Cereblon attenuates DNA damage-induced apoptosis by regulating the transcription-independent function of p53

Cereblon deficiency ameliorates carbon tetrachloride-induced acute hepatotoxicity in HepG2 cells by suppressing MAPK-mediated apoptosis

The liver is vulnerable to various hepatotoxins, including carbon tetrachloride (CCl4), which induces oxidative stress and apoptosis by producing reactive oxygen species (ROS) and activating the mitogen-activated protein kinase (MAPK) pathway. Cereblon (CRBN), a multifunctional protein implicated in various cellular processes, functions in the pathogenesis of various diseases; however, its function in liver injury remains unknown. We established a CRBN-knockout (KO) HepG2 cell line and examined its effect on CCl4-induced hepatocellular damage. CRBN-KO cells exhibited reduced sensitivity to CCl4-induced cytotoxicity, as evidenced by decreased levels of apoptosis markers, such as cleaved caspase-3, and aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities. CRBN deficiency enhanced antioxidant defense, with increased superoxide dismutase activity and glutathione ratios (GSH/GSSG), as well as reduced pro-inflammatory cytokine expression. Mechanistically, the protective effects of CRBN deficiency appeared to involve the attenuation of the MAPK-mediated pathways, particularly through decreased phosphorylation of JNK and ERK. Overall, these results suggest the crucial role of CRBN in mediating the hepatocellular response to oxidative stress and inflammation triggered by CCl4 exposure, offering potential clinical implications for liver injury in a wide range of liver diseases.

Development of decoy oligonucleotide-warheaded chimeric molecules targeting STAT3

Proteolysis-targeting chimera (PROTAC) technology is a disruptive innovation in the drug development community, and over 20 PROTAC molecules are currently under clinical evaluation. These PROTAC molecules contain small-molecule warheads that bind to target proteins. Recently, oligonucleotide-warheaded PROTACs have emerged as a promising new tool to degrade DNA-binding proteins such as transcription factors. In this study, we applied an oligonucleotide-warheaded PROTAC technology to induce the degradation of signal transducer and activator of transcription 3 (STAT3), which is a hard-to-target protein. A double-stranded decoy oligonucleotide specific to STAT3 was conjugated to E3 binders (pomalidomide, VH032, and LCL161) to generate PROTAC molecules that recruited different E3 ubiquitin ligases cereblon (CRBN), von Hippel-Lindau (VHL), and inhibitor of apoptosis protein (IAP), respectively. One of the resulting PROTAC molecules, POM-STAT3, which recruits CRBN, potently induces STAT3 degradation. STAT3 degradation by POM-STAT3 was abolished by scrambling the oligonucleotide sequences of POM-STAT3 and by adding a double-stranded decoy oligonucleotide against STAT3 in a competitive manner, suggesting the significance of oligonucleotide sequences in STAT3 degradation. Moreover, POM-STAT3-induced STAT3 degradation was suppressed by the CRBN binder thalidomide, proteasome inhibitor bortezomib, E1 inhibitor MLN7243, and siRNA-mediated depletion of CRBN, indicating that STAT3 degradation is mediated by the ubiquitin-proteasome system, which involves CRBN as the responsible E3 ubiquitin ligase. Consistent with STAT3 degradation, NCI-H2087 cell viability was severely reduced following POM-STAT3 treatment. Thus, POM-STAT3 is a STAT3 degrader that potentially has cytocidal activity against cancer cells that are highly dependent on STAT3 signaling, which implies that inducing protein degradation by decoy oligonucleotide-warheaded PROTAC molecules could be harnessed to be therapeutic against oncogenic transcription factors.

