Chromatin modifier Hmga2 promotes adult hematopoietic stem cell function and blood regeneration in stress conditions.

The molecular mechanisms governing the response of hematopoietic stem cells (HSCs) to stress insults remain poorly defined. Here, we investigated effects of conditional knock-out or overexpression of Hmga2 (High mobility group AT-hook 2), a transcriptional activator of stem cell genes in fetal HSCs. While Hmga2 overexpression did not affect adult hematopoiesis under homeostasis, it accelerated HSC expansion in response to injection with 5-fluorouracil (5-FU) or in vitro treatment with TNF-α. In contrast, HSC and megakaryocyte progenitor cell numbers were decreased in Hmga2 KO animals. Transcription of inflammatory genes was repressed in Hmga2-overexpressing mice injected with 5-FU, and Hmga2 bound to distinct regions and chromatin accessibility was decreased in HSCs upon stress. Mechanistically, we found that casein kinase 2 (CK2) phosphorylates the Hmga2 acidic domain, promoting its access and binding to chromatin, transcription of anti-inflammatory target genes, and the expansion of HSCs under stress conditions. Notably, the identified stress-regulated Hmga2 gene signature is activated in hematopoietic stem progenitor cells of human myelodysplastic syndrome patients. In sum, these results reveal a TNF-α/CK2/phospho-Hmga2 axis controlling adult stress hematopoiesis.

Extracellular N6 -isopentenyladenosine (i6A) addition induces cotranscriptional i6A incorporation into ribosomal RNAs.

N6 -isopentenyladenosine (i6A), a modified adenosine monomer, is known to induce cell death upon its addition to the culture medium. However, the molecular fate of extracellularly added i6A has yet to be identified. Here we show that i6A addition to cell culture medium results in i6A incorporation into cellular RNA in several cell lines, including the 5-fluorouracil (5-FU)-resistant human oral squamous cell carcinoma cell line FR2-SAS and its parental 5-FU-sensitive cell line SAS. i6A was predominantly incorporated into 18S and 28S rRNAs, and i6A incorporation into total RNA was mostly suppressed by treating these cell lines with an RNA polymerase I (Pol I) inhibitor. i6A was incorporated into RNA even upon inactivation of TRIT1, the only cellular i6A-modifying enzyme. These results indicate that upon cellular uptake of i6A, it is anabolized to be used for Pol I transcription. Interestingly, at lower i6A concentrations, the cytotoxic effect of i6A was substantially more pronounced in FR2-SAS cells than in SAS cells. Moreover, in FR2-SAS cells, i6A treatment decreased the rate of cellular protein synthesis and increased intracellular protein aggregation, and these effects were more pronounced than in SAS cells. Our work provides insights into the molecular fate of extracellularly applied i6A in the context of intracellular nucleic acid anabolism and suggests investigation of i6A as a candidate for a chemotherapy agent against 5-FU-resistant cancer cells.

FOXM1-Mediated Regulation of Reactive Oxygen Species and Radioresistance in Oral Squamous Cell Carcinoma Cells.

Radioresistance is a major obstacle to the successful treatment of oral squamous cell carcinoma (OSCC). To help overcome this issue, we have developed clinically relevant radioresistant (CRR) cell lines generated by irradiating parental cells over time, which are useful for OSCC research. In the present study, we conducted gene expression analysis using CRR cells and their parental lines to investigate the regulation of radioresistance in OSCC cells. Based on gene expression changes over time in CRR cells and parental lines subjected to irradiation, forkhead box M1 (FOXM1) was selected for further analysis in terms of its expression in OSCC cell lines, including CRR cell lines and clinical specimens. We suppressed or upregulated the expression of FOXM1 in OSCC cell lines, including CRR cell lines, and examined radiosensitivity, DNA damage, and cell viability under various conditions. The molecular network regulating radiotolerance was also investigated, especially the redox pathway, and the radiosensitizing effect of FOXM1 inhibitors was examined as a potential therapeutic application. We found that FOXM1 was not expressed in normal human keratinocytes but was expressed in several OSCC cell lines. The expression of FOXM1 was upregulated in CRR cells compared with that detected in the parental cell lines. In a xenograft model and clinical specimens, FOXM1 expression was upregulated in cells that survived irradiation. FOXM1-specific small interfering RNA (siRNA) treatment increased radiosensitivity, whereas FOXM1 overexpression decreased radiosensitivity, and DNA damage was altered significantly under both conditions, as well as the levels of redox-related molecules and reactive oxygen species production. Treatment with the FOXM1 inhibitor thiostrepton had a radiosensitizing effect and overcame radiotolerance in CRR cells. According to these results, the FOXM1-mediated regulation of reactive oxygen species could be a novel therapeutic target for the treatment of radioresistant OSCC; thus, treatment strategies targeting this axis might overcome radioresistance in this disease.

