Vascular effects of vascular endothelial growth factor (VEGF) mutation: implications for magnetic resonance imaging (MRI) of human tumors
2000 - 12 / 2004
Purpose: Vascular endothelial growth factor (VEGF), also called vascular permeability factor, is a secreted homodimeric protein factor that increases blood vessel permeability, stimulates endothelial cell division in vitro, and induces angiogenesis in vivo. VEGF has an important role in tumor angiogenesis, a process that results in a neovascular bed with aberrations in vascular density, structure and function. In addition to its potential as target for anti-cancer therapy, the tumor vascular bed allows for specific tumor visualization via imaging techniques. Especially fast dynamic magnetic resonance imaging (MRI) of tracer uptake in tumor tissue is a promising non-invasive tool for the differentiation between benign and malignant tumors, for demarcation of tumor borders and, potentially, for prediction of radiation sensitivity. The visual parameters that identify malignancy are largely unclear for most tumor types, however, and need further study. In previous experiments we showed that transfection with recombinant VEGF DNA strongly influenced vascular density, architecture and function in human tumor xenografts in nude mice, resulting in changes in the biological behavior of these tumors. To be able to further manipulate tumor angiogenesis, we have now constructed a number of VEGF mutants with modified or antagonistic activity. Tumor cells expressing these mutants produced xenografts in nude mice. This model system proved to be very suited to manipulate vascular parameters and, therefore, allows for studies on the effects of this manipulation on dynamic MRI, that can be directly correlated to biological behavior and radiation sensitivity. In addition, we have developed methods to analyze tumor vascular parameters, blood perfusion, oxygenation status and proliferation after sacrifice of the experimental animal. More recently, improved methods have become available for the analysis of permeability in vivo. Furthermore, a 7 tesla animal MRI facility has become operational in our central animal laboratory. Using this facility, dynamic MR imaging of human glioma xenografts in nude mice was already accomplished. These sensitive methods will be used to analyze tumors with different grades of malignancy. In addition, tumors in which the vascular parameters and the in vivo behavior have been manipulated by transfection with wild-type and mutant VEGF will be investigated to assess what parameters determine the MRI result and estimate the prognostic value of this powerful technique. Plan of investigation: It is the aim of this project to: 1. Investigate tracer uptake as studied by fast dynamic MRI in xenografts of our panel of human melanoma cell lines with a broad spectrum of malignancy, and correlate these findings with vascular density, tumor cell proliferation, oxygenation and percentage of necrosis. 2. Investigate the effects of transfection with wild type and mutant VEGF cDNAs that are known to modulate vascular density, permeability, and biological behavior, on fast dynamic MRI. Attention will focus on the analysis of mutants with modified permeability-inducing activity. 3. Analyze the effects on the above-mentioned vascular parameters in tumors expressing VEGF mutants with antagonist activity in vivo. 4. Study whether VEGF-induced hypervascularity affects tumor hypoxia and, thereby, increases radiation sensitivity. Investigate whether these effects are predictable by dynamic MRI analysis. Possible results: The proposed investigations will improve our understanding of the local vascular parameters that determine the results of tumor imaging. Intravital analysis of vascular parameters can now for the first time be coupled to high-resolution MRI in a mouse tumor model. The combination with transfection with well-defined VEGF mutants gives the unique opportunity to manipulate these vascular parameters in vivo. The use of the established melanoma xenograft model, consisting of a panel of cell lines with increasingly malignant biological behavior, will allow a determination of the potential of imaging techniques as prognostic instrument for the prediction of tumor outgrowth and metastasis. Furthermore, this study may produce indications whether the vascular effects of anti-angiogenesis therapy can be monitored via non-invasive image analysis. Finally, it will be established whether VEGF-induced hypervascularity increases sensitivity to X-irradiation.