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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.

Highlights • Histamine H3 receptor is expressed in pancreatic α-cells. • Histamine H3 receptor negatively regulates glucagon secretion from αTC1.6 cells. • Immepip, a selective H3 receptor agonist, decreases serum glucagon concentration in rats., Pancreatic α-cells secrete glucagon to maintain energy homeostasis. Although histamine has an important role in energy homeostasis, the expression and function of histamine receptors in pancreatic α-cells remains unknown. We found that the histamine H3 receptor (H3R) was expressed in mouse pancreatic α-cells and αTC1.6 cells, a mouse pancreatic α-cell line. H3R inhibited glucagon secretion from αTC1.6 cells by inhibiting an increase in intracellular Ca2+ concentration. We also found that immepip, a selective H3R agonist, decreased serum glucagon concentration in rats. These results suggest that H3R modulates glucagon secretion from pancreatic α-cells.