Overexpression of PAD4 suppresses drug resistance of NSCLC cell lines to gefitinib through inhibiting Elk1-mediated epithelial-mesenchymal transition

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.

Targeting PADI2 as a potential therapeutic strategy against metastasis in oral cancer via suppressing EMT-mediated migration and invasion and CCL3/5-induced angiogenesis

Oral squamous cell carcinoma (OSCC) is a prevalent and aggressive malignancy, with metastasis being the leading cause of death in patients. Unfortunately, therapeutic options for metastatic OSCC remain limited. Peptidylarginine deiminases (PADI) are implicated in various tumorigenesis and metastasis processes across multiple cancers. However, the role of PADI2, a type of PADI, in OSCC is not well understood. This study aimed to explore the impact of PADI2 on epithelial-mesenchymal transition (EMT), angiogenesis, and OSCC metastasis. The effect of PADI2 on EMT was evaluated using cell lines by Western blot analysis with shRNA targeting PADI2. In addition, the selective PADI2 inhibitor AFM32a was used to assess the effect of PADI2 on cancer metastasis and angiogenesis in animal models. Our findings indicated that PADI2 expression correlated with EMT changes, and PADI2 knockdown reversed these changes, reducing cell proliferation, cell migration, and invasion. PADI2 inhibition also diminished tube formation in HUVECs and decreased secretion of angiogenesis-related chemokines CCL3, CCL5 and CCL20. In a mouse model, AFM32a markedly reduced lung metastasis and production of CCL3 and CCL5. Our in vitro and in vivo studies suggested inhibiting PADI2 could prevent OSCC metastasis by impeding EMT and angiogenesis via AKT/mTOR signaling pathway. These results highlight PADI2 as a potential therapeutic target for combating OSCC metastasis.

Citrullinating enzyme PADI4 and transcriptional repressor RING1B bind in cancer cells

Polycomb groups (PcGs) are transcriptional repressors, formed by a complex of several proteins, involved in multicellular development and cancer epigenetics. One of these proteins is the E3 ubiquitin-protein ligase RING1 (or RING1B), associated with the regulation of transcriptional repression and responsible for monoubiquitylation of the histone H2A. On the other hand, PADI4 is one of the human isoforms of a family of enzymes implicated in the conversion of arginine to citrulline, and it is also involved in the development of glioblastoma, among other types of cancers. In this work, we showed the association of PADI4 and RING1B in the nucleus and cytosol in several cancer cell lines by using immunofluorescence and proximity ligation assays. Furthermore, we demonstrated that binding was hampered in the presence of GSK484, an enzymatic PADI4 inhibitor, suggesting that RING1B could bind to the active site of PADI4, as confirmed by protein-protein docking simulations. In vitro and in silico findings showed that binding to PADI4 occurred for the isolated fragments corresponding to both the N-terminal (residues 1-221) and C-terminal (residues 228-336) regions of RING1B. Binding to PADI4 was also hampered by GSK484, as shown by isothermal titration calorimetry (ITC) experiments for the sole N-terminal region, and by both NMR and ITC for the C-terminal one. The dissociation constants between PADI4 and any of the two isolated RING1B fragments were in the low micromolar range (~2-10 μM), as measured by fluorescence and ITC. The interaction between RING1B and PADI4 might imply citrullination of the former, leading to several biological consequences, as well as being of potential therapeutic relevance for improving cancer treatment with the generation of new antigens.

Polyamines: the pivotal amines in influencing the tumor microenvironment

Cellular proliferation, function and survival is reliant upon maintaining appropriate intracellular polyamine levels. Due to increased metabolic needs, cancer cells elevate their polyamine pools through coordinated metabolism and uptake. High levels of polyamines have been linked to more immunosuppressive tumor microenvironments (TME) as polyamines support the growth and function of many immunosuppressive cell types such as MDSCs, macrophages and regulatory T-cells. As cancer cells and other pro-tumorigenic cell types are highly dependent on polyamines for survival, pharmacological modulation of polyamine metabolism is a promising cancer therapeutic strategy. This review covers the roles of polyamines in various cell types of the TME including both immune and stromal cells, as well as how competition for nutrients, namely polyamine precursors, influences the cellular landscape of the TME. It also details the use of polyamines as biomarkers and the ways in which polyamine depletion can increase the immunogenicity of the TME and reprogram tumors to become more responsive to immunotherapy.

