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Glucagon-like peptide-1 protects beta-cells against apoptosis by increasing the activity of an IGF-2/IGF-1 receptor autocrine loop.

Cornu M, Yang JY, Jaccard E, Poussin C, Widmann C, Thorens B - Diabetes (2009)

Bottom Line: We found that IGF-1 receptor expression was markedly reduced in the mutant islets.We found that GLP-1 robustly stimulated IGF-1 receptor expression and Akt phosphorylation and that increased Akt phosphorylation was dependent on IGF-1 but not insulin receptor expression.We demonstrated, both in MIN6 cell line and primary beta-cells, that reducing IGF-1 receptor or IGF-2 expression or neutralizing secreted IGF-2 suppressed GLP-1-induced protection against apoptosis.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Center for Integrative Genomics, University of Lausanne, Lausanne, Switzerland.

ABSTRACT

Objective: The gluco-incretin hormones glucagon-like peptide (GLP)-1 and gastric inhibitory peptide (GIP) protect beta-cells against cytokine-induced apoptosis. Their action is initiated by binding to specific receptors that activate the cAMP signaling pathway, but the downstream events are not fully elucidated. Here we searched for mechanisms that may underlie this protective effect.

Research design and methods: We performed comparative transcriptomic analysis of islets from control and GipR(-/-);Glp-1-R(-/-) mice, which have increased sensitivity to cytokine-induced apoptosis. We found that IGF-1 receptor expression was markedly reduced in the mutant islets. Because the IGF-1 receptor signaling pathway is known for its antiapoptotic effect, we explored the relationship between gluco-incretin action, IGF-1 receptor expression and signaling, and apoptosis.

Results: We found that GLP-1 robustly stimulated IGF-1 receptor expression and Akt phosphorylation and that increased Akt phosphorylation was dependent on IGF-1 but not insulin receptor expression. We demonstrated that GLP-1-induced Akt phosphorylation required active secretion, indicating the presence of an autocrine activation mechanism; we showed that activation of IGF-1 receptor signaling was dependent on the secretion of IGF-2. We demonstrated, both in MIN6 cell line and primary beta-cells, that reducing IGF-1 receptor or IGF-2 expression or neutralizing secreted IGF-2 suppressed GLP-1-induced protection against apoptosis.

Conclusions: An IGF-2/IGF-1 receptor autocrine loop operates in beta-cells. GLP-1 increases its activity by augmenting IGF-1 receptor expression and by stimulating secretion; this mechanism is required for GLP-1-induced protection against apoptosis. These findings may lead to novel ways of preventing beta-cell loss in the pathogenesis of diabetes.

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GLP-1–induced Akt phosphorylation depends on IGF-1R but not IR expression. A: MIN6 cells were transfected with control (Un, lanes 3 and 6), IGF-1R (lanes 4 and 7), or IR-specific (lanes 5 and 8) siRNAs and exposed (+) or not (−) to GLP-1 for 18 h before Western analysis. Lanes 1 and 2 show the induction by GLP-1 of IGF-1R expression in nontransfected cells. Lane 3 shows the basal level of IGF-1R and IR expression in transfected cells; reducing IGF-1R expression (lane 4) but not the IR (lane 5) reduced Akt phosphorylation. Lane 6: GLP-1–treated cells showed higher expression of the IGF-1R but not of the IR; reducing IGF-1R expression (lane 7) but not IR (lane 8) reduced Akt phosphorylation. Bottom panel: Quantitation of the data. Data are means ± SD, n = 3 independent experiments. **P < 0.01; ***P < 0.001. B: MIN6 cells were treated or not for 18 h with GLP-1 to increase IGF-1R expression, then incubated with 2 mmol/l glucose for 2 h, and exposed for 15 min to the indicated concentrations of insulin or IGF-2. IGF-1R and total and phosphorylated Akt were determined by Western blot analysis. C: Quantification of the data in B. *P < 0.05, ***P < 0.001 for IGF-2 vs. insulin; §§P < 0.01, §§§P < 0.001 for IGF-2 in control vs. GLP-1–treated cells; ###P < 0.001 for IGF-2 vs. insulin in GLP-1–treated cells. a.u., arbitrary units.
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Figure 3: GLP-1–induced Akt phosphorylation depends on IGF-1R but not IR expression. A: MIN6 cells were transfected with control (Un, lanes 3 and 6), IGF-1R (lanes 4 and 7), or IR-specific (lanes 5 and 8) siRNAs and exposed (+) or not (−) to GLP-1 for 18 h before Western analysis. Lanes 1 and 2 show the induction by GLP-1 of IGF-1R expression in nontransfected cells. Lane 3 shows the basal level of IGF-1R and IR expression in transfected cells; reducing IGF-1R expression (lane 4) but not the IR (lane 5) reduced Akt phosphorylation. Lane 6: GLP-1–treated cells showed higher expression of the IGF-1R but not of the IR; reducing IGF-1R expression (lane 7) but not IR (lane 8) reduced Akt phosphorylation. Bottom panel: Quantitation of the data. Data are means ± SD, n = 3 independent experiments. **P < 0.01; ***P < 0.001. B: MIN6 cells were treated or not for 18 h with GLP-1 to increase IGF-1R expression, then incubated with 2 mmol/l glucose for 2 h, and exposed for 15 min to the indicated concentrations of insulin or IGF-2. IGF-1R and total and phosphorylated Akt were determined by Western blot analysis. C: Quantification of the data in B. *P < 0.05, ***P < 0.001 for IGF-2 vs. insulin; §§P < 0.01, §§§P < 0.001 for IGF-2 in control vs. GLP-1–treated cells; ###P < 0.001 for IGF-2 vs. insulin in GLP-1–treated cells. a.u., arbitrary units.

