High expression of the cysteine proteinase legumain in colorectal cancer - implications for therapeutic targeting

Abstract

Background: The cysteine proteinase legumain is highly expressed in cancer. Legumain is a potential biomarker and has been suggested to be utilised for prodrug activation in cancer therapy. However, to define the suitability of legumain for such purposes, detailed knowledge of cell type-specific and subcellular expression together with proteolytic activity patterns in tumour tissue is necessary.

Methods: Expression of legumain was examined in a panel of 277 primary tumours from colorectal cancer (CRC) patients using immunohistochemistry. Tumour (cytoplasmic diffuse, cytoplasmic granulated, and nuclear) and stromal cell expression of legumain was quantified, and associations with clinicopathological parameters and outcome were analysed. Additionally, normal colon tissue and spontaneous mouse tumours were stained for legumain.

Results: Legumain was highly expressed in tumour and stromal cells. Nuclear legumain was detected in 30% of the tumours. In colon cancer patients, high legumain expression was associated with overall and metastasis-free survival (OS; MFS) in uni- and multivariate analysis. Nuclear legumain was associated with poor OS, but not MFS in the colon cancer subgroup. Cytoplasmic granulated or diffuse expression was not associated with OS or MFS. Normal epithelial cells exhibited granulated legumain mainly at the apical pole, and legumain was highly expressed in CD68 positive macrophages.

Conclusions: Legumain is a highly expressed proteinase in CRC and associated with poor outcome in colon cancer. Diversified localisation of legumain expression in tumour and stromal cells suggests multiple functions in CRC, representing both a challenge and an opportunity for use in therapeutic targeting.

Keywords: Asparaginyl endopeptidase (AEP); Cell nuclei; Colorectal cancer; Cysteine proteinase; Legumain; Prodrugs; S100A4.