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

p53 is critical for tumor suppression but also elicits detrimental effects when aberrantly overexpressed. Thus, multiple regulators, including RNA-binding protein RBM38, are found to tightly control p53 expression. Interestingly, RBM38 is unique in that it can either suppress or enhance p53 mRNA translation via altered interaction with eIF4E potentially mediated by serine-195 (S195) in RBM38. Thus, multiple RBM38/eIF4E knock-in (KI) cell lines were generated to investigate the significance of eIF4E-RBM38 interaction in controlling p53 activity. We showed that KI of RBM38-S195D or -Y192C enhances, whereas KI of RBM38-S195K/R/L weakens, the binding of eIF4E to p53 mRNA and subsequently p53 expression. We also showed that KI of eIF4E-D202K weakens the interaction of eIF4E with RBM38 and thereby enhances p53 expression, suggesting that D202 in eIF4E interacts with S195 in RBM38. Moreover, we generated an Rbm38 S193D KI mouse model in which human-equivalent serine-193 is substituted with aspartic acid. We showed that S193D KI enhances p53-dependent cellular senescence and that S193D KI mice have a shortened life span and are prone to spontaneous tumors, chronic inflammation, and liver steatosis. Together, we provide in vivo evidence that the RBM38-eIF4E loop can be explored to fine-tune p53 expression for therapeutic development.

Ninjurin 2 Modulates Tumorigenesis, Inflammation, and Metabolism via Pyroptosis.

The nerve injury-induced protein 2 (NINJ2) belongs to a family of homophilic adhesion molecules and was initially found to be involved in nerve regeneration. However, the role of NINJ2 in other cellular processes is not well studied. The Ninj2-deficient mice generated in the current study had a short lifespan and were prone to spontaneous tumors, systemic inflammation, and metabolic defects. Comprehensive carbohydrate and lipid metabolic analyses were performed to better understand the metabolic traits that contribute to these phenotypes. Carbohydrate metabolic analyses showed that NINJ2 deficiency led to defects in monosaccharide metabolism along with accumulation of multiple disaccharides and sugar alcohols. Lipidomic analyses showed that Ninj2 deficiency altered patterns of several lipids, including triglycerides, phospholipids, and ceramides. To identify a cellular process that associated with these metabolic defects, the role of NINJ2 in pyroptosis, a programmed cell death that links cancer, inflammation, and metabolic disorders, was examined. Loss of NINJ2 promoted pyroptosis by activating the NOD-like receptor protein 3 (NLRP3) inflammasome. Taken together, these data reveal a critical role of NINJ2 in tumorigenesis, inflammatory response, and metabolism via pyroptosis.

p73α1, a p73 C-terminal isoform, regulates tumor suppression and the inflammatory response via Notch1.

p73, a p53 family member, undergoes alternative splicing at the 3' end to produce multiple isoforms, but their expression and activity are largely unknown. Thus, CRISPR was used to knock out exon 12 (E12) in human cancer cell lines and mice, leading to isoform switch from p73α to isoform p73α1. We found that p73α1 is naturally expressed and induced by DNA damage. We also found that knockout of E12 suppresses cell growth and migration in H1299 and MIA PaCa-2 cells and promotes cellular senescence in mouse embryonic fibroblasts. Similarly, ectopic expression of p73α1 suppresses cell proliferation, whereas knockdown of p73α1 restores the cell proliferative and migratory capacities of E12−/− cells. Consistently, we found that E12+/− mice are not prone to spontaneous tumors. Instead, E12+/− mice are prone to systemic inflammation and exhibit elevated TNFα expression in inflamed tissues. Moreover, we found that Notch1, a master regulator of the inflammatory response, is regulated by p73α1 and highly expressed in E12−/− cells and inflamed E12+/− mouse tissues. Furthermore, through knockdown of p73α1 and/or Notch1 in E12−/− cells, we found that Notch1 is necessary for p73α1-mediated growth suppression. Together, these data suggest that p73α1 plays a critical role in tumor suppression and the inflammatory response via Notch1.

Ferredoxin reductase is critical for p53-dependent tumor suppression via iron regulatory protein 2.

