Inhibition of Siah2 ubiquitin ligase by vitamin K3 (menadione) attenuates hypoxia and MAPK signaling and blocks melanoma tumorigenesis

LATS2 degradation promoted fibrosis damage and rescued by vitamin K3 in lupus nephritis

BACKGROUND

Lupus nephritis (LN) is the most common complication of systemic lupus erythematosus (SLE). The limited treatment options for LN increase the economic burdens on patients. Because fibrotic progression leads to irreversible renal damage in LN patients and further progresses to chronic kidney disease (CKD) and the end stage of renal disease (ESRD), developing new targets to prevent LN fibrotic progression could lead to a feasible treatment strategy for LN patients.

METHODS

In this study, we examined YAP activation and LATS2 downregulation in LN kidney biopsy samples (LN: n = 8, normal: n = 2) and lupus-prone MRL/lpr mice (n = 8 for each disease stage). The function of LATS2 was further investigated by in situ injection of Ad-LATS2 into mice with LN (n = 6 mice per group). We examined the role of SIAH2-LATS2 regulation by IP-MS and co-IP, and the protective effect of the SIAH2 inhibitor was investigated in mice with LN.

RESULTS

Restoring LATS2 by an adenovirus in vivo alleviated renal fibrotic damage in mice with LN. Moreover, we found that LATS2 was degraded by a K48 ubiquitination-proteasome pathway mediated by SIAH2 and promoted YAP activation to worsen fibrosis progression in LN. The H150 region of the substrate binding domain (SBD) is an important site for SIAH2-LATS2 binding. The SIAH2-specific inhibitor vitamin K3 protected against LN-associated fibrotic damage in vivo.

CONCLUSION

In summary, we identified the SIAH2-LATS2 axis as an attractive intervention target in LN to alter the resistance to fibrosis.

Siah2 inhibitor and the metabolic antagonist Oxamate retard colon cancer progression and downregulate PD1 expression

BACKGROUND

Solid tumors such as colon cancer are characterized by rapid and sustained cell proliferation, which ultimately results in hypoxia, induction of hypoxia-inducible factor-1α (HIF-1α), and activation of glycolysis to promote tumor survival and immune evasion. We hypothesized that a combinatorial approach of menadione (MEN) as an indirect HIF-1α inhibitor and sodium oxamate (OX) as a glycolysis inhibitor may be a promising treatment strategy for colon cancer.

OBJECTIVES

We investigated the potential efficacy of this combination for promoting an antitumor immune response and suppressing tumor growth in a rat model of colon cancer.

METHODS

Colon cancer was induced by once-weekly subcutaneous injection of 20 mg/kg dimethylhydrazine (DMH) for 16 weeks. Control rats received the vehicle and then no further treatment (negative control) or MEN plus OX for 4 weeks (drug control). Dimethylhydrazine-treated rats were then randomly allocated to four groups: DMH alone group and other groups treated with MEN, OX, and a combination of (MEN and OX) for 4 weeks. Serum samples were assayed for the tumor marker carbohydrate antigen (CA19.9), while expression levels of HIF-1α, caspase-3, PHD3, LDH, and PD1 were evaluated in colon tissue samples by immunoassay and qRT-PCR. Additionally, Ki-67 and Siah2 expression levels were examined by immunohistochemistry.

RESULTS

The combination of MEN plus OX demonstrated a greater inhibitory effect on the expression levels of HIF-1α, Siah2, LDH, Ki-67, and PD1, and greater enhancement of caspase-3 and PHD3 expression in colon cancer tissues than either drug alone.

CONCLUSION

Simultaneous targeting of hypoxia and glycolysis pathways by a combination of MEN and OX could be a promising therapy for inhibiting colon cancer cell growth and promoting antitumor immunity [1].

Modulating the Siah2-PHD3-HIF1α axis and/or autophagy potentially retard colon cancer proliferation possibly, due to the damping of colon cancer stem cells

BACKGROUND

Hypoxic microenvironment of colon cancer is associated with HIF-1α upregulation. HIF-1α response elements are responsible for autophagy induction that promotes tumor proliferation. Moreover, HIF-1α induces tumor cell proliferation via maintaining cancer stem cells (CSCs) survival. Siah2 is E3 ubiquitin ligase that indirectly stabilizes HIF-1α. We hypothesized that dual inhibition of Siah2 as well as autophagy could be a promising approach that may inhibit CSCs growth.

AIM OF THE WORK

This study investigated the possible effect of vitamin K3 as a Siah2 inhibitor and hydroxychloroquine as an autophagy inhibitor in colon cancer management. The effect (if any) of these agents on CSCs growth will be also manipulated.

METHODS

Colon cancer was induced by dimethylhydrazine. MDA and GSH were selected as oxidative stress markers, Expression of HIF-1α, Caspase-3, VEGF, MMP-9, EpCAM, SCF, and CA19.9 were assayed using immunoassay. The Western blot technique was used to assess LC3Ⅰ, CD44, and CD133 whereas RT-PCR was used to investigate PHD3 and CD44 in colon tissues. Additionally, Ki-67 and Siah2 were detected immunohistochemically.

RESULTS

vitamin K3 and hydroxychloroquine either alone or in combination downregulated the expression of Siah2 and HIF-1α through upregulating PHD3 in colon tissues. This combination significantly downregulated MDA, Ki-67, VEGF, and MMP-9 expression and upregulated the expression of GSH and caspase-3. LC3Ⅰ was also upregulated. Interestingly, these therapeutic options were correlated with down-regulation of the cancer stem cell marker such as CD44 and EpCAM.

CONCLUSION

Our results suggested that suppression of both Siah2-PHD3-HIF-1α axis and autophagy retard colon cancer proliferation and dampened CSCs.

