Feb. 5, 1998
Viper snakes can kill, but a protein in their venom prevents the spread of tumors in laboratory mice, and a molecular 'portrait' now under development may explain why, according to a University of Delaware scientist profiled in the new issue of Cardiology Today, mailed Feb. 4.
Venom from Macmahon's Viper (Eristocophis macmahoni), found in Afghanistan and Pakistan, contains the protein, eristostatin, which blocks the "metastasis" or spread of tumors in mice injected with cancer cells, notes Mary Ann McLane, an assistant professor in UD's Department of Medical Technology. Her studies of eristostatin's structure, though preliminary, could help pharmaceutical companies develop cancer-fighting drugs, says McLane's collaborator, Stefan Niewiarowski of Temple University Medical School's Department of Physiology and Sol Sherry Thrombosis Research Center.
"The next step," McLane says, "is to find out exactly what it is about the structure of eristostatin that gives it this exciting capability in mice." With UD colleague Mary E. Miele, an assistant professor of medical technology, McLane also plans to study eristostatin's effect on metastatic melanoma cells.
Eristostatin is one of many viper-venom "disintegrins"--proteins that interact with a family of cellular receptors called integrins, McLane explains. Disintegrins are "potent inhibitors of platelet aggregation and cell adhesion," and therefore prevent an early step in blood clotting, Niewiarowski says. Once injected into a victim's bloodstream, disintegrins from viper-snake venom stop the sticky protein, fibrinogen, from binding with platelets. The global quest to better understand disintegrins, launched in the late 1980s, already has resulted in a commercially available anti-platelet drug, based on a synthetic version of these proteins' "three-dimensional scaffolding," McLane notes.
McLane and Niewiarowski got their first glimpse of eristostatin's anti-tumor action several years ago, as part of a project directed by Canadian researcher Vincent L. Morris of the University of Western Ontario. Melanoma cells were injected into cancer-susceptible mice, some of which also received eristostatin. Eleven days later, eristostatin had clearly reduced the average number of liver tumors--from 14.4 among unprotected mice to 0.6 within the treated population, the researchers reported in Experimental Cell Research (Vol. 219, pp. 571-578, 1995).
Since then, McLane's ongoing molecular biology studies of eristostatin have focused on a protruding section of amino acids--the RGD loop, composed of arginine, glycine and aspartic acid--which is known to play a key role in binding with integrins. To learn more about the RGD loop, McLane compares eristostatin with echistatin, a disintegrin from the venom of Echis carinatus, another viper-type snake from the Middle East.
Though the amino-acid sequences of the two proteins are 68 percent identical, McLane says, echistatin exhibits markedly different binding behaviors. Compared to eristostatin, for instance, echistatin is far less effective at preventing fibrinogen from interacting with integrins, and can't prevent melanoma cell metastasis. Echistatin also interacts with receptors on blood vessel walls--a trick that eristostatin has not mastered. "We want to learn what structural differences allow echistatin to be promiscuous, binding to so many receptors, while eristostatin is so selective," McLane explains.
How does she compare the two proteins? First, she manipulates a gene containing the code for echistatin to create mutations within echistatin's RGD loop. Next, she places the resulting DNA (deoxyribonucleic acid) in a bacterial expression system, thereby forcing the bacteria to make the altered snake protein. The goal, she says, is to give echistatin the binding region of eristostatin--one amino acid at a time. Finally, she tests each mutant's ability to interact with receptors on platelets, to identify the exact sequences most critical for binding. She soon will conduct the same tests using lymphocytes, or disease-fighting cells. And, with Miele, she will focus on whether eristostatin can stop cancer cell growth.
McLane's work, funded by the American Heart Association (AHA), thus far suggests that a triple mutation in echistatin can make it act like eristostatin with platelets and endothelial cells. In other words, she says, "Out of 13 amino acids in echistatin's RGD loop, I can change three and make it work like eristostatin." Although she suspects that eristostatin's ability to stop the spread of tumors in mice may be mediated by an integrin receptor, which cancer cells have in common with white blood cells, the exact mechanisms remain a mystery--for now. "This question has been eluding us, but I now have the gene that codes for eristostatin, and I'm optimistic that I will discover some answers." With AHA support, meanwhile, McLane also explores the potential of disintegrins for treating thrombosis--especially arterial thrombosis.
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