Site-Specific Proteasome Inhibitors

Recent advances in small molecule inhibitors of deubiquitinating enzymes

Proteins play a pivotal role in maintaining cellular homeostasis. Their degradation primarily orchestrated through the ubiquitin-proteasome system (UPS) and cellular autophagy. Dysfunction of the UPS is associated with various human diseases, including cancer, autoimmune disorders, and neurodegenerative conditions. Consequently, the UPS has emerged as a promising therapeutic target. Deubiquitinases (DUBs) have garnered significant attention as potential targets for therapeutic intervention due to their role in modulating protein stability and function. This review focuses on recent advancements of DUBs, particularly their relevance in the UPS and their potential as drug targets. Notably, inhibitors targeting specific DUBs, such as USP1, USP7, USP14, and USP30 have shown promise in preclinical and clinical studies for cancer therapy. Additionally, DUB inhibitors have been involved in novel therapeutic approaches lately, including as targets for proteolysis-targeting chimeras (PROTACs) or as tools in deubiquitinase-targeting chimeras (DUBTACs).

Proteasome Inhibitors: Potential in Rheumatoid Arthritis Therapy?

Rheumatoid arthritis (RA) is a chronic autoimmune disease that leads to the destruction of peripheral joint cartilage and bone tissue. Despite the advent of biological therapies in the past decades, the complete remission of RA patients is still out of reach. Therefore, the search for novel therapeutic approaches is still open in the field of RA. Proteasome inhibitors (PIs) were originally designed to be used in hematological malignancies like multiple myeloma. However, evidence has shown that they are potent inhibitors of the NF-κB pathway, which plays a pivotal role in inflammatory processes and RA. Furthermore, inhibition of cell activation and induction of apoptosis was also reported about PIs. In the present review, we summarize the current knowledge about the potential effects of PIs in RA based on reports from animal and human studies. We believe that there is substantial potential in the use of PIs in RA therapy either alone or in combination with the medications already used.

Urolithin A inhibits breast cancer progression via activating TFEB-mediated mitophagy in tumor macrophages

INTRODUCTION

Urolithin A (UA) is a naturally occurring compound that is converted from ellagitannin-like precursors in pomegranates and nuts by intestinal flora. Previous studies have found that UA exerts tumor-suppressive effects through antitumor cell proliferation and promotion of memory T-cell expansion, but its role in tumor-associated macrophages remains unknown.

OBJECTIVES

Our study aims to reveal how UA affects tumor macrophages and tumor cells to inhibit breast cancer progression.

METHODS

Observe the effect of UA treatment on breast cancer progression though in vivo and in vitro experiments. Western blot and PCR assays were performed to discover that UA affects tumor macrophage autophagy and inflammation. Co-ip and Molecular docking were used to explore specific molecular mechanisms.

RESULTS

We observed that UA treatment could simultaneously inhibit harmful inflammatory factors, especially for InterleuKin-6 (IL-6) and tumor necrosis factor α (TNF-α), in both breast cancer cells and tumor-associated macrophages, thereby improving the tumor microenvironment and delaying tumor progression. Mechanistically, UA induced the key regulator of autophagy, transcription factor EB (TFEB), into the nucleus in a partially mTOR-dependent manner and inhibited the ubiquitination degradation of TFEB, which facilitated the clearance of damaged mitochondria via the mitophagy-lysosomal pathway in macrophages under tumor supernatant stress, and reduced the deleterious inflammatory factors induced by the release of nucleic acid from damaged mitochondria. Molecular docking and experimental studies suggest that UA block the recognition of TFEB by 1433 and induce TFEB nuclear localization. Notably, UA treatment demonstrated inhibitory effects on tumor progression in multiple breast cancer models.

CONCLUSION

Our study elucidated the anti-breast cancer effect of UA from the perspective of tumor-associated macrophages. Specifically, TFEB is a crucial downstream target in macrophages.

Swine RNF5 positively regulates the antiviral activity of IFITM1 by mediating the degradation of ABHD16A

Interferon-inducible transmembrane (IFITM) proteins are broad-spectrum antiviral factors that confer cellular resistance to virus invasion. α/β-Hydrolase domain-containing 16A (ABHD16A) has recently been identified as a novel depalmitoylase that can inhibit the antiviral activity of IFITM proteins by catalyzing the depalmitoyl reaction; this pattern may be crucial for the host to avoid damage caused by excessive immune response. However, it remains largely elusive about how host cells regulate the activity of ABHD16A. In the present study, we performed the AlphaFold2-based protein-protein interaction prediction and identified swine E3 ubiquitin ligase ring finger protein 5 (sRNF5) as a sABHD16A-interacting protein and negatively regulated the stability of sABHD16A. Using immunofluorescence and co-immunoprecipitation techniques, we uncovered that sRNF5 targeted sABHD16A for ubiquitination and degradation via the proteasomal pathway at residues K3 and K452. Furthermore, sABHD16A catalyzed the depalmitoylation of sIFITM1, which obstructed the antiviral function of sIFITM1, while sRNF5 caused ubiquitination of sABHD16A, which attenuated the depalmitoylation effect on sIFITM1, and consequently restored the antiviral activity of sIFITM1. Collectively, our findings demonstrate for the first time that sRNF5 positively regulates the antiviral function of sIFITM1 by mediating the degradation of sABHD16A, which expands the biological functions of RNF5 and ABHD16A in immune regulation. Moreover, our work highlights the well-designed interplay between RNF5, ABHD16A, and IFITM, which balances antiviral immune responses to avoid the disorders induced by excessive immune response.

IMPORTANCE

Interferon and interferon-stimulated genes play significant and protective roles in the host's defense against viral infection. IFITM family proteins, which can be strongly induced by interferon, have been identified as the first line of defense to prevent invasion of various viruses. Further analysis reveals the antiviral activity of IFITMs depends on palmitoylation/depalmitoylation. Recently, we reported that ABHD16A, as the first depalmitoylase of IFITMs, negatively regulated the antiviral activity of IFITMs. However, these raise crucial questions: how ABHD16A is regulated and remained in a balanced manner? Here, we show that swine RNF5 attenuates the negative regulation of sIFITM1 against virus invasion by modifying sABHD16A through ubiquitination and guiding sABHD16A for degradation. Thus, sRNF5-sABHD16A interplay plays an indispensable role in regulating immune response and avoiding the disorders induced by elevated interferon levels. Overall, our findings extend the upstream subtle regulatory molecular mechanism of IFITMs and provide potential targets for viral disease therapy.

