UBE1L is a retinoid target that triggers PML/RARalpha degradation and apoptosis in acute promyelocytic leukemia

The diverse repertoire of ISG15: more intricate than initially thought

ISG15, the product of interferon (IFN)-stimulated gene 15, is the first identified ubiquitin-like protein (UBL), which plays multifaceted roles not only as a free intracellular or extracellular molecule but also as a post-translational modifier in the process of ISG15 conjugation (ISGylation). ISG15 has only been identified in vertebrates, indicating that the functions of ISG15 and its conjugation are restricted to higher eukaryotes and have evolved with IFN signaling. Despite the highlighted complexity of ISG15 and ISGylation, it has been suggested that ISG15 and ISGylation profoundly impact a variety of cellular processes, including protein translation, autophagy, exosome secretion, cytokine secretion, cytoskeleton dynamics, DNA damage response, telomere shortening, and immune modulation, which emphasizes the necessity of reassessing ISG15 and ISGylation. However, the underlying mechanisms and molecular consequences of ISG15 and ISGylation remain poorly defined, largely due to a lack of knowledge on the ISG15 target repertoire. In this review, we provide a comprehensive overview of the mechanistic understanding and molecular consequences of ISG15 and ISGylation. We also highlight new insights into the roles of ISG15 and ISGylation not only in physiology but also in the pathogenesis of various human diseases, especially in cancer, which could contribute to therapeutic intervention in human diseases.

Role of ISG15 post-translational modification in immunity against Mycobacterium tuberculosis infection

ISG15 encoded by a type I interferon (IFN) inducible gene mediates an important cellular process called ISGylation. ISGylation emerges as a powerful host tactic against intracellular pathogens like Mycobacterium tuberculosis (Mtb). However, the exact role of ISGylation in immunity remains elusive. To shed light on how ISGylation, which is both interesting and complex, participates in immunity against Mtb, this manuscript summarized the current knowledge about the structural characteristics and targets of ISG15 and how ISGylation cross-talks with other host post-translational modifications to exert its effect.

Which novel agents will have a clinically meaningful impact in AML at diagnosis?

New drug approvals now afford AML physicians a wider choice of initial treatment options than ever before. Although chemotherapy for AML is by no means ready to be replaced entirely by novel agents, the role of traditional cytotoxics in AML therapy is rapidly changing. In particular, biologically targeted agents such as the BCL2 inhibitor venetoclax and inhibitors of FLT3 and IDH mutations stand out as drugs likely to take AML therapy in important new directions. Maximum response and survival benefits likely require combinations of novel agents and chemotherapy or multiple novel agents together. The recently-published phase 3 VIALE-A study demonstrates a very successful example of a new combination approach, which led to venetoclax plus azacitidine establishing itself as the new standard of care for patients unfit for intensive chemotherapy. One could reasonably expect other subsets of AML to benefit from this regimen or other applications of venetoclax combinations. Building on this experience, venetoclax-based regimens also have the potential to replace standard intensive cytarabine/anthracycline "7&3" induction approach for some if not many patients who are fit for induction. This review will describe novel agents with the greatest potential for impactful frontline applications that will change the AML treatment paradigm.

OGP46 Induces Differentiation of Acute Myeloid Leukemia Cells via Different Optimal Signaling Pathways

Acute myelogenous leukemia (AML) is characterized by blockage of cell differentiation leading to the accumulation of immature cells, which is the most prevalent form of acute leukemia in adults. It is well known that all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) are the preferred drugs for acute promyelocytic leukemia (APL). However, they can lead to irreversible resistance which may be responsible for clinical failure after complete remission (CR). Moreover, the differentiation therapy of ATRA-based treatment has not been effective against AML with t(8;21) translocation. Here we aimed to identify the differentiation effect of OGP46 on AML cell lines (HL-60, NB4, and Kasumi-1) and explore its possible mechanisms. We found that OGP46 has significant inhibitory activity against these cells by triggering cell differentiation with cell-cycle exit at G1/G0 and inhibited the colony-formation capacity of the AML cells. It was shown that OGP46 induced the differentiation of NB4 cells via the transcriptional misregulation in cancer signaling pathway by PML-RARα depletion, while it was attributed to the hematopoietic cell lineage and phagosome pathway in Kasumi-1 cells, which are all critical pathways in cell differentiation. These results highlight that OGP46 is an active agent not only in the APL cell line NB4 but also in AML-M2 cell lines, especially Kasumi-1 with t(8;21) translocation. Therefore, OGP46 may be a potential compound for surmounting the differentiation blockage in AML.

[Interferon-regulating activity of the celagrip antiviral drug and its influence on formation of reactive oxygen species and expression of innate immunity genes in the follicular lymphoma patients]

INTRODUCTION

Medicines from the group of interferon inducers (IFNs) "swith on" the synthesis of type 1 interferons (IFN-I) and induce the expression of IFN-stimulated genes (ISGs) that regulate innate immunity reactions and protect the host from infectious agents and the tumour pathology.The purpose of the study was to determine the role of the drug celagrip (CA) in the activation of innate immunity genes and the effect on the production of reactive oxygen species (ROS) in patients with follicular lymphoma (FL).

OBJECTIVES

 to study the intensity of ROS production and the level of expression of the IFN-α2, IFN-λ1, ISG15, BCL2, P53(TP53) and USP18 genes in response to the treatment of blood cells of patients with FL with the preparation of CA.

MATERIAL AND METHODS

The study involved primary cancer patients diagnosed with follicular lymphoma (FL) and healthy volunteers. A kinetic analysis of the dynamics of production of reactive oxygen species (ROS) was performed in whose blood cells, and the expression of the group of genes was determined by real-time PCR in response to CA processing.

RESULTS AND DISCUSSION

ROS production by blood cells of patients with FL and volunteers in the presence of CA significantly decreased (P < 0.05). The level of gene expression of ISG15, P53(TR53) and USP 18 in the group of patients with FL was significantly higher than that in the group of volunteers. When treating blood cells with CA, it becomes possible to divide patients with FL into groups with a positive and negative response in accordance with the level of expression of the USP18 gene. We divided FL patients into groups with a positive and negative response in accordance with the level of USP18 gene expression after treatment of blood cells with CA.

CONCLUSIONS

The CA drug reduces the production of ROS and simultaneously stimulates the activity of the innate immunity genes ISG15, P53(TP53) and USP18 in the blood cells of patients with FL.

E1 Enzymes as Therapeutic Targets in Cancer

Post-translational modifications of cellular substrates with ubiquitin and ubiquitin-like proteins (UBLs), including ubiquitin, SUMOs, and neural precursor cell-expressed developmentally downregulated protein 8, play a central role in regulating many aspects of cell biology. The UBL conjugation cascade is initiated by a family of ATP-dependent enzymes termed E1 activating enzymes and executed by the downstream E2-conjugating enzymes and E3 ligases. Despite their druggability and their key position at the apex of the cascade, pharmacologic modulation of E1s with potent and selective drugs has remained elusive until 2009. Among the eight E1 enzymes identified so far, those initiating ubiquitylation (UBA1), SUMOylation (SAE), and neddylation (NAE) are the most characterized and are implicated in various aspects of cancer biology. To date, over 40 inhibitors have been reported to target UBA1, SAE, and NAE, including the NAE inhibitor pevonedistat, evaluated in more than 30 clinical trials. In this Review, we discuss E1 enzymes, the rationale for their therapeutic targeting in cancer, and their different inhibitors, with emphasis on the pharmacologic properties of adenosine sulfamates and their unique mechanism of action, termed substrate-assisted inhibition. Moreover, we highlight other less-characterized E1s-UBA6, UBA7, UBA4, UBA5, and autophagy-related protein 7-and the opportunities for targeting these enzymes in cancer. SIGNIFICANCE STATEMENT: The clinical successes of proteasome inhibitors in cancer therapy and the emerging resistance to these agents have prompted the exploration of other signaling nodes in the ubiquitin-proteasome system including E1 enzymes. Therefore, it is crucial to understand the biology of different E1 enzymes, their roles in cancer, and how to translate this knowledge into novel therapeutic strategies with potential implications in cancer treatment.

