MCL1 binding to the reverse BH3 motif of P18INK4C couples cell survival to cell proliferation

Overexpression of MCL1 attenuates irritable bowel syndrome by regulating cuproptosis: Screening and validation

Irritable bowel syndrome (IBS) is a type of chronic bowel disorder with a poorly understood pathophysiology. Recently, the imbalance of copper has been reported to influence the progression of IBS, suggesting cuproptosis, a new type of copper-induced cell death, may play a role in IBS. This study found 17 cuproptosis-related differentially expressed genes in IBS through bioinformatic analysis. Six hub genes were identified after the protein-protein interaction network analysis, namely myeloid cell leukemia 1 (MCL1), epidermal growth factor receptor 2, cadherin-associated protein beta 1, solute carrier family 25 members 37, solute carrier family 39 members 14, and six transmembrane epithelial antigens of the prostate 3. We selected MCL1 for further verification. Human normal colon epithelial cell line (NCM460) was used to construct models of IBS or cuproptosis in vitro by lipopolysaccharide (LPS) or LPS combined with copper (II) chloride (CuCl2). We observed that overexpression of MCL1 promoted cell viability and proliferation ability, and inhibited the secretion of inflammatory factors and expression of Bax and caspase-3 of NCM460 cells treated with LPS or LPS combined with CuCl2. In addition, up-regulated MCL1 significantly suppressed the protein levels of ferredoxin 1 and lipoyl synthase, two key regulators of cuproptosis. In conclusion, our study demonstrates that cuproptosis is involved in IBS and identifies a cuproptosis-related gene, MCL1, that helps alleviate IBS by promoting cell growth, reducing inflammation, and suppressing cuproptosis, making it a promising therapeutic target in IBS.

Orientia tsutsugamushi Ank5 promotes NLRC5 cytoplasmic retention and degradation to inhibit MHC class I expression

How intracellular bacteria subvert the major histocompatibility complex (MHC) class I pathway is poorly understood. Here, we show that the obligate intracellular bacterium Orientia tsutsugamushi uses its effector protein, Ank5, to inhibit nuclear translocation of the MHC class I gene transactivator, NLRC5, and orchestrate its proteasomal degradation. Ank5 uses a tyrosine in its fourth ankyrin repeat to bind the NLRC5 N-terminus while its F-box directs host SCF complex ubiquitination of NLRC5 in the leucine-rich repeat region that dictates susceptibility to Orientia- and Ank5-mediated degradation. The ability of O. tsutsugamushi strains to degrade NLRC5 correlates with ank5 genomic carriage. Ectopically expressed Ank5 that can bind but not degrade NLRC5 protects the transactivator during Orientia infection. Thus, Ank5 is an immunoevasin that uses its bipartite architecture to rid host cells of NLRC5 and reduce surface MHC class I molecules. This study offers insight into how intracellular pathogens can impair MHC class I expression.

Targeting Myeloid Leukemia-1 in Cancer Therapy: Advances and Directions

As a tripartite cell death switch, B-cell lymphoma protein 2 (Bcl-2) family members precisely regulate the endogenous apoptosis pathway in response to various cell signal stresses through protein-protein interactions. Myeloid leukemia-1 (Mcl-1), a key anti-apoptotic Bcl-2 family member, is positioned downstream in the endogenous apoptotic pathway and plays a central role in regulating mitochondrial function. Mcl-1 is highly expressed in a variety of hematological malignancies and solid tumors, contributing to tumorigenesis, poor prognosis, and chemoresistance, making it an attractive target for cancer treatment. This Perspective aims to discuss the mechanism by which Mcl-1 regulates apoptosis and non-apoptotic functions in cancer cells and to outline the discovery and optimization process of potent Mcl-1 modulators. In addition, we summarize the structural characteristics of potent inhibitors that bind to Mcl-1 through multiple co-crystal structures and analyze the cardiotoxicity caused by current Mcl-1 inhibitors, providing prospects for rational targeting of Mcl-1.