Chemical and Biological Mechanisms Relevant to the Rescue of MG-132-Treated Neurons by Cysteine

Proteasome dysfunctions are observed in many human pathologies. To study their role and potential treatment strategies, models of proteasome inhibition are widely used in biomedical research. One frequently used tool is the proteasome inhibitor MG-132. It triggers the degeneration of human neurons, and several studies show protection from pathological events by glutathione or its precursors. It has therefore been concluded that glutathione protects cells from proteasome dysfunction. However, an alternative explanation is that MG-132, which is a peptide aldehyde, is chemically inactivated by thiols, and the apparent protection by glutathione from proteasome dysfunction is an artefact. To clarify this issue, we examined the chemical inactivation of MG-132 by thiols and the role of such reactions for neuroprotection. Using mass spectrometry and nuclear magnetic resonance spectroscopy, we found that MG-132 reacted with L-cysteine to form a stable end product and with glutathione to form an unstable intermediate. Using a cell-free proteasome inhibition assay, we found that high concentrations of L-cysteine can scavenge a substantial fraction of MG-132 and thus reduce proteasome inhibition. Glutathione (or N-acetyl-cysteine) did not alter proteasome inhibition (even at high concentrations). In a final step, we studied human neuronal cultures. We exposed them to MG-132, supplemented the culture medium with various thiols, and assessed intracellular L-cysteine concentrations. The transcriptome response pattern also indicated an inhibition of the proteasome by MG-132 in the presence of L-cysteine. We conclude that thiol concentrations that can be reached in cells do not inactivate MG-132 in pathological models. They rather act in a cytoprotective way as antioxidants.

Structure activity relationship for anticancer activities of spirooxindole derivatives: A comprehensive review

Cancer remains one of the leading causes of mortality worldwide, necessitating the continuous search for novel therapeutic agents. Spirooxindole derivatives have recently emerged as a class of compounds with significant potential for cancer treatment owing to their diverse pharmacological activities and unique structural features. The structural diversity of spirooxindole derivatives enables a wide range of modifications, facilitating optimization of their pharmacokinetic and pharmacodynamic properties. Moreover, their ability to interact with multiple molecular targets involved in cancer progression, including kinases, receptors, and enzymes, makes them attractive candidates for multi-targeted therapy. In preclinical studies, numerous spirooxindole derivatives have demonstrated promising antiproliferative activity against various cancer cell lines, including breast, lung, colon, and prostate cancers. Mechanistic investigations have revealed their ability to induce cell cycle arrest and apoptosis and inhibit angiogenesis and metastasis, underscoring their potential as effective anticancer agents. However, challenges such as off-target effects, drug resistance, and limited bioavailability need to be addressed to maximize the therapeutic potential of these compounds. Continued research efforts to elucidate their molecular mechanisms, optimize their pharmacological properties, and conduct rigorous clinical evaluations are warranted to harness their full therapeutic benefits for cancer treatment. This review provides a comprehensive overview of recent advancements in developing spirooxindole derivatives as anticancer agents with structure-activity relationships.

Distinct substrate specificities of the three catalytic subunits of the Trichomonas vaginalis proteasome

The protozoan parasite Trichomonas vaginalis (Tv) causes trichomoniasis, the most common non-viral sexually transmitted infection in the world. Although Tv has been linked to significant health complications, only two closely related 5-nitroimidazole drugs are approved for its treatment. The emergence of resistance to these drugs and lack of alternative treatment options poses an increasing threat to public health, making development of novel anti-Trichomonas compounds an urgent need. The proteasome, a critical enzyme complex found in all eukaryotes has three catalytic subunits, β1, β2, and β5 and has been validated as a drug target to treat trichomoniasis. With the goal of developing tools to study the Tv proteasome, we isolated the enzyme complex and identified inhibitors that preferentially inactivate either one or two of the three catalytic subunits. Using a mass spectrometry-based peptide digestion assay, these inhibitors were used to define the substrate preferences of the β1, β2 and β5 subunits. Subsequently, three model fluorogenic substrates were designed, each specific for one of the catalytic subunits. This novel substrate profiling methodology will allow for individual subunit characterization of other proteasomes of interest. Using the new substrates, we screened a library of 284 peptide epoxyketone inhibitors against Tv and determined the subunits targeted by the most active compounds. The data show that inhibition of the Tv β5 subunit alone is toxic to the parasite. Taken together, the optimized proteasome subunit substrates will be instrumental for understanding the molecular determinants of proteasome specificity and for accelerating drug development against trichomoniasis.

Inhibition of proteolytic and ATPase activities of the proteasome by the BTK inhibitor CGI-1746

Bruton's tyrosine kinase (BTK) inhibitor, ibrutinib, has been shown to synergize in vitro with proteasome inhibitors (PIs) in reducing the viability of cells derived from B cell malignancies, but the mechanism is not known. We report here that an off-target effect of ibrutinib causes synergy because not all BTK inhibitors exhibited the synergistic effect, and those that synergized did so even in cells that do not express BTK. The allosteric BTK inhibitor CGI-1746 showed the strongest synergy. Co-treatment of cells with CGI-1746 increased PI-induced accumulation of ubiquitin conjugates and expression of heat shock proteins and NOXA and decreased a ratio of reduced to oxidized glutathione. CGI-1746, but not other BTK inhibitors, inhibited ATPase activity and all three peptidase activities of the 26S proteasome. The effect demonstrates a conceptually novel mode of proteasome inhibition that may aid the development of more potent PIs.

Dissociation of the nuclear WWOX/TRAF2 switch renders UV/cold shock-mediated nuclear bubbling cell death at low temperatures

BACKGROUND

Normal cells express functional tumor suppressor WW domain-containing oxidoreductase (WWOX), designated WWOXf. UV irradiation induces WWOXf cells to undergo bubbling cell death (BCD) - an event due to the accumulation of nuclear nitric oxide (NO) gas that forcefully pushes the nuclear and cell membranes to form one or two bubbles at room temperature (22 °C) and below. In contrast, when WWOX-deficient or -dysfunctional (WWOXd) cells are exposed to UV and/or cold shock, the cells undergo nuclear pop-out explosion death (POD). We aimed to determine the morphological and biochemical changes in WWOXf cells during BCD versus apoptosis.

METHODS

WWOXf and WWOXd cells were exposed to UV followed by measuring BCD or POD by time-lapse microscopy and/or time-lapse holographic microscopy at 4, 22, or 37 °C to visualize morphological changes. Live cell stains were used to measure the kinetics of nitric oxide (NO) production and Ca2+ influx. Extent of cell death was measured by uptake of propidium iodide and by internucleosomal DNA fragmentation using agarose gel electrophoresis.

