A novel PAD4/SOX4/PU.1 signaling pathway is involved in the committed differentiation of acute promyelocytic leukemia cells into granulocytic cells

Peptidylarginine deiminase (PAD): A promising target for chronic diseases treatment

In 1958, the presence of citrulline in the structure of the proteins was discovered for the first time. Several years later they found that Arginine converted to citrulline during a post-translational modification process by PAD enzyme. Each PAD is expressed in a certain tissue developing a series of diseases such as inflammation and cancers. Among these, PAD2 and PAD4 play a role in the development of rheumatoid arthritis (RA) by producing citrullinated autoantigens and increasing the production of inflammatory cytokines. PAD4 is also associated with the formation of NET structures and thrombosis. In the crystallographic structure, PAD has several calcium binding sites, and the active site of the enzyme consists of different amino acids. Various PAD inhibitors have been developed divided into pan-PAD and selective PAD inhibitors. F-amidine, Cl-amidine, and BB-Cl-amidine are some of pan-PAD inhibitors. AFM-30a and JBI589 are selective for PAD2 and PAD4, respectively. There is a need to evaluate the effectiveness of existing inhibitors more accurately in the coming years, as well as design and production of novel inhibitors targeting highly specific isoforms.

PADI4 negatively regulates RIG-I-mediated antiviral response through deacetylation of IFN-β promoter via HDAC1

The production of type I interferon (IFN) is precisely modulated by host to protect against viral infection efficiently without obvious immune disorders. Elucidating the tight control towards type I IFN production would be helpful to get insight into natural immunity and inflammatory diseases. As yet, however, the mechanisms that regulate IFN-β production, especially the epigenetic regulatory mechanisms, remain poorly explored. This study elucidated the potential function of Peptidylarginine deiminases (PADIs)-mediated citrullination in innate immunity. We identified PADI4, a PADIs family member that can act as an epigenetic coactivator, could repress IFN-β production upon RNA virus infection. Detailed experiments showed that PADI4 deficiency increased IFN-β production and promoted antiviral immune activities against RNA viruses. Mechanistically, the increased PADI4 following viral infection translocated to nucleus and recruited HDAC1 upon binding to Ifnb1 promoter, which then led to the deacetylation of histone H3 and histone H4 for repressing Ifnb1 transcription. Taken together, we identify a novel non-classical role for PADI4 in the regulation of IFN-β production, suggesting its potential as treatment target in inflammatory or autoimmune diseases.

Roles of PADI4 in the expression of cytokines involved in inflammation and adhesion in differentiated NB4 cells treated with ATRA

Differentiation syndrome (DS) is a common complication in patients with acute promyelocytic leukemia (APL) treated with all-trans-retinoic acid (ATRA). However, the target of ATRA during DS in patients with APL remains to be elucidated. Therefore, the current study aimed to investigate the role of peptidylarginine deiminase 4 (PADI4) in the differentiation of ATRA-induced NB4 APL cells. The results showed that PADI4 was significantly upregulated in peripheral blood samples derived from patients with APL DS compared with patients with APL only. In addition, whether ATRA could enhance the expression levels of PADI4 in NB4 cells in vitro was subsequently investigated. The results also showed that PADI4 overexpression promoted the differentiation of NB4 cells treated with ATRA, which was reversed after PADI4 silencing. To uncover the potential mechanisms underlying the above process, PADI4 overexpression induced the secretion of inflammation-related cytokines, such as TNF-α, IL-1β, IL-8, C-C motif chemokine (CCL)2, CCL4, CCR1 and intercellular adhesion molecule-1 in ATRA-treated NB4 cells. However, PADI4 knockdown in the same cells had the opposite effect. The above findings indicated that PADI4 could be involved in the differentiation of ATRA-induced NB4 cells and upregulation of cytokines.

