Atherosclerosis is the build up of plaques in blood vessel walls, composed of fats and the debris of dead cells. Blood vessels are narrowed and weakened, and eventually something important ruptures or blocks, producing a heart attack or stroke. Cholesterols circulate in the bloodstream, attached to low-density lipoprotein (LDL) particles. The immune cells known as monocytes are responsible for ensuring that excess cholesterols stuck in blood vessel walls are removed and returned to the liver to be excreted. They do this by entering blood vessel walls, transforming into macrophages, ingesting the cholesterols, and then handing them off to high-density lipoprotein (HDL) particles.
In older individuals, increased inflammation and oxidative stress causes macrophages to become dysfunctional. Macrophages can be overwhelmed by large amounts of cholesterol, but it takes comparatively little oxidized cholesterol to turn a macrophage into a dysfunctional, inflammatory foam cell, unable to carry out its assigned tasks. Much of an atherosclerotic plaque is made up of the debris of dead macrophages, rich in oxidized cholesterols. Surviving cells signal for aid, calling in more monocytes to destruction. Chronic inflammation in blood vessel tissues makes this feedback loop run that much faster.
A sizable fraction of the chronic inflammation of aging is caused by the presence of senescent cells. These cells are created day in and day out in large numbers, and this is an important part of the normal operation of cellular metabolism. They have important roles in wound healing and cancer suppression, for example. The vast majority of senescent cells either self-destruct or are destroyed by the immune system, but a tiny fraction linger. They secrete a potent mix of signals that disrupt tissue function and produce inflammation. Fortunately, eliminating senescent cells is a going concern, with numerous approaches in human trials or under clinical development. In today's open access paper, researchers demonstrate a novel approach to diminishing the impact of cellular senescence in blood vessel walls, thereby slowing the progression of atherosclerosis. This adds to the existing data that suggests senolytic therapies should produce benefits in this condition.
Senescent cells lose their proliferative potential in response to various stresses. They secrete a variety of pro-inflammatory mediators and proteases, gathered in the senescence-associated secretory phenotype (SASP) that engages the immune system to eliminate senescent cells. Senescent cells accumulate in aging organisms, chronic age-related diseases and benign tumors; conversely, elimination of senescent cells contributes to improve health. They also accumulate in tissues affected by atherosclerosis and their elimination strikingly reduces atherogenicity in animal models. Senescence is thus a link between molecular damage and the altered physiology of aging, and targeting SnC using senolytic drugs appears a promising strategy to reduce the burden of age-related chronic inflammatory diseases, including atherosclerosis.
Angiopoietin like-2 (angptl2) is a member of the SASP and is detectable in most organs of adult mice. Angptl2 is expressed by senescent vascular human endothelial cells (EC), but not quiescent or proliferative EC and is atherogenic when infused in young atherosclerotic (ATX) mouse models. We reported that plasma levels of angptl2 are elevated in patients with cardiovascular diseases (CVD), were associated with endothelial dysfunction, and were predictive of major cardiac adverse events and death. Recently, we reported a strong relationship between arterial expression of p21, a cell cycle inhibitor overexpressed in senescent cells and maintaining growth arrest, and circulating levels of angptl2 in atherosclerotic patients. Senescent EC are activated and promote aggregation of leukocytes, the initiating step of atherogenesis. We therefore hypothesized that down-regulation of vascular angptl2, preferentially in the endothelium of severely dyslipidemic ATX mice would promote endothelial repair and slow atherogenesis.
Here, we report that knockdown of vascular angptl2 by a shRNA (shAngptl2), delivered to the vascular cells via a single injection of an AAV1, slowed atheroma progression in ATX mice. Knockdown of angptl2 was associated with a rapid reduction in the expression of EC senescence-associated p21 accompanied by the increase in Bax/Bcl2 ratio as a marker of apoptosis; subsequently, this was associated with endothelial repair as evidenced by the incorporation of endothelial progenitor CD34+ cells. In addition to our pre-clinical results, we show that vascular ANGPTL2 gene expression is correlated with p21 expression and inflammatory cytokines in the internal mammary artery isolated from severely atherosclerotic patients undergoing a coronary artery bypass surgery. Altogether, our data suggest that targeting vascular angptl2 could be senolytic, delaying the progression of atherosclerosis.