RESULTS

WWOXf cells were exposed to UV and then cold shock, or cold shock and then UV, and cultured at 4, 10, and 22 °C, respectively. Initially, UV induced calcium influx and NO production, which led to nuclear bubbling and final death. Cold shock pretreatment completely suppressed UV-mediated bubbling at 37 °C, so the UV/cold shock-treated cells underwent apoptosis. Without cold shock, UV only induced bubbling at all temperatures, whereas the efficiency of bubbling at 37 °C was reduced by greater than 50%. Morphologically, the WWOXf cell height or thickness was significantly increased during cell division or apoptosis, but the event did not occur in BCD. In comparison, when WWOXd cancer cells received UV or UV/cold shock, these cells underwent NO-independent POD. UV/cold shock effectively downregulated the expression of many proteins such as the housekeeping α-tubulin (> 70%) and β-actin (< 50%), and cortactin (> 70%) in WWOXf COS7 cells. UV/cold shock induced relocation of α-tubulin to the nucleus and nuclear bubbles in damaged cells. UV induced co-translocation of the WWOX/TRAF2 complex to the nuclei, in which the prosurvival TRAF2 blocked the proapoptotic WWOX via its zinc finger domain. Without WWOX, TRAF2 did not relocate to the nuclei. Cold shock caused the dissociation of the WWOX/TRAF2 complex in the nucleus needed for BCD. In contrast, the formation of the WWOX/TRAF2 complex, plus p53, was strengthened at 37 °C required for apoptosis.

CONCLUSIONS

The temperature-sensitive nuclear WWOX/TRAF2 complex acts as a molecular switch, whose dissociation favors BCD at low temperatures, and the association supports apoptosis at 37 °C in UV-treated WWOXf cells.

Vaginal Tritrichomonas foetus infection in mice as an in vivo model for drug development against Trichomonas vaginalis

Trichomonas vaginalis is the causative agent of the common sexually transmitted disease, trichomoniasis, which affects more than a hundred million people worldwide. Metronidazole and tinidazole, agents belonging to the 5-nitroheterocyclic class of antimicrobials, are most often used to treat infection, but increased resistance has been reported and adverse effects of these drugs can be significant. Consequently, an urgent need exists for the development of novel drug entities against trichomoniasis. Critical for antimicrobial drug development is the demonstration of in vivo efficacy. Murine models of vaginal T. vaginalis infection are unreliable for unknown reasons. Meanwhile, murine infections with the related bovine pathogen, Tritrichomonas foetus, tend to be more robust, although susceptibility to different antimicrobials might differ from T. vaginalis. Here, we explored the utility of T. foetus infection as a surrogate model for drug development against T. vaginalis. Four different T. foetus strains caused robust vaginal infection in young mice, while none of four diverse T. vaginalis strains did. Comparison of drug susceptibility profiles revealed that T. foetus and T. vaginalis were similarly susceptible to a range of 5-nitroheterocyclic and gold(I) compounds. By comparison, proteasome inhibitors were 10- to 15-fold less active against T. foetus than T. vaginalis, although one of the proteasome inhibitors, bortezomib, had low micromolar activity or better against multiple strains of both trichomonads. Different strains of T. foetus were used to demonstrate the utility of the murine vaginal infection models for in vivo efficacy testing, including for bortezomib and a gold(I) compound. The differences in susceptibility to proteasome inhibitors may be partially explained by differences in the proteasome subunit sequences between the two trichomonads, although the functional relevance of the proteasome was similar in both organisms. These findings indicate that T. foetus can serve as a reliable surrogate model for T. vaginalis in vitro and in murine infections in vivo, but caution must be exercised for specific drug classes with targets, such as the proteasome, that may display genetic divergence between the trichomonads.

Enhancing the immunogenicity of Wilms tumor 1 epitope in mesothelioma cells with immunoproteasome inhibitors

The immunogenicity of cancer cells is influenced by several factors, including the expression of the major histocompatibility complex class I (MHC-I), antigen expression, and the repertoire of proteasome-produced epitope peptides. The malignant pleural mesothelioma cell line ACC-MEOS-4 (MESO-4) expresses high levels of MHC-I and Wilms tumor 1 (WT1) tumor antigens. Using a functional T cell reporter assay specific for the HLA-A*24:02 restricted WT1 epitope (WT1235, CMTWNQMNL), we searched for factors that augmented the immunogenicity of MESO-4, focusing on proteasomes, which have a central role in the antigen processing machinery. ONX-0914, a selective inhibitor of the immunoproteasome subunit β5i, enhanced immunogenicity dose-dependently at low concentrations without cytotoxicity. In addition, CD8+ T lymphocytes recognizing WT1 showed greater cytotoxicity against MESO-4 pre-treated with ONX-0914. MESO-4 expresses a standard proteasome (SP) and immunoproteasome (IP). Notably, IP has distinct catalytic activity from SP, favoring the generation of antigenic peptides with high affinity for MHC-I in antigen-presenting cells and cancer cells. In vitro, immunoproteasome digestion assay and mass spectrometry analysis showed that IP cleaved WT1235 internally after the hydrophobic residues. Importantly, this internal cleavage of the WT1235 epitope was mitigated by ONX-0914. These results suggest that ONX-0914 prevents the internal destructive cleavage of WT1235 by IP, thereby promoting the specific presentation of the WT1 epitope by MESO-4. In conclusion, selective IP inhibitors might offer a means to modulate cancer cell immunogenicity by directing the presentation of particular tumor epitopes.

Brain-Permeable Immunoproteasome-Targeting Macrocyclic Peptide Epoxyketones for Alzheimer's Disease

Previously, we demonstrated that linear peptide epoxyketones targeting the immunoproteasome (iP) could ameliorate cognitive deficits in mouse models of Alzheimer's disease (AD) independently of amyloid deposition. We also reported the first iP-targeting macrocyclic peptide epoxyketones, which exhibit improved metabolic stability compared with their linear counterparts. Here, we prepared additional macrocyclic peptide epoxyketones and compared them with existing macrocyclic iP inhibitors by assessing Caco2 cell-based permeability and microsomal stability, providing the four best macrocyclic iP inhibitors. We then evaluated the four compounds using the Ames test and the potency assays in BV2 cells, selecting compound 5 as our AD drug lead. When 5 was administered intravenously (40 mg/kg) or orally (150 mg/kg) into healthy BALB/c mice, we observed considerable iP inhibition in the mouse brain, indicating good blood-brain barrier permeability and target engagement. Combined results suggest that 5 is a promising AD drug lead that may need further investigation.

