Abstract A17: A role for the endocytic regulators Rab25 and Rab coupling protein (RCP) in K-Ras dependent colorectal cancer cells

Abstract

Abstract Endocytic transport impacts a wide range of cellular functions, including growth factor signaling, cell polarity, and cell-cell and cellmatrix adhesion. As such, deregulated endocytosis is emerging as a key feature of tumorigenesis. Growth factors and their receptors are important regulators of normal cell survival and proliferation and aberrant growth factor signaling is one of the fundamental elements of tumor growth and progression. Upon activation at the plasma membrane, growth factor receptors are internalized and either trafficked to the lysosome for degradation, or returned to the cell surface via the process of endosomal recycling. Furthermore, growth factor receptors can signal from endocytic compartments en route to the lysosome, and throughout the recycling process. Downstream of growth factor stimulation, signaling from Ras GTPases to effectors such as the mitogen activated protein kinase (MAPK) pathway has also been shown to occur at endocytic compartments. Elucidation of the key regulators of the endocytic process therefore presents a novel opportunity to target certain growth factor signaling pathways. Two such key regulators are the small GTPase Rab25, and its interacting partner Rab coupling protein (RCP). Rab25 and RCP are amplified in certain breast and ovarian cancers, and this amplification is associated with increased aggressiveness of these cancers. Additionally, in mutant p53 cells RCP has been implicated in enhanced EGFR and integrin signaling, resulting in increased invasion. Here, we provide evidence for the importance of Rab25 and RCP in colorectal cancer (CRC) cell lines. Using siRNA to specifically knockdown RCP and Rab25 in a panel of CRC cell lines we have found that RCP and Rab25 knockdown significantly impacts on the proliferation of these lines. Within the panel of CRC cell lines, we have identified subpopulations of K-Ras dependent and K-Ras independent lines. K-Ras dependency appeared to be independent of K-Ras mutation status, and those lines which depend on K-Ras for proliferation also require RCP and Rab25. We are currently investigating the consequences of RCP knockdown on growth factor receptor signaling and subsequent Ras activation. These findings highlight the importance of the regulation of recycling endocytosis in colorectal cancer, and indicate a dependence of colorectal cancer cells on this process. Citation Information: Clin Cancer Res 2010;16(14 Suppl):A17.