Effects of triphenyltin on cytosolic Na(+) and Ca(2+) response to glucose and acetylcholine in pancreatic beta-cells from hamster

Review on endocrine disrupting toxicity of triphenyltin from the perspective of species evolution: Aquatic, amphibious and mammalian

Triphenyltin (TPT) is widely used as a plastic stabilizer, insecticide and the most common fungicide in antifouling coatings. This paper reviewed the main literature evidences on the morphological and physiological changes of animal endocrine system induced by TPT, with emphasis on the research progress of TPT metabolism, neurological and reproductive regulation in animal endocrine system. Similar to tributyltin (TBT), the main effects of TPT on the potential health risks of 25 species of animals, from aquatic animals to mammals, are not only related to exposure dose and time, but also to age, sex and exposed tissue/cells. Moreover, current studies have shown that TPT can directly damage the endocrine glands, interfere with the regulation of neurohormones on endocrine function, and change hormone synthesis and/or the bioavailability (i.e., in the retinoid X receptor and peroxisome proliferator-activated receptor gamma RXR-PPARγ) in target cells. Importantly, TPT can cause biochemical and morphological changes of gonads and abnormal production of steroids, both of which are related to reproductive dysfunction, for example, the imposex of aquatic animals and the irregular estrous cycle of female mammals or spermatogenic disorders of male animals. Therefore, TPT should indeed be regarded as a major endocrine disruptor, which is essential for understanding the main toxic effects on different tissues and their pathogenic effects on endocrine, metabolism, neurological and reproductive dysfunction.

Organotins in obesity and associated metabolic disturbances

The objective of the present study was to review the mechanisms of organotin-induced adipogenesis, obesity, and associated metabolic disturbances. Peroxisome proliferator-activated receptor γ (PPARγ) and retinoid X receptor α (RXRα) activation is considered as the key mechanism of organotin-induced adipogenesis. Particularly, organotin exposure results in increased adipogenesis both in cell and animal models. Moreover, transgenerational inheritance of organotin-induced obese phenotype was demonstrated in vivo. At the same time, the existing data demonstrate that organotin compounds (OTCs) induces aberrant expression of PPARγ-targeted genes, resulting in altered of adipokine, glucose transporter, proinflammatory cytokines levels, and lipid and carbohydrate metabolism. The latter is generally characterized by hyperglycemia and insulin resistance. Other mechanisms involved in organotin-induced obesity may include estrogen receptor and corticosteroid signaling, altered DNA methylation, and gut dysfunction. In addition to cellular effects, organotin exposure may also affect neural circuits of appetite regulation, being characterized by neuropeptide Y (NPY) up-regulation in parallel with of pro-opiomelanocortin (POMC), Agouti-related protein (AgRP), and cocaine and amphetamine regulated transcript (CART) down-regulation in the arcuate nucleus. These changes result in increased orexigenic and reduced anorexigenic signaling, leading to increased food intake. The existing data demonstrate that organotins are potent adipogenic agents, however, no epidemiologic studies have been performed to reveal the association between organotin exposure and obesity and the existing indirect human data are contradictory.

Prioritizing Environmental Chemicals for Obesity and Diabetes Outcomes Research: A Screening Approach Using ToxCast™ High-Throughput Data

BACKGROUND

Diabetes and obesity are major threats to public health in the United States and abroad. Understanding the role that chemicals in our environment play in the development of these conditions is an emerging issue in environmental health, although identifying and prioritizing chemicals for testing beyond those already implicated in the literature is challenging. This review is intended to help researchers generate hypotheses about chemicals that may contribute to diabetes and to obesity-related health outcomes by summarizing relevant findings from the U.S. Environmental Protection Agency (EPA) ToxCast™ high-throughput screening (HTS) program.

OBJECTIVES

Our aim was to develop new hypotheses around environmental chemicals of potential interest for diabetes- or obesity-related outcomes using high-throughput screening data.

METHODS

We identified ToxCast™ assay targets relevant to several biological processes related to diabetes and obesity (insulin sensitivity in peripheral tissue, pancreatic islet and β cell function, adipocyte differentiation, and feeding behavior) and presented chemical screening data against those assay targets to identify chemicals of potential interest.

DISCUSSION

The results of this screening-level analysis suggest that the spectrum of environmental chemicals to consider in research related to diabetes and obesity is much broader than indicated by research papers and reviews published in the peer-reviewed literature. Testing hypotheses based on ToxCast™ data will also help assess the predictive utility of this HTS platform.

CONCLUSIONS

More research is required to put these screening-level analyses into context, but the information presented in this review should facilitate the development of new hypotheses.

CITATION

Auerbach S, Filer D, Reif D, Walker V, Holloway AC, Schlezinger J, Srinivasan S, Svoboda D, Judson R, Bucher JR, Thayer KA. 2016. Prioritizing environmental chemicals for obesity and diabetes outcomes research: a screening approach using ToxCast™ high-throughput data. Environ Health Perspect 124:1141-1154; http://dx.doi.org/10.1289/ehp.1510456.

