Farr, Joshua N.

Cellular senescence is a heterogeneous phenotype characterized primarily in mesenchymal cells, but the extent to which immune cells differ in their senescence phenotype, or “senotype,” is unclear. Here, we applied single-cell approaches alongside both global and cell-specific genetic senolytic mouse models to evaluate the senotype of immune cells in the bone marrow of aging mice. We found that myeloid-lineage cells exhibited the highest expression of p16 and senescence-associated secretory phenotype markers among all immune cell types. In contrast with clearance of p16 + senescent mesenchymal cells, targeted clearance of p16 + myeloid cells in aged mice had only minor effects on age-related bone loss in male mice, with no effects in females. In more detailed analyses, p16 + myeloid cells were only acutely cleared, being repopulated back to basal levels within a short time. This led to a lack of long-lasting reduction in senescent cell burden, unlike when targeting bone mesenchymal cells. In vitro, myeloid-lineage cells differed markedly from mesenchymal cells in the development of a senescent phenotype. Collectively, our findings indicate that aged bone marrow myeloid cells do not achieve the fully developed senescent phenotype originally described in mesenchymal cells, justifying further characterization of senotypes of immune cells across tissues.

Aging plays a major role in the development of numerous chronic diseases, one of which is a marked decline in skeletal health. Beyond diminishing bone mass and strength, mammals of advanced age experience a decline in skeletal mechanotransduction—that is, the ability of the skeleton to respond adaptively to mechanical perturbation. One possibility for the loss of mechanotransduction in bone with aging is an age-associated increase in the population density of senescent cells—those cells that have undergone irreversible cell cycle arrest, resistance to apoptosis, and production of a modified secretome (the SASP) that has damaging effects to nearby healthy (non-senescent) cells. We investigated whether the presence of senescent cells might drive some of the diminished mechanical response observed in aged bone, by testing the hypothesis that the clearance of senescent cells via intermittent senolytic treatment promotes mechanical responsiveness in an aged skeleton. C57BL/6 mice aged 6 months and 22 months were treated weekly with the senolytic cocktail Dasatinib and Quercetin (D + Q) for 1 month, then subjected to low level in vivo mechanical loading of the ulna for 1 week. The 6-month-old mice exhibited a doubling of load-induced ulnar periosteal bone formation when treated with D + Q, compared to vehicle-treated mice, but the periosteal response to loading was not significantly altered by D + Q in the aged (22-month) mice. We further probed the efficacy of D + Q in mechanotransduction by switching to an endocortical model—the axial tibia loading system. Here, the 22-month-old mice had nearly double the load-induced endocortical bone formation compared to vehicle-treated mice. We further assayed the cortical bone gene expression profile in loaded and control tibias from treatment-naïve 6-month and 22-month mice, to determine whether there is significant overlap between mechanically induced signaling genes and SASP genes. We found significant load-induced changes among several SASP genes, suggesting that inhibition of the SASP (i.e., senomorphics) might interfere with mechanical signaling from otherwise healthy cells. In summary, clearance of senescent cells via intermittent D + Q treatment is effective at improving endocortical mechanical responsiveness in the aged skeleton, which is commonly diminished throughout the course of aging.

ABSTRACT Although animal studies have linked cellular senescence to the pathogenesis and complications of type 2 diabetes mellitus (T2DM), there is a paucity of corroborating data in humans. Thus, we measured a previously validated marker for senescent cell burden in humans, T‐cell expression of p16 mRNA, along with additional biomarkers, to compare the senescence phenotypes of postmenopausal control (lean, N = 37) and T2DM ( N = 27) participants. To control for effects of obesity alone, we included a third group of obese but non‐diabetic women ( N = 29) who were matched for body mass index to the T2DM participants. In addition, given the increase in fracture risk in T2DM despite preserved or even increased bone mineral density, we related these senescence biomarkers in the T2DM participants to skeletal microarchitectural parameters. Relative to the lean participants, T‐cell p16 and p21 Cip1 expression was increased in the T2DM, but not the obese, non‐diabetic participants. Expression of p16 and p21 Cip1 was positively associated with HbA1c and an index of skin advanced glycation end‐products. T2DM was also associated with an increase in a number of SASP factors. Among participants with T2DM, women in the highest tertile for T‐cell expression of p16 had significantly reduced tibial cortical area and thickness as compared to those in the lower two tertiles. Overall, our studies link cellular senescence to metabolic and skeletal alterations in T2DM and point to the need for further studies evaluating the role of cellular senescence in mediating skeletal fragility, as well as potentially other complications in T2DM.

