Siah-1L, a novel transcript variant belonging to the human Siah family of proteins, regulates beta-catenin activity in a p53-dependent manner

p53 and Cell Fate: Sensitizing Head and Neck Cancer Stem Cells to Chemotherapy

Head and neck cancers are deadly diseases that are diagnosed annually in approximately half a million individuals worldwide. Growing evidence supporting a role for cancer stem cells (CSCs) in the pathobiology of head and neck cancers has led to increasing interest in identifying therapeutics to target these cells. Apart from the canonical tumor-suppressor functions of p53, emerging research supports a significant role for this protein in physiological stem cell and CSC maintenance and reprogramming. Therefore, p53 has become a promising target to sensitize head and neck CSCs to chemotherapy. In this review, we highlight the role of p53 in stem cell maintenance and discuss potential implications of targeting p53 to treat patients with head and neck cancers.

Identification of potential target genes in pancreatic ductal adenocarcinoma by bioinformatics analysis

Pancreatic ductal adenocarcinoma (PDAC) is one of the most complicated and fatally pathogenic human malignancies. Therefore, there is an urgent need to improve our understanding of the underlying molecular mechanism that drives the initiation, progression, and metastasis of PDAC. The aim of the present study was to identify the key genes and signaling pathways associated with PDAC using bioinformatics analysis. Four transcriptome microarray datasets (GSE15471, GSE55643, GSE62165 and GSE91035) were acquired from Gene Expression Omnibus datasets, which included 226 PDAC samples and 65 normal pancreatic tissue samples. We screened differentially expressed genes (DEGs) with GEO2R and investigated their biological function by Gene Ontology (GO) and Kyoto Encyclopedia of Genes (KEGG) analysis. The overall survival data was obtained from UALCAN, which calculated the data shared with The Cancer Genome Atlas. In addition, a protein-protein interaction (PPI) network of the DEGs was constructed by STRING and Cytoscape software. The four sets of DEGs exhibited an intersection consisting of 205 genes (142 up-regulated and 63 down-regulated), which may be associated with PDAC. GO analysis showed that the 205 DEGs were significantly enriched in the plasma membrane, cell adhesion molecule activity and the Energy pathways, and glycine, serine, threonine metabolism were the most enriched pathways according to KEGG pathway analysis. Kaplan-Meier survival analysis revealed that 22 of 205 common genes were significantly associated with the overall survival of pancreatic cancer patients. In the PPI network and sub-network, DKK1 and HMGA2 were considered as hub genes with high connectivity degrees. DKK1 and HMGA2 are strongly associated with WNT3A and TP53 separately, which indicates that they may play an important role in the Wnt and P53 signaling pathways. Using integrated bioinformatics analysis, we identified DKK1 and HMGA2 as candidate genes in PDAC, which may improve our understanding of the mechanisms of the pathogenesis and integration; the two genes may be therapeutic targets and prognostic markers for PDAC.

Plakoglobin restores tumor suppressor activity of p53R175H mutant by sequestering the oncogenic potential of β-catenin

Tumor suppressor/transcription factor p53 is mutated in over 50% of all cancers. Some mutant p53 proteins have not only lost tumor suppressor activities but they also gain oncogenic functions (GOF). One of the most frequently expressed GOF p53 mutants is Arg175His (p53R175H ) with well-documented roles in cancer development and progression. Plakoglobin is a cell adhesion and signaling protein and a paralog of β-catenin. Unlike β-catenin that has oncogenic function through its role in the Wnt pathway, plakoglobin generally acts as a tumor/metastasis suppressor. We have shown that plakoglobin interacted with wild type and a number of p53 mutants in various carcinoma cell lines. Plakoglobin and mutant p53 interacted with the promoter and regulated the expression of several p53 target genes. Furthermore, plakoglobin interactions with p53 mutants restored their tumor suppressor/metastasis activities in vitro. GOF p53 mutants induce accumulation and oncogenic activation of β-catenin. Previously, we showed that one mechanism by which plakoglobin may suppress tumorigenesis is by sequestering β-catenin's oncogenic activity. Here, we examined the effects of p53R175H expression on β-catenin accumulation and transcriptional activation and their modifications by plakoglobin coexpression. We showed that p53R175H expression in plakoglobin null cells increased total and nuclear levels of β-catenin and its transcriptional activity. Coexpression of plakoglobin in these cells promoted β-catenin's proteasomal degradation, and decreased its nuclear levels and transactivation. Wnt/β-catenin targets, c-MYC and S100A4 were upregulated in p53R175H cells and were downregulated when plakoglobin was coexpressed. Plakoglobin-p53R175H cells also showed significant reduction in their migration and invasion in vitro.

Membrane-bound β-catenin degradation is enhanced by ETS2-mediated Siah1 induction in Helicobacter pylori-infected gastric cancer cells

β-catenin has two different cellular functions: intercellular adhesion and transcriptional activity. The E3 ubiquitin ligase Siah1 causes ubiquitin-mediated degradation of the cytosolic β-catenin and therefore, impairs nuclear translocation and oncogenic function of β-catenin. However, the effect of Siah1 on the cell membrane bound β-catenin has not been studied. In this study, we identified that the carcinogenic bacterium H. pylori increased ETS2 transcription factor-mediated Siah1 protein expression in gastric cancer cells (GCCs) MKN45, AGS and Kato III. Siah1 protein level was also noticeably higher in gastric adenocarcinoma biopsy samples as compared to non-cancerous gastric epithelia. Siah1 knockdown significantly decreased invasiveness and migration of H. pylori-infected GCCs. Although, Siah1 could not increase degradation of the cytosolic β-catenin and its nuclear translocation, it enhanced degradation of the membrane-bound β-catenin in the infected GCCs. This loss of membrane-bound pool of β-catenin was not associated with the proteasomal degradation of E-cadherin. Thus, this work delineated the role of Siah1 in increasing invasiveness of H. pylori-infected GCCs.

