Investigating Cell Death: Developing More Effective Cancer Therapies

BY HEATHER ROCK WOODS

WINTER 2001 - One of the most promising investigations in cancer biology is the study of apoptosis, the natural process of cell death that is important to development and to maintaining tissue health. Some cancer cells have developed mechanisms to inhibit apoptosis and thereby are resistant to many cancer therapies. Understanding the mechanisms of apoptosis can lead to more effective therapies for childern with cancer.

Tracking Success

Critical to advances in treating cancer is an accurate and efficient method to measure the extent to which therapies kill cancer cells. Using innovative radiology techniques in the imaging of cell death, Francis Blankenberg, M.D., a pediatric radiologist at Packard Children's Hospital, has discovered a non-invasive technique to show almost instantly whether chemotherapy is successful.

Francis Blankenberg, M.D., a pediatric radiologist, and Louie Naumovski, M.D., Ph.D., a cell biologist in the Department of Pediatric Oncology and Hematology, are helping to develop more effective chemotherapies by studying the mechanisms of cell death.

He uses a non-toxic human protein called Annexin V that sticks to dying cells like burrs to dog fur. Injected into people and lab animals, this radioactively labeled protein binds to cells undergoing apoptosis, revealing the location and extent of cell death on a single photon emission computer tomography (SPECT) scan. When chemotherapy succeeds, it causes apoptosis in tumor cells. Apoptosis is the first sign of effective treatment and it occurs on a cellular level in just hours -- long before tumors are visibly affected.

"This technique is especially useful for bone marrow diseases. If children don't respond to therapy in the first day or two, we can catch it and switch therapies without having to wait four weeks for a clinical evaluation and biopsy," says Blankenberg, an associate professor of radiology and of pediatrics.

Blankenberg created the technique so children wouldn't need painful bone marrow biopsies, which are usually drawn from the hip, to determine a therapy's progress. "Annexin V imaging also tells you more than a bone marrow biopsy; it tracks cell death over the whole body, and not just at the hip," says Blankenberg, "And, you can follow up everyday because it's totally safe."

Though still two years away from being used in children, Annexin V imaging has been successful in animals and in adult trials. Blankenberg first used the technique to visualize apoptosis in the bone marrow cells of rats responding to chemotherapy. It is now being tested for safety and efficacy in adults to detect cell death from stroke, myocardial infarction, and rejection of heart transplants, among other conditions. Results to date have shown Annexin V imaging to be a reliable indicator of apoptosis.

"I can't think of a single branch of medicine that this won't impact, since apoptosis is a major factor in so many diseases," Blankenberg remarks. "Understanding apoptosis will be for the new century what the discovery of DNA was for the last century."

Combating Chemo-resistant Cells

The imaging techniques developed by Blankenberg are especially valuable to researchers like Louie Naumovski, M.D., Ph.D., a cell biologist and assistant professor in the Department of Pediatrics at Stanford. In collaboration, they are studying how cells react to chemotherapy, and what makes cells live longer or die faster. Naumovski hopes to apply his growing knowledge about the function of several key proteins in apoptosis to help patients who do not respond to treatment.

Chemotherapy works by introducing chemicals that cue apoptosis in cancerous cells. Normally, apoptosis kills off aging or ailing cells to keep the body healthy. Chemotherapy triggers this natural process in cancerous cells, causing them to destroy themselves. But for some children with advanced and aggressive tumors, even high doses of chemotherapy followed by bone marrow transplants -- which are designed to eradicate the cancer cells -- do not succeed or are not possible.

Naumovski's lab has discovered the function of several important proteins in the promotion or hindrance of apoptosis. One protein, 53BP2, has been found to promote cell death. Expression of 53BP2 in tumor cells in the lab makes the cells more easily killed by chemotherapy.

Naumovski has also determined the function of another protein, ARMER, which inhibits apoptosis in tissue culture cells. His team is trying to decrease ARMER's activity in tumor cells so that they become vulnerable to chemotherapy. At the same time, they are attempting to increase its activity in normal cells, in hopes that ARMER would protect the healthy cells that are often killed by chemotherapy, such as those in bone marrow, intestines, gonads, and hair follicles. Explains Naumovski, "The goal is to minimize toxicity to normal cells and to maximize toxicity to tumor cells."

Naumovski's research supports the theory that it is a specific abundance of proteins that inhibit apoptosis and make a malignant cell resistant to chemotherapy. "We think that the process of how a cell responds to chemotherapy is controlled by its genetic makeup and the proteins the cell produces," he says.

As Naumovski illuminates the role of proteins in apoptosis, his patients continue to inspire him.

"We have a wonderful environment here where pediatric oncologists like myself can take care of patients and see what patients' problems are, so that we can focus our research on what we know are major, clinically relevant issues," he says. "There couldn't be a better setting to foster this research that will have such a direct bearing on children's lives."