Fine-tuning p53 activity by modulating the interaction between eukaryotic translation initiation factor eIF4E and RNA-binding protein RBM38

RNA-binding proteins in cellular senescence

Cellular senescence is a state of irreversible cell cycle arrest that is triggered and controlled by various external and/or internal factors. Among them, the regulation of senescence-associated genes is an important molecular event that plays a role in senescence. The regulation of gene expression can be achieved by various types of modulating mechanisms, and RNA-binding proteins (RBPs) are commonly known as critical regulators targeting a global range of transcripts. RBPs bind to RNA-binding motifs of the target transcripts and are involved in post-transcriptional processes such as RNA transport, stabilization, splicing, and decay. These RBPs may also play critical roles in cellular senescence by regulating the expression of senescence-associated genes. The biological functions of RBPs in controlling cellular senescence are being actively studied. Herein, we summarized the RBPs that influence cellular senescence, particularly by regulating processes such as the senescence-associated secretory phenotype, cell cycle, and mitochondrial function.

Tumor suppressor p53 modulates activity-dependent synapse strengthening, autism-like behavior and hippocampus-dependent learning

Synaptic potentiation underlies various forms of behavior and depends on modulation by multiple activity-dependent transcription factors to coordinate the expression of genes necessary for sustaining synaptic transmission. Our current study identified the tumor suppressor p53 as a novel transcription factor involved in this process. We first revealed that p53 could be elevated upon chemically induced long-term potentiation (cLTP) in cultured primary neurons. By knocking down p53 in neurons, we further showed that p53 is required for cLTP-induced elevation of surface GluA1 and GluA2 subunits of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR). Because LTP is one of the principal plasticity mechanisms underlying behaviors, we employed forebrain-specific knockdown of p53 to evaluate the role of p53 in behavior. Our results showed that, while knocking down p53 in mice does not alter locomotion or anxiety-like behavior, it significantly promotes repetitive behavior and reduces sociability in mice of both sexes. In addition, knocking down p53 also impairs hippocampal LTP and hippocampus-dependent learning and memory. Most importantly, these learning-associated defects are more pronounced in male mice than in female mice, suggesting a sex-specific role of p53 in these behaviors. Using RNA sequencing (RNAseq) to identify p53-associated genes in the hippocampus, we showed that knocking down p53 up- or down-regulates multiple genes with known functions in synaptic plasticity and neurodevelopment. Altogether, our study suggests p53 as an activity-dependent transcription factor that mediates the surface expression of AMPAR, permits hippocampal synaptic plasticity, represses autism-like behavior, and promotes hippocampus-dependent learning and memory.

Identification of a First-in-Class Small-Molecule Inhibitor of the EIF4E-RBM38 Complex That Enhances Wild-type TP53 Protein Translation for Tumor Growth Suppression

EIF4E, an mRNA cap-binding protein, is necessary for cap-dependent translation. Overexpression of EIF4E is known to promote cancer development by preferentially translating a group of oncogenic mRNAs. Thus, 4EGI-1, a disruptor of EIF4E-EIF4G1 interaction, was developed to inhibit oncoprotein expression for cancer therapy. Interestingly, RBM38, an RNA-binding protein, interacts with EIF4E on TP53 mRNA, prevents EIF4E from binding to TP53 mRNA cap, and inhibits TP53 expression. Thus, Pep8, an eight amino acid peptide derived from RBM38, was developed to disrupt the EIF4E-RBM38 complex, leading to increased TP53 expression and decreased tumor cell growth. Herein, we have developed a first-in-class small-molecule compound 094, which interacts with EIF4E via the same pocket as does Pep8, dissociates RBM38 from EIF4E, and enhances TP53 translation in RBM38- and EIF4E-dependent manners. Structure-activity relationship studies identified that both the fluorobenzene and ethyl benzamide are necessary for compound 094 to interact with EIF4E. Furthermore, we showed that compound 094 is capable of suppressing three-dimensional tumor spheroid growth in RBM38- and TP53-dependent manners. In addition, we found that compound 094 cooperates with the chemotherapeutic agent doxorubicin and EIF4E inhibitor 4EGI-1 to suppress tumor cell growth. Collectively, we showed that two distinct approaches can be used together to target EIF4E for cancer therapy by enhancing wild-type TP53 expression (094) and by suppressing oncoprotein expression (4EGI-1).