The Possible Mechanisms of the Impaired Insulin Secretion in Hypothyroid Rats

Although the insulin secretion deficit in hypothyroid male rats has been documented, the underling mechanisms of the effect of hypothyroidism on insulin secretion are not clear. Isolated islets of the PTU-induced hypothyroid and control rats were exposed to glibenclamide, acetylcholine, and nifedipine in the presence of glucose concentrations of 2.8 or 8.3 and 16.7 mmol/L. Glucokinase and hexokinase specific activity, glucokinase content, and glucose transporter 2 protein expression were also determined in the isolated islets. Isolated islets from the hypothyroid rats showed a defect in insulin secretion in response to high glucose. In the presence of glibenclamide or acetylcholine, the isolated islets from the hypothyroid and control rats stimulated by glucose concentration of 16.7 mmol/L secreted similar amounts of insulin. In the presence of glucose concentrations of 8.3 mmol/L and 16.7 mmol/L, nifedipine was able to diminish insulin secretion from isolated islets of both groups, indicating that probably the defect may not arise from L type calcium channels or the steps beyond depolarization or the elements involved in the acetylcoline signaling pathway. Glucokinase content and hexokinase specific activity were also the same in the control and hypothyroid groups. On the other hand, glucokinase specific activity and glucose transporter 2 protein expression were significantly (p<0.001 and p<0.01 respectively) lower in the islets isolated from the hypothyroid rats (6.50 ± 0.46 mU/min/mg protein and 0.55 ± 0.09 arbitrary unit) compared to the controls (10.93 ± 0.83 mU/min/mg protein and 0.98 ± 0.07 arbitrary unit) respectively. In conclusion, the results of this study indicated that hypothyroidism reduced insulin secretion from isolated pancreatic islets, which confirms the finding of the previous studies; in addition, the insulin secretion deficit observed in hypothyroid rats may arise from the abnormalities in some parts of the glucose sensor apparatus of the pancreatic islets including glucokinase activity and glucose transporter 2 protein expression.

Biosorption and biodegradation of triphenyltin by Stenotrophomonas maltophilia and their influence on cellular metabolism

Triphenyltin (TPT), an endocrine disruptor, is polluting the global environment through its worldwide use. However, information concerning the mechanisms of TPT biodegradation and cellular metabolism is severely limited. Therefore, these processes were elucidated through experiments involving TPT biosorption and degradation, intracellular metabolite analysis, nutrient use, ion and monosaccharide release, cellular membrane permeability and protein concentration quantification. The results verified that TPT was initially adsorbed by the cell surface of Stenotrophomonas maltophilia and was subsequently transported and degraded intracellularly with diphenyltin and monophenyltin production. Cl(-), Na(+), arabinose and glucose release, membrane permeability and the extracellular protein concentration increased during TPT treatment, whereas K(+) and PO4(3-) utilization and intracellular protein concentration declined. The biosorption, degradation and removal efficiencies of TPT at 0.5mgL(-1) by 0.3gL(-1) viable cells at 10 d were 3.8, 77.8 and 86.2%, respectively, and the adsorption efficiency by inactivated cells was 72.6%.

Triphenyltin impairs insulin secretion by decreasing glucose-induced NADP(H) and ATP production in hamster pancreatic β-cells

Oral administration of triphenyltin chloride (TPT) (6 mg/100g body weight) inhibits insulin secretion by decreasing glucose-induced cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) in pancreatic β-cells of the hamster. To test the possibility that the abnormal level of the [Ca(2+)](i) induced by TPT administration could be due to a defect in the metabolic signal of glucose in the β-cells, we tested the effects of TPT administration on the glucose-induced NAD(P)H and ATP production, and on the changes of membrane potential and [Ca(2+)](i) by glucose and high K(+) in the β-cells. The [Ca(2+)](i) was measured in islet cells loaded with fura-2. TPT administration significantly reduced the NAD(P)H and ATP production, the depolarization of plasma membrane, and insulin secretion by 15 mM glucose in islet cells. TPT administration also reduced the insulin secretion by 10mM dihydroxyacetone and glyceraldehyde. However, TPT administration did not affect the increase of [Ca(2+)](i) and the insulin secretion by 30 mMK(+) or 100 μM tolbutamide, and the membrane potential by 30 mMK(+), and the insulin secretion by 10mM α-ketoisocaproic acid and 0.5mM formycin A, an analog of ATP in the presence of 15 mM glucose. These results suggested that the pathogenesis of TPT-induced hyperglycemia in hamster involves the reduction of [Ca(2+)](i) and insulin secretion in response to K(ATP) channel-dependent depolarization, which is related to the decrease of NAD(P)H and ATP production in pancreatic islet cells after glucose metabolism.

Glucagon-like peptide-1 induces a cAMP-dependent increase of [Na+]i associated with insulin secretion in pancreatic beta-cells

Glucagon-like peptide-1 (GLP-1) elevates the intracellular free calcium concentration ([Ca2+]i) and insulin secretion in a Na+-dependent manner. To investigate a possible role of Na ion in the action of GLP-1 on pancreatic islet cells, we measured the glucose-and GLP-1-induced intracellular Na+ concentration ([Na+]i), [Ca2+]i, and insulin secretion in hamster islet cells in various concentrations of Na+. The [Na+]i and [Ca2+]i were monitored in islet cells loaded with sodium-binding benzofuran isophthalate and fura 2, respectively. In the presence of 135 mM Na+ and 8 mM glucose, GLP-1 (10 nM) strongly increased the [Na+]i, [Ca2+]i, and insulin secretion. In the presence of 13.5 mM Na+, both glucose and GLP-1 increased neither the [Na+]i nor the [Ca2+]i. In a Na+-free medium, GLP-1 and glucose did not increase the [Na+]i. SQ-22536, an inhibitor of adenylate cyclase, and H-89, an inhibitor of PKA, incompletely inhibited the response. In the presence of both 8 mM glucose and H-89, 8-pCPT-2'-O-Me-cAMP, a PKA-independent cAMP analog, increased the insulin secretion and the [Na+]i. Therefore, we conclude that GLP-1 increases the cAMP level via activation of adenylate cyclase, which augments the membrane Na+ permeability through PKA-dependent and PKA-independent mechanisms, thereby increasing the [Ca2+]i and promoting insulin secretion from hamster islet cells.