Mitochondrial cereblon functions as a Lon-type protease

The orally bioavailable GSPT1/2 degrader SJ6986 exhibits in vivo efficacy in acute lymphoblastic leukemia

Advancing cure rates for high-risk acute lymphoblastic leukemia (ALL) has been limited by the lack of agents that effectively kill leukemic cells, sparing normal hematopoietic tissue. Molecular glues direct the ubiquitin ligase cellular machinery to target neosubstrates for protein degradation. We developed a novel cereblon modulator, SJ6986, that exhibits potent and selective degradation of GSPT1 and GSPT2 and cytotoxic activity against childhood cancer cell lines. Here, we report in vitro and in vivo testing of the activity of this agent in a panel of ALL cell lines and xenografts. SJ6986 exhibited similar cytotoxicity to the previously described GSPT1 degrader CC-90009 in a panel of leukemia cell lines in vitro, resulting in apoptosis and perturbation of cell cycle progression. SJ6986 was more effective than CC-90009 in suppressing leukemic cell growth in vivo, partly attributable to favorable pharmacokinetic properties, and did not significantly impair differentiation of human CD34+ cells ex vivo. Genome-wide CRISPR/Cas9 screening of ALL cell lines treated with SJ6986 confirmed that components of the CRL4CRBN complex, associated adaptors, regulators, and effectors were integral in mediating the action of SJ6986. SJ6986 is a potent, selective, orally bioavailable GSPT1/2 degrader that shows broad antileukemic activity and has potential for clinical development.

Targeting CRL4 suppresses chemoresistant ovarian cancer growth by inducing mitophagy

Chemoresistance has long been the bottleneck of ovarian cancer (OC) prognosis. It has been shown that mitochondria play a crucial role in cell response to chemotherapy and that dysregulated mitochondrial dynamics is intricately linked with diseases like OC, but the underlying mechanisms remain equivocal. Here, we demonstrate a new mechanism where CRL4^CUL4A/DDB1 manipulates OC cell chemoresistance by regulating mitochondrial dynamics and mitophagy. CRL4^CUL4A/DDB1 depletion enhanced mitochondrial fission by upregulating AMPKα^Thr172 and MFF^{Ser172/Ser146} phosphorylation, which in turn recruited DRP1 to mitochondria. CRL4^CUL4A/DDB1 loss stimulated mitophagy through the Parkin-PINK1 pathway to degrade the dysfunctional and fragmented mitochondria. Importantly, CRL4^CUL4A/DDB1 loss inhibited OC cell proliferation, whereas inhibiting autophagy partially reversed this disruption. Our findings provide novel insight into the multifaceted function of the CRL4 E3 ubiquitin ligase complex in regulating mitochondrial fission, mitophagy, and OC chemoresistance. Disruption of CRL4^CUL4A/DDB1 and mitophagy may be a promising therapeutic strategy to overcome chemoresistance in OC.

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.

Neddylation regulation of mitochondrial structure and functions

Mitochondria are the powerhouse of a cell. The structure and function of mitochondria are precisely regulated by multiple signaling pathways. Neddylation, a post-translational modification, plays a crucial role in various cellular processes including cellular metabolism via modulating the activity, function and subcellular localization of its substrates. Recently, accumulated data demonstrated that neddylation is involved in regulation of morphology, trafficking and function of mitochondria. Mechanistic elucidation of how mitochondria is modulated by neddylation would further our understanding of mitochondrial regulation to a new level. In this review, we first briefly introduce mitochondria, then neddylation cascade, and known protein substrates subjected to neddylation modification. Next, we summarize current available data of how neddylation enzymes, its substrates (including cullins/Cullin-RING E3 ligases and non-cullins) and its inhibitor MLN4924 regulate the structure and function of mitochondria. Finally, we propose the future perspectives on this emerging and exciting field of mitochondrial research.