SOX4 Mediates ATRA-Induced Differentiation in Neuroblastoma Cells

Neuroblastoma (NB), which is considered to be caused by the differentiation failure of neural crest cells, is the most common extracranial malignant solid tumor in children. The degree of tumor differentiation in patients with NB is closely correlated with the survival rate. To explore the potential targets that mediate NB cell differentiation, we analyzed four microarray datasets from GEO, and the overlapping down- or upregulated DEGs were displayed using Venn diagrams. SOX4 was one of the overlapping upregulated DEGs and was confirmed by RT-qPCR and Western blot in ATRA-treated NGP, SY5Y, and BE2 cells. To clarify whether SOX4 was the target gene regulating NB cell differentiation, the correlation between the expression of SOX4 and the survival of clinical patients was analyzed via the R2 database, SOX4 overexpression plasmids and siRNAs were generated to change the expression of SOX4, RT-qPCR and Western blot were performed to detect SOX4 expression, cell confluence or cell survival was detected by IncuCyte Zoom or CCK8 assay, immunocytochemistry staining was performed to detect cells' neurites, and a cell cycle analysis was implemented using Flow cytometry after PI staining. The results showed that the survival probabilities were positively correlated with SOX4 expression, in which overexpressing SOX4 inhibited NB cell proliferation, elongated the cells' neurite, and blocked the cell cycle in G1 phase, and that knockdown of the expression of SOX4 partially reversed the ATRA-induced inhibition of NB cell proliferation, the elongation of the cells' neurites, and the blocking of the cell cycle in the G1 phase. These indicate that SOX4 may be a target to induce NB cell differentiation.

PAD4 and Its Inhibitors in Cancer Progression and Prognosis

The systemic spread of malignancies and the risk of cancer-associated thrombosis are major clinical challenges in cancer therapy worldwide. As an important post-translational modification enzyme, peptidyl arginine deiminase 4 (PAD4) could mediate the citrullination of protein in different components (including nucleus and cytoplasm, etc.) of a variety of cells (tumor cells, neutrophils, macrophages, etc.), thus participating in gene regulation, neutrophil extracellular trap (NET) and macrophage extracellular trap (MET). Thereby, PAD4 plays an important role in enhancing the growth of primary tumors and facilitating the distant metastasis of cancer cells. In addition, it is related to the formation of cancer-associated thrombosis. Therefore, the development of PAD4-specific inhibitors may be a promising strategy for treating cancer, and it may improve patient prognosis. In this review, we describe PAD4 involvement in gene regulation, protein citrullination, and NET formation. We also discuss its potential role in cancer and cancer-associated thrombosis, and we summarize the development and application of PAD4 inhibitors.

miR‑let‑7c‑3p targeting on Egr‑1 contributes to the committed differentiation of leukemia cells into monocyte/macrophages

In preliminary experiments, it was found that the expression of early growth response-1 (Egr-1) was upregulated during the committed differentiation of leukemia cells into monocytes/macrophages. The cross-analysis of gene chip detection and database prediction indicated that Egr-1 was associated with upstream microRNA (miR)-let-7c-3p, thus the present study focused on the role of the miR-let-7c-3p/Egr-1 signaling axis in the committed differentiation of leukemia cells into monocytes/macrophages. Phorbol 12-myristate 13-acetate (PMA) was used to induce the directed differentiation of human K562 leukemia cells into monocytes/macrophages and the differentiation of K562 leukemia cells was determined by cell morphology observation and expression of differentiation antigens CD11b and CD14 by flow cytometry. The expression levels of Egr-1 and miR-let-7c-3p were detected by reverse transcription-quantitative PCR and the protein expression of Egr-1 was detected by western blotting. The effect of Egr-1 on the differentiation of K562 cells was detected by short interfering (si)RNA interference assay. A dual-luciferase reporter assay was used to detect target binding of miR-let-7c-3p on the 3′UTR of Egr-1. Cell transfection of miR-let-7c-3p mimics and inhibitors was used to modulate the expression of miR-let-7c-3p, as indicated by RT-qPCR assays. Western blotting was also used to examine the effect of miR-let-7c-3p on Egr-1 expression. The PMA-induced differentiation of K562 cells was transfected with miR-let-7c-3p and the expression of differentiation antigen was detected by flow cytometry. A differentiation model of K562 leukemia cells into monocytes/macrophages was induced by PMA, which was indicated by morphological observations and upregulation of CD11b and CD14 antigens. The gene or protein expression of Egr-1 was significantly higher compared with that of the control group, while the expression of miR-let-7c-3p was significantly lower compared with that of the control group. siRNA interference experiments showed that the expression of cell differentiation antigen CD14 in the 100 µg/ml PMA + si-Egr-1 group was significantly lower compared with that in the 100 µg/ml PMA + si-ctrl group. The dual luciferase reporter gene results showed that the luciferase activity of the co-transfected mimic and Egr-1 WT groups was significantly lower than that of the NC control group, while the luciferase activity of the co-transfected mimic and Egr-1 MUT groups was comparable to that of the NC control group. Therefore, the dual-luciferase reporter gene assay confirmed that miR-let-7c-3p can target Egr-1. Western blotting showed that the expression of Egr-1 following transfection with miR-let-7c-3p inhibitor was significantly higher compared with that of the negative control and the expression of Egr-1 after transfection with miR-let-7c-3p mimic was significantly lower than that of the negative control. Following exposure to PMA, the expressions of CD11b and CD14 in the miR-let-7c-3p inhibitor group were significantly higher than those in the miR-let-7c-3p NC group, as indicated by CD11b and CD14 respectively. In conclusion, miR-let-7c-3p could bind to the 3′UTR of Egr-1 and negatively regulated Egr-1 expression. The miR-let-7c-3p/Egr-1 signaling axis was closely associated with the committed differentiation of K562 cells from leukemia cells to monocytes/macrophages.