Genome-Wide Association Study Data Reveal Genetic Susceptibility to Chronic Inflammatory Intestinal Diseases and Pancreatic Ductal Adenocarcinoma Risk

Registry-based epidemiologic studies suggest associations between chronic inflammatory intestinal diseases and pancreatic ductal adenocarcinoma (PDAC). As genetic susceptibility contributes to a large proportion of chronic inflammatory intestinal diseases, we hypothesize that the genomic regions surrounding established genome-wide associated variants for these chronic inflammatory diseases are associated with PDAC. We examined the association between PDAC and genomic regions (±500 kb) surrounding established common susceptibility variants for ulcerative colitis, Crohn's disease, inflammatory bowel disease, celiac disease, chronic pancreatitis, and primary sclerosing cholangitis. We analyzed summary statistics from genome-wide association studies data for 8,384 cases and 11,955 controls of European descent from two large consortium studies using the summary data-based adaptive rank truncated product method to examine the overall association of combined genomic regions for each inflammatory disease group. Combined genomic susceptibility regions for ulcerative colitis, Crohn disease, inflammatory bowel disease, and chronic pancreatitis were associated with PDAC at P values < 0.05 (0.0040, 0.0057, 0.011, and 3.4 × 10-6, respectively). After excluding the 20 PDAC susceptibility regions (±500 kb) previously identified by GWAS, the genomic regions for ulcerative colitis, Crohn disease, and inflammatory bowel disease remained associated with PDAC (P = 0.0029, 0.0057, and 0.0098, respectively). Genomic regions for celiac disease (P = 0.22) and primary sclerosing cholangitis (P = 0.078) were not associated with PDAC. Our results support the hypothesis that genomic regions surrounding variants associated with inflammatory intestinal diseases, particularly, ulcerative colitis, Crohn disease, inflammatory bowel disease, and chronic pancreatitis are associated with PDAC. SIGNIFICANCE: The joint effects of common variants in genomic regions containing susceptibility loci for inflammatory bowel disease and chronic pancreatitis are associated with PDAC and may provide insights to understanding pancreatic cancer etiology.

Endometriosis and nuclear receptors

BACKGROUND

Endometriosis is recognized as a steroid-dependent disorder; however, the precise roles of nuclear receptors (NRs) in steroid responsiveness and other signaling pathways are not well understood.

OBJECTIVE AND RATIONALE

Over the past several years, a number of paradigm-shifting breakthroughs have occurred in the area of NRs in endometriosis. We review and clarify new information regarding the mechanisms responsible for: (i) excessive estrogen biosynthesis, (ii) estrogen-dependent inflammation, (iii) defective differentiation due to progesterone resistance and (iv) enhanced survival due to deficient retinoid production and action in endometriosis. We emphasize the roles of the relevant NRs critical for these pathological processes in endometriosis.

SEARCH METHODS

We conducted a comprehensive search using PubMed for human, animal and cellular studies published until 2018 in the following areas: endometriosis; the steroid and orphan NRs, estrogen receptors alpha (ESR1) and beta (ESR2), progesterone receptor (PGR), steroidogenic factor-1 (NR5A1) and chicken ovalbumin upstream promoter-transcription factor II (NR2F2); and retinoids.

OUTCOMES

Four distinct abnormalities in the intracavitary endometrium and extra-uterine endometriotic tissue underlie endometriosis progression: dysregulated differentiation of endometrial mesenchymal cells, abnormal epigenetic marks, inflammation activated by excess estrogen and the development of progesterone resistance. Endometriotic stromal cells compose the bulk of the lesions and demonstrate widespread epigenetic abnormalities. Endometriotic stromal cells also display a wide range of abnormal NR expression. The orphan NRs NR5A1 and NR2F2 compete to regulate steroid-synthesizing genes in endometriotic stromal cells; NR5A1 dominance gives rise to excessive estrogen formation. Endometriotic stromal cells show an abnormally low ESR1:ESR2 ratio due to excessive levels of ESR2, which mediates an estrogen-driven inflammatory process and prostaglandin formation. These cells are also deficient in PGR, leading to progesterone resistance and defective retinoid synthesis. The pattern of NR expression, involving low ESR1 and PGR and high ESR2, is reminiscent of uterine leiomyoma stem cells. This led us to speculate that endometriotic stromal cells may display stem cell characteristics found in other uterine tissues. The biologic consequences of these abnormalities in endometriotic tissue include intense inflammation, defective differentiation and enhanced survival.

WIDER IMPLICATIONS

Steroid- and other NR-related abnormalities exert genome-wide biologic effects via interaction with defective epigenetic programming and enhance inflammation in endometriotic stromal cells. New synthetic ligands, targeting PGR, retinoic acid receptors and ESR2, may offer novel treatment options.

Inhibition of UBE2L6 attenuates ISGylation and impedes ATRA-induced differentiation of leukemic cells

Ubiquitin/ISG15‐conjugating enzyme E2L6 (UBE2L6) is a critical enzyme in ISGylation, a post‐translational protein modification that conjugates the ubiquitin‐like modifier, interferon‐stimulated gene 15 (ISG15), to target substrates. Previous gene expression studies in acute promyelocytic leukemia (APL) cells showed that all‐trans‐retinoic acid (ATRA) altered the expression of many genes, including UBE2L6 (200‐fold) and other members of the ISGylation pathway. Through gene expression analyses in a cohort of 98 acute myeloid leukemia (AML) patient samples and in primary neutrophils from healthy donors, we found that UBE2L6 gene expression is reduced in primary AML cells compared with normal mature granulocytes. To assess whether UBE2L6 expression is important for leukemic cell differentiation—two cell line models were employed: the human APL cell line NB4 and its ATRA‐resistant NB4R counterpart, as well as the ATRA‐sensitive human AML HL60 cells along with their ATRA‐resistant subclone—HL60R. ATRA strongly induced UBE2L6 in NB4 APL cells and in ATRA‐sensitive HL60 AML cells, but not in the ATRA‐resistant NB4R and HL60R cells. Furthermore, short hairpin (sh)RNA‐mediated UBE2L6 depletion in NB4 cells impeded ATRA‐mediated differentiation, suggesting a functional role for UBE2L6 in leukemic cell differentiation. In addition, ATRA induced ISG15 gene expression in NB4 APL cells, leading to increased levels of both free ISG15 protein and ISG15 conjugates. UBE2L6 depletion attenuated ATRA‐induced ISG15 conjugation. Knockdown of ISG15 in NB4 APL cells inhibited ISGylation and also attenuated ATRA‐induced differentiation. In summary, we demonstrate the functional importance of UBE2L6 in ATRA‐induced neutrophil differentiation of APL cells and propose that this may be mediated by its catalytic role in ISGylation.

Ubiquitin-like modifier-activating enzyme 7 as a marker for the diagnosis and prognosis of breast cancer

Ubiquitin-like modifier-activating enzyme 7 (UBA7) is a specific E1-like ubiquitin-activating enzyme involved in interferon-stimulated gene 15 (ISG15) conjugation. UBA7 expression has been reported to be notably decreased in lung cancer. The present study aimed to investigate the changes in UBA7 expression in breast cancer and the association between UBA7 expression and clinical characteristics, and to elucidate the diagnostic and prognostic significance of UBA7 in breast cancer. The clinical data and RNA-sequencing expression values of 1,104 patients with breast cancer were downloaded from The Cancer Genome Atlas database. The associations between UBA7 expression and clinical characteristics were determined using χ² and Fisher's exact tests. UBA7 expression values were divided into low and high groups using the optimal cut-off value, as determined by the overall survival (OS) value identified via a receiver operating characteristic (ROC) curve analysis, to further study the association between UBA7 expression and clinical characteristics. The diagnostic capability of UBA7 was assessed via ROC analysis, and Kaplan-Meier curve and Cox regression analyses were performed to determine the prognostic value of UBA7. The results demonstrated that UBA7 expression was decreased in breast cancer, and significant differences were observed between groups with regards to vital status, tumor classification, metastasis classification, histological type, sex, molecular subtype, and expression levels of progesterone receptor, estrogen receptor (ER) and human epidermal growth factor receptor 2. Low and high UBA7 expression levels were associated with age, ER expression, menopause status, Tumor-Node-Metastasis classification stage, margin status, vital status, radiation therapy use, OS and relapse-free survival. Furthermore, patients with low UBA7 expression levels had a poor prognosis. UBA7 expression also demonstrated an ability to diagnose patients at all clinical stages. Taken together, the results indicated that UBA7 expression was significantly decreased in breast cancer, and was associated with clinical characteristics and prognosis. Thus, UBA7 can be deemed as a potential biomarker in breast cancer, and may serve as a target in treatment.

