The promyelocytic leukemia protein represses A20-mediated transcription

The Paget's disease of bone risk gene PML is a negative regulator of osteoclast differentiation and bone resorption

Paget's disease of bone (PDB) is characterized by focal increases in bone remodelling. Genome-wide association studies identified a susceptibility locus for PDB tagged by rs5742915, which is located within the PML gene. Here, we have assessed the candidacy of PML as the predisposing gene for PDB at this locus. We found that the PDB-risk allele of rs5742915 was associated with lower PML expression and that PML expression in blood cells from individuals with PDB was lower than in controls. The differentiation, survival and resorptive activity of osteoclasts prepared from Pml-/- mice was increased compared with wild type. Furthermore, the inhibitory effect of IFN-γ on osteoclast formation from Pml-/- was significantly blunted compared with wild type. Bone nodule formation was also increased in osteoblasts from Pml-/- mice when compared with wild type. Although microCT analysis of trabecular bone showed no differences between Pml-/- mice and wild type, bone histomorphometry showed that Pml-/- mice had high bone turnover with increased indices of bone resorption and increased mineral apposition rate. These data indicate that reduced expression of PML predisposes an individual to PDB and identify PML as a novel regulator of bone metabolism. This article has an associated First Person interview with the first author of the paper.

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.

The function, regulation and therapeutic implications of the tumor suppressor protein, PML

The tumor suppressor protein, promyelocytic leukemia protein (PML), was originally identified in acute promyelocytic leukemia due to a chromosomal translocation between chromosomes 15 and 17. PML is the core component of subnuclear structures called PML nuclear bodies (PML-NBs), which are disrupted in acute promyelocytic leukemia cells. PML plays important roles in cell cycle regulation, survival and apoptosis, and inactivation or down-regulation of PML is frequently found in cancer cells. More than 120 proteins have been experimentally identified to physically associate with PML, and most of them either transiently or constitutively co-localize with PML-NBs. These interactions are associated with many cellular processes, including cell cycle arrest, apoptosis, senescence, transcriptional regulation, DNA repair and intermediary metabolism. Importantly, PML inactivation in cancer cells can occur at the transcriptional-, translational- or post-translational- levels. However, only a few somatic mutations have been found in cancer cells. A better understanding of its regulation and its role in tumor suppression will provide potential therapeutic opportunities. In this review, we discuss the role of PML in multiple tumor suppression pathways and summarize the players and stimuli that control PML protein expression or subcellular distribution.

Biphasic regulation of A20 gene expression during human cytomegalovirus infection

BACKGROUND

The A20 ubiquitin-editing enzyme is a target of nuclear factor kappa B (NF-κB) and also plays a key role in regulating the NF-κB signaling pathway. NF-κB activity is increased during human cytomegalovirus (HCMV) infection and HCMV appears to be adapted to this change. To better understand the regulation of NF-κB signaling during HCMV infection, we investigated how A20 expression is controlled during HCMV infection.

METHODS

The expression level of A20 in human fibroblast cells infected with HCMV or UV-inactivated virus (UV-HCMV) was measured by immunoblot analysis, cell staining, and quantitative real-time PCR. Changes of histone modifications on the A20 promoter were determined by chromatin immunoprecipitation assays. Lentiviral vectors were used to knockdown A20 in fibroblast cells.

RESULTS

A20 expression was increased at early times after HCMV infection. This increase of the A20 protein level was promoted by viral gene expression under low viral load conditions. The viral IE1 protein, which is known to activate NF-κB, increased the A20 promoter activity through the upstream NF-κB sites in reporter assays, suggesting that IE1 is at least partly involved in A20 induction. Analysis of A20 expression with a high viral load demonstrated that the A20 regulation by HCMV was biphasic; both A20 protein and mRNA levels were increased at the early stage of infection, but decreased at the late stage. Under high viral load conditions, A20 upregulation was more profound with UV-HCMV than with HCMV, indicating a role of the viral gene product(s) in limiting A20 induction. Consistently, more histone modifications for euchromatin were found on the A20 promoter during UV-HCMV infection than with HCMV infection. A20 knockdown by shRNA reduced HCMV growth.

CONCLUSION

These results suggest that the biphasic regulation of A20 expression may be important for productive HCMV infection.