The emerging roles of E3 ubiquitin ligases in ovarian cancer chemoresistance

Epithelial cancer of the ovary exhibits the highest mortality rate of all gynecological malignancies in women today, since the disease is often diagnosed in advanced stages. While the treatment of cancer with specific chemical agents or drugs is the favored treatment regimen, chemotherapy resistance greatly impedes successful ovarian cancer chemotherapy. Thus, chemoresistance becomes one of the most critical clinical issues confronted when treating patients with ovarian cancer. Convincing evidence hints that dysregulation of E3 ubiquitin ligases is a key factor in the development and maintenance of ovarian cancer chemoresistance. This review outlines recent advancement in our understanding of the emerging roles of E3 ubiquitin ligases in ovarian cancer chemoresistance. We also highlight currently available inhibitors targeting E3 ligase activities and discuss their potential for clinical applications in treating chemoresistant ovarian cancer patients.

The role of Siah2 in tumorigenesis and cancer therapy

Seven in absentia homolog 2 (Siah2), an RING E3 ubiquitin ligases, has been characterized to play the vital role in tumorigenesis and cancer progression. Numerous studies have determined that Siah2 promotes tumorigenesis in a variety of human malignancies such as prostate, lung, gastric, and liver cancers. However, several studies revealed that Siah2 exhibited tumor suppressor function by promoting the proteasome-mediated degradation of several oncoproteins, suggesting that Siah2 could exert its biological function according to different stages of tumor development. Moreover, Siah2 is subject to complex regulation, especially the phosphorylation of Siah2 by a variety of protein kinases to regulate its stability and activity. In this review, we describe the structure and regulation of Siah2 in human cancer. Moreover, we highlight the critical role of Siah2 in tumorigenesis. Furthermore, we note that the potential clinical applications of targeting Siah2 in cancer therapy.

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.

The role of Hypoxia-Inducible Factor-1alpha and its signaling in melanoma

Adaptation to the loss of O2 is regulated via the activity of hypoxia-inducible factors such as Hypoxia-Inducible Factor-1 (HIF-1). HIF-1 acts as a main transcriptional mediator in the tissue hypoxia response that regulates over 1000 genes related to low oxygen tension. The role of HIF-1α in oncogenic processes includes angiogenesis, tumor metabolism, cell proliferation, and metastasis, which has been examined in various malignancies, such as melanoma. Melanoma is accompanied by a high death rate and a cancer type whose incidence has risen over the last decades. The linkage between O2 loss and melanogenesis had extensively studied over decades. Recent studies revealed that HIF-1α contributes to melanoma progression via different signaling pathways such as PI3K/Akt/mTOR, RAS/RAF/MEK/ERK, JAK/STAT, Wnt/β-catenin, Notch, and NF-κB. Also, various microRNAs (miRs) are known to mediate the HIF-1α role in melanoma. Therefore, HIF-1α offers a diagnostic/prognostic biomarker and a candidate for targeted therapy in melanoma.

Targeting ADT-Induced Activation of the E3 Ubiquitin Ligase Siah2 to Delay the Occurrence of Castration-Resistant Prostate Cancer

Siah2 is an E3 ubiquitin ligase that targets androgen receptor (AR) and plays an important role in the development of castration-resistant prostate cancer (CRPC). However, the regulation of Siah2 in prostate cancer (PCa) is largely unknown. In this study, we used AR-dependent and -independent cells lines to investigate the cellular roles of AR and androgen deprivation therapy (ADT) on Siah2 protein levels and E3 ligase activity using Western blotting and co-immunoprecipitation. We also validated our findings using patient samples taken before and after ADT. Finally, we used xenograft tumor models to test the effects of ADT combined with vitamin K3 (Vit K3) on tumor growth in vivo. Our results showed that AR stabilizes Siah2 protein by attenuating its self-ubiquitination and auto-degradation, likely by blocking its E3 ubiquitin ligase activity. Conversely, ADT decreased Siah2 protein expression but enhanced its E3 ligase activity in PCa cells. Notably, the findings that ADT decreasing Siah2 protein expression were verified in a series of paired PCa samples from the same patient. Additionally, we found that ADT-induced Siah2 activation could be abolished by Vit K3. Strikingly, ADT combined with Vit K3 treatment delayed the occurrence of CRPC and dramatically inhibited the growth of tumor xenografts compared with ADT treatment alone. AR is an inhibitor of Siah2 in PCa, and ADT leads to the continuous activation of Siah2, which may contribute to CRPC. Finally, ADT+Vit K3 may be a potential approach to delay the occurrence of CRPC.

Regulation of the SIAH2-HIF-1 Axis by Protein Kinases and Its Implication in Cancer Therapy

The cellular response to hypoxia is a key biological process that facilitates adaptation of cells to oxygen deprivation (hypoxia). This process is critical for cancer cells to adapt to the hypoxic tumor microenvironment resulting from rapid tumor growth. Hypoxia-inducible factor 1 (HIF-1) is a transcription factor and a master regulator of the cellular response to hypoxia. The activity of HIF-1 is dictated primarily by its alpha subunit (HIF-1α), whose level and/or activity are largely regulated by an oxygen-dependent and ubiquitin/proteasome-mediated process. Prolyl hydroxylases (PHDs) and the E3 ubiquitin ligase Von Hippel-Lindau factor (VHL) catalyze hydroxylation and subsequent ubiquitin-dependent degradation of HIF-1α by the proteasome. Seven in Absentia Homolog 2 (SIAH2), a RING finger-containing E3 ubiquitin ligase, stabilizes HIF-1α by targeting PHDs for ubiquitin-mediated degradation by the proteasome. This SIAH2-HIF-1 signaling axis is important for maintaining the level of HIF-1α under both normoxic and hypoxic conditions. A number of protein kinases have been shown to phosphorylate SIAH2, thereby regulating its stability, activity, or substrate binding. In this review, we will discuss the regulation of the SIAH2-HIF-1 axis via phosphorylation of SIAH2 by these kinases and the potential implication of this regulation in cancer biology and cancer therapy.

