Altered expression of the growth and transformation suppressor PML gene in human hepatocellular carcinomas and in hepatitis tissues

Deciphering roles of TRIMs as promising targets in hepatocellular carcinoma: current advances and future directions

Tripartite motif (TRIM) family is assigned to RING-finger-containing ligases harboring the largest number of proteins in E3 ubiquitin ligating enzymes. E3 ubiquitin ligases target the specific substrate for proteasomal degradation via the ubiquitin-proteasome system (UPS), which seems to be a more effective and direct strategy for tumor therapy. Recent advances have demonstrated that TRIM genes associate with the occurrence and progression of hepatocellular carcinoma (HCC). TRIMs trigger or inhibit multiple biological activities like proliferation, apoptosis, metastasis, ferroptosis and autophagy in HCC dependent on its highly conserved yet diverse structures. Remarkably, autophagy is another proteolytic pathway for intracellular protein degradation and TRIM proteins may help to delineate the interaction between the two proteolytic systems. In depth research on the precise molecular mechanisms of TRIM family will allow for targeting TRIM in HCC treatment. We also highlight several potential directions warranted further development associated with TRIM family to provide bright insight into its translational values in hepatocellular carcinoma.

TRIM proteins in hepatocellular carcinoma

The tripartite motif (TRIM) protein family is a highly conserved group of E3 ligases with 77 members known in the human, most of which consist of a RING-finger domain, one or two B-box domains, and a coiled-coil domain. Generally, TRIM proteins function as E3 ligases to facilitate specific proteasomal degradation of target proteins. In addition, E3 ligase independent functions of TRIM protein were also reported. In hepatocellular carcinoma, expressions of TRIM proteins are both regulated by genetic and epigenetic mechanisms. TRIM proteins regulate multiple biological activities and signaling cascades. And TRIM proteins influence hallmarks of HCC. This review systematically demonstrates the versatile roles of TRIM proteins in HCC and helps us better understand the molecular mechanism of the development and progression of HCC.

The translational values of TRIM family in pan-cancers: From functions and mechanisms to clinics

Cancer is the second leading cause of human death across the world. Tripartite motif (TRIM) family, with E3 ubiquitin ligase activities in majority of its members, is reported to be involved in multiple cellular processes and signaling pathways. TRIM proteins have critical effects in the regulation of biological behaviors of cancer cells. Here, we discussed the current understanding of the molecular mechanism of TRIM proteins regulation of cancer cells. We also comprehensively reviewed published studies on TRIM family members as oncogenes or tumor suppressors in the oncogenesis, development, and progression of a variety of types of human cancers. Finally, we highlighted that certain TRIM family members are potential molecular biomarkers for cancer diagnosis and prognosis, and potential therapeutic targets.

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.

Cytoplasmic PML promotes TGF-β-associated epithelial-mesenchymal transition and invasion in prostate cancer

Epithelial-mesenchymal transition (EMT) is a key event that is involved in the invasion and dissemination of cancer cells. Although typically considered as having tumour-suppressive properties, transforming growth factor (TGF)-β signalling is altered during cancer and has been associated with the invasion of cancer cells and metastasis. In this study, we report a previously unknown role for the cytoplasmic promyelocytic leukaemia (cPML) tumour suppressor in TGF-β signalling-induced regulation of prostate cancer-associated EMT and invasion. We demonstrate that cPML promotes a mesenchymal phenotype and increases the invasiveness of prostate cancer cells. This event is associated with activation of TGF-β canonical signalling pathway through the induction of Sma and Mad related family 2 and 3 (SMAD2 and SMAD3) phosphorylation. Furthermore, the cytoplasmic localization of promyelocytic leukaemia (PML) is mediated by its nuclear export in a chromosomal maintenance 1 (CRM1)-dependent manner. This was clinically tested in prostate cancer tissue and shown that cytoplasmic PML and CRM1 co-expression correlates with reduced disease-specific survival. In summary, we provide evidence of dysfunctional TGF-β signalling occurring at an early stage in prostate cancer. We show that this disease pathway is mediated by cPML and CRM1 and results in a more aggressive cancer cell phenotype. We propose that the targeting of this pathway could be therapeutically exploited for clinical benefit.

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.

