| The role of hypoxia in regulating tumor angiogenesis de novo and after therapeutic intervention
With the advent of fluorescent reporter genes, we have been able to conduct a number of novel studies, using skin fold window chambers, to investigate some of the steps involved in very early tumor angiogenesis. Using GFP transfected cell lines, we have serially monitored tumor cell proliferation, migration and interaction with pre-existing vasculature, prior to and after the onset of angiogenesis. We found paracrine survival relationships between tumor and endothelial cells that develop <24hr after transplantation, well prior to overt angiogenesis starts. VEGF, bFGF, Angiopoeitin 2 and interestingly, EPO are involved. Overexpression of Angiopoeitin 2 causes vascular regression and widespread tumor cell apoptosis, as opposed to promotion of angiogenesis.
We have also explored whether hypoxia is a requirement for the onset of angiogenesis. We used doubly transfected tumor lines containing constitutively active RFP and HIF-1 dependent GFP. These studies showed that angiogenesis is initiated before HIF-1 activation. To further prove that hypoxia was not required for nascent angiogenesis, we treated animals with tirapazamine, a selective hypoxic cytotoxin. Tirapazamine delayed onset of HIF-1 expression, but had no effect on angiogenesis initiation. It is likely that HIF-1 independent regulatory pathways controlling VEGF, etc. are responsible for angiogenesis initiation.
We have also explored the question of tumor hypoxia during early tumor angiogenesis by evaluating hemoglobin saturation distributions in vascular networks. These results reveal complex patterns of oxygen delivery to tumors, which undoubtedly gives rise to hypoxia reoxygenation injury. The consequences of this pathophysiology will be discussed.
When tumors are irradiated. HIF-1 is upregulated by a free radical mediated mechanism during a period of improved oxygenation. Superoxide dismutase mimetics greatly sensitized tumor blood vessels to radiation damage.
In summary, cells expressing fluorescent reporter genes provide unique opportunities to learn about key steps in early angiogenesis and tissue responses after treatment.
Biography
Mark W. Dewhirst, DVM, PhD is the Gustavo S. Montana Professor of Radiation Oncology and Director of the Radiation Oncology Program of the Duke Comprehensive Cancer Center . He also holds appointments in the Departments of Pathology and Biomedical Engineering at Duke and in the Department of Anatomy Pathology and Radiology at the School of Veterinary Medicine at North Carolina State University . Dr. Dewhirst joined the faculty of Duke University in 1984 and was promoted to Full Professor in 1993. He received an endowed professorship in 2002. In addition to directing a clinical program grant to study the use of hyperthermia in the treatment of cancer, Dr. Dewhirst has research interests in tumor hypoxia, angiogenesis and drug transport, with an emphasis on translational research. Current work examining the potential role of hyperthermia to augment selective drug delivery to tumors is one example. He has collaborated with faculty in the Engineering School to develop a novel temperature sensitive liposome that has been shown to increase drug delivery to heated sites by a factor of 30 compared with free drug. In the laboratory, he has pioneered new methods for improving tumor oxygenation and has worked with clinical faculty to test these concepts in patients, as means to improve radiation and chemotherapy response. He has recently shown that radiation therapy initiates stabilization of HIF-1, a hypoxia inducible promoter, even in aerobic conditions. HIF-1 stabilization leads to upregulation of angiogenesis, thereby protecting the tumor against radiation damage. This discovery has led to clinical trials targeting HIF-1 stabilization post radiotherapy and chemotherapy as a means to achieve chemo and radiosensitization. Dr. Dewhirst has mentored many students in his career including 16 PhD students, 17 postdoctoral fellows, 8 clinical research fellows and 14 medical students during their research year at Duke. A recipient of the 2001 Wayne Rundles Award, Dr. Dewhirst has over 350 peer-reviewed publications, book chapters and reviews. He has been a Visiting Professor at numerous institutions, including Vanderbilt University , the NCI Radiation Biology and Pediatric Oncology Branches, St. Jude's Children's Research Hospital , the University of Louvain , Memorial Sloan Kettering Cancer Center and the University of Pennsylvania . He has received Prestigious Lectureships such as the A.C. Burton Lecture, University of Western Ontario , 2005, and the Bruce Cain Lecture, New Zealand Cancer Society, 2004. He currently serves on the Editorial Boards of several journals and is Editor-in-Chief of the International Journal of Hyperthermia. He graduated from the University of Arizona in 1971 with a degree in Chemistry and Colorado State University in 1975 and 1979 with DVM and PhD degrees, respectively.
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