Cooperative methylation of human tRNA3Lys at positions A58 and U54 drives the early and late steps of HIV-1 replication.

Retroviral infection requires reverse transcription, and the reverse transcriptase (RT) uses cellular tRNA as its primer. In humans, the TRMT6-TRMT61A methyltransferase complex incorporates N1-methyladenosine modification at tRNA position 58 (m1A58); however, the role of m1A58 as an RT-stop site during retroviral infection has remained questionable. Here, we constructed TRMT6 mutant cells to determine the roles of m1A in HIV-1 infection. We confirmed that tRNA3Lys m1A58 was required for in vitro plus-strand strong-stop by RT. Accordingly, infectivity of VSV-G pseudotyped HIV-1 decreased when the virus contained m1A58-deficient tRNA3Lys instead of m1A58-modified tRNA3Lys. In TRMT6 mutant cells, the global protein synthesis rate was equivalent to that of wild-type cells. However, unexpectedly, plasmid-derived HIV-1 expression showed that TRMT6 mutant cells decreased accumulation of HIV-1 capsid, integrase, Tat, Gag, and GagPol proteins without reduction of HIV-1 RNAs in cells, and fewer viruses were produced. Moreover, the importance of 5,2'-O-dimethyluridine at U54 of tRNA3Lys as a second RT-stop site was supported by conservation of retroviral genome-tRNALys sequence-complementarity, and TRMT6 was required for efficient 5-methylation of U54. These findings illuminate the fundamental importance of tRNA m1A58 modification in both the early and late steps of HIV-1 replication, as well as in the cellular tRNA modification network.

Naringenin potentiates anti-tumor immunity against oral cancer by inducing lymph node CD169-positive macrophage activation and cytotoxic T cell infiltration.

The CD169+ macrophages in lymph nodes are implicated in cytotoxic T lymphocyte (CTL) activation and are associated with improved prognosis in several malignancies. Here, we investigated the significance of CD169+ macrophages in oral squamous cell carcinoma (OSCC). Further, we tested the anti-tumor effects of naringenin, which has been previously shown to activate CD169+ macrophages, in a murine OSCC model. Immunohistochemical analysis for CD169 and CD8 was performed on lymph node and primary tumor specimens from 89 patients with OSCC. We also evaluated the effects of naringenin on two murine OSCC models. Increased CD169+ macrophage counts in the regional lymph nodes correlated with favorable prognosis and CD8+ cell counts within tumor sites. Additionally, naringenin suppressed tumor growth in two murine OSCC models. The mRNA levels of CD169, interleukin (IL)-12, and C-X-C motif chemokine ligand 10 (CXCL10) in lymph nodes and CTL infiltration in tumors significantly increased following naringenin administration in tumor-bearing mice. These results suggest that CD169+ macrophages in lymph nodes are involved in T cell-mediated anti-tumor immunity and could be a prognostic marker for patients with OSCC. Moreover, naringenin is a new potential agent for CD169+ macrophage activation in OSCC treatment.

Role of serum amylase and salivary cytokines in oral complications during chemoradiotherapy.