ELK1/MTOR/S6K1 Pathway Contributes to Acquired Resistance to Gefitinib in Non-Small Cell Lung Cancer

The development of acquired resistance to small molecule tyrosine kinase inhibitors (TKIs) targeting epidermal growth factor receptor (EGFR) signaling has hindered their efficacy in treating non-small cell lung cancer (NSCLC) patients. Our previous study showed that constitutive activation of the 70 kDa ribosomal protein S6 kinase 1 (S6K1) contributes to the acquired resistance to EGFR-TKIs in NSCLC cell lines and xenograft tumors in nude mice. However, the regulatory mechanisms underlying S6K1 constitutive activation in TKI-resistant cancer cells have not yet been explored. In this study, we recapitulated this finding by taking advantage of a gefitinib-resistant patient-derived xenograft (PDX) model established through a number of passages in mice treated with increasing doses of gefitinib. The dissociated primary cells from the resistant PDX tumors (PDX-R) displayed higher levels of phosphor-S6K1 expression and were resistant to gefitinib compared to cells from passage-matched parental PDX tumors (PDX-P). Both genetic and pharmacological inhibition of S6K1 increased sensitivity to gefitinib in PDX-R cells. In addition, both total and phosphorylated mechanistic target of rapamycin kinase (MTOR) levels were upregulated in PDX-R and gefitinib-resistant PC9G cells. Knockdown of MTOR by siRNA decreased the expression levels of total and phosphor-S6K1 and increased sensitivity to gefitinib in PDX-R and PC9G cells. Moreover, a transcription factor ELK1, which has multiple predicted binding sites on the MTOR promoter, was also upregulated in PDX-R and PC9G cells, while the knockdown of ELK1 led to decreased expression of MTOR and S6K1. The chromatin immunoprecipitation (ChIP)-PCR assay showed the direct binding between ELK1 and the MTOR promoter, and the luciferase reporter assay further indicated that ELK1 could upregulate MTOR expression through tuning up its transcription. Silencing ELK1 via siRNA transfection improved the efficacy of gefitinib in PDX-R and PC9G cells. These results support the notion that activation of ELK1/MTOR/S6K1 signaling contributes to acquired resistance to gefitinib in NSCLC. The findings in this study shed new light on the mechanism for acquired EGFR-TKI resistance and provide potential novel strategies by targeting the ELK1/MTOR/S6K1 pathway.

GSK484, an inhibitor of peptidyl arginine deiminase 4, increases the radiosensitivity of colorectal cancer and inhibits neutrophil extracellular traps

INTRODUCTION

Colorectal cancer (CRC) is the third most common malignancy and a major cause of cancer-related deaths. Peptidyl arginine deiminase 4 (PAD4 or PADI4) is expressed in neutrophils that, when activated, can drive the formation of neutrophil extracellular traps (NETs). PAD4 has been found to be upregulated in CRC patients and to predict a poor prognosis. This study is aimed at exploring the role of PAD4 inhibitor (GSK484) in NET formation and radioresistance in CRC.

METHODS

Reverse transcriptase quantitative polymerase chain reaction and western blotting were used to measure PAD4 expression in CRC tissues and cells. GSK484, an inhibitor of PAD4, was investigated in the following functional assays in vitro: western blotting, clonogenic survival, colony formation, TUNEL, flow cytometry and transwell assays. Nude mouse xenograft models were applied to evaluate the effect of GSK484 on tumor growth in CRC in vivo. The formation of NETs influenced by GSK484 was also investigated.

RESULTS

We showed upregulation of PAD4 mRNA and protein in CRC tissues and cells. High expression of PAD4 was related to a poor prognosis in CRC patients. GSK484 treatment promoted the radiosensitivity of CRC cells and induced cell death by promoting DNA double-strand breaks. Rescue experiments further verified that GSK484 inhibited the effects of PAD4 overexpression in irradiated CRC cells. Moreover, GSK484 injection strengthened the radiosensitivity of CRC and inhibited NET formation in vivo.

CONCLUSIONS

PAD4 inhibitor GSK484 promotes the radiosensitivity of CRC and inhibits NET formation in vivo and in vitro.

