Siah proteins: novel drug targets in the Ras and hypoxia pathways

SIAH1 facilitates the migration and invasion of gastric cancer cells through promoting the ubiquitination and degradation of RECK

Siah E3 ubiquitin protein ligase 1 (SIAH1) has been reported to participate in the development of several human cancers, including gastric cancer. However, the effect and mechanism of SIAH1 on the migration and invasion of gastric cancer cells need be further explored. Here, we first analyzed the clinical value of SIAH1 in gastric cancer, and found that SIAH1 was up-regulated in gastric cancer and associated with a poor prognosis. In addition, silencing of SIAH1 significantly inhibited the migration and invasion of gastric cancer cells through inhibiting the expression of matrix metalloproteinase-9 (MMP9), while overexpression of SIAH1 had the opposite effect. Molecularly, we provided the evidence that reversion-inducing cysteine-rich protein with Kazal motifs (RECK) was a potential substrate of SIAH1. We determined that SIAH1 could destabilize RECK through promoting its ubiquitination and degradation via proteasome pathway. We also found RECK was involved in SIAH1-regulated gastric cancer cell migration and invasion. In conclusion, SIAH1 is up-regulated in gastric cancer, which promotes the migration and invasion of gastric cancer cells through regulating RECK-MMP9 pathway.

SIAH2 is specifically expressed during cervical carcinogenesis, and closely relates to the abnormal proliferation of cervical epithelial cells

Background

Cervical cancer is one of the most common malignancies in women worldwide. As a RING type ubiquitin ligase, SIAH2 has been reported to promote the progression of a variety of tumors by interacting with and targeting multiple chaperones and substrates. The aim of this study was to further identify the role and the related molecular mechanisms involved of SIAH2 in cervical carcinogenesis.

Methods and results

Cellular assays in vitro showed that knockdown of SIAH2 inhibited the proliferation, migration and invasion of human cervical cancer cells C33A and SiHa, induced apoptosis, and increased the sensitivity to cisplatin treatment. Knockdown of SIAH2 also inhibited the epithelial-mesenchymal transition and activation of the Akt/mTOR signaling pathway in cervical cancer cells, which were detected by Western blot. Mechanistically, SIAH2, as a ubiquitin ligase, induced the ubiquitination degradation of GSK3β degradation by using coIP. The results of complementation experiments further demonstrated that GSK3β overexpression rescued the increase of cell proliferation and invasion caused by SIAH2 overexpression. Specific expression of SIAH2 appeared in precancerous and cervical cancer tissues compared to inflammatory cervical lesions tissues using immunohistochemical staining. The more SIAH2 was expressed as the degree of cancer progressed. SIAH2 was significantly highly expressed in cervical cancer tissues (44/55, 80 %) compared with precancerous tissues (18/69, 26.1 %). Moreover, the expression level of SIAH2 in cervical cancer tissues was significantly correlated with the degree of cancer differentiation, and cervical cancer tissues with higher SIAH2 expression levels were less differentiated.

Conclusion

Targeting SIAH2 may be beneficial to the treatment of cervical cancer.

SIAH1/CTR9 axis promotes the epithelial-mesenchymal transition of hepatocellular carcinoma

SIAH1 has been reported to participate in several human cancers, including hepatocellular carcinoma (HCC). However, the effect of SIAH1 on the epithelial-mesenchymal transition (EMT) has not been reported in HCC cells. Here, we discovered the inhibitory effect of SIAH1 on HCC cell migration and invasion, which was related with regulating EMT. Molecularly, a yeast two-hybrid experiment indicated that Cln Three Requiring 9 (CTR9) was a potential interacting protein of SIAH1, which was further verified by co-immunoprecipitation assays. Furthermore, SIAH1 inhibited the EMT of HCC cells through negatively regulating CTR9. Importantly, CTR9 was ubiquitinated and degraded by SIAH1 via the proteasome pathway in HCC cells. Additionally, it was showed that SIAH1 mainly mediated the K48-linked polyubiquitination on CTR9. Finally, the protein level of CTR9 was found to be inversely correlated with SIAH1 in human HCC tissues. Summed up all together, these findings reveal that SIAH1/CTR9 axis promotes the EMT of HCC cells and is a promising therapeutic target for HCC therapy.

SIAH1-mediated RPS3 ubiquitination contributes to chemosensitivity in epithelial ovarian cancer

The E3 ligase SIAH1 is deregulated in human cancers and correlated with poor prognosis, but its contributions to chemoresistance in epithelial ovarian cancer (EOC) are not evident. Herein we found that SIAH1 was decreased in EOC tumour tissues and cell lines and negatively correlated with the RPS3 levels. SIAH1 overexpression suppressed tumour cell growth, colony formation, invasion, metastasis, and cisplatin resistance in vivo and in vitro. SIAH1 promoted RPS3 ubiquitination and degradation using the RING-finger domain, and these steps were required for RPS3 localization to the cytoplasm, which led to subsequent NF-κB inactivation and thereby conferred chemosensitivity. Moreover, ectopic expression of RPS3 or depletion of RPS3 ubiquitination mediated by SIAH1 via the K214R mutant significantly impaired cisplatin-induced tumour suppression in cells stably expressing SIAH1. Together, our findings reveal a tumour suppressor function of SIAH1 and provide evidence showing that the SIAH1-RPS3-NF-κB axis may act as an appealing strategy for tackling treatment resistance in EOC.

SIAH1 reverses chemoresistance in epithelial ovarian cancer via ubiquitination of YBX-1

Chemoresistance is a severe outcome among patients with epithelial ovarian cancer (EOC) that leads to a poor prognosis. YBX-1 has been shown to cause treatment failure and cancer progression in EOC. However, strategies that directly target YBX-1 are not yet conceivable. Here, we identified that SIAH1 which was downregulated in chemoresistant EOC samples and cell lines functioned as novel E3 ligases to trigger degradation of YBX-1 at cytoplasm by RING finger domain. Mechanistic studies show that YBX-1 was ubiquitinated by SIAH1 at lys304 that lead to the instability of its target m5C-modified mRNAs, thus sensitized EOC cells to cDDP. Overexpression of SIAH1 enhanced the antitumor efficacy of cisplatin in vitro and in vivo, which were partially impaired by ectopic expression of YBX-1 or depletion of YBX-1 ubiquitination. In summary, our data identify the SIAH1/YBX-1 interaction as a therapeutic target for overcoming EOC chemoresistance.

Siah1 in cancer and nervous system diseases (Review)

The dysregulation of the ubiquitin‑proteasome system will result in the abnormal accumulation and dysfunction of proteins, thus leading to severe diseases. Seven in absentia homolog 1 (Siah1), an E3 ubiquitin ligase, has attracted wide attention due to its varied functions in physiological and pathological conditions, and the numerous newly discovered Siah1 substrates. In cancer and nervous system diseases, the functions of Siah1 as a promoter or a suppressor of diseases are related to the change in cellular microenvironment and subcellular localization. At the same time, complex upstream regulations make Siah1 different from other E3 ubiquitin ligases. Understanding the molecular mechanism of Siah1 will help the study of various signaling pathways and benefit the therapeutic strategy of human diseases (e.g., cancer and nervous system diseases). In the present review, the functions and regulations of Siah1 are described. Moreover, novel substrates of Siah1 discovered in recent studies will be highlighted in cancer and nervous system diseases, providing ideas for future research and clinical targeted therapies using Siah1.