Inflammation-dependent ISG15 upregulation mediates MIA-induced dendrite damages and depression by disrupting NEDD4/Rap2A signaling

BACKGROUND

Maternal immune activation (MIA) is an independent risk factor for psychiatric disorders including depression spectrum in the offsprings, but the molecular mechanism is unclear. Recent studies show that interferon-stimulated gene-15 (ISG15) is involved in inflammation and neuronal dendrite development; here we studied the role of ISG15 in MIA-induced depression and the underlying mechanisms.

METHODS

By vena caudalis injecting polyinosinic: polycytidylic acid (poly I:C) into the pregnant rats to mimic MIA, we used AAV or lentivirus to introduce or silence ISG15 expression. Synaptic plasticity was detected by confocal microscope and Golgi staining. Cognitive performances of the offspring were measured by Open field, Forced swimming and Sucrose preference test.

RESULTS

We found that MIA induced depression-like behaviors with dendrite impairments in the offspring with ISG15 level increased in the offsprings' brain. Overexpressing ISG15 in the prefrontal cortex of neonatal cubs (P0) could mimic dendritic pathology and depressive like behaviors, while downregulating ISG15 rescued these abnormalities in the offsprings. Further studies demonstrated that MIA-induced upregulation of inflammatory cytokines promoted ISG15 expression in the offspring' brain which suppressed Rap2A ubiquitination via NEDD4 and thus induced Rap2A accumulation, while upregulating NEDD4 abolished ISG15-induced dendrite impairments.

CONCLUSIONS

These data reveal that MIA impedes offsprings' dendrite development and causes depressive like behaviors by upregulating ISG15 and suppressing NEDD4/Rap2A signaling. The current findings suggest that inhibiting ISG15 may be a promising intervention of MIA-induced psychiatric disorders in the offsprings.

2-Bromopalmitate targets retinoic acid receptor alpha and overcomes all-trans retinoic acid resistance of acute promyelocytic leukemia

Fatty acid oxidation dependency of leukemia cells has been documented in recent studies. Pharmacologic inhibition of fatty acid oxidation, thereby, displays significant effects in suppressing leukemia. 2-Bromopalmitate, a palmitate analogue, was initially identified as an inhibitor of fatty acid oxidation, and recently recognized as an inhibitor of protein palmitoylation. However, the effects of 2-Bromopalmitate on leukemia and its cellular targets remain obscure. Herein, we discover in cultured cell lines, a transplantable mouse model, and primary blasts that 2-Bromopalmitate presents synergistic differentiation induction with all-trans retinoic acid in acute promyelocytic leukemia. Moreover, 2-Bromopalmitate overcomes all-trans retinoic acid resistance in all-trans retinoic acid-resistant cells and leukemic mice. Mechanistically, 2-Bromopalmitate covalently binds at cysteine 105 and cysteine 174 of retinoic acid receptor alpha (RARα) and stabilizes RARα protein in the presence of all-trans retinoic acid which is known to induce RARα degradation, leading to enhanced transcription of RARα-target genes. Mutation of both cysteines largely abrogates the synergistic effect of 2-Bromopalmitate on all-trans retinoic acid-induced differentiation, demonstrating that 2-Bromopalmitate promotes all-trans retinoic acid-induced differentiation through binding RARα. All-trans retinoic acid-based regimens including arsenic trioxide or chemotherapy, as preferred therapy for acute promyelocytic leukemia, induce adverse events and irreversible resistance. We expect that combining all-trans retinoic acid with 2-Bromopalmitate would be a promising therapeutic strategy for acute promyelocytic leukemia, especially for overcoming all-trans retinoic acid resistance of relapsed acute promyelocytic leukemia patients.

Physiological and pathological implications of retinoid action in the endometrium

Retinol (vitamin A) and its derivatives, collectively known as retinoids, are required for maintaining vision, immunity, barrier function, reproduction, embryogenesis and cell proliferation and differentiation. Despite the fact that most events in the endometrium are predominantly regulated by steroid hormones (estrogens and progesterone), accumulating evidence shows that retinoid signaling is also involved in the development and maintenance of the endometrium, stromal decidualization and blastocyst implantation. Moreover, aberrant retinoid metabolism seems to be a critical factor in the development of endometriosis, a common gynecological disease, which affects up to 10% of reproductive age women and is characterized by the ectopic localization of endometrial-like tissue in the pelvic cavity. This review summarizes recent advances in research on the mechanisms and molecular actions of retinoids in normal endometrial development and physiological function. The potential roles of abnormal retinoid signaling in endometriosis are also discussed. The objectives are to identify limitations in current knowledge regarding the molecular actions of retinoids in endometrial biology and to stimulate new investigations toward the development potential therapeutics to ameliorate or prevent endometriosis symptoms.

PML: Regulation and multifaceted function beyond tumor suppression

Promyelocytic leukemia protein (PML) was originally identified as a fusion partner of retinoic acid receptor alpha in acute promyelocytic leukemia patients with the (15;17) chromosomal translocation, giving rise to PML–RARα and RARα–PML fusion proteins. A body of evidence indicated that PML possesses tumor suppressing activity by regulating apoptosis, cell cycle, senescence and DNA damage responses. PML is enriched in discrete nuclear substructures in mammalian cells with 0.2–1 μm diameter in size, referred to as alternately Kremer bodies, nuclear domain 10, PML oncogenic domains or PML nuclear bodies (NBs). Dysregulation of PML NB formation results in altered transcriptional regulation, protein modification, apoptosis and cellular senescence. In addition to PML NBs, PML is also present in nucleoplasm and cytoplasmic compartments, including the endoplasmic reticulum and mitochondria-associated membranes. The role of PML in tumor suppression has been extensively studied but increasing evidence indicates that PML also plays versatile roles in stem cell renewal, metabolism, inflammatory responses, neural function, mammary development and angiogenesis. In this review, we will briefly describe the known PML regulation and function and include new findings.

Proteomic analysis of RAW macrophages treated with cGAMP or c-di-GMP reveals differentially activated cellular pathways

Global and quantitative analysis of the proteome help to reveal how host cells sense invading bacteria and respond to bacterial signaling molecules. Here, we performed label free quantitative proteomic analysis of RAW macrophages treated with host-derived cGAMP and bacterial-derived c-di-GMP, in an attempt to identify cellular pathways impacted by these dinucleotides and determine if the host responds differentially to these two cyclic dinucleotides. We identified a total of 3811 proteins of which abundances of 404 proteins in cGAMP and 236 proteins in c-di-GMP treated cells were significantly different compared to the control. Many of the proteins that were strongly and commonly upregulated, such as interferon-induced proteins 47, 202 and 204 (Ifi47, Ifi202, Ifi204), ubiquitin-activating enzyme E7 (Uba7), interferon-induced protein with tetratricopeptide repeats 1, 2 or 3 (Ifit1, Ifit2, Ifit3), ubiquitin-like protein ISG15 (ISG15), might be due to the fact that both dinucleotides promote the production of interferons, which induce the expression of many proteins. However, there were also other proteins that were differentially affected by cGAMP or c-di-GMP treatment, including probable ATP-dependent RNA helicase DHX58 (Dhx58), nuclear autoantigen Sp-100 (Sp100), MARCKS-related protein (Marcksl1) and antigen peptide transporter 2 (Tap2). This is probably due to the differential levels of IFNs produced by the dinucleotides or may indicate that non-STING activation might also contribute to the host's response to c-di-GMP and cGAMP. Interestingly Trex1, a nuclease that degrades DNA (an activator of cGAS to produce cGAMP), was upregulated (3.22 fold) upon cGAMP treatment, hinting at a possible feedback loop to regulate cGAMP synthesis. These results lay a foundation for future studies to better characterize and understand the complex c-di-GMP and cGAMP signaling network.

cGAMP modulates proteins involved in antigen presentation and inflammation.

