Oncogenes and Angiogenesis: Signaling Three-Dimensional Tumor Growth

Three-dimensional tumor growth is dependent on the perpetual recruitment of host blood vessels to the tumor site. This recruitment process (mainly via angiogenesis) is thought to be triggered, at least in part, by the very same set of genetic alterations (activated oncogenes, inactivated/lost tumor suppressor genes) as those responsible for other aspects of malignant transformation (e.g., aberrant mitogenesis, resistance to apoptosis). Potent oncogenes are able to deregulate expression of both angiogenesis stimulators and inhibitors in cancer cells. For example, mutant ras expression is associated with increased production of vascular endothelial growth factor (VEGF) and downregulation of thrombospondin-1 (TSP-1). Upregulation of VEGF and angiogenesis can also be induced by constitutive activation of other oncogenic proteins (e.g., EGFR, Raf, MEK, PI3K) acting at various levels on the Ras signaling pathway. The mode and the magnitude of such pro-angiogenic influences can be significantly modified by cell type (fibroblastic or epithelial origin), epigenetic factors (hypoxia, changes in cell density), and/or presence of additional genetic lesions (e.g., preceding loss of p16 or p53 tumor suppressor genes). Activated oncogenes (e.g., ras, src, HER-2) induce co-expression of angiogenic properties concomitantly with several highly selectable traits (increased mitogenesis, resistance to apoptosis), a circumstance that may accelerate selection of the angiogenic phenotype at the cell population level. On the other hand oncogene-induced reduction in growth requirements may also endow tumor cells with a diminished (albeit not abrogated) dependence on (close) proximity to blood vessels, i.e., with reduced vascular dependence. Thus, oncogenes can impact several interconnected aspects of cellular growth, survival, and angiogenesis. Experimental evidence suggests that, in principle, many of these properties (including angiogenesis) can be simultaneously suppressed (and tumor stasis or regression induced) by effective use of the specific oncogene antagonists and signal transduction inhibitors