Shiqin Xiong

FoxO1 Mediates an Autofeedback Loop Regulating SIRT1 Expression

Forkhead transcription factor FoxO1 and the NAD+-dependent histone deacetylase SIRT1 are evolutionarily conserved regulators of the development of aging, oxidative stress resistance, insulin resistance, and metabolism in species ranging from invertebrates to mammals. SIRT1 deacetylates FoxO1 and enables activation of FoxO1 transcription in multiple systems. The functional consequences of the interactions between FoxO1 and SIRT1 remain incompletely understood. Here, we demonstrate that the 1.5-kb rat sirt1 promoter region contains a cluster of five putative FoxO1 core binding repeat motifs (5×IRS-1) and a forkhead-like consensus binding site (FKHD-L). Luciferase promoter assays demonstrate that FoxO1 directly activates SIRT1 promoter activity and that both the IRS-1 and FKHD-L enable FoxO1-dependent SIRT1 transcription. Electrophoretic mobility shift and chromatin immunoprecipitation assays show that FoxO1 binds to the IRS-1 and FKHD-L sites of the SIRT1 promoter. Consistently, FoxO1 overexpression increases SIRT1 expression, and FoxO1 depletion by siRNA reduces SIRT1 expression at both the messenger RNA and protein levels in vascular smooth muscle cells and HEK293 cells. Thus, endogenous FoxO1 is a positive transcriptional regulator of SIRT1. Conversely, SIRT1 promotes FoxO1-driven SIRT1 autotranscription through interacting with and deacetylating FoxO1. Moreover, resveratrol, a plant polyphenol activator of SIRT1, increases FoxO1-dependent SIRT1 transcription activity and thus induces its expression. These findings suggest that positive feedback mechanisms regulate FoxO1-dependent SIRT1 transcription and indicate a previously unappreciated function for FoxO1. This signaling network may coordinate multiple pathways acting upon immune, inflammatory, regenerative, and metabolic processes.

Metformin Beyond Diabetes: Pleiotropic Benefits of Metformin in Attenuation of Atherosclerosis

Background Clinical studies show that metformin attenuates all‐cause mortality and myocardial infarction compared with other medications for type 2 diabetes, even at similar glycemic levels. However, there is paucity of data in the euglycemic state on the vasculoprotective effects of metformin. The objectives of this study are to evaluate the effects of metformin on ameliorating atherosclerosis. Methods and Results Using ApoE−/− C57BL/6J mice, we found that metformin attenuates atherosclerosis and vascular senescence in mice fed a high‐fat diet and prevents the upregulation of angiotensin II type 1 receptor by a high‐fat diet in the aortas of mice. Thus, considering the known deleterious effects of angiotensin II mediated by angiotensin II type 1 receptor, the vascular benefits of metformin may be mediated, at least in part, by angiotensin II type 1 receptor downregulation. Moreover, we found that metformin can cause weight loss without hypoglycemia. We also found that metformin increases the antioxidant superoxide dismutase‐1. Conclusion Pleiotropic effects of metformin ameliorate atherosclerosis and vascular senescence.

Early Endosomal Antigen 1 (EEA1) Is an Obligate Scaffold for Angiotensin II-induced, PKC-α-dependent Akt Activation in Endosomes

Akt/protein kinase B (PKB) activation/phosphorylation by angiotensin II (Ang II) is a critical signaling event in hypertrophy of vascular smooth muscle cells (VSMCs). Conventional wisdom asserts that Akt activation occurs mainly in plasma membrane domains. Recent evidence that Akt activation may take place within intracellular compartments challenges this dogma. The spatial identity and mechanistic features of these putative signaling domains have not been defined. Using cell fractionation and fluorescence methods, we demonstrate that the early endosomal antigen-1 (EEA1)-positive endosomes are a major site of Ang II-induced Akt activation. Akt moves to and is activated in EEA1 endosomes. The expression of EEA1 is required for phosphorylation of Akt at both Thr-308 and Ser-473 as well as for phosphorylation of its downstream targets mTOR and S6 kinase, but not for Erk1/2 activation. Both Akt and phosphorylated Akt (p-Akt) interact with EEA1. We also found that PKC-α is required for organizing Ang II-induced, EEA1-dependent Akt phosphorylation in VSMC early endosomes. EEA1 expression enables PKC-α phosphorylation, which in turn regulates Akt upstream signaling kinases, PDK1 and p38 MAPK. Our results indicate that PKC-α is a necessary regulator of EEA1-dependent Akt signaling in early endosomes. Finally, EEA1 down-regulation or expression of a dominant negative mutant of PKC-α blunts Ang II-induced leucine incorporation in VSMCs. Thus, EEA1 serves a novel function as an obligate scaffold for Ang II-induced Akt activation in early endosomes.

