Platelet EVs contain an active proteasome involved in protein processing for antigen presentation via MHC-I molecules.

In addition to their hemostatic role, platelets play a significant role in immunity. Once activated, platelets release extracellular vesicles (EVs) formed by the budding of their cytoplasmic membranes. Because of their heterogeneity, platelet EVs (PEVs) are thought to perform diverse functions.

Non-lethal proteasome inhibition activates pro-tumorigenic pathways in multiple myeloma cells.

Multiple myeloma (MM) is a haematological malignancy being characterized by clonal plasma cell proliferation in the bone marrow. Targeting the proteasome with specific inhibitors (PIs) has been proven a promising therapeutic strategy and PIs have been approved for the treatment of MM and mantle-cell lymphoma; yet, while outcome has improved, most patients inevitably relapse. As relapse refers to MM cells that survive therapy, we sought to identify the molecular responses induced in MM cells after non-lethal proteasome inhibition.

Structure-Based Design of Inhibitors Selective for Human Proteasome β2c or β2i Subunits.

Subunit-selective proteasome inhibitors are valuable tools to assess the biological and medicinal relevance of individual proteasome active sites. Whereas the inhibitors for the β1c, β1i, β5c, and β5i subunits exploit the differences in the substrate-binding channels identified by X-ray crystallography, compounds selectively targeting β2c or β2i could not yet be rationally designed because of the high structural similarity of these two subunits. Here, we report the development, chemical synthesis, and biological screening of a compound library that led to the identification of the β2c- and β2i-selective compounds LU-002c (4; IC β2c: 8 nM, IC β2i/β2c: 40-fold) and LU-002i (5; IC β2i: 220 nM, IC β2c/β2i: 45-fold), respectively.

Proteasome Inhibition in Multiple Myeloma: Head-to-Head Comparison of Currently Available Proteasome Inhibitors.

Proteasome inhibitors (PIs) are a backbone of multiple myeloma (MM) therapy. The proteasome harbors six proteolytically active subunits (β1, β2, β5), while β5 was identified as rate-limiting and is a primary target of clinically available PIs. The most effective pattern of subunit inhibition provided by these PIs for cytotoxic activity in MM is unknown.

An inhibitor of proteasome β2 sites sensitizes myeloma cells to immunoproteasome inhibitors.

Proteasome inhibitors bortezomib, carfilzomib and ixazomib (approved by the US Food and Drug Administration [FDA]) induce remissions in patients with multiple myeloma (MM), but most patients eventually become resistant. MM and other hematologic malignancies express ubiquitous constitutive proteasomes and lymphoid tissue-specific immunoproteasomes; immunoproteasome expression is increased in resistant patients. Immunoproteasomes contain 3 distinct pairs of active sites, β5i, β1i, and β2i, which are different from their constitutive β5c, β1c, and β2c counterparts.

Milder degenerative effects of Carfilzomib vs. Bortezomib in the Drosophila model: a link to clinical adverse events.

Proteasome inhibitors, e.g. Bortezomib (BTZ) and Carfilzomib (CFZ), have demonstrated clinical efficacy against haematological cancers.

Carfilzomib resistance due to ABCB1/MDR1 overexpression is overcome by nelfinavir and lopinavir in multiple myeloma.

Proteasome inhibitor (PI) carfilzomib (CFZ) has activity superior to bortezomib (BTZ) and is increasingly incorporated in multiple myeloma (MM) frontline therapy and relapsed settings. Most MM patients ultimately experience PI-refractory disease, an unmet medical need with poorly understood biology and dismal outcome. Pharmacologic targeting of ABCB1 improved patient outcomes, including MM, but suffered from adverse drug effects and insufficient plasma concentrations.

Proteasome inhibitor-adapted myeloma cells are largely independent from proteasome activity and show complex proteomic changes, in particular in redox and energy metabolism.

Adaptive resistance of myeloma to proteasome inhibition represents a clinical challenge, whose biology is poorly understood. Proteasome mutations were implicated as underlying mechanism, while an alternative hypothesis based on low activation status of the unfolded protein response was recently suggested (IRE1/XBP1-low model). We generated bortezomib- and carfilzomib-adapted, highly resistant multiple myeloma cell clones (AMO-BTZ, AMO-CFZ), which we analyzed in a combined quantitative and functional proteomic approach.

The novel β2-selective proteasome inhibitor LU-102 synergizes with bortezomib and carfilzomib to overcome proteasome inhibitor resistance of myeloma cells.

Proteasome inhibitor resistance is a challenge for myeloma therapy. Bortezomib targets the β5 and β1 activity, but not the β2 activity of the proteasome. Bortezomib-resistant myeloma cells down-regulate the activation status of the unfolded protein response, and up-regulate β2 proteasome activity.

Use of proteasome inhibitors.

Proteasome inhibitors are indispensable research tools in immunology and cell biology. With numerous proteasome inhibitors available commercially, choosing the appropriate compound for a biological experiment may be challenging, especially for a novice. This unit provides an overview of the proteasome inhibitors commonly used in research.

Structure-based design of β1i or β5i specific inhibitors of human immunoproteasomes.

