| Ras mutations are found in about 30% of human cancers, and approximately 80% of all human tumors are derived from epithelial tissue. However, most of the advances in our understanding of Ras signal transduction have come from studies in fibroblasts. Moreover, many of these studies detailing Ras signaling have come from experimental systems whereby Ras is transiently activated or overexpressed. Because of the lack of studies exploring the role that sustained activation of Ras plays within the context of epithelial cells, this body of work sought to address this gap in knowledge. The results can be summarized in three broad statements.; We previously demonstrated that although Ras and Raf cause transformation of NIH 3T3 fibroblasts, only Ras causes transformation of RIE-1 intestinal epithelial cells. Using these experimental systems, we evaluated the consequences of sustained Ras and Raf activation on the regulation of cell cycle components controlling G1 progression. First, we found that Raf-independent pathways were critical for Ras-mediated deregulation of cyclin D1, p21, and p27 in RIE-1 cells. Second, Ras inactivation of the Rb pathway occurred by distinct mechanisms in the two cell types. Finally, we evaluated the consequences of sustained Ras and Raf activation on the regulation of MAPK family members. We observed that Ras causes downregulation of p38 and upregulation of JNK via a Raf-independent effector pathway(s) that promoted the transformation of RIE-1 cells, in part, via deregulation of cell cycle progression and upregulation of an autocrine growth mechanism.; In summary, these observations are likely to have a significant impact on future signaling paradigms developed in the study of oncogenic Ras signal transduction. The findings here first emphasize the importance of studying the consequences of sustained oncogenic Ras signaling in epithelial model systems. Second, these studies have helped to merge two fields, the study of cell cycle regulation and small GTPases, which were studied independently for quite some time. Finally, key distinctions regarding the role and specificity of MAPK signaling in cellular transformation were established here. Thus, with these new advances, we may be one step closer to developing more effective anti-cancer agents. |
