Stanford Report, July 11, 2001
Researchers discover nicotine stimulates growth of new blood vessels
BY KRISTA CONGER
Nicotine promotes the growth of new blood vessels and can also stimulate tumor growth and the build up of plaque inside arteries, say researchers at Stanford University Medical Center. The finding is the first proof that nicotine affects blood vessel formation.
It suggests that while nicotine treatment may be useful to revive tissue deprived of blood by a stroke or heart attack, physicians should exercise caution when considering the long-term use of nicotine as a treatment. Currently the chemical is a useful tool in smoking-cessation programs and is being studied as a potential therapy for Alzheimer's and Parkinson's diseases, as well as chronic pain.
The scientists, led by John P. Cooke, MD, PhD, tested levels of nicotine similar to those that would be found in a moderate smoker puffing about 20 cigarettes each day. They emphasize, however, that it's difficult to directly compare nicotine's effects with those caused by tobacco smoke, which contains thousands of additional components. The results of the study are published in the July issue of Nature Medicine.
In a series of experiments, the researchers found that nicotine could enhance new blood vessel growth in mice whose hind limbs were artificially starved of oxygen. They also found that lung cancer cells implanted into mice grew more quickly when the mice consumed nicotine in their drinking water. Mice susceptible to developing plaque in the arteries of their hearts also experienced more rapid plaque growth when exposed to nicotine than mice who were not exposed.
The researchers believe nicotine works by binding to a receptor on the surface of endothelial cells that recognizes acetylcholine -- a chemical that nerve cells use to communicate with each other. Endothelial cells line the interior of blood vessels throughout the body. These nicotinic acetylcholine receptors were only recently discovered outside the central nervous system and their function is not well understood. The ability of nicotine to modulate the activity of these receptors in the central nervous system renders it a potential therapy for neurologic disorders.
Researchers speculate that when nicotine binds to the receptor on endothelial cells it stimulates the cells to release a cascade of chemicals that promote the formation of new blood vessels. The new vessels then deliver oxygen and nutrients to the interior of tumors and plaque deposits.
The Stanford researchers were surprised to find how strongly nicotine stimulates new blood vessel growth, a process known as angiogenesis.
"We expected to see that nicotine impairs angiogenesis because it's known that smoking impairs endothelial function," said Chris Heeschen, the first author of the paper and a postdoctoral fellow in cardiovascular medicine in Cooke's laboratory. "But nicotine is not smoking."
Under some experimental conditions the magnitude of nicotine's effect was comparable to that seen with naturally occurring proteins involved in angiogenesis. In every case, chemicals known to inhibit other molecules associated with angiogenesis also blocked the effect of nicotine -- confirming that nicotine affects blood vessel formation.
The researchers began the study by testing whether nicotine could stimulate the division of cultured endothelial cells. They found that nicotine was as effective as vascular endothelial growth factor, or VEGF -- a naturally occurring, well-defined protein that has long been known to enhance endothelial cell proliferation. Cells cultured in the presence of nicotine or VEGF also assembled themselves into whorls in the tissue culture dish -- perhaps a preliminary step to forming new blood vessels. Chemicals that block nicotine's binding to the receptor also blocked proliferation of the cells.
The researchers also found that nicotine enhanced the vascularization of discs implanted under the skin of mice as well as another naturally occurring compound, known as fibroblast growth factor. The discs cause inflammation, which frequently stimulates new blood vessel growth.
To test the ability of nicotine to restore blood flow in oxygen-starved tissue, the researchers tied off a main artery that supplies blood to one hind limb in mice, and then injected nicotine daily into the affected leg. After three weeks the nicotine-injected limbs sported a higher blood vessel density and were receiving more blood flow than those that had been injected with a saline solution.
Finally, the researchers tested nicotine's effect on diseases such as lung cancer and atherosclerosis. They found after only a few days that lung cancer cells implanted under the skin or in the lung tissue of animals who drank nicotine-laced water grew much more quickly and were more densely packed with blood vessels than cancers in animals who were not exposed to nicotine. Mice bred to accumulate plaque in the linings of their arteries also suffered from nicotine exposure. The plaque blockages grew more quickly and were thicker in these mice than in the control animals.
The acceleration of tumor and plaque growth raises a concern about the use of nicotine in patches or gum as an adjunct to smoking-cessation programs. However, Cooke pointed out that "it is critical for people to stop smoking because of the strong evidence that tobacco markedly increases your risk of heart attack, stroke, cancer and lung disease. Therefore, the benefits of nicotine patches and gums far outweigh the risks when the products are used as directed."
Cooke and his colleagues plan to continue their research into the effects of nicotine on angiogenesis.
In addition to Cooke and Heeschen, the other Stanford investigators involved in the study were James Jang, MD; Michael Weis, MD; Anjali Pathak, MD; Shuichiro Kaji; Robert Hu, MD; Philip Tsao, PhD; and Frances Johnson, MD.
The study was funded by the National Heart, Lung and Blood Institute, the Tobacco Related Disease Research Program and the German Research Council. Stanford and the authors have patented the use of nicotine for therapeutic angiogenesis. This patent has been licensed to Endovasc Inc. and may generate royalties for the authors. SR