Study Identifies Cellular Signal Implicated in B-Cell Malignancies

A collaboration between an oncology research lab and a cell metabolism lab at the University of Wisconsin Carbone Cancer Center (UWCCC) has identified the anti-cancer role of a stress response gene in B cell malignancies and provided insight into future treatments.

Dr. John Denu, professor of biomolecular chemistry, and Dr. Peiman Hematti, professor of medicine at UW School of Medicine and Public Health, combined their expertise to determine how the loss of a protein, sirtuin3 (SIRT3), in B cell malignancies promotes growth in those cancerous cells. The findings suggest that treatments which lead to increased SIRT3 levels may be a promising area of future research.

B cells are a subset of white blood cells, and their malignancies include some leukemias and several forms of lymphoma.

“For nearly two decades I've been interested in a family of proteins, the sirtuins, that regulate metabolism and gene expression,” Denu said. “There have been several papers published with discrepancies of if SIRT3 was up- or down-regulated and if that were causative for B cell malignancies, so it was important to look at protein levels to resolve this question.”

Hematti gave Denu anonymous samples from chronic lymphocytic leukemia (CLL) patients, on which their groups ran analyses comparing SIRT3 levels in the leukemia samples to non-cancerous normal B cells.

“The CLL samples had almost no detectable SIRT3 compared to a normal B cell, and similar trends held across other B cell malignancies tested,” Denu said.

He noted, however, that at this point they had only identified a correlation between low SIRT3 levels and malignancy.

“I'm a biochemist, we don't like correlation,” Denu said. “I like to take an observation and follow it up by providing evidence of cause and effect.”

To determine if the reduction of SIRT3 protein directly contributed to the proliferation of malignant B cells, the team looked at the protein targets of SIRT3, including the oxidative stress- response proteins IDH2 and SOD2. These two target proteins are largely responsible for mitigating the effects of reactive oxygen species (ROS) that otherwise damage DNA and proteins and act as a pro-proliferation signal, but only if they are first chemically modified to become active by SIRT3.

Using malignant B cell samples, the researchers showed that IDH2 and SOD2 proteins were present in the cells but inactive, because the low SIRT3 levels meant much less chemical modification could occur. Additionally, they showed that these cells contained much higher than normal levels of ROS. If they re-introduced SIRT3 to the malignant B cells, then IDH2 and SOD2 were active, and the cell's growth rates and level of ROS were greatly reduced.

“We were able to show SIRT3's function is to upregulate the activity of these antioxidant enzymes, and the cancer cells are suppressing SIRT3 levels because that confers a growth advantage,” Denu said.

In the future, Denu and colleagues hope to identify therapeutics that can stabilize SIRT3 levels and treat B cell malignancies, and said that there is evidence that some natural- product small molecules can stimulate the sirtuin family of proteins.

Campus collaborations are often beneficial and are nothing new, but in this case it was especially easy to arrange: Denu's nephew, Ryan Denu, an MD/PhD student in the Medical Scientist Training Program at the UW School of Medicine and Public Health, was doing research in Hematti's lab and contacted his uncle about screening tumor samples for SIRT3 levels.

“Studies such as this one are why we need to partner clinical labs with basic-science labs, and I hope we continue to do that,” Denu said.

The study was published in the Journal of Biological Chemistry and funded in part by a pilot project grant awarded by UWCCC and by NIH grants AG038679 and GM065386. Other UWCCC faculty involved in the study were Drs. Fotis Asimakopoulos, assistant professor of medicine, and David Yang, associate professor of pathology and laboratory medicine.