An Overview on ATP Dependent and Independent Proteases Including an Anterograde to Retrograde Control on Mitochondrial Function; Focus on Diabetes and Diabetic Complications

Mitochondria are the central power stations of the cell involved with a myriad of cell signalling pathways that contribute for whole health status of the cell. It is a well known fact that not only mitochondrial genome encodes for mitochondrial proteins but there are several other mitochondrial specific proteins encoded by nuclear genome which regulate plethora of cell catabolic and anabolic process. Anterograde pathways include nuclear gene encoded proteins and their specific transport into the mitochondria and regulation of mitochondrial homeostasis. The retrograde pathways include crosstalk between the mitochondria and cytoplasmic proteins. Indeed, ATP dependent and independent proteases are identified to be very critical in balancing anterograde to retrograde signalling and vice versa to maintain the cell viability or cell death. Different experimental studies conducted on silencing the genes of these proteases have shown embryonic lethality, cancer cells death, increased hepatic glucose output, insulin tolerance, increased protein exclusion bodies, mitochondrial dysfunction, and defect in mitochondrial biogenesis, increased inflammation, Apoptosis etc. These experimental studies included from eubacteria to eukaryotes. Hence, many lines of theories proposed these proteases are conservative from eubacteria to eukaryotes. However, the regulation of these proteases at gene level is not clearly understood and still research is warranted. In this review, we articulated the origin and regulation of these proteases and the cross talk between the nucleus and mitochondria vice versa, and highlighted the role of these proteases in diabetes and diabetic complications in human diseases.

Hepatitis B Virus HBx Protein Mediates the Degradation of Host Restriction Factors through the Cullin 4 DDB1 E3 Ubiquitin Ligase Complex

The hepatitis B virus (HBV) regulatory HBx protein is required for infection, and its binding to cellular damaged DNA binding protein 1 (DDB1) is critical for this function. DDB1 is an adaptor protein for the cullin 4A Really Interesting New Gene (RING) E3 ubiquitin ligase (CRL4) complex and functions by binding cellular DDB1 cullin associated factor (DCAF) receptor proteins that recruit substrates for ubiquitination and degradation. We compared the proteins found in the CRL4 complex immunoprecipitated from uninfected versus HBV-infected hepatocytes from human liver chimeric mice for insight into mechanisms by which HBV and the cell interact within the CRL4 complex. Consistent with its role as a viral DCAF, HBx was found in the HBV CRL4 complexes. In tissue culture transfection experiments, we showed that HBx expression led to decreased levels of known restriction factor structural maintenance of chromosomes protein 6 (SMC6) and putative restriction factors stromal interaction molecule 1 (STIM1, zinc finger E-box binding homeobox 2 (ZEB2), and proteasome activator subunit 4 (PSME4). Moreover, silencing of these proteins led to increased HBV replication in the HepG2-sodium taurocholate cotransporting polypeptide (NTCP) infection model. We also identified cellular DCAF receptors in CRL4 complexes from humanized mice. Increasing amounts of HBx did not reveal competitive DCAF binding to cullin4 (CUL4)-DDB1 in plasmid-transfected cells. Our results suggest a model in which HBx benefits virus replication by directly or indirectly degrading multiple cellular restriction factors.

CRBN Is a Negative Regulator of Bactericidal Activity and Autophagy Activation Through Inhibiting the Ubiquitination of ECSIT and BECN1

Cereblon (CRBN) as a multifunctional protein has been extensively studied. Here, we show that CRBN is a negative regulator of bactericidal activity and autophagy activation. Mitochondrial localization of CRBN was significantly increased in response to Toll-like receptor 4 (TLR4) stimulation. CRBN interrupted the association of evolutionarily conserved signaling intermediate in Toll pathways (ECSIT)-TNF-receptor associated factor 6 (TRAF6) complex, thereby inhibiting the ubiquitination of ECSIT, which plays a pivotal role for the production of mitochondrial reactive oxygen species (mROS). Subsequently, mROS levels were markedly elevated in CRBN-knockdown (CRBN^KD) THP-1 cells, and that led to resistance against S. typhimurium infection, indicating CRBN is a negative regulator of bactericidal activity through the regulation of mROS. Additionally, CRBN inhibited TRAF6-induced ubiquitination of BECN1 (Beclin 1), and that induced autophagy activation in CRBN^KD THP-1, CRBN-knockout (CRBN^KO) H1299, and CRBN^KO MCF7 cancer cells in response to TLR4 stimulation. Notably, we found that the ability of cancer migration and invasion was significantly enhanced in CRBN^KO H1299 and CRBN^KO MCF7 cancer cells, as compared with those of control cancer cells. Collectively, these results suggest that CRBN is a negative regulator of bactericidal activity and autophagy activation through inhibiting the TRAF6-induced ubiquitination of ECSIT and BECN1, respectively.

