Abstract 781: Sirt1 Modulates Foxo-dependent Transcription and Prevents Smooth Muscle Cell Apoptosis in Response to Oxidative Stress
Background: Forkhead transcription factors represent an important physiological target of phosphatidylinositol-3 kinase/protein kinase B signaling and regulate genes that contribute both to longevity and resistance to various stresses in C. elegans. we previously demonstrated that phosphorylation of FoxO1a regulates vascular smooth muscle cells (VSMC) homeostasis. However, the effect of Foxo1a acetylation remains elusive. Therefore, in the present study we sought to determine the effect of Foxo1a acetylation on VSMC function.
Methods and Results: In cultured human VSMC, Foxo1a was detected in a deacetylated form only. Testing different deacetylase-inhibitors, Foxo1a acetylation was detected following the application of nicotin-amide only, indicating the involvement of a NAD-dependent deacetylase. As demonstrated by co-immunoprecipitation, the class III histone deacetylase SIRT1 physiologically interacts with Foxo1a under native conditions, which was further confirmed by FRET-technique. Activation of Sirt1 by resveratrol increased DNA-binding and transactivation capacity of Foxo1a and further induced expression of Fox1a target genes including GADD45 and Mn-SOD, known to regulate oxidative stress response. In contrast, siRNA-mediated knock-down of Sirt1 as well as the Sirt1 inhibitor sirtinol attenuated DNA-binding and transactivation capacity of Foxo1a, resulting in a reduced expression of Foxo1a target genes. Consistent with these findings, resveratrol significantly reduced VSMC apoptosis in response to oxidative stress, whereas incubation with sirtinol or knock-down of Sirt1 resulted in a significant increase in the apoptotic response. Moreover, embryonic fibroblasts (MEF) isolated from SIRT−/ − mice were more resistant to oxidative stress-induced apoptosis compared to WT-MEF cells.
Conclusion: These results indicate that SIRT1 plays a pivotal role for FOXO function via NAD-dependent deacetylation, thereby contributing to stress resistance of VSMC in response to oxidative stress. Thus, SIRT1-dependent modulation of Foxo1a plays an essential role in apoptotic processes which regulate vascular homeostasis and remodeling.