Inhibition of Siah2 ubiquitin ligase ameliorates monocrotaline-induced pulmonary arterial remodeling through inactivation of YAP

AIMS

It has been shown that up-regulation of E3 ubiquitin ligase seven-in-absentia-homolog 2 (Siah2) and activation of Hippo signaling pathway effector yes-associated protein (YAP) are involved in the development of pulmonary arterial hypertension (PAH). However, it is still unclear whether Siah2 activates YAP in monocrotaline (MCT)-induced PAH rat models.

MAIN METHODS

Intraperitoneal injection of MCT was used to induce PAH rat models. The right ventricular systolic pressure (RVSP), right ventricle hypertrophy index (RVHI), percentage of medial wall thickness (%MT), α-SMA, Ki-67 and TUNEL staining were performed to evaluate the development of PAH. Protein levels of Siah2, Lats1/2, YAP phosphorylation and total YAP, and the subcellular localization of YAP were examined using immunoblotting. Proteasome activity was measured by an assay kit.

KEY FINDINGS

The protein level of Siah2 was significantly increased in MCT-induced PAH rats, this was accompanied with the proteasome-dependent degradation of Lats1/2 and subsequent up-regulation and dephosphorylation of YAP and its nuclear localization. Administration of PAH rats with Siah2 inhibitor Vitamin K3 or proteasome inhibitor MG-132 dramatically suppressed MCT-induced down-regulation of Lats1/2 and activation of YAP, finally reduced RVSP, RVHI, %MT, pulmonary arterial muscularization, pulmonary arterial smooth muscle cells (PASMCs) proliferation and enhanced PASMCs apoptosis in PAH rats.

SIGNIFICANCE

Siah2 contributes to the development of MCT-induced PAH by destabilizing Lats1/2 and subsequently stimulating YAP activation. Inhibition of Siah2 or proteasome alleviates pulmonary arterial remodeling through inactivation of YAP, indicating Siah2 ubiquitin ligase as a novel target might have potential value in the management of PAH.

A KLF4-DYRK2-mediated pathway regulating self-renewal in CML stem cells

Leukemia stem cells are a rare population with a primitive progenitor phenotype that can initiate, sustain, and recapitulate leukemia through a poorly understood mechanism of self-renewal. Here, we report that Krüppel-like factor 4 (KLF4) promotes disease progression in a murine model of chronic myeloid leukemia (CML)-like myeloproliferative neoplasia by repressing an inhibitory mechanism of preservation in leukemia stem/progenitor cells with leukemia-initiating capacity. Deletion of the Klf4 gene severely abrogated the maintenance of BCR-ABL1(p210)-induced CML by impairing survival and self-renewal in BCR-ABL1+ CD150+ lineage-negative Sca-1+ c-Kit+ leukemic cells. Mechanistically, KLF4 repressed the Dyrk2 gene in leukemic stem/progenitor cells; thus, loss of KLF4 resulted in elevated levels of dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 2 (DYRK2), which were associated with inhibition of survival and self-renewal via depletion of c-Myc protein and p53 activation. In addition to transcriptional regulation, stabilization of DYRK2 protein by inhibiting ubiquitin E3 ligase SIAH2 with vitamin K3 promoted apoptosis and abrogated self-renewal in murine and human CML stem/progenitor cells. Altogether, our results suggest that DYRK2 is a molecular checkpoint controlling p53- and c-Myc-mediated regulation of survival and self-renewal in CML cells with leukemic-initiating capacity that can be targeted with small molecules.

Effects of melatonin on production of reactive oxygen species and developmental competence of bovine oocytes exposed to heat shock and oxidative stress during in vitro maturation

SummaryHeat shock may disrupt oocyte function by increasing the generation of reactive oxygen species (ROS). We evaluated the capacity of the antioxidant melatonin to protect oocytes using two models of oxidative stress - heat shock and the pro-oxidant menadione. Bovine cumulus-oocyte complexes (COC) were exposed in the presence or absence of 1 µM melatonin to the following treatments during maturation: 38.5°C, 41°C and 38.5°C+5 µM menadione. In the first experiment, COC were matured for 3 h with 5 µM CellROX® and analyzed by epifluorescence microscopy to quantify production of ROS. The intensity of ROS was greater for oocytes exposed to heat shock and menadione than for control oocytes. Melatonin reduced ROS intensity for heat-shocked oocytes and oocytes exposed to menadione, but not for control oocytes. In the second experiment, COC were matured for 22 h. After maturation, oocytes were fertilized and the embryos cultured for 7.5 days. The proportion of oocytes that cleaved after fertilization was lower for oocytes exposed to heat shock and menadione than for control oocytes. Melatonin increased cleavage for heat-shocked oocytes and oocytes exposed to menadione, but not for control oocytes. Melatonin tended to increase the developmental competence of embryos from heat-shocked oocytes but not for embryos from oocytes exposed to menadione or from control oocytes. In conclusion, melatonin reduced production of ROS of maturing oocytes and protected oocytes from deleterious effects of both stresses on competence of the oocyte to cleave after coincubation with sperm. These results suggest that excessive production of ROS compromises oocyte function.

Identification and characterization of small molecule inhibitors of the ubiquitin ligases Siah1/2 in melanoma and prostate cancer cells

Inhibition of ubiquitin ligases with small molecule remains a very challenging task, given the lack of catalytic activity of the target and the requirement of disruption of its interactions with other proteins. Siah1/2, which are E3 ubiquitin ligases, are implicated in melanoma and prostate cancer and represent high-value drug targets. We utilized three independent screening approaches in our efforts to identify small-molecule Siah1/2 inhibitors: Affinity Selection-Mass Spectrometry, a protein thermal shift-based assay and an in silico based screen. Inhibitors were assessed for their effect on viability of melanoma and prostate cancer cultures, colony formation, prolyl-hydroxylase-HIF1α signaling, expression of selected Siah2-related transcripts, and Siah2 ubiquitin ligase activity. Several analogs were further characterized, demonstrating improved efficacy. Combination of the top hits identified in the different assays demonstrated an additive effect, pointing to complementing mechanisms that underlie each of these Siah1/2 inhibitors.

