Glucagon-Like Peptide 1 Inhibits the Sirtuin Deacetylase SirT1 to Stimulate Pancreatic β-Cell Mass Expansion

GLP-1 inhibits SirT1 deacetylase activity. A: Cells were incubated in the presence or absence of 10 nmol/L GLP-1, 10 mmol/L nicotinamide (NAM), or 10 μmol/L resveratrol (Res). Proteins were extracted after 15 min to perform the in vitro SirT1 deacetylase assay using a fluorogenic SirT1 substrate. B and C: NAD⁺ and NADH levels were determined in extracts from cells (B) or human islets (C) incubated in the presence or absence of 10 nmol/L GLP-1, 10% serum, or 10 μmol/L resveratrol for 15 min. D: SirT1 activity was measured as described in A with the modification that the assay buffer was supplemented with 25 μmol/L NAD⁺. E: Nampt activity was measured by the conversion of ¹⁴C-labeled nicotinamide to [¹⁴C]nicotinamide mononucleotide. The specific Nampt pharmacological inhibitor FK-866 (10 nmol/L) was used as control. F: Total cellular ATP was measured in cells incubated as described above. Means ± SE of three experiments, each comprising duplicates. *P < 0.05.

GLP-1 downregulates SirT1 expression. A and B: Cells (A) and isolated rat islets (B) were incubated in serum-free medium in the absence or presence of GLP-1 or left untreated in complete growth medium for 24 h. SirT1 expression levels were measured by quantitative PCR and normalized to actin. C: SirT1 protein levels were evaluated by Western blot in INS cells treated as described in A and B. D: Densitometry analysis of three different experiments as shown in A and B. *P < 0.05.

Increased dosage of SirT1 suppresses GLP-1–induced β-cell proliferation. A: SirT1 protein levels were evaluated by Western blot after transfection with wild-type (WT) or dominant-negative (DN)-SirT1. GFP was used as control. B: The effect of SirT1 overexpression on β-cell proliferation was evaluated by BrdU incorporation. INS832/13 cells were transfected with WT-SirT1, DN-SirT1, or control GFP for 24 h and then incubated in the presence or absence of GLP-1 (10 nmol/L) or 10% serum for an additional 24-h period. Results represent mean ± SEM of three separate experiments carried out in triplicate. *P < 0.05 compared with the respective control.

The action of exendin-4 on pancreatic β-cell expansion is blunted in SirBACO mice. Wild-type (WT) or SirBACO transgenic mice received daily injections of exendin-4 (10 nmol/kg body wt) or saline for 7 days. Pancreatic β-cell mass and β-cell replication were evaluated by morphometry and immunohistochemistry with anti-insulin and anti-Ki67 antisera. A and B: The relative cross-sectional islet area (A) and the percentage of Ki67-positive β-cells (B) are shown. *P < 0.05 compared with saline controls.