PML-mediated signaling and its role in cancer stem cells

The promyelocytic leukemia (PML) protein, initially discovered as a part of the PML/retinoic acid receptor alpha fusion protein, has been found to be a critical player in oncogenesis and tumor progression. Multiple cellular activities, including DNA repair, alternative lengthening of telomeres, transcriptional control, apoptosis and senescence, are regulated by PML and its featured subcellular structure, the PML nuclear body. In correspondence with its role in many important life processes, PML mediates several complex downstream signaling pathways. The determinant function of PML in tumorigenesis and cancer progression raises the interest in its involvement in cancer stem cells (CSCs), a subpopulation of cancer cells that share properties with stem cells and are critical for tumor propagation. Recently, there are exciting discoveries concerning the requirement of PML in CSC maintenance. Growing evidences strongly suggest a positive role of PML in regulating CSCs in both hematopoietic cancers and solid tumors, whereas the underlying mechanisms may be different and remain elusive. Here we summarize and discuss the PML-mediated signaling pathways in cancers and their potential roles in regulating CSCs.

TRIM involvement in transcriptional regulation

Members of the tripartite motif (TRIM) protein family are found in all multicellular eukaryotes and function in a wide range of cellular processes such as cell cycle regulation, differentiation, development, oncogenesis and viral response. Over the past few years, several TRIM proteins have been reported to control gene expression through regulation of the transcriptional activity of numerous sequence-specific transcription factors. These proteins include the transcriptional intermediary factor 1 (TIF1) regulators, the promyelocytic leukemia tumor suppressor PML and the RET finger protein (RFP). In this chapter, we will consider the molecular interactions made by these TRIM proteins and will attempt to clarify some of the molecular mechanisms underlying their regulatory effect on transcription.

Antiviral TRIMs: friend or foe in autoimmune and autoinflammatory disease?

The concept that viral sensing systems, via their ability to drive pro-inflammatory cytokine and interferon production, contribute to the development of autoimmune and autoinflammatory disease is supported by a wide range of clinical and experimental observations. Recently, the tripartite motif-containing proteins (TRIMs) have emerged as having key roles in antiviral immunity - either as viral restriction factors or as regulators of pathways downstream of viral RNA and DNA sensors, and the inflammasome. Given their involvement in these pathways, we propose that TRIM proteins contribute to the development and pathology of autoimmune and autoinflammatory conditions, thus making them potential novel targets for therapeutic manipulation.

Expression, biological activities and mechanisms of action of A20 (TNFAIP3)

A20 (also known as TNFAIP3) is a cytoplasmic protein that plays a key role in the negative regulation of inflammation and immunity. Polymorphisms in the A20 gene locus have been identified as risk alleles for multiple human autoimmune diseases, and A20 has also been proposed to function as a tumor suppressor in several human B-cell lymphomas. A20 expression is strongly induced by multiple stimuli, including the proinflammatory cytokines TNF and IL-1, and microbial products that trigger pathogen recognition receptors, such as Toll-like receptors. A20 functions in a negative feedback loop, which mediates its inhibitory functions by downregulating key proinflammatory signaling pathways, including those controlling NF-κB- and IRF3-dependent gene expression. Activation of these transcription factors is controlled by both K48- and K63- polyubiquitination of upstream signaling proteins, respectively triggering proteasome-mediated degradation or interaction with other signaling proteins. A20 turns off NF-κB and IRF3 activation by modulating both types of ubiquitination. Induction of K48-polyubiquitination by A20 involves its C-terminal zinc-finger ubiquitin-binding domain, which may promote interaction with E3 ligases, such as Itch and RNF11 that are involved in mediating A20 inhibitory functions. A20 is thought to promote de-ubiquitination of K63-polyubiquitin chains either directly, due to its N-terminal deubiquitinase domain, or by disrupting the interaction between E3 and E2 enzymes that catalyze K63-polyubiquitination. A20 is subject to different mechanisms of regulation, including phosphorylation, proteolytic processing, and association with ubiquitin binding proteins. Here we review the expression and biological activities of A20, as well as the underlying molecular mechanisms.

