Mechanisms of Radiotherapy-Induced Senescence in Endothelial Cells and Strategies to Prevent Radiotherapy-Induced Cardiovascular Diseases

Sneha Chebrolu

 Introduction. Growing evidence suggests ionizing radiation (IR) increases the risk of cardiovascular disease (CVD) in radiotherapy patients.1,2 The effects of radiation damage are not usually observed until a decade after stopping treatment.1 IR-induced endothelial senescence is a major mechanism for premature CVD in radiotherapy patients.1,3 Senescence refers to aged cells that do not replicate but remain metabolically active.1 They produce pro-inflammatory molecules such as IL-6 and ICAM1, which promotes atherosclerosis and other CVD.1,3 Radiation therapy induces endothelial senescence by activating the NF-kB pathway and epigenetically upregulating CD44 expression.4,5 Currently, the drugs used to manage IR-induced CVD are the same as for CVD of different etiologies.6 However, senolytic drugs have been identified as a way to specifically target and reduce viability of senescent cells.7,8 Methods. Human umbilical vein endothelial cells (HUVEC) were used to examine the NF-kB pathway.4 HUVECs received radiation doses of 0-8Gy and were analyzed using MTT assay, Senescence-Associated β-Galactosidase (SA-βGal) assay, RT-qPCR, and immunofluorescence.4 CD44 activation was assessed using human coronary artery endothelial cells irradiated with 10Gy.5 Irradiated cells were stained for SA-βGal to confirm senescence while Western blot, immunofluorescence, and qPCR were used to measure CD44 levels. A quantitative monocyte adhesion assay was utilized to identify monocyte clusters.5 The senolytic drugs were tested in vitro using HUVEC cells and in vivo using murine animal models.8 Results. Nuclear p65 density was greater in irradiated cells and decreased in cells treated with PS1145 (NEMO inhibitor), proving IR activated the NF-kB pathway.4 Irradiated cells had senescent morphology, including larger cell size and abnormal nuclei, and quantitative analysis confirmed increased senescence levels.4 PCR analysis found increased mRNA levels of IL-6 in irradiated endothelial cells.4 Irradiation activated CD44 through demethylation at the CpG3 region.5 Anti-CD44 stain and Western blot showed irradiated cells had increased CD44 expression.5 The monocyte adhesion assay verified monocyte clusters have higher CD44 levels.5 Irradiated cells infected with short hairpin RNAs against CD44 had decreased monocyte adhesion, demonstrating CD44 is critical for adhesion.5 Both in vitro and in vivo testing of senolytic drugs reduced senescent cell viability but combination drug therapy of Quercetin and Dasatinib was more effective than monotherapy.8 Conclusions. Irradiation of endothelial cells induces premature senescence through activation of the NF-kB pathway and CD44 upregulation.4,5 The resulting production of IL-6 and increased adhesion contribute to the accelerated CVD pathogenesis observed in radiotherapy patients.1-5 Senolytic drugs preferentially reduce senescent cells and could be used to decrease IR-induced CVD.7,8

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