Angiogenesis is an essential feature of both physiological and pathological processes, and multiple genetic and environmental factors converge to regulate the formation of new blood vessels. In the well-described progression from normal colonic epithelium to colon cancer, angiogenesis begins early at the stage of the benign adenomatous polyp. Genetic alterations in the K-ras and Wnt pathways that occur at this premalignant stage can regulate the expression of vascular endothelial growth factor (VEGF). As tumors enlarge, hypoxia invariably develops as the metabolic demands outstrip the blood supply. Preliminary studies have revealed that K-ras can interact with and respond to the tumor microenvironment. K-ras cooperates with hypoxia to recruit novel angiogenic pathways that do not require hypoxia-inducible factor-1 (HIF-1). These include the induction of additional angiogenic factors such as interleukin-8 (IL-8) and the novel activation of c-myc to upregulate VEGF. Moreover, K-ras is responsive to local shifts in the balance between pro- and anti-angiogenic factors so that these alternative pathways can be activated in a compensatory manner when classical pathways such as HIF-1 are blocked. This ability of colon tumors to recruit alternative angiogenic factors may explain the resistance that can develop when only a single agent is used for anti-angiogenic therapy. Collectively, these findings suggest that maintenance of the angiogenic phenotype is a dynamic process that is highly responsive to local environmental cues, and there is an important role for K-ras in this process. To address the hypothesis that alterations in K-ras that typically occur in early colonic neoplasia interact with the microenvironment to regulate multiple angiogenic pathways, the following specific aims are proposed: (I) to define the role of K-ras in the induction of IL-8 in colonic epithelial cells, (II) to define the role of K-ras in the hypoxic regulation of VEGF by c-myc in colonic epithelial cells, and (III) to determine whether K-ras induces the expression of alternative angiogenic factors following inhibition of both VEGF and IL-8. A better understanding of the complete spectrum of angiogenic pathways activated in the colonic epithelium is a prerequisite for the rational design and application of targeted anti- angiogenic approaches.
Showing the most recent 10 out of 22 publications