Role of HDAC3-miRNA-CAGE Network in Anti-Cancer Drug-Resistance

Histone modification is associated with resistance to anti-cancer drugs. Epigenetic modifications of histones can regulate resistance to anti-cancer drugs. It has been reported that histone deacetylase 3 (HDAC3) regulates responses to anti-cancer drugs, angiogenic potential, and tumorigenic potential of cancer cells in association with cancer-associated genes (CAGE), and in particular, a cancer/testis antigen gene. In this paper, we report the roles of microRNAs that regulate the expression of HDAC3 and CAGE involved in resistance to anti-cancer drugs and associated mechanisms. In this review, roles of HDAC3-miRNAs-CAGE molecular networks in resistance to anti-cancer drugs, and the relevance of HDAC3 as a target for developing anti-cancer drugs are discussed.

Implication of a novel vitamin K dependent protein, GRP/Ucma in the pathophysiological conditions associated with vascular and soft tissue calcification, osteoarthritis, inflammation, and carcinoma

Gla-rich protein (GRP) or unique cartilage matrix-associated protein (Ucma), the newest member of vitamin K dependent proteins, carries exceptionally high number of γ-carboxyglutamic acid (Gla) residues which contributes to its outstanding capacity of binding with calcium in the extracellular environment indicating its potential role as a global calcium modulator. Recent studies demonstrated a critical function of GRP in the regulation of different pathophysiological conditions associated with vascular and soft tissue calcification including cardiovascular diseases, osteoarthritis, inflammation, and skin and breast carcinomas. These findings established an important relationship between γ-carboxylation of GRP and calcification associated disease pathology suggesting a critical role of vitamin K in the pathophysiological features of various health disorders. This review for the first time summarizes all of the updated findings related to the functional activities of GRP in the pathogenesis of several diseases associated with vascular and soft tissue mineralization, osteoarthritis, inflammation, and carcinoma. The outcome of this review will improve the understanding about the role of GRP in the pathogenesis of tissue calcification and its associated health disorders, which should in turn lead to the design of clinical interventions to improve the condition of patients associated with these health disorders.

Ubiquitin ligases in oncogenic transformation and cancer therapy

The cellular response to external stress signals and DNA damage depends on the activity of ubiquitin ligases (E3s), which regulate numerous cellular processes, including homeostasis, metabolism and cell cycle progression. E3s recognize, interact with and ubiquitylate protein substrates in a temporally and spatially regulated manner. The topology of the ubiquitin chains dictates the fate of the substrates, marking them for recognition and degradation by the proteasome or altering their subcellular localization or assembly into functional complexes. Both genetic and epigenetic alterations account for the deregulation of E3s in cancer. Consequently, the stability and/or activity of E3 substrates are also altered, in some cases leading to downregulation of tumour-suppressor activities and upregulation of oncogenic activities. A better understanding of the mechanisms underlying E3 regulation and function in tumorigenesis is expected to identify novel prognostic markers and to enable the development of the next generation of anticancer therapies. This Review summarizes the oncogenic and tumour-suppressor roles of selected E3s and highlights novel opportunities for therapeutic intervention.

Mechanisms and prevention of UV-induced melanoma

Melanoma is the deadliest form of skin cancer and its incidence is rising, creating a costly and significant clinical problem. Exposure to ultraviolet (UV) radiation, namely UVA (315-400 nm) and UVB (280-315 nm), is a major risk factor for melanoma development. Cumulative UV radiation exposure from sunlight or tanning beds contributes to UV-induced DNA damage, oxidative stress, and inflammation in the skin. A number of factors, including hair color, skin type, genetic background, location, and history of tanning, determine the skin's response to UV radiation. In melanocytes, dysregulation of this UV radiation response can lead to melanoma. Given the complex origins of melanoma, it is difficult to develop curative therapies and universally effective preventative strategies. Here, we describe and discuss the mechanisms of UV-induced skin damage responsible for inducing melanomagenesis, and explore options for therapeutic and preventative interventions.

Vitamin K and cancer

Subclinical vitamin K deficits refer to carboxylation defects of different types of vitamin K-dependent hepatic and extrahepatic so-called Gla proteins without prolongation of the prothrombin time. This condition has been reported in different clinical situations due to insufficient supply or malabsorption of vitamin K as well as drug interactions. This review discusses the effects of different vitamin K subspecies on tumour growth and the possible anti-tumour effects of increased vitamin K intake. Blocking carboxylation of vitamin K-dependent proteins with warfarin anticoagulation - what are the risks/benefits for carcinogenesis? Previous studies on both heparin and low molecular weight heparin blocking of the vitamin K-dependent factors X and II have shown tumour suppressive effects. Vitamin K has anti-inflammatory effects that could also impact carcinogenesis, but little data exists on this subject.

Ubiquitination in melanoma pathogenesis and treatment

Melanoma is one of the most aggressive skin cancers with fiercely increasing incidence and mortality. Since the progressive understanding of the mutational landscape and immunologic pathogenic factors in melanoma, the targeted therapy and immunotherapy have been recently established and gained unprecedented improvements for melanoma treatment. However, the prognosis of melanoma patients remains unoptimistic mainly due to the resistance and nonresponse to current available drugs. Ubiquitination is a posttranslational modification which plays crucial roles in diverse cellular biological activities and participates in the pathogenesis of various cancers, including melanoma. Through the regulation of multiple tumor promoters and suppressors, ubiquitination is emerging as the key contributor and therefore a potential therapeutic target for melanoma. Herein, we summarize the current understanding of ubiquitination in melanoma, from mechanistic insights to clinical progress, and discuss the prospect of ubiquitination modification in melanoma treatment.