Dexamethasone, but Not Vitamin D or A, Dampens the Inflammatory Neutrophil Response to Protect At-risk COVID-19 Patients

Dexamethasone (DEX) was the first drug shown to save lives of critically ill coronavirus disease 2019 (COVID-19) patients suffering from respiratory distress. A hyperactivated state of neutrophils was found in COVID-19 patients compared to non-COVID pneumonia cases. Given the beneficial effects of DEX in COVID-19 patients, we investigated the effects of DEX and of other immunomodulatory drugs vitamin D3 (VD3) and retinoic acid (RA) on neutrophil function. DEX, but not VD3 or RA, significantly inhibited all tested aspects of neutrophil function, e.g., degranulation, intracellular ROS production, CXCL8 release and NETosis. Interestingly, RA displayed the opposite effect by significantly increasing both CXCL8 and NET release by neutrophils. Taken together, these data suggest that the lower COVID-19 mortality in DEX-treated patients may in part be due to the dampening effect of DEX on the inflammatory neutrophil response, which could prevent neutrophil plugs with NETS in the lungs and other inflamed organs of patients.

Peptidylarginine deiminases 4 as a promising target in drug discovery

Peptidylarginine deaminase 4 (PAD4) is a crucial post-translational modifying enzyme catalyzing the conversion of arginine into citrulline residues, and mediating the formation of neutrophil extracellular traps (NETs). PAD4 plays a vital role in the occurrence and development of cardiovascular diseases, autoimmune diseases, and various tumors. Therefore, PAD4 is considered as a promising drug target for disease diagnosis and treatment. More and more efforts are devoted to developing highly efficient and selective PAD4 inhibitors via high-throughput screening, structure-based drug design and structure-activity relationship study. This article outlined the physiological and pathological functions of PAD4, and corresponding representative small molecule inhibitors reported in recent years.

Histone citrullination: a new target for tumors

As the main protein components of chromatin, histones play central roles in gene regulation as spools of winding DNA. Histones are subject to various modifications, including phosphorylation, acetylation, glycosylation, methylation, ubiquitination and citrullination, which affect gene transcription. Histone citrullination, a posttranscriptional modification catalyzed by peptidyl arginine deiminase (PAD) enzymes, is involved in human carcinogenesis. In this study, we highlighted the functions of histone citrullination in physiological regulation and tumors. Additionally, because histone citrullination involves forming neutrophil extracellular traps (NETs), the relationship between NETs and tumors was illustrated. Finally, the clinical application of histone citrullination and PAD inhibitors was discussed.

Effects of an Environmentally Relevant Mixture of Organophosphate Esters Derived From House Dust on Endochondral Ossification in Murine Limb Bud Cultures

Organophosphate esters (OPEs) are used widely as flame retardants and plasticizers but much remains unknown about their potential toxicity. Previously, we reported that 4 individual OPEs suppress endochondral ossification in murine limb bud cultures. However, real-life exposure is to complex OPE mixtures. In the present study, we tested the hypothesis that a Canadian household dust-based OPE mixture will affect endochondral ossification in gestation day 13 CD1 mouse embryo limb buds expressing fluorescent markers for the major cell populations involved in the process: collagen type II alpha 1-enhanced cyan fluorescent protein (proliferative chondrocytes), collagen type X alpha 1-mCherry (hypertrophic chondrocytes), and collagen type I alpha 1-yellow fluorescent protein (osteoblasts). Limbs were cultured for 6 days in the presence of vehicle or dilutions of the OPE mixture (1/1 000 000, 1/600 000, and 1/300 000). All 3 OPE mixture dilutions affected cartilage template development and the progression of endochondral ossification, as indicated by the fluorescent markers. The expression of Sox9, the master regulator of chondrogenesis, was unchanged, but the expression of Runx2 and Sp7, which drive chondrocyte hypertrophy and osteoblastogenesis, was dilution-dependently suppressed. RNA-seq revealed that exposure to the 1/300 000 dilution of the OPE mixture for 24 h downregulated 153 transcripts and upregulated 48 others by at least 1.5-fold. Downregulated transcripts were enriched for those related to the immune system and bone formation. In contrast, upregulated transcripts were enriched for those with stress response functions known to be regulated by ATF4 activation. Thus, exposure to the mixture of OPEs commonly found in house dust may have adverse effects on bone formation.

