Damaged DNA-binding protein 1 (DDB1) interacts with Cdh1 and modulates the function of APC/CCdh1

CRISPR/Cas9-based genome-wide screening for deubiquitinase subfamily identifies USP1 regulating MAST1-driven cisplatin-resistance in cancer cells

Background: Cisplatin is one of the frontline anticancer agents. However, development of cisplatin-resistance limits the therapeutic efficacy of cisplatin-based treatment. The expression of microtubule-associated serine/threonine kinase 1 (MAST1) is a primary factor driving cisplatin-resistance in cancers by rewiring the MEK pathway. However, the mechanisms responsible for MAST1 regulation in conferring drug resistance is unknown.

Methods: We implemented a CRISPR/Cas9-based, genome-wide, dual screening system to identify deubiquitinating enzymes (DUBs) that govern cisplatin resistance and regulate MAST1 protein level. We analyzed K48- and K63-linked polyubiquitination of MAST1 protein and mapped the interacting domain between USP1 and MAST1 by immunoprecipitation assay. The deubiquitinating effect of USP1 on MAST1 protein was validated using rescue experiments, in vitro deubiquitination assay, immunoprecipitation assays, and half-life analysis. Furthermore, USP1-knockout A549 lung cancer cells were generated to validate the deubiquitinating activity of USP1 on MAST1 abundance. The USP1-MAST1 correlation was evaluated using bioinformatics tool and in different human clinical tissues. The potential role of USP1 in regulating MAST1-mediated cisplatin resistance was confirmed using a series of in vitro and in vivo experiments. Finally, the clinical relevance of the USP1-MAST1 axis was validated by application of small-molecule inhibitors in a lung cancer xenograft model in NSG mice.

Results: The CRISPR/Cas9-based dual screening system identified USP1 as a novel deubiquitinase that interacts, stabilizes, and extends the half-life of MAST1 by preventing its K48-linked polyubiquitination. The expression analysis across human clinical tissues revealed a positive correlation between USP1 and MAST1. USP1 promotes MAST1-mediated MEK1 activation as an underlying mechanism that contributes to cisplatin-resistance in cancers. Loss of USP1 led to attenuation of MAST1-mediated cisplatin-resistance both in vitro and in vivo. The combined pharmacological inhibition of USP1 and MAST1 using small-molecule inhibitors further abrogated MAST1 level and synergistically enhanced cisplatin efficacy in a mouse xenograft model.

Conclusions: Overall, our study highlights the role of USP1 in the development of cisplatin resistance and uncovers the regulatory mechanism of MAST1-mediated cisplatin resistance in cancers. Co-treatment with USP1 and MAST1 inhibitors abrogated tumor growth and synergistically enhanced cisplatin efficacy, suggesting a novel alternative combinatorial therapeutic strategy that could further improve MAST1-based therapy in patients with cisplatin-resistant tumors.

Every protagonist has a sidekick: Structural aspects of human xeroderma pigmentosum-binding proteins in nucleotide excision repair

The seven xeroderma pigmentosum proteins (XPps), XPA-XPG, coordinate the nucleotide excision repair (NER) pathway, promoting the excision of DNA lesions caused by exposition to ionizing radiation, majorly from ultraviolet light. Significant efforts are made to investigate NER since mutations in any of the seven XPps may cause the xeroderma pigmentosum and trichothiodystrophy diseases. However, these proteins collaborate with other pivotal players in all known NER steps to accurately exert their purposes. Therefore, in the old and ever-evolving field of DNA repair, it is imperative to reexamine and describe their structures to understand NER properly. This work provides an up-to-date review of the protein structural aspects of the closest partners that directly interact and influence XPps: RAD23B, CETN2, DDB1, RPA (RPA70, 32, and 14), p8 (GTF2H5), and ERCC1. Structurally and functionally vital domains, regions, and critical residues are reexamined, providing structural lessons and perspectives about these indispensable proteins in the NER and other DNA repair pathways. By gathering all data related to the major human xeroderma pigmentosum-interacting proteins, this review will aid newcomers on the subject and guide structural and functional future studies.