Links between the unfolded protein response and the DNA damage response in hypoxia: a systematic review

Hypoxia is a feature of most solid tumours and predicts for poor prognosis. In radiobiological hypoxia (<0.1% O₂) cells become up to three times more resistant to radiation. The biological response to radiobiological hypoxia is one of few physiologically relevant stresses that activates both the unfolded protein and DNA damage responses (UPR and DDR). Links between these pathways have been identified in studies carried out in normoxia. Based in part on these previous studies and recent work from our laboratory, we hypothesised that the biological response to hypoxia likely includes overlap between the DDR and UPR. While inhibition of the DDR is a recognised strategy for improving radiation response, the possibility of achieving this through targeting the UPR has not been realised. We carried out a systematic review to identify links between the DDR and UPR, in human cell lines exposed to <2% O₂. Following PRISMA guidance, literature from January 2010 to October 2020 were retrieved via Ovid MEDLINE and evaluated. A total of 202 studies were included. LAMP3, ULK1, TRIB3, CHOP, NOXA, NORAD, SIAH1/2, DYRK2, HIPK2, CREB, NUPR1, JMJD2B, NRF2, GSK-3B, GADD45a, GADD45b, STAU1, C-SRC, HK2, CAV1, CypB, CLU, IGFBP-3 and SP1 were highlighted as potential links between the hypoxic DDR and UPR. Overall, we identified very few studies which demonstrate a molecular link between the DDR and UPR in hypoxia, however, it is clear that many of the molecules highlighted warrant further investigation under radiobiological hypoxia as these may include novel therapeutic targets to improve radiotherapy response.

E3 ubiquitin ligase-mediated regulation of vertebrate ocular development; new insights into the function of SIAH enzymes

Developmental regulation of the vertebrate visual system has been a focus of investigation for generations as understanding this critical time period has direct implications on our understanding of congenital blinding disease. The majority of studies to date have focused on transcriptional regulation mediated by morphogen gradients and signaling pathways. However, recent studies of post translational regulation during ocular development have shed light on the role of the ubiquitin proteasome system (UPS). This rather ubiquitous yet highly diverse system is well known for regulating protein function and localization as well as stability via targeting for degradation by the 26S proteasome. Work from many model organisms has recently identified UPS activity during various milestones of ocular development including retinal morphogenesis, retinal ganglion cell function as well as photoreceptor homeostasis. In particular work from flies and zebrafish has highlighted the role of the E3 ligase enzyme family, Seven in Absentia Homologue (Siah) during these events. In this review, we summarize the current understanding of UPS activity during Drosophila and vertebrate ocular development, with a major focus on recent findings correlating Siah E3 ligase activity with two major developmental stages of vertebrate ocular development, retinal morphogenesis and photoreceptor specification and survival.

TRAF4, a new substrate of SIAH1, participates in chemotherapy resistance of breast cancer cell by counteracting SIAH1-mediated downregulation of β-catenin

PURPOSE

TRAF4 plays an important role in the development and progression of breast cancer, but its impact on chemotherapy resistance is as yet, however, poorly understood.

METHODS

Western blotting, immunoprecipitation, and immunofluorescence staining were used to identify and verify that TRAF4 was a novel substrate of SIAH1 and prevented SIAH1-mediated β-catenin degradation. Cell proliferation analysis and Flow cytometry analysis were utilized to detect TRAF4's function on the growth-inhibitory effect of etoposide. Immunohistochemistry was used to detect the expression of TRAF4, SIAH1, and β-catenin. Statistical analysis was used to analyze the relationships between them with clinical parameters and curative effect of chemotherapy pathologically.

RESULTS

Our results suggested that TRAF4 prevents SIAH1-mediated β-catenin degradation. TRAF4 was a novel substrate of SIAH1 and the TRAF domain of TRAF4 was critical for binding to SIAH1. TRAF4 reduced the growth-inhibitory effect of etoposide via reducing the number of S-phase cells and suppressing cell apoptosis. Concordantly, we found that breast cancer patients with a low-TRAF4 expression benefited most from chemotherapy, who had higher tumor volume reduction rate and better pathological response, while, the high-TRAF4 expression group had lower tumor volume reduction rate and poor pathological response.

CONCLUSIONS

TRAF4 was a novel substrate of SIAH1 and prevented SIAH1-mediated β-catenin degradation, which explains the protective effect of TRAF4 on β-catenin during cell stress and links TRAF4 to chemotherapy resistance in tumors. These findings implicated a novel pathway for the oncogenic function of TRAF4.

Expression of Notch pathway components (Numb, Itch, and Siah-1) in colorectal tumors: A clinicopathological study

BACKGROUND

The role of the Notch pathway in carcinogenesis and tumor progression has been demonstrated in many organs, including the colon. Accordingly, studies aimed at developing therapies targeting this pathway in various cancers require the identification of several factors that may play a role in regulating Notch-1 expression. Although Numb, Itch, and seven in absentia homolog-1 (Siah-1) have been shown to contribute to the regulation of Notch signaling, their role in colorectal carcinogenesis and tumor progression has not been fully elucidated to date.

AIM

To evaluate Numb, Itch, and Siah-1 expression in colorectal tumors to clarify their relationship with Notch-1 expression and their role in carcinogenesis and tumor behavior.

METHODS

Expression of Notch-1, Numb, Itch, and Siah-1 was investigated in 50 colorectal carcinomas, 30 adenomas, and 20 healthy colonic tissues by immunohistochemistry and quantitative real-time polymerase chain reaction (PCR) analyses.

RESULTS

In contrast to Notch-1, which is expressed at higher levels in tumor tissues and adenomas, expression of Numb, Itch, and Siah-1 was stronger and more frequent in normal mucosa (P < 0.01). There was a positive correlation between Notch-1 expression and high histological grade, the presence of lymph node metastasis, and advanced-stage tumors, whereas expression of Numb, Itch, and Siah-1 was absent or reduced in tumors with these clinicopathological parameters (P < 0.05). In survival analysis, expression of Notch was related to poor prognosis but that of Numb, Itch, and Siah-1 correlated with improved survival (P < 0.05). Multivariate analysis revealed Notch-1 expression and loss of Numb expression to be independent prognostic parameters together with lymph node metastasis (P < 0.05).

CONCLUSION

Our findings support the role of Notch-1 in colorectal carcinoma and indicate that loss of Numb, Itch, and Siah-1 expression is associated with carcinogenesis. Our data also suggest that these three proteins might be involved in the Notch-1 pathway during colorectal carcinoma (CRC) progression and might play an essential role in approaches targeting Notch as novel molecular therapies for CRC.

Ovarian BDNF promotes survival, migration, and attachment of tumor precursors originated from p53 mutant fallopian tube epithelial cells

High-grade serous ovarian carcinoma (HGSOC) is the most lethal gynecological malignancy. New evidence supports a hypothesis that HGSOC can originate from fallopian tube epithelium (FTE). It is unclear how genetic alterations and pathophysiological processes drive the progression of FTE tumor precursors into widespread HGSOCs. In this study, we uncovered that brain-derived neurotrophic factor (BDNF) in the follicular fluid stimulates the tropomyosin receptor kinase B (TrkB)-expressing FTE cells to promote their survival, migration, and attachment. Using in vitro and in vivo models, we further identified that the acquisition of common TP53 gain-of-function (GOF) mutations in FTE cells led to enhanced BDNF/TrkB signaling compared to that of FTE cells with TP53 loss-of-function (LOF) mutations. Different mutant p53 proteins can either increase TrkB transcription or enhance TrkB endocytic recycling. Our findings have demonstrated possible interplays between genetic alterations in FTE tumor precursors (i.e., p53 GOF mutations) and pathophysiological processes (i.e., the release of follicular fluid upon ovulation) during the initiation of HGSOC from the fallopian tube. Our data revealed molecular events underlying the link between HGSOC tumorigenesis and ovulation, a physiological process that has been associated with risk factors of HGSOC.

