Under conditions that cause endothelium injury, such as hypertension, high cholesterol levels and turbulent blood flow, replication of endothelial cells is increased for regenerating the damaged endothelium. 3 However, the regenerated endothelial cells usually reach replicative senescence after a finite number of cell replication. Senescent endothelial cells show diminished vasomotor-regulatory activities and elevated expression of proinflammatory molecules, which contribute to the development of age-associated cardiovascular diseases, such as atherosclerosis. 4 SIRT1 is a class III deacetylase implicated in a wide range of cellular functions. 5 This protein has attracted enormous interest since the discovery of the potential antiaging activities of its yeast homolog, Sir2. In mammalian system, the beneficial activities of SIRT1 against aging diseases, such as those related to neuro-and cardioprotection, have been suggested by many studies. 5,6 Cardiovascular aging represents the largest portion of age-related morbidity and mortality. Therefore, it is particularly important to elucidate whether SIRT1 exerts any antiaging effects in the cardiovascular system.

Cultures from regenerated coronary endothelial cells exhibit genomic changes explaining endothelial dysfunction and suggesting facilitation of coagulation, lipid peroxidation, and extracellular matrix remodeling.

Background-Endothelial senescence represents one of the major characteristics of vascular aging and promotes the development of atherosclerosis. Sirtuin-1 (SIRT1) is an NAD-dependent deacetylase possessing antiaging activities. During the occurrence of endothelial senescence, both the expression and activity of SIRT1 are downregulated. The present study was designed to investigate the molecular mechanisms contributing to the loss-of-SIRT1 function in senescent endothelial cells. Methods and Results-After repetitive passages, primary cultures of porcine aortic endothelial cells exhibited a severe senescence phenotype. Western blotting revealed that phosphorylation of SIRT1 at serine 47 (S47) was significantly enhanced in senescent endothelial cells. S47 phosphorylation was stimulated by agents promoting senescence and attenuated by drugs with antisenescence properties. Mutation of S47 to nonphosphorable alanine (S47A) enhanced whereas replacing S47 with phospho-mimicking aspartic acid (S47D) abolished the antisenescent, growth-promoting, and LKB1-downregulating actions of SIRT1. Phosphorylation at S47 was critically involved in the nuclear retention of SIRT1 but abolished its association with the telomeric repeat-binding factor 2-interacting protein 1. Cyclin-dependent kinase 5 (CDK5) was identified as an SIRT1 kinase modulating S47 phosphorylation. Knockdown or inhibition of CDK5 reduced the number of senescent endothelial cells, promoted nuclear exportation of SIRT1, and attenuated the expression of inflammatory genes in porcine aortic endothelial cells. The truncated regulatory subunit of CDK5, P25, accumulated in senescent porcine aortic endothelial cells and atherosclerotic aortas. Long-term treatment with roscovitine, a CDK5 inhibitor, blocked the development of cellular senescence and atherosclerosis in aortas of hypercholesterolemic apolipoprotein E-deficient mice. Conclusion-CDK5-mediated