Mentions: Because β-cells also express the IR and secrete insulin, we searched for direct evidence that the IGF-1R, and not the IR, was responsible for the induction of Akt phosphorylation. First, we transfected MIN6 cells with IGF-1R– or IR-specific siRNAs before treating the cells with GLP-1 for 18 h and analyzing Akt phosphorylation. Figure 3A (lanes 1 and 2) shows that GLP-1 induced expression of the IGF-1R but not the IR. The receptor-specific, but not the unrelated, siRNAs led to a reduction of either IGF-1R or IR expression in the basal state (in the absence of GLP-1 stimulation; Fig. 2A, lanes 3–5) or after GLP-1 treatment (Fig. 2A, lanes 6–8). Importantly, Akt phosphorylation in the basal state or after GLP-1 treatment was reduced only when the IGF-1R, but not the IR expression, was decreased (lanes 4 and 7 vs. lanes 5 and 8).


Glucagon-like peptide-1 protects beta-cells against apoptosis by increasing the activity of an IGF-2/IGF-1 receptor autocrine loop.

Cornu M, Yang JY, Jaccard E, Poussin C, Widmann C, Thorens B - Diabetes (2009)

GLP-1–induced Akt phosphorylation depends on IGF-1R but not IR expression. A: MIN6 cells were transfected with control (Un, lanes 3 and 6), IGF-1R (lanes 4 and 7), or IR-specific (lanes 5 and 8) siRNAs and exposed (+) or not (−) to GLP-1 for 18 h before Western analysis. Lanes 1 and 2 show the induction by GLP-1 of IGF-1R expression in nontransfected cells. Lane 3 shows the basal level of IGF-1R and IR expression in transfected cells; reducing IGF-1R expression (lane 4) but not the IR (lane 5) reduced Akt phosphorylation. Lane 6: GLP-1–treated cells showed higher expression of the IGF-1R but not of the IR; reducing IGF-1R expression (lane 7) but not IR (lane 8) reduced Akt phosphorylation. Bottom panel: Quantitation of the data. Data are means ± SD, n = 3 independent experiments. **P < 0.01; ***P < 0.001. B: MIN6 cells were treated or not for 18 h with GLP-1 to increase IGF-1R expression, then incubated with 2 mmol/l glucose for 2 h, and exposed for 15 min to the indicated concentrations of insulin or IGF-2. IGF-1R and total and phosphorylated Akt were determined by Western blot analysis. C: Quantification of the data in B. *P < 0.05, ***P < 0.001 for IGF-2 vs. insulin; §§P < 0.01, §§§P < 0.001 for IGF-2 in control vs. GLP-1–treated cells; ###P < 0.001 for IGF-2 vs. insulin in GLP-1–treated cells. a.u., arbitrary units.
© Copyright Policy - creative-commons
Related In: Results  -  Collection