Mammalian genomes encode seven catalytic proteasome subunits, namely, β1c, β2c, β5c (assembled into constitutive 20S proteasome core particles), β1i, β2i, β5i (incorporated into immunoproteasomes), and the thymoproteasome-specific subunit β5t. Extensive research in the past decades has yielded numerous potent proteasome inhibitors including compounds currently used in the clinic to treat multiple myeloma and mantle cell lymphoma. Proteasome inhibitors that selectively target combinations of β1c/β1i, β2c/β2i, or β5c/β5i are available, yet ligands truly selective for a single proteasome activity are scarce.

Cell-line-specific high background in the Proteasome-Glo assay of proteasome trypsin-like activity.

Proteasome-Glo is a homogeneous cell-based assay of proteasomal chymotrypsin-like, trypsin-like, and caspase-like activities using luminogenic substrates, commercially available from Promega. Here we report that the background activity from cleavage of the substrate of the trypsin-like sites by nonproteasomal proteases in multiple breast and lung cancer cell lines exceeds the activity of the proteasome. We also observed substantial background chymotrypsin-like activity in some cell lines.

Toward understanding induction of oxidative stress and apoptosis by proteasome inhibitors.

Significance: Proteasome inhibitors (PIs) are used in the clinic for the treatment of hematopoietic malignancies. PI inhibitors induce endoplasmatic reticulum (ER) stress and oxidative stress, disruption of signaling pathways, mitochondrial dysfunction, and, eventually, cell death by apoptosis. PIs designated as clinical candidates include natural product derivatives and compounds developed by rational design and feature a wide diversity of structural elements.

Relative quantification of proteasome activity by activity-based protein profiling and LC-MS/MS.

Activity-based protein profiling (ABPP) is a functional proteomics technique for directly monitoring the expression of active enzymes in cell extracts and living cells. The technique relies on irreversible inhibitors equipped with reactive groups (warheads) that covalently attach to the active site of enzymes and fluorescent or affinity tags for imaging and purification purposes, respectively. Here, a high-throughput and robust protocol for high-resolution quantitative activity-based proteasome profiling is described.

Incorporation of non-natural amino acids improves cell permeability and potency of specific inhibitors of proteasome trypsin-like sites.

Proteasomes degrade the majority of proteins in mammalian cells by a concerted action of three distinct pairs of active sites. The chymotrypsin-like sites are targets of antimyeloma agents bortezomib and carfilzomib. Inhibitors of the trypsin-like site sensitize multiple myeloma cells to these agents.

Identification and isolation of lantibiotics from culture: a bioorthogonal chemistry approach.

A distinguishing feature of the lantibiotic family of cyclic peptides is the presence of thioethers. Treatment of a lantibiotic with an alkaline solution at high pH gives rise to a β-elimination reaction yielding the corresponding ring opened precursor, containing a dehydro-amino acid residue. We here reveal in a proof-of-concept study that a ring opened lantibiotic (mersacidin) can be captured for pull-down from a culture broth, subsequently released and identified by mass spectrometry.

Two-step bioorthogonal activity-based proteasome profiling using copper-free click reagents: a comparative study.

The development and application of bioorthogonal two-step labeling techniques receives much attention. Employing bifunctional proteasome probe 2 the efficiency of two-step labeling of recently published biotinylated cyclooctynes 3-5 is compared to Staudinger-Bertozzi ligation in cell extracts and living cells. While cyclooctynes 3-5 react faster and at a much lower concentration then the Staudinger-Bertozzi benchmark, background labeling is considerable with these reagents.

Activity-based profiling reveals reactivity of the murine thymoproteasome-specific subunit beta5t.

Epithelial cells of the thymus cortex express a unique proteasome particle involved in positive T cell selection. This thymoproteasome contains the recently discovered beta5t subunit that has an uncharted activity, if any. We synthesized fluorescent epoxomicin probes that were used in a chemical proteomics approach, entailing activity-based profiling, affinity purification, and LC-MS identification, to demonstrate that the beta5t subunit is catalytically active in the murine thymus.

Two-step labeling of endogenous enzymatic activities by Diels-Alder ligation.

A ligation strategy based on the Diels-Alder [4+2] cycloaddition for the two-step activity-based labeling of endogenously expressed enzymes in complex biological samples has been developed. A panel of four diene-derivatized proteasome probes was synthesized, along with a dienophile-functionalized BODIPY(TMR) tag. These probes were applied in a Diels-Alder labeling procedure that enabled us to label active proteasome beta-subunits selectively in cellular extracts and in living cells.

Proteasome selectivity towards Michael acceptor containing oligopeptide-based inhibitors.

The synthesis and biological evaluation of ten Michael acceptors containing potential proteasome inhibitors are described. Cellular targets of azide containing inhibitors and were assessed in HEK293T and RAW264.7 cells by a two step labeling strategy, followed by biotin-pulldown, affinity purification, on-bead tryptic digestion and LC-MS(2) identification.

Incorporation of fluorinated phenylalanine generates highly specific inhibitor of proteasome's chymotrypsin-like sites.

Proteasomal processing is conducted by three individual catalytic subunits, namely beta1, beta2, and beta5. Subunit-specific inhibitors are useful tools in dissecting the role of these individual subunits and are leads toward the development of antitumor agents. We here report that the presence of fluorinated phenylalanine derivatives in peptide based proteasome inhibitors has a profound effect on inhibitor potency and selectivity.