OBJECTIVES: To investigate whether serum amylase can predict the recovery of salivary volume and determine the correlation of the level of cytokines, including epidermal growth factor, hepatocyte growth factor and keratinocyte growth factor, with oral mucositis during chemoradiotherapy for oral cancer. SUBJECTS AND METHODS: This study included 84 patients treated with preoperative chemoradiotherapy followed by curative surgery, following a phase II study protocol. We measured and analysed the correlation of the stimulated saliva volume, serum amylase and cytokines in resting saliva at baseline and 1 month after chemoradiotherapy with oral mucositis levels. RESULTS: We observed a negative correlation between the serum amylase level at the beginning of chemoradiotherapy and the stimulated saliva volume at 1 month after chemoradiotherapy (p = .03). Epidermal growth factor in resting saliva was significantly reduced after chemoradiotherapy (p < .01). The incidence of severe oral mucositis during chemoradiotherapy was significantly higher and negatively associated with the epidermal growth factor and keratinocyte growth factor levels (p = .04, p = .05). CONCLUSIONS: The serum amylase level at the beginning of chemoradiotherapy may be a predictor of the recovery of the saliva volume. Furthermore, cytokines such as epidermal growth factor and keratinocyte growth factor in resting saliva affect the development of oral mucositis during chemoradiotherapy.

Reactive sulfur species regulate tRNA methylthiolation and contribute to insulin secretion.

The 2-methylthio (ms2) modification at A37 of tRNAs is critical for accurate decoding, and contributes to metabolic homeostasis in mammals. However, the regulatory mechanism of ms2 modification remains largely unknown. Here, we report that cysteine hydropersulfide (CysSSH), a newly identified reactive sulfur species, is involved in ms2 modification in cells. The suppression of intracellular CysSSH production rapidly reduced ms2 modification, which was rescued by the application of an exogenous CysSSH donor. Using a unique and stable isotope-labeled CysSSH donor, we show that CysSSH was capable of specifically transferring its reactive sulfur atom to the cysteine residues of ms2-modifying enzymes as well as ms2 modification. Furthermore, the suppression of CysSSH production impaired insulin secretion and caused glucose intolerance in both a pancreatic ß-cell line and mouse model. These results demonstrate that intracellular CysSSH is a novel sulfur source for ms2 modification, and that it contributes to insulin secretion.

Ribosome profiling analysis reveals the roles of DDX41 in translational regulation.

DDX41 mutation has been observed in myeloid malignancies including myelodysplastic syndromes and acute myeloid leukemia, but the underlying causative mechanisms of these diseases have not been fully elucidated. The DDX41 protein is an ATP-dependent RNA helicase with roles in RNA metabolism. We previously showed that DDX41 is involved in ribosome biogenesis by promoting the processing of newly transcribed pre-ribosomal RNA. To build on this finding, in this study, we leveraged ribosome profiling technology to investigate the involvement of DDX41 in translation. We found that DDX41 knockdown resulted in both translationally increased and decreased transcripts. Both gene set enrichment analysis and gene ontology analysis indicated that ribosome-associated genes were translationally promoted after DDX41 knockdown, in part because these transcripts had significantly shorter transcript length and higher transcriptional and translational levels. In addition, we found that transcripts with 5'-terminal oligopyrimidine motifs tended to be translationally upregulated when the DDX41 level was low. Our data suggest that a translationally regulated feedback mechanism involving DDX41 may exist for ribosome biogenesis.

DDX41 coordinates RNA splicing and transcriptional elongation to prevent DNA replication stress in hematopoietic cells.

Myeloid malignancies with DDX41 mutations are often associated with bone marrow failure and cytopenia before overt disease manifestation. However, the mechanisms underlying these specific conditions remain elusive. Here, we demonstrate that loss of DDX41 function impairs efficient RNA splicing, resulting in DNA replication stress with excess R-loop formation. Mechanistically, DDX41 binds to the 5' splice site (5'SS) of coding RNA and coordinates RNA splicing and transcriptional elongation; loss of DDX41 prevents splicing-coupled transient pausing of RNA polymerase II at 5'SS, causing aberrant R-loop formation and transcription-replication collisions. Although the degree of DNA replication stress acquired in S phase is small, cells undergo mitosis with under-replicated DNA being remained, resulting in micronuclei formation and significant DNA damage, thus leading to impaired cell proliferation and genomic instability. These processes may be responsible for disease phenotypes associated with DDX41 mutations.

