The F-box protein FBXO44 mediates BRCA1 ubiquitination and degradation

A high-throughput approach to identify BRCA1-downregulating compounds to enhance PARP inhibitor sensitivity

PARP inhibitors (PARPi) are efficacious in BRCA1-null tumors; however, their utility is limited in tumors with functional BRCA1. We hypothesized that pharmacologically reducing BRCA1 protein levels could enhance PARPi effectiveness in BRCA1 wild-type tumors. To identify BRCA1 downregulating agents, we generated reporter cell lines using CRISPR-mediated editing to tag endogenous BRCA1 protein with HiBiT. These reporter lines enable the sensitive measurement of BRCA1 protein levels by luminescence. Validated reporter cells were used in a pilot screen of epigenetic-modifying probes and a larger screen of more than 6,000 compounds. We identified 7 compounds that could downregulate BRCA1-HiBiT expression and synergize with olaparib. Three compounds, N-acetyl-N-acetoxy chlorobenzenesulfonamide (NANAC), A-443654, and CHIR-124, were validated to reduce BRCA1 protein levels and sensitize breast cancer cells to the toxic effects of olaparib. These results suggest that BRCA1-HiBiT reporter cells hold promise in developing agents to improve the clinical utility of PARPi.

F-box proteins and gastric cancer: an update from functional and regulatory mechanism to therapeutic clinical prospects

Gastric cancer (GC) is a prevalent malignancy characterized by significant morbidity and mortality, yet its underlying pathogenesis remains elusive. The etiology of GC is multifaceted, involving the activation of oncogenes and the inactivation of antioncogenes. The ubiquitin-proteasome system (UPS), responsible for protein degradation and the regulation of physiological and pathological processes, emerges as a pivotal player in GC development. Specifically, the F-box protein (FBP), an integral component of the SKP1-Cullin1-F-box protein (SCF) E3 ligase complex within the UPS, has garnered attention for its prominent role in carcinogenesis, tumor progression, and drug resistance. Dysregulation of several FBPs has recently been observed in GC, underscoring their significance in disease progression. This comprehensive review aims to elucidate the distinctive characteristics of FBPs involved in GC, encompassing their impact on cell proliferation, apoptosis, invasive metastasis, and chemoresistance. Furthermore, we delve into the emerging role of FBPs as downstream target proteins of non-coding RNAs(ncRNAs) in the regulation of gastric carcinogenesis, outlining the potential utility of FBPs as direct therapeutic targets or advanced therapies for GC.

The deubiquitinating enzyme USP4 regulates BRCA1 stability and function

BRCA1 plays a suppressive role in breast tumorigenesis. Ubiquitin-dependent degradation is a common mechanism that regulates BRCA1 protein stability, and several ubiquitin ligases involved have been identified. However, the deubiquitinating enzyme for BRCA1 remains less defined. Here, we report that the deubiquitinase USP4 interacts with, deubiquitinates and stabilizes BRCA1, maintaining the protein level of BRCA1. USP4 knockdown results in a decreased BRCA1 protein level, impairment in homologous recombination mediated double-stranded break repair, and increased genome instability, and confers resistance to DNA damage-inducing agents and PARP inhibitors. Ectopic expression of USP4 stabilizes BRCA1 and reverse the effects caused by USP4 knockdown. Moreover, USP4 is low expressed in human breast cancer tissues and its low expression correlates with poorer survival of patients. Furthermore, we identified several loss-of-function mutations of USP4 in human gynecological cancers, the catalytic activity of which or their interaction with BRCA1 is disrupted. Together, we reveal that USP4 is a deubiquitinase for BRCA1. USP4 positively regulates the stability and function of BRCA1 through de-ubiquitination, and plays important role in the suppression of breast cancer.

Endoplasmic reticulum stress-related signature predicts prognosis and immune infiltration analysis in acute myeloid leukemia

OBJECTIVES

To construct an endoplasmic reticulum stress-related prognostic risk score (RS) model to predict prognosis and perform a preliminary analysis of immune infiltration in patients with acute myeloid leukemia (AML).

METHODS

The whole-genome expression data for AML and endoplasmic reticulum stress (ER stress)-related genes were downloaded from the GEO and GSEA databases, respectively. The samples were divided into death and survival groups, combined with clinical prognosis information. LASSO regression was used to construct a prognostic RS model. The Kaplan-Meier curve method was used to evaluate the association between different risk groups and actual survival prognosis information. A cox regression analysis was used to screen for independent survival prognostic clinical factors and construct a nomogram. CIBERSORT and ssGSEA was used for immune-related analysis.

RESULTS

Eighteen ER-stress related genes were identified and a comprehensive network was constructed. Further, 5 CC, 8 MF, 17 BP, and 2 KEGG pathways were enriched. Ten optimal DEGs were obtained and a prognostic risk model was constructed. Compared to the low RS group, the OS values of the high RS group were significantly lower. A significant correlation between the different risk groups and the actual prognosis was demonstrated. Ten immune cells with significantly different distributions in different risk groups were screened. KEGG enrichment analysis showed that there were 5 signaling pathways in the high-risk group.

CONCLUSIONS

The RS model can effectively predict the prognosis and has clinical implications for the prognosis of AML, combined with the correlation between different RS groups and the immune microenvironment.

The F-box-only protein 44 regulates pregnane X receptor protein level by ubiquitination and degradation

Pregnane X receptor (PXR) is a ligand-activated nuclear receptor that transcriptionally upregulates drug-metabolizing enzymes [e.g., cytochrome P450 3A4 (CYP3A4)] and transporters. Although the regulation of PXR target genes is well-characterized, less is known about the regulation of PXR protein level. By screening an RNAi library, we identified the F-box-only protein 44 (FBXO44) as a novel E3 ligase for PXR. PXR abundance increases upon knockdown of FBXO44, and, inversely, decreases upon overexpression of FBXO44. Further analysis revealed that FBXO44 interacts with PXR, leading to its ubiquitination and proteasomal degradation, and we determined that the F-box associated domain of FBXO44 and the ligand binding domain of PXR are required for the functional interaction. In summary, FBXO44 regulates PXR protein abundance, which has downstream consequences for CYP3A4 levels and drug-drug interactions. The results of this study provide new insight into the molecular mechanisms that regulate PXR protein level and activity and suggest the importance of considering how modulating E3 ubiquitin ligase activities will affect PXR-mediated drug metabolism.

