Aims/hypothesisExposure to environmental pollution has been consistently linked to diabetes incidence in humans, but the potential causative mechanisms remain unclear. Given the critical role of regulated insulin secretion in maintaining glucose homeostasis, environmental chemicals that reach the endocrine pancreas and cause beta cell injury are of particular concern. We propose that cytochrome P450 (CYP) enzymes, which are involved in metabolising xenobiotics, could serve as a useful biomarker for direct exposure of islets to pollutants. Moreover, functional CYP enzymes in islets could also impact beta cell physiology. The aim of this study was to determine whether CYP1A enzymes are activated in islets following direct or systemic exposure to environmental pollutants.MethodsImmortalised liver (HepG2) and rodent pancreatic endocrine cell lines (MIN6, βTC-6, INS1, α-TC1, α-TC3), as well as human islets, were treated in vitro with known CYP1A inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene (3-MC). In addition, mice were injected with either a single high dose of TCDD or multiple low doses of TCDD in vivo, and islets were isolated 1, 7 or 14 days later.ResultsCYP1A enzymes were not activated in any of the immortalised beta or alpha cell lines tested. However, both 3-MC and TCDD potently induced CYP1A1 gene expression and modestly increased CYP1A1 enzyme activity in human islets after 48 h. The induction of CYP1A1 in human islets by TCDD was prevented by cotreatment with a cytokine mixture. After a systemic single high-dose TCDD injection, CYP1A1 enzyme activity was induced in mouse islets ~2-fold, ~40-fold and ~80-fold compared with controls after 1, 7 and 14 days, respectively, in vivo. Multiple low-dose TCDD exposure in vivo also caused significant upregulation of Cyp1a1 in mouse islets. Direct TCDD exposure to human and mouse islets in vitro resulted in suppressed glucose-induced insulin secretion. A single high-dose TCDD injection resulted in lower plasma insulin levels, as well as a pronounced increase in beta cell death.Conclusions/interpretationTransient exposure to TCDD results in long-term upregulation of CYP1A1 enzyme activity in islets. This provides evidence for direct exposure of islets to lipophilic pollutants in vivo and may have implications for islet physiology.Electronic supplementary materialThe online version of this article (10.1007/s00125-019-05035-0) contains peer-reviewed but unedited supplementary material, which is available to authorised users.

epidemiological studies have consistently shown an association between exposure to environmental pollutants and diabetes risk in humans. We have previously shown that direct exposure of mouse and human islets (endocrine pancreas) to the highly persistent pollutant TCDD (2,3,7,8-tetrachlorodibenzop-dioxin) causes reduced insulin secretion ex vivo. Furthermore, a single high-dose of TCDD (200 µg/kg) suppressed both fasting and glucose-induced plasma insulin levels and promoted beta-cell apoptosis after 7 days in male mice. The current study investigated the longer-term effects of a single high-dose TCDD injection (20 µg/kg) on glucose metabolism and beta cell function in male and female C57Bl/6 mice. TCDD-exposed males displayed modest fasting hypoglycemia for ~4 weeks post-injection, reduced fasting insulin levels for up to 6 weeks, increased insulin sensitivity, decreased beta cell area, and increased delta cell area. tcDD-exposed females also had long-term suppressed basal plasma insulin levels, and abnormal insulin secretion for up to 6 weeks. Unlike males, TCDD did not impact insulin sensitivity or islet composition in females, but did cause transient glucose intolerance 4 weeks post-exposure. our results show that a single exposure to dioxin can suppress basal insulin levels longterm in both sexes, but effects on glucose homeostasis are sex-dependent. Diabetes mellitus is one of the leading causes of death worldwide, and its prevalence is increasing at alarming rates 1. Type 2 diabetes (T2D) is the most common form of diabetes, and is characterized by chronic hyperglycemia, peripheral insulin resistance, and insufficient insulin production by pancreatic beta cells 2. The rapid increase in diabetes incidence worldwide cannot be accounted for by genetics and lifestyle alone, suggesting that other environmental factors are likely contributing to diabetes etiology. In particular, epidemiological studies have reported an association between exposure to persistent organic pollutants (POPs) and increased diabetes incidence 3,4. However, while epidemiological studies can uncover correlative associations, basic research in model systems is required to investigate causation. Generally, POPs are lipophilic chemicals that resist degradation, leading to widespread global dispersion, bioaccumulation, and long-term release into the environment 5. Dioxins and dioxin-like compounds are a broad class of POPs that activate the aryl hydrocarbon receptor (AhR), which induces a variety of target genes, including cytochrome P450 (Cyp)1a1 and Cyp1a2. Although CYP enzymes are essential for xenobiotic metabolism, CYP-mediated substrate oxidation can also generate highly reactive intermediate metabolites that cause oxidative stress and DNA/protein damage 6. Our lab has shown that CYP1A enzymes are inducible and functional in human and mouse pancreatic islets, and that these enzymes remain active for at least two weeks following a single acute exposure to the classic dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) 7. These data indic...