The PARP1-Siah1 Axis Controls HIV-1 Transcription and Expression of Siah1 Substrates

Recent studies have revealed a key role of PARP1 that catalyzes the poly-ADP-ribosylation (PARylation) of substrates in regulating gene transcription. We show here that HIV-1 transcriptional activation also requires PARP1 activity. Because efficient HIV-1 transactivation is known to depend on the ELL2-containing super elongation complex (SEC), we investigated the functional relationship between PARP1 and ELL2-SEC in HIV-1 transcriptional control. We show that PARP1 elevates ELL2 protein levels to form more ELL2-SEC in cells. This effect is caused by PARP1's suppression of expression of Siah1, an E3 ubiquitin ligase for ELL2, at both mRNA and protein levels. At the mRNA level, PARP1 coordinates with the co-repressor NCoR to suppress Siah1 transcription. At the protein level, PARP1 promotes Siah1 proteolysis, likely through inducing PARylation-dependent ubiquitination (PARdU) of Siah1. Thus, a PARP1-Siah1 axis activates HIV-1 transcription and controls the expression of ELL2 and other Siah1 substrates.

The role of E3 ubiquitin ligase seven in absentia homolog in the innate immune system: An overview

The innate immune system has been considered as an ancient system and less important than the adaptive immune system. However, the interest in innate immunity has grown significantly in the past few years marked by the identification of Toll-like receptors, a member of pattern recognition receptors (PRRs). The PRRs are crucial for the identification of self- and non-self-antigen and play a role in the initiation of signaling events that activate the effective immune response. These sensor signals through interweaving signaling cascades which result in the production of interferons and cytokines as the effector of immune system. Ubiquitin and ubiquitin-like modifiers (UBLs) actively mediate the rapid and versatile regulatory processes that initiate the activation of the innate immune system cascade. The seven in absentia homolog (SIAH) is a potent RING finger E3 ubiquitin ligase that is known to involve in several stress responses, including hypoxia, oxidative stress, DNA damage stress, and inflammation. In this review, the role of SIAH will be discussed as an E3 ubiquitin ligase on the regulation of innate immune.

PHF19 promotes the proliferation, migration, and chemosensitivity of glioblastoma to doxorubicin through modulation of the SIAH1/β-catenin axis

PHD finger protein 19 (PHF19), a critical component of the polycomb repressive complex 2 (PRC2), is crucial for maintaining the repressive transcriptional activity of several developmental regulatory genes and plays essential roles in various biological processes. Abnormal expression of PHF19 causes dysplasia or serious diseases, including chronic myeloid disorders and tumors. However, the biological functions and molecular mechanisms of PHF19 in glioblastoma (GBM) remain unclear. Here, we demonstrated that PHF19 expression was positively associated with GBM progression, including cell proliferation, migration, invasion, chemosensitivity, and tumorigenesis. Using XAV-939, a Wnt/β-catenin inhibitor, we found that the effects of PHF19 on GBM cells were β-catenin-dependent. We also demonstrated that PHF19 expression was positively correlated with cytoplasmic β-catenin expression. PHF19 stabilized β-catenin by inhibiting the transcription of seven in absentia homolog 1 (SIAH1), an E3 ubiquitin ligase of β-catenin, through direct binding to the SIAH1 promoter region. Taken together, our results revealed the novel PHF19-SIAH1–β-catenin axis as a potential and promising therapeutic target.

Inhibition of Siah2 Ubiquitin Ligase by Vitamin K3 Attenuates Chronic Myeloid Leukemia Chemo-Resistance in Hypoxic Microenvironment

BACKGROUND A hypoxic microenvironment is associated with resistance to tyrosine kinase inhibitors (TKIs) and a poor prognosis in chronic myeloid leukemia (CML). The E3 ubiquitin ligase Siah2 plays a vital role in the regulation of hypoxia response, as well as in leukemogenesis. However, the role of Siah2 in CML resistance is unclear, and it is unknown whether vitaminK3 (a Siah2 inhibitor) can improve the chemo-sensitivity of CML cells in a hypoxic microenvironment. MATERIAL AND METHODS The expression of Siah2 was detected in CML patients (CML-CP and CML-BC), K562 cells, and K562-imatinib-resistant cells (K562-R cells). We measured the expression of PHD3, HIF-1α, and VEGF in both cell lines under normoxia and hypoxic conditions, and the degree of leukemic sensitivity to imatinib and VitaminK3 were evaluated. RESULTS Siah2 was overexpressed in CML-BC patients (n=9) as compared to CML-CP patients (n=13). Similarly, K562-imatinib-resistant cells (K562-R cells) showed a significantly higher expression of Siah2 as compared to K562 cells in a hypoxic microenvironment. Compared to normoxia, under hypoxic conditions, both cell lines had lower PHD3, higher HIF-1α, and higher VEGF expression. Additionally, Vitamin K3 (an inhibitor of Siah2) reversed these changes and promoted a higher degree of leukemic sensitivity to imatinib. CONCLUSIONS Our findings indicate that the Siah2-PHD3- HIF-1α-VEGF axis is an important hypoxic signaling pathway in a leukemic microenvironment. An inhibitor of Siah2, combined with TKIs, might be a promising therapy for relapsing and refractory CML patients.

Phylogenetic analysis of the SINA/SIAH ubiquitin E3 ligase family in Metazoa

Background

The RAS signaling pathway is a pivotal developmental pathway that controls many fundamental biological processes including cell proliferation, differentiation, movement and apoptosis. Drosophila Seven-IN-Absentia (SINA) is a ubiquitin E3 ligase that is the most downstream signaling “gatekeeper” whose biological activity is essential for proper RAS signal transduction. Vertebrate SINA homologs (SIAHs) share a high degree of amino acid identity with that of Drosophila SINA. SINA/SIAH is the most conserved signaling component in the canonical EGFR/RAS/RAF/MAPK signal transduction pathway.

Results

Vertebrate SIAH1, 2, and 3 are the three orthologs to invertebrate SINA protein. SINA and SIAH1 orthologs are found in all major taxa of metazoans. These proteins have four conserved functional domains, known as RING (Really Interesting New Gene), SZF (SIAH-type zinc finger), SBS (substrate binding site) and DIMER (Dimerization). In addition to the siah1 gene, most vertebrates encode two additional siah genes (siah2 and siah3) in their genomes. Vertebrate SIAH2 has a highly divergent and extended N-terminal sequence, while its RING, SZF, SBS and DIMER domains maintain high amino acid identity/similarity to that of SIAH1. But unlike vertebrate SIAH1 and SIAH2, SIAH3 lacks a functional RING domain, suggesting that SIAH3 may be an inactive E3 ligase. The SIAH3 subtree exhibits a high degree of amino acid divergence when compared to the SIAH1 and SIAH2 subtrees. We find that SIAH1 and SIAH2 are expressed in all human epithelial cell lines examined thus far, while SIAH3 is only expressed in a limited subset of cancer cell lines.

