Differential expression of long and short isoforms of tissue transglutaminase 2; Implications for cisplatin resistance in an in-vitro MCF-7 breast cancer model

Despite significant advances in surgery and biology, cancer remains a major health problem because of tumour cell metastasis and resistance to chemotherapeutic drugs. Cisplatin has been described as ‘the penicillin’ of cancer and was the first platinum-based chemotherapeutic drug developed for cancer treatment. However, it has the unfortunate downside of chemoresistance, thus limiting its applicability to the treatment of the more responsive hormone-negative, rather than hormone-positive, metastatic breast cancer. Tissue transglutaminase 2 (TG2), a multifunctional Ca2+ enzyme, belongs to a family of related genes involved in various cellular processes such as the cell cycle, adhesion, protein crosslinking, endocytosis and apoptosis. It is overexpressed in cancer and chemoresistant cells and implicated in encouraging resistance to anti-cancer drugs. The enzyme exists in structurally different isoforms, which are thought to have opposing functions. The full length TG2-L supports cell survival; while the truncated TG2-S form promotes cell death. However, the mechanism has not been investigated in breast cancer. The effect of altering the expression of TG2-L and TG2-S isoforms was investigated in cisplatin-sensitive and cisplatin-resistant hormone-positive MCF-7 breast cancer cells. The cells were treated with non-toxic doses of retinoic acid up to 72 hours in order to raise TG2 expression, while inhibition of TG2 was achieved with cystamine and specific anti-TG2 silencing RNA (siRNA). MTT assay measured cellular viability; flow cytometry measured levels of apoptosis and necrosis, and Western blot analysis measured TG2 isoform expression. The results generated from this study reveal for the first time that retinoic acid treatment of both wild type and cisplatin-resistant cell lines, induced overexpression of TG2-L but had little or no effect on TG-S expression, promoting cell survival and increasing cisplatin chemoresistance in all cases. Conversely, TG2 inhibition with cystamine and siRNA treatment selectively suppressed TG2-L expression (with little effect on TG2-S expression) and reduced cisplatin resistance to basal control levels without affecting cell viability, in all cells tested. Collectively, the results suggest that targeting TG2-L by controlling levels of dietary retinoid consumption, could be a potential therapeutic tool in combating cisplatin resistance in hormone-positive breast cancer patients where, currently, rapid induction of resistance is a barrier to effective treatment and cisplatin therapy.