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The handle http://hdl.handle.net/1887/55958 holds various files of this Leiden University dissertation
Author: Xin, Bo-Tao
Title: Discovery and development of inhibitors selective for human constitutive proteasome and immunoproteasome active sites
Date: 2017-09-27
General Introduction
1.1 The Ubiquitin Proteasome System (UPS)
The ubiquitin proteasome system (UPS) is responsible for the degradation of the majority of cytosolic and nuclear proteins in eukaryotic cells.1 Proteins destined for degradation by the UPS are ubiquitinated by an ATP‐dependent cascade of E1, E2 and E3 enzymes.2 Ubiquitin is a 76‐residue protein that is attached to substrate proteins via an isopeptidic linkage, in which the lysine side chain amine is condensed with the C‐terminal glycine of a ubiquitin moiety.3 Ubiquitin lysines themselves can be modified with ubiquitin moieties, giving rise to a plethora of oligo‐ and poly‐ubiquitylated proteins. Some of these are destined for proteasomal degradation whereas others may guide other cellular functions. Once bound to the 19S caps of 26S proteasomes, ubiquitin chains are removed through the action of one of a number of deubiquitinases. The target proteins are then unfolded and channeled to the inner catalytic chamber of 20S proteasomes where they are processed to give oligopeptides with an average length of 8‐12 amino acids.4 These short peptides can be further degraded into amino acids by aminopeptidases, while some escape further degradation and are displayed by major histocompatibility complex class I (MHCI) molecules on the cell surface.5
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1.2 The Proteasome
Proteasomes are large multiprotein complexes that are expressed by all eukaryotic cells. 26S proteasome particles consist of a 19S regulatory particle and a 20S core particle (CP). 20S CPs are C2‐symmetrical barrel‐shaped multiprotein complexes composed of four stacked rings, each of which contains seven subunits: two inner rings (β1‐7) and two outer rings (α1‐7). In constitutive proteasome CPs, the proteolytic activity resides within three distinct β subunits, namely, β1c (caspase‐like, cleaving preferably after acidic amino acids), β2c (trypsin‐like, cleaving preferably after basic amino acids) and β5c (chymotrypsin‐like, cleaving preferably after hydrophobic amino acids).4 The active sites of the β‐subunits share N‐terminal threonine residue and it can coordinate to a water molecule (Figure 1). The hydroxyl group of the threonine attacks the adjacent peptide bond, which results in the cleavage of the peptide pond and formation of ester bond between proteasome and the substrate. This ester bond could be further cleaved through attack of a water molecule.
In higher vertebrate, these three subunits may be replaced by β1i (LMP2), β2i (MECL1) and β5i (LMP7), giving rise to immunoproteasome core particles (iCPs).6 iCP proteolytic activities feature substrate preferences similar to, but distinct from, their cCP counterparts and are commonly thought to generate peptides with higher affinity for MHCI.7 Cortical thymic epithelial cells uniquely express β5t subunits, which replace β5i in immunoproteasomes to form thymoproteasome core particles (tCPs).8
Figure 1. The catalytic cycle of proteasomal peptide bond hydrolysis.
1.3 Proteasome Inhibitors
Proteasomes are validated drug targets in the treatment of hematological cancers and are considered as promising targets for the development of drugs to treat a variety of immunological disorders, including rheumatoid arthritis and multiple sclerosis.9 Proteasome
inhibitors currently used in the clinic for the treatment of multiple myeloma and mantle cell lymphoma are the C‐terminally modified oligopeptides, bortezomib,10 carfilzomib11 and ixazomib,12 and several analogous structures are in various stages of development. Besides these clinical drugs and drug candidates, numerous proteasome inhibitors have been reported, including both natural products and synthetic compounds.13 Synthetic peptide‐based natural proteasome inhibitors usually consist of 2‐4 amino acids with a N‐terminal cap and a C‐terminal electrophilic trap (also termed the ‘warhead’). The peptide fragment serves as a recognition part that interacts with the non‐primed part of the substrate‐binding channel within proteasome core particles. The electrophile reacts with the N‐terminal and catalytically active threonine residues to form a covalent and either reversible or irreversible linkage. The four most widely used warheads are epoxyketones (EK), vinyl sulfones (VS), boronic acids and aldehydes. Epoxyketones and vinyl sulfones react with proteasome active site threonines to form covalent, irreversible bonds whereas boronic acides and aldehydes form covalent, reversible bonds. The mechanism of action of these electrophiles is show in figure 2.
Figure 2. Mechanism of EK, VS, boronic acid and aldehyde electrophiles is reacting with the active site threonine of proteasomes.
