Thenew study comes on the heels of a June 2003 report from Duke that a new drug, Bevacizumab (trade name Avastin), shrinks tumors and extends survival in patients with colorectal cancer that has spread. Spreading cancer is called "metastatic" cancer, and it is particularly deadly when it reaches the liver. More than 75 percent of colon cancer patients die as a direct result of metastases to the liver, so finding new ways to inhibit liver tumor growth is extremely important, said the Duke researchers.
Bevacizumab and the new treatment approach both work by blocking tumor angiogenesis, the process by which tumor cells grow new blood vessels. Bevacizumab inhibits a protein called vascular endothelial growth factor (VEGF), which malignant cells secrete in order to grow and maintain their blood vessels. When VEGF is blocked by bevacizumab, the tumor's blood supply is diminished and the tumor shrinks and slows its spread.
In the latest Duke study -- presented at the American Association for Cancer Research (AACR)annual meeting in Washington, D.C. -- another Duke team blocked angiogenesis by manipulating the Tie2 receptor on the surface of blood vessel endothelial cells. The Tie2 receptor works in one of two opposing ways: it can either counteract VEGF's message to activate blood vessel growth, or it can prime the blood vessel endothelial cells to respond to VEGF. How Tie-2 responds depends on which protein binds to it.
In tumor cells, the growth-promoting protein called angiopoeitin 2 binds more often to the Tie2 receptor than the growth-inhibiting protein angiopoeitin 1. The binding by angiopoeitin 2 to the Tie2 receptor primes endothelial cells to begin growing.
"In essence, angiopoeitin 2 makes the Tie2 receptor transmit a survival message to blood vessel endothelial cells," said Bryan Clary, Ph.D., assistant professor of surgery at Duke. "Tie2 controls whether the cells responds to VEGF. It is yet another route through which we could block blood vessel formation in tumor cells."
So, the researchers set out to prevent the Tie2 receptor from transmitting its survival signal. Without this signal, the endothelial cells die.
To prevent the Tie2 receptors from activating, the researchers inserted into colon cancer cells "artificial" versions of the VEGF receptor and the Tie2 receptor that mimic the real receptors. Called soluble receptors, they float freely outside the cell instead of sitting on the cells' surface. While floating in the extracellular "matrix," these soluble receptors intercept the proteins -- such as VEGF and angiopoeitin 2 -- that would otherwise dock to the cell surface receptors. Since the protein signals never reach the receptors, the receptors remain unbound. In their unbound state, the receptors cannot trigger the chain of events that commands new blood vessels to grow.
Essentially, soluble receptors are like thieves who intercept the goods before they reach their intended destination, said the researchers. Without the goods, the cellular workings related to blood vessel growth are halted.
"In animals whose cells were expressing soluble Tie2 or the soluble VEGF receptor, the liver metastases grew at a much slower pace," said Mark Dewhirst, Ph.D., D.V.M, a cancer biologist at Duke. "Survival among the mice expressing the soluble receptors was also greater."
The mean survival time increased from 28.6 days in control mice to 46.4 days and 51.3 days, respectively, in mice expressing soluble Tie2 receptor and soluble VEGF receptor. Both of these treatment groups exceeded survival of mice treated with endostatin -- a naturally occurring protein inside endothelial cells that inhibits blood vessel growth. Expression of angiostatin, another naturally occurring protein, did successfully inhibit tumor growth in this model.
"Inhibiting the Tie2 and VEGF receptors worked as well as or better than increasing certain natural angiogenesis inhibitors," said Dewhirst. "This is significant because it demonstrates that putting the brakes on the growth accelerators may be as important as building up the growth promoters."
While multiple agents are currently being tested to block tumor angiogenesis, the Tie2 receptor is a novel target, one that has yet to be fully tapped. The current Duke experiments in mice focused on angiogenesis in the liver because it is a common site for metastasis of many tumors, including colorectal cancer and breast cancer. Moreover, the liver provides a uniquely receptive environment for angiogenesis because it is the only organ in adults that can rapidly regenerate itself. Hence, the liver stores an abundance of blood vessel growth accelerators that are simply awaiting a signal to activate.
"Our Tie2 model is a relevant pre-clinical model that mimics the liver tumor environment in humans," said Clary. "It could represent a new therapeutic modality in the arsenal of drugs used to fight tumor growth and metastasis."