Colorectal cancer (CRC) arises as a result of accumulated mutations in key proteins that regulate cell proliferation, differentiation and death. Large scale sequencing studies have established the predominance of mutations in proteins involved in Wnt signaling pathway in a large number of CRC patients. It was also shown that over 90% of sporadic CRC cases, harboured mutation in APC and/or β-catenin gene both of which are involved in Wnt signaling pathway. One of the proposed mechanisms of intercellular signaling involves exosomes which act as messengers carrying oncoproteins from malignant to non-malignant target cells. In this study, an integrative proteogenomic analysis identified the presence of mutated β-catenin in exosomes secreted by CRC cells. Follow up experiments established that exosomes released from LIM1215 CRC cells stimulated Wnt signaling pathway in a variety of recipient cells. Additionally, exosomes derived from LIM1215 cells promoted proliferation in recipient cells. SILAC-based quantitative proteomics analysis confirmed the transfer of mutant β-catenin to the nucleus of the recipient cells. Furthermore, mouse xenograft of RKO cells were injected intravenously with DiR labelled exosomes and subjected to in vivo tracking of labelled exosomes by IVIS. Mice imaging results showed exosomes distribution to organs with highest accumulation in the liver and spleen followed by the gastro intestinal tract. Interestingly, xenograft tissue lysates of mice injected with exosomes showed increased expression of Wnt target genes confirming the activation of Wnt signaling pathway in vivo. This suggests that circulating exosomes are rapidly taken up by the target cells in the tumor microenvironment and allows for the amplification of the signal.
Significance: The role of exosomes bearing mutant β-catenin in context of Wnt signaling pathway has not been extensively studied yet. This is the first study highlighting the role of exosomes carrying mutant β-catenin in increasing Wnt signaling signaling pathway and proliferation in recipient cells.