Effect of glucagon on insulin secretion through cAMP signaling pathway in MIN6 cells

Objective: To explore the direct regulation effects and mechanisms of glucagon in insulin secretion of MIN6 cells that in the kind of the islet beta cells. Methods ICUE3 and PCDNA3.1 plasmid were transfected to the MIN6 cells by electroporation transfection, and then treated with different concentrations of glucagon (Glg) and glucose (Glu). Biosensor technology that based on the fluorescence resonance energy transfer (FRET) was used to monitor the change of cAMP quantitatively and real-time. The level of cAMP and insulin were measured by the enzyme-linked immunosorbent assay (ELISA). Results: The receptor of Glg was mainly located on the cell membrane in MIN6 cells. Compared with the 0 ng/L Glg group in the Glu-free state, the average value of CFP/YFP increased 4% +/- 0.02 in the 500 ng/L Glg group, and the value in the 1000 ng/L Glg group increased 6% +/- 0.03 (P > 0.05). While in the high-Glu (16.7 mmol/L) state, the value increased 11% +/- 0.02 in the 500 ng/L Glg group, and increased 23% +/- 0.06 in the 1000 ng/L Glg group when compared with the 0 ng/L Glg group(P < 0.01). The levels of the cAMP of 1000 ng/L and 500 ng/L Glg group were higher than those of the 100 ng/L and 0 ng/L Glg group in the condition of Glu-free (81.27 +/- 6.29, 76.73 +/- 2.10,39.45 +/- 2.83, 40.36 +/- 4.20; P < 0.01). The levels of the cAMP of 1000 ng/L, 500 ng/L and 100 ng/L Glg group were higher than those of the 0 ng/L Glg group, at the meanwhile, the levels of the cAMP of 1000 ng/L and 500 ng/L Glg group were also higher than 100 ng/L Glg group in the condition of low-Glu (2.8 mmol/L) (92.91 +/- 7.35, 90.36 +/- 3.15, 65.82 +/- 10.49, 46.73 +/- 1.05; P < 0.01). And this trend in the condition of high-Glu was almost to the low-Glu (106.75 +/- 7.26, 94.18 +/- 2.99, 83.09 +/- 1.16, 55.60 +/- 5.51, P < 0.01). The levels of the insulin of 1000 ng/L, 500 ng/L and 100 ng/L Glg group were higher than those of the 0 ng/L Glg group. While 1000 ng/L Glg group was higher than that of the 500 ng/L and 100 ng/L Glg group in the condition of Glu-free (1844.02 +/- 200.93, 1387.94 +/- 483.12, 1251.817 +/- 60.30, 787.33 +/- 81.72; P < 0.01). The levels of the insulin of 1000 ng/L and 500 ng/L Glg group were higher than those of the 100 ng/L and 0 ng/L Glg group, and the 1000 ng/L and was also higher than 500 ng/L Glg group in the condition of low-Glu (1552.31 +/- 81.20, 1285.62 +/- 131.67, 1020.85 +/- 42.60, 762.89 +/- 26.94, P < 0.01). And this trend in the condition of high-Glu was almost to the low-Glu (1898.337 +/- 169.03, 1399.30 +/- 148.66, 1061.735 +/- 9.13, 972.89 +/- 22.19; P < 0.01). The levels of cAMP and insulin secretion of MIN6 cells had a positive correlation in different Glu conditions (r(2) = 0.559, P < 0.01). Conclusion: Glg may stimulate insulin secretion by increasing cAMP levels in the way of concentration gradient within the islet beta cell lines-MIN6 cells. And the increasing trend was Glu dependent.