When Paths Collide—Redundancy, Angiogenesis, Cancer, and Combinatorial Therapy

By Jason Socrates Bardi

Redundancy may be a classic mark of poor writing, but in other crafts, redundancy is not always, always a bad thing.

Engineers use redundancy as a paradigm to ensure safety in design, and redundancy has evolved in biological systems for apparently the same reason. Having more than one copy of the same gene, for instance, means that people who inherit a mutant form of a certain gene from one parent might still live healthy lives with a healthy gene from their other parent.

But when a set of normal biological processes go awry, redundancy can confound the best efforts of science.

Such is the case in cancer. For years scientists have sought to fight cancer by designing compounds that block angiogenesis—the proliferation of blood vessels that often accompanies the growth of solid cancer tumors. These new blood vessels bring necessary nutrients and oxygen to the hungry tumor cells. Block angiogenesis, the thinking goes, and you can starve a tumor—like drying out a lake by diverting all its tributaries.

But the body has more than one way of proliferating blood vessels. A number of years ago, several scientists including Professor David Cheresh, a member of the Department of Immunology at The Scripps Research Institute (TSRI), described how angiogenesis can be initiated by different growth factors, including vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF).

Significantly, VEGF and bFGF can both induce angiogenesis in tumor cells by coordinating their activities with different "integrin" cell adhesion receptors. VEGF acts with the integrin protein alpha(v)beta(5), and bFGF acts with the integrin alpha(v)beta(3). Designing drugs to target only one of these growth factors/integrin pairs may not be completely effective at stopping angiogenesis or tumor growth because a cancerous cell might simply use the other pathway.

But the two pathways are not entirely distinct from one another.

In a paper published in a recent issue of the journal Science, Cheresh and a team of scientists at TSRI have described how these two pathways intersect in the activation of a protein called Raf—a "kinase" enzyme that is involved in signal transduction. Cheresh and his colleagues describe how endothelial cells use VEGF and bFGF as survival factors to activate Raf as a way of protecting themselves against distinct inducers of apoptosis (programmed cell death) in order to survive—during tissue remodeling events associated with wound repair, inflammation or cancer.

Interestingly, activated Raf does two different things in the cell depending on whether it is activated by VEGF or bFGF. When Raf is activated by bFGF, the activated Raf is driven to the cell's mitochondria, where it protects the cell from stress-mediated death, providing protection against radiation or chemotherapy, which would normally cause a cell to undergo apoptosis.

When Raf is activated by VEGF, on the other hand, the activated Raf protects the cell against receptor-mediated apoptosis, which happens at a site of inflammation, for instance, where the body's cells release chemicals like tumor necrosis factor (TNF) or Fas, which can bind to receptors on otherwise healthy cells and induce cell death. These two pathways of endothelial cell survival probably exist to ensure angiogenesis can take place among remodeling tissues within distinct microenvironments, says Cheresh.

Importantly, tumors induce a blood supply by activating both of these angiogenic pathways. Consequently, Cheresh and his colleagues' findings have a significant impact on how to attack the tumor blood supply: blocking Raf activity might promote endothelial cell death regardless of the angiogenic stimulus.

This idea further validates Raf as a fundamental target for the design of drugs for treating cancer. A year ago, Cheresh and his colleagues published another paper in Science describing an experiment in which they delivered a mutant form of the Raf gene to tumor-bearing blood vessels. The mutant Raf caused endothelial cells within the tumor to die, which was then followed by tumor cell death and regression of large preexisting tumors.

Rather than targeting the growth factors or their receptors, a drug might be designed to target Raf—as a single "bottleneck" downstream of VEGF and bFGF, thereby ensuring blood vessel death regardless of the angiogenic stimulus.

In addition, cancer cells probably use Raf for their own survival in a similar way. Tumor-cell-associated Raf can be activated either by growth factors within the tumor microenvironment or oncogenes within the tumor cells themselves. Once activated, Raf can promote one or both of the pathways leading to cell survival. Therefore, besides validating Raf as a target to block angiogenesis, this work also points the way for designing strategies for combining existing anti-cancer drugs to attack tumors directly.

Combinatorial drug therapy is a treatment paradigm that has proven effective in diseases ranging from AIDS to cancer. But the key, of course, is knowing which drugs to combine. In this case, understanding that Raf in tumor cells can promote survival in the face of distinct inducers of cell death will make it possible to either target Raf directly or combine drugs to effectively ensure that both apoptotic pathways are active. For instance, one might combine a drug like Taxol that induces stress-mediated death with one that promotes receptor-mediated death.

"The combination of those approaches might prove to be very potent or synergistic," says Cheresh. "We think these studies will impact our ability to design and use drugs to block angiogenesis and will also help design and use combinatorial drugs against cancer directly. However, for now we are focusing most of our efforts on targeting Raf itself, which appears to be a key enzyme that proliferating endothelial cells and tumor cells use to stay alive."

To read the article, "Role of Raf in Vascular Protection from Distinct Apoptotic Stimuli" by Alireza Alavi, John D. Hood, Ricardo Frausto, Dwayne G. Stupack, and David A. Cheresh, please see the July 4, 2003 issue of the journal Science, or go to: http://www.sciencemag.org/cgi/content/abstract/301/5629/94.




Schematic showing the differential activation of Raf by FGF and VEGF. Click to enlarge.