Mutual regulation between Polo-like kinase 3 and SIAH2 E3 ubiquitin ligase defines a regulatory network that fine-tunes the cellular response to hypoxia and nickel

Elevated cellular response to hypoxia, which contributes to cell transformation and tumor progression, is a prominent feature of malignant cells in solid tumors. Polo-like kinase 3 (Plk3) is a serine/threonine protein kinase known to inhibit the cellular response to hypoxia and tumorigenesis. Nickel compounds are well-established human carcinogens that induce tumorigenesis partly through their hypoxia-mimicking effects. Despite previous research efforts, the role of Plk3 in the hypoxic response induced by hypoxia or nickel is not completely understood. Here, we show that NiCl₂ (Ni(II)) or hypoxia reduces the protein level and shortens the half-life of cytoplasmic Plk3 in a ubiquitin-proteasome-dependent manner. We identify SIAH2, a RING finger E3 ubiquitin ligase associated with the cellular hypoxic response, to be the ubiquitin E3 ligase that mediates the degradation of Plk3. We show that SIAH2 binds to Plk3 and mediates its ubiquitination primarily through its polo-box domain. We report that USP28, a deubiquitinase known to be inhibitable by Ni(II) or hypoxia, may also contribute to the suppression of the Plk3 protein by Ni(II). We also show that Plk3 in turn suppresses the SIAH2 protein level in a kinase activity-dependent manner. Our study revealed an interesting mutual regulation between Plk3 and SIAH2 and uncovered a regulatory network that functions to fine-tune the cellular hypoxic response. We propose that suppression of Plk3 expression contributes to carcinogenesis and tumor progression induced by nickel compounds.

Melanoma Chemoprevention: Current Status and Future Prospects

The incidence of skin cancers, both nonmelanoma and melanoma, is increasing in the United States. The ultraviolet radiation, mainly from sun, is considered the major cause for these neoplasms. While nonmelanoma skin cancers are far more numerous, melanoma remains the most challenging. This is because melanoma can become extremely aggressive and its incidence is increasing worldwide due to lack of effective early detection, as well as disease recurrence, following both surgery and chemotherapy. Therefore, in addition to better treatment options, newer means are required to prevent melanomas from developing. Chemoprevention is a reasonable cost-effective approach to prevent carcinogenesis by inhibiting the processes of tumor initiation, promotion and progression. Melanoma is a progressive disease, which makes it very suitable for chemopreventive interventions, by targeting the processes and molecular pathways involved in the progression of melanoma. This review discusses the roles of various chemopreventive agents such as NSAIDs, statins, vitamins and dietary agents in melanoma and highlights current advancements and our perspective on future of melanoma chemoprevention. Although considerable preclinical data suggest that melanoma may be prevented or delayed by a numerous chemopreventive agents, we realize there are insufficient clinical studies evaluating their efficacy and long-term safety for human use.

The functional interplay between the HIF pathway and the ubiquitin system - more than a one-way road

The hypoxia inducible factor (HIF) pathway and the ubiquitin system represent major cellular processes that are involved in the regulation of a plethora of cellular signaling pathways and tissue functions. The ubiquitin system controls the ubiquitination of proteins, which is the covalent linkage of one or several ubiquitin molecules to specific targets. This ubiquitination is catalyzed by approximately 1000 different E3 ubiquitin ligases and can lead to different effects, depending on the type of internal ubiquitin chain linkage. The best-studied function is the targeting of proteins for proteasomal degradation. The activity of E3 ligases is antagonized by proteins called deubiquitinases (or deubiquitinating enzymes), which negatively regulate ubiquitin chains. This is performed in most cases by the catalytic removal of these chains from the targeted protein. The HIF pathway is regulated in an oxygen-dependent manner by oxygen-sensing hydroxylases. Covalent modification of HIFα subunits leads to the recruitment of an E3 ligase complex via the von Hippel-Lindau (VHL) protein and the subsequent polyubiquitination and proteasomal degradation of HIFα subunits, demonstrating the regulation of the HIF pathway by the ubiquitin system. This unidirectional effect of an E3 ligase on the HIF pathway is the best-studied example for the interplay between these two important cellular processes. However, additional regulatory mechanisms of the HIF pathway through the ubiquitin system are emerging and, more recently, also the reciprocal regulation of the ubiquitin system through components of the HIF pathway. Understanding these mechanisms and their relevance for the activity of each other is of major importance for the comprehensive elucidation of the oxygen-dependent regulation of cellular processes. This review describes the current knowledge of the functional bidirectional interplay between the HIF pathway and the ubiquitin system on the protein level.

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.

Ubiquitin ligase UBE3C promotes melanoma progression by increasing epithelial-mesenchymal transition in melanoma cells

Melanoma is the most aggressive type of skin cancer, exhibiting extensive local invasion and early distant metastasis. Aberrant expression of ubiquitin-protein ligase E3C (UBE3C) plays a key role in tumor development and progression. In the present study, we analyzed UBE3C expression in samples of cancerous and normal skin tissue. Levels of UBE3C expression were much higher in primary and metastatic melanoma tissues than in normal skin, cutaneous squamous cell carcinoma or basal cell carcinoma. Melanoma cells overexpressing UBE3C frequently exhibited a mesenchymal phenotype, including reduced expression of the epithelial marker E-cadherin and expression of the mesenchymal marker vimentin. Knockdown of UBE3C expression in melanoma cells significantly suppressed melanoma growth and progression. Furthermore, silencing UBE3C led to increased E-cadherin expression and decreased vimentin and Snail1 expression. Thus UBE3C promotes melanoma progression, possibly by inducing epithelial-mesenchymal transition in melanoma cells. Inhibiting UBE3C activity may suppress melanoma invasion and metastasis and may represent a targeted therapeutic approach.

Touch and go: nuclear proteolysis in the regulation of metabolic genes and cancer

The recruitment of transcription factors to promoters and enhancers is a critical step in gene regulation. Many of these proteins are quickly removed from DNA after they completed their function. Metabolic genes in particular are dynamically regulated and continuously adjusted to cellular requirements. Transcription factors controlling metabolism are therefore under constant surveillance by the ubiquitin–proteasome system, which can degrade DNA‐bound proteins in a site‐specific manner. Several of these metabolic transcription factors are critical to cancer cells, as they promote uncontrolled growth and proliferation. This review highlights recent findings in the emerging field of nuclear proteolysis and outlines novel paradigms for cancer treatment, with an emphasis on multiple myeloma.