Abstract Text While the importance of estrogen in driving ER+ breast cancer progression is clear, data from pre-clinical studies exploring the role of tumoral progesterone (PR) and androgen (AR) receptors are conflicting, likely due in part to the promiscuity of steroid receptor ligands, as well as relative levels of steroid hormone receptor expression and of steroid hormones within the tumor milieu. While bone is usually the first site of metastases in ER+ metastatic breast cancer, the roles of tumoral PR and AR (present in 58 percent and 86 percent of clinical ER+ bone metastases [BMET], respectively) in ER+ breast cancer progression at this site have not been well studied. Studies were undertaken to examine the effects of ovariectomy (OVX), a model of surgical menopause that significantly reduces endogenous progesterone and androgen levels, in a pre-clinical model of human ER+ bone metastases that is dependent on exogenous 17ß-estradiol (E2) supplementation. Effects of OVX (at 9-weeks of age) were ascertained using skeletally mature female nude mice supplemented with a modest dose of E2 (0.36 mg 60 day pellets) prior to intracardiac inoculation (at 10 weeks of age) with bone metastasis-derived MCF7 cells (43.4M cells) that form robust E2- and TGFß-dependent osteolytic metastasis in vivo, and express PR and AR in vitro. Osteolytic bone metastatic lesions in E2-supplemented mice were reduced in both incidence and size in OVX (vs ovary intact) mice. Anti-tumoral effects of OVX were not limited to bone; E2-driven orthotopic tumor progression was similarly reduced. Histologic assessment of cytokeratin-positive breast cancer tumor cells within bone revealed a small but statistically significant reduction in Ki67-positivity in OVX (vs ovary -intact) mice that correlated with osteolytic lesion size. In addition, PR and AR expression in BMET lesions, which was readily detected by IHC in ovary-intact mice, was markedly reduced in OVX mice. While additional experiments are underway to query possible roles of specific reproductive hormones altered in OVX mice, these results are consistent with the postulate that tumoral AR and PR signaling may promote ER+ bone metastases progression in this model. In addition, while ovariectomy is a common pre-clinical means of reducing estrogen to model natural menopause in women, resultant changes in steroid receptor signaling are clearly much more complex. Funding: NIH 5R25CA275753, 5P30CA023074 Date of Presentation October 17, 2024

Abstract Text While the importance of estrogen in driving ER+ breast cancer progression is clear, data from pre-clinical studies exploring the role of tumoral progesterone (PR) and androgen (AR) receptors are conflicting, likely due in part to the promiscuity of steroid receptor ligands, as well as relative levels of steroid hormone receptor expression and of steroid hormones within the tumor milieu. While bone is usually the first site of metastases in ER+ metastatic breast cancer, the roles of tumoral PR and AR (present in 58 percent and 86 percent of clinical ER+ bone metastases [BMET], respectively) in ER+ breast cancer progression at this site have not been well studied. Studies were undertaken to examine the effects of ovariectomy (OVX), a model of surgical menopause that significantly reduces endogenous progesterone and androgen levels, in a pre-clinical model of human ER+ bone metastases that is dependent on exogenous 17ß-estradiol (E2) supplementation. Effects of OVX (at 9-weeks of age) were ascertained using skeletally mature female nude mice supplemented with a modest dose of E2 (0.36 mg 60 day pellets) prior to intracardiac inoculation (at 10 weeks of age) with bone metastasis-derived MCF7 cells (43.4M cells) that form robust E2- and TGFß-dependent osteolytic metastasis in vivo, and express PR and AR in vitro. Osteolytic bone metastatic lesions in E2-supplemented mice were reduced in both incidence and size in OVX (vs ovary intact) mice. Anti-tumoral effects of OVX were not limited to bone; E2-driven orthotopic tumor progression was similarly reduced. Histologic assessment of cytokeratin-positive breast cancer tumor cells within bone revealed a small but statistically significant reduction in Ki67-positivity in OVX (vs ovary -intact) mice that correlated with osteolytic lesion size. In addition, PR and AR expression in BMET lesions, which was readily detected by IHC in ovary-intact mice, was markedly reduced in OVX mice. While additional experiments are underway to query possible roles of specific reproductive hormones altered in OVX mice, these results are consistent with the postulate that tumoral AR and PR signaling may promote ER+ bone metastases progression in this model. In addition, while ovariectomy is a common pre-clinical means of reducing estrogen to model natural menopause in women, resultant changes in steroid receptor signaling are clearly much more complex. Funding: NIH 5R25CA275753, 5P30CA023074 Date of Presentation October 17, 2024