Multi-scale modeling of cell survival and death mediated by the p53 network: a systems pharmacology framework

The determination of cell fate is a key regulatory process for the development of complex organisms that are controlled by distinct genes in mammalian cells.

Cellular reprogramming in skin cancer

Early primitive stem cells have long been viewed as the cancer cells of origin (tumor initiating target cells) due to their intrinsic features of self-renewal and longevity. However, emerging evidence suggests a surprising capacity for normal committed cells to function as reserve stem cells upon reprogramming as a consequence of tissue damage resulting in inflammation and wound healing. This results in an alternative concept positing that tumors may originate from differentiated cells that can re-acquire stem cell properties due to genetic or epigenetic reprogramming. It is likely that both models are correct, and that a continuum of potential cells of origin exists, ranging from early primitive stem cells to committed progenitor or even terminally differentiated cells. A combination of the nature of the target cell and the specific types of gene mutations introduced determine tumor cell lineage, as well as potential for malignant conversion. Evidence from mouse skin models of carcinogenesis suggests that initiated cells at different stages within a stem cell hierarchy have varying degrees of requirement for reprogramming (e.g. inflammation stimuli), depending on their degree of differentiation. This article will present evidence in favor of these concepts that has been developed from studies of several mouse models of skin carcinogenesis.

p53, Stem Cells, and Reprogramming: Tumor Suppression beyond Guarding the Genome

p53 is well recognized as a potent tumor suppressor. In its classic role, p53 responds to genotoxic insults by inducing cell cycle exit or programmed cell death to limit the propagation of cells with corrupted genomes. p53 is also implicated in a variety of other cellular processes in which its involvement is less well understood including self-renewal, differentiation, and reprogramming. These activities represent an emerging area of intense interest for cancer biologists, as they provide potential mechanistic links between p53 loss and the stem cell-like cellular plasticity that has been suggested to contribute to tumor cell heterogeneity and to drive tumor progression. Despite accumulating evidence linking p53 loss to stem-like phenotypes in cancer, it is not yet understood how p53 contributes to acquisition of "stemness" at the molecular level. Whether and how stem-like cells confer survival advantages to propagate the tumor also remain to be resolved. Furthermore, although it seems reasonable that the combination of p53 deficiency and the stem-like state could contribute to the genesis of cancers that are refractory to treatment, direct linkages and mechanistic underpinnings remain under investigation. Here, we discuss recent findings supporting the connection between p53 loss and the emergence of tumor cells bearing functional and molecular similarities to stem cells. We address several potential molecular and cellular mechanisms that may contribute to this link, and we discuss implications of these findings for the way we think about cancer progression.

Siah1/SIP regulates p27(kip1) stability and cell migration under metabolic stress

p27(kip1) has been implicated in cell cycle regulation, functioning as an inhibitor of cyclin-dependent kinase activity. In addition, p27 was also shown to affect cell migration, with accumulation of cytoplasmic p27 associated with tumor invasiveness. However, the mechanism underlying p27 regulation as a cytoplasmic protein is poorly understood. Here we show that glucose starvation induces proteasome-dependent degradation of cytoplasmic p27, accompanied by a decrease in cell motility. We also show that the glucose limitation-induced p27 degradation is regulated through an ubiquitin E3 ligase complex involving Siah1 and SIP/CacyBP. SIP (-/-) embryonic fibroblasts have increased levels of cytosolic p27 and exhibit increased cell motility compared to wild-type cells. These observations suggest that the Siah1/SIP E3 ligase complex regulates cell motility through degradation of p27.

Excessive activity of apolipoprotein B mRNA editing enzyme catalytic polypeptide 2 (APOBEC2) contributes to liver and lung tumorigenesis

Apolipoprotein B mRNA editing enzyme catalytic polypeptide 2 (APOBEC2) was originally identified as a member of the cytidine deaminase family with putative nucleotide editing activity. To clarify the physiologic and pathologic roles, and the target nucleotide of APOBEC2, we established an APOBEC2 transgenic mouse model and investigated whether APOBEC2 expression causes nucleotide alterations in host DNA or RNA sequences. Sequence analyses revealed that constitutive expression of APOBEC2 in the liver resulted in significantly high frequencies of nucleotide alterations in the transcripts of eukaryotic translation initiation factor 4 gamma 2 (Eif4g2) and phosphatase and tensin homolog (PTEN) genes. Hepatocellular carcinoma developed in 2 of 20 APOBEC2 transgenic mice at 72 weeks of age. In addition, constitutive APOBEC2 expression caused lung tumors in 7 of 20 transgenic mice analyzed. Together with the fact that the proinflammatory cytokine tumor necrosis factor-α induces ectopic expression of APOBEC2 in hepatocytes, our findings indicate that aberrant APOBEC2 expression causes nucleotide alterations in the transcripts of the specific target gene and could be involved in the development of human hepatocellular carcinoma through hepatic inflammation.

Key signaling nodes in mammary gland development and cancer: β-catenin

β-Catenin plays important roles in mammary development and tumorigenesis through its functions in cell adhesion, signal transduction and regulation of cell-context-specific gene expression. Studies in mice have highlighted the critical role of β-catenin signaling for stem cell biology at multiple stages of mammary development. Deregulated β-catenin signaling disturbs stem and progenitor cell dynamics and induces mammary tumors in mice. Recent data showing deregulated β-catenin signaling in metaplastic and basal-type tumors suggest a similar link to reactivated developmental pathways and human breast cancer. The present review will discuss β-catenin as a central transducer of numerous signaling pathways and its role in mammary development and breast cancer.