Myeloid Cell Leukemia 1 Small Molecule Inhibitor S63845 Synergizes with Cisplatin in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is an aggressive cancer that lacks specific molecular targets that are often used for therapy. The refractory rate of TNBC to broad-spectrum chemotherapy remains high; however, the combination of newly developed treatments with the current standard of care has delivered promising anti-tumor effects. One mechanism employed by TNBC to avoid cell death is the increased expression of the anti-apoptotic protein, myeloid cell leukemia 1 (MCL1). Multiple studies have demonstrated that increased MCL1 expression enables resistance to platinum-based chemotherapy. In addition to suppressing apoptosis, we recently demonstrated that MCL1 also binds and negatively regulates the transcriptional activity of TP73. TP73 upregulation is a critical driver of cisplatin-induced DNA damage response, and ultimately, cell death. We therefore sought to determine if the coadministration of an MCL1-targeted inhibitor with cisplatin could produce a synergistic response in TNBC. This study demonstrates that the MCL1 inhibitor, S63845, combined with cisplatin synergizes by inducing apoptosis while also decreasing proliferation in a subset of TNBC cell lines. The use of combined MCL1 inhibitors with cisplatin in TNBC effectively initiates TAp73 anti-tumor effects on cell cycle arrest and apoptosis. This observation provides a molecular profile that can be exploited to identify sensitive TNBCs.

Polypyrimidine tract binding protein 1 (PTBP1) contains a novel regulatory sequence, the rBH3, that binds the prosurvival protein MCL1

The maturation of RNA from its nascent transcription to ultimate utilization (e.g., translation, miR-mediated RNA silencing, etc.) involves an intricately coordinated series of biochemical reactions regulated by RNA-binding proteins (RBPs). Over the past several decades, there has been extensive effort to elucidate the biological factors that control specificity and selectivity of RNA target binding and downstream function. Polypyrimidine tract binding protein 1 (PTBP1) is an RBP that is involved in all steps of RNA maturation and serves as a key regulator of alternative splicing, and therefore, understanding its regulation is of critical biologic importance. While several mechanisms of RBP specificity have been proposed (e.g., cell-specific expression of RBPs and secondary structure of target RNA), recently, protein-protein interactions with individual domains of RBPs have been suggested to be important determinants of downstream function. Here, we demonstrate a novel binding interaction between the first RNA recognition motif 1 (RRM1) of PTBP1 and the prosurvival protein myeloid cell leukemia-1 (MCL1). Using both in silico and in vitro analyses, we demonstrate that MCL1 binds a novel regulatory sequence on RRM1. NMR spectroscopy reveals that this interaction allosterically perturbs key residues in the RNA-binding interface of RRM1 and negatively impacts RRM1 association with target RNA. Furthermore, pulldown of MCL1 by endogenous PTBP1 verifies that these proteins interact in an endogenous cellular environment, establishing the biological relevance of this binding event. Overall, our findings suggest a novel mechanism of regulation of PTBP1 in which a protein-protein interaction with a single RRM can impact RNA association.

The first evidence of biological activity for free Hypusine, an enigmatic amino acid discovered in the '70s

Hypusine amino acid [N^ε-(4-amino-2-hydroxybutyl)-lysine] was first isolated in 1971 from bovine brain extracts. Hypusine originates from a post-translational modification at the eukaryotic translation initiation factor 5A (eIF5A), a protein produced by archaebacteria and eukaryotes. The eIF5A protein is the only one described containing the hypusine residue, which is essential for its activity. Hypusine as a free amino acid is a consequence of proteolytic degradation of eIF5A. Herein, we showed, for the first time, evidence of biological activity for the free hypusine. C6 rat glioma cells were treated with hypusine, and different cellular parameters were evaluated. Hypusine treatment significantly reduced C6 cell proliferation and potently suppressed their clonogenic capacity without leading to apoptosis. Hypusine also decreased the Eif5A transcript content and the global protein synthesis profile that may occur due to negative feedback in response to high hypusine concentration, controlling the content of newly synthesized eIF5A, which can affect the translation process. Besides, hypusine treatment also altered cellular metabolism by changing the pathways for energy production, reducing cellular respiration coupled with oxidative phosphorylation, and increasing the anaerobic metabolism. These observed results and the relationship between eIF5A and tumor processes led us to test the combination of hypusine with the chemotherapeutic drug temozolomide. Combining temozolomide with hypusine reduced the MTT conversion to the same levels as those observed using double temozolomide dosage alone, demonstrating a synergetic action between the compounds. Thus, since 1971, this is the first study showing evidence of biological activity for hypusine not associated with being an essential component of the eiF5A protein. Finding out the molecular targets of hypusine are the following efforts to completely characterize its biological activity.