Steerable Microneedles Enabling Deep Delivery of Photosensitizers and CRISPR/Cas9 Systems for Effective Combination Cancer Therapy

Although gene therapy has shown prospects in treating triple-negative breast cancer, it is insufficient to treat such a malignant tumor. Herein, nanoparticles (NPs)-embedded dissolving microneedles (IR780-PL/pFBXO44@MNs) with steerable and flectional property were developed to achieve the codelivery of FBXO44-targeted CRISPR/Cas9 plasmids (pFBXO44) and hydrophobic photosensitizers. For improved NP penetration in tumor tissue, collagenase@MNs were preapplied to degrade the tumor matrix. Under light irradiation, IR780 exhibited remarkable phototherapy, while the escape efficiency of NPs from lysosomes was improved. pFBXO44 was subsequently released in tumor cell cytoplasm via reducing the disulfide bonds of NPs, which could specifically knock out the FBXO44 gene to inhibit the migration and invasion of tumor cells. As a result, tumor cells were eradicated, and lung metastasis was effectively suppressed. This micelle-incorporated microneedle platform broadens the potential of combining gene editing and photo synergistic cancer therapy.

Metabolic rewiring and epigenetic reprogramming in leptin receptor-deficient db/db diabetic nephropathy mice

BACKGROUND

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease in the United States. Emerging evidence suggests that mitochondrial metabolism and epigenetics play an important role in the development and progression of DN and its complications. For the first time, we investigated the regulation of cellular metabolism, DNA methylation, and transcriptome status by high glucose (HG) in the kidney of leptin receptor-deficient db/db mice using multi-omics approaches.

METHODS

The metabolomics was performed by liquid-chromatography-mass spectrometry (LC-MS), while epigenomic CpG methylation coupled with transcriptomic gene expression was analyzed by next-generation sequencing.

RESULTS

LC-MS analysis of glomerular and cortex tissue samples of db/db mice showed that HG regulated several cellular metabolites and metabolism-related signaling pathways, including S-adenosylmethionine, S-adenosylhomocysteine, methionine, glutamine, and glutamate. Gene expression study by RNA-seq analysis suggests transforming growth factor beta 1 (TGFβ1) and pro-inflammatory pathways play important roles in early DN. Epigenomic CpG methyl-seq showed HG revoked a list of differentially methylated regions in the promoter region of the genes. Integrated analysis of DNA methylation in the promoter regions of genes and gene expression changes across time points identified several genes persistently altered in DNA methylation and gene expression. Cyp2d22, Slc1a4, and Ddah1 are some identified genes that could reflect dysregulated genes involved in renal function and DN.

CONCLUSION

Our results suggest that leptin receptor deficiency leading to HG regulates metabolic rewiring, including SAM potentially driving DNA methylation and transcriptomic signaling that could be involved in the progression of DN.

The Role of Genetic Mutations in Mitochondrial-Driven Cancer Growth in Selected Tumors: Breast and Gynecological Malignancies

There is an increasing understanding of the molecular and cytogenetic background of various tumors that helps us better conceptualize the pathogenesis of specific diseases. Additionally, in many cases, these molecular and cytogenetic alterations have diagnostic, prognostic, and/or therapeutic applications that are heavily used in clinical practice. Given that there is always room for improvement in cancer treatments and in cancer patient management, it is important to discover new therapeutic targets for affected individuals. In this review, we discuss mitochondrial changes in breast and gynecological (endometrial and ovarian) cancers. In addition, we review how the frequently altered genes in these diseases (BRCA1/2, HER2, PTEN, PIK3CA, CTNNB1, RAS, CTNNB1, FGFR, TP53, ARID1A, and TERT) affect the mitochondria, highlighting the possible associated individual therapeutic targets. With this approach, drugs targeting mitochondrial glucose or fatty acid metabolism, reactive oxygen species production, mitochondrial biogenesis, mtDNA transcription, mitophagy, or cell death pathways could provide further tailored treatment.

BRCA1 deficiency in triple-negative breast cancer: Protein stability as a basis for therapy

Mutations in breast cancer-associated 1 (BRCA1) increase the lifetime risk of developing breast cancer by up to 51% over the risk of the general population. Many aspects of this multifunctional protein have been revealed, including its essential role in homologous recombination repair, E3 ubiquitin ligase activity, transcriptional regulation, and apoptosis. Although most studies have focused on BRCA1 deficiency due to mutations, only a minority of patients carry BRCA1 mutations. A recent study has suggested an expanded definition of BRCA1 deficiency with reduced BRCA1 levels, which accounts for almost half of all triple-negative breast cancer (TNBC) patients. Reduced BRCA1 levels can result from epigenetic modifications or increased proteasomal degradation. In this review, we discuss how this knowledge of BRCA1 function and regulation of BRCA1 protein stability can help overcome the challenges encountered in the clinic and advance current treatment strategies for BRCA1-related breast cancer patients, especially focusing on TNBC.

Phosphorylation of BRCA1 by ATM upon double-strand breaks impacts ATM function in end-resection: A potential feedback loop

BRCA1 maintains genome stability by promoting homologous recombination (HR)-mediated DNA double-strand break (DSB) repair. Mutation of mouse BRCA1-S1152, corresponding to an ATM phosphorylation site in its human counterpart, resulted in increased genomic instability and tumor incidence. In this study, we report that BRCA1-S1152 is part of a feedback loop that sustains ATM activity. BRCA1-S1152A mutation impairs recruitment of the E3 ubiquitin ligase SKP2. This in turn attenuates NBS1-K63 ubiquitination by SKP2 at DSB, impairs sustained ATM activation, and ultimately leads to deficient end resection, the commitment step in the HR repair pathway. Auto-phosphorylation of human ATM at S1981 is known to be important for its kinase activation; we mutated the corresponding amino acid residue in mouse ATM (S1987A) to characterize potential roles of mouse ATM-S1987 in the BRCA1-SKP2-NBS1-ATM feedback loop. Unexpectedly, MEFs carrying the ATM-S1987A knockin mutation maintain damage-induced ATM kinase activation, suggesting a species-specific function of human ATM auto-phosphorylation.

Prediction of Response to Radiotherapy by Characterizing the Transcriptomic Features in Clinical Tumor Samples across 15 Cancer Types

Purpose

Radiotherapy (RT) is one of the major cancer treatments. However, the responses to RT vary among individual patients, partly due to the differences of the status of gene expression and mutation in tumors of patients. Identification of patients who will benefit from RT will improve the efficacy of RT. However, only a few clinical biomarkers were currently used to predict RT response. Our aim is to obtain gene signatures that can be used to predict RT response by analyzing the transcriptome differences between RT responder and nonresponder groups.

Materials and Methods

We obtained transcriptome data of 1664 patients treated with RT from the TCGA database across 15 cancer types. First, the genes with a significant difference between RT responder (R group) and nonresponder groups (PD group) were identified, and the top 100 genes were used to build the gene signatures. Then, we developed the predictive model based on binary logistic regression to predict patient response to RT.