Positive selection of cereblon modified function including its E3 ubiquitin ligase activity and binding efficiency with AMPK

Cereblon (CRBN) is a substrate receptor for an E3 ubiquitin ligase that directly binds to target proteins resulting in cellular activities, such as energy metabolism, membrane potential regulation, and transcription factor degradation. Genetic mutations in human CRBN lead to intellectual disabilities. In addition, it draws pathological attention because direct binding with immunomodulatory drugs can cure multiple myeloma (MM) and lymphocytic leukemia. To further explore the function of CRBN, we focused on its molecular evolution. Since CRBN interacts directly with its substrates and is widely conserved in vertebrates, evolutionary study to identify the selective pressure on CRBN that occur during CRBN-substrate interaction is an effective approach to search for a novel active site. Using mammalian CRBN sequences, dN/dS analysis was conducted to detect positive selection. By multiple sequence alignment we found that the residue at position 366 was under positive selection. This residue is present in the substrate-binding domain of CRBN. Most mammals harbor cysteine at position 366, whereas rodents and chiroptera have serine at this site. Subsequently, we constructed a C366S human CRBN to confirm the potential of positive selection. Auto-ubiquitination activity occurs in E3 ubiquitin ligases, including CRBN, and increased in C366S CRBN, which lead to the conclusion that E3 ubiquitin ligase activity may have changed over the course of mammalian evolution. Furthermore, binding with AMP-activated protein kinase was augmented when the substitution was present, which is supported by coevolution analysis. These results suggest that the molecular evolution of CRBN occurred through codon-based positive selection, providing a new approach to investigate CRBN function.

Antioxidant Activity of Ge-132, a Synthetic Organic Germanium, on Cultured Mammalian Cells

Ge-132 is a synthetic organic germanium that is used as a dietary supplement. The antioxidant activity of Ge-132 on cultured mammalian cells was investigated in this study. First, Ge-132 cytotoxicity on mammalian cultured cells was determined by measuring lactate dehydrogenase (LDH) levels. Ge-132 had no cytotoxic effect on three different cell lines. Second, the cell proliferative effect of Ge-132 was determined by measuring ATP content of whole cells and counting them. Ge-132 treatment of Chinese hamster ovary (CHO-K1) and SH-SY5Y cells promoted cell proliferation in a dose-dependent manner. Finally, antioxidant activity of Ge-132 against hydrogen peroxide-induced oxidative stress was determined by measuring the levels of intracellular reactive oxygen species (ROS) and carbonylated proteins. Pre-incubation of CHO-K1 and SH-SY5Y cells with Ge-132 suppressed intracellular ROS production and carbonylated protein levels induced by hydrogen peroxide. Our results suggest that Ge-132 has antioxidant activity against hydrogen peroxide-induced oxidative stress.

The Neuroprotective Effect of Thalidomide against Ischemia through the Cereblon-mediated Repression of AMPK Activity

Thalidomide was originally used as a sedative and found to be a teratogen, but now thalidomide and its derivatives are widely used to treat haematologic malignancies. Accumulated evidence suggests that thalidomide suppresses nerve cell death in neurologic model mice. However, detailed molecular mechanisms are unknown. Here we examined the molecular mechanism of thalidomide’s neuroprotective effects, focusing on its target protein, cereblon (CRBN), and its binding protein, AMP-activated protein kinase (AMPK), which plays an important role in maintaining intracellular energy homeostasis in the brain. We used a cerebral ischemia rat model of middle cerebral artery occlusion/reperfusion (MCAO/R). Thalidomide treatment significantly decreased the infarct volume and neurological deficits of MCAO/R rats. AMPK was the key signalling protein in this mechanism. Furthermore, we considered that the AMPK–CRBN interaction was altered when neuroprotective action by thalidomide occurred in cells under ischemic conditions. Binding was strong between AMPK and CRBN in normal SH-SY5Y cells, but was weakened by the addition of H₂O₂. However, when thalidomide was administered at the same time as H₂O₂, the binding of AMPK and CRBN was partly restored. These results suggest that thalidomide inhibits the activity of AMPK via CRBN under oxidative stress and suppresses nerve cell death.