Regulation of SOX10 stability via ubiquitination-mediated degradation by Fbxw7α modulates melanoma cell migration

Dysregulation of SOX10 was reported to be correlated with the progression of multiple cancer types, including melanocytic tumors and tumors of the nervous system. However, the mechanisms by which SOX10 is dysregulated in these tumors are poorly understood. In this study, we report that SOX10 is a direct substrate of Fbxw7α E3 ubiquitin ligase, a tumor suppressor in multiple cancers. Fbxw7α promotes SOX10 ubiquitination-mediated turnover through CPD domain of SOX10. Besides, GSK3β phosphorylates SOX10 at CPD domain and facilitates Fbxw7α-mediated SOX10 degradation. Moreover, SOX10 protein levels were inversely correlated with Fbxw7α in melanoma cells, and modulation of Fbxw7α levels regulated the expression of SOX10 and its downstream gene MIA. More importantly, SOX10 reversed Fbxw7α-mediated suppression of melanoma cell migration. This study provides evidence that the tumor suppressor Fbxw7α is the E3 ubiquitin ligase responsible for the degradation of SOX10, and suggests that reduced Fbxw7α might contribute to the upregulation of SOX10 in melanoma cells.

TEL2 suppresses metastasis by down-regulating SERPINE1 in nasopharyngeal carcinoma

Metastasis is the major cause of treatment failure in patients with nasopharyngeal carcinoma (NPC). However, the molecular mechanisms of NPC metastasis are poorly understood. Here, using our customized gene microarray containing all of the known human transcription factors and the current markers for epithelial-mesenchymal transition, we report that TEL2 was down-regulated in highly metastatic NPC cells and the metastatic tissues in lymph node. Mechanistically, TEL2 inhibits the cell migration and invasion in vitro and metastasis in vivo by releasing its direct suppression on the SERPINE1 promoter in NPC. Consistently, an inverse correlation was observed between the protein levels of TEL2 and SERPINE1 using clinical NPC samples. Collectively, we have provided the first evidence that TEL2 plays a key role in NPC metastasis by directly down-regulating SERPINE1, and that this novel axis of TEL2 / SERPINE1 may be valuable to develop new strategies for treating NPC patients with metastasis.

LncRNA PANDAR regulates the G1/S transition of breast cancer cells by suppressing p16(INK4A) expression

It has been reported that lncRNA PANDAR (promoter of CDKN1A antisense DNA damage-activated RNA) is induced as a result of DNA damage, and it regulates the reparation of DNA damage. In this study, we investigated the role of lncRNA PANDAR in the progression of breast cancer and found that PANDAR was up-regulated in breast cancer tissues and cell lines. The knockdown of PANDAR suppresses G1/S transition of breast cancer cells. We demonstrated mechanistically that the regulation of G1/S transition by PANDAR was partly due to the transcriptional modulation of p16^INK4A. Moreover, we showed that PANDAR impacted p16^INK4A expression by regulating the recruitment Bmi1 to p16^INK4A promoter. To our knowledge, this is the first study which showed the functional roles and mechanisms of PANDAR in regulating the progression of breast cancer. The PANDAR/Bmi1/p16^INK4A axis could serve as novel targets for breast cancer therapy.

The CUL4-DDB1 ubiquitin ligase complex controls adult and embryonic stem cell differentiation and homeostasis

Little is known on post-transcriptional regulation of adult and embryonic stem cell maintenance and differentiation. Here we characterize the role of Ddb1, a component of the CUL4-DDB1 ubiquitin ligase complex. Ddb1 is highly expressed in multipotent hematopoietic progenitors and its deletion leads to abrogation of both adult and fetal hematopoiesis, targeting specifically transiently amplifying progenitor subsets. However, Ddb1 deletion in non-dividing lymphocytes has no discernible phenotypes. Ddb1 silencing activates Trp53 pathway and leads to significant effects on cell cycle progression and rapid apoptosis. The abrogation of hematopoietic progenitor cells can be partially rescued by simultaneous deletion of Trp53. Conversely, depletion of DDB1 in embryonic stem cell (ESC) leads to differentiation albeit negative effects on cell cycle and apoptosis. Mass spectrometry reveals differing protein interactions between DDB1 and distinct DCAFs, the substrate recognizing components of the E3 complex, between cell types. Our studies identify CUL4-DDB1 complex as a novel post-translational regulator of stem and progenitor maintenance and differentiation.

