The characterization of the human Siah-1 promoter(1)

Host cell factors stimulate HIV-1 transcription by antagonizing substrate-binding function of Siah1 ubiquitin ligase to stabilize transcription elongation factor ELL2

The Siah1 and Siah2 ubiquitin ligases are implicated in diverse biological processes ranging from cellular stress responses, signaling to transcriptional regulation. A key substrate of Siah1 is ELL2, which undergoes proteolysis upon polyubiquitination. ELL2 stimulates transcriptional elongation and is a subunit of the Super Elongation Complex (SEC) essential for HIV-1 transactivation. Previously, multiple transcriptional and post-translational mechanisms are reported to control Siah's expression and activity. Here we show that the activity of Siah1/2 can also be suppressed by host cell factor 1 (HCF1), and the hitherto poorly characterized HCF2, which themselves are not degraded but can bind and block the substrate-binding domain (SBD) of Siah1/2 to prevent their autoubiquitination and trans-ubiquitination of downstream targets including ELL2. This effect stabilizes ELL2 and enhances the ELL2-SEC formation for robust HIV-1 transactivation. Thus, our study not only identifies HCF1/2 as novel activators of HIV-1 transcription through inhibiting Siah1 to stabilize ELL2, but also reveals the SBD of Siah1/2 as a previously unrecognized new target for HCF1/2 to exert this inhibition.

Characterization and functional analysis of SIAH1 during skin and hair follicle development in the angora rabbit (Oryctolagus cuniculus)

BACKGROUND

Seven in absentia homolog 1 (SIAH1) is an E3 ubiquitin ligase containing a RING-finger domain and a key regulator of normal development. Skin and hair follicle development is a complex and special process of morphogenesis involving multiple signaling pathways. SIAH1 is enriched in the Wnt signaling pathway and potentially related to hair follicle cycle and skin development. This study aims to provide evidence for the role of SIAH1 in skin and hair development.

RESULTS

Full-length cloning and analysis of SIAH1 was conducted to better understand its function. Phylogenetically, the sequence of SIAH1 in the rabbit shares the greatest homology with Home sapiens, Pongo abelii and Mus mulatta. Based on the rabbit hair follicle synchronization model, we found that the expression level of SIAH1 in the regressive period of the rabbit hair cycle is significantly lower than in the active growth and rest periods. In addition, the mRNA expression levels of skin and hair follicle development-related genes changed significantly when SIAH1 was overexpressed and silenced. After SIAH1 overexpression, the expression levels of WNT2, LEF1 and FGF2 decreased, and those of SFRP2 and DKK1 increased (P < 0.05). After interference of SIAH1, the expression levels of WNT2, LEF1 and FGF2 increased (P < 0.05), and SFRP2 and DKK1 decreased.

CONCLUSIONS

SIAH1 can affect skin and hair follicle development and exert an inhibitory effect. These results could provide foundamental insights into the role of SIAH1 as a target gene in rabbit skin and hair follicle development.

Seven in Absentia E3 Ubiquitin Ligases: Central Regulators of Neural Cell Fate and Neuronal Polarity

During neural development, neural precursors transition from a proliferative state within their germinal niches to a migratory state as they relocate to their final laminar positions. Transitions across these states are coupled with dynamic alterations in cellular polarity. This key feature can be seen throughout the developing vertebrate brain, in which neural stem cells give rise to multipolar or unpolarized transit-amplifying progenitors. These transit-amplifying progenitors then expand to give rise to mature neuronal lineages that become polarized as they initiate radial migration to their final laminar positions. The conventional understanding of the cellular polarity regulatory program has revolved around signaling cascades and transcriptional networks. In this review, we discuss recent discoveries concerning the role of the Siah2 ubiquitin ligase in initiating neuronal polarity during cerebellar development. Given the unique features of Siah ubiquitin ligases, we highlight some of the key substrates that play important roles in cellular polarity and propose a function for the Siah ubiquitin proteasome pathway in mediating a post-translational regulatory network to control the onset of polarization.

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.

Regulators and effectors of Siah ubiquitin ligases

The Siah ubiquitin ligases are members of the RING finger E3 ligases. The Siah E3s are conserved from fly to mammals. Primarily implicated in cellular stress responses, Siah ligases play a key role in hypoxia, through the regulation of HIF-1α transcription stability and activity. Concomitantly, physiological conditions associated with varying oxygen tension often highlight the importance of Siah, as seen in cancer and neurodegenerative disorders. Notably, recent studies also point to the role of these ligases in fundamental processes including DNA damage response, cellular organization and polarity. This review summarizes the current understanding of upstream regulators and downstream effectors of Siah.