Phylogenetic analysis of the MCL1 BH3 binding groove and rBH3 sequence motifs in the p53 and INK4 protein families

B-cell lymphoma 2 (Bcl-2) proteins are central, conserved regulators of apoptosis. Bcl-2 family function is regulated by binding interactions between the Bcl-2 homology 3 (BH3) motif in pro-apoptotic family members and the BH3 binding groove found in both the pro-apoptotic effector and anti-apoptotic Bcl-2 family members. A novel motif, the reverse BH3 (rBH3), has been shown to interact with the anti-apoptotic Bcl-2 homolog MCL1 (Myeloid cell leukemia 1) and have been identified in the p53 homolog p73, and the CDK4/6 (cyclin dependent kinase 4/6) inhibitor p18INK4c, (p18, cyclin-dependent kinase 4 inhibitor c). To determine the conservation of rBH3 motif, we first assessed conservation of MCL1's BH3 binding groove, where the motif binds. We then constructed neighbor-joining phylogenetic trees of the INK4 and p53 protein families and analyzed sequence conservation using sequence logos of the rBH3 locus. This showed the rBH3 motif is conserved throughout jawed vertebrates p63 and p73 sequences and in chondrichthyans, amphibians, mammals, and some reptiles in p18. Finally, a potential rBH3 motif was identified in mammalian and osteichthyan p19INK4d (p19, cyclin dependent kinase 4 inhibitor d). These findings demonstrate that the interaction between MCL1 and other cellular proteins mediated by the rBH3 motif may be conserved throughout jawed vertebrates.

Longitudinal drug synergy assessment using convolutional neural network image-decoding of glioblastoma single-spheroid cultures

Background

In recent years, drug combinations have become increasingly popular to improve therapeutic outcomes in various diseases, including difficult to cure cancers such as the brain cancer glioblastoma. Assessing the interaction between drugs over time is critical for predicting drug combination effectiveness and minimizing the risk of therapy resistance. However, as viability readouts of drug combination experiments are commonly performed as an endpoint where cells are lysed, longitudinal drug-interaction monitoring is currently only possible through combined endpoint assays.

Methods

We provide a method for massive parallel monitoring of drug interactions for 16 drug combinations in 3 glioblastoma models over a time frame of 18 days. In our assay, viabilities of single neurospheres are to be estimated based on image information taken at different time points. Neurosphere images taken on the final day (day 18) were matched to the respective viability measured by CellTiter-Glo 3D on the same day. This allowed to use of machine learning to decode image information to viability values on day 18 as well as for the earlier time points (on days 8, 11, and 15).

Results

Our study shows that neurosphere images allow us to predict cell viability from extrapolated viabilities. This enables to assess of the drug interactions in a time window of 18 days. Our results show a clear and persistent synergistic interaction for several drug combinations over time.

Conclusions

Our method facilitates longitudinal drug-interaction assessment, providing new insights into the temporal-dynamic effects of drug combinations in 3D neurospheres which can help to identify more effective therapies against glioblastoma.

Understanding MCL1: from cellular function and regulation to pharmacological inhibition

Myeloid cell leukemia-1 (MCL1), an antiapoptotic member of the BCL2 family characterized by a short half-life, functions as a rapid sensor that regulates cell death and other relevant processes that include cell cycle progression and mitochondrial homeostasis. In cancer, MCL1 overexpression contributes to cell survival and resistance to diverse chemotherapeutic agents; for this reason, several MCL1 inhibitors are currently under preclinical and clinical development for cancer treatment. However, the nonapoptotic functions of MCL1 may influence their therapeutic potential. Overall, the complexity of MCL1 regulation and function represent challenges to the clinical application of MCL1 inhibitors. We now summarize the current knowledge regarding MCL1 structure, regulation, and function that could impact the clinical success of MCL1 inhibitors.

Structural Details of BH3 Motifs and BH3-Mediated Interactions: an Updated Perspective

Apoptosis is a mechanism of programmed cell death crucial in organism development, maintenance of tissue homeostasis, and several pathogenic processes. The B cell lymphoma 2 (BCL2) protein family lies at the core of the apoptotic process, and the delicate balance between its pro- and anti-apoptotic members ultimately decides the cell fate. BCL2 proteins can bind with each other and several other biological partners through the BCL2 homology domain 3 (BH3), which has been also classified as a possible Short Linear Motif and whose distinctive features remain elusive even after decades of studies. Here, we aim to provide an updated overview of the structural features characterizing BH3s and BH3-mediated interactions (with a focus on human proteins), elaborating on the plasticity of BCL2 proteins and the motif properties. We also discussed the implication of these findings for the discovery of interactors of the BH3-binding groove of BCL2 proteins and the design of mimetics for therapeutic purposes.