Abnormalities in oxygen sensing define early and late onset preeclampsia as distinct pathologies

BACKGROUND

The pathogenesis of preeclampsia, a serious pregnancy disorder, is still elusive and its treatment empirical. Hypoxia Inducible Factor-1 (HIF-1) is crucial for placental development and early detection of aberrant regulatory mechanisms of HIF-1 could impact on the diagnosis and management of preeclampsia. HIF-1α stability is controlled by O(2)-sensing enzymes including prolyl hydroxylases (PHDs), Factor Inhibiting HIF (FIH), and E3 ligases Seven In Absentia Homologues (SIAHs). Here we investigated early- (E-PE) and late-onset (L-PE) human preeclamptic placentae and their ability to sense changes in oxygen tension occurring during normal placental development.

METHODS AND FINDINGS

Expression of PHD2, FIH and SIAHs were significantly down-regulated in E-PE compared to control and L-PE placentae, while HIF-1α levels were increased. PHD3 expression was increased due to decreased FIH levels as demonstrated by siRNA FIH knockdown experiments in trophoblastic JEG-3 cells. E-PE tissues had markedly diminished HIF-1α hydroxylation at proline residues 402 and 564 as assessed with monoclonal antibodies raised against hydroxylated HIF-1α P402 or P564, suggesting regulation by PHD2 and not PHD3. Culturing villous explants under varying oxygen tensions revealed that E-PE, but not L-PE, placentae were unable to regulate HIF-1α levels because PHD2, FIH and SIAHs did not sense a hypoxic environment.

CONCLUSION

Disruption of oxygen sensing in E-PE vs. L-PE and control placentae is the first molecular evidence of the existence of two distinct preeclamptic diseases and the unique molecular O(2)-sensing signature of E-PE placentae may be of diagnostic value when assessing high risk pregnancies and their severity.

Siah1 proteins enhance radiosensitivity of human breast cancer cells

Background

Siah proteins play an important role in cancer progression. We evaluated the effect of Siah1, its splice variants Siah1L and the Siah1 mutant with the RING finger deleted (Siah1ΔR) on radiosensitization of human breast cancer cells.

Methods

The status of Siah1 and Siah1L was analysed in five breast cancer cell lines. To establish stable cells, SKBR3 cells were transfected with Siah1, Siah-1L and Siah1ΔR. Siah1 function was suppressed by siRNA in MCF-7 cells. The impact of Siah1 overexpression and silencing on apoptosis, proliferation, survival, invasion ability and DNA repair was assessed in SKBR3 and MCF-7 cells, also in regards to radiation.

Results

Siah1 and Siah1L mRNA expression was absent in four of five breast cancer cells lines analysed. Overexpression of Siah1 and Siah1L enhanced radiation-induced apoptosis in stable transfected SKBR3 cells, while Siah1ΔR failed to show this effect. In addition, Siah1 and Siah1L significantly reduced cell clonogenic survival and proliferation. Siah1L sensitization enhancement ratio values were over 1.5 and 4.0 for clonogenic survival and proliferation, respectively, pointing to a highly cooperative and potentially synergistic fashion with radiation. Siah1 or Siah1L significantly reduced invasion ability of SKBR3 and suppressed Tcf/Lef factor activity. Importantly, Siah1 siRNA demonstrated opposite effects in MCF-7 cells. Siah1 and Siah1L overexpression resulted in inhibition of DNA repair as inferred by increased levels of DNA double-strand breaks in irradiated SKBR3 cells.

Conclusion

Our results reveal for the first time how overexpression of Siah1L and Siah1 can determine radiosensitivity of breast cancer cells. These findings suggest that development of drugs augmenting Siah1 and Siah1L activity could be a novel approach in improving tumor cell kill.

Identification and preliminary functional analysis of alternative splicing of Siah1 in Xenopus laevis

Siah proteins are vertebrate homologs of the Drosophila 'seven in absentia' gene. In this study, we characterized two splicing forms, Siah1a and Siah1b, of the Xenopus seven in absentia homolog 1 gene (Siah1). Overexpression of xSiah1a led to severe suppression of embryo cleavage, while that of xSiah1b was not effective even at a high dose. Competition analysis demonstrated that co-expression of xSiah1a and 1b generated the same phenotype as overexpression of xSiah1a alone, suggesting that xSiah1b does not interfere with the function of xSiah1a. Since xSiah1b has an additional 31 amino acids in the N-terminus compared to xSiah1a, progressive truncation of xSiah1b from the N-terminus showed that inability of xSiah1b to affect embryo cleavage was associated with the length of the N-terminal extension of extra amino acids. The possible implication of this finding is discussed.

Distinct expression patterns of the E3 ligase SIAH-1 and its partner Kid/KIF22 in normal tissues and in the breast tumoral processes

SIAH proteins are the human members of an highly conserved family of E3 ubiquitin ligases. Several data suggest that SIAH proteins may have a role in tumor suppression and apoptosis. Previously, we reported that SIAH-1 induces the degradation of Kid (KIF22), a chromokinesin protein implicated in the normal progression of mitosis and meiosis, by the ubiquitin proteasome pathway. In human breast cancer cells stably transfected with SIAH-1, Kid/KIF22 protein level was markedly reduced whereas, the Kid/KIF22 mRNA level was increased. This interaction has been further elucidated through analyzing SIAH and Kid/KIF22 expression in both paired normal and tumor tissues and cell lines. It was observed that SIAH-1 protein is widely expressed in different normal tissues, and in cells lines but showing some differences in western blotting profiles. Immunofluorescence microscopy shows that the intracellular distribution of SIAH-1 and Kid/KIF22 appears to be modified in human tumor tissues compared to normal controls. When mRNA expression of SIAH-1 and Kid/KIF22 was analyzed by real-time PCR in normal and cancer breast tissues from the same patient, a large variation in the number of mRNA copies was detected between the different samples. In most cases, SIAH-1 mRNA is decreased in tumor tissues compared to their normal counterparts. Interestingly, in all breast tumor tissues analyzed, variations in the Kid/KIF22 mRNA levels mirrored those seen with SIAH-1 mRNAs. This concerted variation of SIAH-1 and Kid/KIF22 messengers suggests the existence of an additional level of control than the previously described protein-protein interaction and protein stability regulation. Our observations also underline the need to re-evaluate the results of gene expression obtained by qRT-PCR and relate it to the protein expression and cellular localization when matched normal and tumoral tissues are analyzed.