The natural product, epoxomicin, is the archetypal peptide epoxyketone proteasome inhibitor (Figure 3). Epoxomicin is a highly specific proteasome inhibitor with no off‐targets known to date. This specificity may be due to its mechanism of action: two electrophiles are present in the N‐terminal 2‐amino‐alcohol that makes up the active site residue.14 Indeed, both threonine alcohol and amine are covalently modified upon reaction with epoxomicin and related peptide epoxyketones. Originally and based on crystal studies the covalent adduct was thought to be a morpholine ring, however, recent structural studies proteasomes complexed to epoxyketones suggest that a seven‐membered oxazepane ring can be formed instead (Figure 2).15 Vinyl sulfones inhibit proteasomes by conjugate addition of the hydroxyl moiety of the threonine in the active site to the vinyl sulfone to form a stable ether linkage.16 Common off‐targets of peptide vinyl sulfones are cysteine proteases, which in a similar process react to
Chapter 1
give stable thioether adducts. Peptide boronic acids react as a Lewis base with the Lewis acidic threonine‐alcohol to form stable, tetrahedral adducts that however collapse upon denaturing proteasomes and as such are formal reversible inhibitors.17 Depending on the nature of the peptide sequence, the residence time of peptide boronic acids varies such that, while some are to all intends and purposes irreversible (the time of their presence in proteasome active sites surpasses that of proteasome half‐life), others are (slowly) reversible. Off‐targets of peptide boronic acids may be found in the (extensive) serine hydrolase family. Peptide aldehydes finally form a reversible hemi‐acetal bond with the active site threonine alcohol.18 Peptide aldehydes (with as example Z‐Leu3‐Al, or MG‐132) were the first designed, synthetic proteasome inhibitors. Today they are however considered inferior in comparison to peptide epoxyketones, peptide vinyl sulfones and peptide boronic acids because they are less active, relatively unstable and moreover have many off targets in the serine hydrolase and cysteine protease families.
Figure 3. Structures of representative compounds with various electrophiles: EK (epoxomicin), VS (Z‐L3‐VS), boronic acid (bortezomib) and aldehyde (MG132).
In the first years, efforts in the development of proteasome inhibitors were either directed on pan‐proteasome inhibitors or on the development of inhibitors active against proteasomal chymotryptic sites. In the last few years, however, inhibitors that target selectively β1c/1i (NC‐001),19 β2c/2i (LU‐102)20 and β5c/5i (NC‐005)21 have been reported (Figure 4A). After the successful development of these selective inhibitors, research has been focusing on the development of inhibitors that selectively and potently target one out of the six catalytically active subunits of human cCPs and iCPs together.22 Recently, and at the basis of the research described in this Thesis, a set of inhibitors were disclosed (Figure 4B),23 by means of which one selected activity out of the six embedded in human cCPs and iCPs combined can be blocked.
Figure 4. Structures of selective proteasome inhibitors.
As well, a set of activity‐based probes with complementary fluorescent properties and reporting, in a single activity‐based protein profiling (ABPP) assay, on β1c/1i, β2c/2i and β5c/5i, respectively, was described (Figure 5). With this ABPP assay, the activity and selectivity of proteasome inhibitors, including those developed in the course of the research described in this Thesis, can be monitored with relative ease.23
Chapter 1
Figure 5. A) Structures of Cy5‐NC‐001, BODIPY(FL)‐LU‐112 and BODIPY(TMR)‐NC‐005‐VS. B) Selective inhibition of β‐subunits in Raji cell lysates by subunit‐specific proteasome inhibitors assayed by competitive ABPP using cocktail APBs.
1.4 Aim and outline of this thesis
This thesis presents the development and optimization of subunit‐selective proteasome inhibitors and their corresponding activity‐based probes (ABPs), building on the inhibitor‐ and probe set depicted in Figure 4. Chapter 2 provides a concise overview of methods for the asymmetric synthesis of non‐canonical aliphatic ‐amino acids – structures featuring in several of the optimized oligopeptide vinyl sulfones and oligopeptide epoxyketones presented in subsequent chapters. In Chapter 3, the design, synthesis and evaluation as potential proteasome inhibitors of a series of C‐terminally modified oligopeptides featuring structural variations on the epoxyketone theme is presented. Chapter 4 describes the incorporation of the constrained peptidomimetic, 5‐methylpyridin‐2‐one into peptide vinyl sulfones and peptide epoxyketones as well as the consequences of this modification on proteasome inhibition potency. The structure‐based design of β5c selective inhibitors of human constitutive proteasomes including a stereoselective synthesis of the Boc‐cis‐BiCha‐OH and Boc‐trans‐BiCha‐OH amino acids is subject of Chapter 5. Chapter 6 presents the synthesis and evaluation of a focused compound library designed to encompass β2c‐selective proteasome inhibitors. Chapter 7 reports on the discovery of a β2i‐selective inhibitor and activity‐based probe. In Chapter 8, efforts to attain new reagents for in vitro and in situ two‐step labeling of individual proteasome activities is described. Chapter 9 provides a summary of the research described in this Thesis and reflects on some future research directions presents. One major outstanding question is the development of an inhibitor selective for the seventh human proteasome activity, the thymoproteasome‐specific β5t. Amongst others, this final chapter reports on results obtained after screening of an in‐house available library of C‐terminally modified oligopeptides, including all proteasome inhibitors currently available at Leiden University for the presence of β5t selective inhibitors.
1.5 References
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