RKIP Suppresses Breast Cancer Metastasis to the Bone by Regulating Stroma-Associated Genes

In the past decade cancer research has recognized the importance of tumorstroma interactions for the progression of primary tumors to an aggressive and invasive phenotype and for colonization of new organs in the context of metastasis. The dialogue between tumor cells and the surrounding stroma is a complex and dynamic phenomenon, as many cell types and soluble factors are involved. While the function of many of the players involved in this cross talk have been studied, the regulatory mechanisms and signaling pathways that control their expression haven’t been investigated in depth. By using a novel, interdisciplinary approach applied to the mechanism of action of the metastasis suppressor, Raf kinase inhibitory protein (RKIP), we identified a signaling pathway that suppresses invasion and metastasis through regulation of stroma-associated genes. Conceptually, the approach we developed uses a master regulator and expression arrays from breast cancer patients to formulate hypotheses based on clinical data. Experimental validation is followed by further bioinformatic analysis to establish the clinical significance of discoveries. Using RKIP as an example we show here that this multi-step approach can be used to identify gene regulatory mechanisms that affect tumor-stroma interactions that in turn influence metastasis to the bone or other organs. 1. Introduction Under normal physiological conditions, the stromal compartment of epithelial tissue regulates homeostasis by maintaining the proper architecture and nutrient levels required for epithelial function. It also serves as an important barrier to cell transformation. However in response to lesions (i.e., wounding) the stromal compartment undergoes changes including the recruitment and activation of fibroblasts, immune, and endothelial cells that in turn provide growth, and matrix remodeling factors, as well as a new blood supply. Similar changes in the stromal compartment have been shown to occur during tumor growth and the importance of the stromal compartment, called “the tumor microenvironment,” in modulating and driving cancer progression has become increasingly evident [1]. The tumor microenvironment has become the subject of intense therapeutic and prognostic interest as its phenotypic and molecular characteristics have been correlated with disease-free survival in multiple tumor types [2]. It is believed that during the first phase of carcinogenesis the tumor microenvironment initially reacts to suppress malignant transformation by maintaining tissue architecture and differentiation. As cancer