Specification of the NF-kappaB transcriptional response by p65 phosphorylation and TNF-induced nuclear translocation of IKK epsilon

Here we investigated the regulation of NF-κB activity by post-translational modifications upon reconstitution of NF-κB p65-deficient cells with the wild-type protein or phosphorylation-defect mutants. Analysis of NF-κB target gene expression showed that p65 phosphorylations alone or in combination function to direct transcription in a highly target gene-specific fashion, a finding discussed here as the NF-κB barcode hypothesis. High-resolution microscopy and surface rendering revealed serine 536 phosphorylated p65 predominantly in the cytosol, while serine 468 phosphorylated p65 mainly localized in nuclear speckles. TNF stimulation resulted in the translocation of the cytosolic p65 kinase IKKε to the nucleus and also to promyelocytic leukemia (PML) nuclear bodies. This inducible IKKε translocation was dependent on p65 phosphorylation and was prevented by the oncogenic PML-RARα fusion protein. Chromatin immunoprecipitation experiments revealed the inducible association of IKKε to the control regions of several NF-κB target genes. In the nucleus, the kinase contributes to the expression of a subset of NF-κB-regulated genes, thus revealing a novel role of IKKε for the control of nuclear NF-κB activity.

Impaired TNFalpha-induced A20 expression in E1A/Ras-transformed cells

Background:

Tumour necrosis factor (TNF) is capable of activating the cell death pathway, and has been implicated in killing transformed cells. However, TNF also activates survival signals, including NF-κB activation and the subsequent expression of anti-apoptotic genes, leading to protection against TNF toxicity.

Methods:

In this study, we show that, although untransformed mouse embryonic fibroblasts (MEFs) were resistant to TNF killing, E1A/Ras-transformed MEFs were susceptible to extensive apoptosis induced by TNF. The key factors for determining TNF sensitivity were explored by comparing wild-type and E1A/Ras-transformed MEFs.

Results:

TNF signalling to NF-κB and to its target genes such as IκBα seemed to be mostly intact in E1A/Ras-transformed cells. Instead, the induction of A20 was completely abolished in E1A/Ras-transformed MEFs, although A20 is known to be NF-κB dependent. Reintroduction of A20 into E1A/Ras-transformed MEFs rescued these cells from TNF-induced death and reduced the formation of the FADD/caspase-8 complex. This impaired A20 induction in E1A/Ras MEFs was not because of the stabilisation of p53 or a defective TNF-induced p38 and Jun N-terminal kinase (JNK) signalling. Consistently, we found a reduced A20 promoter activity but normal NF-κB activity in TNF-treated E1A/Ras MEFs. However, Bcl-3 seemed to have a role in the transactivation of the A20 promoter in E1A/Ras cells.

Conclusions:

Our results suggest that specific inhibition of certain survival factors, such as A20, may determine the sensitivity to TNF-induced apoptosis in transformed cells such as E1A/Ras MEFs.

Promyelocytic leukemia protein induces apoptosis due to caspase-8 activation via the repression of NFkappaB activation in glioblastoma

Promyelocytic leukemia (PML) protein plays an essential role in the induction of apoptosis; its expression is reduced in various cancers. As the functional roles of PML in glioblastoma multiforme (GBM) have not been clarified, we assessed the expression of PML protein in GBM tissues and explored the mechanisms of PML-regulated cell death in GBM cells. We examined the PML mRNA level and the expression of PML protein in surgical GBM specimens. PML-regulated apoptotic mechanisms in GBM cells transfected with plasmids expressing the PML gene were examined. The protein expression of PML was significantly lower in GBM than in non-neoplastic tissues; approximately 10% of GBM tissues were PML-null. The PML mRNA levels were similar in both tissue types. The overexpression of PML activated caspase-8 and induced apoptosis in GBM cells. In these cells, PML decreased the expression of transactivated forms of NFkappaB/p65, and c-FLIP gene expression was suppressed. Therefore, PML-induced apoptosis resulted from the suppression of the transcriptional activity of NFkappaB/p65. PML overexpression decreased phosphorylated IkappaBalpha and nuclear NFkappaB/p65 and increased the expression of the suppressor of cytokine signaling (SOCS-1). A proteasome inhibitor blocked the reduction of activated p65 by PML. The reduction of PML is associated with the pathogenesis of GBM. PML induces caspase-8-dependent apoptosis via the repression of NFkappaB activation by which PML facilitates the proteasomal degradation of activated p65 and the sequestration of p65 with IkappaBalpha in the cytoplasm. This novel mechanism of PML-regulated apoptosis may represent a therapeutic target for GBM.