Targeting epigenetic regulators to overcome drug resistance in cancers

Drug resistance is mainly responsible for cancer recurrence and poor prognosis. Epigenetic regulation is a heritable change in gene expressions independent of nucleotide sequence changes. As the common epigenetic regulation mechanisms, DNA methylation, histone modification, and non-coding RNA regulation have been well studied. Increasing evidence has shown that aberrant epigenetic regulations contribute to tumor resistance. Therefore, targeting epigenetic regulators represents an effective strategy to reverse drug resistance. In this review, we mainly summarize the roles of epigenetic regulation in tumor resistance. In addition, as the essential factors for epigenetic modifications, histone demethylases mediate the histone or genomic DNA modifications. Herein, we comprehensively describe the functions of the histone demethylase family including the lysine-specific demethylase family, the Jumonji C-domain-containing demethylase family, and the histone arginine demethylase family, and fully discuss their regulatory mechanisms related to cancer drug resistance. In addition, therapeutic strategies, including small-molecule inhibitors and small interfering RNA targeting histone demethylases to overcome drug resistance, are also described.

DNA and histone modifications as potent diagnostic and therapeutic targets to advance non-small cell lung cancer management from the perspective of 3P medicine

Lung cancer has a very high mortality in females and males. Most (~ 85%) of lung cancers are non-small cell lung cancers (NSCLC). When lung cancer is diagnosed, most of them have either local or distant metastasis, with a poor prognosis. In order to achieve better outcomes, it is imperative to identify the molecular signature based on genetic and epigenetic variations for different NSCLC subgroups. We hypothesize that DNA and histone modifications play significant roles in the framework of predictive, preventive, and personalized medicine (PPPM; 3P medicine). Epigenetics has a significant impact on tumorigenicity, tumor heterogeneity, and tumor resistance to chemotherapy, targeted therapy, and immunotherapy. An increasing interest is that epigenomic regulation is recognized as a potential treatment option for NSCLC. Most attention has been paid to the epigenetic alteration patterns of DNA and histones. This article aims to review the roles DNA and histone modifications play in tumorigenesis, early detection and diagnosis, and advancements and therapies of NSCLC, and also explore the connection between DNA and histone modifications and PPPM, which may provide an important contribution to improve the prognosis of NSCLC. We found that the success of targeting DNA and histone modifications is limited in the clinic, and how to combine the therapies to improve patient outcomes is necessary in further studies, especially for predictive diagnostics, targeted prevention, and personalization of medical services in the 3P medicine approach. It is concluded that DNA and histone modifications are potent diagnostic and therapeutic targets to advance non-small cell lung cancer management from the perspective of 3P medicine.

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.

PAD4-dependent citrullination of nuclear translocation of GSK3β promotes colorectal cancer progression via the degradation of nuclear CDKN1A

Peptidylarginine deiminase 4 (PAD4), a Ca²⁺-dependent enzyme, catalyzes the conversion of arginine to citrulline and has been strongly associated with many malignant tumors. However, the molecular mechanisms of PAD4 in the development and progression of colorectal cancer (CRC) remain unclearly defined. In our study, PAD4 expression was increased in CRC tissues and cells, and was closely related to tumor size, lymph node metastasis. Moreover, the transcription factor KLF9 directly bound to PADI4 gene promoter, leading to overexpression of PAD4 in CRC cells, which augmented cell growth and migration. We revealed that PAD4 interacted with and citrullinated glycogen synthase kinase-3β (GSK3β) in CRC cells, and GSK3β Arg-344 was the dominating PAD4-citrullination site. Furthermore, IgL2 and catalytic domains of PAD4 directly bound to the kinase domain of GSK3β in CRC cells. Mechanistically, PAD4 promoted the transport of GSK3β from the cytoplasm to the nucleus, thereby increasing the ubiquitin-dependent proteasome degradation of nuclear cyclin-dependent kinase inhibitor 1 (CDKN1A). Our study is the first to reveal the details of a critical PAD4/GSK3β/CDKN1A signaling axis for CRC progression, and provides evidence that PAD4 is a potential diagnosis biomarker and therapeutic target in CRC.

PADI4 promotes epithelial-mesenchymal transition(EMT) in gastric cancer via the upregulation of interleukin 8

Background

Gastric cancer (GC) is one of the deadliest tumours due to its ability to metastasize. The Epithelial–to-mesenchymal transition plays a crucial role in promoting the GC metastasis, which increases the migration and metastasis of tumour cells. Peptidyl arginine deiminase IV (PADI4) is a susceptibility gene for gastric carcinoma. The aim of this study was to evaluate the functional roles of PADI4 in gastric cancer.