Cytoplasmic PML: from molecular regulation to biological functions

The tumor suppressor promyelocytic leukemia protein (PML) is predominantly localized in the nucleus, where it is essential for the formation and stabilization of the PML nuclear bodies (PML-NBs). PML-NBs are involved in the regulation of numerous cellular functions, such as tumorigenesis, DNA damage and antiviral responses. Despite its nuclear localization, a small portion of PML has been found in the cytoplasm. A number of studies recently demonstrated that the cytoplasmic PML (cPML) has diverse functions in many cellular processes including tumorigenesis, metabolism, antiviral responses, cell cycle regulation, and laminopothies. In this prospective, we will summarize the current viewpoints on the regulation and biological significance of cPML and discuss the important questions that still need to be further answered.

Polymorphisms in microRNA target sites modulate risk of lymphoblastic and myeloid leukemias and affect microRNA binding

BACKGROUND

MicroRNA dysregulation is a common event in leukemia. Polymorphisms in microRNA-binding sites (miRSNPs) in target genes may alter the strength of microRNA interaction with target transcripts thereby affecting protein levels. In this study we aimed at identifying miRSNPs associated with leukemia risk and assessing impact of these miRSNPs on miRNA binding to target transcripts.

METHODS

We analyzed with specialized algorithms the 3' untranslated regions of 137 leukemia-associated genes and identified 111 putative miRSNPs, of which 10 were chosen for further investigation. We genotyped patients with acute myeloid leukemia (AML, n = 87), chronic myeloid leukemia (CML, n = 140), childhood acute lymphoblastic leukemia (ALL, n = 101) and healthy controls (n = 471). Association between SNPs and leukemia risk was calculated by estimating odds ratios in the multivariate logistic regression analysis. For miRSNPs that were associated with leukemia risk we performed luciferase reporter assays to examine whether they influence miRNA binding.

RESULTS

Here we show that variant alleles of TLX1_rs2742038 and ETV6_rs1573613 were associated with increased risk of childhood ALL (OR (95% CI) = 3.97 (1.43-11.02) and 1.9 (1.16-3.11), respectively), while PML_rs9479 was associated with decreased ALL risk (OR = 0.55 (0.36-0.86). In adult myeloid leukemias we found significant associations between the variant allele of PML_rs9479 and decreased AML risk (OR = 0.61 (0.38-0.97), and between variant alleles of IRF8_ rs10514611 and ARHGAP26_rs187729 and increased CML risk (OR = 2.4 (1.12-5.15) and 1.63 (1.07-2.47), respectively). Moreover, we observed a significant trend for an increasing ALL and CML risk with the growing number of risk genotypes with OR = 13.91 (4.38-44.11) for carriers of ≥3 risk genotypes in ALL and OR = 4.9 (1.27-18.85) for carriers of 2 risk genotypes in CML. Luciferase reporter assays revealed that the C allele of ARHGAP26_rs187729 creates an illegitimate binding site for miR-18a-3p, while the A allele of PML_rs9479 enhances binding of miR-510-5p and the C allele of ETV6_rs1573613 weakens binding of miR-34c-5p and miR-449b-5p.

CONCLUSIONS

Our study implicates that microRNA-binding site polymorphisms modulate leukemia risk by interfering with the miRNA-mediated regulation. Our findings underscore the significance of variability in 3' untranslated regions in leukemia.

Promyelocytic leukaemia protein links DNA damage response and repair to hepatitis B virus-related hepatocarcinogenesis

DNA damage response and repair pathways are important barriers to carcinogenesis. Here, we show that promyelocytic leukaemia (PML, also known as TRIM19), involved in sensing DNA damage and executing homologous recombination repair, is down-regulated in non-tumour liver cells surrounding hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC). No PML mutation or deletion was found in HBV-infected liver or HCC cells. Immunohistochemical analysis of liver biopsies from patients with breast or liver cancer and HBV reactivation after chemotherapy revealed PML up-regulation and HBV exacerbation in normal liver tissue in response to DNA damage (functional PML), PML down-regulation in HCC peritumour cells associated with high HBsAg accumulation and low HBV replication activity (suppressive PML), and heterogeneous nuclear PML expression in HCC cells that lost HBV DNA and HBsAg and were non-reactive to DNA damage (dysregulated PML). Loss of PML in HBsAg-transgenic mice promoted chromosome breaks in liver cells and accelerated the accumulation of body and liver fat and the development of a liver steatosis-dysplasia-adenoma-carcinoma sequence in an inflammation-independent and male-predominant manner, compared to PML knock-out or HBsAg-transgenic mice during the same time period. These results indicate that PML deficiency facilitates genomic instability and promotes HBsAg-related hepatocarcinogenesis, which also involves androgen and lipid metabolism. These findings uncover a novel PML link between HBV-related tumourigenesis, DNA repair, and metabolism.