Results

We identified a series of differentially expressed genes between the two groups, which are involved in cell proliferation, migration, invasion, EMT, and DNA damage repair pathway. Among them, MDC1, UCP2, and RBM45 have been demonstrated to be involved in DNA damage repair and radiosensitivity. Our analysis revealed that the predictive model was highly specific for distinguishing the R and PD patients in different cancer types with an area under the curve (AUC) ranging from 0.772 to 0.972. It also provided a more accurate prediction than that from a single-gene signature for the overall survival (OS) of patients.

Conclusion

The predictive model has a potential clinical application as a biomarker to help physicians create optimal treatment plans. Furthermore, some of the genes identified here may be directly involved in radioresistance, providing clues for further studies on the mechanism of radioresistance.

CRL4-DCAF8L2 E3 ligase promotes ubiquitination and degradation of BARD1

BARD1 is a tumor suppressor that is necessary for the functioning and stability of BRCA1, with which it forms a heterodimer and participates in the repair of DNA double-strand breaks. The cellular level of BARD1 and its interaction with BRCA1 are crucial for BRCA1/BARD1 function in homologous recombination and tumor suppression. However, the regulatory mechanism underpinning the stability of BARD1 is largely unclear. In this study, we identified DCAF8L2, a DDB1-Cullin associated factor (DCAF) associated with CRL4 E3 ligase, as a negative regulator of BARD1. Mechanistically, DCAF8L2 interacts with and targets BARD1 for ubiquitination and degradation. In addition, the interaction of DCAF8L2 with BARD1 through the RING domain could compete with the dimerization of BRCA1 and BARD1, leading to increased cellular uncoupling of BARD1 and BRCA1, subjecting the latter to degradation. The overexpression of DCAF8L2 compromises the homologous recombination process and confers cells with increased sensitivity to DNA damage. Furthermore, DCAF8L2 was aberrantly expressed in breast cancer cell lines. Our findings suggest that DCAF8L2 may play an oncogenic role in the pathogenesis of breast cancer, possibly by negative regulation of BARD1.

The BRCA1/BARD1 ubiquitin ligase and its substrates

Mutations in breast cancer type 1 susceptibility protein (BRCA1) and its heterodimeric binding partner BARD1 confer a high risk for the development of breast and ovarian cancers. The sole enzymatic function of the BRCA1/BARD1 complex is as a RING-type E3 ubiquitin (Ub) ligase, leading to the deposition of Ub signals onto a variety of substrate proteins. Distinct types of Ub signals deposited by BRCA1/BARD1 (i.e. degradative vs. non-degradative; mono-Ub vs. poly-Ub chains) on substrate proteins mediate aspects of its function in DNA double-stranded break repair, cell-cycle regulation, and transcriptional regulation. While cancer-predisposing mutations in both subunits lead to the inactivation of BRCA1/BARD1 ligase activity, controversy remains as to whether its Ub ligase activity directly inhibits tumorigenesis. Investigation of BRCA1/BARD1 substrates using rigorous, well-validated mutants and experimental systems will ultimately clarify the role of its ligase activity in cancer and possibly establish prognostic and diagnostic metrics for patients with mutations. In this review, we discuss the Ub ligase function of BRCA1/BARD1, highlighting experimental approaches, mechanistic considerations, and reagents that are useful in the study of substrate ubiquitylation. We also discuss the current understanding of two well-established BRCA1/BARD1 substrates (nucleosomal H2A and estrogen receptor α) and several recently discovered substrates (p50, NF2, Oct1, and LARP7). Lessons from the current body of work should provide a road map to researchers examining novel substrates and biological functions attributed to BRCA1/BARD1 Ub ligase activity.

A Hypoxia-Induced SCFFBXL1 E3 Ligase Ubiquitinates and Degrades the MEN1 Tumor Suppressor to Promote Colorectal Cancer Tumorigenesis

Purpose

Emerging evidence has shown that SKP1-cullin-1-F-box-protein (SCF) E3 ligases contribute to the pathogenesis of different cancers by mediating the ubiquitination and degradation of tumor suppressors. However, the functions of SCF E3 ligases in the pathogenesis of colorectal cancer (CRC) remain obscure.

Materials and Methods

The cancerous and adjacent noncancerous tissues from CRC patients were collected, and protein levels were analyzed. Lentiviral short hairpin RNA (shRNA) and plasmid transfection were used to knock down and overexpress gene expression in CRC cell lines. Immunoprecipitation (IP), mass spectrometry, and co-IP analyses were used to determine protein interactions and the assembly of the SCF complex. Cell proliferation, migration, and tumor xenograft assays were performed to examine the effects of SCF members on CRC cell growth in vitro and in vivo.

Results

Hypoxia activated the docking of HIF1a onto the CUL1 promoter and induced CUL1 expression in CRC cells. CUL1 coupled with RBX1, SKP1, and FBXL1 to assemble the SCFFBXL1 complex in CRC biopsies and cells. The SCFFBXL1 E3 ligase specifically ubiquitinated and degraded the MEN1 tumor suppressor. Knockdown of HIF1a or SCFFBXL1 members, or blockage of SCFFBXL1 by two inhibitors (DT204 and SZLP1-41) caused the accumulation of MEN1 protein and led to a significant decrease in cell proliferation and migration in vitro and tumor growth in vivo.

Conclusion

The SCFFBXL1 E3 ligase is required for the ubiquitination of MEN1, and disruption of this complex may represent a new therapeutic strategy for the treatment of CRC.

BRCA1 degradation in response to mitochondrial damage in breast cancer cells

BRCA1 is a well-studied tumor suppressor involved in the homologous repair of DNA damage, whereas PINK1, a mitochondrial serine/threonine kinase, is known to be involved in mitochondrial quality control. Genetic mutations of PINK1 and Parkin cause autosomal recessive early-onset Parkinson's disease. We found that in breast cancer cells, the mitochondrial targeting reagents, which all induce mitochondrial depolarization along with PINK1 upregulation, induced proteasomal BRCA1 degradation. This BRCA1 degradation was dependent on PINK1, and BRCA1 downregulation upon mitochondrial damage caused DNA double-strand breaks. BRCA1 degradation was mediated through the direct interaction with the E3 ligase Parkin. Strikingly, BRCA1 and PINK1/Parkin expression were inversely correlated in cancerous mammary glands from breast cancer patients. BRCA1 knockdown repressed cancer cell growth, and high BRCA1 expression predicted poor relapse-free survival in breast cancer patients. These observations indicate a novel mechanism by which mitochondrial damage is transmitted to the nucleus, leading to BRCA1 degradation.