Synthesis of novel glutarimide ligands for the E3 ligase substrate receptor Cereblon (CRBN): Investigation of their binding mode and antiproliferative effects against myeloma cell lines

To expand the chemical toolkit for targeted protein degradation, we report the generation of a new series of non-thalidomide Cereblon (CRBN) ligands. Readily available 2-methylidene glutarimide was converted to a series of 2-((hetero)aryl(methyl))thio glutarimides via the thio-Michael addition reaction. The compounds thus synthesized were evaluated for their affinity to the thalidomide-binding domain of human CRBN and their binding modes studied via X-ray crystallography. This helped identify several promising glutarimide derivatives which bind stronger to CRBN compared to thalidomide and contain a functional group which permits further chemical conjugation. Oxidation of the sulfur atom in a select group of 2-((hetero)aryl(methyl))thio glutarimides produced the respective sulfones which were found to possess a markedly stronger antiproliferative profile against multiple myeloma cell lines and a sophisticated structural binding mode with additional hydrogen bonding interactions. The newly identified Cereblon ligands form the basis for the synthesis of novel PROTAC protein degraders.

The ubiquitination-dependent and -independent functions of cereblon in cancer and neurological diseases

Cereblon (CRBN) mediates the teratogenic effect of thalidomide in zebrafish, chicken, and humans. It additionally modulates the anti-myeloma effect of the immunomodulatory drugs (IMiDs) thalidomide, lenalidomide, and pomalidomide. IMiDs bind to CRBN and recruit neo-substrates for their ubiquitination and proteasome-mediated degradation, which significantly expands the application of proteolysis-targeting chimeras (PROTACs) for targeted drug discovery. However, the underlying molecular mechanisms by which CRBN mediates the teratogenicity and anti-myeloma effect of IMiDs are not fully elucidated. Furthermore, the normal physiological functions of endogenous CRBN have not been extensively studied, which precludes the thorough assessment of side effects of the CRBN ligand-based PROTACs in the treatment of cancer and neurological diseases. To advance our understanding of the diverse functions of CRBN, in this review, we will survey the ubiquitination-dependent and -independent functions of CRBN, summarize recent advances in the discovery of constitutive and neo-substrates of CRBN, and explore the molecular functions of CRBN in cancer treatment and in the development of neurological diseases. We will also discuss the potential future directions towards the identification of CRBN substrates and interacting proteins, and CRBN-ligand-based drug discovery in the treatment of cancer and neurological diseases.

A proximity biotinylation-based approach to identify protein-E3 ligase interactions induced by PROTACs and molecular glues

Proteolysis-targeting chimaeras (PROTACs) as well as molecular glues such as immunomodulatory drugs (IMiDs) and indisulam are drugs that induce interactions between substrate proteins and an E3 ubiquitin ligases for targeted protein degradation. Here, we develop a workflow based on proximity-dependent biotinylation by AirID to identify drug-induced neo-substrates of the E3 ligase cereblon (CRBN). Using AirID-CRBN, we detect IMiD-dependent biotinylation of CRBN neo-substrates in vitro and identify biotinylated peptides of well-known neo-substrates by mass spectrometry with high specificity and selectivity. Additional analyses reveal ZMYM2 and ZMYM2-FGFR1 fusion protein-responsible for the 8p11 syndrome involved in acute myeloid leukaemia-as CRBN neo-substrates. Furthermore, AirID-DCAF15 and AirID-CRBN biotinylate neo-substrates targeted by indisulam and PROTACs, respectively, suggesting that this approach has the potential to serve as a general strategy for characterizing drug-inducible protein-protein interactions in cells.

Molecular Mechanisms of Cereblon-Interacting Small Molecules in Multiple Myeloma Therapy

