Upregulation of the MET transcript is consistently associated with invasion and tumor budding in colorectal cancer

Background: The c-MET proto-oncogene is frequently overexpressed (50-60%), amplified (1-3%), and mutated (1-3%) in colorectal cancer (CRC). Hepatocyte growth factor (HGF)-dependent and independent activation of c-MET has been associated with increased survival and resistance to targeted therapies. This study aimed to investigate the role of the HGF/c-MET axis in regulating migration/invasion in CRC, using pre-clinical models and clinical samples. Methods: In order to model CRC tumour cell invasion, we have generated invasive CRC subpopulations using Boyden Invasion chambers. To model the CRC microenvironment, we have used a range of co-culture techniques with CRC cells and colon fibroblasts. Migration/invasion was determined using xCELLigence System (Roche). c-MET expression in parental and invasive cell lines was measured using Western blotting and qRT-PCR. c-MET expression in CRC FFPE tissues was measured using IHC and RNAScope®. Results: We identified marked upregulated expression of c-MET at both the protein and transcript levels in our invasive CRC cell line models. Importantly, both parental and invasive subpopulations were found to be inherently dependent on c-MET for migration, as RNAi against c-MET abrogated migration/invasion in both parental and invasive models. We also demonstrated that stimulation of CRC cells with rh-HGF resulted in increased CRC cell migration/invasion. In addition, co-culture of CRC cells with colonic myofibroblasts, resulted in marked increases in migratory and invasive capacity, and this was dependent on HGF/c-MET signaling. Interestingly, stimulation with myofibroblast conditioned medium or HGF promotes rapid degradation of c-MET at the protein level, followed by recycling, while MET transcript remains unaltered, illustrating a dynamic expression of c-MET protein in response to activation. We further showed that MET is transcriptionally upregulated in tumour budding foci at the invasive front of a cohort of stage III CRC tumors. Intriguingly, c-MET protein levels do not correlate with the transcript, most likely due to a similar protein degradation process observed in our aforementioned in vitro models. Conclusions: We show for the first time a key role for transcriptional upregulation of MET as a molecular driver of tumour invasion, both in vitro and in stage III CRC tumours.