Bone mesenchymal stem cell secretion of sRANKL/OPG/M-CSF in response to macrophage-mediated inflammatory response influences osteogenesis on nanostructured Ti surfaces

Although it has been well established that osteogenic differentiation of bone mesenchymal stem cells (bMSCs) as well as osteoclastic differentiation of macrophages can be manipulated by the nanostructure of biomaterial surfaces, the interactions among the effects of the surface on immune cells and bMSCs remained unknown. Therefore, in this study, the osteogenic behaviors and secretion of osteoclastogenesis-related cytokines of human bMSCs on TiO₂ nanotubular (NT) surfaces in conditioned medium (CM) generated by macrophages cultured on the respective NT surfaces (NT-CM) were analyzed. Although bMSCs showed consistent osteogenic behaviors on the NT5 and NT20 surfaces in both standard culture medium and both types of NT-CM, collagen synthesis and extracellular matrix mineralization were partially impeded on the NT20 surface in NT20-CM and bMSC cytokine secretions on the NT20 surface in NT20-CM elicited remarkable multinuclear giant cell and osteoclast formation compared with that observed on the NT5 surface in NT5-CM. After implantation in vivo, mineralized bone formation was significantly delayed around the NT20 implant compared with the NT5 implant, but both surfaces contributed to good bone formation after 12 weeks. The results obtained in this study advance our understanding of the confounding influence of the implant surface nanostructure, macrophage inflammatory response, and osteogenic differentiation of bMSCs as well as the retro-regulative effects of bMSCs on the osteoclastic differentiation of macrophages, and the culture system based on different NT surfaces and CM generated on the respective surfaces may provide a systematic research model for evaluating the performance of endosseous implants as well as a prospective approach for improving implant osseointegration via immune-regulation.

Activation of the CRABPII/RAR pathway by curcumin induces retinoic acid mediated apoptosis in retinoic acid resistant breast cancer cells

Due to the anti-proliferative and anti-apoptotic effects of retinoic acid (RA), this hormone has emerged as a target for several diseases, including cancer. However, development of retinoid resistance is a critical issue and efforts to understand the retinoid signaling pathway may identify useful biomarkers for future clinical trials. Apoptotic responses of RA are exhibited through the cellular RA-binding protein II (CRABPII)/retinoic acid receptor (RAR) signaling cascade. Delivery of RA to RAR by CRABPII enhances the transcriptional activity of genes involved in cell death and cell cycle arrest. The purpose of this study was to investigate the role of curcumin in sensitizing RA-resistant triple-negative breast cancer (TNBC) cells to RA-mediated apoptosis. We provide evidence that curcumin upregulates the expression of CRABPII, RARβ and RARγ in two different TNBC cell lines. Co-treatment of the cells with curcumin and RA results in increased apoptosis as demonstrated by elevated cleavage of poly(ADP-ribose) polymerase and cleaved caspase-9. Additionally, silencing CRABPII reverses curcumin sensitization of TNBC cells to the apoptotic inducing effects of RA. These findings provide mechanistic insights into sensitizing TNBC cells to RA-mediated cell death by curcumin-induced upregulation of the CRABPII/RAR pathway.

Positive feedback regulation of p53 transactivity by DNA damage-induced ISG15 modification

p53 plays a pivotal role in tumour suppression under stresses, such as DNA damage. ISG15 has been implicated in the control of tumorigenesis. Intriguingly, the expression of ISG15, UBE1L and UBCH8 is induced by DNA-damaging agents, such as ultraviolet and doxorubicin, which are known to induce p53. Here, we show that the genes encoding ISG15, UBE1L, UBCH8 and EFP, have the p53-responsive elements and their expression is induced in a p53-dependent fashion under DNA damage conditions. Furthermore, DNA damage induces ISG15 conjugation to p53 and this modification markedly enhances the binding of p53 to the promoters of its target genes (for example, CDKN1 and BAX) as well as of its own gene by promoting phosphorylation and acetylation, leading to suppression of cell growth and tumorigenesis. These findings establish a novel feedback circuit between p53 and ISG15-conjugating system for positive regulation of the tumour suppressive function of p53 under DNA damage conditions.

The ‘genome guardian' p53 has a well-established role in suppressing tumour development after DNA damage. Here the authors show that expression of the ubiquitin-like protein ISG15 is regulated by p53 which in turn is modified by ISG15 to enhance binding to target gene promoters.

CRABP-II- and FABP5-independent responsiveness of human glioblastoma cells to all-trans retinoic acid

Glioblastomas respond differently to all-trans retinoic acid (RA) for unknown reasons. Because CRABP-II and FABP5 mediate RA intracellular signaling respectively and lead to distinct biological consequences, their expression patterns in different grades of astrocytomas and the glioblastoma cells lines LN18, LN428 and U251 were examined to identify potential correlations with RA sensitivities. The response of glioblastoma cells to RA, decitabine or the FABP5 competitive inhibitor, BMS309403, was analyzed. CRABP-II and FABP5 were expressed to varying degrees by the 84-astrocytoma cases examined. Treatment of LN428, U251 and LN18 cells with RA failed to suppress their growth; however, U251 proliferation was inhibited by decitabine. The combination of decitabine and RA suppressed the growth of all three cell lines and induced significant apoptosis of LN428 and U251 cells. Both CRABP-II and FABP5 were transcribed in the three cell lines but FABP5 proteins were undetectable in U251 cells. The ratio of CRABP-II to FABP5 was not altered after RA, decitabine or RA and decitabine treatment and the resistance of cells to RA was not reversed by BMS309403 treatment. In conclusion, CRABP-II and FABP5 expression patterns are neither related to the tumor grades nor correlated with RA sensitivity. Additional molecular factors may be present that determines the sensitivity of glioblastoma cells to RA. Dicitabine may improve the sensitivity of glioblastoma cells to RA, however, its underlying mechanism and its in vivo feasibility need to be investigated.

Abscisic-acid-induced cellular apoptosis and differentiation in glioma via the retinoid acid signaling pathway

Retinoid acid (RA) plays critical roles in regulating differentiation and apoptosis in a variety of cancer cells. Abscisic acid (ABA) and RA are direct derivatives of carotenoids and share structural similarities. Here we proposed that ABA may also play a role in cellular differentiation and apoptosis by sharing a similar signaling pathway with RA that may be involved in glioma pathogenesis. We reported for the first time that the ABA levels were twofold higher in low-grade gliomas compared with high-grade gliomas. In glioma tissues, there was a positive correlation between the ABA levels and the transcription of cellular retinoic acid-binding protein 2 (CRABP2) and a negative correlation between the ABA levels and transcription of fatty acid-binding protein 5 (FABP5). ABA treatment induced a significant increase in the expression of CRABP2 and a decrease in the expression of peroxisome proliferator-activated receptor (PPAR) in glioblastoma cells. Remarkably, both cellular apoptosis and differentiation were increased in the glioblastoma cells after ABA treatment. ABA-induced cellular apoptosis and differentiation were significantly reduced by selectively silencing RAR-α, while RAR-α overexpression exaggerated the ABA-induced effects. These results suggest that ABA may play a role in the pathogenesis of glioma by promoting cellular apoptosis and differentiation through the RA signaling pathway.

Synergistic targeted therapy for acute promyelocytic leukaemia: a model of translational research in human cancer

Acute promyelocytic leukaemia (APL), the M3 subtype of acute myeloid leukaemia, was once a lethal disease, yet nowadays the majority of patients with APL can be successfully cured by molecularly targeted therapy. This dramatic improvement in the survival rate is an example of the advantage of modern medicine. APL is characterized by a balanced reciprocal chromosomal translocation fusing the promyelocytic leukaemia (PML) gene on chromosome 15 with the retinoic acid receptor α (RARα) gene on chromosome 17. It has been found that all-trans-retinoic acid (ATRA) or arsenic trioxide (ATO) alone exerts therapeutic effect on APL patients with the PML-RARα fusion gene, and the combination of both drugs can act synergistically to further enhance the cure rate of the patients. Here, we provide an insight into the pathogenesis of APL and the mechanisms underlying the respective roles of ATRA and ATO. In addition, treatments that lead to more effective differentiation and apoptosis of APL cells, including leukaemia-initiating cells, and more thorough eradication of the disease will be discussed. Moreover, as a model of translational research, the development of a cure for APL has followed a bidirectional approach of 'bench to bedside' and 'bedside to bench', which can serve as a valuable example for the diagnosis and treatment of other malignancies.

