The secretome of highly metastatic cells as a source of biomarkers and metastatic effectors in colorectal cancer patients.

Abstract

Introduction: Colorectal cancer (CRC) is the second leading cause of cancer death in developed countries, mainly as a consequence of metastatic spread. Secreted proteins are essential to communicate the cancer cells with the tumor- microenvironment, favoring tumor progression and metastasis. The conditioned medium or cellular secretome, including exosomes, has demonstrated to be a rich source of metastatic effectors and biomarkers of metastatization in different tumors. Our goal was to identify proteins altered in the secreted fraction of highly metastatic cells in order to determine altered biological pathways and potential biomarkers of prognosis. Material and methods: A quantitative label-free proteomic analysis was carried out on the secretome of highly and poorly metastatic CRC cell lines from different genetic background. Proteomic results were validated using unbiased transcriptomic analyses. Hazard ratios and long-rank tests for the differentially-secreted proteins were determined in four different external datasets in order to select the proteins with clinical relevance for further analysis. Gene ontology was performed to determine the biological processes altered in highly metastatic cells. Results: 221 differentially-secreted proteins were found to be significantly associated to metastasis. Some corresponding genes were able to predict the overall and progression free survival in stage II and stage III CRC patients. In addition, they showed higher expression in the stem cell-like and CMS4 subtypes of CRC, associated to worse prognosis. These genes were also associated to deficient mismatch repair, CpG-island methylator positive status and BRAF mutation. It is also remarkable that a significant number of altered proteins were involved in cholesterol metabolism, suggesting an enhancement of the LDL uptake and metabolism in highly metastatic cells. Extracellular matrix constituents involved in cell adhesion and migration were also overrepresented in the highly metastatic fraction. Conclusion: Secretome of highly metastatic cells is enriched in proteins involved in essential pathways for metastasis such us adhesion, migration and lipid metabolism. In addition, several proteins with a robust and significant prognosis value were detected howing the value of the fraction as a source of biomarkers.