ABSTRACT There is an increasing need for biomarkers of senescent cell burden to facilitate the selection of participants for clinical trials. p16 Ink4a is encoded by the CDKN2A locus, which produces five variant transcripts in humans, two of which encode homologous p16 proteins: p16 Inka4a , encoded by p16_variant 1 , and p16ɣ, encoded by p16_variant 5. While distinct quantitative polymerase chain reaction primers can be designed for p16_variant 5, primers for p16_variant 1 also measure p16_variant 5 ( p16_variant 1 + 5 ). In a recent clinical trial evaluating the effects of the senolytic combination, dasatinib + quercetin (D + Q), on bone metabolism in postmenopausal women, we found that women in the highest tertile for T‐cell expression of p16_variant 5 had the most robust skeletal responses to D + Q. Importantly, the assessment of p16_variant 5 was more predictive of these responses than p16_variant 1 + 5 . Here, we demonstrate that in vitro, p16_variant 1 + 5 increased rapidly (Week 1) following the induction of DNA damage, whereas p16_variant 5 increased later (Week 4), suggesting that p16_variant 5 becomes detectable only when the abundance of senescent cells reaches some threshold. Further analysis identified a SASP panel in plasma that performed as well in identifying postmenopausal women with a positive skeletal response to D + Q. Collectively, our findings provide further support for the T‐cell p16_variant 5 assay as a biomarker for selecting participants in clinical trials of senolytic interventions. In addition, our data indicate that correlated plasma SASP markers could be used in lieu of the more technically challenging T‐cell p16 assay. Trial Registration: ClinicalTrials.gov identifier: NCT04313634.

Abstract Disclosure: K. Yu: None. J. Saini: None. J.N. Farr: None. F. Vanessa: None. S. Khosla: None. I. Bancos: Advisory Board Member; Self; AstraZeneca, Adrenas, Neurocrine, Spruce, Recordati, Xeris, Novo Nordisk, HRA Pharmaceuticals, Sparrow, Diurnal, Corcept. Research Investigator; Self; HRA Pharmaceuticals, Recordati. Importance: Patients with both mild autonomous cortisol secretion (MACS) and overt Cushing syndrome (CS) demonstrated high prevalence of aging-related morbidity and an increased mortality. Objective: To determine whether p16INK4a in T cell, a cell cycle enzyme that associated with chronological aging and cell senescence, is associated with hypercortisolism and hypercortisolism-related comorbidities. Design, Setting, and Participants: In this single-center cross-sectional study, we prospectively recruited consecutive patients with hypercortisolism between July 1, 2020 and August 24 2022. Patients were included if they were diagnosed with Cushing syndrome (CS) or mild autonomous cortisol secretion (MACS). Age- (± 2 years) and sex-matched referent subjects without known adrenal disorders were included if abdominal imaging was available. Participants on exogenous glucocorticoids within 3 months prior were excluded. Exposure: Presence of hypercortisolism (MACS and CS). Main Outcomes and Measures: Primary outcomes included p16INK4a, advanced glycation end products (AGE) reader result, Frailty Index score, frailty (defined as frailty index>0.31), and physical performances in patients and matched referents at index date. Secondary outcomes included correlation between biological age markers, hormonal workup and the risk of comorbidities. p16INK4a expression analysis was adjusted for age, sex, smoking, and alcohol use for all analyses except for associations with CVD and frailty. Results: Of 85 patients (87% women, mean age 53 years), 51 (60%) had MACS (34 had matched referents), 34 (40%) had overt CS (16 had matched referents). Patients with CS demonstrated a median 1.9-fold higher p16INK4a expression (P<.001), and patients with MACS showed a median of 1.70-fold p16INK4a expression (P=.013), when compared to referent subjects. Age- and sex-adjusted p16INK4a expression was associated with frailty index score (r=0.23, P=.035), time-up-and-go test (r=0.28, P=.048), gait speed (r=-0.36, P=.035), but not with AGE reader result, 6-minute walk test, or hand grip measurement. The p16INK4a expression was associated with history of cardiovascular disease (age-adjusted odds ratio (aOR)=3.70, 95% confidence interval (CI), 1.20-11.43) and frailty (aOR=2.27, 95%CI, 1.04-4.98). Conclusion and relevance: We demonstrated evidence of cell senescence in both patients with CS and patients with MACS. Further studies need to determine the impact of surgical, cortisol-lowering, and senolytic medical therapies in MACS on cell senescence and associated aging-related comorbidities. Presentation: 6/1/2024

