Scientists supported by the National Institute of Dental and Craniofacial Research (NIDCR), part of the National Institutes of Health, have added a key new piece to the puzzle of how tumor cells induce new blood vessels to form and fuel their abnormal growth, a well-known process called angiogenesis.
As published in this months issue of the journal Cancer Cell, the scientists found that in addition to the well-known strategy of secreting proteins to trigger angiogenesis, tumor cells also physically attach to a protein displayed on the surfaces of cells that line the walls of our blood vessels. This physical interaction, like a finger pushing a button, sends a signal within these cells to grow and sprout new capillaries.
The finding, while technical in nature, has potentially major implications for anti-angiogenic therapy, one of the hottest areas in cancer research. Dr. Cun-Yu Wang, a scientist at the University of Michigan and senior author on the paper, said the finding suggests a future anti-angiogenic strategy of blocking not only the secreted molecules but also the cell-to-cell contact.
Wang said these early data also suggest the intriguing possibility of directing growth-inhibiting drugs at the normal blood vessel cells to stop angiogenesis. Its well established that tumor cells can become resistant to chemotherapy, said Dr. Wang. For endothelial cells, which are the cells that line the walls of the blood vessels, there is no indication that resistance is a problem. Its an intriguing idea, and one that we think might be well worth pursuing.
Wang said his group began a few years ago studying a secreted protein called hepatocyte growth factor, or HGF, and its role in helping head and neck tumors to turn cancerous. HGF does so, in part, by helping to induce nearby blood vessels to grow misguidedly toward and eventually into the developing tumor for nourishment. Still unanswered was exactly how HGF sets the angiogenic process in motion, said Dr. Qinghua Zeng, lead author on the paper and a researcher at the University of Michigan, noting that HGF also has a pro-angiogenic effect in other tumor types. We needed to connect the molecular dots.
Zeng and his colleagues conducted a series of experiments under carefully controlled laboratory conditions to determine whether, as they suspected, HGF stimulates head and neck tumor cells to release pro-angiogenic proteins. To their surprise, they found that was not the case. Tumor cells stimulated by HGF strongly promoted the formation of a capillary-like network compared with secreted factors induced by HGF alone. At this point, we didnt have any idea of what was going on, said Wang. But we started to think that it must involve the direct interaction between the tumor and endothelial cells.
Wang said thats where luck entered the picture. He and his colleagues decided to take a closer look at a vast body of data that they had generated a few years earlier showing thousands of genes that HGF activates in head and neck tumor cells. The gene that was among the most expressed is called jagged1, which is known to bind to a specific protein on the surface of endothelial cells. I thought, Oh, this makes sense, said Wang. The jagged1 protein is not secreted but is displayed on the surface of the tumor cells. I speculated that HGF induced jagged1 levels to increase, leading to a direct surface to surface interaction between the tumor and endothelial cells.
Wangs hunch also made good intuitive sense for another reason. The jagged1 protein bound in a hand-in-glove manner to a protein on the endothelial cells called notch. Other laboratories have shown that notch plays a key role during human development in forming blood vessels. Oddly, Wang noted, the possible role of notch in tumor angiogenesis has not been well studied.
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