The origins and evolution of the p53 family of genes

A common ancestor to the three p53 family members of human genes p53, p63, and p73 is first detected in the evolution of modern-day sea anemones, in which both structurally and functionally it acts to protect the germ line from genomic instabilities in response to stresses. This p63/p73 common ancestor gene is found in almost all invertebrates and first duplicates to produce a p53 gene and a p63/p73 ancestor in cartilaginous fish. Bony fish contain all three genes, p53, p63, and p73, and the functions of these three transcription factors diversify in the higher vertebrates. Thus, this gene family has preserved its structural features and functional activities for over one billion years of evolution.

A genomewide study identifies the Wnt signaling pathway as a major target of p53 in murine embryonic stem cells

Both p53 and the Wnt signaling pathway play important roles in regulating the differentiation of mouse embryonic stem cells (mESCs). However, it is not known whether they directly and/or functionally crosstalk in mESCs. Here we report a surprising antidifferentiation function of p53 in mESCs through directly regulating the Wnt signaling pathway. A chromatin-immunoprecipitation-based microarray (ChIP-chip) and gene expression microarray assays reveal that the Wnt signaling pathway is significantly (P value, 0.000048) overrepresented in p53-regulated genes in mESCs. The expression of five Wnt ligand genes is robustly induced by various genotoxic and nongenotoxic insults in a p53-dependent manner. Moreover, the induction of these Wnt genes is greatly attenuated in mouse embryonic fibroblast (MEF) cells and ESC-derived neural stem/progenitor cells, suggesting that the induction is mESC specific. It is established that the activation of the Wnt signaling pathway inhibits the differentiation of mESCs. Consistent with this notion, we detected an antidifferentiation activity from the conditioned medium (CM) collected from UV (UV)-treated mESCs. This antidifferentiation activity can be lowered by either the addition of Wnt antagonists into the CM or the reduction of p53 levels in UV-treated mESCs. Therefore, reminiscent of its dual functions on death and survival in somatic cells, p53 appears to regulate both prodifferentiation and antidifferentiation programs in mESCs. Our findings uncover a direct and functional connection between p53 and the Wnt signaling pathway, and expand the catalog of p53 regulated genes in mESCs.

Attenuation of proteolysis-mediated cyclin E regulation by alternatively spliced Parkin in human colorectal cancers

Parkin has a critical role in the ubiquitin-proteasome system as an E3-ligase targeting several substrates. Our recent finding that Parkin-deficient mice are susceptible to tumorigenesis provided evidence that Parkin is a tumor suppressor gene. Dysfunction of the Parkin gene is frequently observed in various human cancers, but the mechanism underlying the cell cycle disruption induced by Parkin dysfunction that leads to carcinogenesis is not known. Here, we demonstrated that Parkin expression in colonic epithelial cells is regulated in a cell cycle-associated manner. Epidermal growth factor (EGF) stimulation upregulated Parkin gene expression in human colon cells. Inhibition of the phosphoinositide 3-kinase [PI(3)K]-Akt-dependent pathways suppressed growth factor-induced Parkin expression. The expression of alternatively spliced Parkin isoforms with various deletions spanning exons 3-6 was detected in 18 of 43 (42%) human colorectal cancer tissues. Wild-type Parkin induced the degradation of cyclin E protein, but the alternatively spliced Parkin identified in colon cancers showed defective proteolysis of cyclin E. These findings indicate that Parkin expression is induced by growth factor stimulation and is involved in the cell cycle regulation of colon cells. Tumor-specific expression of alternatively spliced Parkin isoforms might contribute to enhanced cell proliferation through the attenuation of proteolysis-mediated cyclin E regulation in human colorectal cancers.

Radiobiologic response of medulloblastoma cell lines: involvement of beta-catenin?

Medulloblastoma (MB) is the most common brain malignancy in children. Whole neural axis irradiation is the treatment of choice, but it often results in long-term neurocognitive and developmental impairment. Only insights into MB biology will lead to improved therapeutic outcome. Wingless (WNT) signalling deregulation occurs in up to 25% of sporadic tumors, but the specific role of nuclear beta-catenin and its involvement in the radioresponse remains unsettled. Therefore we studied the gamma-radiation response of two MB cell lines from cellular and molecular points of view. Our data show that the p53 wild-type cell line is more sensitive to ionizing radiations (IR) than the p53 mutated line, but apoptosis is also induced in p53-mutated cells, suggesting an alternative p53-independent mechanism. In addition, this study is the first to demonstrate that gamma-rays trigger the WNT system in our in vitro models. Further studies are required to test if this could explain the radiosensitivity of MB and the favorable prognostic value of nuclear beta-catenin in this tumor.

Control of HIPK2 stability by ubiquitin ligase Siah-1 and checkpoint kinases ATM and ATR

The tumour suppressor HIPK2 is an important regulator of cell death induced by DNA damage, but how its activity is regulated remains largely unclear. Here we demonstrate that HIPK2 is an unstable protein that colocalizes and interacts with the E3 ubiquitin ligase Siah-1 in unstressed cells. Siah-1 knockdown increases HIPK2 stability and steady-state levels, whereas Siah-1 expression facilitates HIPK2 polyubiquitination, degradation and thereby inactivation. During recovery from sublethal DNA damage, HIPK2, which is stabilized on DNA damage, is degraded through a Siah-1-dependent, p53-controlled pathway. Downregulation of Siah-1 inhibits HIPK2 degradation and recovery from damage, driving the cells into apoptosis. We have also demonstrated that DNA damage triggers disruption of the HIPK2-Siah-1 complex, resulting in HIPK2 stabilization and activation. Disruption of the HIPK2-Siah-1 complex is mediated by the ATM/ATR pathway and involves ATM/ATR-dependent phosphorylation of Siah-1 at Ser 19. Our results provide a molecular framework for HIPK2 regulation in unstressed and damaged cells.