DOI:http://dx.doi.org/10.7554/eLife.07539.001

Stem cells can develop into other types of cells via a process called “differentiation”. When a stem cell divides in two, it typically produces another stem cell and a cell that goes on to differentiate. Hematopoietic stem cells (or HSCs) are found in the bone marrow and give rise to all blood cells throughout the life of an organism. It is therefore crucial that they divide correctly to maintain the balance between renewing their numbers and making new types of cells.

Many studies have investigated how stem cells are maintained, but there are still major gaps in our knowledge. Recent research suggested that the cell’s “ubiquitin-proteasome system” might be important for regulating stem cell division. This system rapidly degrades proteins, thereby regulating protein abundance in cells. Enzymes known as E3 ligases form part of this system, and recognize proteins to be marked for destruction with a small protein tag.

Gao et al. have now observed that a component of an E3 ligase called DDB1 is highly expressed in hematopoietic stem cells. Further experiments revealed that genetically engineered mice that lack DDB1 in their population of blood cells die soon after they are born and have fewer blood cells. Gao et al. next inhibited the production of DDB1 in adult mice. This stopped the adult mice’s hematopoietic stem cells from dividing, and the mice died because their bone marrow couldn’t produce new blood cells. These results show that DDB1 is necessary for stem cells to renew their numbers and differentiate into blood cells in both developing and adult animals.

Next, Gao et al. investigated the how DDB1 regulates stem cell division, and discovered that a protein called p53, which is a key player in controlling cell division, is regulated by DDB1. Under normal conditions, p53 levels are kept low in cells. However, in the absence of DDB1, the levels of p53 rise, which triggers the death of the hematopoietic stem cells.

Further experiments revealed that not all dividing cells undergo cell death with the loss of DDB1. Instead, Gao et al. found that rapidly dividing embryonic stem cells differentiate when DDB1 is lost but do not die. These findings suggest that specific components of the ubiquitin ligase complex play a key role in deciding a stem cell’s fate. In the future, identifying these components will further our understanding of the decision of stem cells to differentiate.

DOI:http://dx.doi.org/10.7554/eLife.07539.002

Down-regulation of prostate stem cell antigen (PSCA) by Slug promotes metastasis in nasopharyngeal carcinoma

Distant metastasis and local recurrence are still the major causes for failure of treatment in patients with nasopharyngeal carcinoma (NPC), making it urgent to further elicit the molecular mechanisms of NPC metastasis. Using a gene microarray including transcription factors and known markers for cancer stem cells, prostate stem cell antigen (PSCA) was found to be significantly down-regulated in metastatic NPC in lymph node, compared to its primary tumour, and in NPC cell lines with high metastatic ability compared to those with low metastatic ability. NPC patients with low PSCA expression had a consistently poor metastasis-free survival (p = 0.003). Knockdown and overexpression of PSCA respectively enhanced and impaired the migration and invasion in vitro and the lung metastasis in vivo of NPC cells. Mechanistically, the enhancement of NPC metastasis by knocking down PSCA probably involved epithelial-mesenchymal transition (EMT), by up-regulating N-cadherin and ZEB1/2 and by activating RhoA. The down-regulation of PSCA in NPC cells resulted directly from the binding of Slug to the PSCA promoter. PSCA may be a potential diagnostic marker and therapeutic target for patients with NPC.