Objective Exposure to persistent organic pollutants is consistently associated with increased diabetes risk in humans. We investigated the short- and long-term impact of transient low-dose dioxin (2,3,7,8-tetrachlorodibenzo- p -dioxin, TCDD) exposure during pregnancy and lactation on glucose homeostasis and beta cell function in female mice, including their response to a metabolic stressor later in life. Methods Female mice were injected with either corn oil (CO; vehicle control) or 20 ng/kg/d TCDD 2x/week throughout mating, pregnancy and lactation, and then tracked for 6–10 weeks after chemical exposure stopped. A subset of CO- and TCDD-exposed dams was then transferred to a 45% high-fat diet (HFD) or remained on a standard chow diet for an additional 11 weeks to assess the long-term effects of TCDD on adaptability to a metabolic stressor. To summarize, female mice were transiently exposed to TCDD and then subsequently tracked beyond when TCDD had been excreted to identify lasting metabolic effects of TCDD exposure. Results TCDD-exposed dams were hypoglycemic at birth but otherwise had normal glucose homeostasis during and post-TCDD exposure. However, TCDD-exposed dams on a chow diet were modestly heavier than controls starting 5 weeks after the last TCDD injection, and their weight gain accelerated after transitioning to a HFD. TCDD-exposed dams also had an accelerated onset of hyperglycemia, impaired glucose-induced plasma insulin levels, reduced islet size, increased MAFA -ve beta cells, and increased proinsulin accumulation following HFD feeding compared to controls. Overall, our study demonstrates that low-dose TCDD exposure during pregnancy has minimal effects on metabolism during the period of active exposure, but has detrimental long-term effects on metabolic adaptability to HFD feeding. Conclusions Our study suggests that transient low-dose TCDD exposure in female mice impairs metabolic adaptability to HFD feeding, demonstrating that dioxin exposure may be a contributing factor to obesity and diabetes pathogenesis in females.

Objective Human studies consistently show an association between exposure to persistent organic pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, aka “dioxin”), and increased diabetes risk. We previously showed that a single high-dose TCDD exposure (20 µg/kg) decreased plasma insulin levels in male and female mice in vivo, but effects on glucose homeostasis were sex-dependent. The current study assessed whether prolonged exposure to a physiologically relevant low-dose of TCDD impacts glucose homeostasis and/or the islet phenotype in a sex-dependent manner in chow-fed or high fat diet (HFD)-fed mice. Methods Male and female mice were exposed to 20 ng/kg/d TCDD 2x/week for 12 weeks and simultaneously fed standard chow or a 45% HFD. Glucose homeostasis was assessed by glucose and insulin tolerance tests, and glucose-induced plasma insulin levels were measured in vivo. Histological analysis was performed on pancreas from male and female mice, and islets were isolated from females for Tempo-Seq® analysis. Results Low-dose TCDD exposure did not lead to adverse metabolic consequences in chow-fed male or female mice, or in HFD-fed males. However, TCDD accelerated the onset of HFD-induced hyperglycemia and impaired glucose-induced plasma insulin levels in female mice. TCDD caused a modest increase in islet area in males but reduced % beta cell area within islets in females. RNAseq analysis revealed abnormal changes to endocrine and metabolic pathways in TCDDHFD females. Conclusions Our data suggest that prolonged low-dose TCDD exposure has minimal effects on glucose homeostasis and islet morphology in chow-fed male and female mice, but promotes maladaptive metabolic responses in HFD-fed females.

Exposure to environmental pollutants is consistently associated with increased diabetes risk in humans. In male mice, acute dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD) exposure supresses insulin secretion. This study investigated the long-term effects of a single TCDD injection (20 µg/kg) on glucose metabolism and beta cell function in male and female mice. TCDD-exposed males displayed modest fasting hypoglycemia for ~4 weeks post-injection, reduced fasting insulin levels for up to 6 weeks, increased insulin sensitivity, and decreased beta cell area. TCDD-exposed females also had long-term suppressed basal plasma insulin levels, and abnormal insulin secretion for up to 6 weeks. Unlike males, TCDD did not impact insulin sensitivity or beta cell area in females, but did cause transient glucose intolerance 4 weeks post-exposure. Our results show that a single exposure to dioxin can supress basal insulin levels long-term in both sexes, but effects on metabolism are sex-dependent.