A comparative study on interactions of cisplatin and ruthenium arene anticancer complexes with metallothionein using MALDI-TOF-MS

Abstract

Metallothioneins (MTs) are a group of low molecular weight (6–7 KDa) proteins featured by high cysteine content which allows the proteins to bind to a diverse range of metals. MTs, of which the gene transcription can be induced by a variety of stimuli including hormones, cytokines and metal ions, have important biological functions such as storage, trafficking and homeostasis of metal ions, detoxification of heavy metals, resistance to ionizing radiation, scavenging hydroxyl radical, etc. Importantly, it has been reported that MTs play a role in oncogenesis and cancer prognosis, and are implicated in resistance of cancer cells to anticancer metallodrug cisplatin. In this work, we present a comparative study on interactions between MTs and cisplatin and ruthenium arene anticancer complexes using MALDI-TOF-MS. The results show that cisplatin coordinates to MT-I and MT-II at either pH 3.0 or 7.4, and exhibits a higher affinity to MT-II than to MT-I. The MT–Pt complexes increase significantly in content with decrease in pH values of solutions, indicative that cisplatin competes with zinc for coordination to cysteine residues on MTs and interferes with the binding of zinc to the proteins. While the ruthenium arene anticancer complexes [(η 6-arene)Ru(en)Cl]PF 6 (arene = benzene or biphenyl, en = ethylenediamine) hardly bind to MTs at acidic pH and coordinate to MTs at a much lower level than cisplatin at neutral pH, which may account for the less toxicity and lack of cross-resistance to cisplatin for this class of ruthenium anticancer complexes. With the MT–Pt complexes as model protein complexes, DDT, the widely used unfolding thiol-containing agent in proteomic research was shown to reduce the coordination of platinum to MTs significantly, implying that DDT used as unfolding reagent at high concentration may cause dissociation of bound metallodrug and should be applied with care.