Hypoxia-induced β-catenin downregulation involves p53-dependent activation of Siah-1

Solid tumors contain extensive hypoxic areas and it is of considerable importance to decipher the potential role of hypoxia in signaling pathway regulation. In the present study, we examined the impact of hypoxia on β-catenin status and the mechanisms involved. Hypoxia significantly decreased β-catenin protein, but had no effect on glycogen synthase kinase (GSK)-3β or adenomatous polyposis coli (APC) levels. However, hypoxia-induced β-catenin downregulation seemed to require APC but not GSK-3β. Further investigation revealed that hypoxia significantly upregulated Siah-1, the human homolog of Drosophila seven in absentia. In addition, hypoxia augmented the interaction between β-catenin and SIP and Skp1. Silencing of Siah-1, as well as the use of a dominant negative Siah-1 mutant, attenuated these responses to hypoxia and rescued β-catenin transactivation. The Siah-1-mediated degradation of β-catenin during hypoxia may involve p53, but not hypoxia-inducible factor-1, activation. Together, the results suggest that hypoxia downregulates β-catenin by increasing Siah-1 expression in a p53-dependent manner.

E2F1 represses beta-catenin/TCF activity by direct up-regulation of Siah1

Transcription factor E2F1 is a key regulator of cell proliferation and apoptosis. Its activity is strictly controlled by the pRB/E2F pathway. In the majority of cancer cells, however, this pathway is frequently found deregulated, and the underlying mechanism involving transcriptional control by E2F1 has not yet been fully elucidated. Here we report the identification of two putative E2F1-binding sites located upstream from Siah1 transcription start site (+1). Chromatin immunoprecipitation assay reveals that transcription factor E2F1 is capable of binding to the putative sites, and luciferase reporter assay shows that E2F1 can activate transcription from the Siah1 promoter. Ectopic expression of E2F1 elevates the Siah1 level, hence suppressing the beta-catenin/TCF activity. Consistently, knock-down of endogenous E2F1 by a shRNA strategy results in reduced expression of Siah1. Moreover, repression of beta-catenin/TCF activity by E2F1 can be attenuated by shRNA-based repression of endogenous Siah1, implying that Siah1 is a bona fide E2F1 target gene, which at least partly, mediates the suppression of beta-catenin/TCF signalling pathway.

The Modular Structure of SIP Facilitates Its Role in Stabilizing Multiprotein Assemblies,

Siah-interacting protein (SIP) was identified as a novel adaptor that physically links the E3 ubiquitin ligase activity of Siah-1 with Skp1 and Ebi F-Box protein in the degradation of beta-catenin, a transcriptional activator of TCF/LEF genes. In this study, we have used solution NMR spectroscopy to characterize the domain structure of SIP, which includes a novel helical hairpin domain at the N-terminus flexibly linked to a CS domain and an unstructured carboxy terminal SGS domain. These studies have been complemented by mapping the sites of functionally important protein-protein interactions involving Siah-1 and Skp1 to individual domains of SIP. NMR-based chemical shift perturbation assays show that Siah-1 interacts with the flexible linker between SIP N and CS domains. This site for interaction in the linker does not perturb residues in the structured region at the N-terminus but does appear to restrict the rotational freedom of the SIP CS domain in the context of the full-length protein. In contrast, Skp1 engages the SIP CS domain exclusively through weak interactions that are not coupled to the other domains. The principal role of the modular structure of SIP appears to be in bringing these two proteins into physical proximity and orchestrating the orientation required for polyubiquitination of beta-catenin in the intact SCF-type complex.