Peroxisome Proliferator-Activated Receptor γ Coactivator-1α Is a Central Negative Regulator of Vascular Senescence

Objective—Cellular senescence influences organismal aging and increases predisposition to age-related diseases, in particular cardiovascular disease, a leading cause of death and disability worldwide. Peroxisome proliferator-activated receptor &ggr; coactivator-1&agr; (PGC-1&agr;) is a master regulator of mitochondrial biogenesis and function, oxidative stress, and insulin resistance. Senescence is associated with telomere and mitochondrial dysfunction and oxidative stress, implying a potential causal role of PGC-1&agr; in senescence pathogenesis. Approach and Results—We generated a PGC-1&agr;+/–/apolipoprotein E–/– mouse model and showed that PGC-1&agr; deficiency promotes a vascular senescence phenotype that is associated with increased oxidative stress, mitochondrial abnormalities, and reduced telomerase activity. PGC-1&agr; disruption results in reduced expression of the longevity-related deacetylase sirtuin 1 (SIRT1) and the antioxidant catalase, and increased expression of the senescence marker p53 in aortas. Further, angiotensin II, a major hormonal inducer of vascular senescence, induces prolonged lysine acetylation of PGC-1&agr; and releases the PGC-1&agr;–FoxO1 complex from the SIRT1 promoter, thus reducing SIRT1 expression. The phosphorylation-defective mutant PGC-1&agr; S570A is not acetylated, is constitutively active for forkhead box O1-dependent SIRT1 transcription, and prevents angiotensin II–induced senescence. Acetylation of PGC-1&agr; by angiotensin II interrupts the PGC-1&agr;–forkhead box O1–SIRT1 feed-forward signaling circuit leading to SIRT1 and catalase downregulation and vascular senescence. Conclusions—PGC-1&agr; is a primary negative regulator of vascular senescence. Moreover, the central role of posttranslational modification of PGC-1&agr; in regulating angiotensin II–induced vascular senescence may inform development of novel therapeutic strategies for mitigating age-associated diseases, such as atherosclerosis.

PGC-1α Modulates Telomere Function and DNA Damage in Protecting against Aging-Related Chronic Diseases

Cellular senescence and organismal aging predispose age-related chronic diseases, such as neurodegenerative, metabolic, and cardiovascular disorders. These diseases emerge coincidently from elevated oxidative/electrophilic stress, inflammation, mitochondrial dysfunction, DNA damage, and telomere dysfunction and shortening. Mechanistic linkages are incompletely understood. Here, we show that ablation of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) accelerates vascular aging and atherosclerosis, coinciding with telomere dysfunction and shortening and DNA damage. PGC-1α deletion reduces expression and activity of telomerase reverse transcriptase (TERT) and increases p53 levels. Ectopic expression of PGC-1α coactivates TERT transcription and reverses telomere malfunction and DNA damage. Furthermore, alpha lipoic acid (ALA), a non-dispensable mitochondrial cofactor, upregulates PGC-1α-dependent TERT and the cytoprotective Nrf-2-mediated antioxidant/electrophile-responsive element (ARE/ERE) signaling cascades, and counteracts high-fat-diet-induced, age-dependent arteriopathy. These results illustrate the pivotal importance of PGC-1α in ameliorating senescence, aging, and associated chronic diseases, and may inform novel therapeutic approaches involving electrophilic specificity.