The expression and function of E3 ligase SIAH2 in acute T lymphoblastic leukemia

INTRODUCTION

The seven in absentia homolog 2 (SIAH2) protein plays a significant role in human cancer by regulating hypoxia-inducible factor-a (HIF-1α); however, its role in T-cell acute lymphoblastic leukemia (T-ALL) is less clear.

METHODS

Immunofluorescence evaluation of SIAH2 protein expression and location were conducted in Jurkat cell (a T-ALL cell line) as well as in bone marrow mononuclear cells (BMMNCs) from T-ALL and idiopathic thrombocytopenic purpura (ITP) patients. The expression of SIAH2 mRNA was also examined by quantitative real-time PCR (qRT-PCR) in these cells. Lentivirus-packed shRNA targeting on SIAH2 (Lv-shSIAH2) was used to knock down SIAH2 expression in Jurkat cells. Cell proliferation, apoptosis, invasion and protein levels were then determined by CCK-8 assay, annexin V-PI assay, transwell and Western blotting, respectively.

RESULTS

The mRNA expression of SIAH2 in BMMNCs from primary T-ALL patients was significantly higher than cells from ITP patients (P=0.0312); There were significant positive associations between SIAH2 expression and the extramedullary infiltration (EMI) (P=0.0003), especially with the mediastinal lymph node metastasis (P=0.0168) and the pleural effusion (P=0.014). However, SIAH2 expression in T-ALL BMMNCs was not correlated with age, gender, white cell count or the clinical risk classification. SIAH2 knockdown by shRNA led to increased apoptosis and decreased proliferation, migration and invasion of Jurkat cells. Moreover, Prolyl Hydroxylase (PHD), P27 and Caspase3 were upregulated and HIF-1α, VEGF, VEGF Receptor 2, MMP-13, CyclinE1, C-myc and BCL2 were downregulated in SIAH2 knockdown Jurkat cells.

CONCLUSIONS

Our results suggest that SIAH2 regulates multi processes in T-ALL and may be an attractive therapeutic target.

SIAH2 protein expression in breast cancer is inversely related with ER status and outcome to tamoxifen therapy

Our previous study demonstrated that high mRNA levels for Seven in Absentia Homolog 2 (SIAH2) correlated with high Estrogen Receptor (ER) mRNA levels and with longer progression-free survival (PFS) after first-line tamoxifen. Others showed high SIAH2 protein levels in ER-negative breast cancer associated with an unfavorable relapse-free survival. In the current study, we investigated SIAH2 protein expression to clarify the discrepancy between protein and mRNA findings and to determine its diagnostic value in breast cancer patients. Tissue microarrays (TMAs) containing core specimens of primary breast tumors were immunohistochemically stained for SIAH2 protein. The TMAs analyzed a cohort of 746 patients with primary breast cancer (PBC) and a cohort of 245 patients with ER-positive metastatic breast cancer (MBC) treated with first-line tamoxifen. SIAH2 staining was scored for intensity and proportion of positive tumor cells and evaluated for its relationship with metastasis-free survival (MFS) and PFS. Multivariate survival analyses included traditional prognostic or predictive factors, respectively. The PBC-cohort had 263 patients with high SIAH2 protein expression and decreased expression of ER protein and mRNA levels (P = 0.005 and P = 0.003, respectively). High SIAH2 levels correlated with significant unfavorable MFS in lymph node negative, ER-positive breast cancer patients. The MBC-cohort had 86 patients with increased SIAH2 protein expression. High SIAH2 expression was associated with an unfavorable PFS after first-line tamoxifen in multivariate analyses (HR = 1.45; 95% CI, 1.07-1.96; P = 0.015). In conclusion, SIAH2 protein expression is especially observed in ER-negative tumors. Its prognostic value in breast cancer does not add to current prognostic markers. The proportion of SIAH2-positive cells can be used as biomarker to predict tamoxifen treatment failure in MBC patients.

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.

Novel therapy for locally advanced triple-negative breast cancer

To evaluate a novel therapy for triple-negative breast cancer (TNBC), the biological responses to vitamin K3 (VK3) should be considered with the understanding of the features of breast cancer. In human breast cancer cell lines, the effects of VK3 on cell growth inhibition and the cellular signaling pathway were determined by MTT assay and western blotting. In the in vivo study, a subcutaneous tumor model of breast cancer was created, VK3 was injected into the subcutaneous tumors, and tumor size was measured. The IC₅₀ of VK3 for breast cancer cells was calculated to be 11.3–25.1 μM. VK3 induced phosphorylation of whole tyrosine and epidermal growth factor receptor. VK3 mediated phosphorylation of extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) for 30 min. ERK but not JNK phosphorylation was maintained for at least 6 h. In contrast, another antioxidant agent, catalase, showed no effect on either ERK phosphorylation or growth inhibition. On built-up tumors under the skin of mice, local treatment with VK3 was effective in a time- and dose-dependent manner, and the experiments for total tumor volume also showed a dose-dependent effect of VK3. The expression of phosphorylated ERK was clearly detected at 10.9 times the control in tumor tissue, whereas ethanol itself showed no effect. In conclusion, ERK plays a critical role in VK3-induced growth inhibition, and it will be the focus of next steps in the development of molecular therapy for TNBC.

Vitamins and Melanoma

A tremendous amount of information was published over the past decades in relation to the role of vitamins in various neoplastic diseases. In particular, several studies showed an inverse relationship between selected vitamins intake and cancer risk. In this review we will focus on the role played by vitamins in melanoma with particular regard to vitamin A, D, K, E and C. Given that vitamin supplementation is easy, convenient, and readily accepted by patients, in the future the use of vitamins in chemoprevention and therapy of melanoma could be encouraged if supported by pre-clinical and clinical evidence.