Conclusion

Through phylogenetic analyses of metazoan SINA and SIAH E3 ligases, we identified many invariant and divergent amino acid residues, as well as the evolutionarily conserved functional motifs in this medically relevant gene family. Our phylomedicinal study of this unique metazoan SINA/SIAH protein family has provided invaluable evolution-based support towards future effort to design logical, potent, and durable anti-SIAH-based anticancer strategies against oncogenic K-RAS-driven metastatic human cancers. Thus, this method of evolutionary study should be of interest in cancer biology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-1024-x) contains supplementary material, which is available to authorized users.

The SIAH E3 ubiquitin ligases promote Wnt/β-catenin signaling through mediating Wnt-induced Axin degradation

In this study, Ji et al. identify SIAH1/2 (SIAH) as the E3 ligase mediating Wnt-induced Axin degradation. Their results suggest that Wnt-induced dissociation of the Axin/GSK3 complex allows SIAH to interact with Axin and promote its degradation, which represents an important feed-forward mechanism to achieve sustained Wnt/β-catenin signaling.

The Wnt/β-catenin signaling pathway plays essential roles in embryonic development and adult tissue homeostasis. Axin is a concentration-limiting factor responsible for the formation of the β-catenin destruction complex. Wnt signaling itself promotes the degradation of Axin. However, the underlying molecular mechanism and biological relevance of this targeting of Axin have not been elucidated. Here, we identify SIAH1/2 (SIAH) as the E3 ligase mediating Wnt-induced Axin degradation. SIAH proteins promote the ubiquitination and proteasomal degradation of Axin through interacting with a VxP motif in the GSK3-binding domain of Axin, and this function of SIAH is counteracted by GSK3 binding to Axin. Structural analysis reveals that the Axin segment responsible for SIAH binding is also involved in GSK3 binding but adopts distinct conformations in Axin/SIAH and Axin/GSK3 complexes. Knockout of SIAH1 blocks Wnt-induced Axin ubiquitination and attenuates Wnt-induced β-catenin stabilization. Our data suggest that Wnt-induced dissociation of the Axin/GSK3 complex allows SIAH to interact with Axin not associated with GSK3 and promote its degradation and that SIAH-mediated Axin degradation represents an important feed-forward mechanism to achieve sustained Wnt/β-catenin signaling.

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.

The substrate binding domains of human SIAH E3 ubiquitin ligases are now crystal clear

BACKGROUND

Seven in absentia homologs (SIAHs) comprise a family of highly conserved E3 ubiquitin ligases that play an important role in regulating signalling pathways in tumorigenesis, including the DNA damage repair and hypoxia response pathways. SIAH1 and SIAH2 have been found to function as a tumour repressor and a proto-oncogene, respectively, despite the high sequence identity of their substrate binding domains (SBDs). Ubiquitin-specific protease USP19 is a deubiquitinase that forms a complex with SIAHs and counteracts the ligase function. Much effort has been made to find selective inhibitors of the SIAHs E3 ligases. Menadione was reported to inhibit SIAH2 specifically.

METHODS

We used X-ray crystallography, peptide array, bioinformatic analysis, and biophysical techniques to characterize the structure and interaction of SIAHs with deubiquitinases and literature reported compounds.

RESULTS

We solved the crystal structures of SIAH1 in complex with a USP19 peptide and of the apo form SIAH2. Phylogenetic analysis revealed the SIAH/USP19 complex is conserved in evolution. We demonstrated that menadione destabilizes both SIAH1 and SIAH2 non-specifically through covalent modification.

CONCLUSIONS

The SBDs of SIAH E3 ligases are structurally similar with a subtle stability difference. USP19 is the only deubiquitinase that directly binds to SIAHs through the substrate binding pocket. Menadione is not a specific inhibitor for SIAH2.

GENERAL SIGNIFICANCE

The crystallographic models provide structural insights into the substrate binding of the SIAH family E3 ubiquitin ligases that are critically involved in regulating cancer-related pathways. Our results suggest caution should be taken when using menadione as a specific SIAH2 inhibitor.

SIAH ubiquitin ligases regulate breast cancer cell migration and invasion independent of the oxygen status

Seven-in-absentia homolog (SIAH) proteins are evolutionary conserved RING type E3 ubiquitin ligases responsible for the degradation of key molecules regulating DNA damage response, hypoxic adaptation, apoptosis, angiogenesis, and cell proliferation. Many studies suggest a tumorigenic role for SIAH2. In breast cancer patients SIAH2 expression levels correlate with cancer aggressiveness and overall patient survival. In addition, SIAH inhibition reduced metastasis in melanoma. The role of SIAH1 in breast cancer is still ambiguous; both tumorigenic and tumor suppressive functions have been reported. Other studies categorized SIAH ligases as either pro- or antimigratory, while the significance for metastasis is largely unknown. Here, we re-evaluated the effects of SIAH1 and SIAH2 depletion in breast cancer cell lines, focusing on migration and invasion. We successfully knocked down SIAH1 and SIAH2 in several breast cancer cell lines. In luminal type MCF7 cells, this led to stabilization of the SIAH substrate Prolyl Hydroxylase Domain protein 3 (PHD3) and reduced Hypoxia-Inducible Factor 1α (HIF1α) protein levels. Both the knockdown of SIAH1 or SIAH2 led to increased apoptosis and reduced proliferation, with comparable effects. These results point to a tumor promoting role for SIAH1 in breast cancer similar to SIAH2. In addition, depletion of SIAH1 or SIAH2 also led to decreased cell migration and invasion in breast cancer cells. SIAH knockdown also controlled microtubule dynamics by markedly decreasing the protein levels of stathmin, most likely via p27(Kip1). Collectively, these results suggest that both SIAH ligases promote a migratory cancer cell phenotype and could contribute to metastasis in breast cancer.

Suppression of cell migration is promoted by miR-944 through targeting of SIAH1 and PTP4A1 in breast cancer cells

Background

Aberrant expression of microRNAs has been associated with migration of tumor cells. In this study, we aimed to investigate the biological significance of miR-944 whose function is unknown in breast cancer.

Methods

MiR-944 expression in breast cancer cells and tumors was evaluated by Taqman qRT-PCR assays. Transcriptional profiling of MDA-MB-231 cells expressing miR-944 was performed using DNA microarrays. Cell viability, migration and invasion were assessed by MTT, scratch/wound-healing and transwell chamber assays, respectively. The luciferase reporter assay was used to evaluate targeting of SIAH1, PTP4A1 and PRKCA genes by miR-944. SIAH1 protein levels were measured by Western blot. Silencing of SIAH1 gene was performed by RNA interference using shRNAs.

Results

Our data showed that miR-944 expression was severely repressed in clinical specimens and breast cancer cell lines. Suppression of miR-944 levels was independent of hormonal status and metastatic potential of breast cancer cells. Gain-of-function analysis indicated that miR-944 altered the actin cytoskeleton dynamics and impaired cell migration and invasion. Genome-wide transcriptional profiling of MDA-MB-231 cells that ectopically express miR-944 showed that 15 genes involved in migration were significantly repressed. Notably, luciferase reporter assays confirmed the ability of miR-944 to bind the 3´UTR of SIAH1 and PTP4A1 genes, but not PRKCA gene. Congruently, an inverse correlation between miR-944 and SIAH1 protein expression was found in breast cancer cells. Moreover, SIAH1 was upregulated in 75 % of miR-944-deficient breast tumors. Finally, SIAH1 gene silencing by RNA interference significantly impaired cell migration of breast cancer cells.