Ferredoxin reductase (FDXR), a target of p53, modulates p53-dependent apoptosis and is necessary for steroidogenesis and biogenesis of iron-sulfur clusters. To determine the biological function of FDXR, we generated a Fdxr-deficient mouse model and found that loss of Fdxr led to embryonic lethality potentially due to iron overload in developing embryos. Interestingly, mice heterozygous in Fdxr had a short life span and were prone to spontaneous tumors and liver abnormalities, including steatosis, hepatitis, and hepatocellular carcinoma. We also found that FDXR was necessary for mitochondrial iron homeostasis and proper expression of several master regulators of iron metabolism, including iron regulatory protein 2 (IRP2). Surprisingly, we found that p53 mRNA translation was suppressed by FDXR deficiency via IRP2. Moreover, we found that the signal from FDXR to iron homeostasis and the p53 pathway was transduced by ferredoxin 2, a substrate of FDXR. Finally, we found that p53 played a role in iron homeostasis and was required for FDXR-mediated iron metabolism. Together, we conclude that FDXR and p53 are mutually regulated and that the FDXR-p53 loop is critical for tumor suppression via iron homeostasis.

Mice deficient in poly(C)-binding protein 4 are susceptible to spontaneous tumors through increased expression of ZFP871 that targets p53 for degradation.

Poly(C)-binding protein 4 (PCBP4), also called MCG10 and a target of p53, plays a role in the cell cycle and is implicated in lung tumor suppression. Here, we found that PCBP4-deficient mice are prone to lung adenocarcinoma, lymphoma, and kidney tumor and that PCBP4-deficient mouse embryo fibroblasts (MEFs) exhibit enhanced cell proliferation but decreased cellular senescence. We also found that p53 expression is markedly reduced in PCBP4-deficient MEFs and mouse tissues, suggesting that PCBP4 in turn regulates p53 expression. To determine how PCBP4 regulates p53 expression, PCBP4 targets were identified by RNA immunoprecipitation followed by RNA sequencing (RNA-seq). We found that the transcript encoding ZFP871 (zinc finger protein 871; also called ZNF709 in humans) interacts with and is regulated by PCBP4 via mRNA stability. Additionally, we found that ZFP871 physically interacts with p53 and MDM2 proteins. Consistently, ectopic expression of ZFP871 decreases-whereas knockdown of ZFP871 increases-p53 protein stability through a proteasome-dependent degradation pathway. Moreover, loss of ZFP871 reverses the reduction of p53 expression by lack of PCBP4, and thus increased expression of ZFP871 is responsible for decreased expression of p53 in the PCBP4-deficient MEFs and mouse tissues. Interestingly, we found that, like PCBP4, ZFP871 is also regulated by DNA damage and p53. Finally, we showed that knockdown of ZFP871 markedly enhances p53 expression, leading to growth suppression and apoptosis in a p53-dependent manner. Thus, the p53-PCBP4-ZFP871 axis represents a novel feedback loop in the p53 pathway. Together, we hypothesize that PCBP4 is a potential tissue-specific tumor suppressor and that ZFP871 is part of MDM2 and possibly other ubiquitin E3 ligases that target p53 for degradation.

Development and evaluation of an external quality control and internal quality control containing real-time RT-PCR assay for the detection of o'nyong-nyong virus.

O'nyong-nyong fever is a mosquito-borne tropical viral disease while few molecular diagnostic tools have been established for its surveillance until now. In the current study, a single-step, dual-color real-time reverse transcription polymerase chain reaction (RT-PCR) assay which contained both external quality control (EQC) and internal quality control (IQC) prepared by armored RNA technique was developed and evaluated for the detection of o'nyong-nyong virus (ONNV). Results showed that the assay was established successfully without cross-reaction with genetically related or symptom-alike diseases, which showed high specificity of the assay. The coefficient of variation of the assay was 0.97%, far less than 5%, indicating good repeatability of the assay. The lower limit of detection of the assay could reach as low as 100 copies of genome equivalent. During evaluation, the assay could correctly detect ONNV from spiked human serum samples and Anopheles species mosquito samples, while no ONNV positive was observed either from serum samples of patients with acute febrile illness or from local Anopheles species mosquitoes, suggesting no ONNV had been transmitted locally. In conclusion, the assay could potentially provide a valuable platform for ONNV molecular detection, which may improve the preparedness for future o'nyong-nyong fever outbreaks.