Regulation of Mertk Surface Expression via ADAM17 and γ-Secretase Proteolytic Processing

Mertk, a type I receptor tyrosine kinase and member of the TAM family of receptors, has important functions in promoting efferocytosis and resolving inflammation under physiological conditions. In recent years, Mertk has also been linked to pathophysiological roles in cancer, whereby, in several cancer types, including solid cancers and leukemia/lymphomas. Mertk contributes to oncogenic features of proliferation and cell survival as an oncogenic tyrosine kinase. In addition, Mertk expressed on macrophages, including tumor-associated macrophages, promotes immune evasion in cancer and is suggested to act akin to a myeloid checkpoint inhibitor that skews macrophages towards inhibitory phenotypes that suppress host T-cell anti-tumor immunity. In the present study, to better understand the post-translational regulation mechanisms controlling Mertk expression in monocytes/macrophages, we used a PMA-differentiated THP-1 cell model to interrogate the regulation of Mertk expression and developed a novel Mertk reporter cell line to study the intracellular trafficking of Mertk. We show that PMA treatment potently up-regulates Mertk as well as components of the ectodomain proteolytic processing platform ADAM17, whereas PMA differentially regulates the canonical Mertk ligands Gas6 and Pros1 (Gas6 is down-regulated and Pros1 is up-regulated). Under non-stimulated homeostatic conditions, Mertk in PMA-differentiated THP1 cells shows active constitutive proteolytic cleavage by the sequential activities of ADAM17 and the Presenilin/γ-secretase complex, indicating that Mertk is cleaved homeostatically by the combined sequential action of ADAM17 and γ-secretase, after which the cleaved intracellular fragment of Mertk is degraded in a proteasome-dependent mechanism. Using chimeric Flag-Mertk-EGFP-Myc reporter receptors, we confirm that inhibitors of γ-secretase and MG132, which inhibits the 26S proteasome, stabilize the intracellular fragment of Mertk without evidence of nuclear translocation. Finally, the treatment of cells with active γ-carboxylated Gas6, but not inactive Warfarin-treated non-γ-carboxylated Gas6, regulates a distinct proteolytic itinerary-involved receptor clearance and lysosomal proteolysis. Together, these results indicate that pleotropic and complex proteolytic activities regulate Mertk ectodomain cleavage as a homeostatic negative regulatory event to safeguard against the overactivation of Mertk.

An evolutionarily conserved ubiquitin ligase drives infection and transmission of flaviviruses

Mosquito-borne flaviviruses such as dengue (DENV) and Zika (ZIKV) cause hundreds of millions of infections annually. The single-stranded RNA genome of flaviviruses is translated into a polyprotein, which is cleaved equally into individual functional proteins. While structural proteins are packaged into progeny virions and released, most of the nonstructural proteins remain intracellular and could become cytotoxic if accumulated over time. However, the mechanism by which nonstructural proteins are maintained at the levels optimal for cellular fitness and viral replication remains unknown. Here, we identified that the ubiquitin E3 ligase HRD1 is essential for flaviviruses infections in both mammalian hosts and mosquitoes. HRD1 directly interacts with flavivirus NS4A and ubiquitylates a conserved lysine residue for ER-associated degradation. This mechanism avoids excessive accumulation of NS4A, which otherwise interrupts the expression of processed flavivirus proteins in the ER. Furthermore, a small-molecule inhibitor of HRD1 named LS-102 effectively interrupts DENV2 infection in both mice and Aedes aegypti mosquitoes, and significantly disturbs DENV transmission from the infected hosts to mosquitoes owing to reduced viremia. Taken together, this study demonstrates that flaviviruses have evolved a sophisticated mechanism to exploit the ubiquitination system to balance the homeostasis of viral proteins for their own advantage and provides a potential therapeutic target to interrupt flavivirus infection and transmission.

B-Myb deficiency boosts bortezomib-induced immunogenic cell death in colorectal cancer

B-Myb has received considerable attention for its critical tumorigenic function of supporting DNA repair. However, its modulatory effects on chemotherapy and immunotherapy have rarely been reported in colorectal cancer. Bortezomib (BTZ) is a novel compound with chemotherapeutic and immunotherapeutic effects, but it fails to work in colorectal cancer with high B-Myb expression. The present study was designed to investigate whether B-Myb deletion in colorectal cancer could potentiate the immune efficacy of BTZ against colorectal cancer and to clarify the underlying mechanism. Stable B-Myb knockdown was induced in colorectal cancer cells, which increased apoptosis of the cancer cells relative to the control group in vitro and in vivo. We found that BTZ exhibited more favourable efficacy in B-Myb-defective colorectal cancer cells and tumor-bearing mice. BTZ treatment led to differential expression of genes enriched in the p53 signaling pathway promoted more powerful downstream DNA damage, and arrested cell cycle in B-Myb-defective colorectal cancer. In contrast, recovery of B-Myb in B-Myb-defective colorectal cancer cells abated BTZ-related DNA damage, cell cycle arrest, and anticancer efficacy. Moreover, BTZ promoted DNA damage-associated enhancement of immunogenicity, as indicated by potentiated expression of HMGB1 and HSP90 in B-Myb-defective cells, thereby driving M1 polarization of macrophages. Collectively, B-Myb deletion in colorectal cancer facilitates the immunogenic death of cancer cells, thereby further promoting the immune efficacy of BTZ by amplifying DNA damage. The present work provides an effective molecular target for colorectal cancer immunotherapy with BTZ.

Proteasome Inhibitors in Multiple Myeloma: Biological Insights on Mechanisms of Action or Resistance Informed by Functional Genomics

During the last 20 years, proteasome inhibitors have been a cornerstone for the therapeutic management of multiple myeloma (MM). This review highlights how MM research has evolved over time in terms of our understanding of the mechanistic basis for the pronounced clinical activity of proteasome inhibitors in MM, compared with the limited clinical applications of this drug class outside the setting of plasma cell dyscrasias.

PSMA7 promotes the malignant proliferation of esophageal cancer

Background

It is important to explore novel molecules that play a key role in esophageal cancer (ESCA) progression.

Methods

Two ESCA tissue expression profile microarrays (GSE92396 and GSE17351) data from GEO were downloaded, and differentially expressed genes (DEGs) were analyzed using GEO2R. The DEGs common to both microarrays were analyzed for protein-protein interactions, KEGG and GO. The altered expression of proteasome 20S subunit α 7 (PSMA7) in ESCA tissues was analyzed using information from publicly available databases (GEO, TCGA, TNMplot). PSMA7 was overexpressed or knocked down in Eca109 and KYSE150 cells using transfection, and the effects on cell proliferation, migration, invasion and apoptosis were examined using CCK-8, Transwell, and flow cytometry experiments.