Conclusions

Our results pointed out that miR-944 is a novel upstream negative regulator of SIAH1 and PTP4A1 genes and provided for the first time evidence for its functional role in migration and invasion of breast cancer cells. They also suggest that miR-944 restoration may represent a potential strategy for breast cancer therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2470-3) contains supplementary material, which is available to authorized users.

CHK2 stability is regulated by the E3 ubiquitin ligase SIAH2

The serine threonine checkpoint kinase 2 (CHK2) is a critical protein involved in the DNA damage-response pathway, which is activated by phosphorylation inducing cellular response such as DNA repair, cell-cycle regulation or apoptosis. Although CHK2 activation mechanisms have been amply described, very little is known about degradation control processes. In the present study, we identify the ubiquitin E3 ligase SIAH2 as an interaction partner of CHK2, which mediates its ubiquitination and proteasomal degradation. CHK2 degradation is independent of both its activation and its kinase activity, but also of the phosphorylation in S456. We show that SIAH2-deficient cells present CHK2 accumulation together with lower ubiquitination levels. Accordingly, SIAH2 depletion by siRNA increases CHK2 levels. In response to DNA damage induced by etoposide, interaction between both proteins is disrupted, thus avoiding CHK2 degradation and promoting its stabilization. We also found that CHK2 phosphorylates SIAH2 at three residues (Thr26, Ser28 and Thr119), modifying its ability to regulate certain substrates. Cellular arrest in the G2/M phase induced by DNA damage is reverted by SIAH2 expression through the control of CHK2 levels. We observed that hypoxia decreases CHK2 levels in parallel to SIAH2 induction. Similarly, we provide evidence suggesting that resistance to apoptosis induced by genotoxic agents in cells subjected to hypoxia could be partly explained by the mutual regulation between both proteins. These results indicate that SIAH2 regulates CHK2 basal turnover, with important consequences on cell-cycle control and on the ability of hypoxia to alter the DNA damage-response pathway in cancer cells.

Overexpression of Siah2 Is Associated With Poor Prognosis in Patients With Epithelial Ovarian Carcinoma

Objectives

Seven in absentia homolog 2 (Siah2) is an E3 ubiquitin ligase that is expressed in mammals and is homologous to seven in absentia in Drosophila. Siah2 is involved in the progression of many malignancies. However, the role of Siah2 in ovarian cancer remains unclear. This study aims to evaluate the prognostic value of Siah2 expression for epithelial ovarian carcinoma (EOC) patients.

Materials and Methods

Immunohistochemical analysis was conducted using 32 normal ovarian specimens and 122 ovarian carcinoma specimens, respectively. We analyzed the correlations of Siah2 expression with the clinicopathological factors and prognosis of ovarian cancer patients. χ2 Analysis, Kaplan-Meier method, and multivariate Cox proportional hazard analysis were conducted for statistical analyses.

Results

Immunohistochemical staining demonstrated that the expression of Siah2 was higher in the EOC tissues than in the normal tissues. High Siah2 expression positively correlated with histological grade and lymph node metastasis but not with age, histologic type, International Federation of Gynecology and Obstetrics staging, and CA125. Patients with positive Siah2 expression showed lower overall survival and disease-free survival rates than those with negative Siah2 expression (P < 0.05 for both). Multivariate Cox analysis indicated that Siah2 was an independent parameter for overall survival (hazards ratio, 2.166; 95% confidence interval, 1.182–3.970; P = 0.012) and disease-free survival (hazards ratio, 1.819; 95% confidence interval, 1.030–3.216; P = 0.039).

Conclusions

Siah2 is possibly involved in tumor development and progression in EOC. Thus, Siah2 is a promising biomarker for predicting the prognosis of ovarian cancer patients and may serve as a novel target for treating ovarian carcinoma.

The ubiquitin ligase Siah is a novel regulator of Zeb1 in breast cancer

Elucidating the mechanisms that underlie metastasis is of paramount importance to understanding tumor progression and to the development of novel therapeutics. Epithelial to Mesenchymal Transition (EMT) plays a vital role in tumor cell dissemination and is regulated by a core cassette of transcription factors. Despite recent advances, the molecular pathways that regulate the EMT program have not yet been fully delineated. We show that Siah ubiquitin ligases regulate Zeb1 protein, a key EMT transcription factor. The induction of EMT in breast cancer cells leads to the down-regulation of Siah, while the loss of Siah induces a mesenchymal phenotype, concurrent with an up-regulation of Zeb1. Overexpression of Siah in vitro mediates Zeb1 degradation, which can be blocked with a Siah peptide inhibitor. Thus, this work demonstrates that Siah is a novel regulator of EMT. This work is the first to identify a mechanism of post-translational regulation of the key Epithelial to Mesenchymal Transition transcription factor Zeb1.

The Expression of the Ubiquitin Ligase SIAH2 (Seven In Absentia Homolog 2) Is Increased in Human Lung Cancer

OBJECTIVES

Lung cancer is the leading cause of cancer-related deaths worldwide. Overall 5-year survival has shown little improvement over the last decades. Seven in absentia homolog (SIAH) proteins are E3 ubiquitin ligases that mediate proteasomal protein degradation by poly-ubiquitination. Even though SIAH proteins play a key role in several biological processes, their role in human cancer remains controversial. The aim of the study was to document SIAH2 expression pattern at different levels (mRNA, protein level and immunohistochemistry) in human non-small cell lung cancer (NSCLC) samples compared to surrounding healthy tissue from the same patient, and to analyse the association with clinicopathological features.

MATERIALS AND METHODS

One hundred and fifty-two samples from a patient cohort treated surgically for primary lung cancer were obtained for the study. Genic and protein expression levels of SIAH2 were analysed and compared with clinic-pathologic variables.

RESULTS

The present study is the first to analyze the SIAH2 expression pattern at different levels (RNA, protein expression and immunohistochemistry) in non-small cell lung cancer (NSCLC). We found that SIAH2 protein expression is significantly enhanced in human lung adenocarcinoma (ADC) and squamous cell lung cancer (SCC). Paradoxically, non-significant changes at RNA level were found, suggesting a post-traductional regulatory mechanism. More importantly, an increased correlation between SIAH2 expression and tumor grade was detected, suggesting that this protein could be used as a prognostic biomarker to predict lung cancer progression. Likewise, SIAH2 protein expression showed a strong positive correlation with fluorodeoxyglucose (2-deoxy-2(18F)fluoro-D-glucose) uptake in primary NSCLC, which may assist clinicians in stratifying patients at increased overall risk of poor survival. Additionally, we described an inverse correlation between the expression of SIAH2 and the levels of one of its substrates, the serine/threonine kinase DYRK2.

CONCLUSIONS

Our results provide insight into the potential use of SIAH2 as a novel target for lung cancer treatment.