The Steroidogenic Enzyme AKR1C3 Regulates Stability of the Ubiquitin Ligase Siah2 in Prostate Cancer Cells

Re-activation of androgen receptor (AR) activity is the main driver for development of castration-resistant prostate cancer. We previously reported that the ubiquitin ligase Siah2 enhanced AR transcriptional activity and prostate cancer cell growth. Among the genes we found to be regulated by Siah2 was AKR1C3, which encodes a key androgen biosynthetic enzyme implicated in castration-resistant prostate cancer development. Here, we found that Siah2 inhibition in CWR22Rv1 prostate cancer cells decreased AKR1C3 expression as well as intracellular androgen levels, concomitant with inhibition of cell growth in vitro and in orthotopic prostate tumors. Re-expression of either wild-type or catalytically inactive forms of AKR1C3 partially rescued AR activity and growth defects in Siah2 knockdown cells, suggesting a nonenzymatic role for AKR1C3 in these outcomes. Unexpectedly, AKR1C3 re-expression in Siah2 knockdown cells elevated Siah2 protein levels, whereas AKR1C3 knockdown had the opposite effect. We further found that AKR1C3 can bind Siah2 and inhibit its self-ubiquitination and degradation, thereby increasing Siah2 protein levels. We observed parallel expression of Siah2 and AKR1C3 in human prostate cancer tissues. Collectively, our findings identify a new role for AKR1C3 in regulating Siah2 stability and thus enhancing Siah2-dependent regulation of AR activity in prostate cancer cells.

miR-335 Targets SIAH2 and Confers Sensitivity to Anti-Cancer Drugs by Increasing the Expression of HDAC3

We previously reported the role of histone deacetylase 3 (HDAC3) in response to anti-cancer drugs. The decreased expression of HDAC3 in anti-cancer drug-resistant cancer cell line is responsible for the resistance to anti-cancer drugs. In this study, we investigated molecular mechanisms associated with regulation of HDAC3 expression. MG132, an inhibitor of proteasomal degradation, induced the expression of HDAC3 in various anti-cancer drug-resistant cancer cell lines. Ubiquitination of HDAC3 was observed in various anti-cancer drug-resistant cancer cell lines. HDAC3 showed an interaction with SIAH2, an ubiquitin E3 ligase, that has increased expression in various anti-cancer drug-resistant cancer cell lines. miRNA array analysis showed the decreased expression of miR-335 in these cells. Targetscan analysis predicted the binding of miR-335 to the 3'-UTR of SIAH2. miR-335-mediated increased sensitivity to anti-cancer drugs was associated with its effect on HDAC3 and SIAH2 expression. miR-335 exerted apoptotic effects and inhibited ubiquitination of HDAC3 in anti-cancer drug-resistant cancer cell lines. miR-335 negatively regulated the invasion, migration, and growth rate of cancer cells. The mouse xenograft model showed that miR-335 negatively regulated the tumorigenic potential of cancer cells. The down-regulation of SIAH2 conferred sensitivity to anti-cancer drugs. The results of the study indicated that the miR-335/SIAH2/HDAC3 axis regulates the response to anti-cancer drugs.

Ferroxitosis: a cell death from modulation of oxidative phosphorylation and PKM2-dependent glycolysis in melanoma

Reliance on glycolysis is a characteristic of malignancy, yet the development of resistance to BRAF inhibitors in melanoma is associated with gain of mitochondrial function. Concurrent attenuation of oxidative phosphorylation and HIF-1α/PKM2-dependent glycolysis promotes a non-apoptotic, iron- and oxygen-dependent cell death that we term ferroxitosis. The redox cycling agent menadione causes a robust increase in oxygen consumption, accompanied by significant loss of intracellular ATP and rapid cell death. Conversely, either hypoxic adaptation or iron chelation prevents menadione-induced ferroxitosis. Ectopic expression of K213Q HIF-1α mutant blunts the effects of menadione. However, knockdown of HIF-1α or PKM2 restores menadione-induced cytotoxicity in hypoxia. Similarly, exposure of melanoma cells to shikonin, a menadione analog and a potential PKM2 inhibitor, is sufficient to induce ferroxitosis under hypoxic conditions. Collectively, our findings reveal that ferroxitosis curtails metabolic plasticity in melanoma.

Inhibition of synaptophysin ubiquitination may improve the intelligent drop due to high glucose and hypoxia

Synaptophysin (SYP) is a synaptic vesicle membrane protein and involved in the release of neurotransmitters, synaptic plasticity, and formation and recycling of synaptic vesicles. SYP may serve as a specific marker of synaptic proteins. Our results showed high glucose and hypoxia could significantly impair the intelligence and the SYP level was positively related to the degree of intelligence in animals. Ubiquitination may influence the SYP expression. In our study, an ubiquitination inhibitor (MG-132) was injected intraperitoneally. Results showed this could increase SYP expression in the brain to improve the intelligent drop due to high glucose and hypoxia in animals. SYP ubiquitination is closely associated with E3 ubiquitin ligase, a mammalian homologue of seven in absentia. In our study, lentivirus expressing siah shRNA was injected into the cerebral ventricle to down-regulate siah expression, and results showed the SYP expression in the brain increased markedly. In addition, we also found that VitK3 could increase the SYP expression in the brain to improve the intelligent drop due to high glucose and hypoxia.

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.

Putative role of HIF transcriptional activity in melanocytes and melanoma biology

Hypoxia-inducible factor-1α (HIF-1α) is a highly oxygen sensitive bHLH protein that is part of the heterodimeric HIF-1 transcription factor. Under hypoxic stress, HIF-1 activity is induced to control expression of multiple downstream target genes, including vascular endothelial growth factor (VEGF). The normal epidermis exists in a constant mild hypoxic microenvironment and constitutively expresses HIF-1α and HIF-2α. Expression of HIF-1α and/or HIF-2α has been suggested to correlate with the increased malignant potential of melanocytes, therefore, failures of melanoma therapies may be partially linked to high HIF activity. Notably, melanomas that have the V600E BRAF mutation exhibit increased HIF-1α expression. We have utilized a bioinformatics approach to identify putative hypoxia response elements (HREs) in a set of genes known to participate in the process of melanogenesis (includingTRPM1, SLC45A2, HRAS, C-KIT, PMEL and CRH). While some of the mechanistic links between these genes and the HIF pathway have been previously explored, others await further investigation. Although agents targeting HIF activity have been proposed as novel treatment modalities for melanoma, there are currently no clinical trials in progress to test their efficacy in melanoma.