SIRT2 promotes BRCA1-BARD1 heterodimerization through deacetylation

The breast cancer type I susceptibility protein (BRCA1) and BRCA1-associated RING domain protein I (BARD1) heterodimer promote genome integrity through pleiotropic functions, including DNA double-strand break (DSB) repair by homologous recombination (HR). BRCA1-BARD1 heterodimerization is required for their mutual stability, HR function, and role in tumor suppression; however, the upstream signaling events governing BRCA1-BARD1 heterodimerization are unclear. Here, we show that SIRT2, a sirtuin deacetylase and breast tumor suppressor, promotes BRCA1-BARD1 heterodimerization through deacetylation. SIRT2 complexes with BRCA1-BARD1 and deacetylates conserved lysines in the BARD1 RING domain, interfacing BRCA1, which promotes BRCA1-BARD1 heterodimerization and consequently BRCA1-BARD1 stability, nuclear retention, and localization to DNA damage sites, thus contributing to efficient HR. Our findings define a mechanism for regulation of BRCA1-BARD1 heterodimerization through SIRT2 deacetylation, elucidating a critical upstream signaling event directing BRCA1-BARD1 heterodimerization, which facilitates HR and tumor suppression, and delineating a role for SIRT2 in directing DSB repair by HR.

Minten et al. show that SIRT2, a sirtuin deacetylase and tumor suppressor protein, promotes BRCA1-BARD1 heterodimerization through deacetylation of BARD1 at conserved lysines within its RING domain. These findings elucidate a critical upstream signaling event directing BRCA1-BARD1 heterodimerization, which facilitates HR and tumor suppression.

MiRNAs: A Powerful Tool in Deciphering Gynecological Malignancies

Accumulated evidence on the clinical roles of microRNAs (miRNAs) in cancer prevention and control has revealed the emergence of new genetic techniques that have improved the understanding of the mechanisms essential for pathology induction and progression. Comprehension of the modifications and individual differences of miRNAs and their interactions in the pathogenesis of gynecological malignancies, together with an understanding of the phenotypic variations have considerably improved the management of the diagnosis and personalized treatment for different forms of cancer. In recent years, miRNAs have emerged as signaling molecules in biological pathways involved in different categories of cancer and it has been demonstrated that these molecules could regulate cancer-relevant processes, our focus being on malignancies of the gynecologic tract. The aim of this paper is to summarize novel research findings in the literature regarding the parts that miRNAs play in cancer-relevant processes, specifically regarding gynecological malignancy, while emphasizing their pivotal role in the disruption of cancer-related signaling pathways.

N-Terminal Targeting of Regulator of G Protein Signaling Protein 2 for F-Box Only Protein 44-Mediated Proteasomal Degradation

Regulator of G protein signaling (RGS) proteins are negative modulators of G protein signaling that have emerged as promising drug targets to improve specificity and reduce side effects of G protein-coupled receptor-related therapies. Several small molecule RGS protein inhibitors have been identified; however, enhancing RGS protein function is often more clinically desirable but presents a challenge. Low protein levels of RGS2 are associated with various pathologies, including hypertension and heart failure. For this reason, RGS2 is a prominent example wherein enhancing its function would be beneficial. RGS2 is rapidly ubiquitinated and proteasomally degraded, providing a point of intervention for small molecule RGS2-stabilizing compounds. We previously identified a novel cullin-RING E3 ligase utilizing F-box only protein 44 (FBXO44) as the substrate recognition component. Here, we demonstrate that RGS2 associates with FBXO44 through a stretch of residues in its N terminus. RGS2 contains four methionine residues close to the N terminus that can act as alternative translation initiation sites. The shorter translation initiation variants display reduced ubiquitination and proteasomal degradation as a result of lost association with FBXO44. In addition, we show that phosphorylation of Ser³ may be an additional mechanism to protect RGS2 from FBXO44-mediated proteasomal degradation. These findings contribute to elucidating mechanisms regulating steady state levels of RGS2 protein and will inform future studies to develop small molecule RGS2 stabilizers. These would serve as novel leads in pathologies associated with low RGS2 protein levels, such as hypertension, heart failure, and anxiety. SIGNIFICANCE STATEMENT: E3 ligases provide a novel point of intervention for therapeutic development, but progress is hindered by the lack of available information about specific E3-substrate pairs. Here, we provide molecular detail on the recognition of regulator of G protein signaling protein 2 (RGS2) by its E3 ligase, increasing the potential for rational design of small molecule RGS2 protein stabilizers. These would be clinically useful in pathologies associated with low RGS2 protein levels, such as hypertension, heart failure, and anxiety.

p53 Affects PGC1α Stability Through AKT/GSK-3β to Enhance Cisplatin Sensitivity in Non-Small Cell Lung Cancer

Drug resistance greatly limits the therapeutic efficacy of treatment of non-small cell lung cancer (NSCLC). One of the important factors is the dysfunction of tumor suppressor p53. Recent studies have suggested that p53 suppresses tumors by regulating number of mitochondrial proteins, including peroxisome proliferator-activated receptor coactivator (PGC1α). Although several studies have confirmed the interaction between p53 and PGC1α, the precise mechanism has not been completely determined in NSCLC. In this study, we investigated the specific signaling between p53 and PGC1α to improve anti-tumor drug effects on NSCLC. We found that low expression of p53 and high expression of PGC1α correlated with shorter survival time of NSCLC patients. In vitro experiments confirmed that NCI-H1299 (p53-null) cells had high levels of PGC1α and were insensitive to cisplatin (CDDP). When PGC1α was knocked down, the sensitivity to cisplatin was increased. Notably, the stability of PGC1α is an important mechanism in its activity regulation. We demonstrated that p53 decreased the stability of PGC1α via the ubiquitin proteasome pathway, which was mediated by protein kinase B (AKT) inhibition and glycogen synthase kinase (GSK-3β) activation. Therefore, p53 may regulate the stability of PGC1α through the AKT/GSK-3β pathway, thus affect the chemosensitivity of NSCLC.

Non-canonical regulation of homologous recombination DNA repair by the USP9X deubiquitylase

In order to prevent the deleterious effects of genotoxic agents, cells have developed complex surveillance mechanisms and DNA repair pathways that allow them to maintain genome integrity. The ubiquitin-specific protease 9X (USP9X) contributes to genome stability during DNA replication and chromosome segregation. Depletion of USP9X leads to DNA double-strand breaks, some of which are triggered by replication fork collapse. Here, we identify USP9X as a novel regulator of homologous recombination (HR) DNA repair in human cells. By performing cellular HR reporter, irradiation-induced focus formation and colony formation assays, we show that USP9X is required for efficient HR. Mechanistically, we show USP9X is important to sustain the expression levels of key HR factors, namely BRCA1 and RAD51 through a non-canonical regulation of their mRNA abundance. Intriguingly, we find that the contribution of USP9X to BRCA1 and RAD51 expression is independent of its known catalytic activity. Thus, this work identifies USP9X as a regulator of HR, demonstrates a novel mechanism by which USP9X can regulate protein levels, and provides insights in to the regulation of BRCA1 and RAD51 mRNA.This article has an associated First Person interview with the first author of the paper.