Results

284 common DEGs were identified, and 10 core proteins, HSP90AA1, AURKA, CDC6, PCNA, MCM5, KAT2B, GRB2, MYBL2, PSMA7, and CKAP5, involved in ESCA progression were identified. PSMA7 mRNA level was significantly increased in ESCA tissues. PSMA7 overexpression significantly promoted the proliferation, migration and invasion of Eca109 and KYSE150 cells, and significantly promoted apoptosis. In contrast, PSMA7 knockdown inhibited their proliferation and motility, and significantly suppressed apoptosis.

Conclusion

This study analyzed multiple proteins that may play a key role in ESCA progression, and identified the pro-cancer role of PSMA7.

Dihydrocelastrol induces antitumor activity and enhances the sensitivity of bortezomib in resistant multiple myeloma by inhibiting STAT3-dependent PSMB5 regulation

Multiple myeloma (MM) is characterized by excessive aggregation of B-cell-derived malignant plasma cells in the hematopoietic system of bone marrow. Previously, we synthesized an innovative molecule named dihydrocelastrol (DHCE) from celastrol, a triterpene purified from medicinal plant Tripterygium wilfordii. Herein, we explore the therapeutic properties and latent signal transduction mechanism of DHCE action in bortezomib (BTZ)-resistant (BTZ-R) MM cells. In this study, we first report that DHCE shows antitumor activities in vitro and in vivo and exerts stronger inhibitory effects than celastrol on BTZ-R cells. We find that DHCE inhibits BTZ-R cell viability by promoting apoptosis via extrinsic and intrinsic pathways and suppresses BTZ-R MM cell proliferation by inducing G0/G1 phase cell cycle arrest. In addition, inactivation of JAK2/STAT3 and PI3K/Akt pathways are involved in the DHCE-mediated antitumor effect. Simultaneously, DHCE acts synergistically with BTZ on BTZ-R cells. PSMB5, a molecular target of BTZ, is overexpressed in BTZ-R MM cells compared with BTZ-S MM cells and is demonstrated to be a target of STAT3. Moreover, DHCE downregulates PSMB5 overexpression in BTZ-R MM cells, which illustrates that DHCE overcomes BTZ resistance through increasing the sensitivity of BTZ in resistant MM via inhibiting STAT3-dependent PSMB5 regulation. Overall, our findings imply that DHCE may become a potential therapeutic option that warrants clinical evaluation for BTZ-R MM.

Proteasome Inhibitors against Glioblastoma-Overview of Molecular Mechanisms of Cytotoxicity, Progress in Clinical Trials, and Perspective for Use in Personalized Medicine

Proteasome inhibitors are moieties targeting the proteolytic activity of a proteasome, with demonstrated efficacy in certain hematological malignancies and candidate drugs in other types of cancer, including glioblastoma (GBM). They disturb the levels of proteasome-regulated proteins and lead to the cell cycle inhibition and apoptosis of GBM cells. The accumulation of cell cycle inhibitors p21 and p27, and decreased levels of prosurvival molecules NFKB, survivin, and MGMT, underlie proteasome inhibitors' cytotoxicity when used alone or in combination with the anti-GBM cytostatic drug temozolomide (TMZ). The evidence gathered in preclinical studies substantiated the design of clinical trials that employed the two most promising proteasome inhibitors, bortezomib and marizomib. The drug safety profile, maximum tolerated dose, and interaction with other drugs were initially evaluated, mainly in recurrent GBM patients. A phase III study on newly diagnosed GBM patients who received marizomib as an adjuvant to the Stupp protocol was designed and completed in 2021, with the Stupp protocol receiving patients as a parallel control arm. The data from this phase III study indicate that marizomib does not improve the PFS and OS of GBM patients; however, further analysis of the genetic and epigenetic background of each patient tumor may shed some light on the sensitivity of individual patients to proteasome inhibition. The mutational and epigenetic makeup of GBM cells, like genetic alterations to TP53 and PTEN, or MGMT promoter methylation levels may actually determine the response to proteasome inhibition.

Proteasome inhibitors as anticancer agents

INTRODUCTION

The therapeutic targeting of the ubiquitin-proteasome pathway (UPP) through inhibitors of the 20S proteasome core proteolytic activities has revolutionized the treatment of hematological malignancies and is paving the way for its extension to solid tumors.

AREAS COVERED

This review covers the progress made in the field of proteasome inhibitors, ranging from the first-generation bortezomib to the latest second-generation inhibitors such as carfilzomib and ixazomib as well as the proteasome inhibitors in clinical phase such as oprozomib and marizomib. The development of selective and potent proteasome inhibitors with improved pharmacological properties is described from the synthesis to their basic biological, and clinical validation.

EXPERT OPINION

Proteasome inhibitors have transformed the treatment landscape for hematological malignancies and hold great promise for cancer therapy. Combination therapies targeting multiple pathways, the development of novel inhibitors or 'hybrid-inhibitors,' and the optimization of treatment protocols are key areas for future exploration. The extension of proteasome inhibitors for the treatment of solid tumors, and their ability to pass the blood-brain barrier open new possibilities for treating central nervous system cancers. However, managing adverse effects, particularly those affecting the central nervous system, remains a critical consideration and a strategic 'working on' aspect for the near future.

TCH-165 attenuates cardiac ischaemia/reperfusion injury by balancing mitochondrial dynamics via increasing proteasome activity

The proteasome is the main complex responsible for maintaining intracellular protein homeostasis, impairment of which is associated with cardiac ischaemia/reperfusion (I/R) injury. The small molecule TCH-165 has been found to activate the 20S proteasome to remove disordered proteins in multiple myeloma and glioblastoma. However, the preventive effect of TCH-165 against I/R-mediated cardiac impairment in mice remains largely unknown. Here, a cardiac I/R model was established in mice. Heart function was assessed with echocardiography. Cardiac infarction, myocyte death, and superoxide level were evaluated by 2,3,5-triphenyltetrazolium chloride (TTC)-Evans blue staining, terminal deoxynucleotidyl transferase-mediated dUTP nick and labelling (TUNEL) assay and immunostaining, respectively. Our results showed that TCH-165 treatment markedly ameliorated I/R-mediated cardiac dysfunction and decreased the infarct size, apoptosis, and superoxide levels. Mechanistically, TCH-165 increased immunoproteasome subunit expression/activity, increasing pro-fission protein dynamin-1-like protein (DNM1L, also known as DRP1) degradation and the expression of the pro-fusion proteins mitofusin 1/2 (Mfn1/2) and thereby leading to mitochondrial fission/fusion balance. In vitro experiments confirmed that inhibition of proteasome activity by epoxomicin abolished the protective effect of TCH-165 against hypoxia/reoxygenation (H/R)-induced increases in cardiomyocyte apoptosis, superoxide production and mitochondrial fission. In summary, TCH-165 is a newly discovered inducer of immunoproteasome activity that exerts a preventive effect against cardiac I/R damage by targeting Drp1 degradation, indicating that it may be as a potential therapeutic candidate for ischaemic heart disease.