Differentiation of HL-60 cells in serum-free hematopoietic cell media enhances the production of neutrophil extracellular traps

Neutrophil extracellular traps (NETs) are web-like structures made of chromatin and have been identified to have a role in the host's immune defense. Differentiated human promyelocytic leukemia HL-60 cells (dHL-60) have been used to study the mechanisms of NETs formation, as neutrophils have a short lifespan that limits their use. However, dHL-60 cells are inefficient at generating NETs and therefore are not ideal replacements for neutrophils in studying of NET formation. In the present study, the optimal cell culture conditions and differentiation time that result in the most effective release of NETs from dHL-60 cells upon stimulation were determined. HL-60 cells were cultured in serum (FBS) or serum-free (X-VIVO) medium and differentiated using all-trans retinoic acid (ATRA) or dimethyl sulfoxide (DMSO). dHL-60 cells were stimulated with phorbol 12-myristate 13-acetate (PMA) or Ca²⁺ ionophore (CI). Cell differentiation and apoptosis, as well as the formation of reactive oxygen species (ROS) and citrullinated histone H3 (citH3) were analyzed using flow cytometry. NETs were visualized using fluorescence microscopy and NET quantification was performed using PicoGreen. Induction of HL-60 cells for five days produced the best results in terms of differentiation markers and cell viability. Both ATRA- and DMSO-induced dHL-60 cells were able to release NETs upon PMA and CI stimulation; dHL-60 cells in serum-free medium produced more NETs than those in serum-containing medium. DMSO-dHL-60 (X-VIVO) cells were most efficient at producing NETs and ROS upon stimulation with PMA, while ATRA-dHL-60 (X-VIVO) cells were most efficient at producing NETs and citH3 upon stimulation with CI. It was concluded that DMSO-dHL-60 (X-VIVO) may be a model for the study of ROS-high NETosis and ATRA-dHL-60 (X-VIVO) may be suitable for ROS-low NETosis.

Studying Neutrophil Function in vitro: Cell Models and Environmental Factors

Neutrophils are the most abundant immune cell type in the blood and constitute the first line of defense against invading pathogens. Despite their important role in many diseases, they are challenging to study due to their short life span and the inability to cryopreserve or expand them in vitro. Thus, research into neutrophils has to rely on cells freshly isolated from peripheral blood of human donors, introducing donor-dependent variation in the experimental data. To counteract these problems, researchers tried to develop adequate cell models, such as cell lines. For those functional studies that cannot rely on cell models, a standardization of protocols regarding neutrophil purification and culturing could be a solution. In this review, we provide an overview of the most commonly used models for neutrophil function (HL-60, PLB-985, NB4, Kasumi-1 and induced pluripotent stem cells). In addition, we describe the effects of glucose concentration, pH, oxygen tension and temperature on neutrophil function.

Evolving insights on histone methylome regulation in human acute myeloid leukemia pathogenesis and targeted therapy