Cullin Ring Ubiquitin Ligases (CRLs) in Cancer: Responses to Ionizing Radiation (IR) Treatment

Treatment with ionizing radiation (IR) remains the cornerstone of therapy for multiple cancer types, including disseminated and aggressive diseases in the palliative setting. Radiotherapy efficacy could be improved in combination with drugs that regulate the ubiquitin-proteasome system (UPS), many of which are currently being tested in clinical trials. The UPS operates through the covalent attachment of ATP-activated ubiquitin molecules onto substrates following the transfer of ubiquitin from an E1, to an E2, and then to the substrate via an E3 enzyme. The specificity of ubiquitin ligation is dictated by E3 ligases, which select substrates to be ubiquitylated. Among the E3s, cullin ring ubiquitin ligases (CRLs) represent prototypical multi-subunit E3s, which use the cullin subunit as a central assembling scaffold. CRLs have crucial roles in controlling the cell cycle, hypoxia signaling, reactive oxygen species clearance and DNA repair; pivotal factors regulating the cancer and normal tissue response to IR. Here, we summarize the findings on the involvement of CRLs in the response of cancer cells to IR, and we discuss the therapeutic approaches to target the CRLs which could be exploited in the clinic.

CAND1 regulates lunapark for the proper tubular network of the endoplasmic reticulum

Endoplasmic reticulum (ER) tubules connect each other by three-way junctions, resulting in a tubular ER network. Oligomerization of three-way junction protein lunapark (Lnp) is important for its localization and the three-way junction stability. On the other hand, Lnp has an N-terminal ubiquitin ligase activity domain, which is also important for the three-way junction localization. To understand the mode of action of Lnp, we isolated Cullin-associated and neddylation-dissociated 1 (CAND1), a regulator of Skp1-Cul1-F-box (SCF) ubiquitin ligase, as a Lnp-binding protein by affinity chromatography. CAND1 and Lnp form a higher-molecular-weight complex in vitro, while they do not co-localize at the three-way junctions. CAND1 reduces the auto-ubiquitination activity of Lnp. CAND1 knockdown enhances proteasomal degradation of Lnp and reduces the tubular ER network in mammalian cells. These results suggest that CAND1 has the potency to promote the formation of the higher-molecular-weight complex with Lnp and reduce the auto-ubiquitination activity of Lnp, thereby regulating the three-way junction stability of the tubular ER network.

USP9X stabilizes BRCA1 and confers resistance to DNA-damaging agents in human cancer cells

BRCA1, a multifunctional protein with an important role in DNA double‐strand break repair by homologous recombination (HR), is subjected to ubiquitin‐dependent degradation. To date, several E3 ubiquitin ligases have been identified to govern BRCA1 stability, but the deubiquitinase that counteracts its turnover remains undefined. In this study, we report that the ubiquitin‐specific protease 9X (USP9X) is a bona fide deubiquitinase for BRCA1 in human cancer cells. Reciprocal immunoprecipitation assays demonstrated that USP9X interacted with BRCA1. Depletion of USP9X by short interfering RNAs or inhibition of USP9X by the small‐molecular inhibitor WP1130 significantly reduced BRCA1 protein abundance, without affecting its mRNA levels. In contrast, overexpression of wild‐type USP9X, but not its deubiquitinase activity‐defective mutant (C1566S), resulted in an upregulation of BRCA1 protein levels. Moreover, USP9X depletion reduced the half‐life of BRCA1, accompanied by an increase in its ubiquitination. HR assays showed that knockdown of USP9X significantly reduced HR efficiency, which was partially rescued by reintroduction of BRCA1 into USP9X‐depleted cells. In support of these findings, USP9X knockdown significantly enhanced sensitivity to PARP inhibitor Olaparib and methyl methanesulfonate (MMS). Collectively, these results establish USP9X as a deubiquitinase for BRCA1 and reveal a previously unrecognized role of USP9X in the regulation of HR repair and the sensitivity of cancer cells to DNA‐damaging agents.

BRCA Genes: The Role in Genome Stability, Cancer Stemness and Therapy Resistance

Carcinogenesis is a multistep process, and tumors frequently harbor multiple mutations regulating genome integrity, cell division and death. The integrity of cellular genome is closely controlled by the mechanisms of DNA damage signaling and DNA repair. The association of breast cancer susceptibility genes BRCA1 and BRCA2 with breast and ovarian cancer development was first demonstrated over 20 years ago. Since then the germline mutations within these genes were linked to genomic instability and increased risk of many other cancer types. Genomic instability is an engine of the oncogenic transformation of non-tumorigenic cells into tumor-initiating cells and further tumor evolution. In this review we discuss the biological functions of BRCA1 and BRCA2 genes and the role of BRCA mutations in tumor initiation, regulation of cancer stemness, therapy resistance and tumor progression.

The emerging role for Cullin 4 family of E3 ligases in tumorigenesis

As a member of the Cullin-RING ligase family, Cullin-RING ligase 4 (CRL4) has drawn much attention due to its broad regulatory roles under physiological and pathological conditions, especially in neoplastic events. Based on evidence from knockout and transgenic mouse models, human clinical data, and biochemical interactions, we summarize the distinct roles of the CRL4 E3 ligase complexes in tumorigenesis, which appears to be tissue- and context-dependent. Notably, targeting CRL4 has recently emerged as a noval anti-cancer strategy, including thalidomide and its derivatives that bind to the substrate recognition receptor cereblon (CRBN), and anticancer sulfonamides that target DCAF15 to suppress the neoplastic proliferation of multiple myeloma and colorectal cancers, respectively. To this end, PROTACs have been developed as a group of engineered bi-functional chemical glues that induce the ubiquitination-mediated degradation of substrates via recruiting E3 ligases, such as CRL4 (CRBN) and CRL2 (pVHL). We summarize the recent major advances in the CRL4 research field towards understanding its involvement in tumorigenesis and further discuss its clinical implications. The anti-tumor effects using the PROTAC approach to target the degradation of undruggable targets are also highlighted.