Methods

The expression of PADI4 was examined by qRT-PCR, western blot and immunohistochemistry. In addition, the functional roles of PADI4 were explored by over-expression PADI4 plasmids in gastric cancer cells.

Results

We found that the expression of PADI4 was up-regulated in GC. PADI4 overexpression in GC cells increased the proliferation, migration, metastasis, clone forming ability, and tumorigenic ability, but reduced the apoptosis ability. The Multi-Analyte ELISArray Kit results showed that interleukin 8 (IL-8) is upregulated in PADI4-overexpressing gastric cells. Using short interfering RNA (siRNA) to silence the expression of IL-8, we demonstrated that IL-8 silencing significantly inhibited the increased migratory capacity in PADI4-overexpressing GC cells.

Conclusions

Our data suggest that PADI4 accelerate metastasis by promoting IL-8 expression in gastric cancer cells, indicating that it is a new PADI4/IL-8 signalling pathway in metastatic GC.

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.

PADI1 contributes to EMT in PAAD by activating the ERK1/2-p38 signaling pathway

Background

Peptidylarginine deiminase 1 (PADI1) has been reported to promote tumorigenesis in breast cancer. However, the functional role of PADI1 in pancreatic ductal adenocarcinoma (PAAD) has remained elusive until now.

Methods

The expression pattern of PADI1 in PAAD tissues and normal tissues was analyzed using The Cancer Genome Atlas (TCGA) dataset. A Kaplan-Meier curve analysis was performed to evaluate the prognostic value of PADI1 in PAAD patients. PADI1 was knocked down in CFPAN-1 and HPAC cells, and overexpressed in PANC-1 and Bxpc-3 cells by RNA interference. A wound-healing assay was performed to analyze relative cell migration distance. Cell migration and invasion were assessed by a Transwell assay. Related protein expression levels were measured by western blot and immunofluorescence.

Results

The bioinformatics analysis showed that PADI1 was overexpressed in PAAD tissues and associated with a poor survival prognosis. The knockdown of PADI1 suppressed cell migration and invasion, and activated the ERK1/2-p38 signaling pathway in CFPAN-1 and HPAC cells. The overexpression of PADI1 produced the opposite results in PANC-1 and Bxpc-3 cells. Additionally, treatment with an MEK1/2 inhibitor significantly attenuated the effects of PADI1 knockdown on cell migration, invasion, the epithelial-mesenchymal transition (EMT) process, and p-ERK1/2 and p38 expression in CFPAN-1 and HPAC cells.

Conclusions

Our data suggested that PADI1 may function as an oncogene in regulating metastasis in vitro in PAAD.

The PAD4 inhibitor GSK484 enhances the radiosensitivity of triple-negative breast cancer

Triple-negative breast cancer (TNBC) accounts for approximately 10-20% of all breast cancers and is one of the leading causes of mortality among females. Radiotherapy is essential during the treatment of breast cancer. Growing evidence has indicated that peptidyl arginine deiminase-4 (PAD4) inhibitor can alleviate the development of multiple cancers, including breast cancer, through inhibiting cell proliferation. GSK484 is considered to be a highly potent PAD4-selective inhibitors. However, the potential role and mechanism of GSK484 in TNBC remain unclear. In this study, we intended to explore the effects of GSK484 on the radiosensitivity of TNBC cell lines (MDA-MB-231 and BT-549). We found that the pretreatment of GSK484 enhanced irradiation (IR)-induced inhibitory effects on cell proliferation, migration and invasion. Besides, our findings revealed that GSK484 facilitated TNBC cell apoptosis. IR treatment-induced increase of the protein level of ATG5 and ATG7, and decrease of p62 protein level were countervailed by GSK484. In addition, GSK484 enhanced DNA damage induced by IR. Moreover, in vivo experiments demonstrated that the combined treatment of IR and GSK484 showed an obvious decline of tumor growth in contrast to IR-alone or GSK484-alone treatment. Overall, GSK484 may serve as a radiosensitizer of TNBC through inhibiting IR-induced autophagy.