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.

Hypoxic regulation of glutamine metabolism through HIF1 and SIAH2 supports lipid synthesis that is necessary for tumor growth

Recent reports have identified a phenomenon by which hypoxia shifts glutamine metabolism from oxidation to reductive carboxylation. We now identify the mechanism by which HIF-1 activation results in a dramatic reduction in the activity of the key mitochondrial enzyme complex α ketoglutarate dehydrogenase (αKGDH). HIF-1 activation promotes SIAH2 targeted ubiquitination and proteolysis of the 48 kDa splice variant of the E1 subunit of the αKGDH complex (OGDH2). Knockdown of SIAH2 or mutation of the ubiquitinated lysine residue on OGDH2 (336KA) reverses the hypoxic drop in αKGDH activity, stimulates glutamine oxidation, and reduces glutamine-dependent lipid synthesis. 336KA OGDH2-expressing cells require exogenous lipids or citrate for growth in hypoxia in vitro and fail to grow as model tumors in immunodeficient mice. Reversal of hypoxic mitochondrial function may provide a target for the development of next-generation anticancer agents targeting tumor metabolism.

The role of mitogen- and stress-activated protein kinase pathways in melanoma

Recent discoveries have increased our comprehension of the molecular signaling events critical for melanoma development and progression. Many oncogenes driving melanoma have been identified, and most of them exert their oncogenic effects through the activation of the RAF/MEK/ERK mitogen-activated protein kinase (MAPK) pathway. The c-Jun N-terminal kinase (JNK) and p38 MAPK pathways are also important in melanoma, but their precise role is not clear yet. This review summarizes our current knowledge on the role of the three main MAPK pathways, extracellular regulated kinase (ERK), JNK, and p38, and their impact on melanoma biology. Although the results obtained with BRAF inhibitors in melanoma patients are impressive, several mechanisms of acquired resistance have emerged. To overcome this obstacle constitutes the new challenge in melanoma therapy. Given the major role that MAPKs play in melanoma, understanding their functions and the interconnection among them and with other signaling pathways represents a step forward toward this goal.

Effects of vitamin K3 and K5 on proliferation, cytokine production, and regulatory T cell-frequency in human peripheral-blood mononuclear cells

AIMS

The effects of vitamin K (VK) derivatives VK3 and VK5 on human immune cells have not been extensively investigated. We examined the effects of VK3 and VK5 on proliferation, apoptosis, cytokine production, and CD4+CD25+Foxp3+ regulatory T (Treg) cell-frequency in human peripheral blood mononuclear cells (PBMCs) activated by T cell mitogen in vitro.

MAIN METHODS

Anti-proliferative effects of VK3 and VK5 on T-cell mitogen activated PBMCs were assessed by WST assay procedures. Apoptotic cells were determined as Annexin V positive/propidium iodide (PI) negative cells. Cytokine concentrations in the supernatant of the culture medium were measured with bead-array procedures followed by analysis with flow cytometry. The CD4+CD25+Foxp3+Treg cells in mitogen-activated PBMCs were stained with fluorescence-labeled specific antibodies followed by flow cytometry.

KEY FINDINGS

VK3 and VK5 suppressed the mitogen-activated proliferation of PBMCs significantly at 10-100μM (p<0.05). The data also suggest that VK3 and VK5 promote apoptosis in the mitogen-activated T cells. VK3 and VK5 significantly inhibited the production of tumor necrosis factor (TNF) α, interleukin (IL)-4, -6, and -10 from the activated PBMCs at 10-100μM (p<0.05). In contrast, VK3 and VK5 significantly increased Treg cell-frequency in the activated PBMCs at concentrations more than 10μM (p<0.001).

SIGNIFICANCE

Our data suggest that VK3 and VK5 attenuate T cell mediated immunity by inhibiting the proliferative response and inducing apoptosis in activated T cells.

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.

Seven in absentia homolog 2 (Siah2) protein is a regulator of NF-E2-related factor 2 (Nrf2)

Under pathological conditions such as ischemia-reperfusion, Nrf2 acts as a key regulator of cellular oxidative response. Provided that Nrf2 is sensitive to hypoxia during ischemia, Nrf2 may affect reactive oxygen species metabolism during reoxygenation. In this study, hypoxia suppressed Nrf2 protein, and its hypoxic suppression was not recovered with knockdown of the Nrf2 repressor Keap1. Moreover, an Nrf2 mutant lacking the Keap1 binding domain was suppressed under hypoxia, suggesting that Keap1 does not contribute to hypoxic Nrf2 suppression. HIF-1α and Siah2 are both key regulators of hypoxic responses. Hypoxia induced the Siah2 protein. Although inhibition or knockdown of Siah2 prevented the suppression of Nrf2, knockdown of HIF-1α did not. Moreover, Siah2 interacted with Nrf2 through a binding motif, suggesting that Siah2 contributes to the suppression of Nrf2. Some cytosolic kinases also play important roles in Nrf2 regulation. In this study, PKC phosphorylates serine residues of Nrf2 during hypoxia. Knockdown of Siah2 rescued hypoxic decreases in an Nrf2 mutant that mimicked phosphorylation at serine 40 or lacked this phosphorylation site, suggesting that Siah2 contributes to the degradation of Nrf2 irrespective of its phosphorylation status. Moreover, knockdown of Siah2 attenuated ubiquitination of the Nrf2 mutant, suggesting that association of Siah2 with Nrf2 causes proteasome-mediated degradation of Nrf2.

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.