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

Beta-catenin is a potent oncogenic protein whose cytoplasmic accumulation is a frequent event in cancer cells. The level of beta-catenin is regulated by two mechanisms: the adenomatous polyposis coli/Axin/glycogen synthase kinase 3beta-dependent degradation pathway and the Siah-1/Siah interacting protein/Ebi-mediated degradation pathway. In this study, we have investigated the functional significance of p53-inducible human Siah-family protein expression in the regulation of beta-catenin activity. We show here by reverse-transcriptase polymerase chain reaction that two mRNA transcripts, designated human Siah-1 and Siah-1L, are generated from the human Siah-1 locus. Interestingly, the expression of Siah-1L was upregulated by p53, whereas human Siah-1 expression was constant. Furthermore, introduction of exogenous Siah-1L protein downregulated beta-catenin protein and promoted apoptosis induced by anticancer drugs in cancer cells that lack endogenous p53. Thus, Siah-1L represents a new member of the human Siah family that is induced in response to p53 and plays an important role in the regulation of beta-catenin activity in tumor cells. These findings also suggest new strategies for restoring tumor suppressive pathways lost in cancer cells that have suffered p53 inactivation.

Siah-1b is a direct transcriptional target of p53: identification of the functional p53 responsive element in the siah-1b promoter

Siah proteins are E3 ubiquitin ligases. They are homologues of the Drosophila seven in absentia (Sina), a protein required for the R7 photoreceptor development. We have previously found that the expression of human siah-1 and its mouse homologue siah-1b are induced by p53 during apoptosis and tumor reversion. So far, no evidence that the siah-1b gene is a direct transcriptional target of p53 has been provided. In the present study we investigate this issue. Northern blot analysis with a specific probe demonstrates an increase in siah-1b transcription on activation of endogenous and inducible exogenous p53. To explore whether this effect is directly mediated by p53 we analyzed 20 kb of chromosome X DNA, containing the siah-1b locus. A p53-binding site was identified in the siah-1b promoter, located at nucleotides -2155/-2103 relative to the translational start site. This site is composed of two half-sites, conforming to the p53-binding consensus sequence but separated by a nonclassical 33-bp spacer. In luciferase assays, p53 induces a substantial increase in siah-1b promoter activity. Gel shift and DNase-I-footprinting studies, combined with mutational analysis and chromatin immunoprecipitation, indicate that p53 effectively binds the siah-1b promoter in vitro and in vivo. Thus, the siah-1b gene is a direct transcriptional target of p53.

SIAH1 inactivation correlates with tumor progression in hepatocellular carcinomas

Accumulation of loss of heterozygosity (LOH) on chromosome 16 is frequently observed in human hepatocellular carcinomas (HCCs). To identify tumor-suppressor genes (TSGs) involved in hepatocarcinogenesis, we performed deletion mapping of chromosome 16 in 59 HCCs. Three commonly deleted regions, located in 16q12.1, 16q22.1, and 16q24.2, were observed. Because there has been no study on LOH at locus 16q12.1 in HCCs, we focused on this region. By searching the Human Genome Database at the National Center for Biotechnology Information web site, we identified 14 known genes in 16q12.1 as TSG candidates. Among these, the expression of SIAH1 was markedly downregulated in HCCs, and inactivation of SIAH1 expression was associated with LOH at 16q12.1. A mutation analysis of SIAH1 revealed no somatic mutations, but one single nucleotide polymorphism was found among the 35 HCCs investigated. Subsequently, we evaluated the relation between SIAH1 expression, confirmed by semiquantitative RT-PCR, and clinicopathological parameters in HCCs. SIAH1 was significantly downregulated in advanced HCCs, including poorly differentiated tumors, larger tumors, and tumors in advanced stages. These findings suggest that inactivation of SIAH1 plays an important role in HCC progression.

Normal p53 function in primary cells deficient for Siah genes

Overexpression studies have suggested that Siah1 proteins may act as effectors of p53-mediated cellular responses and as regulators of mitotic progression. We have tested these hypotheses using Siah gene knockout mice. Siah1a and Siah1b were not induced by activation of endogenous p53 in tissues, primary murine embryonic fibroblasts (MEFs) or thymocytes. Furthermore, primary MEFs lacking Siah1a, Siah1b, Siah2, or both Siah2 and Siah1a displayed normal cell cycle progression, proliferation, p53-mediated senescence, and G(1) phase cell cycle arrest. Primary thymocytes deficient for Siah1a, Siah2, or both Siah2 and Siah1a, E1A-transformed MEFs lacking Siah1a, Siah1b, or Siah2, and Siah1b-null ES cells all underwent normal p53-mediated apoptosis. Finally, inhibition of Siah1b expression in Siah2 Siah1a double-mutant cells failed to inhibit cell division, p53-mediated induction of p21 expression, or cell cycle arrest. Our loss-of-function experiments do not support a general role for Siah genes in p53-mediated responses or mitosis.