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 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.

The spliceosome is a large ribonucleoprotein complex that catalyzes the removal of introns from RNA polymerase II-transcribed RNAs. Spliceosome assembly occurs in a stepwise manner through specific intermediates referred to as pre-spliceosome complexes E, A, B, B* and C. It has been reported that small molecule inhibitors of the spliceosome that target the SF3B1 protein component of complex A lead to the accumulation of cells in the G1 and G2/M phases of the cell cycle. Here we performed a comprehensive flow cytometry analysis of the effects of isoginkgetin (IGG), a natural compound that interferes with spliceosome assembly at a later step, complex B formation. We found that IGG slowed cell cycle progression in multiple phases of the cell cycle (G1, S and G2) but not M phase. This pattern was somewhat similar to but distinguishable from changes associated with an SF3B1 inhibitor, pladienolide B (PB). Both drugs led to a significant decrease in nascent DNA synthesis in S phase, indicative of an S phase arrest. However, IGG led to a much more prominent S phase arrest than PB while PB exhibited a more pronounced G1 arrest that decreased the proportion of cells in S phase as well. We also found that both drugs led to a comparable decrease in the proportion of cells in M phase. This work indicates that spliceosome inhibitors affect multiple phases of the cell cycle and that some of these effects vary in an agent-specific manner despite the fact that they target splicing at similar stages of spliceosome assembly.

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.