Thalidomide analogues (or immunomodulatory imide drugs, IMiDs) are cornerstones in the treatment of multiple myeloma (MM). These drugs bind Cereblon (CRBN), a receptor for the Cullin-ring 4 ubiquitin-ligase (CRL4) complex, to modify its substrate specificity. IMiDs mediate CRBN-dependent engagement and proteasomal degradation of ‘neosubstrates’, Ikaros (IKZF1) and Aiolos (IKZF3), conveying concurrent antimyeloma activity and T-cell costimulation. There is now a greater understanding of physiological CRBN functions, including endogenous substrates and chaperone activity. CRISPR Cas9-based genome-wide screening has further elucidated the complex cellular machinery implicated in IMiD sensitivity, including IKZF1/3-independent mechanisms. New-generation IMiD derivatives with more potent anti-cancer properties—the CELMoDs (Cereblon E3 ligase modulators)—are now being evaluated. Rational drug design also allows ‘hijacking’ of CRL4^CRBN utilising proteolysis targeting chimeras (PROTACs) to convey entirely distinct substrate repertoires. As all these chemotypes—thalidomide, IMiDs, CELMoDs and PROTACs—engage CRBN and modify its functions, we describe them here in aggregate as ‘CRBN-interacting small molecules’ (CISMs). In this review, we provide a contemporary summary of the biological consequences of CRBN modulation by CISMs. Detailed molecular insight into CRBN–CISM interactions now provides an opportunity to more effectively target previously elusive cancer dependencies, representing a new and powerful tool for the implementation of precision medicine.

The Impact of Induction Regimes on Immune Responses in Patients with Multiple Myeloma

Current standard frontline therapy for newly diagnosed patients with multiple myeloma (NDMM) involves induction therapy, autologous stem cell transplantation (ASCT), and maintenance therapy. Major efforts are underway to understand the biological and the clinical impacts of each stage of the treatment protocols on overall survival statistics. The most routinely used drugs in the pre-ASCT "induction" regime have different mechanisms of action and are employed either as monotherapies or in various combinations. Aside from their direct effects on cancer cell mortality, these drugs are also known to have varying effects on immune cell functionality. The question remains as to how induction therapy impacts post-ASCT immune reconstitution and anti-tumor immune responses. This review provides an update on the known immune effects of melphalan, dexamethasone, lenalidomide, and bortezomib commonly used in the induction phase of MM therapy. By analyzing the actions of each individual drug on the immune system, we suggest it might be possible to leverage their effects to rationally devise more effective induction regimes. Given the genetic heterogeneity between myeloma patients, it may also be possible to identify subgroups of patients for whom particular induction drug combinations would be more appropriate.

Long-term depletion of cereblon induces mitochondrial dysfunction in cancer cells

Cereblon (CRBN) is a multi-functional protein that acts as a sub-strate receptor of the E3 ligase complex and a molecular chaperone. While CRBN is proposed to function in mitochondria, its specific roles are yet to be established. Here, we showed that knockdown of CRBN triggers oxidative stress and calcium overload in mitochondria, leading to disruption of mitochondrial membrane potential. Notably, long-term CRBN depletion using PROteolysis TArgeting Chimera (PROTAC) induced irreversible mitochondrial dysfunction, resulting in cell death. Our collective findings indicate that CRBN is required for mitochondrial homeostasis in cells.

Ablation of CRBN induces loss of type I collagen and SCH in mouse skin by fibroblast senescence via the p38 MAPK pathway

Cereblon (CRBN) is a substrate receptor of the cullin-RING E3 ubiquitin ligase (CRL) complex that mediates the ubiquitination of several substrates. In this study, CRBN knockout (KO) mice exhibited decreased levels of stratum corneum hydration (SCH) and collagen I expression with an elevated protein level of matrix metalloprotease 1 (MMP1). The absence of cereblon in the skin of CRBN KO mice mimics the damage caused by narrowband ultraviolet B (NB-UVB). The primary CRBN deficient mouse embryonic fibroblasts (MEFs) undergo G2/M-arrested premature senescence via protein signaling of p38 MAPK and its dependent p53/p21pathway. The absence of CRBN induced the markers of cellular senescence, such as the senescence-associated heterochromatin foci (SAHF), SA-β-Gal staining, and p21 upregulation while the ectopic expression of CRBN reversed the phenotypes of SA-β-Gal staining and p21 upregulation. Reversion of the decreased protein level of collagen I was demonstrated after the reintroduction of the CRBN gene back into CRBN KO MEFs, validating the promising role of CRBN as a potential regulator for the function of the skin barrier and its cellular homeostasis.

