ii) the phase of brain development at which the agent exerts disruption, whether prenatal, postnatal, or adolescent, and (iii) the age of expression of structural and functional abnormalities. Clinical staging may be assumed at any or each developmental phase. The present perspective offers both a challenge to bring further order to diagnosis, intervention, and prognosis and a statement regarding the extreme complexities and interwoven intricacies of epigenetic factors, biomarkers, and neurobehavioral entities that aggravate currents notions of the neuropsychiatric disorders.