PADs in cancer: Current and future

Protein arginine deiminases (PADs), is a group of calcium-dependent enzymes, which play crucial roles in citrullination, and can catalyze arginine residues into citrulline. This chemical reaction induces citrullinated proteins formation with altered structure and function, leading to numerous pathological diseases, including inflammation and autoimmune diseases. To date, multiple studies have provided solid evidence that PADs are implicated in cancer progression. Nevertheless, the findings on PADs functions in tumors are too complex to understand due to its involvements in variable signaling pathways. The increasing interest in PADs has heightened the need for a comprehensive description for its role in cancer. The present study aims to identify the gaps in present knowledge, including its structures, biological substrates and tissue distribution. Since several irreversible inhibitors for PADs with good potency and selectivity have been explored, the mechanisms on the dysregulation in tumors remain poorly understood. The present study discusses the relationship between PADs and tumor apoptosis, EMT formation and metastasis as well as the implication of neutrophil extracellular traps (NETs) in tumorigenesis. In addition, the potential uses of citrullinated antigens for immunotherapy were proposed.

Overexpression of CTEN is associated with gefitinib resistance in non-small cell lung cancer

COOH-terminus tensin-like molecule (CTEN) is a member of the tensin family, which is considered to be one of the novel proto-oncogenes involved in tumorigenesis and cancer progression. However, the mechanisms of CTEN in acquired resistance of non-small cell lung cancer (NSCLC) remain relatively unknown. The aim of the present study was to understand the roles of CTEN in acquired gefitinib resistance of NSCLC. The present study investigated the expression level of CTEN using reverse transcription-quantitative polymerase chain reaction and Western blot analysis. Cell Counting kit-8 and colony-formation assays were performed to evaluate the proliferative and colony-formative abilities of PC9 and PC9/GR cells in vitro. Mouse xenograft models were used to assess the growth of PC9/GR cells in vivo. A gefitinib-resistant NSCLC cell line (PC9/GR) was established, and the protein and mRNA expression levels of CTEN were observed to be higher in PC9/GR cells than in PC9 cells. Notably, the sensitivity of PC9/GR cells to gefitinib was observed to be decreased when CTEN was overexpressed, while PC9/GR cells with CTEN-downregulation showed markedly enhanced sensitivity to gefitinib. In vitro proliferation and colony formation assays revealed that increased CTEN markedly promoted the cell proliferative and colony-forming capacities of PC9 and PC9/GR cells, and CTEN-silencing inhibited the cell proliferative and colony-forming abilities of the PC9 and PC9/GR cells. Notably, deficient expression of CTEN notably retarded the growth of PC9/GR xenografts in vivo. In addition, the plasma mRNA expression of CTEN was notably elevated in patients with NSCLC with acquired gefitinib resistance. Overexpression of CTEN is associated with acquired gefitinib resistance in NSCLC. CTEN may be investigated as a potential therapeutic target for the treatment of patients with NSCLC with acquired gefitinib resistance.

TAT-Modified Gold Nanoparticles Enhance the Antitumor Activity of PAD4 Inhibitors

Purpose

Histone citrullination by peptidylarginine deiminases 4 (PAD4) regulates the gene expression of tumor suppressor. In our previously study, YW3-56 (356) was developed as a potent PAD4 inhibitor for cancer therapy with novel function in the autophagy pathway. To enhance the antitumor activity, the PAD4 inhibitor 356 was modified by the well-established cationic penetrating peptide RKKRRQRRR (peptide TAT) and gold nanoparticles to obtain 356-TAT-AuNPs which could enhance the permeability of chemical drug in solid tumor.

Methods

356-TAT-AuNPs were prepared, and their morphology were characterized. The antitumor activity of 356-TAT-AuNPs was evaluated in vitro and in vivo.

Results

356-TAT-AuNPs exhibited higher anticancer activity against HCT-116, MCF-7 and A549 cells than 356 and 356-AuNPs. Compared with 356 and 356-AuNPs, 356-TAT-AuNPs entered the cytoplasm and nuclear, exhibited stronger anticancer activity by increasing apoptosis, inducing autophagy and inhibiting of histone H3 citrullination, and in HCT-116 xenograft mouse model, 356-TAT-AuNPs could improve the antitumor activity.

Conclusion

The modified AuNPs with peptide TAT as drug delivery system are potent in delaying tumor growth and could be a powerful vehicle for profitable anticancer drug development. We believe that peptide TAT modification strategy may provide a simple and valuable method for improving antitumor activity of PAD4 inhibitors for clinical use.

