Short communication Selected cytotoxic gold compounds cause significant inhibition of 20S proteasome catalytic activities Nicola Micale a, ⁎, Tanja Schirmeister b , Roberta Ettari c , Maria A. Cinellu d , Laura Maiore e , Maria Serratrice d , Chiara Gabbiani f , Lara Massai g , Luigi Messori g, ⁎⁎ a Department of Drug Sciences and Health Products, University of Messina, Viale Annunziata, 98168 Messina, Italy b Institute of Pharmacy and Biochemistry, University of Mainz, Staudinger Weg 5, D-55099 Mainz, Germany c Department of Pharmaceutical Sciences, University of Milan, Via Mangiagalli 25, 20122 Milan, Italy d Department of Chemistry and Pharmacy, University of Sassari, Via Vienna 2, 07100 Sassari, Italy e Department of Chemical and Geological Sciences, University of Cagliari, S.S. 554, 09042 Monserrato, CA, Italy f Department of Chemistry and Industrial Chemistry, via Risorgimento 35, 56126 Pisa, Italy g Laboratory of “Metals in Medicine” (METMED), Department of Chemistry “Ugo Schiff”, University of Florence, Via della Lastruccia 3, Sesto Fiorentino, 50019 Firenze, Italy abstract article info Article history: Received 16 April 2014 Received in revised form 31 July 2014 Accepted 1 August 2014 Available online 11 August 2014 Keywords: Proteasome Gold compounds Anticancer drugs Enzyme inhibition Six structurally diverse cytotoxic gold compounds are reported to cause profound and differential inhibition of the three main catalytic activities of purified 20S proteasome whilst auranofin, an established gold(I) drug in clinical use, is nearly ineffective. In particular, the gold(I) complex [(pbiH)Au(PPh 3 )]PF 6 , turns out to be the most potent inhibitor of all three enzyme activities with sub-micromolar IC 50 values. The present results further support the view that proteasome inhibition may play a major – yet not exclusive – role in the cytotoxic actions of gold based anticancer agents. © 2014 Elsevier Inc. All rights reserved. The ubiquitin/proteasome system (UPS hereafter) is a complex molecular machinery specifically devoted to the turnover of intracellu- lar proteins in eukaryotic cells; owing to the discovery of UPS and to the assessment of its biological relevance, Hershko and Ciechanover were awarded the 2004 Nobel Prize in Chemistry [1]. The proteasome most exclusively used in mammals is the cytosolic 26S proteasome, ~2 MDa in molecular mass; it contains one 20S core particle capped by two 19S regulatory subunits (Fig. 1). The 20S core is hollow and forms a cavity where ubiquitin-tagged proteins are de- graded. Each end of the core particle associates with a 19S regulatory subunit containing multiple ATPase sites and ubiquitin binding sites; this structure is capable of recognising poly-ubiquitinated proteins that are then transferred to the catalytic interior [2]. Three main enzyme activities were identified in the proteasome, namely the chymotryptic- like (CT-L), the caspase-like (C-L; also known as post glutamyl-peptide hydrolyzing, PGPH) and the tryptic-like (T-L). These enzymatic activities are performed by distinct inner subunits (β5, β2, and β1, respectively) characterised by N-terminal catalytic threonine residues [3,4]. The CT-L activity is considered the most important one; its inhibition is com- monly associated to relevant pro-apoptotic and antiproliferative effects. Targeting the CT-L active site has long been considered as sufficient to develop new candidate drugs for cancer treatment; yet, inhibition of multiple active sites is usually required to decrease markedly protein degradation and produce more relevant biological effects [5]. So, beyond the β5 active site, it is also important to target either the β1 or β2 active sites, considered as co-targets of cancer drugs [6–8]. Specific assays were developed to monitor independently the three individual catalytic activities of the proteasome and a number of selective inhibi- tors were identified [9–11]. The UPS-dependent degradation pathway plays an essential role both in up-regulation of cell proliferation and down-regulation of cell death in human cancer cells and represents a validated druggable target. In vitro and in vivo experimental and clinical results clearly doc- umented the use of proteasome inhibitors as potential anticancer drugs [12]. Proteasome inhibition in cancer cells leads to accumulation of pro-apoptotic proteins followed by induction of cell death [13]. The clinical efficacy of the proteasome inhibitor bortezomib towards multiple myeloma and other haematological malignancies provided the conclusive “proof of concept” that targeting the proteasome is a fea- sible and innovative strategy in cancer treatment [14]. Currently, there Journal of Inorganic Biochemistry 141 (2014) 79–82 ⁎ Corresponding author. Tel.: +39 090 676 6419; fax: +39 090 676 6402. ⁎⁎ Corresponding author. Tel.: +39 055 4573388; fax: +39 055 4573385. E-mail addresses: nmicale@unime.it (N. Micale), luigi.messori@unifi.it (L. Messori). http://dx.doi.org/10.1016/j.jinorgbio.2014.08.001 0162-0134/© 2014 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Journal of Inorganic Biochemistry journal homepage: www.elsevier.com/locate/jinorgbio