Inflammation and tumor development are closely linked processes, but they are considered to be mechanistically separate, as the cytokine-producing immune cells are distinct from the proliferating tumor cells. This proposal focuses on an unusual class of potential tumor suppressors, the ?B-Ras 1 and 2 proteins that control both inflammatory and proliferative signaling. While previous studies have shown that ?B-Ras proteins can inhibit inflammatory signaling through the NF-??B pathway, we recently demonstrated that ?B-Ras proteins inhibit signaling by the Ral GTPase, and hence their deletion leads to unrestrained Ral activation, and consequent cell proliferation. Therefore, ?B-Ras proteins form a unique molecular bridge between inflammation and cancer. Such a role for ?B-Ras proteins is corroborated by observations that the level of ?B-Ras proteins is reduced in many different human cancers, and heterozygous loss of the ?B-Ras 1 gene is observed at a high frequency in many different human epithelial tumors. Intriguingly, reconstitution of ?B-Ras proteins in human cancer cell lines with low ?B-Ras levels leads to suppression of anchorage-independent proliferation. These observations open up a series of intriguing questions that we intend to address with the experiments proposed in this application.
In Aim 1, we will use cutting-edge bioinformatics to investigate the correlation between loss of ?B-Ras and human tumors. To this end, we will perform pan-cancer genomic analysis on large patient data sets available through The Cancer Genome Atlas and the International Cancer Genome Consortium. This will allow us not only to determine whether loss of ?B-Ras is a true driver of human carcinogenesis, but also to identify any cooperative networks in which ?B-Ras proteins may be involved. A preliminary analysis has already indicated that loss of ?B-Ras may synergize with loss of the central tumor suppressor p53 across multiple human cancers.
In Aim 2, we will employ two different mouse models of carcinogenesis to genetically dissect the contributions of ?B-Ras loss to tumorigenesis, using a conditional ?B-Ras knock-out mouse we have recently created. Specifically, we will investigate (i) whether loss of ?B-Ras enhances tumor burden associated with loss of p53 in a model of lung cancer; and (ii) whether loss of ?B-Ras in macrophages and/or epithelial cells increases tumorigenesis in a model of colitis-associated colon cancer. Finally, Aim 3 is designed to elucidate the molecular mechanisms underlying the regulation of Ral signaling by ?B-Ras proteins, as well as the downstream consequences of this regulation for gene expression in EGF-stimulated epithelial cells. Given the role of ?B-Ras proteins as regulators of two separate, central pathways involved in cancer development, the experiments proposed herein will significantly advance our understanding of human carcinogenesis and potentially provide a basis for novel therapeutic approaches.
The development of cancer is a complex interplay between a large number of variables that include oncogenes, tumor suppressors and inflammation in the tumor microenvironment. The experiments described in this proposal are designed to elucidate the role of a novel class of tumor suppressors, the ?B-Ras proteins that bridge inflammation and cancer by regulating two central cellular signaling pathways through separate mechanisms. Understanding how ?B-Ras proteins function in tumor development will not only help us to assess their potential as targets for novel cancer therapies, but also deepen our understanding of the relative contributions of inflammatory and proliferative pathways to carcinogenesis.