Ubiquitination by HUWE1 in tumorigenesis and beyond

Ubiquitination modulates a large repertoire of cellular functions and thus, dysregulation of the ubiquitin system results in multiple human diseases, including cancer. Ubiquitination requires an E3 ligase, which is responsible for substrate recognition and conferring specificity to ubiquitination. HUWE1 is a multifaceted HECT domain-containing ubiquitin E3 ligase, which catalyzes both mono-ubiquitination and K6-, K48- and K63-linked poly-ubiquitination of its substrates. Many of the substrates of HUWE1 play a crucial role in maintaining the homeostasis of cellular development. Not surprisingly, dysregulation of HUWE1 is associated with tumorigenesis and metastasis. HUWE1 is frequently overexpressed in solid tumors, but can be downregulated in brain tumors, suggesting that HUWE1 may possess differing cell-specific functions depending on the downstream targets of HUWE1. This review introduces some important discoveries of the HUWE1 substrates, including those controlling proliferation and differentiation, apoptosis, DNA repair, and responses to stress. In addition, we review the signaling pathways HUWE1 participates in and obstacles to the identification of HUWE1 substrates. We also discuss up-to-date potential therapeutic designs using small molecules or ubiquitin variants (UbV) against the HUWE1 activity. These molecular advances provide a translational platform for future bench-to-bed studies. HUWE1 is a critical ubiquitination modulator during the tumor progression and may serve as a possible therapeutic target for cancer treatment.

Regulating BRCA1 protein stability by cathepsin S-mediated ubiquitin degradation

Cathepsin S (CTSS) is a cysteine protease that is thought to play a role in many physiological and pathological processes including tumor growth, angiogenesis, and metastasis; it has been identified as a radiation response gene. Here, we examined the role of CTSS in regulating the DNA damage response in breast cancer cells. Activating CTSS (producing the cleavage form of the protein) by radiation induced proteolytic degradation of BRCA1, which ultimately suppressed DNA double-strand break repair activity. Depletion of CTSS by RNAi or expression of a mutant type of CTSS enhanced the protein stability of BRCA1 by inhibiting its ubiquitination. CTSS interacted with the BRCT domain of BRCA1 and facilitated ubiquitin-mediated proteolytic degradation of BRCA1, which was tightly associated with decreased BRCA1-mediated DNA repair activity. Treatment with a pharmacological CTSS inhibitor inhibited proteolytic degradation of BRCA1 and restored BRCA1 function. Depletion of CTSS by shRNA delayed tumor growth in a xenograft mouse model, only in the presence of functional BRCA1. Spontaneously uced rat mammary tumors and human breast cancer tissues with high levels of CTSS expression showed low BRCA1 expression. From these data, we suggest that CTSS inhibition is a good strategy for functional restoration of BRCA1 in breast cancers with reduced BRCA1 protein stability.

Identification of aberrantly expressed F-box proteins in squamous-cell lung carcinoma

PURPOSE

F-box proteins, as components of the Skp1-Cullin 1-F-box protein (SCF) E3 ubiquitin ligase, can specifically bind to substrates and regulate multiple tumor behaviors. However, the role of F-box proteins in squamous-cell lung carcinoma (SqCLC) has not been established.

METHODS

We identified the differentially expressed F-box protein-encoding genes in SqCLC by analyzing data from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. Prognosis data were evaluated using the Kaplan-Meier (KM) plotter website. The FBXO5 and FBXO45 mRNA levels were analyzed by real time RT-PCR. The impact of the inhibition of these genes with si-RNA on apoptosis and migration was also investigated.

RESULTS

The FBXO45 and FBXO5 genes were significantly up-regulated in SqCLC compared with normal lung (p values = 0.002 and 0.025, respectively). FBXO45 was significantly elevated in each tumorigenic step, including dysplasia, in situ and SqCLC. The RT-PCR analysis results showed that FBXO5 and FBXO45 were elevated in cancer tissues (p values = 0.024 and 0.004, respectively). Overexpression of FBXO5 and FBXO45 was associated with shorter overall survival (OS) in the SqCLC patients from the K-M plotter database [FBXO5 HR: 1.53 (1.03-2.28), p = 0.036]; [FBXO45 HR: 1.47 (1.03-2.08), p = 0.030]. The GO and KEGG pathway analysis showed that FBXO5 and FBXO45 were associated with cell cycle and adhesion, respectively. Knockdown of FBXO5 leads to increased apoptosis, while knockdown of FBXO45 facilitates the process of epithelial-mesenchymal transition (EMT).

CONCLUSIONS

Our results provide evidence that FBXO45 and FBXO5 may play a key role in tumorigenesis and prognosis of SqCLC.

F-box proteins in epigenetic regulation of cancer

Epigenetic abnormalities are now realized as important as genetic alterations in contributing to the initiation and progression of cancer. Recent advancements in the cancer epigenetics field have identified extensive alterations of the epigenetic network in human cancers, including histone modifications and DNA methylation. F-box proteins, the substrate receptors of SCF (SKP1-Cullin1-F-box protein) E3 ubiquitin ligases, can directly and indirectly affect the balance of epigenetic regulation. In this brief review, we discuss our current understanding of F-box proteins in cellular epigenetic regulation and how dysregulation of these processes contribute to cancer development.

Ubiquitin-specific protease 21 stabilizes BRCA2 to control DNA repair and tumor growth

Tumor growth relies on efficient DNA repair to mitigate the detrimental impact of DNA damage associated with excessive cell division. Modulating repair factor function, thus, provides a promising strategy to manipulate malignant growth. Here, we identify the ubiquitin-specific protease USP21 as a positive regulator of BRCA2, a key mediator of DNA repair by homologous recombination. USP21 interacts with, deubiquitinates and stabilizes BRCA2 to promote efficient RAD51 loading at DNA double-strand breaks. As a result, depletion of USP21 decreases homologous recombination efficiency, causes an increase in DNA damage load and impairs tumor cell survival. Importantly, BRCA2 overexpression partially restores the USP21-associated survival defect. Moreover, we show that USP21 is overexpressed in hepatocellular carcinoma, where it promotes BRCA2 stability and inversely correlates with patient survival. Together, our findings identify deubiquitination as a means to regulate BRCA2 function and point to USP21 as a potential therapeutic target in BRCA2-proficient tumors.BRCA2 is essential for the repair of DNA damage; therefore, defects in BRCA2 are associated with tumorigenesis but also with increased susceptibility to genotoxic stress. Here the authors show that USP21 regulates the ability of tumor cells to repair damaged DNA by regulating BRCA2 stability.

