Post-translational modification of SIRT1 during endothelial senescence and vascular aging : molecular mechanisms and pathophysiological implications

Abstract

Endothelial senescence represents one of the major characteristics of vascular aging contributing to the development of cardiovascular diseases. SIRT1 is a NAD+-dependent enzyme catalyzing the deacetylation reaction of various signaling molecules and exerts beneficial effects against aging-associated pathologies. SIRT1 is a potent regulator antagonizing endothelial senescence. Both expression and activity of SIRT1 are down-regulated in senescent endothelial cells. However, the molecular mechanisms underlying the loss-of-SIRT1 function during the occurrence of endothelial senescence remain unknown. The present study reveals that phosphorylation at serine 47(S47) contributes to the loss-of-SIRT1 function during endothelial senescence. In both replicative and premature senescent endothelial cells, increased phosphorylation of SIRT1 at S47 was closely associated with the severity of cellular senescence. Replacing serine 47 residue with a phospho-mimicking aspartic acid residue impaired the anti-senescence activity of this protein. In addition, phosphorylation of SIRT1 at serine 47 inhibited its nuclear-cytoplasmic shuttling and protein-protein interactions with LKB1, a senescence-promoting kinase and telomeric repeat-binding factor 2–interacting protein 1, a telomere and inflammation regulator. As a result, the anti-inflammatory function of SIRT1 was also abolished by phosphorylation at serine 47. Cyclin dependent kinase 5 (CDK5) was identified as an upstream kinase responsible for phosphorylation of SIRT1 at serine 47. During the endothelial senescence, the activity of this kinase was up-regulated which was attributed to the augmented P25, a regulatory subunit of CDK5. Inhibition of this kinase by roscovitine, a CDK5 inhibitor, decreased the phosphorylation of SIRT1 at serine 47, reduced cellular senescence, promoted the cytoplasmic translocation of SIRT1 and attenuated the inflammation in endothelial cells triggered by tumor necrosis factor α. Moreover, the kinase activity of CDK5 was significantly elevated in aorta tissues of apolipoprotein E–deficient mice. Chronic administration of roscovitine alleviated endothelial senescence, vascular inflammation and the development of arterial atherosclerosis. These results collectively suggest that CDK 5 is responsible for the phosphorylation of SIRT1 at serine 47, which impairs the anti-senescence activity of enzyme and contributes to loss-of-SIRT1 function during vascular aging. By inhibiting this kinase, SIRT1 function can be improved, in turn preventing the development of endothelial senescence and slowing down the process of vascular aging. In addition to phosphorylation, I have also performed a preliminary study on the ubiquitination of SIRT1. The results demonstrated that SIRT1 ubiquitination was mediated by a Cullin-1-RING E3 ligase complex. Knocking down of cullin-1 enhanced SIRT1 protein expression, promoted proliferation and inhibited senescence in endothelial cells. This discovery may provide novel insights on the anti-vascular aging therapeutic development based on SIRT1 modification.