Emerging Cellular Functions of Cytoplasmic PML

The tumor suppressor promyelocytic leukemia protein (PML) is located primarily in the nucleus, where it is the scaffold component of the PML nuclear bodies (PML-NBs). PML-NBs regulate multiple cellular functions, such as apoptosis, senescence, DNA damage response, and resistance to viral infection. Despite its nuclear localization, a small portion of PML has been identified in the cytoplasm. The cytoplasmic PML (cPML) could be originally derived from the retention of exported nuclear PML (nPML). In addition, bona fide cPML isoforms devoid of nuclear localization signal (NLS) have also been identified. Recently, emerging evidence showed that cPML performs its specific cellular functions in tumorigenesis, glycolysis, antiviral responses, laminopothies, and cell cycle regulation. In this review, we will summarize the emerging roles of cPML in cellular functions.

Defective DNA damage response and repair in liver cells expressing hepatitis B virus surface antigen

Hepatitis B virus (HBV) is implicated in liver cancer. The aim of this study was to find out whether HBV or its components [HBV surface antigen (HBsAg), HBV core protein (HBc), and HBV X protein (HBx)] could interfere with the host DNA damage response and repair pathway. The full HBV genome or individual HBV open-reading frame (ORF) was introduced into HepG2 cells to examine the effect on host genomic stability, DNA repair efficacy in response to double-strand DNA damage, and DNA damage-induced cell death. Responses to apoptosis induction in the HBV ORF-transfected HepG2 cells were also compared with those in HBV-positive and HBV-negative human hepatocellular carcinoma (HCC) cells. In the absence of HBV replication, accumulation of HBsAg in liver cells without other HBV proteins enhanced DNA repair protein and tumor suppressor promyelocytic leukemia (PML) degradation, which resulted in resistance to apoptosis induction and deficient double-strand DNA repair. However, HBsAg-positive cells exhibited increased cell death with exposure to the poly(ADP-ribose) polymerase inhibitor that blocks single-strand DNA repair. These results indicate that suppression of PML by HBsAg disrupts cellular mechanisms that respond to double-strand DNA damage for DNA repair or apoptosis induction, which may facilitate hepatocarcinogenesis and open up a synthetic lethality strategy for HBsAg-positive HCC treatment.

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.

Pondering the puzzle of PML (promyelocytic leukemia) nuclear bodies: can we fit the pieces together using an RNA regulon?

The promyelocytic leukemia protein PML and its associated nuclear bodies are hot topics of investigation. This interest arises for multiple reasons including the tight link between the integrity of PML nuclear bodies and several disease states and the impact of the PML protein and PML nuclear bodies on proliferation, apoptosis and viral infection. Unfortunately, an understanding of the molecular underpinnings of PML nuclear body function remains elusive. Here, a general overview of the PML field is provided and is extended to discuss whether some of the basic tenets of "PML-ology" are still valid. For instance, recent findings suggest that some components of PML nuclear bodies form bodies in the absence of the PML protein. Also, a new model for PML nuclear body function is proposed which provides a unifying framework for its effects on diverse biochemical pathways such as Akt signaling and the p53-Mdm2 axis. In this model, the PML protein acts as an inhibitor of gene expression post-transcriptionally via inhibiting a network node in the eIF4E RNA regulon. An example is given for how the PML RNA regulon model provided the basis for the development of a new anti-cancer strategy being tested in the clinic.