Ubiquitin Ligase Nrdp1 Controls Autophagy-Associated Acrosome Biogenesis and Mitochondrial Arrangement during Spermiogenesis

Autophagy is critical to acrosome biogenesis and mitochondrial quality control, but the underlying mechanisms remain unclear. The ubiquitin ligase Nrdp1/RNF41 promotes ubiquitination of the mitophagy-associated Parkin and interacts with the pro-autophagic protein SIP/CacyBP. Here, we report that global deletion of Nrdp1 leads to formation of the round-headed sperm and male infertility by disrupting autophagy. Quantitative proteome analyses demonstrated that the expression of many proteins associated with mitochondria, lysosomes, and acrosomes was dysregulated in either spermatids or sperm of the Nrdp1-deficient mice. Deletion of Nrdp1 increased the levels of Parkin but decreased the levels of SIP, the mitochondrial fission protein Drp1 and the mitochondrial protein Tim23 in sperm, accompanied by the inhibition of autophagy, the impairment of acrosome biogenesis and the disruption of mitochondrial arrangement in sperm. Thus, our results uncover an essential role of Nrdp1 in spermiogenesis and male fertility by promoting autophagy, providing important clues to cope with the related male reproductive diseases.

Detection and biochemical characterization of circulating proteasomes in dog plasma

A growing body of evidence convincingly indicates that proteasomes are not located exclusively within cells but also in different extracellular compartments. In humans, in fact, this large multimeric protease has been identified in many body fluids and secretions such as blood, urine, tears, sweat, saliva, milk, and cerebrospinal and pericardial fluid. Intriguingly, the exact origins of these extracellular proteasomes as well as the specific biological functions they perform are largely unknown. As no data on this important subject is yet available in domestic animals, the present study was undertaken to investigate the presence of extracellular proteasomes in canine blood. As a result, for the first time, circulating proteasomes could be clearly detected in the plasma of a cohort of 20 healthy dogs. Furthermore, all three main proteasomal peptidase activities were measured and characterized using fluorogenic peptides and highly specific inhibitors. Finally, the effect of ATP and PA28 family activators on this circulating proteasome was investigated. Collectively, our data indicate that at least a part of the proteasome present in dog plasma consists of a particle that in vitro displays the enzymatic properties of the 20S proteasome.

Prospects and Limitations of High-Resolution Single-Particle Cryo-Electron Microscopy

Single particle cryo-electron microscopy (cryo-EM) has matured into a robust method for the determination of biological macromolecule structures in the past decade, complementing X-ray crystallography and nuclear magnetic resonance. Constant methodological improvements in both cryo-EM hardware and image processing software continue to contribute to an exponential growth in the number of structures solved annually. In this review, we provide a historical view of the many steps that were required to make cryo-EM a successful method for the determination of high-resolution protein complex structures. We further discuss aspects of cryo-EM methodology that are the greatest pitfalls challenging successful structure determination to date. Lastly, we highlight and propose potential future developments that would improve the method even further in the near future.

High-Throughput/High Content Imaging Screen Identifies Novel Small Molecule Inhibitors and Immunoproteasomes as Therapeutic Targets for Chordoma

Chordomas account for approximately 1-4% of all malignant bone tumors and 20% of primary tumors of the spinal column. It is a rare disease, with an incidence estimated to be approximately 1 per 1,000,000 people. The underlying causative mechanism of chordoma is unknown, which makes it challenging to treat. Chordomas have been linked to the T-box transcription factor T (TBXT) gene located on chromosome 6. The TBXT gene encodes a protein transcription factor TBXT, or brachyury homolog. Currently, there is no approved targeted therapy for chordoma. Here, we performed a small molecule screening to identify small chemical molecules and therapeutic targets for treating chordoma. We screened 3730 unique compounds and selected 50 potential hits. The top three hits were Ribociclib, Ingenol-3-angelate, and Duvelisib. Among the top 10 hits, we found a novel class of small molecules, including proteasomal inhibitors, as promising molecules that reduce the proliferation of human chordoma cells. Furthermore, we discovered that proteasomal subunits PSMB5 and PSMB8 are increased in human chordoma cell lines U-CH1 and U-CH2, confirming that the proteasome may serve as a molecular target whose specific inhibition may lead to better therapeutic strategies for chordoma.

The establishment of mitotic errors-driven senescence depends on autophagy

We and others have reported that senescence onset is accompanied by genomic instability that is evident by several defects, such as aneuploidy or erroneous mitosis features. Here, we report that these defects also appear in young cells upon oxidative insult. We provide evidence that these errors could be the consequence of oxidative stress (OS)- either exogenous or senescence-associated - overriding the spindle assembly checkpoint (SAC). Young cells treated with Η₂Ο₂ as well as older cells fail to maintain mitotic arrest in the presence of spindle poisons and a significant higher percentage of them have supernumerary centrosomes and centrosome related anomalous characteristics. We also report that aging is escorted by expression modifications of SAC components, and especially of Bub1b/BubR1. Bub1b/BubR1 has been previously reported to decrease naturally upon aging. Here, we show that there is an initial increase in Bub1b/BubR1 levels, feasibly as part of the cells' response against OS-driven genomic instability, that is followed by its autophagy dependent degradation. This provides an explanation that was missing regarding the molecular entity responsible for the downregulation of Bub1b/BubR1 upon aging, especially since it is well established, by us and others, that the proteasome function decays as cells age. These results, not only serve the previously reported notion of a shift from proteasome to autophagy-dependent degradation upon aging, but also provide a mechanistic insight for mitotic errors-driven senescence. We believe that our conclusions deepen our understanding regarding the homeostatic function of autophagy that serves the establishment of senescence as a barrier against cellular transformation.