Mutant p53 antagonizes p63/p73-mediated tumor suppression via Notch1.

p53 is the most frequently mutated gene in human cancers and mutant p53 has a gain of function (GOF) that promotes tumor progression and therapeutic resistance. One of the major GOF activities of mutant p53 is to suppress 2 other p53 family proteins, p63 and p73. However, the molecular basis is not fully understood. Here, we examined whether mutant p53 antagonizes p63/p73-mediated tumor suppression in vivo by using mutant p53-R270H knockin and TAp63/p73-deficient mouse models. We found that knockin mutant p53-R270H shortened the life span of p73+/- mice and subjected TAp63+/- or p73+/- mice to T lymphoblastic lymphomas (TLBLs). To unravel the underlying mechanism, we showed that mutant p53 formed a complex with Notch1 intracellular domain (NICD) and antagonized p63/p73-mediated repression of HES1 and ECM1. As a result, HES1 and ECM1 were overexpressed in TAp63+/- ;p53R270H/- and p73+/- ;p53R270H/- TLBLs, suggesting that normal function of HES1 and ECM1 in T cell activation is hyperactivated, leading to lymphomagenesis. Together, our data reveal a previously unappreciated mechanism by which GOF mutant p53 hijacks the p63/p73-regulated transcriptional program via the Notch1 pathway.

Thiamin and riboflavin status with related enzyme activities in pulmonary tuberculosis with diabetes mellitus in Shandong province of China.

BACKGROUND AND OBJECTIVES: Poor nutritional status is a common finding in pulmonary tuberculosis (TB) patients with and without type 2 diabetes mellitus (T2DM), thiamin (VB-1) and riboflavin (VB-2) are coenzymes important for the activation of many enzymes involved in improving nutritional status. We aimed to investigate enzymatic activities and the associations between VB-1 and VB-2, and their relations to nutritional status in TB and TB+T2DM patients. METHODS AND STUDY DESIGN: This was a cross-sectional study that prospectively enrolled TB 40 patients with or without T2DM respectively from the Chest Hospital of Qingdao and 76 healthy controls with similar age and gender distributions were recruited from the medical center of the affiliated hospital of Qingdao Medical College. The erythrocyte transketolase activation coefficient (ETKac, for VB-1 deficiency), the glutathione reductase activation coefficient (EGRac, for VB-2 deficiency), and metabolic enzyme activities were analyzed. RESULTS: VB-1 and VB-2 deficiency rates were higher, and enzyme activities were lower in TB and TB+T2DM relative to control group. ETKac and EGRac were negatively correlated with enzyme activities, either with body mass index (BMI), while enzyme activities were positively associated with BMI. CONCLUSIONS: VB-1 and VB-2 concentrations were lower in TB patients with or without T2DM relative to controls, with concomitant reductions in the activity levels of key metabolic enzymes. Significant correlations were observed between VB-1 and VB-2 concentrations and the activity of these metabolic enzymes, they all correlated with nutrition status. VB-1 and VB-2 concentrations may thus impact metabolic enzyme activity and thereby influence nutritional status.

Microglia-Derived Olfactomedin-like 3 Is a Potent Angiogenic Factor in Primary Mouse Brain Endothelial Cells: A Novel Target for Glioblastoma.

Neoangiogenesis, a hallmark feature of all malignancies, is robust in glioblastoma (GBM). Vascular endothelial growth factor (VEGF) has long been regarded as the primary pro-angiogenic molecule in GBM. However, anti-VEGF therapies have had little clinical efficacy, highlighting the need to explore VEGF-independent mechanisms of neoangiogenesis. Olfactomedin-like 3 (OLFML3), a secreted glycoprotein, is an established proangiogenic factor in many cancers, but its role in GBM neoangiogenesis is unknown. To gain insight into the role of OLFML3 in microglia-mediated angiogenesis, we assessed endothelial cell (EC) viability, migration and differentiation following (1) siRNA knockdown targeting endogenous EC Olfml3 and (2) EC exposure to human recombinant OLFML3 (rhOLFML3; 10 ng/mL, 48 h), and conditioned medium (CM) from isogenic control and Olfml3−/− microglia (48 h). Despite a 70% reduction in Olfml3 mRNA levels, EC angiogenic parameters were not affected. However, exposure to both rhOLFML3 and isogenic control microglial CM increased EC viability (p < 0.01), migration (p < 0.05) and differentiation (p < 0.05). Strikingly, these increases were abolished, or markedly attenuated, following exposure to Olfml3−/− microglial CM despite corresponding increased microglial secretion of VEGF-A (p < 0.0001). Consistent with reports in non-CNS malignancies, we have demonstrated that OLFML3, specifically microglia-derived OLFML3, promotes VEGF-independent angiogenesis in primary brain microvascular ECs and may provide a complementary target to mitigate neovascularization in GBM.