Functional interactions between the Epstein-Barr virus BZLF1 protein and the promyelocytic leukemia protein

The Epstein-Barr virus immediate-early protein BZLF1 (Z) has been shown to alter the cellular localization of the promyelocytic leukemia (PML) protein. PML has important implications for growth control, apoptosis, anti-viral effects and many more processes. Here we further examined the relationship between PML and the Epstein-Barr virus Z protein. We examined the effect of Z expression on PML protein levels, and the effect of increased PML protein levels on Z-mediated dispersion of PML bodies. We found that increased levels of PML protein, such as through interferon treatment, were able to suppress Z-mediated PML body dispersion. We also studied the consequences of PML dispersion by Z, by examining p21 transactivation, A20 transactivation, and MHC Class I presentation levels in Z-expressing cells. We found that, while Z-mediated dispersion of PML did not affect MHC Class I presentation, it did alter p21 and A20 expression. In addition, we found that increased levels of PML were able to prevent Z protein binding to mitotic chromosomes. Our work implies that the balance of PML and Z levels in cells may affect how each protein functions.

Overexpression of PML induced apoptosis in bladder cancer cell by caspase dependent pathway

The promyelocytic leukemia gene (PML) encodes a growth/tumor suppressor protein that is essential for the induction of apoptosis in response to various apoptotic signals. The mechanism by which PML plays a role in the regulation of cell death is still unknown. Our previous study demonstrated that overexpression of PML suppress the growth of bladder cancer cells by inducing apoptosis and cell cycle arrest. To further elucidate the mechanism of PML induced apoptosis in bladder cancer, we constructed a PML inducible stable cell line. We found that the increased expression of PML significantly inhibit the growth of the UM-UC-2/PML clone cells and present apparent massive apoptosis in 24 h post-induction, while the UM-UC-2/PMEP4 cells are not. We also examined the effect of PML on the cell cycle distribution in UM-UC-2 cells. We showed overexpression of PML cause a cell cycle arrest in G1 phase. In additional, increased expression of PML in bladder cancer UM-UC-2 cells reduce Survivin expression and up regulated Caspase-3, and cleaved PARP expression, these suggested that PML might regulate apoptosis through Caspase dependent pathways. Our results demonstrate a novel mechanism of PML-induced apoptosis by down-regulation of Survivin and activation of Caspase dependent pathway.

Promyelocytic leukemia nuclear bodies associate with transcriptionally active genomic regions

The promyelocytic leukemia (PML) protein is aggregated into nuclear bodies that are associated with diverse nuclear processes. Here, we report that the distance between a locus and its nearest PML body correlates with the transcriptional activity and gene density around the locus. Genes on the active X chromosome are more significantly associated with PML bodies than their silenced homologues on the inactive X chromosome. We also found that a histone-encoding gene cluster, which is transcribed only in S-phase, is more strongly associated with PML bodies in S-phase than in G0/G1 phase of the cell cycle. However, visualization of specific RNA transcripts for several genes showed that PML bodies were not themselves sites of transcription for these genes. Furthermore, knock-down of PML bodies by RNA interference did not preferentially change the expression of genes closely associated with PML bodies. We propose that PML bodies form in nuclear compartments of high transcriptional activity, but they do not directly regulate transcription of genes in these compartments.

Role of PML and the PML-nuclear body in the control of programmed cell death

PML is a tumor suppressor implicated in leukemia and cancer pathogenesis. PML epitomizes a multiprotein nuclear structure, the PML-nuclear body (PML-NB), whose proper formation and function depends on PML. Studies in knockout (KO) mice and cells unraveled an essential pleiotropic role for PML in multiple p53-dependent and -independent apoptotic pathways. As a result, Pml(-/-) mice and cells are protected from apoptosis triggered by a number of stimuli such as ionizing radiation, interferon, ceramide, Fas and TNF. It is becoming apparent that PML and the PML-NB act as molecular hubs for the induction and/or reinforcement of programmed cell death through a selective and dynamic regulation of proapoptotic transcriptional events. In addition, recent observations propose a role for PML in checkpoint responses upon DNA damage. Moreover, PML and the PML-NB have also been implicated in the control of genomic stability and DNA repair. Here, we will discuss the molecular mechanisms by which PML regulates these processes and the implication of these findings for cancer pathogenesis and therapy.