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Figure 3: GLP-1–induced Akt phosphorylation depends on IGF-1R but not IR expression. A: MIN6 cells were transfected with control (Un, lanes 3 and 6), IGF-1R (lanes 4 and 7), or IR-specific (lanes 5 and 8) siRNAs and exposed (+) or not (−) to GLP-1 for 18 h before Western analysis. Lanes 1 and 2 show the induction by GLP-1 of IGF-1R expression in nontransfected cells. Lane 3 shows the basal level of IGF-1R and IR expression in transfected cells; reducing IGF-1R expression (lane 4) but not the IR (lane 5) reduced Akt phosphorylation. Lane 6: GLP-1–treated cells showed higher expression of the IGF-1R but not of the IR; reducing IGF-1R expression (lane 7) but not IR (lane 8) reduced Akt phosphorylation. Bottom panel: Quantitation of the data. Data are means ± SD, n = 3 independent experiments. **P < 0.01; ***P < 0.001. B: MIN6 cells were treated or not for 18 h with GLP-1 to increase IGF-1R expression, then incubated with 2 mmol/l glucose for 2 h, and exposed for 15 min to the indicated concentrations of insulin or IGF-2. IGF-1R and total and phosphorylated Akt were determined by Western blot analysis. C: Quantification of the data in B. *P < 0.05, ***P < 0.001 for IGF-2 vs. insulin; §§P < 0.01, §§§P < 0.001 for IGF-2 in control vs. GLP-1–treated cells; ###P < 0.001 for IGF-2 vs. insulin in GLP-1–treated cells. a.u., arbitrary units.
Mentions: Because β-cells also express the IR and secrete insulin, we searched for direct evidence that the IGF-1R, and not the IR, was responsible for the induction of Akt phosphorylation. First, we transfected MIN6 cells with IGF-1R– or IR-specific siRNAs before treating the cells with GLP-1 for 18 h and analyzing Akt phosphorylation. Figure 3A (lanes 1 and 2) shows that GLP-1 induced expression of the IGF-1R but not the IR. The receptor-specific, but not the unrelated, siRNAs led to a reduction of either IGF-1R or IR expression in the basal state (in the absence of GLP-1 stimulation; Fig. 2A, lanes 3–5) or after GLP-1 treatment (Fig. 2A, lanes 6–8). Importantly, Akt phosphorylation in the basal state or after GLP-1 treatment was reduced only when the IGF-1R, but not the IR expression, was decreased (lanes 4 and 7 vs. lanes 5 and 8).

Bottom Line: We found that IGF-1 receptor expression was markedly reduced in the mutant islets.We found that GLP-1 robustly stimulated IGF-1 receptor expression and Akt phosphorylation and that increased Akt phosphorylation was dependent on IGF-1 but not insulin receptor expression.We demonstrated, both in MIN6 cell line and primary beta-cells, that reducing IGF-1 receptor or IGF-2 expression or neutralizing secreted IGF-2 suppressed GLP-1-induced protection against apoptosis.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Center for Integrative Genomics, University of Lausanne, Lausanne, Switzerland.

ABSTRACT

Objective: The gluco-incretin hormones glucagon-like peptide (GLP)-1 and gastric inhibitory peptide (GIP) protect beta-cells against cytokine-induced apoptosis. Their action is initiated by binding to specific receptors that activate the cAMP signaling pathway, but the downstream events are not fully elucidated. Here we searched for mechanisms that may underlie this protective effect.

Research design and methods: We performed comparative transcriptomic analysis of islets from control and GipR(-/-);Glp-1-R(-/-) mice, which have increased sensitivity to cytokine-induced apoptosis. We found that IGF-1 receptor expression was markedly reduced in the mutant islets. Because the IGF-1 receptor signaling pathway is known for its antiapoptotic effect, we explored the relationship between gluco-incretin action, IGF-1 receptor expression and signaling, and apoptosis.

Results: We found that GLP-1 robustly stimulated IGF-1 receptor expression and Akt phosphorylation and that increased Akt phosphorylation was dependent on IGF-1 but not insulin receptor expression. We demonstrated that GLP-1-induced Akt phosphorylation required active secretion, indicating the presence of an autocrine activation mechanism; we showed that activation of IGF-1 receptor signaling was dependent on the secretion of IGF-2. We demonstrated, both in MIN6 cell line and primary beta-cells, that reducing IGF-1 receptor or IGF-2 expression or neutralizing secreted IGF-2 suppressed GLP-1-induced protection against apoptosis.

Conclusions: An IGF-2/IGF-1 receptor autocrine loop operates in beta-cells. GLP-1 increases its activity by augmenting IGF-1 receptor expression and by stimulating secretion; this mechanism is required for GLP-1-induced protection against apoptosis. These findings may lead to novel ways of preventing beta-cell loss in the pathogenesis of diabetes.

Show MeSH
Related in: MedlinePlus