Targeting of the E3 ubiquitin-protein ligase HUWE1 impairs DNA repair capacity and tumor growth in preclinical multiple myeloma models

HAPSTR1 localizes HUWE1 to the nucleus to limit stress signaling pathways

HUWE1 is a large, enigmatic HECT-domain ubiquitin ligase implicated in the regulation of diverse pathways, including DNA repair, apoptosis, and differentiation. How HUWE1 engages its structurally diverse substrates and how HUWE1 activity is regulated are unknown. Using unbiased quantitative proteomics, we find that HUWE1 targets substrates in a largely cell-type-specific manner. However, we identify C16orf72/HAPSTR1 as a robust HUWE1 substrate in multiple cell lines. Previously established physical and genetic interactions between HUWE1 and HAPSTR1 suggest that HAPSTR1 positively regulates HUWE1 function. Here, we show that HAPSTR1 is required for HUWE1 nuclear localization and nuclear substrate targeting. Nuclear HUWE1 is required for both cell proliferation and modulation of stress signaling pathways, including p53 and nuclear factor κB (NF-κB)-mediated signaling. Combined, our results define a role for HAPSTR1 in gating critical nuclear HUWE1 functions.

The giant E3 ligase HUWE1 is linked to tumorigenesis, spermatogenesis, intellectual disability, and inflammatory diseases

E3 ubiquitin ligases determine the substrate specificity and catalyze the ubiquitination of lysine residues. HUWE1 is a catalytic HECT domain-containing giant E3 ligase that contains a substrate-binding ring structure, and mediates the ubiquitination of more than 40 diverse substrates. HUWE1 serves as a central node in cellular stress responses, cell growth and death, signal transduction, etc. The expanding atlas of HUWE1 substrates presents a major challenge for the potential therapeutic application of HUWE1 in a particular disease. In addition, HUWE1 has been demonstrated to play contradictory roles in certain aspects of tumor progression in either an oncogenic or a tumor-suppressive manner. We recently defined novel roles of HUWE1 in promoting the activation of multiple inflammasomes. Inflammasome activation-mediated immune responses might lead to multifunctional effects on tumor therapy, inflammation, and autoimmune diseases. In this review, we summarize the known substrates and pleiotropic functions of HUWE1 in different types of cells and models, including its involvement in development, cancer, neuronal disorder and infectious disease. We also discuss the advances in cryo-EM-structural analysis for a functional-mechanistic understanding of HUWE1 in modulating the multitudinous diverse substrates, and introduce the possibility of revisiting the comprehensive roles of HUWE1 in multiple aspects within one microenvironment, which will shed light on the potential therapeutic application of targeting giant E3 ligases like HUWE1.

Genomic Instability and Replicative Stress in Multiple Myeloma: The Final Curtain?

Simple Summary

Genomic instability is recognized as a driving force in most cancers as well as in the haematological cancer multiple myeloma and remains among the leading cause of drug resistance. Several evidences suggest that replicative stress exerts a fundamental role in fuelling genomic instability. Notably, cancer cells rely on a single protein, ATR, to cope with the ensuing DNA damage. In this perspective, we provide an overview depicting how replicative stress represents an Achilles heel for multiple myeloma, which could be therapeutically exploited either alone or in combinatorial regimens to preferentially ablate tumor cells.

Abstract

Multiple Myeloma (MM) is a genetically complex and heterogeneous hematological cancer that remains incurable despite the introduction of novel therapies in the clinic. Sadly, despite efforts spanning several decades, genomic analysis has failed to identify shared genetic aberrations that could be targeted in this disease. Seeking alternative strategies, various efforts have attempted to target and exploit non-oncogene addictions of MM cells, including, for example, proteasome inhibitors. The surprising finding that MM cells present rampant genomic instability has ignited concerted efforts to understand its origin and exploit it for therapeutic purposes. A credible hypothesis, supported by several lines of evidence, suggests that at the root of this phenotype there is intense replicative stress. Here, we review the current understanding of the role of replicative stress in eliciting genomic instability in MM and how MM cells rely on a single protein, Ataxia Telangiectasia-mutated and Rad3-related protein, ATR, to control and survive the ensuing, potentially fatal DNA damage. From this perspective, replicative stress per se represents not only an opportunity for MM cells to increase their evolutionary pool by increasing their genomic heterogeneity, but also a vulnerability that could be leveraged for therapeutic purposes to selectively target MM tumor cells.

