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Metabolism-disrupting chemicals (MDCs) are endocrine disruptors with obesogenic and/or diabetogenic action. There is mounting evidence linking exposure to MDCs to increased susceptibility to diabetes. Despite the important role of glucagon in glucose homeostasis, there is little information on the effects of MDCs on α-cells. Furthermore, there are no methods to identify and test MDCs with the potential to alter α-cell viability and function. Here, we used the mouse α-cell line αTC1-9 to evaluate the effects of MDCs on cell viability and glucagon secretion. We tested six chemicals at concentrations within human exposure (from 0.1 pM to 1 µM): bisphenol-A (BPA), tributyltin (TBT), perfluorooctanoic acid (PFOA), triphenylphosphate (TPP), triclosan (TCS), and dichlorodiphenyldichloroethylene (DDE). Using two different approaches, MTT assay and DNA-binding dyes, we observed that BPA and TBT decreased α-cell viability via a mechanism that depends on the activation of estrogen receptors and PPARγ, respectively. These two chemicals induced ROS production, but barely altered the expression of endoplasmic reticulum (ER) stress markers. Although PFOA, TPP, TCS, and DDE did not alter cell viability nor induced ROS generation or ER stress, all four compounds negatively affected glucagon secretion. Our findings suggest that αTC1-9 cells seem to be an appropriate model to test chemicals with metabolism-disrupting activity and that the improvement of the test methods proposed herein could be incorporated into protocols for the screening of diabetogenic MDCs.

Glucose‐dependent insulinotropic polypepide (GIP) was first extracted from porcine gut mucosa and identified as “incretin” decades ago. Though early studies have shown the possible GIP isoforms by gel filtration profiles from porcine or human intestinal extracts analyzed by radioimmunoassay (RIA), GIP is currently believed to consist of 42 amino acids (GIP1‐42), which are released from gut K‐cells and promote postprandial insulin release. In fact, GIP1‐42 is usually processed from proGIP by the action of prohormone convertase (PC) 1/3 in the gut. GIP expression is occasionally found in the intestinal glucagon‐like peptide‐1‐secreting cells, suggesting gene expression of both GIP and proglucagon can co‐exist in identical cells. However, GIP1‐42 immunoreactivity is rarely found in α‐cells or other pancreatic endocrine cells of wild‐type mammals. Interestingly, we found that short‐form GIP1‐30 is expressed in and released from pancreatic α‐cells and a subset of enteroendocrine cells through proGIP processing by PC2. GIP1‐30 is also insulinotropic and modulates glucose‐stimulated insulin secretion in a paracrine manner. It is also suggested that short‐form GIP1‐30 possibly plays a crucial role for the islet development. It has not been well elucidated whether expression of GIP1‐30 is modulated in the diabetic status, and whether GIP1‐30 might have therapeutic potentials. Our preliminary data suggest that short‐form GIP1‐30 might play important roles in glucose metabolism.

Accumulating evidence supports the "glucagonocentric hypothesis", in which antecedent α-cell failure and inhibition of glucagon secretion are responsible for diabetes progression. Protein kinase C (PKC) is involved in glucagon secretion from α-cells, although which PKC isozyme is involved and the mechanism underlying this PKC-regulated glucagon secretion remains unknown. Here, the involvement of PKCδ in the onset and progression of diabetes was elucidated. Immunofluorescence studies revealed that PKCδ was expressed and activated in α-cells of STZ-induced diabetic model mice. Phorbol 12-myristate 13-acetate (PMA) stimulation significantly augmented glucagon secretion from isolated islets. Pre-treatment with quercetin and rottlerin, PKCδ signaling inhibitors, significantly suppressed the PMA-induced elevation of glucagon secretion. While Go6976, a Ca2+-dependent PKC selective inhibitor did not suppress glucagon secretion. Quercetin suppressed PMA-induced phosphorylation of Tyr311 of PKCδ in isolated islets. However, quercetin itself had no effect on either glucagon secretion or glucagon mRNA expression. Our data suggest that PKCδ signaling inhibitors suppressed glucagon secretion. Elucidation of detailed signaling pathways causing PKCδ activation in the onset and progression of diabetes followed by the augmentation of glucagon secretion could lead to the identification of novel therapeutic target molecules and the development of novel therapeutic drugs for diabetes. J. Med. Invest. 64: 122-128, February, 2017.