Reduced membranous beta-catenin protein expression is associated with metastasis and poor prognosis in squamous cell carcinoma of the esophagus

OBJECTIVES

The aim of this study was to evaluate, by immunohistochemical analysis, the protein expression of beta-catenin and p53 in resected esophageal squamous cell carcinoma specimens. The clinical relevance and prognostic significance of the expression of these proteins were also analyzed.

METHODS

Immunohistochemistry was performed on paraffin-embedded tissue specimens from 68 resected esophageal squamous cell carcinoma tumor specimens to detect the expression of beta-catenin and p53. The correlation between the results of immunoexpression and the clinicopathologic parameters and patient survival was processed statistically.

RESULTS

Reduced membranous beta-catenin expression was noted in 43 (63.2%) of 68 tumor specimens. Increased expression of p53 was observed in 43 (63.2%) of 68 specimens. Reduced membranous beta-catenin protein expression was associated with the presence of distant metastasis (P = .006). Patients with reduced membranous beta-catenin expression had a worse prognosis than patients with normal membranous beta-catenin expression (P = .005). Patients with combined increased p53 and reduced membranous beta-catenin protein expression had the worst prognosis (P = .012). In a multivariate survival analysis, reduced membranous beta-catenin expression and nodal involvement were independent prognostic factors (P = .004 and .019, respectively).

CONCLUSIONS

Immunohistochemical analysis revealed that reduced membranous beta-catenin protein expression was associated with the presence of distant metastasis and a poor prognosis in patients with esophageal squamous cell carcinoma. Combined increased p53 and reduced membranous beta-catenin protein expression indicated a very poor prognosis in patients with esophageal squamous cell carcinoma. Further investigation is needed to understand the roles of beta-catenin and p53 in the tumorigenesis and metastasis of esophageal squamous cell carcinoma.

E3 ubiquitin ligase SIAH1 mediates ubiquitination and degradation of TRB3

Tribbles 3 homolog (TRB3) is recently identified as a scaffold-like regulator of various signal transducers and has been implicated in several processes including insulin signaling, NF-kappaB signaling, lipid metabolism and BMP signaling. To further understand cellular mechanisms of TRB3 regulation, we performed a yeast two-hybrid screen to identify novel TRB3 interacting proteins and totally obtained ten in-frame fused preys. Candidate interactions were validated by co-immunoprecipitation assays in mammalian cells. We further characterized the identified proteins sorted by Gene Ontology Annotation. Its interaction with the E3 ubiquitin ligase SIAH1 was further investigated. SIAH1 could interact with TRB3 both in vitro and in vivo. Importantly, SIAH1 targeted TRB3 for proteasome-dependent degradation. Cotransfection of SIAH1 could withdraw up-regulation of TGF-beta signaling by TRB3, suggesting SIAH1-induced degradation of TRB3 represents a potential regulatory mechanism for TGF-beta signaling.

SIAH-1 interacts with the Kaposi's sarcoma-associated herpesvirus-encoded ORF45 protein and promotes its ubiquitylation and proteasomal degradation

Kaposi's sarcoma-associated herpesvirus (KSHV), also referred to as human herpesvirus 8, is a potentially tumorigenic virus implicated in the etiology of Kaposi's sarcoma, primary effusion lymphoma, and some forms of multicentric Castleman's disease. The open reading frame 45 (ORF45) protein, encoded by the KSHV genome, is capable of inhibiting virus-dependent interferon induction and appears to be essential for both early and late stages of infection. In the present study, we show, both in yeast two-hybrid assays and in mammalian cells, that the ORF45 protein interacts with the cellular ubiquitin E3 ligase family designated seven in absentia homologue (SIAH). We provide evidence that SIAH-1 promotes the degradation of KSHV ORF45 through a RING domain-dependent mechanism and via the ubiquitin-proteasome system. Furthermore, our data indicate the involvement of SIAH-1 in the regulation of the expression of ORF45 in KSHV-infected cells. Since the availability of KSHV ORF45 is expected to influence the course of KSHV infection, our findings identify a novel biological role for SIAH proteins as modulators of virus infection.

Coiled-coil domain containing 85B suppresses the beta-catenin activity in a p53-dependent manner

Aberrant accumulation of beta-catenin is closely related to carcinogenesis. Mutations in the p53 gene are reported to induce the aberrant accumulation of beta-catenin in the absence of dysfunction in the glycogen synthase kinase 3beta (GSK3beta)-mediated degradation pathway, but the mechanism remains incompletely understood. Here, we show that human coiled-coil domain containing 85B (CCDC85B) is induced by p53 and regulates beta-catenin activity via interaction with the T-cell factor 4 in the nucleus. Moreover, CCDC85B enhances the degradation of beta-catenin and suppresses tumor cell growth. In conclusion, we revealed that CCDC85B-induced degradation of beta-catenin is independent of GSK3beta and other p53-inducible products, Siah-1L, suggesting that CCDC85B constitutes the one of the frameworks of p53-induced multiple regulatory pathways for beta-catenin activity.