Loss of DDB1 Leads to Transcriptional p53 Pathway Activation in Proliferating Cells, Cell Cycle Deregulation, and Apoptosis in Zebrafish Embryos

DNA damage-binding protein 1 (DDB1) is a large subunit of the heterodimeric DDB complex that recognizes DNA lesions and initiates the nucleotide excision repair process. DDB1 is also a component of the CUL4 E3 ligase complex involved in a broad spectrum of cellular processes by targeted ubiquitination of key regulators. Functions of DDB1 in development have been addressed in several model organisms, however, are not fully understood so far. Here we report an ENU induced mutant ddb1 allele (ddb1^m863) identified in zebrafish (Danio rerio), and analyze its effects on development. Zebrafish ddb1 is expressed broadly, both maternally and zygotically, with enhanced expression in proliferation zones. The (ddb1^m863 mutant allele affects the splice acceptor site of exon 20, causing a splicing defect that results in truncation of the 1140 amino acid protein after residue 800, lacking part of the β-propeller domain BPC and the C-terminal helical domain CTD. ddb1^m863 zygotic mutant embryos have a pleiotropic phenotype, including smaller and abnormally shaped brain, head skeleton, eyes, jaw, and branchial arches, as well as reduced dopaminergic neuron groups. However, early forming tissues develop normally in zygotic ddb1^m863 mutant embryos, which may be due to maternal rescue. In ddb1^m863 mutant embryos, pcna-expressing proliferating cell populations were reduced, concurrent with increased apoptosis. We also observed a concomitant strong up-regulation of transcripts of the tumor suppressor p53 (tp53) and the cell cycle inhibitor cdkn1a (p21a/b^CIP1/WAF1) in proliferating tissues. In addition, transcription of cyclin genes ccna2 and ccnd1 was deregulated in ddb1^m863 mutants. Reduction of p53 activity by anti-sense morpholinos alleviated the apoptotic phenotype in ddb1^m863 mutants. These results imply that Ddb1 may be involved in maintaining proper cell cycle progression and viability of dividing cells during development through transcriptional mechanisms regulating genes involved in cell cycle control and cell survival.

Targeting the anaphase-promoting complex/cyclosome (APC/C)- bromodomain containing 7 (BRD7) pathway for human osteosarcoma

Osteosarcoma is the most common primary malignant bone tumor in childhood and adolescence and has a propensity for local invasion and early lung metastasis. However, the current therapies often result in chemoresistance, and a therapeutic target is not available in the clinic for osteosarcoma. Here, we report that BRD7 forms a complex with the anaphase-promoting complex/cyclosome (APC/C) and is degraded by APC/C^cdh1 and APC/C^cdc20 during the cell cycle. Moreover, BRD7 is a tumor suppressor in osteosarcoma, and the BRD7 mutant resistant to degradation by APC/C is more efficient than the wild-type protein at suppressing proliferation, colony formation, and tumor growth of osteosarcoma in vitro and in vivo. The combination of proTAME, an inhibitor of APC/C, with chemotherapeutic drugs efficiently targets osteosarcoma in vitro. Furthermore, there is a strong inverse correlation of protein levels between BRD7 and Cdh1 or Cdc20, and lower BRD7 expression is an indicator for poor prognosis in patients with osteosarcoma. Collectively, our results indicate that targeting the APC/C-BRD7 pathway may be a novel strategy for treating osteosarcoma.

miR-148b functions as a tumor suppressor in non-small cell lung cancer by targeting carcinoembryonic antigen (CEA)

Aberrant expression of miR-148b has been found in several types of cancer, but its expression and potential biologic role in non-small cell lung cancer (NSCLC) are still largely unknown. Here, we found that miR-148b was commonly under-expressed in human non-small celllung cancer (NSCLC) specimens and cell lines. The overexpression of miR-148b dramatically suppressed NSCLC cell proliferation and migration. Furthermore, miR-148b could regulate carcinoembryonic antigen (CEA) expression by luciferase reporter assay. On the other hand, CEA was widely up-regulated in NSCLC specimens, and its mRNA levels were inversely correlated with miR-148b expression. These suggest that CEA expression may be regulated by miR-148b. Collectively, our findings indicate miR-148b is low expression in NSCLC cells, which results in CEA overexpression and disease progression in NSCLC patients.

