217 MET ECD shedding and use of targeted therapies Biochemical evidence for MET ectodomain shedding in oral squamous cell carcinoma Knowing that MET ectodomain shedding is associated with poor prognosis and possibly potentiates MET’s oncogenic potential (40, 44, 45), it was investigated whether this phenomenon occurs in OSCC using valid C- and N-terminal MET antibodies in parallel (42, 46). When analyzing the results of this study, it was assumed that the existence of CTFs with molecular weights reciprocal to those observed for the NTFs described by Athauda et al. (40) alongside p145β under reducing conditions could be considered as evidence for the existence of MET ectodomain shedding (Table 1). First, it was established that MET expressing OSCC cell lines (SCC-4, SCC-25, CAL-27 and UM-SCC-14C) are subjective to ectodomain shedding by detecting CTFs (i.e. p70β, p90β, and p95β) under reducing conditions and detecting N-terminal MET immunoreactivity by performing an ELISA on the culture media (42). The detection of CTFs (i.e. p95β, p90β, p70β, p60β, and p55β) under reducing conditions illustrated that ectodomain shedding also occurs in fresh frozen tissues of surgically removed primary OSCC (42). Use of biological duplicates revealed inconsistencies in terms of CTF detection, suggesting that ectodomain shedding occurs heterogeneously across a tumor (42). The existence and heterogeneity of ectodomain shedding was confirmed through comparison of C- and N-terminal membranous MET immunoreactivites on formalin-fixed paraffin-embedded whole tissue sections, leading to the identification of regions positive for membranous C-terminal MET immunoreactivity, yet devoid of N-terminal MET immunoreactivity, which were considered to be positive for (MET-EC-) (42, 47). Taking everything into consideration it was concluded that MET ectodomain shedding occurs in OSCC (42, 47). To our knowledge, none of the genetic aberrations known in HNSCC lead to enhanced ectodomain shedding and stabilization of MET-EC- on the membrane. Therefore, we propose a model for the molecular mechanism underlying MET ectodomain shedding below. Crosstalk between G-protein-coupled and EGF receptors enhances MET ectodomain shedding MET ectodomain shedding has been described to be regulated at the post-translational level by a crosstalk between G-protein-coupled and epidermal growth factor receptors (GPCRs and EGFRs). In 2001, using NH2-specific MET antibodies, Nath 7
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