Hepatitis B virus X protein differentially affects the ubiquitin-mediated proteasomal degradation of beta-catenin depending on the status of cellular p53

Abnormal accumulation of beta-catenin is considered to be a strong driving force in hepatocellular carcinogenesis; however, the mechanism of beta-catenin accumulation in tumours is unclear. Here, it was demonstrated that hepatitis B virus X protein (HBx) differentially regulates the level of beta-catenin through two ubiquitin-dependent proteasome pathways depending on p53 status. In the presence of p53, HBx downregulated beta-catenin through the activation of a p53-Siah-1 proteasome pathway. For this purpose, HBx upregulated Siah-1 expression at the transcriptional level via activation of p53. In the absence of p53, however, HBx stabilized beta-catenin through the inhibition of a glycogen synthase kinase-3beta-dependent pathway. Interestingly, HBx variants with a Pro-101 to Ser substitution were unable to activate p53 and thus could stabilize beta-catenin irrespective of p53 status. Based on these findings, a model of beta-catenin regulation by HBx is proposed whereby the balance between the two opposite activities of HBx determines the overall expression level of beta-catenin. Differential regulation of beta-catenin by HBx depending on host (p53 status) and viral factors (HBx sequence variation) helps not only to explain the observation that cancers accumulating beta-catenin also exhibit a high frequency of p53 mutations but also to understand the contradictory reports on the roles of HBx during hepatocellular carcinogenesis.

Siah-1S, a novel splice variant of Siah-1 (seven in absentia homolog), counteracts Siah-1-mediated downregulation of beta-catenin

Siah-1 (seven in absentia homolog) is known to cause indirect degradation of beta-catenin through formation of a complex with Siah-interacting protein (SIP), Skp1 and Ebi. Here, we report the characterization of a novel splice variant of human Siah-1, designated Siah-1S, which is produced by an alternative splicing mechanism. The novel intron/exon junctions used to generate Siah-1S follow a non-conventional CT-AC rule. Siah-1S exhibits an even shorter half-life than Siah-1 and is able to catalyse self-ubiquitination that results in its subsequent degradation by proteasome. Siah-1S is shown to upregulate beta-catenin-dependent Tcf/Lef transcriptional activation and antagonize Siah-1's potentiation effect on the apoptosis induced by etoposide in MCF-7 cells. Additionally, Siah-1S is found to interact with Siah-1 to form heterodimer or with itself to form homodimer. Unlike homodimer Siah-1*Siah-1, neither Siah-1*Siah-1S nor Siah-1S*Siah-1S is able to bind to Siah-1-interacting protein, which may explain the underlying mechanism for Siah-1S's dominant negative effect on Siah-1. Importantly, results from in vitro soft agar assay demonstrated that Siah-1S displays a promotion effect on cells tumorigenicity.

p53 downstream target DDA3 is a novel microtubule-associated protein that interacts with end-binding protein EB3 and activates beta-catenin pathway

We have previously identified mouse DDA3 as a p53-inducible gene. To explore the functional role of DDA3, we screened a mouse brain cDNA library by the yeast two-hybrid assay, and identified the microtubule plus-end binding protein EB3 as a DDA3-interacting protein. Binding of DDA3 to EB3 was verified by glutathione S-transferase (GST) pull-down assay and subcellular colocalization; co-immunoprecipitation further indicated that interaction of these two proteins within cells required intact microtubules. Domains of DDA3-EB3 interaction were mapped by GST pull-down assay to amino acids 118-241 and 242-329 of DDA3 and the N- and C-termini of EB3. Immunofluorescence analysis revealed colocalization of DDA3 with microtubules in various cell phases, and regions encompassing aa 118-241 and 242-329 contained microtubule-interacting and bundling activities. In vitro microtubule-binding assay showed that DDA3 and EB3 associated directly with microtubules, and cooperated with each other for microtubule binding. In addition, DDA3 bound to the EB3 interacting partner adenomatous polyposis coli 2 (APC2), a homolog of the tumor suppressor APC, which is a component of the beta-catenin destruction complex. Ectopic expression of DDA3 and EB3 enhanced beta-catenin-dependent transactivation and cyclin D1 production, whereas knockdown of endogenous DDA3 or EB3 inhibited beta-catenin-mediated transactivation and the ability of cells to form colonies. Together, our results identify DDA3 as a novel microtubule-associated protein that binds to EB3, and implicate DDA3 and EB3 in the beta-catenin-mediated growth signaling.

IKKα stabilizes cytosolic β-catenin by inhibiting both canonical and non-canonical degradation pathways

Beta-catenin is a bi-functional protein. It is not only a major component of the cellular adhesion machinery, but is also a transcription co-activator of the Wnt signaling pathway. The cytosolic levels of the beta-catenin protein, as well as its subcellular localization, are tightly regulated due to its oncogenic potentials. Two independent pathways are found to regulate beta-catenin. The canonical pathway is induced by the Axin/adenomatous polyposis coli (APC)/glycogen synthase kinase-3beta (GSK-3beta) complex which is dependent on GSK-3beta phosphorylation. The non-canonical pathway is mediated by p53-induced Siah-1 which is GSK-3beta phosphorylation-independent. Recently, several studies reported that IkappaB kinase alpha (IKKalpha) could stabilize beta-catenin and stimulate beta-catenin/T cell factor (Tcf)-dependent transcription. Here we report that IKKalpha could inhibit beta-catenin degradation mediated not only by the Axin/APC/GSK-3beta complex, but also by the Siah-1 pathway. Consistently, we found that IKKalpha abolished the inhibition of beta-catenin/Tcf-dependent transcription by Siah-1. Furthermore, we found that IKKalpha interacted with beta-catenin and inhibited beta-catenin ubiquitination. Taken together, our results provide a new insight into IKKalpha-mediated beta-catenin stabilization.