Downregulation of MCT1 inhibits tumor growth, metastasis and enhances chemotherapeutic efficacy in osteosarcoma through regulation of the NF-κB pathway

Monocarboxylate transporter isoform 1 (MCT1) is an important member of the proton-linked MCT family and has been reported in an array of human cancer cell lines and primary human tumors. MCT1 expression is associated with developing a new therapeutic approach for cancer. In this study, we initially showed that MCT1 is expressed in a variety of human osteosarcoma cell lines. Moreover, we evaluated the therapeutic response of targeting MCT1 using shRNA or MCT1 inhibitor. Inhibiting MCT1 delayed tumor growth in vitro and in vivo, including in an orthotopic model of osteosarcoma. Targeting MCT1 greatly enhanced the sensitivity of human osteosarcoma cells to the chemotherapeutic drugs adriamycin (ADM). In addition, we observed that MCT1 knockdown significantly suppressed the metastatic activity of osteosarcoma, including wound healing, invasion and migration. Further mechanistic studies revealed that the antitumor effects of targeting MCT1 might be related to the NF-κB pathway. Immunochemistry assay showed that MCT1 was an independent positive prognostic marker in osteosarcoma patients. In conclusion, our data, for the first time, demonstrate that MCT1 inhibition has antitumor potential which is associated with the NF-κB pathway, and high MCT1 expression predicates poor overall survival in patients with osteosarcoma.

CUL4B promotes replication licensing by up-regulating the CDK2-CDC6 cascade

CUL4B up-regulates CDK2 by repressing miR-372 and miR-373, leading to increased phosphorylation and stabilization of CDC6, thus promoting replication licensing.

Cullin-RING ubiquitin ligases (CRLs) participate in the regulation of diverse cellular processes including cell cycle progression. Mutations in the X-linked CUL4B, a member of the cullin family, cause mental retardation and other developmental abnormalities in humans. Cells that are deficient in CUL4B are severely selected against in vivo in heterozygotes. Here we report a role of CUL4B in the regulation of replication licensing. Strikingly, CDC6, the licensing factor in replication, was positively regulated by CUL4B and contributed to the loading of MCM2 to chromatin. The positive regulation of CDC6 by CUL4B depends on CDK2, which phosphorylates CDC6, protecting it from APC^CDH1-mediated degradation. Thus, aside being required for cell cycle reentry from quiescence, CDK2 also contributes to pre-replication complex assembly in G1 phase of cycling cells. Interestingly, the up-regulation of CDK2 by CUL4B is achieved via the repression of miR-372 and miR-373, which target CDK2. Our findings thus establish a CUL4B–CDK2–CDC6 cascade in the regulation of DNA replication licensing.

β-phenylethyl isothiocyanate reverses platinum resistance by a GSH-dependent mechanism in cancer cells with epithelial-mesenchymal transition phenotype

Platinum (Pt)-based chemotherapy is an important regimen in the clinical treatment of cancer, but development of drug resistance presents a major challenge. One key mechanism involved in resistance to Pt drugs is the decrease of intracellular Pt due to the drug efflux through the glutathione (GSH)-mediated export, and this is particularly significant in cancer cells with stem-cell like properties. In the present study, we showed that two Pt-resistant human cancer cell lines exhibited stem-cell like EMT properties, had high cellular GSH and accumulated significantly less cellular Pt compared to their parental cells, and failed to undergo apoptosis when exposed to Pt at the drug concentrations toxic to the parental cells. Importantly, we found that the natural compound β-phenylethyl isothiocyanate (PEITC) was able to effectively abolish this drug resistant mechanism by effective depletion of cellular GSH, leading to a significant increase in cellular Pt as well as DNA-bound Pt. A combination of PEITC and Pt showed a striking synergistic anticancer activity both in vitro and in vivo, as evidenced by an increase in drug-induced apoptosis, a loss of colony formation capacity, and significant suppression of tumor growth in mice. Taken together, our study shows a promising therapeutic strategy to overcome drug resistance to platinum-based chemotherapy and may potentially have broad implications in clinical treatment of cancer.

