Clogging the Ubiquitin-Proteasome Machinery with Marine Natural Products: Last Decade Update

Abstract

The ubiquitin-proteasome pathway (UPP) is the central protein degradation system in eukaryotic cells, playing a key role in homeostasis maintenance, through proteolysis of regulatory and misfolded (potentially harmful) proteins. As cancer cells produce proteins inducing cell proliferation and inhibiting cell death pathways, UPP inhibition has been exploited as an anticancer strategy to shift the balance between protein synthesis and degradation towards cell death. Over the last few years, marine invertebrates and microorganisms have shown to be an unexhaustive factory of secondary metabolites targeting the UPP. These chemically intriguing compounds can inspire clinical development of novel antitumor drugs to cope with the incessant outbreak of side effects and resistance mechanisms induced by currently approved proteasome inhibitors (e.g., bortezomib). In this review, we report about (a) the role of the UPP in anticancer therapy, (b) chemical and biological properties of UPP inhibitors from marine sources discovered in the last decade, (c) high-throughput screening techniques for mining natural UPP inhibitors in organic extracts. Moreover, we will tell about the fascinating story of salinosporamide A, the first marine natural product to access clinical trials as a proteasome inhibitor for cancer treatment.

Keywords: cancer; high-throughput screening; lead compounds; marine; natural products; proteasome; salinosporamide; secondary metabolites; ubiquitin.

Figure 1

Protein degradation through the ubiquitin-proteasome pathway. The ubiquitin-activating enzyme E1 uses ATP to adenylate ubiquitin, yielding a ubiquitin-adenylate adduct and pyrophosphate. Ubiquitin is then transferred from E1 to the ubiquitin-conjugating enzyme E2. The ubiquitin ligase E3 transfers ubiquitin to a target substrate and a polyubiquitin chain is attached to tag the protein for degradation by the proteasome complex. Ubiquitin is released by deubiquitinating enzymes and the protein is degraded into oligopeptides. Ub, ubiquitin; ATP, adenosine triphosphate; ADP, adenosine diphosphate; AMP, adenosine monophosphate; PPi, inorganic pyrophosphate; DUB, deubiquitinase.

Figure 2

Key cellular targets affected by proteasome inhibition.

Figure 3

Mechanism of action of proteasome inhibitors. (a) Epoxyketones react with the N-terminal threonine (Thr-1) of the catalytically active β-subunits in a two-step mechanism. Initially a 1,2-addition of the γ-OH group of Thr-1 (Thr1Oγ) to the carbonyl group of the epoxyketone leads to the formation of a hemiketal derivative; then, the α-amino-group of Thr-1 makes a nucleophilic attack at C-2 of the epoxide moiety to give a stable morpholine adduct. (b) In peptide boronates, the boron atom covalently interacts with the nucleophilic oxygen lone pair of Thr1Oγ, yielding a tetrahedral boronate adduct. This adduct is in turn stabilized by an acidic boronate OH group, which hydrogen-bridges Thr-1 amine atom. (c) The side-chain hydroxy group of Thr-1 cleaves the lactone ring of β-lactone proteasome inhibitors, yielding the proteasome-inhibitor complex; then, in the case of salinosporamides, the newly formed tertiary alcohol displaces the chlorine atom, leading to the formation of a tetrahydrofuran ring, which in turn coordinates the α-amino group of Thr-1. (d) The carbonyl moiety of peptide aldehydes is the electrophilic site which undergoes nucleophilic attack by Thr-1 to form an hemiacetal adduct; (e) Vinyl ketones harbor a Michael system which undergoes 1,4-nucleophilic addition with the Thr1Oγ, forming a one-step irreversible covalent adduct.

Figure 4

The five classes of proteasome inhibitors. Proteasome inhibitors possess an electrophilic group for covalent binding to the proteasome. The reactive pharmacophore of each inhibitor is highlighted in red.

Figure 5

(a) Percentage distribution of natural UPP inhibitors discovered from marine sources in the last decade (2007–2018); (b) Distribution of pharmacological targets of reviewed UPP inhibitors.