Identification of novel defective HCV clones in liver transplant recipients with recurrent HCV infection

Patients with recurrent hepatitis C after liver transplantation usually have a high viral load and are generally resistant to interferon (IFN)-alpha2b plus ribavirin (RBV) therapy. However, it remains unclear whether pretreatment viral titre determines the effectiveness of combination therapy, especially in patients with a high viral load. The aim of this study was to identify the viral factors associated with a sustained virological response (SVR) to antiviral therapy in patients with recurrent hepatitis C after living-donor liver transplantation. Twenty-three patients with recurrent hepatitis C received combination therapy of IFN-alpha2b plus RBV. SVR was achieved in 7 of the 23 patients (30.4%). Predictive factors for SVR included a 2 log10 decline in Hepatitis C virus (HCV) RNA at 2 weeks after the start of therapy and disappearance of HCV RNA at 4 or 24 weeks after the start of therapy. As the pretreatment high viral load showed no association with SVR, we asked whether other viral factor was associated with the response to the combination therapy in transplant recipients. We found the several novel defective HCV clones in 4 of 12 recipients' sera. All defective HCV clones had deletions in the envelope region. Interestingly, no patients with defective clones showed a prompt decrease in HCV RNA after the start of IFN-alpha2b plus RBV therapy. Thus, early decline in serum HCV RNA after treatment was closely associated with SVR. The circulating defective HCV clones are present and might be associated with the response to the combination therapy in patients with recurrent hepatitis after liver transplantation.

Critical function for SIP, a ubiquitin E3 ligase component of the beta-catenin degradation pathway, for thymocyte development and G1 checkpoint

Beta-catenin has been implicated in thymocyte development because of its function as a coactivator of Tcf/LEF-family transcription factors. Previously, we discovered a novel pathway for p53-induced beta-catenin degradation through a ubiquitin E3 ligase complex involving Siah1, SIP (CacyBP), Skp1, and Ebi. To gain insights into the physiological relevance of this new degradation pathway in vivo, we generated mutant mice lacking SIP. We demonstrate here that SIP-/- thymocytes have an impaired pre-TCR checkpoint with failure of TCRbeta gene rearrangement and increased apoptosis, resulting in reduced cellularity of the thymus. Moreover, the degradation of beta-catenin in response to DNA damage is significantly impaired in SIP-/- cells. SIP-/- embryonic fibroblasts show a growth-rate increase resulting from defects in G1 arrest. Thus, the beta-catenin degradation pathway mediated by SIP defines an essential checkpoint for thymocyte development and cell-cycle progression.

Anti-viral protein APOBEC3G is induced by interferon-alpha stimulation in human hepatocytes

Apolipoprotein B mRNA-editing enzyme catalytic-polypeptide 3G (APOBEC3G) is a potent inhibitor of infection by a wide range of retroviruses. Although recent reports have suggested that human APOBEC3G exerts antiviral activity against hepatitis B virus, APOBEC3G expression is normally low in the human liver. To clarify the role of APOBEC3G in cellular defenses against hepatitis viruses, the regulation of the APOBEC3G expression was investigated in human hepatocytes. Endogenous transcripts of nine APOBEC family members were barely detectable in quiescent liver cells. However, APOBEC3G was significantly up-regulated in response to interferon-alpha (IFN-alpha) stimulation in HepG2, Huh-7, and primary human hepatocytes. IFN regulatory factor elements that are important for IFN-inducible promoter activity were identified 5' upstream from the human APOBEC3G gene. Our findings provided the first evidence showing that APOBEC3G is induced by IFN stimulation in human hepatocytes and thus could be involved in host defense mechanisms directed against hepatitis viruses.

Expression of APOBEC2 is transcriptionally regulated by NF-kappaB in human hepatocytes

Apolipoprotein B mRNA-editing enzyme catalytic subunit 2 (APOBEC2) is a member of the nucleic-acid-editing enzymes. However, the physiological function of APOBEC2 remains unclear. We demonstrate that APOBEC2 expression is strongly enhanced in response to both tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta. Inhibition of NF-kappaB activation invariably blocks TNF-alpha-induced APOBEC2 expression. The promoter region of APOBEC2 contains functional NF-kappaB response elements in the 5' untranslated region of the gene at -625/-616. These results show that APOBEC2 expression is regulated by pro-inflammatory cytokines via NF-kappaB activation and suggest a possible role of APOBEC2 in the pathophysiology of hepatic inflammation.

Expression of activation-induced cytidine deaminase in human hepatocytes during hepatocarcinogenesis

Activation-induced cytidine deaminase (AID) plays a role as a genome mutator in activated B cells, and inappropriate expression of AID has been implicated in the immunopathological phenotype of human B-cell malignancies. Notably, we found that the transgenic mice overexpressing AID developed lung adenocarcinoma and hepatocellular carcinoma (HCC), suggesting that ectopic expression of AID can lead to tumorigenesis in epithelial tissues as well. To examine the involvement of AID in the development of human HCC, we analyzed the AID expression and its correlation with mutation frequencies of the p53 gene in liver tissues from 51 patients who underwent resection of primary HCCs. The specific expression, inducibility by cytokine stimulation and mutagenic activity of AID were investigated in cultured human hepatocytes. Only trace amounts of AID transcripts were detected in the normal liver; however, endogenous AID was significantly upregulated in both HCC and surrounding noncancerous liver tissues with underlying chronic hepatitis or liver cirrhosis (p < 0.05). Most liver tissues with underlying chronic inflammation with endogenous AID upregulation already contained multiple genetic changes in the p53 gene. In both hepatoma cell lines and cultured human primary hepatocytes, the expression of AID was substantially induced by TGF-beta stimulation. Aberrant activation of AID in hepatocytes resulted in accumulation of multiple genetic alterations in the p53 gene. Our findings suggest that the aberrant expression of AID is observed in human hepatocytes with several pathological settings, including chronic liver disease and HCC, which might enhance the genetic susceptibility to mutagenesis leading to hepatocarcinogenesis.

β-Catenin Status Predicts a Favorable Outcome in Childhood Medulloblastoma: The United Kingdom Children's Cancer Study Group Brain Tumour Committee

Purpose

Identifying pathobiological correlates of clinical behavior or therapeutic response currently represents a key challenge for medulloblastoma research. Nuclear accumulation of the β-catenin protein is associated with activation of the Wnt/Wg signaling pathway, and mutations affecting components of this pathway have been reported in approximately 15% of sporadic medulloblastomas. We tested the hypothesis that nuclear immunoreactivity for β-catenin is a prognostic marker in medulloblastoma, and assessed the relationship between nuclear β-catenin immunoreactivity and mutations of CTNNB1 and APC.