Loss of Siah2 does not impact angiogenic potential of murine endothelial cells

Angiogenesis is triggered in response to hypoxia under many circumstances, from healthy cells and tissues during embryogenesis to pathological conditions like the formation of new blood vessels to supply tumours and promote invasive cancer. Siah2 has been shown to regulate the hypoxia pathway upstream of hypoxia-induced transcription factor subunit Hif-1alpha, and therefore may play an important role in angiogenesis in response to hypoxic stress in endothelial cells. This study aims to investigate the basic function of Siah2 in endothelial cells under hypoxia and to test the ability of Siah2 deficient cells to mount an angiogenic response when deprived of oxygen. We and others have previously shown that Siah2 is crucial for mediating the hypoxic response in many different cell types studied. In this study however, we describe that Siah2(-/-) endothelial cells have an intact hypoxic signalling pathway, including Hif-1alpha stabilisation and gene expression, the first report of a tissue or cell lineage in which the loss of Siah2 does not seem to impact hypoxic response signalling. In mice, the infiltration of Siah2(-/-) endothelial cells into a Matrigel plug containing a VEGF-A attractant was similar compared with wildtype endothelial cells. Ex vivo however, there was a reduced capacity of Siah2(-/-) aorta to form tubes or new vessels. Thus, we conclude that Siah2 is not essential for the hypoxic response of endothelial cells.

Ubiquitin in regulation of spindle apparatus and its positioning: implications in development and disease

Emerging data implicates ubiquitination, a post-translational modification, in regulating essential cellular events, one of them being mitosis. In this review we discuss how various E3 ligases modulate the cortical proteins such as dynein, LGN, NuMa, Gα, along with polymerization, stability, and integrity of spindles. These are responsible for regulating symmetric cell division. Some of the ubiquitin ligases regulating these proteins include PARK2, BRCA1/BARD1, MGRN1, SMURF2, and SIAH1; these play a pivotal role in the correct positioning of the spindle apparatus. A direct connection between developmental or various pathological disorders and the ubiquitination mediated cortical regulation is rather speculative, though deletions or mutations in them lead to developmental disorders and disease conditions.

SIAH2 antagonizes TYK2-STAT3 signaling in lung carcinoma cells

The Janus tyrosine kinases JAK1-3 and tyrosine kinase-2 (TYK2) are frequently hyperactivated in tumors. In lung cancers JAK1 and JAK2 induce oncogenic signaling through STAT3. A putative role of TYK2 in these tumors has not been reported. Here, we show a previously not recognized TYK2-STAT3 signaling node in lung cancer cells. We reveal that the E3 ubiquitin ligase seven-in-absentia-2 (SIAH2) accelerates the proteasomal degradation of TYK2. This mechanism consequently suppresses the activation of STAT3. In agreement with these data the analysis of primary non-small-cell lung cancer (NSCLC) samples from three patient cohorts revealed that compared to lung adenocarcinoma (ADC), lung squamous cell carcinoma (SCC) show significantly higher levels of SIAH2 and reduced STAT3 phosphorylation levels. Thus, SIAH2 is a novel molecular marker for SCC. We further demonstrate that an activation of the oncologically relevant transcription factor p53 in lung cancer cells induces SIAH2, depletes TYK2, and abrogates the tyrosine phosphorylation of STAT1 and STAT3. This mechanism appears to be different from the inhibition of phosphorylated JAKs through the suppressor of cytokine signaling (SOCS) proteins. Our study may help to identify molecular mechanisms affecting lung carcinogenesis and potential therapeutic targets.

Significance of expression of RIN1 and SIAH2 proteins in gastric carcinoma

AIM: To examine the expression of Ras and Rab interactor 1 (RIN1) and seven in absentia homologue 2 (SIAH2) proteins in gastric carcinoma and to analyze their relationship with clinicopathological factors.

METHODS: The expression of RIN1 and SIAH2 was detected by immunohistochemistry in 80 gastric carcinoma and 40 normal gastric tissues.

RESULTS: The positive rates of RIN1 and SIAH2 protein expression were significantly higher in gastric carcinoma than in normal gastric tissues (61.2% vs 32.5%, 60.0% vs 27.5%, P < 0.05 for both). In gastric carcinoma, RIN1 expression was related to tumor size, clinical stage, histopathological grade, lymph node metastasis and depth of invasion (P < 0.05 for all), but not to patients' age or gender (P > 0.05 for both); SIAH2 expression was related to clinical stage, histopathological grade, lymph node metastasis and depth of invasion (P < 0.05 for all), but not to tumor size (P > 0.05). There was a positive correlation between the expression of RIN1 and SIAH2 in gastric carcinoma (r = 0.607, P < 0.01).

CONCLUSION: The overexpression of RIN1 and SIAH2 might play an important role in the progression, invasion and metastasis of gastric carcinoma, and they may be used as early diagnostic markers and therapeutic targets for gastric carcinoma.

Investigating the molecular basis of Siah1 and Siah2 E3 ubiquitin ligase substrate specificity

The Siah1 and Siah2 E3 ubiquitin ligases play an important role in diverse signaling pathways and have been shown to be deregulated in cancer. The human Siah1 and Siah2 isoforms share high sequence similarity but possess contrary roles in cancer, with Siah1 more often acting as a tumor suppressor while Siah2 functions as a proto-oncogene. The different function of Siah1 and Siah2 in cancer is likely due to the ubiquitination of distinct substrates. Hence, we decided to investigate the molecular basis of the substrate specificity, utilizing the well-characterized Siah2 substrate PHD3. Using chimeric and mutational approaches, we identified critical residues in Siah2 that promote substrate specificity. Thus, we have found that four residues in the N-terminal region of the Siah2 substrate binding domain (SBD) (Ser132, His150, Pro155, Tyr163) are critical for substrate specificity. In the C-terminal region of the SBD, a single residue, Leu250, was identified to promote the specific binding of Siah2 SBD to PHD3. Our study may help to overcome the challenges in the identification of Siah2 specific inhibitors.

Fine tuning of the UPR by the ubiquitin ligases Siah1/2

The endoplasmic reticulum (ER) responds to changes in intracellular homeostasis through activation of the unfolded protein response (UPR). Yet, it is not known how UPR-signaling coordinates adaptation versus cell death. Previous studies suggested that signaling through PERK/ATF4 is required for cell death. We show that high levels of ER stress (i.e., ischemia-like conditions) induce transcription of the ubiquitin ligases Siah1/2 through the UPR transducers PERK/ATF4 and IRE1/sXBP1. In turn, Siah1/2 attenuates proline hydroxylation of ATF4, resulting in its stabilization, thereby augmenting ER stress output. Conversely, ATF4 activation is reduced upon Siah1/2 KD in cultured cells, which attenuates ER stress-induced cell death. Notably, Siah1a(+/-)::Siah2(-/-) mice subjected to neuronal ischemia exhibited smaller infarct volume and were protected from ischemia-induced death, compared with the wild type (WT) mice. In all, Siah1/2 constitutes an obligatory fine-tuning mechanism that predisposes cells to death under severe ER stress conditions.

Siah ubiquitin ligases modulate nodal signaling during zebrafish embryonic development

Siah2 is a zebrafish homologue of mammalian Siah family. Siah acts as an E3 ubiquitin ligase that binds proteins destined for degradation. Extensive homology between siah and Drosophila Siah homologue (sina) suggests their important physiological roles during embryonic development. However, detailed functional studies of Siah in vertebrate development have not been carried out. Here we report that Siah2 specifically augments nodal related gene expression in marginal blastomeres at late blastula through early gastrula stages of zebrafish embryos. Siah2 dependent Nodal signaling augmentation is confirmed by cell-based reporter gene assays using 293T cells and 3TPluciferase reporter plasmid. We also established a molecular hierarchy of Siah as a upstream regulator of FoxH1/Fast1 transcriptional factor in Nodal signaling. Elevated expression of nodal related genes by overexpression of Siah2 was enough to override the inhibitory effects of atv and lft2 on the Nodal signaling. In particular, E3 ubiquitin ligase activity of Siah2 is critical to limit the duration and/or magnitude of Nodal signaling. Additionally, since the embryos injected with Siah morpholinos mimicked the atv overexpression phenotype at least in part, our data support a model in which Siah is involved in mesendoderm patterning via modulating Nodal signaling.

