Senescent cells may underlie clotting predisposition, suggest the findings from a recent study by researchers at the Buck Institute for Research on Aging. Using unbiased proteomics, the team identified over 300 proteins secreted by senescent human cells, 44 of which are associated with hemostasis. In addition, elimination of senescent cells was shown to prevent the pro-coagulation side effects of chemotherapy in mice.1 The work highlights the potential that senolytic drugs which electively eliminate senescent cells could have on reducing both age- and chemotherapy-related thrombosis.
Hemostasis and thrombosis
Hemostasis is a tightly regulated physiological process that stops excessive bleeding after an injury. During primary hemostasis, a platelet plug forms at the site of the injury, which is then strengthened and stabilized during secondary hemostasis by the deposition of insoluble fibrin. Dysregulation of the mechanisms underlying these processes can result in pathological bleeding or thrombosis.2
"The incidence of venous thrombosis, which includes deep vein thrombosis and pulmonary embolism is extremely low until the age of 45, when it begins to rise rapidly. Over time it becomes a major risk factor for death. By 80, the condition affects five to six people per thousand individuals," said Judith Campisi, PhD, Buck professor and senior co-author of the study in a press release. "Blood clots are also a serious side effect of chemotherapy, which sets off a cascade of senescence in those undergoing treatment. That's why blood thinners, which carry their own risks, are often included in treatment protocols." Although the links between DNA-damaging cancer therapies and cellular senescence have been studied previously, a link between cellular senescence and thrombosis has not yet been identified.
What is cellular senescence?
A senescent cell is a cell which has exited the cell cycle and is no longer proliferating, caused by exposure to stress such as DNA damage, oncogene activation, mitochondrial dysfunction or chemotherapeutic agents. This response can be beneficial, by suppressing tumorigenesis or accelerating wound healing for example, as well as being deleterious by promoting cancer cell proliferation or impairing tissue regeneration. It is thought that these responses are mediated by the senescence-associated secretory phenotype (SASP), the complex mix of growth factors, cytokines and chemokines that senescent cells produce and secrete.3 As cellular senescence is associated with the occurrence of several diseases, increased knowledge of the SASP and its effects could help in the development of novel senolytic drugs.
Profiling the SASP
In this study, published in Cell Reports, the researchers set out to profile the SASP using a mass spectrometry-based technique - stable isotope labelling with amino acids (SILACs). Conditioned media (CM) from quiescent and senescent primary human foreskin (HCA2) fibroblasts were analyzed, leading to the detection of 1,047 proteins. Of the SASP factors identified, 343 were significantly increased in senescent cells compared to quiescent cells. Subsequent bioinformatic analyses suggested that several of the SASP factors might be involved in hemostasis. These results were validated by determining the levels of the 11 hemostasis-related factors identified by SILACs by western blotting. All 11 were found at increased levels in CM from senescent cells compared to quiescent, suggesting that senescent cells may be involved in the process of hemostasis.
Thrombosis and senescence
Following on from this, the researchers used a DNA-damaging chemotherapeutic, doxorubicin (DOXO), to induce cells to senesce, and then compared SASP factors from these cells with quiescent cells. Increased secretion of SASP factors was associated with increased levels of mRNAs encoding hemostasis-related factors in the DOXO treated cells. Transgenic mice (p16-3MR) were then used to investigate whether the same effects would be seen in vivo. The number of senescent cells in the mice’s livers increased after treatment with DOXO and were efficiently eliminated by ganciclovir (GCV). A similar effect was seen in mRNA levels of several hemostasis-related genes.
Taken together, the study’s results suggest that chemotherapy-induced senescence could be involved in blood clotting, and that these effects could potentially be reduced by SASP-suppressing therapies.