Acute myeloid leukemia (AML) is an aggressive, disseminated hematological malignancy associated with clonal selection of aberrant self-renewing hematopoietic stem cells and progenitors and poorly differentiated myeloid blasts. The most prevalent form of leukemia in adults, AML is predominantly an age-related disorder and accounts for more than 10,000 deaths per year in the United States alone. In comparison to solid tumors, AML has an overall low mutational burden, albeit more than 70% of AML patients harbor somatic mutations in genes encoding epigenetic modifiers and chromatin regulators. In the past decade, discoveries highlighting the role of DNA and histone modifications in determining cellular plasticity and lineage commitment have attested to the importance of epigenetic contributions to tumor cell de-differentiation and heterogeneity, tumor initiation, maintenance, and relapse. Orchestration in histone methylation levels regulates pluripotency and multicellular development. The increasing number of reversible methylation regulators being identified, including histone methylation writer, reader, and eraser enzymes, and their implications in AML pathogenesis have widened the scope of epigenetic reprogramming, with multiple drugs currently in various stages of preclinical and clinical trials. AML methylome also determines response to conventional chemotherapy, as well as AML cell interaction within a tumor-immune microenvironment ecosystem. Here we summarize the latest developments focusing on molecular derangements in histone methyltransferases (HMTs) and histone demethylases (HDMs) in AML pathogenesis. AML-associated HMTs and HDMs, through intricate crosstalk mechanisms, maintain an altered histone methylation code conducive to disease progression. We further discuss their importance in governing response to therapy, which can be used as a biomarker for treatment efficacy. Finally we deliberate on the therapeutic potential of targeting aberrant histone methylome in AML, examine available small molecule inhibitors in combination with immunomodulating therapeutic approaches and caveats, and discuss how future studies can enable posited epigenome-based targeted therapy to become a mainstay for AML treatment.

DNA demethylation increases NETosis

Neutrophil extracellular traps (NETs) occur during the development of autoimmune diseases, cancer and diabetes. A novel form of cell death that is induced by NETs is called NETosis. Although these diseases are known to have an epigenetic component, epigenetic regulation of NETosis has not previously been explored. In the present study, we investigated the effects of epigenetic change, especially DNA demethylation, on NETosis in neutrophil-like cells differentiated from HL-60 cells, which were incubated for 72 h in the presence of 1.25% DMSO. DMSO-differentiated neutrophil-like cells tended to have increased methylation of genomic DNA. NETosis in the neutrophil-like cells was induced by the treatment with A23187, calcium ionophore, and increased by the addition of the DNMT inhibitor 5-azacytidine (Aza) during differentiation. Interestingly, Aza-stimulated neutrophil-like cell induced NETosis without treatment with A23187. Although reactive oxygen species (ROS), especially superoxide and hypochlorous acid, are important in NETosis induction, treatment with Aza decreased production of ROS, while mitochondria ROS scavenger tended to decrease Aza-induced NETosis. Moreover, the genomic DNA in Aza-stimulated neutrophil-like cell was demethylated, and the expression of peptidylarginine deiminase4 (PAD4) and citrullinated histone H3 (R2+R8+R17) was increased, but myeloperoxidase expression was unaffected. Additionally, PAD4 inhibition tended to decrease Aza-induced NETosis. The DNA demethylation induced by the DNMT inhibitor in neutrophil-like cells enhanced spontaneous NETosis through increasing PAD4 expression and histone citrullination. This study establishes a relationship between NETosis and epigenetics for the first time, and indicates that various diseases implicated to have an epigenetic component might be exacerbated by excessive NETosis also under epigenetic control.

Peptidylarginine Deiminase IV Regulates Breast Cancer Stem Cells via a Novel Tumor Cell-Autonomous Suppressor Role

Peptidylarginine deiminases (PADI) catalyze posttranslational modification of many target proteins and have been suggested to play a role in carcinogenesis. Citrullination of histones by PADI4 was recently implicated in regulating embryonic stem and hematopoietic progenitor cells. Here, we investigated a possible role for PADI4 in regulating breast cancer stem cells. PADI4 activity limited the number of cancer stem cells (CSC) in multiple breast cancer models in vitro and in vivo. Mechanistically, PADI4 inhibition resulted in a widespread redistribution of histone H3, with increased accumulation around transcriptional start sites. Interestingly, epigenetic effects of PADI4 on the bulk tumor cell population did not explain the CSC phenotype. However, in sorted tumor cell populations, PADI4 downregulated expression of master transcription factors of stemness, NANOG and OCT4, specifically in the cancer stem cell compartment, by reducing the transcriptionally activating H3R17me2a histone mark at those loci; this effect was not seen in the non-stem cells. A gene signature reflecting tumor cell-autonomous PADI4 inhibition was associated with poor outcome in human breast cancer datasets, consistent with a tumor-suppressive role for PADI4 in estrogen receptor-positive tumors. These results contrast with known tumor-promoting effects of PADI4 on the tumor stroma and suggest that the balance between opposing tumor cell-autonomous and stromal effects may determine net outcome. Our findings reveal a novel role for PADI4 as a tumor suppressor in regulating breast cancer stem cells and provide insight into context-specific effects of PADI4 in epigenetic modulation. SIGNIFICANCE: These findings demonstrate a novel activity of the citrullinating enzyme PADI4 in suppressing breast cancer stem cells through epigenetic repression of stemness master transcription factors NANOG and OCT4.

