Colorectal cancer cell derived exosomes activate Wnt signaling pathway

S Mathivanan

La Trobe University, Melbourne

Exosomes are extracellular vesicles that are secreted by many cell types under both physiological and pathological conditions. These extracellular vesicles are implicated in cell-cell communication by transferring the cargo or by the transport of ligands to the recipient cells. Exosomes contain many oncoproteins and influence the downstream signaling pathway of the recipient cells by the non-selective transfer of these oncoproteins. It is established that over 90% of sporadic colorectal cancer (CRC) cases, harboured mutation in APC and/or β-catenin gene both of which are involved in Wnt signaling pathway. Whilst mutations in the gene APC often results in loss of function, β-catenin mutations especially at the residues critical for its degradation renders it constitutively active. Here, we show that such mutant β-catenin can be transported via exosomes and activate Wnt signaling pathway in the recipient cells. Firstly, 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. SILAC-based quantitative proteomics analysis confirmed the transfer of mutant β-catenin to the nucleus of the recipient cells. Furthermore, DiR labelled exosomes were injected intravenously in mouse implanted with RKO CRC cells and subjected to in vivo tracking by IVIS at four different time points. Interestingly, tumor tissue lysates of mice injected with exosomes showed increased expression of Wnt target genes confirming the activation of Wnt signaling pathway in vivo. This is the first report to our knowledge implicating exosomal mutant β-catenin in the activation of Wnt signaling pathway both in vitro and in vivo.