Studies by vascular biologists at The Johns Hopkins Hospital could lead to new treatments for vascular disease.This work was led by Dan Berkowitz, M.B.B.Ch., and Lewis Romer, M.D., both professors of anesthesiology and critical care medicine at the Johns Hopkins University School of Medicine. The studies focus on the balance between (good) nitric oxide, and (bad) oxidants—both important regulators of the inner lining of blood vessels, called the endothelium.
Cardiovascular disease is the leading cause of mortality in the Western world. The Berkowitz-Romer lab team is moving toward new therapies for treating cardiovascular disease that could be used in conjunction with statins and exercise, says Berkowitz.
Researchers have come to recognize the significance of the endothelium, which serves as an interface between the blood flowing through blood vessels and the tissues receiving nutrients from the blood, making it a “major determinant of the balance of blood flow and nutrient distribution,” says Berkowitz.
A damaged or poorly functioning endothelium can lead to inflammation, fat accumulation and buildup of plaque on the surface of the vessel, clogging arteries and causing heart attacks, strokes, diabetic vascular disease and erectile dysfunction.
As researchers have zeroed in on the characteristics of a healthy endothelium, nitric oxide has emerged as the hero. It keeps platelets from clotting, prevents inflammation and broadly regulates “blood vessel’s essential missions,” says Romer. But if nitric oxide is the hero, then arginase 2 is the villain. If too much of this enzyme is present and active in the endothelium, nitric oxide production will be suppressed. Oxidized LDL, the “bad” cholesterol, is one source of injury that can lead to an increase in the production of arginase 2.
The challenge for the Romer-Berkowitz team was figuring out how to keep arginase 2 levels down. Using human cells and mouse tissue, the team has identified two strategies for controlling arginase 2 in vascular endothelium: blocking its rapid release in response to injury and limiting arginase 2 levels for a longer term through blocking the gene that makes this enzyme.
In the first study, they showed that injuring the endothelium by exposing it to oxidized LDL releases arginase 2 from its primary location in mitochondria and causes it to flood into the cytoplasm, impeding nitric oxide availability and causing a sharp rise in harmful reactive oxidants. This work also showed that knockout of the arginase 2 gene protected against endothelial dysfunction and reduced blood vessel clogging in mice.
In the second study, the team tested about 15 variations of medicines that block another family of enzymes, called histone deacetylases (HDACs). These enzymes regulate genes and control how much of a particular protein is produced—some of them are in use as cancer treatments. The Romer-Berkowitz team found that a particular member of this family, HDAC2, controls levels of arginase 2 gene activity. “When you block HDAC2, you increase arginase 2, and when you boost HDAC2, you can decrease arginase 2,” they say.
Statins improve blood flow and protect the endothelium by reducing cholesterol levels and increasing nitric oxide production. Exercise improves blood flow by using the heart muscle to move blood more quickly. For people at risk of cardiovascular disease, this research represents a new hope for a therapy that could be used in conjunction with statins and exercise.
The study “OxLDL Triggers Retrograde Translocation Of Arginase 2 via ROCK and Mitochondrial Processing Peptidase” was authored by Deepesh Pandey, Anil Bhunia, Young Oh, Fumin Chang, Yehudit Bergman, Jae Kim, Janna Serbo, Tatiana Boronina, Robert Cole, Jennifer Van Eyk, Dan Berkowitz and Lewis Romer of the Johns Hopkins University School of Medicine and by Alan Remaley of the National Institutes of Health. It is in press in the journal Circulation Research.
The study “Transcriptional Regulation of Endothelial Arginase 2 by HDAC2” was authored by Deepesh Pandey, Gautam Sikka, Yehudit Bergman, Jae Kim, Lewis Romer and Dan Berkowitz of the Johns Hopkins University School of Medicine and by Sungwoo Ryoo of Kangwon National University in Korea. It was published in the July 2014 issue of the journal Arteriosclerosis Thrombosis and Vascular Biology.
These studies were funded by the National Heart, Lung and Blood Institute (grant HL089668), an American Heart Association Postdoctoral Fellow Award (13POST16810011) and the National Science Foundation (grant MCB-0923661).