Promyelocytic leukemia protein 4 induces apoptosis by inhibition of survivin expression

The promyelocytic leukemia protein (PML) plays an essential role in multiple pathways of apoptosis. Our previous study showed that PML enhances tumor necrosis factor-induced apoptosis by inhibiting the NFkappaB survival pathway. To continue exploring the mechanism of PML-induced apoptosis, we performed a DNA microarray screening of PML target genes using a PML-inducible stable cell line. We found that Survivin was one of the downstream target genes of PML. Cotransfection experiments demonstrated that PML4 repressed transactivation of the Survivin promoter in an isoform-specific manner. Western blot analysis demonstrated that induced PML expression down-regulated Survivin. Inversely, PML knockdown by siRNA up-regulated Survivin expression. A substantial increase in Survivin expression was found in PML-deficient cells. Re-expression of PML in PML-/- mouse embryo fibroblasts down-regulated the expression of Survivin. Furthermore, cells arrested at the G2/M cell cycle phase expressed a high level of Survivin and a significantly lower level of PML. Overexpression of PML in A549 cells reduced Survivin expression leading to massive apoptotic cell death associated with activation of procaspase 9, caspase 3, and caspase 7. Together, our results demonstrate a novel mechanism of PML-induced apoptosis by down-regulation of Survivin.

PML colocalizes with and stabilizes the DNA damage response protein TopBP1

The PML tumor suppressor gene is consistently disrupted by t(15;17) in patients with acute promyelocytic leukemia. Promyelocytic leukemia protein (PML) is a multifunctional protein that plays essential roles in cell growth regulation, apoptosis, transcriptional regulation, and genome stability. Our study here shows that PML colocalizes and associates in vivo with the DNA damage response protein TopBP1 in response to ionizing radiation (IR). Both PML and TopBP1 colocalized with the IR-induced bromodeoxyuridine single-stranded DNA foci. PML and TopBP1 also colocalized with Rad50, Brca1, ATM, Rad9, and BLM. IR and interferon (IFN) coinduce the expression levels of both TopBP1 and PML. In PML-deficient NB4 cells, TopBP1 was unable to form IR-induced foci. All-trans-retinoic acid induced reorganization of the PML nuclear body (NB) and reappearance of the IR-induced TopBP1 foci. Inhibition of PML expression by siRNA is associated with a significant decreased in TopBP1 expression. Furthermore, PML-deficient cells express a low level of TopBP1, and its expression cannot be induced by IR or IFN. Adenovirus-mediated overexpression of PML in PML(-/-) mouse embryo fibroblasts substantially increased TopBP1 expression, which colocalized with the PML NBs. These studies demonstrated a mechanism of PML-dependent expression of TopBP1. PML overexpression induced TopBP1 protein but not the mRNA expression. Pulse-chase labeling analysis demonstrated that PML overexpression stabilized the TopBP1 protein, suggesting that PML plays a role in regulating the stability of TopBP1 in response to IR. Together, our findings demonstrate that PML regulates TopBP1 functions by association and stabilization of the protein in response to IR-induced DNA damage.

Promyelocytic leukemia protein sensitizes tumor necrosis factor alpha-induced apoptosis by inhibiting the NF-kappaB survival pathway

The promyelocytic leukemia protein (PML) is a growth/tumor suppressor essential for induction of apoptosis by diverse apoptotic stimuli. The mechanism by which PML regulates cell death remains unclear. In this study we found that ectopic expression of PML potentiates cell death by apoptosis in the tumor necrosis factor alpha (TNFalpha)-resistant cell line U2OS and other cell lines. Treatment with TNFalpha significantly sensitized these cells to apoptosis in a p53-independent manner. PML/TNFalpha-induced cell death is associated with DNA fragmentation, activation of caspase-3, -7, and -8, and degradation of DNA fragmentation factor/inhibitor of CAD. PML/TNFalpha-induced cell death could be blocked by the caspase-8 inhibitors CrmA and c-FLIP but not by Bcl-2. These findings indicate that this cell death event is initiated through the death receptor-dependent apoptosis pathway. PML is a transcriptional repressor of NF-kappaB by interacting with RelA/p65 and prevents its binding to the cognate enhancer through the C terminus. Coimmunoprecipitation and double-color immunofluorescence staining demonstrated that PML physically interacts with RelA/p65 in vivo and the two proteins colocalized at the endogenous levels. Overexpression of NF-kappaB rescued cell death induced by PML/TNFalpha. Furthermore, PML(-/-) mouse embryo fibroblasts are more resistant to TNFalpha-induced apoptosis. Together this study defines a novel mechanism by which PML induces apoptosis through repression of the NF-kappaB survival pathway.