Concordant expression of proto-oncogene promyelocytic leukemia and major histocompatibility antigen HLA class I in human hepatocellular carcinoma

Many malignant cancer cells downregulate human leukocyte antigen (HLA) class I antigen expression to evade T cell recognition. However, hepatocellular carcinoma (HCC) is exceptional to the general findings in cancer cells, and the mechanisms for its upregulation remain unclear. It has been reported that promyelocytic leukemia (PML) proto-oncogene controls the transcription of multiple class I antigen presentation genes in murine cancer cells. To find out the functional role of PML gene on the increased HLA class I antigen expression in HCC cells, we analyzed the expression of proto-oncogene PML and multiple class I antigen presentation genes in HCC specimens obtained in China. The results showed concordant changes of proto-oncogene PML and cell surface HLA-A expression in 44 paraffin-embedded HCC tissues. Furthermore, co-upregulated expression of PML genes and class I antigen presentation genes could be detected in 9 of 15 fresh HCC tissues by reverse transcription polymerase chain reaction (RT-PCR). In addition, studies using HCC cell lines showed that increased expression of HLA class I molecules paralleled with PML upregulation were detected in QGY-7701 HCC cell line with RT-PCR, western blot, and flow cytometry, and that the overexpression of exogenous PML in a low-expression class I cell line BEL-7405 could induce the expression of multiple class I antigen-presenting molecule genes and slightly but significantly increase the expression of cell surface HLA class I molecules. In conclusion, the expression of proto-oncogene PML and HLA class I molecules were concordantly upregulated and the expression of PML gene might be one of the mechanisms that leads to the increased expression of class I antigen in HCC.

Loss of promyelocytic leukemia protein in human gastric cancers

To clarify the clinical implications of promyelocytic leukemia (PML) expression in gastric carcinomas, the expression of PML was analyzed in large series of gastric carcinoma by immunohistochemistry, western blotting and reverse transcription-PCR. PML protein expression was reduced or abolished in gastric carcinomas (31.7 and 10.6%, respectively) by immunohistochemistry. PML protein loss was associated with more lymphatic invasion, higher pTNM stage, and worse patient survival. Only one gastric carcinoma cell line showed loss of PML, and the PML protein re-appeared after the treatment of proteasome inhibitor in this cell line. We conclude that PML protein loss occurs in a minority of gastric carcinomas during carcinogenesis and progression, and suggest the proteasome-dependent pathway as a mechanism of PML protein loss.

Nuclear structure in cancer cells

Nuclear architecture - the spatial arrangement of chromosomes and other nuclear components - provides a framework for organizing and regulating the diverse functional processes within the nucleus. There are characteristic differences in the nuclear architectures of cancer cells, compared with normal cells, and some anticancer treatments restore normal nuclear structure and function. Advances in understanding nuclear structure have revealed insights into the process of malignant transformation and provide a basis for the development of new diagnostic tools and therapeutics.

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.

Promyelocytic leukaemia (PML) protein expression in human placenta and choriocarcinoma

Promyelocytic leukaemia (PML) protein, the product of the pml gene, is heterogeneously expressed in various normal and neoplastic tissues, and the fusion of the pml gene with retinoic acid receptor-alpha is believed to be a central mechanism in acute PML tumourigenesis. As PML is important for controlling major cellular processes, such as growth and differentiation, it is believed that it plays an important role during human gestation. The human placenta is a critical organ for the maintenance of gestation, but the expression pattern and functional significance of PML in the placenta have not been documented. The present study has therefore investigated the expression of PML in the human placenta and in choriocarcinoma, and has observed the biological effects following the overexpression of PML in choriocarcinoma cell lines (BeWo and JEG-3). In the human placenta, PML expression was readily found in villous stromal fibroblasts, capillary endothelial cells, Hofbauer cells, and occasionally in amnion cells. Moreover, immunoblotting of placental lysates demonstrated increased PML expression with increasing gestation. Interestingly, PML expression was confined to intermediate trophoblasts and syncytiotrophoblastic giant cells at the placental site (placental site giant cells) in the trophoblastic cell population. Intermediate trophoblasts at non-placental sites, and villous cytotrophoblasts and syncytiotrophoblasts consistently did not express PML. Further screening of PML expression in hydatidiform moles (n = 4) and choriocarcinomas (n = 7) also revealed selective PML expression in intermediate trophoblastic cells and syncytiotrophoblastic cells, but not in the cytotrophoblastic populations, which corresponds well with observations in the placental bed. Adenoviral transduction of PML resulted in a marked reduction in cell growth in both choriocarcinoma cell lines, which was associated with increased apoptosis. The findings of the present study strongly suggest that PML plays an important role in human placental development and growth, and in the pathobiology of trophoblasts and trophoblastic neoplasia.

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.