PADI4 modulates the invasion and migration of osteosarcoma cells by down-regulation of epithelial-mesenchymal transition

Osteosarcoma (OS) is the most common type of primary bone malignancy with high recurrence and metastasis. Peptidylarginine deiminase 4 (PADI4), as an important protein post-translational modification enzyme, has been identified as a potential regulator in the invasion and migration in several types of tumors. The role of PADI4 in osteosarcoma metastasis remains unknown. In this study, we revealed significant positive correlation between PADI4 and pulmonary metastasis of osteosarcoma. Wound-healing and transwell assay indicated that PADI4 induced invasion and migration of osteosarcoma cell in vitro while PADI4 inhibitor has repressive effect. PADI4 mutation with no deimination activity exhibited no significant effect on invasion and migration of osteosarcoma cells. Moreover, we evaluated the effect of PADI4 on expression of the markers of epithelial-mesenchymal transition and results showed that PADI4 promoted EMT while PADI4 inhibitor suppressed EMT in osteosarcoma cells. We also detected the expression of PADI4 and E-Cadherin in the tissues of osteosarcoma patients with or without pulmonary metastasis. Results showed positive relationship between the expression of PADI4 and osteosarcoma metastasis. In contrast, the expression of E-Cadherin exhibited negative correlation with PADI4 and osteosarcoma metastasis. Our research offered a novel link between PADI4 and osteosarcoma metastasis and demonstrated PADI4 as a promising target for treatment of osteosarcoma metastasis.

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.

Colorectal cancer liver metastatic growth depends on PAD4-driven citrullination of the extracellular matrix

Citrullination of proteins, a post-translational conversion of arginine residues to citrulline, is recognized in rheumatoid arthritis, but largely undocumented in cancer. Here we show that citrullination of the extracellular matrix by cancer cell derived peptidylarginine deiminase 4 (PAD4) is essential for the growth of liver metastases from colorectal cancer (CRC). Using proteomics, we demonstrate that liver metastases exhibit higher levels of citrullination and PAD4 than unaffected liver, primary CRC or adjacent colonic mucosa. Functional significance for citrullination in metastatic growth is evident in murine models where inhibition of citrullination substantially reduces liver metastatic burden. Additionally, citrullination of a key matrix component collagen type I promotes greater adhesion and decreased migration of CRC cells along with increased expression of characteristic epithelial markers, suggesting a role for citrullination in promoting mesenchymal-to-epithelial transition and liver metastasis. Overall, our study reveals the potential for PAD4-dependant citrullination to drive the progression of CRC liver metastasis.

Lung Cancer Therapy Targeting Histone Methylation: Opportunities and Challenges

Lung cancer is one of the most common malignancies. In spite of the progress made in past decades, further studies to improve current therapy for lung cancer are required. Dynamically controlled by methyltransferases and demethylases, methylation of lysine and arginine residues on histone proteins regulates chromatin organization and thereby gene transcription. Aberrant alterations of histone methylation have been demonstrated to be associated with the progress of multiple cancers including lung cancer. Inhibitors of methyltransferases and demethylases have exhibited anti-tumor activities in lung cancer, and multiple lead candidates are under clinical trials. Here, we summarize how histone methylation functions in lung cancer, highlighting most recent progresses in small molecular inhibitors for lung cancer treatment.

MicroRNA-873 inhibits colorectal cancer metastasis by targeting ELK1 and STRN4

MicroRNAs (miRNAs) are a group of small non-coding RNAs that directly bind to the 3ʹ-untranslated-region (3ʹUTR) of mRNA, thereby blocking gene expression post-transcriptionally. Accumulating evidence prove that microRNA-873 (miR-873) functions as a promoter or suppressor in various cancers, while whether it affects the progression of colorectal cancer (CRC) is yet unknown. Here we found that miR-873 was downregulated in human CRC clinical samples, mouse CRC specimens and cell lines with high metastatic potential. We also demonstrated that low miR-873 expression was closely associated with poor prognosis of CRC. Overexpressing miR-873 suppressed proliferation and metastasis of CRC cells both in vitro and in vivo, while inhibiting miR-873 expression promoted the proliferation, migration and invasion in vitro. Moreover, miR-873 exerted its function by perturbing the ERK-CyclinD1 pathway and the epithelial-mesenchymal transition (EMT) process. Furthermore, we revealed that miR-873 acted as a tumor-suppressive microRNA by directly binding to the 3ʹUTRs of ELK1 and STRN4 and suppressed their expression. Our study uncovered an inhibitory role of miR-873 in CRC progression and might provide a promising marker for CRC diagnosis and prognosis.

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.