Caspase-8 Inhibition Prevents the Cleavage and Degradation of E3 Ligase Substrate Receptor Cereblon and Potentiates Its Biological Function

Cereblon (CRBN), a substrate receptor of cullin 4-RING E3 ligase (CRL4), mediates the ubiquitination and degradation of constitutive substrates and immunomodulatory drug-induced neo-substrates including MEIS2, c-Jun, CLC1, IKZF1/3, CK1α, and SALL4. It has been reported that CRBN itself could be degraded through the ubiquitin-proteasome system by its associated or other cullin-RING E3 ligases, thus influencing its biological functions. However, it is unknown whether the CRBN stability and its biological function could be modulated by caspases. In this study, using model cell lines, we found that activation of the death receptor using tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) leads to the decreased CRBN protein level. Through pharmacological inhibition and activation of caspase-8 (CASP-8), we disclosed that CASP-8 regulates CRBN cleavage in cell lines. Site mapping experiments revealed that CRBN is cleaved after Asp9 upon CASP-8 activation, resulting in the reduced stability. Using myeloma as a model system, we further revealed that either inhibition or genetic depletion of CASP-8 enhances the anti-myeloma activity of lenalidomide (Len) by impairing CRBN cleavage, leading to the attenuated IKZF1 and IKZF3 protein levels and the reduced viability of myeloma cell lines and primary myeloma cells from patients. The present study discovered that the stability of the substrate receptor of an E3 ligase can be modulated by CASP-8 and suggested that administration of CASP-8 inhibitors enhances the overall effectiveness of Len-based combination therapy in myeloma.

The NMDAR GluN1-1a C-terminus binds to CaM and regulates synaptic function

The binding of calmodulin (CaM) to NMDAR (N-methyl-D-aspartate receptor) GluN1 C-terminus is required for cacium (Ca²⁺)/calmodulin (CaM)-dependent inactivation of NMDAR. Previously, we found that GluN1 C-terminus translocated to nucleus, and regulated synaptic transmission. However, whether GluN1 C-terminus containing the nuclear localization signaling regulates the cellular distribution of CaM, and the CaM binding are not clear. In this study, we found that the 10 positive residues of GluN1 C-terminus determined the translocation of CaM to the nucleus. RNA sequencing data showed that CaM could regulate the genes expression of multiple cell surface membrane receptors. This was confirmed by electrophysiology data that the 10A mutation of GluN1 C-terminus increased the NMDAR/AMAPR-mediated synaptic transduction.

Cereblon Promotes the Ubiquitination and Proteasomal Degradation of Interleukin Enhancer-Binding Factor 2

Interleukin enhancer-binding factor 2 (ILF2) forms a heterodimer with interleukin enhancer-binding factor 3 (ILF3) via double-stranded RNA-binding motif and zinc finger associated domain and thus regulates gene expression and cancer cell growth. However, how ILF2 is degraded in cells remains elusive. In this work, using stable isotope labeling by amino acids in cell culture (SILAC) quantitative proteomics, we find that ILF2 is downregulated in cells expressing cereblon (CRBN). Using affinity purification and immunoblotting analysis, we demonstrate that CRBN interacts with ILF2 and functions as a substrate receptor of the cullin-4 RING E3 ligase complex. Biochemical experiments disclose that CRBN expression reduces ILF2 protein level and this reduction is diminished when the proteasome is inhibited. Upon protein synthesis inhibition, the degradation of ILF2 is enhanced by CRBN. Moreover, CRBN promotes the ubiquitination of ILF2 and thus results in the ubiquitin-mediated proteasomal degradation. Analyses of previously identified post-translational modification sites and the crystal structure of ILF2 discover the potential ubiquitination sites on ILF2. Through mutagenesis and biochemical experiments, we further reveal that the K45R mutation completely abolishes the effect of CRBN on ILF2, suggesting that this is the key residue responsible for its ubiquitination. Taken together, we identify an E3 ligase that regulates ILF2 and uncover a molecular pathway for its degradation. This work might be helpful to elucidate the molecular mechanism by which CRBN regulates diverse cellular functions.