Retinoids and rexinoids in cancer prevention: from laboratory to clinic

Early in the age of modern medicine the consequences of vitamin A deficiency drew attention to the fundamental link between retinoid-dependent homeostatic regulation and malignant hyperproliferative diseases. The term "retinoid" includes a handful of endogenous and a large group of synthetic derivatives of vitamin A. These multifunctional lipid-soluble compounds directly regulate target genes of specific biological functions and critical signaling pathways to orchestrate complex functions from vision to development, metabolism, and inflammation. Many of the retinoid activities on the cellular level have been well characterized and translated to the regulation of processes like differentiation and cell death, which play critical roles in the outcome of malignant transformation of tissues. In fact, retinoid-based differentiation therapy of acute promyelocytic leukemia was one of the first successful examples of molecularly targeted treatment strategies. The selectivity, high receptor binding affinity and the ability of retinoids to directly modulate gene expression programs present a distinct pharmacological opportunity for cancer treatment and prevention. However, to fully exploit their potential, the adverse effects of retinoids must be averted. In this review we provide an overview of the biology of retinoid (activated by nuclear retinoic acid receptors [RARs]) and rexinoid (engaged by nuclear retinoid X receptors [RXRs]) action concluded from a long line of preclinical studies, in relation to normal and transformed states of cells. We will also discuss the past and current uses of retinoids in the treatment of malignancies, the potential of rexinoids in the cancer prevention setting, both as single agents and in combinations.

Vitamin A, cancer treatment and prevention: the new role of cellular retinol binding proteins

Retinol and vitamin A derivatives influence cell differentiation, proliferation, and apoptosis and play an important physiologic role in a wide range of biological processes. Retinol is obtained from foods of animal origin. Retinol derivatives are fundamental for vision, while retinoic acid is essential for skin and bone growth. Intracellular retinoid bioavailability is regulated by the presence of specific cytoplasmic retinol and retinoic acid binding proteins (CRBPs and CRABPs). CRBP-1, the most diffuse CRBP isoform, is a small 15 KDa cytosolic protein widely expressed and evolutionarily conserved in many tissues. CRBP-1 acts as chaperone and regulates the uptake, subsequent esterification, and bioavailability of retinol. CRBP-1 plays a major role in wound healing and arterial tissue remodelling processes. In the last years, the role of CRBP-1-related retinoid signalling during cancer progression became object of several studies. CRBP-1 downregulation associates with a more malignant phenotype in breast, ovarian, and nasopharyngeal cancers. Reexpression of CRBP-1 increased retinol sensitivity and reduced viability of ovarian cancer cells in vitro. Further studies are needed to explore new therapeutic strategies aimed at restoring CRBP-1-mediated intracellular retinol trafficking and the meaning of CRBP-1 expression in cancer patients' screening for a more personalized and efficacy retinoid therapy.

Curcumin restores sensitivity to retinoic acid in triple negative breast cancer cells

Background

A major obstacle in the use of retinoid therapy in cancer is the resistance to this agent in tumors. Retinoic acid facilitates the growth of mammary carcinoma cells which express high levels of fatty acid-binding protein 5 (FABP5). This protein delivers retinoic acid to peroxisome proliferator-activated receptor β/δ (PPARβ/δ) that targets genes involved in cell proliferation and survival. One approach to overcome resistance of mammary carcinoma cells to retinoic acid is to target and suppress the FABP5/ PPARβ/δ pathway. The objective of this research was to investigate the effect of curcumin, a polyphenol extract from the plant Curcuma longa, on the FABP5/ PPARβ/δ pathway in retinoic acid resistant triple negative breast cancer cells.

Methods

Cell viability and proliferation of triple negative breast cancer cell lines (MDA-MB-231 and MD-MB-468) treated with curcumin and/or retinoic was analyzed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and 5-bromo-2’-deoxyuridine (BrdU). Expression level of FABP5 and PPARβ/δ in these cells treated with curcumin was examined by Western Blotting analysis and Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR). Effect of curcumin and retinoic acid on PPARβ/δ target genes, PDK1and VEGF-A were also examined using qRT-PCR. Western Blotting was utilized to examine the protein expression level of the p65 subunit of NF-κB.

Results

Treatment of retinoic acid resistant triple negative breast cancer cells with curcumin sensitized these cells to retinoic acid mediated growth suppression, as well as suppressed incorporation of BrdU. Further studies demonstrated that curcumin showed a marked reduction in the expression level of FABP5 and PPARβ/δ. We provide evidence that curcumin suppresses p65, a transcription factor known to regulate FABP5. The combination of curcumin with retinoic acid suppressed PPARβ/δ target genes, VEGF-A and PDK1.

Conclusions

Curcumin suppresses the expression level of FABP5 and PPARβ/δ in triple negative mammary carcinoma cells. By targeting the FABP5/PPARβ/δ pathway, curcumin prevents the delivery of retinoic acid to PPARβ/δ and suppresses retinoic acid-induced PPARβ/δ target gene, VEGF-A. Our data demonstrates that suppression of the FABP5/ PPARβ/δ pathway by curcumin sensitizes retinoic acid resistant triple negative breast cancer cells to retinoic acid mediated growth suppression.

USP18 is crucial for IFN-γ-mediated inhibition of B16 melanoma tumorigenesis and antitumor immunity

Background

Interferon (IFN)-γ-mediated immune response plays an important role in tumor immunosurveillance. However, the regulation of IFN-γ-mediated tumorigenesis and immune response remains elusive. USP18, an interferon stimulating response element, regulates IFN-α-mediated signaling in anti-viral immune response, but its role in IFN-γ-mediated tumorigenesis and anti-tumor immune response is unknown.

Method

In this study, USP18 in tumorigenesis and anti-tumor immune response was comprehensively appraised in vivo by overexpression or downregulation its expression in murine B16 melanoma tumor model in immunocompetent and immunodeficient mice.

Results

Ectopic expression or downregulation of USP18 in B16 melanoma tumor cells inhibited or promoted tumorigenesis, respectively, in immunocompetent mice. USP18 expression in B16 melanoma tumor cells regulated IFN-γ-mediated immunoediting, including upregulating MHC class-I expression, reducing tumor cell-mediated inhibition of T cell proliferation and activation, and suppressing PD-1 expression in CD4⁺ and CD8⁺ T cells in tumor-bearing mice. USP18 expression in B16 melanoma tumor cells also enhanced CTL activity during adoptive immunotherapy by prolonging the persistence and enhancing the activity of adoptively transferred CTLs and by reducing CTL exhaustion in the tumor microenvironment. Mechanistic studies demonstrated that USP18 suppressed tumor cell-mediated immune inhibition by activating T cells, inhibiting T-cell exhaustion, and reducing dendritic cell tolerance, thus sensitizing tumor cells to immunosurveillance and immunotherapy.

Conclusion

These findings suggest that stimulating USP18 is a feasible approach to induce B16 melanoma specific immune response.

Triple A therapy: the molecular underpinnings of the unique sensitivity of leukemic promyelocytes to anthracyclines, all-trans-retinoic acid and arsenic trioxide

If looking for a mnemonic to remember the relevant facts about acute promyelocytic leukemia (APL), one just has to remember that APL is a disease of A's. It is acute and it is highly sensitive to treatment with anthracyclines, all-trans-retinoic acid (RA) and arsenic trioxide (ATO). The presence of fusions involving the retinoic acid receptor alpha (RARA) is without question the central player driving APL and dictating the response of this disease to these therapeutic agents. However, beyond this knowledge, the molecular mechanisms that contribute to the complicated pathogenesis and the response to treatment of APL are not completely defined. As more is understood about this hematological malignancy, there are more opportunities to refine and improve treatment based on this knowledge. In this review article, we discuss the response of APL to these "A" therapies.

Sertraline increases the survival of retinoic acid induced neuronal cells but not glial cells from human mesenchymal stem cells

An increase in the number of viable in vitro differentiated neuronal cells is important for their use in clinics. A proportion of differentiated cells lose their viability before being used, and therefore we decided to use a pharmacological agent, sertraline, to increase neural cell differentiation and their survival. Purified endometrial stem cells (EnSCs) were examined for neuronal and glial cell specific markers after retinoic acid (RA) and sertraline treatment via RT-PCR, immunocytochemistry and Western blot analysis. The survival of differentiated cells was measured by MTT assay and the frequency of apoptosis, demonstrated by caspase-3-like activity. EnSCs were differentiated into neuronal cells after RA induction. Sertraline increased neuronal cell differentiation by 1.2-fold and their survival by 1.4-fold, and decreased from glial cell differentiation significantly. The findings indicate that sertraline could be used to improve the in vitro differentiation process of stem cells into neuronal cells, and may be involved in regenerative pharmacology in future.