Citrullination in Cancer

Posttranslational modifications of proteins have been implicated in pathogenesis of numerous diseases. Arginine deimination (also known as citrullination) has a principal role in progression of rheumatoid arthritis through generation of autoantibodies and exacerbation of the inflammatory response. Recently, multiple research groups provided solid evidence of citrullination being in control of cancer progression; however, there is no comprehensive overview of these findings. This article summarizes and critically reviews the influence of citrullination on different aspects of tumor biology, including (i) regulation of apoptosis and differentiation, (ii) promoting EMT and metastasis, and (iii) potential use of citrullinated antigens for immunotherapy. In addition, (iv) the role of citrullination as a cancer biomarker and (v) implication of neutrophil extracellular traps in tumorigenesis are discussed. In summary, current findings testify to the significance of arginine deimination in tumor biology and thus more basic and translational studies are needed to further explore this topic.

Mathematical models of amino acid panel for assisting diagnosis of children acute leukemia

BACKGROUND

The altered concentrations of amino acids were found in the bone marrow or blood of leukemia patients. Metabolomics technology combining mathematical model of biomarkers could be used for assisting the diagnosis of pediatric acute leukemia (AL).

METHODS

The concentrations of 17 amino acids was measured by targeted liquid chromatograph-tandem mass spectrometry in periphery blood collected using dried blood spots. After evaluation, the mathematical models were further evaluated by prospective clinical validation cohort for AL diagnosis.

RESULTS

The concentrations of 13 in 17 amino acids were statistically different between the periphery blood dried serum dots measured by targeted LC-MS/MS. The receiver operating characteristic analysis for the models of amino acid panel showed that the area under curve for AL diagnosis were 0.848, 0.834 and 0.856 by SVM, RF and XGBoost. The Kappa values in further prospectively evaluated clinical cohort were 0.697, 0.703 and 0.789 (p > 0.05) respectively, and the accuracies for the models were 84.86%, 85.20% and 89.46% respectively with further clinical validation.

CONCLUSIONS

The established mathematical model is a faster, cheaper and more convenient way than conventional methods, and no significant difference on the effect of diagnosis comparing with conventional methods. The mathematical model can be clinically useful for assisting pediatric AL diagnosis.

β-Cypermethrin and its metabolite 3-phenoxybenzoic acid induce cytotoxicity and block granulocytic cell differentiation in HL-60 cells

The most widely used type II pyrethroid is β-cypermethrin (β-CYP), and 3-phenoxybenzoic acid (3-PBA) is one of its primary metabolites. Although CYP has been shown to pose toxic effects in some immune cells, as of now the immunotoxicity of CYP on immune progenitor cells has not been well studied. In this study, we evaluated the immunotoxicity of β-CYP and 3-PBA on the human promyelocytic leukemia cell line, HL-60. Both β-CYP and 3-PBA reduced cell viability. In addition, both β-CYP and 3-PBA stimulated the intrinsic apoptotic pathway in a dose- and time-dependent manner, while only β-CYP induced cell cycle arrest in G1 stage. Moreover, exposure to β-CYP and 3-PBA at 100 μM inhibited all-trans retinoic acid (ATRA)-induced mRNA expressions of the granulocytic differentiation-related genes, CD11b and CSF-3R. Furthermore, exposure to β-CYP and 3-PBA resulted in a downregulation of the granulocytic differentiation promoting transcriptional factors, PU.1 and C/EBPε. Furthermore, we found that β-CYP and 3-PBA exposure led to elevated levels of cellular reactive oxygen species (ROS), and that pretreatment with N-acetylcysteine (NAC) blocked the toxic effects caused by β-CYP and 3-PBA. The results obtained in the present study provide evidence showing the immunotoxic effects of β-CYP and 3-PBA on promyelocytic cells as well as its possible underlying mechanism.