Patients and Methods

Medulloblastomas from children entered onto the International Society for Pediatric Oncology (SIOP)/United Kingdom Children's Cancer Study Group (UKCCSG) PNET3 trial (n = 109) were examined for β-catenin immunoreactivity, and where tissue was available, evidence of CTNNB1 and APC mutations. The results were correlated with clinicopathologic variables, principally outcome.

Results

Children with medulloblastomas that showed a nucleopositive β-catenin immunophenotype (27 of 109; 25%) had significantly better overall (OS) and event-free (EFS) survivals than children with tumors that showed either membranous/cytoplasmic β-catenin immunoreactivity or no immunoreactivity (P = .0015 and P = .0026, respectively). For β-catenin nucleopositive and nucleonegative medulloblastomas, 5-year OS was 92.3% (95% CI, 82% to 100%) versus 65.3% (95% CI, 54.8 to 75.7%), and 5-year EFS was 88.9% (95% CI, 77% to 100%) versus 59.5% (95% CI, 48.8 to 70.2%), respectively. Mutations in CTNNB1 were found exclusively among medulloblastomas that demonstrated nuclear β-catenin immunoreactivity, but no evidence of APC mutation was found in these cases. All children with β-catenin nucleopositive large cell/anaplastic medulloblastomas and β-catenin nucleopositive medulloblastomas presenting with metastatic disease are alive at least 5 years postdiagnosis.

Conclusion

Nuclear accumulation of β-catenin appears to be a marker of favorable outcome in medulloblastoma, and should be investigated further in large group-wide trials.

Wnt signalling and prostate cancer

The Wnt signalling pathway plays a role in the direction of embryological development and maintenance of stem cell populations. Heritable alterations in genes encoding molecules of the Wnt pathway, including mutation and epigenetic events, have been demonstrated in a variety of cancers. It has been proposed that disruption of this pathway is a significant step in the development of many tumours. Interactions between beta-catenin--the effector molecule of the Wnt pathway--and the androgen receptor highlight the pathway's relevance to urological malignancy. Mutation or altered expression of Wnt genes in tumours may give prognostic information and treatments are being developed which target this pathway.

p53 in neuronal apoptosis

The tumor suppressor and transcription factor p53 is a key modulator of cellular stress responses, and activation of p53 can trigger apoptosis in many cell types including neurons. Apoptosis is a form of programmed cell death that occurs in neurons during development of the nervous system and may also be responsible for neuronal deaths that occur in neurological disorders such as stroke, and Alzheimer's and Parkinson's diseases. p53 production is rapidly increased in neurons in response to a range of insults including DNA damage, oxidative stress, metabolic compromise, and cellular calcium overload. Target genes induced by p53 in neurons include those encoding the pro-apoptotic proteins Bax and the BH3-only proteins PUMA and Noxa. In addition to such transcriptional control of the cell death machinery, p53 may more directly trigger apoptosis by acting at the level of mitochondria, a process that can occur in synapses (synaptic apoptosis). Preclinical data suggest that agents that inhibit p53 may be effective therapeutics for several neurodegenerative conditions.

The p53 pathway: positive and negative feedback loops

The p53 pathway responds to stresses that can disrupt the fidelity of DNA replication and cell division. A stress signal is transmitted to the p53 protein by post-translational modifications. This results in the activation of the p53 protein as a transcription factor that initiates a program of cell cycle arrest, cellular senescence or apoptosis. The transcriptional network of p53-responsive genes produces proteins that interact with a large number of other signal transduction pathways in the cell and a number of positive and negative autoregulatory feedback loops act upon the p53 response. There are at least seven negative and three positive feedback loops described here, and of these, six act through the MDM-2 protein to regulate p53 activity. The p53 circuit communicates with the Wnt-beta-catenin, IGF-1-AKT, Rb-E2F, p38 MAP kinase, cyclin-cdk, p14/19 ARF pathways and the cyclin G-PP2A, and p73 gene products. There are at least three different ubiquitin ligases that can regulate p53 in an autoregulatory manner: MDM-2, Cop-1 and Pirh-2. The meaning of this redundancy and the relative activity of each of these feedback loops in different cell types or stages of development remains to be elucidated. The interconnections between signal transduction pathways will play a central role in our understanding of cancer.

p53 prevents maturation of T cell development to the immature CD4-CD8+ stage in Bcl11b-/- mice

Signaling pathways such as the pre-TCR and Wnt pathways regulate alpha/beta T cell differentiation in thymus. Mice lacking an essential component of the pre-TCR exhibit arrest at the (CD4(-)CD8(-)) (CD44(-)CD25(+)) stage (DN3) of thymocyte development, and introduction of p53 deficiency into those mice abrogates this arrest, resulting in transition to the (CD4(+)CD8(+)) double-positive (DP) stage. This paper examines the effect of inactivation of p53 on thymocyte development in Bcl11b(-/-) mice that exhibit arrest at the DN3 or (CD4(-)CD8(+)) immature single-positive (ISP) stage. No DP thymocytes were detected in thymocytes of adoptive transfer experiments in scid mice that were derived from p53(-/-)Bcl11b(-/-) precursors but ISP thymocytes increased in the proportion and in the cell number approximately three times higher than those from Bcl11b(-/-) precursors. Consistently, the level of apoptosis decreased to the level of wild-type precursors. These results suggest that inactivation of p53 is sufficient for DN3 thymocytes to differentiate into the ISP, but not to DP, stage of thymocyte development in Bcl11b(-/-) mice. This provides evidence for a novel p53-mediated checkpoint that regulates the transition from the DN3 to ISP stage of thymocyte development.