Repression of exogenous gene expression by the retinoic acid target gene G0S2

The G0/G1 switch gene 2 (G0S2) is rapidly induced by all-trans-retinoic acid (RA)-treatment of acute promyelocytic leukemia (APL) and other cells. G0S2 regulates lipolysis via inhibition of adipose triglyceride lipase (ATGL). This study found that retinoic acid receptor (RAR), but not retinoid X receptor (RXR) agonists induced G0S2 expression in APL cells. Novel G0S2 functions were uncovered that included repression of exogenous gene expression and transcriptional activity. Transient G0S2 transfection repressed the activities of multiple reporter constructs (including the retinoid-regulated species RARβ, UBE1L and G0S2); this occurred in diverse cell contexts. This inhibition was antagonized by siRNA-mediated G0S2 knockdown. To determine the inhibitory effects were not due to transient G0S2 expression, G0S2 was stably overex-pressed in cells without appreciable basal G0S2 expression. As expected, this repressed transcriptional activities. Intriguingly, transfection of G0S2 did not affect endogenous RARβ, UBE1L or G0S2 expression. Hence, only exogenously expressed genes were affected by G0S2. The domain responsible for this repression was localized to the G0S2 hydrophobic domain (HD). This was the same region responsible for the ability of G0S2 to inhibit ATGL activity. Whether an interaction with ATGL accounted for this new G0S2 activity was studied. Mimicking the inhibition of ATGL by oleic acid treatment that increased lipid droplet size or ATGL siRNA knockdown did not recapitulate G0S2 repressive effects. Engineered gain of ATGL expression did not rescue G0S2 transcriptional repression either. Thus, transcriptional repression by G0S2 did not depend on the ability of G0S2 to inhibit ATGL. Subcellular localization studies revealed that endogenous and exogenously-expressed G0S2 proteins were localized to the cytoplasm, particularly in the perinuclear region. Expression of a mutant G0S2 species that lacked the HD domain altered cytosolic G0S2 localization. This linked G0S2 subcellular localization to G0S2 transcriptional repression. The potential mechanisms responsible for this G0S2 repression are examined.

Post-translational modifications of PML: consequences and implications

The tumor suppressor promyelocytic leukemia protein (PML) predominantly resides in a structurally distinct sub-nuclear domain called PML nuclear bodies. Emerging evidences indicated that PML actively participates in many aspects of cellular processes, but the molecular mechanisms underlying PML regulation in response to stress and environmental cues are not complete. Post-translational modifications, such as SUMOylation, phosphorylation, acetylation, and ubiquitination of PML add a complex layer of regulation to the physiological function of PML. In this review, we discuss the fast-moving horizon of post-translational modifications targeting PML.

IFNs, ISGylation and cancer: Cui prodest?

IFNs are cytokines that segregate viral infections, modulate the immune responses and influence tumor cells survival. These options are under the control of ISGs (Interferon Stimulated Genes) which expression is propelled by IFNs. To the ISGs family belong all the components of the molecular machinery that modifies proteins by the addition of the ubiquitin-like protein ISG15, in a process known as ISGylation. Despite alterations in the components of this machinery are frequently observed in cancer, the contribution of ISG15 and of ISGylation to tumor growth and resistance to chemotherapy is unclear and debated. With the aim of elucidating this point, in this review we have discussed about recent data pointing to a dysregulation of the IFN signaling and the ISGylation system in cancer.

Evidence for the ubiquitin protease UBP43 as an antineoplastic target

New pharmacologic targets are needed for lung cancer. One candidate pathway to target is composed of the E1-like ubiquitin-activating enzyme (UBE1L) that associates with interferon-stimulated gene 15 (ISG15), which complexes with and destabilizes cyclin D1. Ubiquitin protease 43 (UBP43/USP18) removes ISG15 from conjugated proteins. This study reports that gain of UBP43 stabilized cyclin D1, but not other D-type cyclins or cyclin E. This depended on UBP43 enzymatic activity; an enzymatically inactive UBP43 did not affect cyclin D1 stability. As expected, small interfering RNAs that reduced UBP43 expression also decreased cyclin D1 levels and increased apoptosis in a panel of lung cancer cell lines. Forced cyclin D1 expression rescued UBP43 apoptotic effects, which highlighted the importance of cyclin D1 in conferring this. Short hairpin RNA-mediated reduction of UBP43 significantly increased apoptosis and reduced murine lung cancer growth in vitro and in vivo after transplantation of these cells into syngeneic mice. These cells also exhibited increased response to all-trans-retinoic acid, interferon, or cisplatin treatments. Notably, gain of UBP43 expression antagonized these effects. Normal-malignant human lung tissue arrays were examined independently for UBP43, cyclin D1, and cyclin E immunohistochemical expression. UBP43 was significantly (P < 0.01) increased in the malignant versus normal lung. A direct relationship was found between UBP43 and cyclin D1 (but not cyclin E) expression. Differential UBP43 expression was independently detected in a normal-malignant tissue array with diverse human cancers. Taken together, these findings uncovered UBP43 as a previously unrecognized antineoplastic target.

Chemosensitivity is controlled by p63 modification with ubiquitin-like protein ISG15

Identification of the cellular mechanisms that mediate cancer cell chemosensitivity is important for developing new cancer treatment strategies. Several chemotherapeutic drugs increase levels of the posttranslational modifier ISG15, which suggests that ISGylation could suppress oncogenesis. However, how ISGylation of specific target proteins controls tumorigenesis is unknown. Here, we identified proteins that are ISGylated in response to chemotherapy. Treatment of a human mammary epithelial cell line with doxorubicin resulted in ISGylation of the p53 family protein p63. An alternative splice variant of p63, ΔNp63α, suppressed the transactivity of other p53 family members, and its expression was abnormally elevated in various human epithelial tumors, suggestive of an oncogenic role for this variant. We showed that ISGylation played an essential role in the downregulation of ΔNp63α. Anticancer drugs, including doxorubicin, induced ΔNp63α ISGylation and caspase-2 activation, leading to cleavage of ISGylated ΔNp63α in the nucleus and subsequent release of its inhibitory domain to the cytoplasm. ISGylation ablated the ability of ΔNp63α to promote anchorage-independent cell growth and tumor formation in vivo as well to suppress the transactivities of proapoptotic p53 family members. These findings establish ISG15 as a tumor suppressor via its conjugation to ΔNp63α and provide a molecular rationale for therapeutic use of doxorubicin against ΔNp63α-mediated cancers.

TRIM32 promotes neural differentiation through retinoic acid receptor-mediated transcription

Retinoic acid (RA), a metabolite of vitamin A, plays versatile roles in development, differentiation, cell cycles and regulation of apoptosis by regulating gene transcription through nuclear receptor activation. Ubiquitinylation, which is one of the post-translational modifications, appears to be involved in the transcriptional activity of intranuclear receptors including retinoic acid receptor α (RARα). Mutations in the tripartite motif-containing protein 32 gene (TRIM32; also known as E3 ubiquitin-protein ligase) have been reported to be responsible for limb-girdle muscular dystrophy type 2H in humans, and its encoded protein has been shown to interact with several other important proteins. In this study, we found that TRIM32 interacts with RARα and enhances its transcriptional activity in the presence of RA. We also found that overexpression of TRIM32 in mouse neuroblastoma cells and embryonal carcinoma cells promoted stability of RARα, resulting in enhancement of neural differentiation. These findings suggest that TRIM32 functions as one of the co-activators for RARα-mediated transcription, and thereby TRIM32 is a potential therapeutic target for developmental disorders and RA-dependent leukemias.

PMLRARα binds to Fas and suppresses Fas-mediated apoptosis through recruiting c-FLIP in vivo

Defective Fas signaling leads to resistance to various anticancer therapies. Presence of potential inhibitors of Fas which could block Fas signaling can explain cancer cells resistance to apoptosis. We identified promyelocytic leukemia protein (PML) as a Fas-interacting protein using mass spectrometry analysis. The function of PML is blocked by its dominant-negative form PML-retinoic acid receptor α (PMLRARα). We found PMLRARα interaction with Fas in acute promyelocytic leukemia (APL)-derived cells and APL primary cells, and PML-Fas complexes in normal tissues. Binding of PMLRARα to Fas was mapped to the B-box domain of PML moiety and death domain of Fas. PMLRARα blockage of Fas apoptosis was demonstrated in U937/PR9 cells, human APL cells and transgenic mouse APL cells, in which PMLRARα recruited c-FLIP(L/S) and excluded procaspase 8 from Fas death signaling complex. PMLRARα expression in mice protected the mice against a lethal dose of agonistic anti-Fas antibody (P < .001) and the protected tissues contained Fas-PMLRARα-cFLIP complexes. Taken together, PMLRARα binds to Fas and blocks Fas-mediated apoptosis in APL by forming an apoptotic inhibitory complex with c-FLIP. The presence of PML-Fas complexes across different tissues implicates that PML functions in apoptosis regulation and tumor suppression are mediated by direct interaction with Fas.