Inhibition of cotranslational translocation by apratoxin S4: Effects on oncogenic receptor tyrosine kinases and the fate of transmembrane proteins produced in the cytoplasm

Receptor tyrosine kinases (RTKs) have become major targets for anticancer therapy. However, resistance and signaling pathway redundancy has been problematic. The marine-derived apratoxins act complementary to direct kinase inhibitors by downregulating the levels of multiple of these receptors and additionally prevent the secretion of growth factors that act on these receptors by targeting Sec61α, therefore interfering with cotranslational translocation. We have profiled the synthetic, natural product-inspired apratoxin S4 against panels of cancer cells characterized by differential sensitivity to RTK inhibitors due to receptor mutations, oncogenic KRAS mutations, or activation of compensatory pathways. Apratoxin S4 was active at low-nanomolar to sub-nanomolar concentrations against panels of lung, head and neck, bladder, and pancreatic cancer cells, concomitant with the downregulation of levels of several RTKs, including EGFR, MET and others. However, the requisite concentration to inhibit certain receptors varied, suggesting some differential substrate selectivity in cellular settings. This selectivity was most pronounced in breast cancer cells, where apratoxin S4 selectively targeted HER3 over HER2 and showed greater activity against ER+ and triple negative breast cancer cells than HER2+ cancer cells. Depending on the breast cancer subtype, apratoxin S4 differentially downregulated transmembrane protein CDCP1, which is linked to metastasis and invasion in breast cancer and modulates EGFR activity. We followed the fate of CDCP1 through proteomics and found that nonglycosylated CDCP1 associates with chaperone HSP70 and HUWE1 that functions as an E3 ubiquitin ligase and presumably targets CDCP1, as well as potentially other substrates inhibited by apratoxins, for proteasomal degradation. By preventing cotranslational translocation of VEGF and other proangiogenic factors as well as VEGFR2 and other receptors, apratoxins also possess antiangiogenic activity, which was validated in endothelial cells where downregulation of VEGFR2 was observed, extending the therapeutic scope to angiogenic diseases.

Genome Instability in Multiple Myeloma: Facts and Factors

Multiple myeloma (MM) is a malignant neoplasm of terminally differentiated immunoglobulin-producing B lymphocytes called plasma cells. MM is the second most common hematologic malignancy, and it poses a heavy economic and social burden because it remains incurable and confers a profound disability to patients. Despite current progress in MM treatment, the disease invariably recurs, even after the transplantation of autologous hematopoietic stem cells (ASCT). Biological processes leading to a pathological myeloma clone and the mechanisms of further evolution of the disease are far from complete understanding. Genetically, MM is a complex disease that demonstrates a high level of heterogeneity. Myeloma genomes carry numerous genetic changes, including structural genome variations and chromosomal gains and losses, and these changes occur in combinations with point mutations affecting various cellular pathways, including genome maintenance. MM genome instability in its extreme is manifested in mutation kataegis and complex genomic rearrangements: chromothripsis, templated insertions, and chromoplexy. Chemotherapeutic agents used to treat MM add another level of complexity because many of them exacerbate genome instability. Genome abnormalities are driver events and deciphering their mechanisms will help understand the causes of MM and play a pivotal role in developing new therapies.

The long journey to bring a Myc inhibitor to the clinic

The oncogene Myc is deregulated in the majority of human tumors and drives numerous hallmarks of cancer. Despite its indisputable role in cancer development and maintenance, Myc is still undrugged. Developing a clinical inhibitor for Myc has been particularly challenging owing to its intrinsically disordered nature and lack of a binding pocket, coupled with concerns regarding potentially deleterious side effects in normal proliferating tissues. However, major breakthroughs in the development of Myc inhibitors have arisen in the last couple of years. Notably, the direct Myc inhibitor that we developed has just entered clinical trials. Celebrating this milestone, with this Perspective, we pay homage to the different strategies developed so far against Myc and all of the researchers focused on developing treatments for a target long deemed undruggable.