Inhibition of Transglutaminase 2 activity increases cisplatin cytotoxicity in a model of human hepatocarcinoma chemotherapy

Transglutaminase 2 (TG2) is a ubiquitous multifunctional enzyme whose expression has been found to be altered in numerous studies of apoptosis and cell survival; its activity has been found to be increased in many types of cancer, where it is often over-expressed. Cisplatin has long been used as an effective therapeutic drug to treat numerous cancers. Although its activity is based on cross-linking of DNA, cisplatin may also operate via other mechanisms that involve modification and alteration in the activity of protein and RNA modulators of the cell cycle and apoptotic processes; these mechanisms are less well characterised. In this study, we investigated the effects of cisplatin-induced apoptosis on TG2 expression and activity in the human hepatocarcinoma (HepG2) cell line. Through a combination of Western blotting, enzymatic activity assays, flow cytometry and fluorescence microscopy we provide evidence that TG2 is inhibited during initiation of apoptosis by cisplatin, an observation that was reversed by increasing the expression of TG2, by treating cells with retinoic acid. We also report, for the first time, that cisplatin can directly inhibit transglutaminase activity in vitro. Collectively, these studies increase our understanding of the mechanism(s) of action of cisplatin, as cisplatin–mediated reduction in TG2 activity appears to act as an early activator of apoptosis during chemotherapeutic treatment of hepatocarcinoma cells. This observation suggests an explanation as to how increased levels of TG2 activity in cancer cells could contribute to chemotherapeutic resistance to cisplatin, and so has implications for novel approaches to cisplatin therapy