Combining techniques for screening and evaluating interaction terms on high-dimensional time-to-event data

Background

Molecular data, e.g. arising from microarray technology, is often used for predicting survival probabilities of patients. For multivariate risk prediction models on such high-dimensional data, there are established techniques that combine parameter estimation and variable selection. One big challenge is to incorporate interactions into such prediction models. In this feasibility study, we present building blocks for evaluating and incorporating interactions terms in high-dimensional time-to-event settings, especially for settings in which it is computationally too expensive to check all possible interactions.

Results

We use a boosting technique for estimation of effects and the following building blocks for pre-selecting interactions: (1) resampling, (2) random forests and (3) orthogonalization as a data pre-processing step. In a simulation study, the strategy that uses all building blocks is able to detect true main effects and interactions with high sensitivity in different kinds of scenarios. The main challenge are interactions composed of variables that do not represent main effects, but our findings are also promising in this regard. Results on real world data illustrate that effect sizes of interactions frequently may not be large enough to improve prediction performance, even though the interactions are potentially of biological relevance.

Conclusion

Screening interactions through random forests is feasible and useful, when one is interested in finding relevant two-way interactions. The other building blocks also contribute considerably to an enhanced pre-selection of interactions. We determined the limits of interaction detection in terms of necessary effect sizes. Our study emphasizes the importance of making full use of existing methods in addition to establishing new ones.

The role and therapeutic implications of RING-finger E3 ubiquitin ligases in hepatocellular carcinoma

Increasing evidence indicates that deregulation of RING-finger ubiquitin-protein ligases (E3s) involves in the development of hepatocellular carcinoma (HCC). These RING-finger E3s serve as oncoproteins or tumor suppressors in HCC under specific conditions. In this review, we summarize current knowledge about abnormal RING-finger E3s and their clinical significance in the development of HCC, and discuss parts of critical substrates for these RING-finger E3s in detail. Furthermore, in light of success of Bortezomib in treating hematological malignancies, we describe the preclinical and clinical studies of therapeutic approaches targeting aberrant RING-finger E3s in HCC.

Two high-resolution structures of the human E3 ubiquitin ligase Siah1

Siah1 is an E3 ubiquitin ligase that contributes to proteasome-mediated degradation of multiple targets in key cellular processes and which shows promise as a therapeutic target in oncology. Structures of a truncated Siah1 bound to peptide-based inhibitors have been reported. Here, new crystallization conditions have allowed the determination of a construct encompassing dual zinc-finger subdomains and substrate-binding domains at significantly higher resolution. Although the crystals appear isomorphous, two structures present distinct states in which the spatial orientation of one zinc-finger subdomain differs with respect to the rest of the dimeric protein. Such a difference, which is indicative of conformational freedom, infers potential biological relevance related to recognition of binding partners. The crystallization conditions and improved models of Siah1 may aid future studies investigating Siah1-ligand complexes.

The many faces of C/EBPδ and their relevance for inflammation and cancer

The CCAAT/enhancer binding protein delta (CEBPD, C/EBPδ) is a transcription factor that modulates many biological processes including cell differentiation, motility, growth arrest, proliferation, and cell death. The diversity of C/EBPδ's functions depends in part on the cell type and cellular context and can have opposing outcomes. For example, C/EBPδ promotes inflammatory signaling, but it can also inhibit pro-inflammatory pathways, and in a mouse model of mammary tumorigenesis, C/EBPδ reduces tumor incidence but promotes tumor metastasis. This review highlights the multifaceted nature of C/EBPδ's functions, with an emphasis on pathways that are relevant for cancer and inflammation, and illustrates how C/EBPδ emerged from the shadow of its family members as a fascinating “jack of all trades.” Our current knowledge on C/EBPδ indicates that, rather than being essential for a specific cellular process, C/EBPδ helps to interpret a variety of cues in a cell-type and context-dependent manner, to adjust cellular functions to specific situations. Therefore, insights into the roles and mechanisms of C/EBPδ signaling can lead to a better understanding of how the integration of different signaling pathways dictates normal and pathological cell functions and physiology.

Structure-based design of covalent Siah inhibitors

The E3 ubiquitin ligase Siah regulates key cellular events that are central to cancer development and progression. A promising route to Siah inhibition is disrupting its interactions with adaptor proteins. However, typical of protein-protein interactions, traditional unbiased approaches to ligand discovery did not produce viable hits against this target, despite considerable effort and a multitude of approaches. Ultimately, a rational structure-based design strategy was successful for the identification of Siah inhibitors in which peptide binding drives specific covalent bond formation with the target. X-ray crystallography, mass spectrometry, and functional data demonstrate that these peptide mimetics are efficient covalent inhibitors of Siah and antagonize Siah-dependent regulation of Erk and Hif signaling in the cell. The proposed strategy may result useful as a general approach to the design of peptide-based inhibitors of other protein-protein interactions.

Siah2 regulates tight junction integrity and cell polarity through control of ASPP2 stability

Changes in cell adhesion and polarity are closely associated with epithelial cell transformation and metastatic capacity. The tumor suppressor protein ASPP2 has been implicated in control of cell adhesion and polarity, through its effect on the PAR complex. Here we demonstrate that under hypoxic conditions the ubiquitin ligase Siah2 controls ASPP2 availability, with concomitant effect on epithelial cell polarity. LC-MS/MS analysis identified ASPP2 and ASPP1 as Siah2 interacting proteins. Biochemical analysis confirmed this interaction and mapped degron motifs within ASPP2, which are required for Siah2-mediated ubiquitination and proteasomal-dependent degradation. Inhibition of Siah2 expression increases ASPP2 levels and enhances ASPP2-dependent maintenance of TJ integrity and polarized architecture in 3D organotypic culture. Conversely, increase of Siah2 expression under hypoxia decreases ASPP2 levels and the formation of apical polarity in 3D culture. In all, our studies demonstrate the role of Siah2 in regulation of TJ integrity and cell polarity under hypoxia, through its regulation of ASPP2 stability.

PML Degradation: Multiple Ways to Eliminate PML

The promyelocytic leukemia tumor suppressor gene (PML) critically regulates several cellular functions that oppose tumorigenesis such as oncogene-induced senescence, apoptosis, the response to DNA damage and to viral infections. PML deficiency occurs commonly in a broad spectrum of human cancers through mechanisms that involve its aberrant ubiquitination and degradation. Furthermore, several viruses encode viral proteins that promote viral replication through degradation of PML. These observations suggest that restoration of PML should lead to potent antitumor effects or antiviral responses. In this review we will summarize the mechanisms involved in PML degradation with the intent to highlight novel therapeutic strategies to trigger PML restoration.

Siah: a promising anticancer target

Siah ubiquitin ligases play important roles in a number of signaling pathways involved in the progression and spread of cancer in cell-based models, but their role in tumor progression remains controversial. Siah proteins have been described to be both oncogenic and tumor suppressive in a variety of patient cohort studies and animal cancer models. This review collates the current knowledge of Siah in cancer progression and identifies potential methods of translation of these findings into the clinic. Furthermore, key experiments needed to close the gaps in our understanding of the role Siah proteins play in tumor progression are suggested.

