Functional characterisation and role of Tissue Transglutaminase during the progression of colorectal cancer

Abstract

Colorectal cancer (CRC) is the 4th most lethal cancer worldwide. Currently available chemotherapy treatments used in combination with surgery have toxic effects which limit their prolonged usage. Thus, there is still great need for less toxic and more specific treatments, alongside better predictive and prognostic disease markers. Tissue Transglutaminase (TG2) is a multi-functional enzyme whose role in cancer can be either tumour-promoting or tumour-suppressing depending on cell type and intracellular localisation. There is a large body of literature dissecting TG2 tumour-promoting role, whereas very little is known about its tumour-suppressing functions. MicroRNAs are small RNA molecules with translation regulation functions. Their altered expression in cancer causes abnormal translation of their target mRNAs. The aim of this work was to fully characterise TG2 in a unique in vitro model of CRC progression, to assess by which mechanisms it acts a tumour suppressor, and how TG2 expression is regulated.The in vitro CRC model used consisted of two cell lines: SW480 (derived by the primary tumour site of a CRC patient and expressing very high TG2), and SW620 (derived by a lymph node metastasis of the same patient and expressing very little TG2).Silencing of TG2 in SW480 directly promoted cell invasion on Transwell system, whereas transfecting TG2 in SW620 prevented it. Compared to SW620, in SW480 TG2 is found more SUMOylated at the leading edges of cells, and TG2 levels are positively correlated with ?-catenin levels, suggesting a role for TG2 in maintaining cell-cell junctions and regulating motility. Silencing TG2 in SW480 and transfecting TG2 in SW620 show that TG2 levels are positively correlated with expression of HLA?I; this effect may be linked to tumour immune evasion. To understand how TG2 expression is regulated, in silico and experimental analysis were performed which identified miR-19a as a regulator. Transfection of miR-19a in SW480 directly downregulated TG2 resulting in cell invasion.Given that exogenous administration of TG2 would not represent a viable option due to its systemic expression and pleiotropic functions, these observations provide a rationale for sequestering miR-19a in primary CRC tumour in order to prevent downregulation of TG2 and thus metastasis.<br/