Premature aging induced by radiation exhibits pro‐atherosclerotic effects mediated by epigenetic activation ofCD44 expression

Abstract

Age is undoubtedly a major risk factor for heart disease. However, the reason for this is not entirely clear. In the course of our investigation into the mechanism of radiation-induced cardiovascular disease, we made several unexpected findings that inform us on this question. We observed that human coronary endothelial cells, while being able to initiate repair of radiation-induced DNA damage, often fail to complete the repair and become senescent. Such radiation-induced cellular aging occurs through a mutation-independent route. Endothelial cells that aged naturally through replication or as a result of radiation exhibited indistinguishable characteristics. The promoter regions of the CD44 gene in aging endothelial cells become demethylated, and the proteins are highly expressed on the cell surface, making the cells adhesive for monocytes. Adhesion is a cardinal feature that recruits monocytes to the endothelium, allowing them to infiltrate the vessel wall and initiate atherosclerosis. The epigenetic activation of CD44 expression is particularly significant as it causes persistent elevated CD44 protein expression, making senescent endothelial cells chronically adhesive. In addition to understanding why cardiovascular disease increases with age, these observations provide insights into the puzzling association between radiation and cardiovascular disease and highlight the need to consider premature aging as an additional risk of radiation to human health.

Circulatory disease mortality in the Massachusetts tuberculosis fluoroscopy cohort study

“…The somewhat high (albeit nonsignificant) risks for hypertensive heart disease and CeVD if RBM dose is used (Table 6) (weakly) suggest that dose to this tissue may not be relevant for these endpoints. There is biological data suggesting radiation-associated senescence of monocytes [39], and a some-what similar mechanism based on monocyte cell killing in the arterial intima suggests that the arterial intima may be causally associated with initiating atheroma in the arterial wall [40] (although there are many other stages between that point and plaque rupture [41,42]), so that mean arterial dose might be the most relevant organ or tissue dose for studying circulatory disease. Several recent reviews [8,9,28,43] describe the abundant radiobiological reasons for considering the studies of moderate and low doses separately from studies of high doses.…”

Quantitation of Endothelial Cell Adhesiveness <em>In Vitro</em>

“…This method has been used with est2-immortalised human coronary endothelial cells and similar results were also obtained with primary human coronary endothelial cells 10 , demonstrating that it is not specific to just a particular cell line. Also, the adoption of this method of assaying adhesiveness of endothelial cells do not preclude subjecting the same cells to the standard adhesion assay that measures fluorescence of labeled immunocytes.…”

“…The monocyte adhesion assay described above was used successfully in experiments designed to study the biological effects of ionizing radiation on endothelial monolayers 10 . Although this is not the only method available to assess the adhesiveness of an endothelial monolayer, it is the only method that allows the quantification of the proportion or percentage of endothelial cells within a monolayer that is adhesive.…”

“…After the adhesion assay and cells on the coverslip have been fixed in 10% formalin for 15 min at RT, subject the coverslips to immunofluorescence using standard procedure 10 and antibodies of choice.…”

“…In addition, these flow systems also recapitulate the effect of shear force on the endothelial cells. While the advantages of such systems are clear and very attractive, it is also important to realize that while endothelial cell adhesiveness can be greatly augmented, as is the case in acute inflammation, by factors such as TNFα, some other activators, such as ionizing radiation 10 elicit changes that are not readily detected within in vitro experimental time frames by these very stringent systems. While such minute changes of endothelial cell adhesiveness are easily missed or dismissed in vitro, it is not necessarily benign in vivo, where such small changes within a life time, which is characteristic of chronic inflammation, can exert very significant outcomes.…”