Thrombospondin-1 signaling through CD47 inhibits cell cycle progression and induces senescence in endothelial cells

CD47 signaling in endothelial cells has been shown to suppress angiogenesis, but little is known about the link between CD47 and endothelial senescence. Herein, we demonstrate that the thrombospondin-1 (TSP1)-CD47 signaling pathway is a major mechanism for driving endothelial cell senescence. CD47 deficiency in endothelial cells significantly improved their angiogenic function and attenuated their replicative senescence. Lack of CD47 also suppresses activation of cell cycle inhibitors and upregulates the expression of cell cycle promoters, leading to increased cell cycle progression. Furthermore, TSP1 significantly accelerates replicative senescence and associated cell cycle arrest in a CD47-dependent manner. These findings demonstrate that TSP1-CD47 signaling is an important mechanism driving endothelial cell senescence. Thus, TSP1 and CD47 provide attractive molecular targets for treatment of aging-associated cardiovascular dysfunction and diseases involving endothelial dysregulation.Endothelial cell (EC) senescence is accompanied with vascular dysfunction, including arterial stiffening and remodeling,¹ impaired angiogenesis,^{2, 3} reduced endothelial repair capability and increased incidence of cardiovascular disease.^{4, 5, 6} Cellular senescence can occur in vivo or in vitro in response to various stressors,^{7, 8, 9, 10} leading to suppression of cell proliferation. EC senescence has been reported to contribute to the pathogenesis of age-associated vascular diseases, such as atherosclerosis.¹¹ Thus, further understanding the mechanisms of EC senescence may help to identify effective targets for antisenescence therapy and treatment aging-associated cardiovascular disorders.Previous studies have shown that the secreted matricellular protein thrombospondin-1 (TSP1) is as potent inhibitor of angiogenesis¹² and its antiangiogenic activity is mediated by its receptors, CD36^{13, 14} and CD47.^{15, 16} CD47 is a ubiquitously expressed transmembrane protein that serves as a ligand for signal regulatory protein-α and is a signaling receptor of TSP1. The TSP1-CD47 pathway has an important role in several fundamental cellular functions, including proliferation, apoptosis, inflammation and atherosclerotic response.¹⁷ Ligation of CD47 by TSP1 has been shown to inhibit nitric oxide (NO)/cGMP signaling in vascular cells, leading to suppression of angiogenic responses.¹⁶ Recently, it was reported that lack of CD47 expression in ECs may enable these cells to spontaneously gain characteristics of embryonic stem cells.¹⁸ However, the potential role of CD47 in regulation of EC senescence has not been well explored. The present study was initiated to determine the role and mechanisms of TSP1-CD47 signaling pathway in regulating cell cycle progression and replicative senescence of ECs.