Pan Liu1,2,3, Jia Song1,2,3, He Liu1,2,3, Fei Yan1,2,3, Tianyi He1,2,3, Lingshu Wang1,2,3, Huying Shen4, Xinguo Hou5,6,7, Li Chen8,9,10. 1. Department of Endocrinology, Qilu Hospital of Shandong University, Jinan, 250000, China. 2. Institute of Endocrinology and Metabolism, Shandong University, Jinan, 250000, China. 3. Key Laboratory of Endocrinology and Metabolism, Shandong Province in Medicine & Health, Jinan, 250000, China. 4. Baylor College of Medicine, Houston, TX, USA. 5. Department of Endocrinology, Qilu Hospital of Shandong University, Jinan, 250000, China. houxinguo@foxmail.com. 6. Institute of Endocrinology and Metabolism, Shandong University, Jinan, 250000, China. houxinguo@foxmail.com. 7. Key Laboratory of Endocrinology and Metabolism, Shandong Province in Medicine & Health, Jinan, 250000, China. houxinguo@foxmail.com. 8. Department of Endocrinology, Qilu Hospital of Shandong University, Jinan, 250000, China. chenli3@medmail.com.cn. 9. Institute of Endocrinology and Metabolism, Shandong University, Jinan, 250000, China. chenli3@medmail.com.cn. 10. Key Laboratory of Endocrinology and Metabolism, Shandong Province in Medicine & Health, Jinan, 250000, China. chenli3@medmail.com.cn.
Abstract
PURPOSE: Proglucagon is expressed in both pancreatic alpha cells and intestinal epithelial L cells and is cleaved into glucagon and glucagon-like peptide-1 (GLP-1) by different prohormone convertases (PCs). Recent studies have shown that α-cells can also secrete GLP-1, which may improve islet function. However, little is known about the factors influencing GLP-1 secretion by α cells. In this study, we investigated whether insulin promotes GLP-1 secretion by α cells, as well as the mechanisms underlying this phenomenon. METHODS: We cultured the alpha-cell line In-R1-G9 in low- or high-glucose medium in the presence or absence of insulin to determine the influence of glucose concentrations on the actions of insulin. We also treated In-R1-G9 cells with insulin for different times and at different doses. Then GLP-1 and glucagon protein expression levels were estimated. Moreover, ERK and phosphatidylinositol-3-kinase/AKT (PI3K/AKT) pathway activity levels and prohormone convertase expression levels were evaluated to elucidate the mechanism underlying the effects of insulin on GLP-1 secretion by α-cells. RESULTS: Insulin promoted GLP-1 secretion in a time- and dose-dependent manner under high-glucose conditions. Inhibiting the PI3K/AKT pathway with LY294002 and the Ras/mitogen-activated protein kinase (RAS/MAPK) pathway with PD98059 reduced GLP-1 secretion, respectively, in inhibitor-treated cells compared with insulin-treated cells. Moreover, insulin increased prohormone convertase 1/3 expression levels in the corresponding group of IN-R1-G9 cells compared with the control group of cells. CONCLUSION: Insulin facilitates GLP-1 secretion by pancreatic alpha cells by inducing PC1/3 expression under high-glucose conditions, a phenomenon that may be associated mainly with PI3K/AKT pathway activation.
PURPOSE: Proglucagon is expressed in both pancreatic alpha cells and intestinal epithelial L cells and is cleaved into glucagon and glucagon-like peptide-1 (GLP-1) by different prohormone convertases (PCs). Recent studies have shown that α-cells can also secrete GLP-1, which may improve islet function. However, little is known about the factors influencing GLP-1 secretion by α cells. In this study, we investigated whether insulin promotes GLP-1 secretion by α cells, as well as the mechanisms underlying this phenomenon. METHODS: We cultured the alpha-cell line In-R1-G9 in low- or high-glucose medium in the presence or absence of insulin to determine the influence of glucose concentrations on the actions of insulin. We also treated In-R1-G9 cells with insulin for different times and at different doses. Then GLP-1 and glucagon protein expression levels were estimated. Moreover, ERK and phosphatidylinositol-3-kinase/AKT (PI3K/AKT) pathway activity levels and prohormone convertase expression levels were evaluated to elucidate the mechanism underlying the effects of insulin on GLP-1 secretion by α-cells. RESULTS:Insulin promoted GLP-1 secretion in a time- and dose-dependent manner under high-glucose conditions. Inhibiting the PI3K/AKT pathway with LY294002 and the Ras/mitogen-activated protein kinase (RAS/MAPK) pathway with PD98059 reduced GLP-1 secretion, respectively, in inhibitor-treated cells compared with insulin-treated cells. Moreover, insulin increased prohormone convertase 1/3 expression levels in the corresponding group of IN-R1-G9 cells compared with the control group of cells. CONCLUSION:Insulin facilitates GLP-1 secretion by pancreatic alpha cells by inducing PC1/3 expression under high-glucose conditions, a phenomenon that may be associated mainly with PI3K/AKT pathway activation.
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