MCL1 alternative polyadenylation is essential for cell survival and mitochondria morphology

Alternative polyadenylation in the 3' UTR (3' UTR-APA) is a mode of gene expression regulation, fundamental for mRNA stability, translation and localization. In the immune system, it was shown that upon T cell activation, there is an increase in the relative expression of mRNA isoforms with short 3' UTRs resulting from 3' UTR-APA. However, the functional significance of 3' UTR-APA remains largely unknown. Here, we studied the physiological function of 3' UTR-APA in the regulation of Myeloid Cell Leukemia 1 (MCL1), an anti-apoptotic member of the Bcl-2 family essential for T cell survival. We found that T cells produce two MCL1 mRNA isoforms (pA1 and pA2) by 3' UTR-APA. We show that upon T cell activation, there is an increase in both the shorter pA1 mRNA isoform and MCL1 protein levels. Moreover, the less efficiently translated pA2 isoform is downregulated by miR-17, which is also more expressed upon T cell activation. Therefore, by increasing the expression of the more efficiently translated pA1 mRNA isoform, which escapes regulation by miR-17, 3' UTR-APA fine tunes MCL1 protein levels, critical for activated T cells' survival. Furthermore, using CRISPR/Cas9-edited cells, we show that depletion of either pA1 or pA2 mRNA isoforms causes severe defects in mitochondria morphology, increases apoptosis and impacts cell proliferation. Collectively, our results show that MCL1 alternative polyadenylation has a key role in the regulation of MCL1 protein levels upon T cell activation and reveal an essential function for MCL1 3' UTR-APA in cell viability and mitochondria dynamics.

Stress Relief Techniques: p38 MAPK Determines the Balance of Cell Cycle and Apoptosis Pathways

Protein signaling networks are formed from diverse and inter-connected cell signaling pathways converging into webs of function and regulation. These signaling pathways both receive and conduct molecular messages, often by a series of post-translation modifications such as phosphorylation or through protein-protein interactions via intrinsic motifs. The mitogen activated protein kinases (MAPKs) are components of kinase cascades that transmit signals through phosphorylation. There are several MAPK subfamilies, and one subfamily is the stress-activated protein kinases, which in mammals is the p38 family. The p38 enzymes mediate a variety of cellular outcomes including DNA repair, cell survival/cell fate decisions, and cell cycle arrest. The cell cycle is itself a signaling system that precisely controls DNA replication, chromosome segregation, and cellular division. Another indispensable cell function influenced by the p38 stress response is programmed cell death (apoptosis). As the regulators of cell survival, the BCL2 family of proteins and their dynamics are exquisitely sensitive to cell stress. The BCL2 family forms a protein-protein interaction network divided into anti-apoptotic and pro-apoptotic members, and the balance of binding between these two sides determines cell survival. Here, we discuss the intersections among the p38 MAPK, cell cycle, and apoptosis signaling pathways.

The multiple mechanisms of MCL1 in the regulation of cell fate

MCL1 (myeloid cell leukemia-1) is a widely recognized pro-survival member of the Bcl-2 (B-cell lymphoma protein 2) family and a promising target for cancer therapy. While the role MCL1 plays in apoptosis is well defined, its participation in emerging non-apoptotic signaling pathways is only beginning to be appreciated. Here, we synthesize studies characterizing MCL1s influence on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that MCL1 plays in cellular homeostasis regulation. Throughout this review, we discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy Bcl-2 homology 3 (BH3)-mimetics in the future.

In this review Widden and Placzek synthesize studies characterizing the influence that myeloid cell leukemia-1 (MCL1) has on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that it plays in cellular homeostasis regulation. They discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy BH3-mimetics in the future.

MCL1 binds and negatively regulates the transcriptional function of tumor suppressor p73

MCL1, an anti-apoptotic protein that controls chemosensitivity and cell fate through its regulation of intrinsic apoptosis, has been identified as a high-impact target in anti-cancer therapeutic development. With MCL1-specific inhibitors currently in clinical trials, it is imperative that we understand the roles that MCL1 plays in cells, especially when targeting the Bcl-2 homology 3 (BH3) pocket, the central region of MCL1 that mediates apoptotic regulation. Here, we establish that MCL1 has a direct role in controlling p73 transcriptional activity, which modulates target genes associated with DNA damage response, apoptosis, and cell cycle progression. This interaction is mediated through the reverse BH3 (rBH3) motif in the p73 tetramerization domain, which restricts p73 assembly on DNA. Here, we provide a novel mechanism for protein-level regulation of p73 transcriptional activity by MCL1, while also framing a foundation for studying MCL1 inhibitors in combination with platinum-based chemotherapeutics. More broadly, this work expands the role of Bcl-2 family signaling beyond cell fate regulation.