Current and Novel Therapeutical Approaches of Classical Homocystinuria in Childhood With Special Focus on Enzyme Replacement Therapy, Liver-Directed Therapy and Gene Therapy

Classical homocystinuria is a hereditary defect of the enzyme cystathionine beta synthase, which is produced in the liver. If this enzyme fails, the synthesis pathway of cysteine from methionine is interrupted, leading to the accumulation of homocysteine in the blood plasma and homocysteine in the urine. After birth, the children are unremarkable except for the characteristic laboratory findings. Symptoms rarely appear before the second year of life. The most common symptom is a prolapse of the crystalline lens. This finding is seen in 70% of untreated 10-year-old affected individuals. As the earliest symptom, psychomotor retardation occurs in the majority of patients already during the first two years of life. Limiting factors in terms of life expectancy are thromboembolism, peripheral arterial disease, myocardial infarction, and stroke. These symptoms are due to the damage to the vessels caused by the elevated amino acid levels. About 30% suffer a thromboembolic event by the age of 20, about half by the age of 30. This review focus on present and new therapeutical approaches like the role of enzyme replacement with presentation of different novel targets in research like pegtibatinase, pegtarviliase, CDX-6512, erymethionase, chaperones, proteasome inhibitors and probiotic treatment with SYNB 1353. Furthermore, we analyze the role of liver-directed therapy with three dimensional (3D) bioprinting, liver bioengineering of liver organoids in vitro and liver transplantation. The role of different gene therapy options to treat and cure this extremely rare disease in childhood will be discussed.

Discovery of Highly Selective Inhibitors of the Human Constitutive Proteasome β5c Chymotryptic Subunit

We describe our discovery and development of potent and highly selective inhibitors of human constitutive proteasome chymotryptic activity (β5c). Structure-activity relationship studies of the novel class of inhibitors focused on optimization of N-cap, C-cap, and side chain of the chemophore asparagine. Compound 32 is the most potent and selective β5c inhibitor in this study. A docking study provides a structure rationale for potency and selectivity. Kinetic studies show a reversible and noncompetitive inhibition mechanism. It enters the cells to engage the proteasome target, potently and selectively kills multiple myeloma cells, and does so by synergizing with a β5i-selective inhibitor.

HMGCS2-Induced Autophagic Degradation of Tau Involves Ketone Body and ANKRD24

BACKGROUND

Accumulation of hyperphosphorylated Tau (pTau) contributes to the formation of neurofibrillary tangles in Alzheimer's disease (AD), and targeting Tau/pTau metabolism has emerged as a therapeutic approach. We have previously reported that mitochondrial 3-hydroxy-3-methylglutaryl-COA synthase 2 (HMGCS2) is involved in AD by promoting autophagic clearance of amyloid-β protein precursor via ketone body-associated mechanism, whether HMGCS2 may also regulate Tau metabolism remains elusive.

OBJECTIVE

The present study was to investigate the role of HMGCS2 in Tau/p degradation.

METHODS

The protein levels of Tau and pTau including pT217 and pT181, as well as autophagic markers LAMP1 and LC3-II were assessed by western blotting. The differentially regulated genes by HMGCS2 were analyzed by RNA sequencing. Autophagosomes were assessed by transmission electron microscopy.

RESULTS

HMGCS2 significantly decreased Tau/pTau levels, which was paralleled by enhanced formation of autophagic vacuoles and prevented by autophagic regulators chloroquine, bafilomycin A1, 3-methyladenine, and rapamycin. Moreover, HMGCS2-induced alterations of LAMP1/LC3-II and Tau/pTau levels were mimicked by ketone body acetoacetate or β-hydroxybutyrate. Further RNA-sequencing identified ankyrin repeat domain 24 (ANKRD24) as a target gene of HMGCS2, and silencing of ANKRD24 reduced LAMP1/LC3-II levels, which was accompanied by the altered formation of autophagic vacuoles, and diminished the effect of HMGCS2 on Tau/pTau.

CONCLUSION

HMGCS2 promoted autophagic clearance of Tau/pTau, in which ketone body and ANKRD24 played an important role.

Modulatory effect of MG-132 proteasomal inhibition on boar sperm motility during in vitro capacitation

A series of biochemical and biophysical changes during sperm capacitation initiates various signaling pathways related to protein phosphorylation leading to sperm hyperactivation, simultaneously with the regulation of proteasomal activity responsible for protein degradation and turnover. Our study aimed to unveil the role of the proteasome in the regulation of boar sperm motility, hyperactivated status, tyrosine phosphorylation, and total protein ubiquitination. The proteolytic activity of the 20S proteasomal core was inhibited by MG-132 in concentrations of 10, 25, 50, and 100 μM; and monitored parameters were analyzed every hour during 3 h of in vitro capacitation (IVC). Sperm motility and kinematic parameters were analyzed by Computer Assisted Sperm Analysis (CASA) during IVC, showing a significant, negative, dose-dependent effect of MG-132 on total and progressive sperm motility (TMOT, PMOT, respectively). Furthermore, proteasomal inhibition by 50 and 100 μM MG-132 had a negative impact on velocity-based kinematic sperm parameters (VSL, VAP, and VCL). Parameters related to the progressivity of sperm movement (LIN, STR) and ALH were the most affected by the highest inhibitor concentration (100 μM). Cluster analysis revealed that the strongest proteasome-inhibiting treatment had a significant effect (p ≤ 0.05) on the hyperactivated sperm subpopulation. The flow cytometric viability results proved that reduced TMOT and PMOT were not caused by disruption of the integrity of the plasma membrane. Neither the protein tyrosine phosphorylation profile changes nor the accumulation of protein ubiquitination was observed during the course of capacitation under proteasome inhibition. In conclusion, inhibition of the proteasome reduced the ability of spermatozoa to undergo hyperactivation; however, there was no significant effect on the level of protein tyrosine phosphorylation and accumulation of ubiquitinated proteins. These effects might be due to the presence of compensatory mechanisms or the alteration of various ubiquitin-proteasome system-regulated pathways.

Targeting immunoproteasome in neurodegeneration: A glance to the future

The immunoproteasome is a specialized form of proteasome equipped with modified catalytic subunits that was initially discovered to play a pivotal role in MHC class I antigen processing and immune system modulation. However, over the last years, this proteolytic complex has been uncovered to serve additional functions unrelated to antigen presentation. Accordingly, it has been proposed that immunoproteasome synergizes with canonical proteasome in different cell types of the nervous system, regulating neurotransmission, metabolic pathways and adaptation of the cells to redox or inflammatory insults. Hence, studying the alterations of immunoproteasome expression and activity is gaining research interest to define the dynamics of neuroinflammation as well as the early and late molecular events that are likely involved in the pathogenesis of a variety of neurological disorders. Furthermore, these novel functions foster the perspective of immunoproteasome as a potential therapeutic target for neurodegeneration. In this review, we provide a brain and retina-wide overview, trying to correlate present knowledge on structure-function relationships of immunoproteasome with the variety of observed neuro-modulatory functions.