Skp1 in lung cancer: clinical significance and therapeutic efficacy of its small molecule inhibitors

Skp1 is an essential adaptor protein of the Skp1-Cul1-F-box protein complex and is able to stabilize the conformation of some ubiquitin E3 ligases. However, the role Skp1 plays during tumorigenesis remains unclear and Skp1-targeting agent is lacking. Here we showed that Skp1 was overexpressed in 36/64 (56.3%) of non-small cell lung cancers, and elevated Skp1 was associated with poor prognosis. By structure-based high-throughput virtual screening, we found some Skp1-targeting molecules including a natural compound 6-O-angeloylplenolin (6-OAP). 6-OAP bound Skp1 at sites critical to Skp1-Skp2 interaction, leading to dissociation and proteolysis of oncogenic E3 ligases NIPA, Skp2, and β-TRCP, and accumulation of their substrates Cyclin B1, P27 and E-Cadherin. 6-OAP induced prometaphase arrest and exerted potent anti-lung cancer activity in two murine models and showed low adverse effect. These results indicate that Skp1 is critical to lung cancer pathogenesis, and Skp1 inhibitor inactivates crucial oncogenic E3 ligases and exhibits significant therapeutic potentials.

Skp1: Implications in cancer and SCF-oriented anti-cancer drug discovery

In the last decade, the ubiquitin proteasome system (UPS), in general, and E3 ubiquitin ligases, in particular, have emerged as valid drug targets for the development of novel anti-cancer therapeutics. Cullin RING Ligases (CRLs), which can be classified into eight groups (CRL1-8) and comprise approximately 200 members, represent the largest family of E3 ubiquitin ligases which facilitate the ubiquitination-derived proteasomal degradation of a myriad of functionally and structurally diverse substrates. S phase kinase-associated protein 1 (Skp1)-Cullin1-F-Box protein (SCF) complexes are the best characterized among CRLs, which play crucial roles in numerous cellular processes and physiological dysfunctions, such as in cancer biology. Currently, there is growing interest in developing SCF-targeting anti-cancer therapies for clinical application. Indeed, the research in this field has seen some progress in the form of cullin neddylation- and Skp2-inhibitors. However, it still remains an underdeveloped area and needs to design new strategies for developing improved form of therapy. In this review, we venture a novel strategy that rational pharmacological targeting of Skp1, a central regulator of SCF complexes, may provide a novel avenue for SCF-oriented anti-cancer therapy, expected: (i) to simultaneously address the critical roles that multiple SCF oncogenic complexes play in cancer biology, (ii) to selectively target cancer cells with minimal normal cell toxicity, and (iii) to offer multiple chemical series, via therapeutic interventions at the Skp1 binding interfaces in SCF complex, thereby maximizing chances of success for drug discovery. In addition, we also discuss the challenges that might be posed regarding rational pharmacological interventions against Skp1.

Ubiquitination-mediated degradation of cell cycle-related proteins by F-box proteins

F-box proteins, subunits of SKP1-cullin 1-F-box protein (SCF) type of E3 ubiquitin ligase complexes, have been validated to play a crucial role in governing various cellular processes such as cell cycle, cell proliferation, apoptosis, migration, invasion and metastasis. Recently, a wealth of evidence has emerged that F-box proteins is critically involved in tumorigenesis in part through governing the ubiquitination and subsequent degradation of cell cycle proteins, and dysregulation of this process leads to aberrant cell cycle progression and ultimately, tumorigenesis. Therefore, in this review, we describe the critical role of F-box proteins in the timely regulation of cell cycle. Moreover, we discuss how F-box proteins involve in tumorigenesis via targeting cell cycle-related proteins using biochemistry studies, engineered mouse models, and pathological gene alternations. We conclude that inhibitors of F-box proteins could have promising therapeutic potentials in part through controlling of aberrant cell cycle progression for cancer therapies.

Damage-induced BRCA1 phosphorylation by Chk2 contributes to the timing of end resection

The BRCA1 tumor suppressor plays an important role in homologous recombination (HR)-mediated DNA double-strand-break (DSB) repair. BRCA1 is phosphorylated by Chk2 kinase upon γ-irradiation, but the role of Chk2 phosphorylation is not understood. Here, we report that abrogation of Chk2 phosphorylation on BRCA1 delays end resection and the dispersion of BRCA1 from DSBs but does not affect the assembly of Mre11/Rad50/NBS1 (MRN) and CtIP at DSBs. Moreover, we show that BRCA1 is ubiquitinated by SCF(Skp2) and that abrogation of Chk2 phosphorylation impairs its ubiquitination. Our study suggests that BRCA1 is more than a scaffold protein to assemble HR repair proteins at DSBs, but that Chk2 phosphorylation of BRCA1 also serves as a built-in clock for HR repair of DSBs. BRCA1 is known to inhibit Mre11 nuclease activity. SCF(Skp2) activity appears at late G1 and peaks at S/G2, and is known to ubiquitinate phosphodegron motifs. The removal of BRCA1 from DSBs by SCF(Skp2)-mediated degradation terminates BRCA1-mediated inhibition of Mre11 nuclease activity, allowing for end resection and restricting the initiation of HR to the S/G2 phases of the cell cycle.

The Structural Differences between a Glycoprotein Specific F-Box Protein Fbs1 and Its Homologous Protein FBG3

The Skp1-Cul1-F-box protein (SCF) complex catalyzes protein ubiquitination in diverse cellular processes and is one of the best-characterized ubiquitin ligases. F-box proteins determine the substrate specificities of SCF ubiquitin ligases. Among these, Fbs1/FBG1/FBXO2, Fbs2/FBG2/FBXO6, and Fbs3/FBG5/FBXO27 recognize the N-glycans of glycoproteins, whereas FBG3/FBXO44 has no sugar-binding activity, despite the high sequence homology and conservation of the residues necessary for oligosaccharide binding between Fbs1–3 and FBG3. Here we determined the crystal structure of the Skp1–FBG3 complex at a resolution of 2.6 Å. The substrate-binding domain of FBG3 is composed of a 10-stranded antiparallel β-sandwich with three helices. Although the overall structure of FBG3 is similar to that of Fbs1, the residues that form the Fbs1 carbohydrate-binding pocket failed to be superposed with the corresponding residues of FBG3. Structure-based mutational analysis shows that distinct hydrogen bond networks of four FBG3 loops, i.e., β2-β3, β5-β6, β7-β8, and β9-β10, prevent the formation of the carbohydrate-binding pocket shown in Fbs1.

Deregulation of F-box proteins and its consequence on cancer development, progression and metastasis

F-box proteins are substrate receptors of the SCF (SKP1-Cullin 1-F-box protein) E3 ubiquitin ligase that play important roles in a number of physiological processes and activities. Through their ability to assemble distinct E3 ubiquitin ligases and target key regulators of cellular activities for ubiquitylation and degradation, this versatile group of proteins is able to regulate the abundance of cellular proteins whose deregulated expression or activity contributes to disease. In this review, we describe the important roles of select F-box proteins in regulating cellular activities, the perturbation of which contributes to the initiation and progression of a number of human malignancies.