Deubiquitinase USP48 promotes ATRA-induced granulocytic differentiation of acute promyelocytic leukemia cells

All-trans retinoic acid (ATRA) has been used for the treatment of acute promyelocytic leukemia (APL). However, its molecular mechanisms of action are unclear. Ubiquitin-specific protease 48 (USP48) is a deubiquitinase enzyme that can post-translationally remove ubiquitin molecules from substrates. In the present study, the role of USP48 in ATRA-induced differentiation of APL cells was studied. The expression of USP48 decreased following ATRA treatment. Functionally, overexpression of USP48 using electroporation-mediated delivery inhibited the proliferation of APL cells and promoted ATRA-mediated differentiation. The inverse observations were made upon siRNA-mediated knockdown of USP48. Furthermore, the expression of USP48 was increased in the nucleus upon ATRA exposure for ≤24 h, suggesting that USP48 was translocated into the nucleus. Interestingly, regulation of p65, a substrate of USP48, did not contribute to the downstream mechanism of ATRA-induced differentiation of APL cells. In addition, upstream mechanistic studies demonstrated that the expression of USP48 was regulated by microRNA-301a-3p. In conclusion, the present study highlights the function of USP48 in the ATRA-induced granulocytic differentiation of APL cells and provides a theoretical basis for identifying novel targets for differentiation therapy of APL.

RARα2 and PML-RAR similarities in the control of basal and retinoic acid induced myeloid maturation of acute myeloid leukemia cells

Treatment of acute promyelocytic leukemia (APL) with all-trans retinoic acid (ATRA) is the first example of targeted therapy. In fact, the oncogenic fusion-protein (PML-RAR) typical of this leukemia contains the retinoid-nuclear-receptor RARα. PML-RAR is responsible for the differentiation block of the leukemic blast. Besides PML-RAR, two endogenous RARα proteins are present in APL blasts, i.e. RARα1 and RARα2. We developed different cell populations characterized by PML-RAR, RARα2 and RARα1 knock-down in the APL-derived NB4 cell-line. Unexpectedly, silencing of PML-RAR and RARα2 results in similar increases in the constitutive expression of several granulocytic differentiation markers. This is accompanied by enhanced expression of the same granulocytic markers upon exposure of the NB4 blasts to ATRA. Silencing of PML-RAR and RARα2 causes also similar perturbations in the whole genome gene-expression profiles of vehicle and ATRA treated NB4 cells. Unlike PML-RAR and RARα2, RARα1 knock-down blocks ATRA-dependent induction of several granulocytic differentiation markers. Many of the effects on myeloid differentiation are confirmed by over-expression of RARα2 in NB4 cells. RARα2 action on myeloid differentiation does not require the presence of PML-RAR, as it is recapitulated also upon knock-down in PML-RAR-negative HL-60 cells. Thus, relative to RARα1, PML-RAR and RARα2 exert opposite effects on APL-cell differentiation. These contrasting actions may be related to the fact that both PML-RAR and RARα2 interact with and inhibit the transcriptional activity of RARα1. The interaction surface is located in the carboxy-terminal domain containing the D/E/F regions and it is influenced by phosphorylation of Ser-369 of RARα1.

The influence of agents differentiating HL-60 cells toward granulocyte-like cells on their ability to release neutrophil extracellular traps

Studies on neutrophil extracellular traps (NETs) are challenging as neutrophils live shortly and easily become activated. Thus, availability of a cell line model closely resembling the functions of peripheral blood neutrophils would be advantageous. Our purpose was to find a compound that most effectively differentiates human promyelocytic leukemia (HL-60) cells toward granulocyte-like cells able to release NETs. HL-60 cells were differentiated with all-trans retinoic acid (ATRA), dimethyl sulfoxide (DMSO) or dimethylformamide (DMF) and stimulated with phorbol 12-myristate 13-acetate (PMA) or calcium ionophore A23187 (CI). Cell differentiation, phagocytosis and calcium influx were analyzed by flow cytometry. Reactive oxygen species production and NETs release were measured fluorometrically and analyzed microscopically. LC3-II accumulation and histone 3 citrullination were analyzed by western blot. ATRA most effectively differentiated HL-60 cells toward granulocyte-like cells. ATRA-dHL-60 cells released NETs only upon PMA stimulation, DMSO-dHL-60 cells only post CI stimulation, while DMF-dHL-60 cells formed NETs in response to both stimuli. Oxidative burst was induced in ATRA-, DMSO- and DMF-dHL-60 cells post PMA stimulation and only in DMF-dHL-60 cells post CI stimulation. Increased histone 3 citrullination was observed in stimulated DMSO- and DMF-, but not in ATRA-dHL-60 cells. The calcium influx was diminished in ATRA-dHL-60 cells. Significant increase in autophagosomes formation was observed only in PMA-stimulated DMF-dHL-60 cells. Phagocytic index was higher in ATRA-dHL-60 cells than in control, DMSO- and DMF-dHL-60 cells. We conclude that ATRA, DMSO and DMF differentiate HL-60 in different mechanisms. DMF is the best stimulus for HL-60 cell differentiation for NETs studies.

