Scientist Identifies Key Element in Heart Cell Death

A scientist in the Department of Surgery at The Ohio State University Medical Center has identified a key element in the process that causes the death of heart cells after ischemia and reperfusion, or blockage and restoration of blood flow. Reperfusion injury occurs when patients with blocked arteries receive medical or surgical treatments to restore blood flow. The resulting restoration of blood, along with the oxygen it carries, causes tissue damage and weakens the heart’s function.

Pedram Ghafourifar, Pharm.D., Ph.D.

Pedram Ghafourifar, Pharm.D., Ph.D., associate professor of surgery and director of basic science research in the Division of General Vascular Surgery, and his colleagues have identified cardiac cell mitochondria as the source of the signal that causes cell death following hypoxia, or a shortage of oxygen, and subsequent reoxygenation. Cardiac cell mitochondria are the principal energy source in the cells.

Their study, the results of which were published in the October 2007 issue of the Journal of Molecular and Cellular Cardiology, shows for the first time that heart mitochondria respond to varying oxygen concentrations during hypoxia and reoxygenation. The importance of Ghafourifar’s findings was highlighted by Dr. Louis Ignarro, the 1998 Nobel laureate, in an editorial that preceded Ghafourifar’s paper.

The researchers hope that identifying the origin of the signal that causes cell death will help them find a way to stop the signal and reduce the damage associated with restoring blood flow to the heart.

“This form of cardiac cell death is a major medical and health issue,” Ghafourifar says. “The patient has severe pain from the loss of blood flow and oxygen to the heart, so we cannot do anything other than clear that artery to restore the blood and oxygen. But when that is done, cardiac cells start to die. It’s a paradox. The mitochondria have been suspected in this process, but to date, we haven’t known for sure.”

Ghafourifar’s lab developed a technique allowing researchers to investigate isolated mitochondria in real time during reoxygenation. Using chemical probes and this novel technique, called dual wavelength excitation spectrophoto-fluorometry, they observed that as soon as the hypoxic mitochondria were subjected to reoxygenation, calcium increased in the mitochondria.

“Calcium levels went up like never before, which is unusual, because mitochrondria typically are able to tightly maintain a low level of calcium,” he says.

Next, the calcium stimulated an enzyme that generated toxic levels of the free radical nitric oxide in the mitochondria. Then the excess of nitric oxide in mitochondria led to the release of a mitochondrial protein, which resulted in cell death.

The enzyme in this process is called mitochondrial nitric oxide synthase, which was discovered and reported by Ghafourifar’s lab in 1997. Because researchers don’t know the cause of the calcium increase during reoxygenation of the heart, Ghafourifar and his colleagues have focused on the enzyme.

“The next immediate step is finding how we can inhibit this enzyme, so it doesn’t generate excess nitric oxide during the reoxygenation phase,” he says. “We’re developing experimental drugs that can be delivered at the time of reperfusion or just before. Some seem to be successful in selectively inhibiting the enzyme.”

The identification of the enzyme as a trigger of cell death could influence a wide range of therapeutic options, Ghafourifar says, for disease processes characterized by cell death, and for others in which cells refuse to die when they should.

“Cell death is involved in a variety of diseases that don’t seem to be related,” he says. “In cancer, cells do not die. In Parkinson’s and Alzheimer’s disease, cells die earlier than we want them to. If we figure out how cell death happens, we can put up a fight against a number of diseases.”