Abstract Histone deacetylase 3 (Hdac3) is an epigenetic regulator of gene expression and interacts with skeletal transcription factors such as Runx2. We previously reported that conditional deletion of Hdac3 in Osterix-Cre recombinase-expressing osteoprogenitor cells (Hdac3 CKOOsx) caused osteopenia and increased marrow adiposity, both hallmarks of skeletal aging. We also showed that Runx2+ cells within osteogenic cultures of Hdac3-depleted bone marrow stromal cells (BMSCs) contain lipid droplets (LDs). Cellular senescence, a nonproliferative metabolically active state, is associated with increased marrow adiposity, bone loss, and aging. In this study, we sought to determine if Hdac3 depleted Runx2+ pre-osteoblasts from young mice exhibit chromatin changes associated with early cellular senescence and how these events correlate with the appearance of LDs. We first confirmed that BMSCs from Hdac3 CKOOsx mice have more Runx2 + LD+ cells compared with controls under osteogenic conditions. We then measured senescence-associated distention of satellite (SADS) DNA and telomere-associated foci (TAFs) in Hdac3 CKOOsx and control BMSCs. In situ, Runx2+ cells contained more SADS per nuclei in Hdac3 CKOOsx femora than in controls. Runx2+ BMSCs from Hdac3 CKOOsx mice also contained more SADS and TAFs per nuclei than Runx2+ cells from age-matched control mice in vitro. SADs and TAFs were present at similar levels in Runx2 + LD+ cells and Runx2 + LD− cells from Hdac3 CKOOsx mice. Hdac inhibitors also increased the number of SADS in Runx2 + LD+ and Runx2 + LD− WT BMSCs. Senolytics reduced viable cell numbers in Hdac3 CKOOsx BMSC cultures. These data demonstrate that the depletion of Hdac3 in osteochondral progenitor cells triggers LD formation and early events in cellular senescence in Runx2+ BMSCs through mutually exclusive mechanisms.

Cells expressing features of senescence, including upregulation of p21 and p16, appear transiently following tissue injury, yet the properties of these cells or how they contrast with age-induced senescent cells remains unclear. Here, we used skeletal injury as a model and identified the rapid appearance following fracture of p21 + cells expressing senescence markers, mainly as osteochondroprogenitors (OCHs) and neutrophils. Targeted genetic clearance of p21 + cells suppressed senescence-associated signatures within the fracture callus and accelerated fracture healing. By contrast, p21 + cell clearance did not alter bone loss due to aging; conversely, p16 + cell clearance, known to alleviate skeletal aging, did not affect fracture healing. Following fracture, p21 + neutrophils were enriched in signaling pathways known to induce paracrine stromal senescence, while p21 + OCHs were highly enriched in senescence-associated secretory phenotype factors known to impair bone formation. Further analysis revealed an injury-specific stem cell–like OCH subset that was p21 + and highly inflammatory, with a similar inflammatory mesenchymal population (fibro-adipogenic progenitors) evident following muscle injury. Thus, intercommunicating senescent-like neutrophils and mesenchymal progenitor cells were key regulators of tissue repair in bone and potentially across tissues. Moreover, our findings established contextual roles of p21 + versus p16 + senescent/senescent-like cells that may be leveraged for therapeutic opportunities.