Targeted Reduction of Senescent Cell Burden Alleviates Focal Radiotherapy-Related Bone Loss

Abstract

Clinical radiotherapy treats life‐threatening cancers, but the radiation often affects neighboring normal tissues including bone. Acute effects of ionizing radiation include oxidative stress, DNA damage, and cellular apoptosis. We show in this study that a large proportion of bone marrow cells, osteoblasts, and matrix‐embedded osteocytes recover from these insults only to attain a senescent profile. Bone analyses of senescence‐associated genes, senescence‐associated beta‐galactosidase (SA‐β‐gal) activity, and presence of telomere dysfunction‐induced foci (TIF) at 1, 7, 14, 21, and 42 days post–focal radiation treatment (FRT) in C57BL/6 male mice confirmed the development of senescent cells and the senescence‐associated secretory phenotype (SASP). Accumulation of senescent cells and SASP markers were correlated with a significant reduction in bone architecture at 42 days post‐FRT. To test if senolytic drugs, which clear senescent cells, alleviate FRT‐related bone damage, we administered the senolytic agents, dasatinib (D), quercetin (Q), fisetin (F), and a cocktail of D and Q (D+Q). We found moderate alleviation of radiation‐induced bone damage with D and Q as stand‐alone compounds, but no such improvement was seen with F. However, the senolytic cocktail of D+Q reduced senescent cell burden as assessed by TIF+ osteoblasts and osteocytes, markers of senescence (p16 Ink4a and p21), and key SASP factors, resulting in significant recovery in the bone architecture of radiated femurs. In summary, this study provides proof of concept that senescent cells play a role in radiotherapy‐associated bone damage, and that reduction in senescent cell burden by senolytic agents is a potential therapeutic option for alleviating radiotherapy‐related bone deterioration. © 2020 American Society for Bone and Mineral Research.

Therapy-Induced Senescence: An “Old” Friend Becomes the Enemy

“…Dasatinib + Quercetin -Senescent HUVEC, senescent preadipocytes in vitro -Senescent MEFs, senescent bone marrow-derived murine mesenchymal stem cells in vivo -SA-β-gal positive muscle and fat tissue of irradiated single mouse limb -Progeroid Ercc1(−/∆) mice [186] -Senescent lung fibroblasts and epithelial cells in bleomycin-induced lung injury/idiopathic pulmonary fibrosis mouse model [18] -Senescent alveolar epithelial type (AT)II ex vivo in bleomycin-induced lung injury/idiopathic pulmonary fibrosis mouse model. [187] -Senescent medial aortal cells of aging mice and hypercholesterolemia (atherosclerosis) mouse models [188] -Senescent hepatocytes of dietary hepatic steatosis mouse model [189] -Radiation-induced senescent preadipocytes in vivo -Senescent cells in freshly isolated human omental adipose tissue of obese individuals ex vivo [190] -Arteriovenous fistula-chronic kidney disease mouse model [191] -20-month-old, transgenic tau NFT -Mapt 0/0 mice [192] -Aβ plaque-associated senescent oligodendrocyte progenitor cells in vivo -ZsGreen/APPPS1 p16 INK4 reporter Alzheimer's disease mouse model -Radiation-induced senescent N2a cells [193] -Uterine fibrosis mouse model [194] -Telomere dysfunction-induced senescent osteoblasts and osteocytes [195] Navitoclax (ABT263) -Radiation-induced, replication-exhausted and Ras-induced senescent WI38 fibroblasts in vitro -Radiation-induced senescent human IMR90 fibroblasts, human renal epithelial cells and mouse embryonic fibroblasts in vitro -Radiation-induced senescence in p16-3MR transgenic mouse model [196] -Radiation-induced senescent human umbilical vein epithelial cells, IMR90 human lung fibroblasts and mouse embryonic fibroblasts [197] -Radiation-induced senescent type II alveolar epithelial cells in vitro and in vivo [198] -Senescent pancreatic tissue of i4F mouse model [199] -Replicative-exhausted human mesenchymal stromal cells [200] -Aging-induced senescent cardiac myocytes -Myocardial infarction mouse model [201] -Senescent murine pancreatic β-cells in vitro and in vivo [202] -Aging mouse bone marrow stromal cells [203] -WIPI1 and SLITKR4 overexpression-induced senescent uterine leiomyoma spheroid model ex vivo [204]…”

“…Early results from the NCT02848131 clinical trial have demonstrated that the D+Q combination has successfully cleared senescent adipocytes in humans [224]. D+Q was shown to effectively clear senescent cells in several in vitro, in vivo and ex vivo in several disease models including Alzheimer's disease, obesity and radiation-induced tissue damage [190][191][192][193][194][195] (Table 2). Finally, it is important to mention that other preclinical evidence has suggested that D+Q might not be as effective in the clearance of senescent tumor cells as for aging-related pathologies [225].…”

DOT1L modulates the senescence-associated secretory phenotype through epigenetic regulation of IL1A

“…Numerous strides have been made towards clearing senescent cells in order to restrain the detrimental effects of cellular senescence. Indeed, selective targeting of senescent cells using genetic models or senolytics has clearly demonstrated that inhibition of the SASP in both cancer, aging, and other models is beneficial (Bussian et al, 2018;Chandra et al, 2020;Chang et al, 2016;Demaria et al, 2017;Jeon et al, 2017;Justice et al, 2019;Perrott et al, 2017;Suvakov et al, 2019;Wiley et al, 2018;Xu et al, 2015). While inhibitors of NF-κβ suppress the SASP, they also reverse the senescence-associated cell cycle arrest (Chien et al, 2011a), which would not be desirable in the context of cancer.…”

Current advances in regulation of bone homeostasis

“…Recently, there have been several reports describing the role of cellular senescence in the pathophysiology of osteoporosis via altered expression of SASP factors 111,112 . Senescent cells accumulate in the bone microenvironment with aging and clearance of these cells using genetic and pharmaceutical interventions improves bone quality in aged mice 113,114 . In advanced aging, multiple cell types such as containing B cells, T cells, myeloid cells, osteoprogenitors, osteoblasts and osteocytes in the bone microenvironment become senescent, 115,116 although senescent myeloid cells and senescent osteocytes predominantly develop the SASP, a pro‐inflammatory environment 117 .…”