Bone Marrow Adiposity in Models of Radiation- and Aging-Related Bone Loss Is Dependent on Cellular Senescence

Oxidative stress-induced reactive oxygen species, DNA damage, apoptosis, and cellular senescence have been associated with reduced osteoprogenitors in a reciprocal fashion to bone marrow adipocyte tissue (BMAT); however, a direct (causal) link between cellular senescence and BMAT is still elusive. Accumulation of senescent cells occur in naturally aged and in focally radiated bone tissue, but despite amelioration of age- and radiation-associated bone loss after senescent cell clearance, molecular events that precede BMAT accrual are largely unknown. Here we show by RNA-Sequencing data that BMAT-related genes were the most upregulated gene subset in radiated bones of C57BL/6 mice. Using focal radiation as a model to understand age-associated changes in bone, we performed a longitudinal assessment of cellular senescence and BMAT. Using real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR), RNA in situ hybridization of p21 transcripts and histological assessment of telomere dysfunction as a marker of senescence, we observed an increase in senescent cell burden of bone cells from day 1 postradiation, without the presence of BMAT. BMAT was significantly elevated in radiated bones at day 7, confirming the qRT-PCR data in which most BMAT-related genes were elevated by day 7, and the trend continued until day 42 postradiation. Similarly, elevation in BMAT-related genes was observed in bones of aged mice. The senolytic cocktail of Dasatinib (D) plus Quercetin (Q) (ie, D + Q), which clears senescent cells, reduced BMAT in aged and radiated bones. MicroRNAs (miRNAs or miRs) linked with senescence marker p21 were downregulated in radiated and aged bones, whereas miR-27a, a miR that is associated with increased BMAT, was elevated both in radiated and aged bones. D + Q downregulated miR-27a in radiated bones at 42 days postradiation. Overall, our study provides evidence that BMAT occurrence in oxidatively stressed bone environments, such as radiation and aging, is induced following a common pathway and is dependent on the presence of senescent cells. © 2022 American Society for Bone and Mineral Research (ASBMR).

Clearance of Senescent Cells From Injured Muscle Abrogates Heterotopic Ossification in Mouse Models of Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease caused by mutations in activin A receptor type I/activin-like kinase 2 (ACVR1/ALK2), a bone morphogenetic protein (BMP) type I receptor, resulting in the formation of extraskeletal or heterotopic ossification (HO) and other features consistent with premature aging. During the first decade of life, episodic bouts of inflammatory swellings (flare-ups) occur, which are typically triggered by soft tissue trauma. Through an endochondral process, these exacerbations ultimately result in skeletal muscles, tendons, ligaments, fascia, and aponeuroses transforming into ectopic bone, rendering movement impossible. We have previously shown that soft tissue injury causes early FOP lesions characterized by cellular hypoxia, cellular damage, and local inflammation. Here we show that muscle injury in FOP also results in senescent cell accumulation, and that senescence promotes tissue reprogramming toward a chondrogenic fate in FOP muscle but not wild-type (WT) muscle. Using a combination of senolytic drugs we show that senescent cell clearance and reduction in the senescence associated secretory phenotype (SASP) ameliorate HO in mouse models of FOP. We conclude that injury-induced senescent cell burden and the SASP contribute to FOP lesion formation and that tissue reprogramming in FOP is mediated by cellular senescence, altering myogenic cell fate toward a chondrogenic cell fate. Furthermore, pharmacological removal of senescent cells abrogates tissue reprogramming and HO formation. Here we provide proof-of-principle evidence for senolytic drugs as a future therapeutic strategy in FOP. © 2021 American Society for Bone and Mineral Research (ASBMR).

Bone Aging, Cellular Senescence, and Osteoporosis

Changes in aging bone that lead to osteoporosis are mediated at multiple levels, including hormonal alterations, skeletal unloading, and accumulation of senescent cells. This pathological interplay is superimposed upon medical conditions, potentially bone-wasting medications, modifiable and unmodifiable personal risk factors, and genetic predisposition that accelerate bone loss with aging. In this study, the focus is on bone hemostasis and its dysregulation with aging. The major physiological changes with aging in bone and the role of cellular senescence in contributing to age-related osteoporosis are summarized. The aspects of bone aging are reviewed including remodeling deficits, uncoupling phenomena, inducers of cellular senescence related to bone aging, roles of the senescence-associated secretory phenotype, radiation-induced bone loss as a model for bone aging, and the accumulation of senescent cells in the bone microenvironment as a predominant mechanism for age-related osteoporosis. The study also addresses the rationale and potential for therapeutic interventions based on the clearance of senescent cells or suppression of the senescence-associated secretory phenotype. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The significance of the senescence pathway in breast cancer progression

Invasive breast cancer develops through prolonged accumulation of multiple genetic changes. The progression to a malignant phenotype requires overriding of growth inhibition. It is evident that some breast cancers have an inherited basis, and both hereditary and sporadic cancers appear to involve molecular mechanisms that are linked to the cell cycle. Frequently, changes in the molecular pathways with gene deletions, point mutations and/or overexpression of growth factors can be seen in these cancers. Recent evidence also implicates the senescence pathway in breast carcinogenesis. It has a barrier effect towards excessive cellular growth, acting as the regulator of tumour initiation and progression. Later in carcinogenesis, acquisition of the senescence associated secretory phenotype may instead promote tumour progression by stimulating growth and transformation in adjacent cells. This two-edge role of senescence in cancer directs more investigations into the effects of the senescence pathway in the development of malignancy. This review presents the current evidence on the roles of senescence molecular pathways in breast cancer and its progression.