Is PPARbeta/delta a Retinoid Receptor?

The broad ligand-binding characteristic of PPARβ/δ has long hampered identification of physiologically-meaningful ligands for the receptor. The observations that the activity of PPARβ/δ is supported by fatty acid binding protein 5 (FABP5), which directly delivers ligands from the cytosol to the receptor, suggest that bona fide PPARβ/δ ligands both activate the receptor, and trigger the nuclear translocation of FABP5. Using these criteria, it was recently demonstrated that all-trans-retinoic acid (RA), the activator of the classical retinoic acid receptor RAR, also serves as a ligand for PPARβ/δ. Partitioning of RA between its two receptors was found to be regulated by FABP5, which delivers it to PPARβ/δ, and cellular RA binding protein II (CRABP-II), which targets it to RAR. Consequently, RA activates PPARβ/δ in cells that display a high FABP5/CRABP-II expression ratio. It remains to be clarified whether compounds other than RA may also serve as endogenous activators for this highly promiscuous protein.

Differentiation therapy of acute myeloid leukemia

Acute Myeloid Leukemia (AML) is a predominant acute leukemia among adults, characterized by accumulation of malignantly transformed immature myeloid precursors. A very attractive way to treat myeloid leukemia, which is now called 'differentiation therapy', was proposed as in vitro studies have shown that a variety of agents stimulate differentiation of the cell lines isolated from leukemic patients. One of the differentiation-inducing agents, all-trans retinoic acid (ATRA), which can induce granulocytic differentiation in myeloid leukemic cell lines, has been introduced into clinics to treat patients with acute promyelocytic leukemia (APL) in which a PML-RARA fusion protein is generated by a t(15;17)(q22;q12) chromosomal translocation. Because differentiation therapy using ATRA has significantly improved prognosis for patients with APL, many efforts have been made to find alternative differentiating agents. Since 1,25-dihydroxyvitamin D3 (1,25D) is capable of inducing in vitro monocyte/macrophage differentiation of myeloid leukemic cells, clinical trials have been performed to estimate its potential to treat patients with AML or myelodysplastic syndrome (MDS). Unfortunately therapeutic concentrations of 1,25D can induce potentially fatal systemic hypercalcemia, thus limiting clinical utility of that compound. Attempts to overcome this problem have focused on the synthesis of 1,25D analogs (VDAs) which retain differentiation inducing potential, but lack its hypercalcemic effects. This review aims to discuss current problems and potential solutions in differentiation therapy of AML.

Interferon-stimulated gene 15 and the protein ISGylation system

Interferon-stimulated gene 15 (ISG15) is one of the most upregulated genes upon Type I interferon treatment or pathogen infection. Its 17  kDa protein product, ISG15, was the first ubiquitin-like modifier identified, and is similar to a ubiquitin linear dimer. As ISG15 modifies proteins in a similar manner to ubiquitylation, protein conjugation by ISG15 is termed ISGylation. Some of the primary enzymes that promote ISGylation are also involved in ubiquitin conjugation. The process to remove ISG15 from its conjugated proteins, termed de-ISGylation, is performed by a cellular ISG15-specific protease, ubiquitin-specific proteases with molecular mass 43 kDa (UBP43)/ubiquitin-specific proteases 18. Relative to ubiquitin, the biological function of ISG15 is still poorly understood, but ISG15 appears to play important roles in various biological and cellular functions. Therefore, there is growing interest in ISG15, as the study of free ISG15 and functional consequences of ISGylation/de-ISGylation may identify useful therapeutic targets. This review highlights recent discoveries and remaining questions important to understanding the biological functions of ISG15.

Molecular mechanisms of leukemia-associated protein degradation

Chemical biology, using small molecules as probes to study the cellular signaling network, has developed rapidly in recent years. The interaction between chemistry and biology not only provides new insight into the understanding of cellular activities, but also generates new lead compounds for the treatment of diseases. Transcription factors and kinases such as retinoic acid receptor-alpha (RARα), acute myeloid leukemia 1 (AML1), CAAT/enhancer-binding protein α (C/EBPα), c-myc, and c-abl play important roles in the differentiation of hematopoietic stem/progenitor cells. Abnormalities in these proteins may cause the dysregulation of hematopoiesis and even the occurrence of leukemia. Ubiquitin-mediated protein degradation represents a critical mechanism in regulating the cellular levels and functions of these proteins. Thus, targeting protein degradation has been emerging as an important strategy to conquer malignant diseases. In this review, we will summarize the recent advances in the understanding of the roles of protein degradation in leukemia, with an emphasis on the mechanisms revealed by small molecules.

Blockade of the ubiquitin protease UBP43 destabilizes transcription factor PML/RARα and inhibits the growth of acute promyelocytic leukemia

More effective treatments for acute promyelocytic leukemia (APL) are needed. APL cell treatment with all-trans-retinoic acid (RA) degrades the chimeric, dominant-negative-acting transcription factor promyelocytic leukemia gene (PML)/RARα, which is generated in APL by chromosomal translocation. The E1-like ubiquitin-activating enzyme (UBE1L) associates with interferon-stimulated gene ISG15 that binds and represses PML/RARα protein. Ubiquitin protease UBP43/USP18 removes ISG15 from conjugated proteins. In this study, we explored how RA regulates UBP43 expression and the effects of UBP43 on PML/RARα stability and APL growth, apoptosis, or differentiation. RA treatment induced UBE1L, ISG15, and UBP43 expression in RA-sensitive but not RA-resistant APL cells. Similar in vivo findings were obtained in a transgenic mouse model of transplantable APL, and in the RA response of leukemic cells harvested directly from APL patients. UBP43 knockdown repressed PML/RARα protein levels and inhibited RA-sensitive or RA-resistant cell growth by destabilizing the PML domain of PML/RARα. This inhibitory effect promoted apoptosis but did not affect the RA differentiation response in these APL cells. In contrast, elevation of UBP43 expression stabilized PML/RARα protein and inhibited apoptosis. Taken together, our findings define the ubiquitin protease UBP43 as a novel candidate drug target for APL treatment.

Pharicin B stabilizes retinoic acid receptor-α and presents synergistic differentiation induction with ATRA in myeloid leukemic cells

All-trans retinoic acid (ATRA), a natural ligand for the retinoic acid receptors (RARs), induces clinical remission in most acute promyelocytic leukemia (APL) patients through the induction of differentiation and/or eradication of leukemia-initiating cells. Here, we identify a novel natural ent-kaurene diterpenoid derived from Isodon pharicus leaves, called pharicin B, that can rapidly stabilize RAR-α protein in various acute myeloid leukemic (AML) cell lines and primary leukemic cells from AML patients, even in the presence of ATRA, which is known to induce the loss of RAR-α protein. Pharicin B also enhances ATRA-dependent the transcriptional activity of RAR-α protein in the promyelocytic leukemia-RARα-positive APL cell line NB4 cells. We also showed that pharicin B presents a synergistic or additive differentiation-enhancing effect when used in combination with ATRA in several AML cell lines and, especially, some primary leukemic cells from APL patients. In addition, pharicin B can overcome retinoid resistance in 2 of 3 NB4-derived ATRA-resistant subclones. These findings provide a good example for chemical biology-based investigations of pathophysiological and therapeutic significances of RAR-α and PML-RAR-α proteins. The effectiveness of the ATRA/pharicin B combination warrants further investigation on their use as a therapeutic strategy for AML patients.

