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The β6 subunit of the αvβ6 integrin heterodimer has long been an enigma in cancer biology though recent research has provided many new insights into its biology. Collectively, these findings include discovery of the transcriptional, translational and cell biological mechanisms by which β6 acts, the identification of the cellular influences β6 exerts upon the cell proteome, the characterisation of multiple β6-centric pro-metastatic signalling systems and the search for pharmacological therapies (industry and academia) targeted against β6. Once expressional restriction is overcome in early colorectal cancer (CRC), epithelial cell surface restricted αvβ6 can physically interact with, and activate, known oncoproteins, and has the potential to enable the cross-talk through non-canonical signal transduction pathways, resulting in the adoption of an invasive/metastatic phenotype. This recent research has identified numerous interconnections and potential feedback loops, highlighting the fact that the expression of the β6 subunit may initiate a cascade of downstream effects on the CRC cell rather than acting through a single mechanism. We here review these recent studies and postulate that the existence of a cell surface uPAR/αvβ6/TGFβ "metastasome" interactome in/on a proportion of colorectal cancer cells, where β6 expression sequesters and activates multiple systems at the invasive front of tumour lesions, promoting cancer metastasis and hence explaining why β6 has been correlated with reduced patient survival in CRC.

BackgroundCurrent methods widely deployed for colorectal cancers (CRC) screening lack the necessary sensitivity and specificity required for population-based early disease detection. Cancer-specific protein biomarkers are thought to be produced either by the tumor itself or other tissues in response to the presence of cancers or associated conditions. Equally, known examples of cancer protein biomarkers (e.g., PSA, CA125, CA19-9, CEA, AFP) are frequently found in plasma at very low concentration (pg/mL-ng/mL). New sensitive and specific assays are therefore urgently required to detect the disease at an early stage when prognosis is good following surgical resection. This study was designed to meet the longstanding unmet clinical need for earlier CRC detection by measuring plasma candidate biomarkers of cancer onset and progression in a clinical stage-specific manner. EDTA plasma samples (1 μL) obtained from 75 patients with Dukes’ staged CRC or unaffected controls (age and sex matched with stringent inclusion/exclusion criteria) were assayed for expression of 92 human proteins employing the Proseek® Multiplex Oncology I proximity extension assay. An identical set of plasma samples were analyzed utilizing the Bio-Plex Pro™ human cytokine 27-plex immunoassay.ResultsSimilar quantitative expression patterns for 13 plasma antigens common to both platforms endorsed the potential efficacy of Proseek as an immune-based multiplex assay for proteomic biomarker research. Proseek found that expression of Carcinoembryonic Antigen (CEA), IL-8 and prolactin are significantly correlated with CRC stage.ConclusionsCEA, IL-8 and prolactin expression were found to identify between control (unaffected), non-malignant (Dukes’ A + B) and malignant (Dukes’ C + D) stages.Electronic supplementary materialThe online version of this article (doi:10.1186/s12014-015-9081-x) contains supplementary material, which is available to authorized users.

Urokinase plasminogen activator receptor (uPAR) and the epithelial integrin αvβ6 are thought to individually play critical roles in cancer metastasis. These observations have been highlighted by the recent discovery (by proteomics) of an interaction between these two molecules, which are also both implicated in the epithelial-mesenchymal transition (EMT) that facilitates escape of cells from tissue barriers and is a common signature of cancer metastases. In this study, orthogonal in cellulo and in vitro functional proteomic approaches were used to better characterize the uPAR·αvβ6 interaction. Proximity ligation assays (PLA) confirmed the uPAR·αvβ6 interaction on OVCA429 (ovarian cancer line) and four different colon cancer cell lines including positive controls in cells with de novo β6 subunit expression. PLA studies were then validated using peptide arrays, which also identified potential physical sites of uPAR interaction with αvβ6, as well as verifying interactions with other known uPAR ligands (e.g., uPA, vitronectin) and individual integrin subunits (i.e., αv, β1, β3, and β6 alone). Our data suggest that interaction with uPAR requires expression of the complete αβ heterodimer (e.g., αvβ6), not individual subunits (i.e., αv, β1, β3, or β6). Finally, using in silico structural analyses in concert with these functional proteomics studies, we propose and demonstrate that the most likely unique sites of interaction between αvβ6 and uPAR are located in uPAR domains II and III.

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