07 Jul Key Study Shifts Focus To Smooth Muscle Cells In Atherosclerotic Heart Disease
Medical Research: What is the background for this study?
Dr. Owens: The leading cause of death in the USA and worldwide is cardiovascular disease with many of the clinical consequences including heart attacks (myocardial infarctions) and strokes being secondary consequences of atherosclerosis, commonly referred to as hardening of the arteries. Importantly, a heart attack is not caused by gradual narrowing of a large coronary artery by the atherosclerotic plaque, but rather is caused by acute rupture of a plaque that results in a catastrophic thrombotic event that can completely occlude a major coronary artery shutting off blood supply to a major heart region. Similarly, rupture of a plaque can result in formation of a thrombus that breaks off and circulates to a cerebral vessel where it can occlude blood flow to a brain region leading to a stroke. As such, it is critical to understand the mechanisms that regulate the stability of plaques, and the likelihood of plaque rupture.
The general dogma among clinicians and cardiovascular researchers has been that atherosclerotic plaques that have an abundance of macrophages and macrophage-derived foam cells relative to smooth muscle cells (SMC), the cells that normally line all of your blood vessels, are less stable and more prone to rupture with subsequent clinical consequences. However, the evidence for this is based on use of methods that are unreliable in identifying which cells within the plaque are truly derived from macrophages versus SMC, and even more importantly, what mechanisms regulate phenotypic transitions of these cells that are critical in the pathogenesis of this disease. Indeed, results of studies in cultured smooth muscle cells and macrophages have shown that each cell can express markers of the other cell type in response to stimuli likely to be present within advanced atherosclerotic lesions while down-regulating expression of their typical cell selective markers. As such, previous studies in the field have likely mis-identified which cell is which in many cases.
The goals of our studies were to clearly identify which cells within advanced atherosclerotic lesions are derived from SMC, to determine the various phenotypes exhibited by these cells and their functional role in lesion pathogenesis, and to determine what regulates these phenotypic transitions.
Medical Research: What are the main findings?
Dr. Owens: Using a rigorous SMC-specific lineage tracing mouse developed by our lab, we determined that >80% of SMC within advanced atherosclerotic lesions of ApoE knockout hyperlipidemic mice cannot be identified using typical SMC markers such as SM alpha-actin (SMaA), the marker used in virtually all previous studies in the field. Moreover, we observed that approximately 1/3 of cells within lesions that express macrophage markers are of SMC not myeloid origin and that SMC-derived lesion cells that are negative for SMaA also express markers of mesenchymal stem cells and/or myofibroblasts. Importantly, using a novel single cell epigenetic SMC lineage tracing method previously developed by our lab, we also showed that transition of smooth muscle cells to macrophage-like cells also is highly prevalent within advanced human coronary artery atherosclerotic lesions comprising about 20% of cells previously thought to be macrophages.
Finally, we show that SMC specific knockout of the stem cell pluripotency gene Klf4 did not result in a change in the number of smooth muscle cells within lesions but resulted in these cells undergoing transition to a phenotype that was atheroprotective as evidenced by lesions that were much smaller in size, that contained far fewer SMC-derived macrophage-like cells, and which exhibited multiple indices of increased plaque stability including a thickened fibrous cap. As such, loss of one gene, Klf4, in one cell type, SMC, had a rather profound positive impact on the pathogenesis of atherosclerosis.
Medical Research: What should clinicians and patients take away from your report?
Dr. Owens: Taken together, results of our studies indicate that smooth muscle cells-derived cells play a much more important role in atherosclerosis pathogenesis than has generally been appreciated but that SMC-derived lesion cells can exhibit detrimental as well as beneficial properties depending on the nature of their phenotypic transitions. Results also clearly establish that most previous studies of atherosclerosis have mis-identified many of the SMC and macrophages within lesions of both man and animal models, as well as the extent to which these cells contribute to formation of foam cells and plaque stability.
Medical Research: What recommendations do you have for future research as a result of this study?
Dr. Owens: Our results clearly establish that use of conventional markers of smooth muscle cells and macrophages are insufficient for identifying these cell types within lesions. Future studies need to be much more careful in interpreting results if the experimental approach does not include rigorous lineage tracing.
However, of greatest significance, studies are the first to our knowledge to demonstrate that therapeutic targeting of smooth muscle cells within lesions represents a viable means of enhancing plaque stability to reduce the probability of plaque rupture with possible myocardial infarction or stroke. That is, can we identify therapeutic agents that induce SMC to undergo changes in phenotype that are beneficial in promoting stability of advanced atherosclerosis plaques?
This represents a paradigm shift for the atherosclerosis field since therapies to date have largely been focused on drugs such as statins that control blood lipids which do modestly reduce disease prevalence and/or anti-inflammatory strategies targeting macrophages and other immune cells which have largely failed, including a number of recent $500M+ clinical trials that showed either no significant benefit or detrimental effects. A key goal for the future is to identify the factors and mechanisms that can promote beneficial changes in smooth muscle cells phenotype that can either augment or replace these more conventional anti-atherosclerotic therapies.
Dr. Gary K Owens Ph.D Robert M. Berne Cardiovascular Research Center (2015). Key Study Shifts Focus To Smooth Muscle Cells In Atherosclerotic Heart Disease