Unregulated cell growth seems to be a driver behind the growth of atherosclerotic plaques, changing the traditional story of plaque formation. The rapid cell growth in the arterial wall is similar to pre-cancerous growth in other tissues.
For decades, the standard story of how atherosclerotic plaque forms focused on the accumulation of cholesterol and inflammatory cells in arterial walls. Now, researchers at the Stanford School of Medicine have identified another factor that initiates the formation of these plaques: inflamed smooth muscle cells in the linings of arterial walls that multiply in an unregulated way.
The culprit behind many heart attacks and strokes, atherosclerotic plaques are unhealthy masses of oxidized cholesterol, immune cells and dead tissue that form within arterial walls. When atherosclerotic plaques grow large or rupture they can impede blood supply to vital tissues and organs, making them a lethal threat.
“Cardiovascular disease remains the world’s No. 1 killer, despite widespread use of cholesterol-lowering medicines,” said Nicholas Leeper, MD, professor of surgery at Stanford. “Recent studies have indicated that the standard dogma about how atherosclerosis happens wasn’t really capturing the whole story.”
In 1973, researchers proposed that a substantial portion of atherosclerotic plaques were made up of arterial blood vessel cells that had expanded over time, and that these abnormally growing cells could be driving the growth of the plaque. But for decades, that idea was sidelined in favor of findings pointing to the importance of cholesterol in plaque formation.
Renewed interest in old finding
Recently, Leeper and his colleagues renewed their interest in the old finding. They demonstrated that these abnormal blood vessel cells were protected from removal by the immune system, indicating that the cells could be part of the disease process underlying plaque formation.
To find out how these rapidly proliferating cells arise, sparking the chain of events that leads to plaque formation, Leeper and his colleagues looked at blood vessel muscle cells in a plaque using “rainbow” mice, which can be triggered so that every cell in their bodies expresses a particular color of a fluorescent protein. A single cell can be marked the color red, for instance. As it and its offspring divide, all the exact copies, or clones, that arise will colored red. The researchers showed that early in the formation of a plaque, a single cell in the smooth muscle cell layer of the artery starts to proliferate more than it should, leading to the growth of a lesion in the vessel wall that can constrict blood flow.
Normally, such poorly regulated cell growth can be controlled by immune cells that eat the outlaw cells. But in addition to losing regulatory features that control cell growth, the smooth muscle cells arm themselves with CD47, a cell marker that gives off a “don’t eat me” signal to immune cells.
“The combination of hyperproliferative growth and increased expression of ‘don’t eat me’ signals that frustrate immune mechanisms that would otherwise control them give these cells a competitive advantage over normal smooth muscle cells,” Leeper said.
A paper describing the research was published online June 15 in the Proceedings of the National Academy of Sciences. Leeper is the senior author. Postdoctoral fellow Ying Wang, PhD, is the lead author.
‘Spewing out inflammatory factors’
Leeper and his colleagues identified an additional characteristic of these rogue cells that leads to plaque formation. “The cells are not only proliferating in an uncontrolled way, they are also spewing out inflammatory factors,” Leeper said. When the researchers looked at the genes turned on and off in proliferating cells, they found that they produced high levels of a molecule called C3, a key part of the immune system that pumps up immune activity and inflammation. A systemic increase in C3 is known to be a risk factor in cardiovascular disease.
Moving beyond the mouse model and into human patients, they confirmed that C3 production is increased in a subset of smooth muscle blood vessel cells in human plaques.
Finally, the researchers analyzed plaque samples from patients with coronary artery disease and confirmed that human plaques had cells that had gene expression profiles similar to those of the plaque-initiating smooth muscle cells in the mouse model.
“This research confirms that plaques are founded on a clone of arterial smooth muscle cells that are rapidly dividing, evading immune control, and producing prodigious amounts of pro-inflammatory signals,” Leeper said.
The unregulated clonal expansion of cells in atherosclerotic plaques parallels that of poorly regulated cell growth in cancerous or pre-cancerous tissues, the researchers said.
“The clonal expansion of smooth muscle cells looks a lot like what we find in other pre-cancerous diseases like myelodysplastic syndrome,” which is caused by a clone of blood-forming stem cells taking over the blood forming system, said professor Irving Weissman, MD, a co-author on the paper and the Virginia and DK Ludwig Professor for Clinical Investigation in Cancer Research. “In theory, given enough time, this might become a cancer of the arteries, but it kills people through plaque formation long before that.”
Similarity to cancer cells
Cancer cells exhibit the same increase in CD47 “don’t eat me” signals that prevent them from being consumed by immune cells. Research in Weissman’s laboratory and evidence from clinical trials have shown that blocking the CD47 signal leads the immune system to attack cancer cells, and Leeper’s group has shown that blocking CD47 could also lead to reduction in the size of atherosclerotic plaques.
“If we block the CD47 signaling, these plaques start to go away,” Leeper said. There is currently no FDA-approved therapy to block CD47 signaling, but anti-CD47 therapies are in clinical trials as a cancer therapy. Clinical trials of anti-CD47 therapy against atherosclerosis are likely to follow, Leeper said. Researchers are also still working to understand how the unregulated blood vessel cell growth relates to the accumulation of cholesterol and other materials in plaques.
Leeper said that the proliferative smooth muscle cell model of atherosclerotic plaque formation has implications that point to other possible treatments. “These multiplying cells can give rise to different varieties of smooth muscle cell, some bad and some good,” Leeper said. “In the future we may be able to bias the reproduction of these cells away from the bad, toward the good.”
Other Stanford scientists involved in the research are postdoctoral scholars Vivek Nanda, PhD, Daniel Direnzo, PhD, Kathryn Howe, MD, PhD, Pavlos Tsantilas, MD, Anne Eberhard, MD, and Kai-Uwe Jarr, MD; senior scientists Jianqin Ye, MD, PhD, and Yoko Kojima, MD, PhD; research fellow Alyssa Flores; undergraduates Sophia Xiao and Noah Tsao; graduate student Abhiram Rao; James Priest, MD, assistant professor of pediatrics; and Erik Ingelsson, MD, PhD, professor of cardiovascular medicine.
Weissman is the director of the Stanford Institute for Stem Cell Biology and Regenerative Medicine, as well as the director of the Ludwig Center for Cancer Stem Cell Research and Medicine.
Researchers at Icahn School of Medicine, Tartu University Hospital in Estonia, University Medical Center Utrecht and the University of Virginia also contributed to the work
The research was supported by the National Institutes of Health (grants R35 HL144475, R01 HL125224, R01HL123370, and R01HL125863), the American Heart Association, the Virginia and DK Ludwig Fund for Cancer Research, the Swedish Research Council and Heart Lung Foundation, the Deutsche Forschungsgemeinschaft and the Fondation Leducq.
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