Independent Roles of Estrogen Deficiency and Cellular Senescence in the Pathogenesis of Osteoporosis: Evidence in Young Adult Mice and Older Humans

Abstract

Estrogen deficiency is a seminal mechanism in the pathogenesis of osteoporosis. Mounting evidence, however, establishes that cellular senescence, a fundamental mechanism that drives multiple age‐related diseases, also causes osteoporosis. Recently, we systematically identified an accumulation of senescent cells, characterized by increased p16Ink4a and p21Cip1 levels and development of a senescence‐associated secretory phenotype (SASP), in mouse bone/marrow and human bone with aging. We then demonstrated that elimination of senescent cells prevented age‐related bone loss using multiple approaches, eg, treating old mice expressing a “suicide” transgene, INK‐ATTAC, with AP20187 to induce apoptosis of p16Ink4a‐senescent cells or periodically treating old wild‐type mice with “senolytics,” ie, drugs that eliminate senescent cells. Here, we investigate a possible role for estrogen in the regulation of cellular senescence using multiple approaches. First, sex steroid deficiency 2 months after ovariectomy (OVX, n = 15) or orchidectomy (ORCH, n = 15) versus sham surgery (SHAM, n = 15/sex) in young adult (4‐month‐old) wild‐type mice did not alter senescence biomarkers or induce a SASP in bone. Next, in elderly postmenopausal women, 3 weeks of estrogen therapy (n = 10; 74 ± 5 years) compared with no treatment (n = 10; 78 ± 5 years) did not alter senescence biomarkers or the SASP in human bone biopsies. Finally, young adult (4‐month‐old) female INK‐ATTAC mice were randomized (n = 17/group) to SHAM+Vehicle, OVX+Vehicle, or OVX+AP20187 for 2 months. As anticipated, OVX+Vehicle caused significant trabecular/cortical bone loss compared with SHAM+Vehicle. However, treatment with AP20187, which eliminates senescent cells in INK‐ATTAC mice, did not rescue the OVX‐induced bone loss or alter senescence biomarkers. Collectively, our data establish independent roles of estrogen deficiency and cellular senescence in the pathogenesis of osteoporosis, which has important implications for testing novel senolytics for skeletal efficacy, as these drugs will need to be evaluated in preclinical models of aging as opposed to the current FDA model of prevention of OVX‐induced bone loss. © 2019 American Society for Bone and Mineral Research.

Accelerated osteocyte senescence and skeletal fragility in mice with type 2 diabetes

“…Body mass (grams) was recorded on all mice at the onset of the lead-in phase (age 3 months) and at baseline (age 4 months), midpoint (age 5.5 months), and endpoint (age 7 months) of the experimental phase. At baseline and endpoint of the experimental phase, body composition (i.e., whole-body lean grams and fat mass grams) was assessed in vivo using quantitative Echo magnetic resonance imaging (EchoMRI-100) in mice that were nonanesthetized and conscious, as described (56).…”

“…Skeletal imaging. All imaging and analysis were performed in a blinded fashion as described by our group previously (11,12,56) and as detailed in Supplemental Methods.…”

Senolytic drugs: from discovery to translation

“…Senescent cells can accumulate at aetiological sites of multiple diseases throughout the lifespan. For example, in humans, senescent cells accumulate in adipose tissue in diabetes and obesity, in the hippocampi and frontal cortex in Alzheimer's disease (AD), the substantia nigra in Parkinson's disease (PD), bone and marrow in age-related osteoporosis, lungs in idiopathic pulmonary fibrosis, liver in cirrhosis, retinae in macular degeneration, plaques in psoriasis, kidneys in diabetic kidney disease, endothelium in pre-eclampsia, and the heart and major arteries in cardiovascular disease, amongst many other conditions [6,7,13,19,23,[70][71][72][73][74][75][76][77][78][79][80].…”

Effect of menopausal hormone therapy on proteins associated with senescence and inflammation

“…In response to DNA damage, senescence serves as an anticancer mechanism and may also have beneficial functions in embryogenesis, parturition, and tissue repair (Behnia et al, 2015;Demaria et al, 2014;Munoz-Espin et al, 2013). However, senescent cells that are not cleared efficiently by the immune system disrupt tissue function, which increases the vulnerability to the onset and progression of a host of age-related diseases, including pulmonary dysfunction, cardiovascular disorders, osteoporosis, neurodegeneration, and diabetes (Baker et al, 2011;Farr et al, 2019;Musi et al, 2018;Palmer et al, 2019;Schafer et al, 2017). In part, the deleterious effects of senescent cells are mediated by the SASP.…”

“…For example, estrogen decreases cell senescence in endothelial progenitor cells, and activates estrogen receptor alpha (ERα) to inhibit cell senescence-like phenotypes in human epithelial cells (Liu et al, 2016). Estrogen also slows deficits associated with aging and cell senescence in bone, such as declining bone density (Farr et al, 2019). However, little is known regarding how cell senescence might be modified by natural changes in hormone concentrations, such as those that occur during menopause, and how this might be modulated by hormone therapies.…”