Expression of promyelocytic leukemia protein increases during the differentiation of human neuroblastoma cells

The promyelocytic leukemia (PML) protein, whose fusion with retinoic acid receptor alpha is responsible for the tumorigenesis of acute promyelocytic leukemia, acts as a tumor suppressor in various types of human cancers. We analyzed the expression patterns of PML, in both primary neuroblastic tumors ( n=20) and two human neuroblastoma (NB) cell lines, SMS-KCNR (KCNR) and SH-SY5Y (SY5Y). The expression of PML, revealed as speckled or microgranular staining in the nuclei, was positively correlated with the differentiation status of NB cells in vivo, and was upregulated during the differentiation of KCNR and SY5Y cells following retinoic acid treatment. Screening of PML expression in human brain and sympathetic ganglia showed restricted expression of PML in mature neurons and glial cells, a result that was consistent with that in differentiated NB tumors. All these findings strongly suggest that increased PML expression is associated with growth inhibition and differentiation of human NB cells, and that it is of critical significance in the biology of NBs and in human nervous system development.

Differential expression of the suppressor PML and Ki-67 identifies three subtypes of human nasopharyngeal carcinoma

The promyelocytic leukaemia (PML) gene, which encodes a transformation and growth suppressor, was found to regulate transcription and apoptosis. PML was first identified at the chromosomal translocation break-points t(15;17) of acute promyelocytic leukaemia and the gene product may mediate cell-cycle control and apoptosis. PML was found to interact with the co-transactivator CREB binding protein (CBP) and the apoptotic-modulator Bax. To determine if PML, CBP and Bax may be involved in solid tumours, such as the nasopharyngeal carcinoma (NPC), a rare neoplasia that is prevalent in Southern China, the expression of these proteins and the proliferation marker Ki-67 was analysed by immunohistochemical staining. Expression of PML in the PML-oncogenic domain (POD) or nuclear bodies in most NPC was inversely correlated with the expression of Ki-67. In addition, based on PML expression patterns in NPC three subtypes could be identified, namely, Subtype-1, with strong PML expression in POD structures and with low Ki-67 staining; Subtype-2, where PML was expressed in a homogeneously diffused pattern, but with a low intensity in the tumour cells; while Ki-67 was expressed in a moderate number of cells and Subtype-3, where the majority of tumour cells were PML-negative, while a considerable number of tumour cells were strongly labelled with Ki-67. Furthermore, CBP was present in most of the NPC cells with moderate-strong nuclear staining, while the expression in non-tumour cells were relatively weak. However, there was no direct correlation between PML and CBP expression in the NPC examined. In addition, there was low or no expression of Bax in the NP and NPC. This is, to our knowledge, the first report describing PML and CBP expression in NPC and our data strongly suggests that PML and CBP, but not Bax, may play a role in the transformed phenotypes of NPC.

The RING domains of the promyelocytic leukemia protein PML and the arenaviral protein Z repress translation by directly inhibiting translation initiation factor eIF4E

The promyelocytic leukemia protein (PML) is a mammalian regulator of cell growth which is characteristically disrupted in acute promyelocytic leukemia and by a variety of viruses. PML contains a RING domain which is required for its growth-suppressive and antiviral properties. Although normally nuclear, in certain pathogenic conditions, including arenaviral infection, PML is relocated to the cytoplasm, where its functions are poorly understood. Here, we observe that PML and arenavirus protein Z use regions around the first zinc-binding site of their respective RING domains to directly interact, with sub-micromolar affinity, with the dorsal surface of translation initiation factor eIF4E, representing a novel mode of eIF4E recognition. PML and Z profoundly reduce the affinity of eIF4E for its substrate, the 5' 7-methyl guanosine cap of mRNA, by over 100-fold. Association with the dorsal surface of eIF4E and direct antagonism of mRNA cap binding by PML and Z lead to direct inhibition of translation. These activities of the RING domains of PML and Z do not involve ubiquitin-mediated protein degradation, in contrast to many RINGs which have been observed to do so. Although PML and Z have well characterized physiological functions in regulation of growth and apoptosis, this work establishes the first discrete biochemical mechanism which underlies the biological activities of their RING domains. Thus, we establish PML and Z as translational repressors, with potential contributions to the pathogenesis of acute promyelocytic leukemia and variety of viral infections.