Differential gene expression in bovine cells infected with wild type and leaderless foot-and-mouth disease virus

The leader proteinase (L(pro)) of foot-and-mouth disease virus (FMDV) plays a critical role in viral pathogenesis. Molecular studies have demonstrated that L(pro) inhibits translation of host capped mRNAs and transcription of some genes involved in the innate immune response. We have used microarray technology to study the gene expression profile of bovine cells infected with wild type (WT) or leaderless FMDV. Thirty nine out of approximately 22,000 bovine genes were selectively up-regulated by 2 fold or more in leaderless versus WT virus infected cells. Most of the up-regulated genes corresponded to IFN-inducible genes, chemokines or transcription factors. Comparison of promoter sequences suggested that host factors NF-kappaB, ISGF3G and IRF1 specifically contributed to the differential expression, being NF-kappaB primarily responsible for the observed changes. Our results suggest that L(pro) plays a central role in the FMDV evasion of the innate immune response by inhibiting NF-kappaB dependent gene expression.

ISG15 and immune diseases

ISG15, the product of interferon (IFN)-stimulated gene 15, is the first identified ubiquitin-like protein, consisting of two ubiquitin-like domains. ISG15 is synthesized as a precursor in certain mammals and, therefore, needs to be processed to expose the C-terminal glycine residue before conjugation to target proteins. A set of three-step cascade enzymes, an E1 enzyme (UBE1L), an E2 enzyme (UbcH8), and one of several E3 ligases (e.g., EFP and HERC5), catalyzes ISG15 conjugation (ISGylation) of a specific protein. These enzymes are unique among the cascade enzymes for ubiquitin and other ubiquitin-like proteins in that all of them are induced by type I IFNs or other stimuli, such as exposure to viruses and lipopolysaccharide. Mass spectrometric analysis has led to the identification of several hundreds of candidate proteins that can be conjugated by ISG15. Some of them are type I IFN-induced proteins, such as PKR and RIG-I, and some are the key regulators that are involved in IFN signaling, such as JAK1 and STAT1, implicating the role of ISG15 and its conjugates in type I IFN-mediated innate immune responses. However, relatively little is known about the functional significance of ISG15 induction due to the lack of information on the consequences of its conjugation to target proteins. Here, we describe the recent progress made in exploring the biological function of ISG15 and its reversible modification of target proteins and thus in their implication in immune diseases.

Differentiation therapy of leukemia: 3 decades of development

A characteristic feature of leukemia cells is a blockade of differentiation at a distinct stage in cellular maturation. In the 1970s and 1980s, studies demonstrating the capabilities of certain chemicals to induce differentiation of hematopoietic cell lines fostered the concept of treating leukemia by forcing malignant cells to undergo terminal differentiation instead of killing them through cytotoxicity. The first promising reports on this notion prompted a review article on this subject by us 25 years ago. In this review, we revisit this interesting field of study and report the progress achieved in the course of nearly 3 decades. The best proof of principle for differentiation therapy has been the treatment of acute promyelocytic leukemia with all-trans retinoic acid. Attempts to emulate this success with other nuclear hormone ligands such as vitamin D compounds and PPARgamma agonists or different classes of substances such as hematopoietic cytokines or compounds affecting the epigenetic landscape have not been successful on a broad scale. However, a multitude of studies demonstrating partial progress and improvements and, finally, the new powerful possibilities of forward and reverse engineering of differentiation pathways by manipulation of transcription factors support the continued enthusiasm for differentiation therapy of leukemia in the future.

UBE1L causes lung cancer growth suppression by targeting cyclin D1

UBE1L is the E1-like ubiquitin-activating enzyme for the IFN-stimulated gene, 15-kDa protein (ISG15). The UBE1L-ISG15 pathway was proposed previously to target lung carcinogenesis by inhibiting cyclin D1 expression. This study extends prior work by reporting that UBE1L promotes a complex between ISG15 and cyclin D1 and inhibited cyclin D1 but not other G1 cyclins. Transfection of the UBE1L-ISG15 deconjugase, ubiquitin-specific protein 18 (UBP43), antagonized UBE1L-dependent inhibition of cyclin D1 and ISG15-cyclin D1 conjugation. A lysine-less cyclin D1 species was resistant to these effects. UBE1L transfection reduced cyclin D1 protein but not mRNA expression. Cycloheximide treatment augmented this cyclin D1 protein instability. UBE1L knockdown increased cyclin D1 protein. UBE1L was independently retrovirally transduced into human bronchial epithelial and lung cancer cells. This reduced cyclin D1 expression and clonal cell growth. Treatment with the retinoid X receptor agonist bexarotene induced UBE1L and reduced cyclin D1 immunoblot expression. A proof-of-principle bexarotene clinical trial was independently examined for UBE1L, ISG15, cyclin D1, and Ki-67 immunohistochemical expression profiles in pretreatment versus post-treatment tumor biopsies. Increased UBE1L with reduced cyclin D1 and Ki-67 expression occurred in human lung cancer when a therapeutic bexarotene intratumoral level was achieved. Thus, a mechanism for UBE1L-mediated growth suppression was found by UBE1L-ISG15 preferentially inhibiting cyclin D1. Molecular therapeutic implications are discussed.

Granulocytic differentiation of HL-60 promyelocytic leukemia cells is associated with increased expression of Cul5

The human HL-60 promyelocytic leukemia cell line has been widely used as a model for studying granulocytic differentiation. All-trans retinoic acid (ATRA) treatment of HL-60 cells promotes granulocytic differentiation and is effective as differentiation therapy for patients with acute promyelocytic leukemia. The identification of genes that are transcriptionally regulated by ATRA has provided insight into granulocytic differentiation and differentiation therapy. The Asb-2 (ankyrin repeat SOCS box 2) gene has previously been identified as a transcriptional target in ATRA-treated HL-60 cells. The ASB-2 protein forms an E3 ubiquitin ligase complex with the proteins, Cul5, regulator of cullin 2 (ROC2), and elongin B and C. The purpose of this study was to determine if there is increased expression of Cul5 during granulocytic differentiation of HL-60 cells. To induce granulocytic differentiation, HL-60 cells were treated for 5 d with ATRA and differentiation was confirmed by examining superoxide anion production, nuclear morphology, and changes in the expression of CD11b, CD13, and CD15. Quantitative real-time RT-PCR was used to measure Cul5 mRNA expression and also the expression of other components of the E3 ubiquitin ligase (ASB-2, ROC2, elongin B and C). Granulocytic differentiation of HL-60 cells was associated with a 1.6-, 1.7-, and 23-fold statistically significant (P <or= 0.05) increase in mRNA expression for Cul5, ROC2, and ASB-2, respectively. No significant change was found in elongin B and C mRNA expression. Using Western blot analysis, the expression of Cul5 protein was increased 6.5-fold with granulocytic differentiation of the HL-60 cells. Increased expression of multiple components of the Cul5-containing E3 ubiquitin ligase complex with ATRA treatment of HL-60 cells indicates that this complex may play an important role in granulocytic differentiation.

G0S2 is an all-trans-retinoic acid target gene

All-trans-retinoic acid (RA) treatment of acute promyelocytic leukemia (APL) cases expressing the t(15;17) product, PML/RARalpha, is a successful example of differentiation therapy. Uncovering RA target genes is of considerable interest in APL. This study comprehensively examines in APL cells transcriptional and post-transcriptional regulation of the novel candidate RA target gene, G0S2, the G0/G1 switch gene. Reverse transcription (RT)-polymerase chain reaction (PCR) and heteronuclear PCR assays performed +/- treatment with the protein synthesis inhibitor cycloheximide (CHX) revealed G0S2 induction within 3 h of RA-treatment. Treatment with the RNA synthesis inhibitor actinomycin D did not implicate G0S2 transcript stabilization in the RA-mediated increase of G0S2 mRNA expression. Promoter elements of G0S2 were cloned into a reporter plasmid and retinoic acid receptor (RAR) co-transfection assays confirmed transcriptional activation after RA-treatment. Consistent with G0S2 being a direct RA target gene, retinoic acid response element (RARE) half-sites were found in this promoter. Mutation of these sites blocked RA-transcriptional activation of G0S2. To extend analyses to the protein expression level, a polyclonal anti-G0S2 antibody was derived and detected murine and human G0S2 species. G0S2 protein was rapidly induced in cultured NB4-S1 human APL cells and in APL transgenic mice treated with RA. An RAR pan-antagonist confirmed dependence on RARs for this induction. That these findings are clinically relevant was shown by analyses of APL cells derived directly from patients. These leukemic cells induced both a prominent increase in the cellular differentiation marker nitrotetrazolium blue (NBT) staining and marked increase in G0S2 expression. Taken together, these findings indicate G0S2 is an RA target gene. The functional role of G0S2 in retinoid response of APL warrants further study.