Retinoic Acid-Induced Transglutaminase 2 Expression Reduces Sensitivity to Cisplatin in the Hormone-Positive MCF-7 Breast Cancer Cell Model

Cisplatin is an effective chemotherapeutic drug, but is limited both by its toxicity and its tendency to induce drug resistance rapidly in some patients. Tissue transglutaminase 2 (TG2), which is overexpressed in various cancers, has two main isoforms: a long (TG2-L) and a short form (TG2-S). While TG2-L supports cell survival, conversely, TG2-S promotes cell death. Evidence increasingly suggests that TG2 may be a suitable target for combating chemoresistance in a variety of human cancers. Here, we show that cisplatin toxicity towards wild-type MCF-7 breast cancer cells is associated with reduced TG2-L and TG2-S expression, whereas approximately doubling the TG2-L expression through the retinoic acid pre-treatment of these cells induces survival in the presence of cisplatin at levels similar to those seen in long-term cisplatin-co-cultured cells, which have reduced sensitivity. The treatment of cisplatin-surviving cells with cisplatin alone did not significantly alter the levels of either TG2 isoform, whereas the cisplatin challenge of cisplatin-surviving MCF-7 cells following 20 µM retinoic acid pre-treatment resulted in increased levels of TG2-L, increased TG2 enzyme activity, and no significant change in TG2-S levels, with increased cell survival. These findings suggest a subtype-specific regulatory effect of RA in cisplatin-surviving MCF-7 cells, with TG2-L upregulated at higher RA concentrations, potentially contributing to altered cisplatin sensitivity. Anti-TG2 siRNA silencing reduced cisplatin IC50 to base levels in both wild-type and cisplatin-surviving MCF-7 cells, supporting the notion that the modulation of TG2 expression could offer a significant benefit to cisplatin efficacy. Preventing excessive retinoic acid exposure may also be a mechanism for maximising cisplatin efficacy, considering TG2 modulation.