Expression and clinical significance of SIAH in laryngeal squamous cell carcinoma

SIAH is widely expressed in lot of kinds of tumors. It plays a significant role in human cancer, but its clinicopathologic and prognostic significance in laryngeal squamous cell carcinoma (LSCC) has not yet been elucidated. The SIAH expression was examined at mRNA and protein levels by real-time quantitative polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemistry in LSCC tissues and adjacent normal larynx tissues. Statistical analyses were applied to test the associations between SIAH expression, clinicopathologic factors, and prognosis. Western blots and RT-PCR showed that the expression level of SIAH was lower in LSCC tissues than in adjacent normal larynx tissues. By immunohistochemical analysis, reduced expression of SIAH was found in 71.67 % LSCCs. After multivariate analysis, along with pathologic differentiation, the protein expression level of SIAH was an independent and significant predictive factor (P = 0.04). Furthermore, patients with SIAH-low tumors had a shorter disease-free survival and overall survival (P = 0.002 and P = 0.045, respectively). Our study suggests that SIAH protein expression is a valuable biomarker for LSCC. Low expression of SIAH is associated with poor disease-free survival and overall survival in LSCC patients.

Expression of seven-in-absentia homologue 1 and hypoxia-inducible factor 1 alpha: novel prognostic factors of nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is an EBV-associated cancer. We analysed Siah1 expression as well as LMP1 and HIF1α expression by immuno-histochemical staining in 74 NPC biopsy specimens and found that the expression of Siah1 was significantly correlated with advanced tumour status and stage. Moreover, Siah1-positive and HIF1α-positive cases had significantly worse prognoses. The expression score for LMP1 was remarkably correlated with that of Siah1, whereas there was little correlation between LMP1 expression and the other markers evaluated. This is the first study to evaluate the pattern and clinical significance of Siah1 and HIF1α expression in NPC, and such an evaluation is valuable for identifying those patients at a high risk for a poor prognosis.

SIAH ubiquitin ligases target the nonreceptor tyrosine kinase ACK1 for ubiquitinylation and proteasomal degradation

Activated Cdc42-associated kinase 1 (ACK1) is a nonreceptor tyrosine kinase linked to cellular transformation. The aberrant regulation of ACK1 promotes tumor progression and metastasis. Therefore, ACK1 is regarded as a valid target in cancer therapy. Seven in absentia homolog (SIAH) ubiquitin ligases facilitate substrate ubiquitinylation that targets proteins to the proteasomal degradation pathway. Here we report that ACK1 and SIAH1 from Homo sapiens interact in a yeast two-hybrid screen. Protein-protein interaction studies and protein degradation analyses using deletion and point mutants of ACK1 verify that SIAH1 and the related SIAH2 interact with ACK1. The association between SIAHs and ACK1 depends on the integrity of a highly conserved SIAH-binding motif located in the far C-terminus of ACK1. Furthermore, we demonstrate that the interaction of ACK1 with SIAH1 and the induction of proteasomal degradation of ACK1 by SIAH1 are independent of ACK1's kinase activity. Chemical inhibitors blocking proteasomal activity corroborate that SIAH1 and SIAH2 destabilize the ACK1 protein by inducing its proteasomal turnover. This mechanism apparently differs from the lysosomal pathway targeting ACK1 after stimulation with the epidermal growth factor. Our data also show that ACK1, but not ACK1 mutants lacking the SIAH binding motif, has a discernable negative effect on SIAH levels. Additionally, knockdown approaches targeting the SIAH2 mRNA uncover specifically that the induction of SIAH2 expression, by hormonally-induced estrogen receptor (ER) activation, decreases the levels of ACK1 in luminal human breast cancer cells. Collectively, our data provide novel insights into the molecular mechanisms modulating ACK1 and they position SIAH ubiquitin ligases as negative regulators of ACK1 in transformed cells.

KRAS mutations in lung cancer

Epidermal growth factor receptor (EGFR) gene mutations and increased EGFR copy numbers have been associated with a favorable response to EGFR tyrosine kinase inhibitors (TKI) in patients with non-small-cell lung cancer (NSCLC), and several markers have been identified that predict response to treatment. Lung adenocarcinomas also harbor activating mutations in the downstream GTPase, v-Ki-ras2 Kirsten rat sarcoma viral oncogene (KRAS), and mutations in EGFR and KRAS appear to be mutually exclusive. Even though KRAS mutations were identified in NSCLC tumors more than 20 years ago, we have only just begun to appreciate the clinical value of determining KRAS tumor status. Recent studies indicate that patients with mutant KRAS tumors fail to benefit from adjuvant chemotherapy and do not respond to EGFR inhibitors. There is a clear need for therapies specifically developed for patients with KRAS-mutant NSCLC. In this review, we summarize the clinical and pathologic characteristics of patients with NSCLC and with KRAS mutations, describe work that explores the predictive and prognostic influence of KRAS mutations, and provide an overview of the "synthetic lethal" interactions and current approaches to targeting KRAS-mutant NSCLC.

SIAH-mediated ubiquitination and degradation of acetyl-transferases regulate the p53 response and protein acetylation

Posttranslational modification of proteins by lysine acetylation regulates many biological processes ranging from signal transduction to chromatin compaction. Here we identify the acetyl-transferases CBP/p300, Tip60 and PCAF as new substrates for the ubiquitin E3 ligases SIAH1 and SIAH2. While CBP/p300 can undergo ubiquitin/proteasome-dependent degradation by SIAH1 and SIAH2, the two other acetyl-transferases are exclusively degraded by SIAH2. Accordingly, SIAH-deficient cells show enhanced protein acetylation, thus revealing SIAH proteins as indirect regulators of the cellular acetylation status. Functional experiments show that Tip60/PCAF-mediated acetylation of the tumor suppressor p53 is antagonized by the p53 target gene SIAH2 which mediates ubiquitin/proteasome-mediated degradation of both acetyl-transferases and consequently diminishes p53 acetylation and transcriptional activity. The p53 kinase HIPK2 mediates hierarchical phosphorylation of SIAH2 at 5 sites, which further boosts its activity as a ubiquitin E3 ligase for several substrates and therefore dampens the late p53 response.

SIAH proteins: critical roles in leukemogenesis

The delicate balance between the synthesis and the degradation of proteins ensures cellular homeostasis. Proteases act in an irreversible manner and therefore have to be strictly regulated. The ubiquitin-proteasome system (UPS) is a major pathway for the proteolytic degradation of cellular proteins. As dysregulation of the UPS is observed in most cancers including leukemia, the UPS is a valid target for therapeutic intervention strategies. Ubiquitin-ligases selectively bind substrates to target them for poly-ubiquitinylation and proteasomal degradation. Therefore, pharmacological modulation of these proteins could allow a specific level of control. Increasing evidence accumulates that ubiquitin-ligases termed mammalian seven in absentia homologs (SIAHs) are not only critical for the pathogenesis of solid tumors but also for leukemogenesis. However, the relevance and therapeutic potential of SIAH-dependent processes has not been fully elucidated. Here, we summarize functions of SIAH ubiquitin-ligases in leukemias, how they select leukemia-relevant substrates for proteasomal degradation, and how the expression and activity of SIAH1 and SIAH2 can be modulated in vivo. We also discuss that epigenetic drugs belonging to the group of histone deacetylase inhibitors induce SIAH-dependent proteasomal degradation to accelerate the turnover of leukemogenic proteins. In addition, our review highlights potential areas for future research on SIAH proteins.