UM/Sylvester Researcher Shows Enzyme Can Fight Cancer
Jaime R. Merchan, M.D., M.M.Sc., assistant professor of clinical medicine in the Department of Medicine, Division of Hematology-Oncology at the University of Miami Sylvester Comprehensive Cancer Center, has published an important study in the Journal of the National Cancer Institute on the workings of proteases in tumors. Proteases are enzymes that cleave, or split, proteins and they are known to play a role in the growth and spread of cancer. “There is a widely accepted dogma or paradigm that proteases are bad,” said Merchan, who recently joined the faculty of UM/Sylvester from the Mayo Clinic in Minnesota. “Our study provides evidence that proteases of the plasminogen activator system may negatively regulate tumor progression and that the tumor-delaying effects are directly due to their proteolytic activity.”
Merchan and his colleagues studied two proteases that are suspected of playing a role in the growth of new blood vessels to tumors: tissue plasminogen activator, or tPA, and urokinase, or uPA. Many oncologists have long suspected that protease enzymes help tumors grow new blood supplies and that has fueled research into protease inhibitors as anti-cancer drugs. They can be very effective against viruses – for example, at least nine different protease inhibitors are approved for use against HIV/AIDS. Protease inhibitors block the ability of the virus to replicate, preventing its spread. Many investigators thought this might be a good strategy against cancer as well.
In a series of experiments Dr. Merchan and his colleagues genetically engineered breast cancer cells to produce the proteases tPA and uPA in excess of the levels that tumor cells produce. When those tumor cells were implanted into the mammary glands of mice, Merchan observed that tumors with tPA overexpression grew at about half the rate of the control (untreated) breast tumors and that the uPA tumors grew even slower – at about one-fourth the rate of the regular tumors. The protease-overexpressing tumors were also less likely to metastasize, or spread. And they found in animal models that, compared to untreated tumors, overall survival was one-third longer in the tPA group, and twice as long in the uPA group. “The survival data was very significant,” said Merchan.
To determine whether the anti-tumor effects were due to the enzymatic activity of the proteases, the researchers genetically modified urokinase (uPA) to make it “proteolytically inactive.” Tumors expressing the proteolytically inactive urokinase grew faster than tumors exposed to “active” urokinase and at about the same rate as untreated control tumors.
“We observed that tumors overexpressing urokinase and tPA had fewer tumor blood vessels than tumors not overexpressing the proteases, and had less tumor proliferation,” said Merchan. Curiously, the proteases only stopped tumors in animal models, not in isolated cancer cells in the lab. “That led us to conclude that the proteases overexpressed by the tumors are not directly toxic to the tumor cells, but that they block tumor growth indirectly.”
Merchan has two theories about how the proteases work. The overexpression of proteases may disrupt the stroma, or protein and cell tissue that make up the microenvironment around the tumor. That would create an unfriendly environment for cancer. Another possibility is that the proteases may break apart some proteins in the tumor stroma which are precursors of smaller proteins and peptides that block blood vessel growth to tumors. When the proteases cleave those cells they may be converted to enzymes that fight, rather than encourage, a new blood supply.
This study builds on a previous report (JNCI, 2003) in which Merchan and his team discovered that the protease tPA blocks the growth of new blood vessels when mixed with human plasma. Moreover, when patients with cancer were treated with tPA and a drug called captopril, the plasma of those patients changed. Plasma that normally would induce angiogenesis, or new blood vessel growth, instead blocked it. Merchan next plans to study these proteases in other types of cancer and to try to explain the cellular and molecular mechanisms behind the antitumor effects of proteases. He hopes to have more data within a year.
“This study challenges the current dogma that proteases are ‘bad,’ and that in order to treat tumors, proteases have to be blocked,” said Merchan. “Based on our results, we say that proteases are neither ‘good’ nor ‘bad’ but that their positive or negative effects depend mainly on the balance between the proteases and their inhibitors, a balance that is tightly regulated in space and time. Our data strongly suggest that if we shift that balance in favor of proteases (by ‘forcing’ tumors to produce more proteases), tumor growth may be blocked.”
This is a new way of thinking about proteases and the hope is to translate it into a viable therapy. “The important question which is being asked in our laboratories is just when during tumor progression does it make sense to shift the proteolytic balance in favor of proteases,” Merchan said. “Our results caution against the widespread use of protease inhibitors, especially urokinase inhibitors, against cancer.”
Merchan’s study is published in the June 6 issue of the Journal of the National Cancer Institute and is available online at this link: http://jncicancerspectrum.oxfordjournals.org/jnci/current.shtml.