Expression of promyelocytic leukaemia protein in thyroid neoplasms

AIMS

Promyelocytic leukaemia protein (PML) is an oncoprotein involved in the pathogenesis of acute promyelocytic leukaemia and is localized in distinct PML nuclear bodies. Our previous observation of overexpression of the PML in hormone-sensitive normal tissues and malignant solid tumours, including the thyroid, led to this analysis of the PML expression in various thyroid neoplasms to characterize the importance of the PML in thyroid carcinogenesis.

METHODS AND RESULTS

Immunohistochemistry was performed on paraffin-embedded tissue samples from 106 thyroid neoplasms after antigen retrieval by microwave. Immunoblotting was done with fresh frozen tissues in a few tumours. The PML was strongly expressed in all papillary carcinomas in diffuse or ball-shaped patterns. In the follicular neoplasms, the PML expression was variable, but there was no significant difference between adenomas and carcinomas. In the medullary carcinomas, the PML expression was either not detectable or was lower than in non-neoplastic thyroids. Quantitatively different expression of the PML in various thyroid neoplasms was confirmed by immunoblotting.

CONCLUSION

A significant difference of the PML expression according to the type of thyroid neoplasms suggests that the PML is important in papillary thyroid carcinomas, and furthermore, that PML expression may be used in differential diagnosis of thyroid neoplasms.

Lack of expression for the suppressor PML in human small cell lung carcinoma

The promyelocytic leukemia (PML) gene, which encodes a transformation and growth suppressor, was first identified at the chromosomal translocation break point t(15;17) in acute promyelocytic leukemia (PML). To determine if the PML gene might be involved in other neoplasias such as lung cancer, PML expression was analyzed by immunohistochemical staining and in situ hybridization. Considerable PML protein expression in the PML-oncogenic domain (POD) structure was found in adenocarcinomas (ADC) and squamous cell carcinomas (SCC) of the lung, but was almost completely absent in all the small cell lung carcinomas (SCLC) examined. In situ hybridization showed that both mRNA and DNA of PML were present in SCLC and in normal lung, suggesting that the decreased protein expression was due to either a defect in translation or protein instability, rather than the consequence of decreased transcription or gene deletion. Double staining showed that PML expression was inversely correlated with the proliferation marker Ki-67 and positively correlated with levels of apoptotic cells in these tumors. To determine if the precursor cells of SCLC, the neuroendocrine-producing cells, express PML, double labeling was performed with PML and chromogranin A, a bio-marker for neuroendocrine cells. Neuroendocrine cells from normal tissues were found to be PML positive, indicating that the lack of PML protein in SCLC is associated with the tumorigenic phenotype and is not the result of cell-lineage specificity. Thus, the decreased PML expression may play an important role in SCLC development.

Altered expression of the suppressors PML and p53 in glioblastoma cells with the antisense-EGF-receptor

Gene amplification and enhanced expression of the epidermal growth factor receptor (EGFR) represent the major molecular genetic alteration in glioblastomas and it may play an essential role in cell growth and in the carcinogenic process. On the other hand, the nuclear suppressor proteins PML and p53 are also known to play critical roles in cancer development and in suppressing cell growth. Here we report that, in glioblastoma cells with defective EGFR function, the expressions of both promyelocytic leukaemia (PML) and p53 were altered. Cells that were transfected with the antisense-cDNA of EGFR were found to have more cells in G1 and fewer cells in S phase. In addition, the transfected cells were found to be non-responsive to EGF-induced cell growth. Interestingly, the expression of the suppressors p53 and PML were found to be significantly increased by immunohistochemical assay in the antisense-EGFR cells. Moreover, the PML expression in many of the cells was converted from the nuclear dot pattern into fine-granulated staining pattern. In contrast, the expressions of other cell cycle regulated genes and proto-oncogene, including the cyclin-dependent kinase 4 (cdk4), retinoblastoma, p16INK4a and p21H-ras, were not altered. These data indicate that there are specific inductions of PML and p53 proteins which may account for the increase in G1 and growth arrest in antisense-EGFR treated cells. It also indicates that the EGF, p53 and PML transduction pathways were linked and they may constitute an integral part of an altered growth regulatory programme. The interactions and cross-talks of these critical molecules may be very important in regulating cell growth, differentiation and cellular response to treatment in glioblastomas.