The RNA-binding protein RBM24 regulates lipid metabolism and SLC7A11 mRNA stability to modulate ferroptosis and inflammatory response

Lipids play a critical role in many cellular processes by serving as structural components of cell membranes or functioning as energy fuel and signaling molecules. The RNA-binding proteins RBM24 and RBM38 share an identical RNA-binding domain and thereby, regulate a group of same targets, such as p21. However, it is not certain whether RBM24 and RBM38 participates in lipid homeostasis. Here, lipidomic analysis showed that a deficiency in RBM24 or RBM38 leads to altered lipid metabolism, with more profound alteration by loss of RBM24 in MCF7 cells. We also showed that mice deficient in RBM24 were prone to chronic inflammation and liver steatosis, but not spontaneous tumors. These data let us speculate whether RBM24 regulates ferroptosis, a programmed cell death that links inflammation and liver steatosis via lipid peroxidation. Indeed, we found that over-expression of RBM24 protected, whereas knockout of RBM24 sensitized, cells to Erastin-induced ferroptosis by modulating the mRNA stability of SLC7A11, a ferroptosis inhibitor. Moreover, we showed that knockdown of SLC7A11 reversed the effect of RBM24 on ferroptosis. Together, our study revealed that RBM24 regulates lipid metabolism and SLC7A11 mRNA stability to modulate ferroptosis and inflammatory response.

Environmental BPDE induced human trophoblast cell apoptosis by up-regulating lnc-HZ01/p53 positive feedback loop

Human trophoblast cell apoptosis may induce miscarriage. Trophoblast cells are sensitive to environmental BaP-7,8-dihydrodiol-9,10-epoxide (BPDE). However, how BPDE induces human trophoblast cell apoptosis is still largely elusive. In this work, we used BPDE-treated human trophoblast cells and villous tissues collected from recurrent miscarriage and health control groups to explore the underlying mechanism of BPDE-induced human trophoblast cell apoptosis. Continued with our recent work, we found that lncRNA HZ01 (lnc-HZ01) could induce human trophoblast cell apoptosis. In mechanism, lnc-HZ01 up-regulated p53 expression level by suppressing its MDM2-mediated proteasomal degradation. Meanwhile, we found that p53 acted as lnc-HZ01 transcription factor and promoted lnc-HZ01 transcription. Thus, lnc-HZ01 and p53 composed a positive feedback loop in human trophoblast cells. In normal trophoblast cells, relatively low levels of lnc-HZ01 and p53 suppressed p53/caspase-3 apoptosis pathway, giving normal pregnancy. Upon BPDE exposure, BPDE up-regulated the expression levels of lnc-HZ01 and p53, triggered this positive feedback loop, activated the p53/caspase-3 apoptosis pathway, and then induced miscarriage. Collectively, we discovered new mechanism by which lnc-HZ01 regulated BPDE-induced human trophoblast cell apoptosis, providing scientific basis for the diagnosis and treatment of unexplained recurrent miscarriage.

Optimization of eIF4E-Binding Peptide Pep8 to Disrupt the RBM38-eIF4E Complex for Induction of p53 and Tumor Suppression

Interaction of RNA-binding protein RBM38 with eIF4E on p53 mRNA is known to suppress p53 mRNA translation, which can be disrupted by an 8-amino acid peptide (Pep8-YPYAASPA) derived from RBM38, leading to induction of p53 and tumor suppression. Here, we rationally designed multiple Pep8 derivatives and screened for their binding affinities towards eIF4E in silico. We showed that several key residues within Pep8 are necessary for its structure and function. We identified a shortened 7-amino acid peptide (Pep7-PSAASPV) that has the highest affinity towards eIF4E and is the most potent inducer of p53 expression. We found that iRGD is an effective vehicle to deliver Pep7 inside of cells for induction of p53 expression and growth suppression as compared to other cell penetrating peptides (Penetratin and Pep-1). We found that peptide cyclization enhances Pep8 affinity for eIF4E, induction of p53 and tumor cell growth suppression. We also found that the ability of Pep7 to induce p53 expression and growth suppression is conserved in cells derived from canine osteosarcoma, a spontaneous tumor model frequently used for testing the feasibility of a therapeutic agent for human cancer. Moreover, we showed that both human and canine osteosarcoma cells, which are notoriously resistant to radiation therapy, were sensitized by Pep7 to radiation-induced growth suppression and cell death. Together, our data suggest that Pep7 may be explored to sensitize tumors to radiation therapy.