Overexpression of SOX4 correlates with poor prognosis of acute myeloid leukemia and is leukemogenic in zebrafish

The SOX4 transcription factor is a key regulator of embryonic development, cell-fate decision, cellular differentiation and oncogenesis. Abnormal expression of SOX4 is related to malignant tumor transformation and cancer metastasis. However, no reports are available regarding the clinical significance of SOX4 in acute myeloid leukemia (AML) and the role of SOX4 in leukemogenesis. In the current study, we found that AML patients with low bone marrow (BM) SOX4 expression had higher remission rates and longer overall survival than those with high SOX4 expression, regardless of age, white blood cell count at diagnosis, karyotype profile and NPM1/FLT3-ITD status. To elucidate the role of SOX4 in leukemogenesis, we generated a transgenic zebrafish model that overexpressed human SOX4 in the myeloid lineage Tg(spi1-SOX4-EGFP). These transgenic zebrafish showed, at 5 months of age, increased myelopoiesis with dedifferentiation in kidney marrow. At 9 months of age, their kidney structure was significantly effaced and distorted by increased infiltration of myeloid progenitor cells. These results suggest that SOX4 is not only an independent prognostic factor of AML, but also an important molecular factor in leukemogenesis.

Role of citrullination modification catalyzed by peptidylarginine deiminase 4 in gene transcriptional regulation

Peptidylarginine deiminase 4 (PADI4), a new histone modification enzyme, which converts both arginine and monomethyl-arginine to citrulline, has gained massive attention in recent years as a potential regulator of gene transcription. Recent studies have shown that arginine residues R2, R8, R17, and R26 in the H3 tail and R3 in the H4 tail can be deiminated by PADI4. This kind of histone post-translational modification has the potential to antagonize histone methylation and coordinate with histone deacetylation to regulate gene transcription. PADI4 also deiminates non-histone proteins, such as p300, NPM1, ING4, RPS2, and DNMT3A. PADI4 has been shown to involve in cell apoptosis and differentiation. Moreover, PADI4 can interact with tumor suppressor p53 and regulate the transcriptional activity of p53. Dysregulation of PADI4 is implicated in a variety of diseases, including rheumatoid arthritis, tumor development, and multiple sclerosis. A wide variety of PADI4 inhibitors have been identified. Further understanding of PADI4 functions may lead to novel diagnostic and therapeutic approaches in these diseases. This review summarizes the recent progress in the study of the regulation mechanism of PADI4 on gene transcription and the major physiological functions of PADI4 in human diseases.

Effect of NADPH-cytochrome P450 reductase on all-trans-retinoic acid efficacy and cytochrome P450 26A1 expression in human myeloid leukaemia HL-60 cells

OBJECTIVES

All-trans-retinoic acid (ATRA), a naturally occurring metabolite of vitamin A, has been shown to have great potential as an antitumorigenic drug to treat acute leukaemia by promoting cancer cell differentiation. Cytochrome P450 oxidoreductase (POR) is the only obligate electron donor for all of the microsomal cytochrome P450 enzymes including CYP26A1 which is highly specific for ATRA metabolism and efficacy in human myeloid leukaemia cells. In this study, we aimed to investigate the effect of POR on ATRA efficacy and CYP26A1 expression in human myeloid leukaemia HL-60 cells.

METHODS

Stably expressed POR and POR-RNAi HL-60 cell lines were established by transfecting POR overexpression or RNAi (RNA interference) vectors mediated by lentivirus. The protein expression of POR and CYP26A1 was examined by Western blot. The potential roles of POR on ATRA efficacy in HL-60 cells were explored by cell viability assay, cell cycle distribution, cellular differentiation and apoptosis analysis.

KEY FINDINGS

All-trans-retinoic acid treatment caused the expression of POR upregulation and CYP26A1 downregulation in dose- and time-dependent manners. POR overexpression decreased CYP26A1 expression in HL-60 cells. When POR gene was interfered, the downregulation of CYP26A1 expression by ATRA was abolished. In addition, POR overexpression in HL-60 cells significantly compromised ATRA-induced cell proliferation inhibition, cell cycle arrest, differentiation and apoptosis, whereas downregulation of POR significantly potentiated ATRA effects.

CONCLUSIONS

Our study therefore suggested that POR played an important role in regulating ATRA efficacy and CYP26A1 expression in HL-60 cells.