hSSB1 regulates both the stability and the transcriptional activity of p53

The tumor suppressor p53 is essential for several cellular processes that are involved in the response to diverse genotoxic stress, including cell cycle arrest, DNA repair, apoptosis and senescence. Studies of the regulation of p53 have mostly focused on its stability and transactivation; however, new regulatory molecules for p53 have also been frequently identified. Here, we report that human ssDNA binding protein SSB1 (hSSB1), a novel DNA damage-associated protein, can interact with p53 and protect p53 from ubiquitin-mediated degradation. Furthermore, hSSB1 also associates with the acetyltransferase p300 and is required for efficient transcriptional activation of the p53 target gene p21 by affecting the acetylation of p53 at lysine382. Functionally, the hSSB1 knockdown-induced abrogation of the G2/M checkpoint is partially dependent on p53 or p300. Collectively, our results indicate that hSSB1 may regulate DNA damage checkpoints by positively modulating p53 and its downstream target p21.

Human KIAA1018/FAN1 nuclease is a new mitotic substrate of APC/C(Cdh1)

A recently identified protein, FAN1 (FANCD2-associated nuclease 1, previously known as KIAA1018), is a novel nuclease associated with monoubiquitinated FANCD2 that is required for cellular resistance against DNA interstrand crosslinking (ICL) agents. The mechanisms of FAN1 regulation have not yet been explored. Here, we provide evidence that FAN1 is degraded during mitotic exit, suggesting that FAN1 may be a mitotic substrate of the anaphase-promoting cyclosome complex (APC/C). Indeed, Cdh1, but not Cdc20, was capable of regulating the protein level of FAN1 through the KEN box and the D-box. Moreover, the up- and down-regulation of FAN1 affected the progression to mitotic exit. Collectively, these data suggest that FAN1 may be a new mitotic substrate of APC/C^Cdh1 that plays a key role during mitotic exit.

MiR-138 suppressed nasopharyngeal carcinoma growth and tumorigenesis by targeting the CCND1 oncogene

The microRNA miR-138 is dysregulated in several human cancers, but the underlying mechanism remains largely unknown. Here, we report that miR-138 is commonly underexpressed in nasopharyngeal carcinoma (NPC) specimens and NPC cell lines. The ectopic expression of miR-138 dramatically suppressed cell proliferation and colony formation in vitro and inhibited tumorigenesis in vivo. Moreover, we identified the cyclin D1 (CCND1) gene as a novel direct target of miR-138. In consistent with the knocked-down expression of CCND1, overexpression of miR-138 inhibited cell growth and cell cycle progression in NPC cells. Furthermore, CCND1 was widely upregulated in NPC tumors, and its mRNA levels were inversely correlated with miR-138 expression. Taken together, our findings suggest that miR-138 might be a tumor suppressor in NPC, which is exerted partially by inhibiting CCND1 expression. The identification of functional miR-138 in NPC and its direct link to CCND1 might provide good candidates for developing diagnostic markers and therapeutic applications for NPC.

Glycogen synthase kinase-3β, NF-κB signaling, and tumorigenesis of human osteosarcoma

Background

Glycogen synthase kinase-3β (GSK-3β), a serine/threonine protein kinase, may function as a tumor suppressor or an oncogene, depending on the tumor type. We sought to determine the biological function of GSK-3β in osteosarcoma, a rare pediatric cancer for which the identification of new therapeutic targets is urgent.

Methods

We used cell viability assays, colony formation assays, and apoptosis assays to analyze the effects of altered GSK-3β expression in U2OS, MG63, SAOS2, U2OS/MTX300, and ZOS osteosarcoma cell lines. Nude mice (n = 5–8 mice per group) were injected with U2OS/MTX300, and ZOS cells to assess the role of GSK-3β in osteosarcoma growth in vivo and to evaluate the effects of inhibitors and/or anticancer drugs on tumor growth. We used an antibody array, polymerase chain reaction, western blotting, and a luciferase reporter assay to establish the effect of GSK-3β inhibition on the nuclear factor-κB (NF-κB) pathway. Immunochemistry was performed on primary tumor specimens from osteosarcoma patients (n = 74) to determine the relationship of GSK-3β activity with overall survival.

