Aims: Recently, incretin therapy has attracted increasing attention because of its potential use in tissue-protective therapy. Neuron-derived orphan receptor 1 (NOR1) is a nuclear orphan receptor that regulates vascular smooth muscle cell (VSMC) proliferation. In the present study, we investigated the vascular-protective effect of Exendin-4 (Ex-4), a glucagon-like peptide-1 receptor agonist, by inhibiting NOR1 expression in VSMCs.
Methods: We classified 7-week-old male 129X1/SvJ mice into control group and Ex-4 low- and high-dose-treated groups fed normal or high-fat diets, respectively. Endothelial denudation injuries were induced in the femoral artery at 8 weeks of age, followed by the evaluation of neointima formation at 12 weeks of age. To evaluate VSMC proliferation, bromodeoxyuridine incorporation assay and cell cycle distribution analysis were performed. NOR1 and cell cycle regulators were detected using immunohistochemistry, western blotting, quantitative reverse-transcription polymerase chain reaction, and luciferase assays.
Results: Ex-4 treatment reduced vascular injury-induced neointima formation compared with controls. In terms of VSMCs occupying the neointima area, VSMC numbers and NOR1-expressing proliferative cells were significantly decreased by Ex-4 in a dose-dependent manner in both diabetic and non-diabetic mice. In vitro experiments using primary cultured VSMCs revealed that Ex-4 attenuated NOR1 expression by reducing extracellular signal-regulated kinase-mitogen-activated protein kinase and cAMP-responsive element-binding protein phosphorylations. Furthermore, in the cell cycle distribution analysis, serum-induced G1–S phase entry was significantly attenuated by Ex-4 treatment of VSMCs by inhibiting the induction of S-phase kinase-associated protein 2.
Conclusion: Ex-4 attenuates neointima formation after vascular injury and VSMC proliferation possibly by inhibiting NOR1 expression.