Mdm2 is a target and mediator of IRP2 in cell growth control.

Iron is an essential element to all living organisms and plays a vital role in many cellular processes, such as DNA synthesis and energy production. The Mdm2 oncogene is an E3 ligase and known to promote tumor growth. However, the role of Mdm2 in iron homeostasis is not certain. Here, we showed that Mdm2 expression was increased by iron depletion but decreased by iron repletion. We also showed that Iron Regulatory Protein 2 (IRP2) mediated iron-regulated Mdm2 expression. Specifically, Mdm2 expression was increased by ectopic IRP2 but decreased by knockdown or knockout of IRP2 in human cancer cells as well as in mouse embryonic fibroblasts. In addition, we showed that IRP2-regulated Mdm2 expression was independent of tumor suppressor p53. Mechanistically, we found that IRP2 stabilized Mdm2 transcript via binding to an iron response element (IRE) in the 3'UTR of Mdm2 mRNA. Finally, we showed that Mdm2 is required for IRP2-mediated cell proliferation and Mdm2 expression is highly associated with IRP2 in both the normal and cancerous liver tissues. Together, we uncover a novel regulation of Mdm2 by IRP2 via mRNA stability and that the IRP2-Mdm2 axis may play a critical role in cell growth.

FDXR regulates TP73 tumor suppressor via IRP2 to modulate aging and tumor suppression.

Ferredoxin reductase (FDXR) is a mitochondrial flavoprotein that initiates electron transport from NADPH to several cytochromes P450 via two electron carriers, ferredoxin 1 (FDX1) and FDX2. FDXR is the sole ferredoxin reductase in humans and plays a critical role in steroidogenesis and biosynthesis of heme and iron-sulfur clusters. However, much less is known about the role of FDXR in cancer. Here, we show that FDXR plays a role in tumorigenesis by modulating expression of the tumor suppressor p73. By using genetically modified mouse models, we recently showed that mice deficient in either Fdxr or Trp73 had a shorter lifespan and were prone to spontaneous tumors as compared with wild-type (WT) mice. Interestingly, compound Trp73 +/- ;Fdxr +/- mice lived longer and developed fewer tumors when compared with Fdxr +/- or Trp73 +/- mice. Moreover, we found that cellular senescence was increased in Trp73 +/- and Fdxr +/- mouse embryonic fibroblasts (MEFs), which was further increased in Trp73 +/- ;Fdxr +/- MEFs, as compared with that in WT MEFs. As FDXR is regulated by p73, we examined whether there was a feedback regulation between p73 and FDXR. Indeed, we found that Trp73 expression was decreased by loss of Fdxr in MEFs and that FDXR is required for p73 expression in multiple human cancer cell lines independent of p53. Mechanistically, we found that loss of FDXR, via FDX2, increased expression of iron-binding protein 2 (IRP2), which subsequently repressed TP73 mRNA stability. We also showed that TP73 transcript contained an iron response element in its 3'UTR, which was required for IRP2 to destabilize TP73 mRNA. Together, these data reveal a novel regulation of p73 by FDXR via IRP2 and that the FDXR-p73 axis plays a critical role in aging and tumor suppression. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Glycogen synthase kinase 3 promotes p53 mRNA translation via phosphorylation of RNPC1.

