EpCAM proteolysis: new fragments with distinct functions?

Abstract

EpCAM [epithelial cell adhesion molecule; CD326 (cluster of differentiation 326)] is highly expressed on epithelium-derived tumours and can play a role in cell proliferation. Recently, RIP (regulated intramembrane proteolysis) has been implicated as the trigger for EpCAM-mediated proliferative signalling. However, RIP does not explain all EpCAM-derived protein fragments. To shed light on how proteolytic cleavage is involved in EpCAM signalling, we characterized the protein biochemically using antibodies binding to three different EpCAM domains. Using a newly generated anti-EpCAM antibody, we find that EpCAM can be cleaved at multiple positions within its ectodomain in addition to described peptides, revealing that EpCAM is processed via distinct proteolytic pathways. Here, we report on four new peptides, but also discuss the previously described cleavage products to provide a comprehensive picture of EpCAM cleavage at multiple positions. The complex regulation of EpCAM might not only result in the absence of full-length EpCAM, but the newly formed EpCAM-derived proteins may have their own signalling properties.

Predominant expression of truncated EpCAM is associated with a more aggressive phenotype and predicts poor overall survival in colorectal cancer

“…The staining patterns obtained in the present study might reflect retention of EpEX at the plasma membrane or in the interstitium in the vicinity of tumor cells. In addition, only small amounts of soluble EpEX in sera of cancer patients as well as in the supernatant of cell lines in vitro could be detected …”

“…Regulated intramembrane proteolysis (RIP) seems to be a critical mechanism for oncogenic activation of EpCAM, resulting in cleavage of its intracellular domain EpICD which also has been proposed as a novel biomarker for treatment response with EpCAM‐targeted agents . Only recently, we and others could identify two different membranous EpCAM variants namely full‐length EpCAM (EpCAM MF ; EpCAM membranous full‐length ) and its truncated variant EpCAM MT (EpCAM membranous truncated ) within the most common human epithelial malignancies . Moreover, loss of the intracellular domain EpICD significantly correlated with poor outcome in patients with pancreatic and breast cancer …”

Crystal structure and its bearing towards an understanding of key biological functions of EpCAM

“…However, the g-cleavages are preceded by the cleavages within the extracellular part therefore we speculate that the cleavage-induced destabilization of the extracellular region conformationally and/or sterically paves the way for g-cleavages within the TM part. Recently, additional cleavage sites at the extracellular part were identified: a cleavage at the mouth of the central groove in cis-dimer within a region connecting the bC1 to the a2 helix of the CD, and a several cleavages within and near the SCD (Figs 1b and 2a) 49 . The role of all these cleavages and the exact mechanism by which they occur remains to be elucidated.…”

Expression of EpCAMMFand EpCAMMTvariants in human carcinomas

“…Hence, the cleaved intracellular domain EpICD has been proposed as a novel marker for treatment response in EpCAM positive tumours8 and there is now growing evidence that subcellular compartmental accumulation of EpICD may be involved in development of epithelial carcinomas 9. Very recently, different variants of the EpCAM protein have been described in cell lines with different biological functions 10. A better characterisation of the expression of different EpCAM variants that may predict response to EpCAM-targeted therapies is crucial to optimise the selection of patients for these treatments.…”