Mouse Monoclonal Antibodies Generated from Full Length Human Cereblon: Detection of Cereblon Protein in Patients with Multiple Myeloma

Immunomodulatory drugs (IMiDs) are profoundly active compounds in the treatment of patients with multiple myeloma (MM). However, despite the fact that treatment with IMiDs has dramatically improved survival for patients with MM, the majority of MM patients develop IMiDs resistance over time. We have found that expression of functional cereblon is required for IMiDs' action. In addition, it has been reported that cells expressing high levels of cereblon are resistant to proteasome inhibitor, implying that patients with high levels of cereblon should be resistant to proteasome inhibitor. If the above conclusions are correct, cereblon could be considered as a biomarker to determine which standard regimens should be used to treat patients with MM. Unfortunately, the conclusions mentioned above have not been clinically confirmed. In order to confirm these conclusions, we have generated three highly specific mouse monoclonal antibodies (mAbs) against full-length human cereblon. These mAbs can be used to do western blot, immunoprecipitation and immunohistochemistry staining. In addition, their epitopes have been precisely determined and the peptides covering their epitopes completely blocked the antibody binding to cereblon in western blot analysis or in immunohistochemistry staining of MM patients' specimens.

A Brighter Side to Thalidomide: Its Potential Use in Immunological Disorders

Thalidomide and its derivatives are immunomodulatory drugs (IMiDs) known for their sedative, teratogenic, anti-angiogenic, and anti-inflammatory properties. Commonly used in the treatment of cancers such as multiple myeloma and myelodysplastic syndrome (MDS), IMiDs have also been used in the treatment of an inflammatory skin pathology associated with Hansen's disease/leprosy. They have also shown promise in the treatment of autoimmune disorders including systemic lupus erythmatosus (SLE) and inflammatory bowel disease (IBD). Recent structural and experimental observations have revolutionized our understanding of these properties by revealing the fundamental molecular events underpinning IMiD activity. We review these findings, their relevance to IMiD therapy in immunological disorders, and discuss how further research might unlock the vast clinical potential of these compounds.

Ohgata, the Single Drosophila Ortholog of Human Cereblon, Regulates Insulin Signaling-dependent Organismic Growth

Cereblon (CRBN) is a substrate receptor of the E3 ubiquitin ligase complex that is highly conserved in animals and plants. CRBN proteins have been implicated in various biological processes such as development, metabolism, learning, and memory formation, and their impairment has been linked to autosomal recessive non-syndromic intellectual disability and cancer. Furthermore, human CRBN was identified as the primary target of thalidomide teratogenicity. Data on functional analysis of CRBN family members in vivo, however, are still scarce. Here we identify Ohgata (OHGT), the Drosophila ortholog of CRBN, as a regulator of insulin signaling-mediated growth. Using ohgt mutants that we generated by targeted mutagenesis, we show that its loss results in increased body weight and organ size without changes of the body proportions. We demonstrate that ohgt knockdown in the fat body, an organ analogous to mammalian liver and adipose tissue, phenocopies the growth phenotypes. We further show that overgrowth is due to an elevation of insulin signaling in ohgt mutants and to the down-regulation of inhibitory cofactors of circulating Drosophila insulin-like peptides (DILPs), named acid-labile subunit and imaginal morphogenesis protein-late 2. The two inhibitory proteins were previously shown to be components of a heterotrimeric complex with growth-promoting DILP2 and DILP5. Our study reveals OHGT as a novel regulator of insulin-dependent organismic growth in Drosophila.