The RNPC1 RNA-binding protein, also called Rbm38, is a target of p53 and a repressor of p53 mRNA translation. Thus, the p53-RNPC1 loop is critical for modulating p53 tumor suppression, but it is not clear how the loop is regulated. Here, we showed that RNPC1 is phosphorylated at Ser195 by glycogen synthase kinase 3 (GSK3). We also showed that GSK3 promotes p53 mRNA translation through phosphorylation of RNPC1. Interestingly, we found that the phosphor-mimetic mutant S195D and the deletion mutant Δ189-204, which lacks the GSK3 phosphorylation site, are unable to repress p53 mRNA translation due to loss of interaction with eukaryotic translation factor eIF4E on p53 mRNA. Additionally, we found that phosphorylated RNPC1, RNPC1-S195D, and RNPC1(Δ189-204) promote p53 mRNA translation through interaction with eukaryotic translation factor eIF4G, which then facilitates the assembly of the eIF4F complex on p53 mRNA. Furthermore, we showed that upon inhibition of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway, GSK3 is activated, leading to increased RNPC1 phosphorylation and increased p53 expression in a RNPC1-dependent manner. Together, we postulate that the p53-RNPC1 loop can be explored to increase or decrease p53 activity for cancer therapy.

Microglia-Derived Olfactomedin-like 3 Promotes Pro-Tumorigenic Microglial Function and Malignant Features of Glioma Cells.

Under the influence of transforming growth factor-beta (TGFß), glioma-associated microglia produce molecules that promote glioma growth and invasion. Olfactomedin-like 3 (Olfml3), a novel, secreted glycoprotein, is known to promote several non-CNS cancers. While it is a direct TGFß1 target gene in microglia, the role of microglia-derived OLFML3 in glioma progression is unknown. Here, we tested the hypotheses that microglial Olfml3 is integral to the pro-tumorigenic glioma-associated microglia phenotype and promotes glioma cell malignancy. Using an Olfml3 knockout microglial cell line (N9), we demonstrated that Olfml3 is a direct target gene of all TGFß isoforms in murine microglia. Moreover, loss of Olfml3 attenuated TGFß-induced restraint on microglial immune function and production of cytokines that are critical in promoting glioma cell malignancy. Importantly, microglia-derived OLFML3 directly contributes to glioma cell malignancy through increased migration and invasion. While exposure to conditioned medium (CM) from isogenic control microglia pre-treated with TGFß increased mouse glioma cell (GL261) migration and invasion, this effect was abolished with exposure to CM from TGFß-treated Olfml3-/- microglia. Taken together, our data suggest that Olfml3 may serve as a gatekeeper for TGFß-induced microglial gene expression, thereby promoting the pro-tumorigenic microglia phenotype and glioma cell malignancy.

Serine 195 phosphorylation in the RNA-binding protein Rbm38 increases p63 expression by modulating Rbm38's interaction with the Ago2-miR203 complex.

The p63 transcription factor, a p53 family protein, regulates genes involved in various cellular processes, including cell growth and differentiation. We previously showed that RNA-binding motif protein (Rbm38) is a p63 target and, in turn, regulates p63α mRNA stability by binding to the AU/U-rich element in its 3'UTR. Interestingly, Rbm38 can be phosphorylated at serine 195, altering its ability to regulate mRNA translation. However, whether the Ser-195 phosphorylation affects Rbm38's ability to destabilize p63 mRNA remains unclear. Here, using MCF7 and HaCaT cells, we showed that ectopic expression of phosphomimetic Rbm38-S195D increases, whereas WT Rbm38 and nonphosphorylatable Rbm38-S195A decrease p63α protein and transcript levels. We also found that upon activation of glycogen synthase kinase 3ß (GSK3ß), phosphorylation of Rbm38 at Ser-195 is increased, enhancing p63α expression in an Rbm38-dependent manner. To confirm this, we generated mouse embryo fibroblasts (MEFs) in which Ser-193 in mouse Rbm38 (equivalent to Ser-195 in human Rbm38) was substituted with aspartic acid (Rbm38S193D/S193D ) or alanine (Rbm38S193A/S193A ). We observed that the p63 transcript level was increased in Rbm38S193D/S193D MEFs, but decreased in Rbm38S193A/S193A MEFs. Mechanistically, we found that WT Rbm38, but not Rbm38-S195D, is required for p63 mRNA degradation mediated by microRNA 203 (miR203). Furthermore, we noted that Argonaute 2 (Ago2), a key regulator in microRNA-mediated mRNA decay, associates with WT Rbm38, and this association was reduced by Ser-195 phosphorylation. Together, our results reveal a critical mechanism by which Ser-195 phosphorylation in Rbm38 increases p63 expression by attenuating the association of Rbm38 with the Ago2-miR203 complex.

