You might recall that in 2011, researchers observed a five year gain in life expectancy versus the general population in a study of 121 older people with osteoporosis who were treated with bisphosphonates. This is a large effect size, at the upper end of what is typically observed in studies of exercise and physical fitness. Other, larger trials have observed reduced mortality with bisphosphonate treatment. A modest degree of effort has gone into attempting to understand the mechanisms that might be involved.
One possible candidate mechanism is the clearance of senescent cells and suppression of the harmful inflammatory signaling produced by lingering senescent cells present in old tissues. Selective destruction of senescent cells via senolytic drugs has been shown to produce impressive degrees of rejuvenation in aged mice. With this in mind, in today's open access paper researchers demonstrate that the bisphosphonate drug zoledronate is in fact either senolytic to a meaningful degree, or acts in other ways to reduce the generation of inflammatory signaling by senescent cells. This is quite interesting.
In addition to reducing fracture risk, zoledronate has been found in some studies to decrease mortality in humans and extend lifespan and healthspan in animals. Because senescent cells accumulate with aging and contribute to multiple co-morbidities, the non-skeletal actions of zoledronate could be due to senolytic (killing of senescent cells) or senomorphic (inhibition of the secretion of the senescence-associated secretory phenotype [SASP]) actions.
To test this, we first performed in vitro senescence assays using human lung fibroblasts and DNA repair-deficient mouse embryonic fibroblasts, which demonstrated that zoledronate killed senescent cells with minimal effects on non-senescent cells. Next, in aged mice treated with zoledronate or vehicle for 8 weeks, zoledronate significantly reduced circulating SASP factors, including CCL7, IL-1β, TNFRSF1A, and TGFβ1 and improved grip strength. Analysis of publicly available RNAseq data from CD115+ pre-osteoclastic cells isolated from mice treated with zoledronate demonstrated a significant downregulation of senescence/SASP genes. To establish that these cells are potential senolytic/senomorphic targets of zoledronate, we used single cell proteomic analysis and demonstrated that zoledronate significantly reduced the number of pre-osteoclastic cells and decreased protein levels of p16, p21, and SASP markers in these cells without affecting other immune cell populations.
Collectively, our findings demonstrate that zoledronate has senolytic effects in vitro and modulates senescence/SASP biomarkers in vivo. These data point to the need for additional studies testing zoledronate and/or other bisphosphonate derivatives for senotherapeutic efficacy.