Results

Osteosarcoma cells with low levels of inactive p-Ser9-GSK-3β formed colonies in vitro and tumors in vivo more readily than cells with higher levels and cells in which GSK-3β had been silenced formed fewer colonies and smaller tumors than parental cells. Silencing or pharmacological inhibition of GSK-3β resulted in apoptosis of osteosarcoma cells. Inhibition of GSK-3β resulted in inhibition of the NF-κB pathway and reduction of NF-κB-mediated transcription. Combination treatments with GSK-3β inhibitors, NF-κB inhibitors, and chemotherapy drugs increased the effectiveness of chemotherapy drugs in vitro and in vivo. Patients whose osteosarcoma specimens had hyperactive GSK-3β, and nuclear NF-κB had a shorter median overall survival time (49.2 months) compared with patients whose tumors had inactive GSK-3β and NF-κB (109.2 months).

Conclusion

GSK-3β activity may promote osteosarcoma tumor growth, and therapeutic targeting of the GSK-3β and/or NF-κB pathways may be an effective way to enhance the therapeutic activity of anticancer drugs against osteosarcoma.

Arabidopsis WD repeat domain55 Interacts with DNA damaged binding protein1 and is required for apical patterning in the embryo

CUL4-RING ubiquitin E3 ligases (CRL4s) were recently shown to exert their specificity through the binding of various substrate receptors, which bind the CUL4 interactor DNA damaged binding protein1 (DDB1) through a WDxR motif. In a segregation-based mutagenesis screen, we identified a WDxR motif-containing protein (WDR55) required for male and female gametogenesis and seed development. We demonstrate that WDR55 physically interacts with Arabidopsis thaliana DDB1A in planta, suggesting that WDR55 may be a novel substrate recruiter of CRL4 complexes. Examination of mutants revealed a failure in the fusion of the polar cells in embryo sac development, in addition to embryo and endosperm developmental arrest at various stages ranging from the zygote stage to the globular stage. wdr55-2 embryos suggest a defect in the transition to bilateral symmetry in the apical embryo domain, further supported by aberrant apical embryo localization of DORNROESCHEN, a direct target of the auxin response factor protein monopteros. Moreover, the auxin response pattern, as determined using the synthetic auxin-responsive reporter ProDR5:green fluorescent protein, was shifted in the basal embryo and suspensor but does not support a strong direct link to auxin response. Interestingly, the observed embryo and endosperm phenotype is reminiscent of CUL4 or DDB1A/B loss of function and thus may support a regulatory role of a putative CRL4(WDR55) E3 ligase complex.

miR-124 suppresses multiple steps of breast cancer metastasis by targeting a cohort of pro-metastatic genes in vitro

Metastasis is a multistep process involving modification of morphology to suit migration, reduction of tumor cell adhesion to the extracellular matrix, increase of cell mobility, tumor cell resistance to anoikis, and other steps. MicroRNAs are well-suited to regulate tumor metastasis due to their capacity to repress numerous target genes in a coordinated manner, thereby enabling their intervention at multiple steps of the invasion-metastasis cascade. In this study, we identified a microRNA exemplifying these attributes, miR-124, whose expression was reduced in aggressive MDA-MB-231 and SK-3rd breast cancer cells. Downregulation of miR-124 expression in highly aggressive breast cancer cells contributed in part to DNA hypermethylation around the promoters of the three genes encoding miR-124. Ectopic expression of miR-124 in MDA-MB-231 cells suppressed metastasis-related traits including formation of spindle-like morphology, migratory capacity, adhesion to fibronectin, and anoikis. These findings indicate that miR-124 suppresses multiple steps of metastasis by diverse mechanisms in breast cancer cells and suggest a potential application of miR-124 in breast cancer treatment.