Precisely Targeted Nano-Controller of PD-L1 Level for Non-Small Cell Lung Cancer Spinal Metastasis Immunotherapy

Although immune checkpoint inhibitors (ICIs) have been widely applied to treat non-small cell lung cancer (NSCLC), a significant proportion of patients, especially those with spinal metastasis (NSCLC-SM), are insensitive to anti-programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) ICIs. A drug delivery nano-controller of PD-L1 that targets NSCLC-SM can solve this problem, however, none have been developed to date. In this study, it is shown that integrin β3 (β3-int) is strongly upregulated in NSCLC-SM. Its inhibitor RGDyK promotes PD-L1 ubiquitination, indicating the potential application of RGDyK as a new PD-L1 inhibitor in nano-controller and a targeting peptide for NSCLC-SM treatment. According to the synergistic effect of photodynamic therapy and ICIs on T-cell activation through the release of tumor antigens, RGDyK-modified and zinc protoporphyrin (ZnPP)-loaded mesoporous silicon nanoparticles (ZnPP@MSN-RGDyK) are fabricated. The ZnPP@MSN-RGDyK nanoparticles precisely target β3-int to inhibit PD-L1, exhibiting high photodynamic therapy efficiency, and excellent immunotherapeutic effects in an NSCLC-SM mouse model. Collectively, the findings indicate that ZnPP@MSN-RGDyK is a promising immunotherapeutic agent for treating NSCLC-SM.

Polo-Like Kinase 2 Plays an Essential Role in Cytoprotection against MG132-Induced Proteasome Inhibition via Phosphorylation of Serine 19 in HSPB5

Protein homeostasis, including protein folding, refolding, and degradation, is thought to decline with aging. HSPB5 (also known as αB-crystallin) prevents target protein aggregation as a molecular chaperone and exhibits a cytoprotective function against various cell stresses. To elucidate the effect of HSPB5 on endoplasmic reticulum (ER) stress, we searched for novel binding proteins of HSPB5 using the proximity-dependent biotin labeling method. Proteins presumed to interact with HSPB5 in cells treated with the proteasome inhibitor MG132 were identified by a reversible biotin-binding capacity method combining tamavidin2-REV magnetic beads and mass spectrometry. We discovered a new binding protein for HSPB5, polo-like kinase 2 (PLK2), which is an apoptosis-related enzyme. The expression of PLK2 was upregulated by MG132 treatment, and it was co-localized with HSPB5 near the ER in L6 muscle cells. Inhibition of PLK2 decreased ER stress-induced phosphorylation of serine 19 in HSPB5 and increased apoptosis by activation of caspase 3 under ER stress. Overexpression of HSPB5 (WT) suppressed the ER stress-induced caspase 3 activity, but this was not observed with phospho-deficient HSPB5 (3A) mutants. These results clarify the role of HSPB5 phosphorylation during ER stress and suggest that the PLK2/HSPB5 pathway plays an essential role in cytoprotection against proteasome inhibition-induced ER stress.

Isoform-specific and ubiquitination dependent recruitment of Tet1 to replicating heterochromatin modulates methylcytosine oxidation

Oxidation of the epigenetic DNA mark 5-methylcytosine by Tet dioxygenases is an established route to diversify the epigenetic information, modulate gene expression and overall cellular (patho-)physiology. Here, we demonstrate that Tet1 and its short isoform Tet1s exhibit distinct nuclear localization during DNA replication resulting in aberrant cytosine modification levels in human and mouse cells. We show that Tet1 is tethered away from heterochromatin via its zinc finger domain, which is missing in Tet1s allowing its targeting to these regions. We find that Tet1s interacts with and is ubiquitinated by CRL4(VprBP). The ubiquitinated Tet1s is then recognized by Uhrf1 and recruited to late replicating heterochromatin. This leads to spreading of 5-methylcytosine oxidation to heterochromatin regions, LINE 1 activation and chromatin decondensation. In summary, we elucidate a dual regulation mechanism of Tet1, contributing to the understanding of how epigenetic information can be diversified by spatio-temporal directed Tet1 catalytic activity.

The Histone Deacetylase Inhibitor ITF2357 (Givinostat) Targets Oncogenic BRAF in Melanoma Cells and Promotes a Switch from Pro-Survival Autophagy to Apoptosis

Histone deacetylase inhibitors (HDACI) are epigenetic compounds that have been widely considered very promising antitumor agents. Here, we focus on the effects of the pan-HDAC inhibitor ITF2357 (Givinostat) in comparison with SAHA (Vorinostat) in melanoma cells bearing BRAF V600E oncogenic mutation. Our results indicate both ITF2357 and SAHA dose-dependently reduce the viability of BRAF-mutated SK-MEL-28 and A375 melanoma cells. The comparison of IC50 values revealed that ITF2357 was much more effective than SAHA. Interestingly, both inhibitors markedly decreased oncogenic BRAF protein expression levels, ITF2357 being the most effective compound. Moreover, the BRAF decrease induced by ITF2357 was accompanied by a decrease in the level of phospho-ERK1/2. The inhibitor of upstream MEK activity, U0126, reduced ERK1/2 phosphorylation and dramatically potentiated the antitumor effect of ITF2357, exacerbating the reduction in the BRAF level. ITF2357 stimulated an early pro-survival autophagic response, which was followed by apoptosis, as indicated by apoptotic markers evaluation and the protective effects exerted by the pan-caspase inhibitor z-VADfmk. Overall, our data indicate for the first time that ITF2357 targets oncogenic BRAF in melanoma cells and induces a switch from autophagy to classic apoptosis, thus representing a possible candidate in melanoma targeted therapy.

Regulating Proteasome Activity

Strictly controlled degradation of the proteome is a key factor in maintaining cellular homeostasis and allows a rapid and effective response to a variety of different stress challenges [...]

Localized Proteasomal Degradation: From the Nucleus to Cell Periphery

The proteasome is responsible for selective degradation of most cellular proteins. Abundantly present in the cell, proteasomes not only diffuse in the cytoplasm and the nucleus but also associate with the chromatin, cytoskeleton, various membranes and membraneless organelles/condensates. How and why the proteasome gets to these specific subcellular compartments remains poorly understood, although increasing evidence supports the hypothesis that intracellular localization may have profound impacts on the activity, substrate accessibility and stability/integrity of the proteasome. In this short review, I summarize recent advances on the functions, regulations and targeting mechanisms of proteasomes, especially those localized to the nuclear condensates and membrane structures of the cell, and I discuss the biological significance thereof in mediating compartmentalized protein degradation.