Thoracic aortic aneurysm/dissection (TAAD) is characterized by excessive smooth muscle cell (SMC) loss, extra cellular matrix (ECM) degradation and inflammation. However, the mechanism whereby signaling leads to SMC loss is unclear. We used senescence-associated (SA)-beta-gal staining and analysis of expression of senescence-related proteins (p53, p21, p19) to show that excessive mechanical stretch (20% elongation, 3600 cycles/h, 48 h) induced SMC senescence. SMC senescence was also detected in TAAD specimens from both mice and humans. High-performance liquid chromatography and luciferin-luciferase-based assay revealed that excessive mechanical stretch increased adenosine diphosphate (ADP) release from SMCs both in vivo and in vitro. Elevated ADP induced SMC senescence while genetic knockout of the ADP receptor, P2Y G protein-coupled receptor 12 (P2ry12), in mice protected against SMC senescence and inflammation. Both TAAD formation and rupture were significantly reduced in P2ry12(-/-) mice. SMCs from P2ry12(-/-) mice were resistant to senescence induced by excessive mechanical stretch or ADP treatment. Mechanistically, ADP treatment sustained Ras activation, whereas pharmacological inhibition of Ras protected against SMC senescence and reduced TAAD formation. Taken together, excessive mechanical stress may induce a sustained release of ADP and promote SMC senescence via P2ry12-dependent sustained Ras activation, thereby contributing to excessive inflammation and degeneration, which provides insights into TAAD formation and progression. (C) 2016 Elsevier Ltd. All rights reserved.