Ninjurin 1 has two opposing functions in tumorigenesis in a p53-dependent manner.

WT p53 is critical for tumor suppression, whereas mutant p53 promotes tumor progression. Nerve injury-induced protein 1 (Ninj1) is a target of p53 and forms a feedback loop with p53 by repressing p53 mRNA translation. Here, we show that loss of Ninj1 increased mutant p53 expression and, subsequently, enhanced cell growth and migration in cells carrying a mutant p53. In contrast, loss of Ninj1 inhibited cell growth and migration in cells carrying a WT p53. To explore the biological significance of Ninj1, we generated a cohort of Ninj1-deficient mice and found that Ninj1+/- mice were prone to systemic inflammation and insulitis, but not to spontaneous tumors. We also found that loss of Ninj1 altered the tumor susceptibility in both mutant p53 and p53-null background. Specifically, in a mutant p53(R270H) background, Ninj1 deficiency shortened the lifespan, altered the tumor spectrum, and increased tumor burden, likely via enhanced expression of mutant p53. In a p53-null background, Ninj1 deficiency significantly increased the incidence of T-lymphoblastic lymphoma. Taken together, our data suggest that depending on p53 genetic status, Ninj1 has two opposing functions in tumorigenesis and that the Ninj1-p53 loop may be targeted to manage inflammatory diseases and cancer.

Translational repression of p53 by RNPC1, a p53 target overexpressed in lymphomas.

The p53 pathway is critical for tumor suppression, as the majority of human cancer has a faulty p53. Here, we identified RNPC1, a p53 target and a RNA-binding protein, as a critical regulator of p53 translation. We showed that ectopic expression of RNPC1 inhibited, whereas knockdown of RNPC1 increased, p53 translation under normal and stress conditions. We also showed that RNPC1 prevented cap-binding protein eIF4E from binding p53 mRNA via its C-terminal domain for physical interaction with eIF4E, and its N-terminal domain for binding p53 mRNA. Consistent with this, we found that RNPC1 directly binds to p53 5' and 3'untranslated regions (UTRs). Importantly, we showed that RNPC1 inhibits ectopic expression of p53 in a dose-dependent manner via p53 5' or 3' UTR. Moreover, we showed that loss of RNPC1 in mouse embryonic fibroblasts increased the level of p53 protein, leading to enhanced premature senescence in a p53-dependent manner. Finally, to explore the clinical relevance of our finding, we showed that RNPC1 was frequently overexpressed in dog lymphomas, most of which were accompanied by decreased expression of wild-type p53. Together, we identified a novel p53-RNPC1 autoregulatory loop, and our findings suggest that RNPC1 plays a role in tumorigenesis by repressing p53 translation.

RNA-binding Protein PCBP2 Regulates p73 Expression and p73-dependent Antioxidant Defense.

TAp73, a member of the p53 family tumor suppressors, plays a critical rule in tumor suppression and neuronal development. However, how p73 activity is controlled at the posttranscriptional level is not well understood. Here, we showed that TAp73 activity is regulated by RNA-binding protein PCBP2. Specifically, we found that knockdown or knock-out of PCBP2 reduces, whereas ectopic expression of PCBP2 increases, TAp73 expression. We also showed that PCBP2 is necessary for p73 mRNA stability via the CU-rich elements in p73 3'-UTR. To uncover the biological relevance of PCBP2-regulated TAp73 expression, we showed that ectopic expression of PCBP2 inhibits, whereas knockdown or knock-out of PCBP2 increases, the production of reactive oxygen species (ROS) in a TAp73-dependent manner. Additionally, we found that glutaminase 2 (GLS2), a modulator of p73-dependent antioxidant defense, is also involved in PCBP2-regulated ROS production. Moreover, we generated PCBP2-deficient mice and primary mouse embryonic fibroblasts (MEFs) and showed that loss of PCBP2 leads to decreased p73 expression and, subsequently, increased ROS production and accelerated cellular senescence. Together, our data suggest that PCBP2 regulates p73 expression via mRNA stability and p73-dependent biological function in ROS production and cellular senescence.

