FRACTALKINE-FRACTALKINE RECEPTOR AXIS IN HUMAN COLORECTAL CANCER

Abstract

Although the presence of genetic and epigenetic abnormalities is important for tumor occurrence, the formation of an inflammatory microenvironment plays a pivotal role in tumor progression. Even if the link between cancer and inflammation has been widely accepted in the last decade, no inflammation-associated markers or potential targets have yet been translated into clinical practice. Human colorectal cancer (CRC), the second cause of cancer-related death in Western countries, represents a paradigm of the cancer-related inflammation. The aim of this study is to provide an overview of the inflammatory mediators expressed in tumor microenvironment of CRC, in order to select and characterize new candidates for clinical or therapeutic strategies. In this study, the RNA profile of a large panel of inflammatory genes, in particular cytokines, chemokines and receptors, has been analysed in eight surgical tumor samples and corresponding normal mucosa, from CRC patients. Several chemokines and receptors were found to be strongly up regulated in tumor tissues. The expression of selected genes, such as CCL3, CCL4, CXCL8 and CX3CL1, was further investigated and confirmed in a larger number of samples. Further studies focus on a specific chemokine-receptor pair: CX3CL1 and CX3CR1. CX3CL1 is a transmembrane protein acting both as an adhesion molecule as well as a chemokine when cleaved by specific protease. It has a single receptor, CX3CR1, which has been primarily studied on immune cells. A number of reports indicate that the CX3CL1-CX3CR1 axis is involved in the pathogenesis of different chronic inflammatory conditions, but its role in cancer has not been elucidated. By immunohistochemistry, we analysed the expression of CX3CL1 and CX3CR1 in a series of 144 paraffin-embedded CRC tissues. We found that CX3CL1 and CX3CR1 were already faintly expressed by healthy epithelial cells, and their expression progressively increased in pre-cancerous lesions and in tumor tissues, with a heterogeneous staining. Based on the median score value of CX3CL1 and CX3CR1 staining, we divided our case list in high and low expressing tumors. Correlating these results with patients’ clinical and pathological features, we found that patients with high expression of CX3CL1 and CX3CR1 showed a trend to better prognosis. Of great interest, the concomitant high expression of both CX3CL1 and CX3CR1 (positive-axis tumors) resulted in significantly increased disease-specific survival and disease-free survival. Given the role of CX3CL1 in the recruitment of immune cells at sites of inflammation, we investigated whether high expression of CX3CL1 correlated with an increased amount of tumor infiltrating lymphocytes (CD3+ cells, known to be protective in CRC) or macrophages (CD68+ cells). To our surprise, CX3CL1 expression by tumor cells did not correlate either with the density of CD3+ nor CD68+ cells at tumor invasive front, indicating that the CX3CL1-associated favourable prognosis is not mediated via the recruitment of these immune cells. Since CX3CL1 is…