F-box protein interactions with the hallmark pathways in cancer

F-box proteins (FBP) are the substrate specifying subunit of Skp1-Cul1-FBP (SCF)-type E3 ubiquitin ligases and are responsible for directing the ubiquitination of numerous proteins essential for cellular function. Due to their ability to regulate the expression and activity of oncogenes and tumour suppressor genes, FBPs themselves play important roles in cancer development and progression. In this review, we provide a comprehensive overview of FBPs and their targets in relation to their interaction with the hallmarks of cancer cell biology, including the regulation of proliferation, epigenetics, migration and invasion, metabolism, angiogenesis, cell death and DNA damage responses. Each cancer hallmark is revealed to have multiple FBPs which converge on common signalling hubs or response pathways. We also highlight the complex regulatory interplay between SCF-type ligases and other ubiquitin ligases. We suggest six highly interconnected FBPs affecting multiple cancer hallmarks, which may prove sensible candidates for therapeutic intervention.

FBXO44-Mediated Degradation of RGS2 Protein Uniquely Depends on a Cullin 4B/DDB1 Complex

The ubiquitin-proteasome system for protein degradation plays a major role in regulating cell function and many signaling proteins are tightly controlled by this mechanism. Among these, Regulator of G Protein Signaling 2 (RGS2) is a target for rapid proteasomal degradation, however, the specific enzymes involved are not known. Using a genomic siRNA screening approach, we identified a novel E3 ligase complex containing cullin 4B (CUL4B), DNA damage binding protein 1 (DDB1) and F-box protein 44 (FBXO44) that mediates RGS2 protein degradation. While the more typical F-box partners CUL1 and Skp1 can bind FBXO44, that E3 ligase complex does not bind RGS2 and is not involved in RGS2 degradation. These observations define an unexpected DDB1/CUL4B-containing FBXO44 E3 ligase complex. Pharmacological targeting of this mechanism provides a novel therapeutic approach to hypertension, anxiety, and other diseases associated with RGS2 dysregulation.

It is not all about BRCA: Cullin-Ring ubiquitin Ligases in ovarian cancer

Mutations in the BRCA1 and BRCA2 genes predispose individuals to the development of breast and ovarian cancers. As a result, biochemical functions of BRCA1 and BRCA2 proteins are being characterised in great detail. These studies have prompted the use of PARP inhibitors to treat BRCA1 and BRCA2-deficient ovarian cancers. This example of synthetic lethality represents a conceptual progress made recently in the approach to cancer treatment and is being currently tested in multiple clinical trials. Other than BRCA1 and BRCA2, many other factors might influence the survival of ovarian cancer patients. Currently, ovarian cancer remains the fifth most common cancer in the United Kingdom among women. Recent evidence suggests benefit in the modulation of the ubiquitin-proteasome system for the treatment of ovarian cancer. In this manuscript, we review the role of Cullin-Ring ubiquitin Ligases (CRLs) in the pathogenesis of ovarian cancer and their potential therapeutic exploitation. CRLs comprise a large family of proteins that, like kinases, might represent ideal candidates for targeted therapy and provide a large repertoire for the development of new anti-cancer compounds.

TUSC4 functions as a tumor suppressor by regulating BRCA1 stability

BRCA1 expression is lost frequently in breast cancers in which it promotes malignant development. In the present study, we performed a global expression analysis of breast cancer cells in which the tumor-suppressor candidate gene TUSC4 was silenced to gain insights into its function. TUSC4 silencing affected genes involved in cell cycle and cell death, which have broad reaching influence on cancer development. Most importantly, we found a cluster pattern of gene-expression profiles in TUSC4-silenced cells that defined a homologous recombination (HR) repair defect signature. Mechanistic investigations indicated that TUSC4 protein could physically interact with the E3 ligase Herc2, which prevents BRCA1 degradation through the ubiquitination pathway. TUSC4 silencing enhanced BRCA1 polyubiquitination, leading to its degradation and a marked reduction in HR repair efficiency. Notably, ectopic expression of TUSC4 suppressed the proliferation, invasion, and colony formation of breast cancer cells in vitro and tumorigenesis in vivo. Furthermore, TUSC4 silencing was sufficient to transform normal mammary epithelial cells and to enhance sensitivity to PARP inhibitors. Our results provide a set of genetic and biologic proofs that TUSC4 functions as a bona fide tumor suppressor by regulating the protein stability and function of BRCA1 in breast cancer.

Roles of F-box proteins in cancer

F-box proteins, which are the substrate-recognition subunits of SKP1-cullin 1-F-box protein (SCF) E3 ligase complexes, have pivotal roles in multiple cellular processes through ubiquitylation and subsequent degradation of target proteins. Dysregulation of F-box protein-mediated proteolysis leads to human malignancies. Notably, inhibitors that target F-box proteins have shown promising therapeutic potential, urging us to review the current understanding of how F-box proteins contribute to tumorigenesis. As the physiological functions for many of the 69 putative F-box proteins remain elusive, additional genetic and mechanistic studies will help to define the role of each F-box protein in tumorigenesis, thereby paving the road for the rational design of F-box protein-targeted anticancer therapies.

HUWE1 interacts with BRCA1 and promotes its degradation in the ubiquitin-proteasome pathway

The cellular BRCA1 protein level is essential for its tumor suppression activity and is tightly regulated through multiple mechanisms including ubiquitn-proteasome system. E3 ligases are involved to promote BRCA1 for ubiquitination and degradation. Here, we identified HUWE1/Mule/ARF-BP1 as a novel BRCA1-interacting protein involved in the control of BRCA1 protein level. HUWE1 binds BRCA1 through its N-terminus degron domain. Depletion of HUWE1 by siRNA-mediated interference significantly increases BRCA1 protein levels and prolongs the half-life of BRCA1. Moreover, exogenous expression of HUWE1 promotes BRCA1 degradation through the ubiquitin-proteasome pathway, which could explain an inverse correlation between HUWE1 and BRCA1 levels in MCF10F, MCF7 and MDA-MB-231 breast cancer cells. Consistent with a functional role for HUWE1 in regulating BRCA1-mediated cellular response to DNA damage, depletion of HUWE1 by siRNA confers increased resistance to ionizing radiation and mitomycin. These data indicate that HUWE1 is a critical negative regulator of BRCA1 and suggest a new molecular mechanism for breast cancer pathogenesis.