Mice deficient in Rbm38, a target of the p53 family, are susceptible to accelerated aging and spontaneous tumors.

RNA-binding motif protein 38 (Rbm38), also called RNPC1 [RNA-binding region (RNP1, RRM) containing 1], is a target of the p53 family and modulates p53 expression via mRNA translation. To investigate the biological function of Rbm38 in vivo, we generated an Rbm38-null mouse model. We showed that mice deficient in Rbm38 exhibit signs of accelerated aging and are prone to hematopoietic defects and spontaneous tumors. To determine the biological significance of the p53-Rbm38 loop, we showed that Rbm38 deficiency enhances accumulation of p53 induced by ionizing radiation (IR) and sensitizes mice to IR-induced lethality in a p53-dependent manner. Most importantly, Rbm38 deficiency markedly decreases the tumor penetrance in mice heterozygous for p53 via enhanced p53 expression. Interestingly, we found that Rbm38 deficiency shortens the life span of, and promotes lymphomagenesis in, mice deficient in p53. These results provide genetic evidence that Rbm38 is necessary for normal hematopoiesis and for suppressing accelerated aging and tumorigenesis. Thus, the p53-Rbm38 axis might be explored for extending longevity and for tumor suppression.

Ninjurin1, a target of p53, regulates p53 expression and p53-dependent cell survival, senescence, and radiation-induced mortality.

The tumor suppressor protein p53 plays a crucial role in coordinating cellular processes, such as cell cycle arrest, apoptosis, and senescence. The nerve injury-induced protein 1 (Ninjurin1, Ninj1) is a homophilic adhesion molecule and involved in nerve regeneration. Interestingly, Ninj1 is found to be overexpressed in human cancer, but its role in tumorigenesis is not clear. Here, we found that Ninj1 is transcriptionally regulated by p53 and can be induced by DNA damage in a p53-dependent manner. We also found that knockout or knockdown of Ninj1 increases p53 expression potentially through enhanced p53 mRNA translation. In addition, we found that Ninj1 deficiency suppresses cell proliferation but enhances apoptosis and premature senescence in a p53-dependent manner. Consistent with this, we found that mice heterozygous in ninj1 are hypersensitive to ionizing radiation-induced lethality, along with increased expression of p53 in thymus. Taken together, we provided evidence that Ninj1 is a p53 target and modulates p53 mRNA translation and p53-dependent premature senescence, cell proliferation, apoptosis, and radiation-induced mortality in vitro and in vivo. Thus, we postulate that as a membrane adhesion molecule, Ninj1 is an ideal target to regulate p53 activity via the p53-Ninj1 loop.

Rbm24, an RNA-binding protein and a target of p53, regulates p21 expression via mRNA stability.

p21, a cyclin-dependent kinase inhibitor, is necessary for proper control of the cell cycle and premature senescence. Thus, p21 expression needs to be tightly controlled. In this study, we found that Rbm24, an RNA-binding protein and a target gene of the p53 protein, can regulate p21 expression via mRNA stability. Specifically, we showed that Rbm24 is induced by DNA damage and Mdm2 inhibitor Nutlin-3. We also found that p53 protein binds to and activates the promoter of the Rbm24 gene. Moreover, we found that overexpression of Rbm24 increases, whereas knockdown of Rbm24 decreases, p21 mRNA and protein expression. In addition, we demonstrated that overexpression of Rbm24 enhances the half-life of p21 transcript. Consistent with this, we provided evidence that Rbm24 binds to the 3'-untranslated region (3'-UTR) of p21 transcript and an AU/U-rich element in the p21 3'-UTR is necessary for Rbm24 to increase p21 expression. Finally, we showed that the RNA recognition motif in Rbm24 is required for binding to p21 transcript and subsequently for inducing p21 expression. Altogether, we uncovered that Rbm24 is a novel player in the p53 pathway, which may be explored to restore proper cell cycle control in p53-deficient tumors via p21.