The Tumour Suppressor CYLD Is Required for Clathrin-Mediated Endocytosis of EGFR and Cetuximab-Induced Apoptosis in Head and Neck Squamous Cell Carcinoma.

Epidermal growth factor receptor (EGFR) is frequently overexpressed in head and neck squamous cell carcinoma (HNSCC) and is a target for the therapeutic antibody cetuximab (CTX). However, because only some patients have a significant clinical response to CTX, identification of its predictive biomarkers and potentiation of CTX-based therapies are important. We have recently reported a frequent downregulation of cylindromatosis (CYLD) in primary HNSCC, which led to increased cell invasion and cisplatin resistance. Here, we show that CYLD located mainly in lipid rafts was required for clathrin-mediated endocytosis (CME) and degradation of the EGFR induced by EGF and CTX in HNSCC cells. The N-terminus containing the first cytoskeleton-associated protein-glycine domain of CYLD was responsible for this regulation. Loss of CYLD restricted EGFR to lipid rafts, which suppressed CTX-induced apoptosis without impeding CTX's inhibitory activity against downstream signalling pathways. Disruption of the lipid rafts with cholesterol-removing agents overcame this resistance by restoring CME and the degradation of EGFR. Regulation of EGFR trafficking by CYLD is thus critical for the antitumour activity of CTX. Our findings suggest the usefulness of a combination of cholesterol-lowering drugs with anti-EGFR antibody therapy in HNSCC.

miR-30a attenuates drug sensitivity to 5-FU by modulating cell proliferation possibly by downregulating cyclin E2 in oral squamous cell carcinoma.

We aimed to determine the functional role of the miRNA, which affects drug sensitivity to 5-FU in oral squamous cell carcinoma (OSCC), using two types of 5-FU-resistant and parental OSCC cell lines. MiRNA microarray data showed that miR-30a was significantly upregulated in two resistant cell lines. Therefore, we investigated the effects and molecular mechanism of miR-30a on 5-FU sensitivity. Stable overexpression of miR-30a in parental OSCC cells decreased cell proliferation and attenuated drug sensitivity to 5-FU. Cell cycle analysis indicated that miR-30a overexpression increased the proportion of G1 phase cells and decreased the proportion of S phase cells. MiR-30a knockdown using siRNA reversed the effects of miR-30a overexpression. DNA microarray analysis using miR-30a-overexpressing cell lines and a TargetScan database search showed that cyclin E2 (CCNE2) is a target of miR-30a. A luciferase reporter assay confirmed that a miR-30a mimic interacted with the specific binding site in the 3' UTR of CCNE2. CCNE2 knockdown with siRNA in OSCC cells yielded decreased drug sensitivity to 5-FU, similar to miR-30a overexpressing cells. These findings suggest that miR-30a in OSCC may be a novel biomarker of 5-FU-resistant tumors, as well as a therapeutic target for combating resistance.

FTO Demethylates Cyclin D1 mRNA and Controls Cell-Cycle Progression.

N6-Methyladenosine (m6A) modification is the major chemical modification in mRNA that controls fundamental biological processes, including cell proliferation. Herein, we demonstrate that fat mass and obesity-associated (FTO) demethylates m6A modification of cyclin D1, the key regulator for G1 phase progression and controls cell proliferation in vitro and in vivo. FTO depletion upregulates cyclin D1 m6A modification, which in turn accelerates the degradation of cyclin D1 mRNA, leading to the impairment of G1 progression. m6A modification of cyclin D1 oscillates in a cell-cycle-dependent manner; m6A levels are suppressed during the G1 phase and enhanced during other phases. Low m6A levels during G1 are associated with the nuclear translocation of FTO from the cytosol. Furthermore, nucleocytoplasmic shuttling of FTO is regulated by casein kinase II-mediated phosphorylation of FTO. Our results highlight the role of m6A in regulating cyclin D1 mRNA stability and add another layer of complexity to cell-cycle regulation.