Does the Heart Contain Stem Cells?
Jul 22, 2005 - 12:54:38 AM
Steven Houser, Ph.D., Director of Cardiovascular Center for Temple University School of Medicine and Senior Associate Dean of Research, is sold on the idea that the heart - like the skin - contains its own stem cells: cells that are self-renewing and can be differentiated into different types of heart tissue. It's a controversial subject in cardiovascular circles, but for Houser, who spent thirty years studying the molecular biology of heart cells, the stakes are worth it when it comes to combating congestive heart failure (CHF).
CHF, which afflicts 4.8 million Americans, occurs when the heart no longer has the strength to pump blood efficiently, resulting in an increasingly diminished quality of life. While a wide variety of well-understood conditions such as high blood pressure or excess weight can lead to CHF, a cure for the debilitating disease is less clear. Invasive surgical procedures such as a bypass operation or the insertion of a stent can extend a patient's life, but the essential damage to the heart caused by the condition remains. Pharmacological therapies to improve the function of cardiac muscle cells improve heart function, but the treatments often lead to fatal arrhythmias, canceling out any benefits. To Houser, the problem proved frustrating.
"Cells were dying in the heart and there were simply not enough new cells to replace them," he says. "They were tired soldiers who could no longer work efficiently, and there were no new soldiers coming to take their place."
Although stem cells have been found in many other organs in the body, including the brain, many cardiac researchers remain unconvinced that the heart contains stem cells. For one, stem cells can over multiply and cause tumors, and the heart rarely gets tumors. These facts and the observations that adult cardiac myocytes do not have the capacity to proliferate have led most investigators to accept the dogma that no new cardiac myocytes are manufactured in the normal heart.
Houser respectfully disagrees. Abandoning his prior cell research, he joined forces with one of the foremost investigators in cardiac stem cells, Pierro Anversa, Ph.D. professor of medicine and Director of the Cardiovascular Institute at the New York Medical College at Valhalla, New York. Anversa, who has been on the forefront of stem cell research for the past five years, has suggested that heart cells undergo an ongoing turnover fueled by cardiac stem cells. In June of this year, Anversa published a study that actually identified cardiac stem cells in animal models that repaired tissue damaged by a heart attack.
One element that convinced Houser of Anversa's work was his own research into how the heart reacts under the stress of hypertensive diseases that can lead to congestive heart failure. Early in the disease, the heart muscle mass increases and the chambers stretch in a vain attempt to increase contracting power. While part of the enlargement is due to increased muscle mass, the question of how the chambers grow is less certain.
The traditional view holds that cardiac cells simply grow larger to accommodate the increased need, but Houser and Anversa developed a different theory - that spurred by the cardiac stem cells, cardiomyocytes actually increase in number in their response to the heart's traumatic condition.
"It was striking that with hypertension there were actually more cardiomyocytes than were originally lost," says Houser. "New myocytes were forming in excess of the cells that were dying."
And if that were true, these were the new soldiers to replace those missing in action.
To test this theory, Houser, with the help of Anversa, has received a new NIH grant to study if there are autologous stem cells in the heart. The two researchers have arrived at a deceptively simple idea. After inducing hypertension in an animal model to produce a distressed heart they will study the heart tissue and count cells, first in the normal heart and then in a heart that must work harder to develop excess pressure. If, according to the scientists' thesis, there are more cardiomyocytes in the heart as opposed to simply larger cells, they will conclude that stem cells had a hand in an attempt to repair and restore the heart.
If their hunch proves correct, the implications for treatment of heart disease are profound. In the future, for example, Houser envisions people banking their own stem cells, so that when a problem arises, new tissue can be made by injecting these cells into the damaged heart. The idea is simple, use the patients own cardiac stem cells to repair their damaged heart.
"We've made a tremendous impact on cardiovascular diseases such as congestive heart failure through the hard work of physicians, better service, stents and bypasses," he says. "But what we need to do now is to reverse this disease rather than just slow its progression."
Although his colleagues may remain skeptical, Houser has committed himself to the stem cell model. Inducing cells to promote repair can answer the question that has haunted his career.
"We don't know how to fix the broken heart, but stem cells might be a large part of the answer."
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