UBE2M Is a Stress-Inducible Dual E2 for Neddylation and Ubiquitylation that Promotes Targeted Degradation of UBE2F.

UBE2M and UBE2F are two family members of neddylation E2 conjugating enzyme that, together with E3s, activate CRLs (Cullin-RING Ligases) by catalyzing cullin neddylation. However, whether and how two E2s cross-talk with each other are largely unknown. Here, we report that UBE2M is a stress-inducible gene subjected to cis-transactivation by HIF-1 and AP1, and MLN4924, a small molecule inhibitor of E1 NEDD8-activating enzyme (NAE), upregulates UBE2M via blocking degradation of HIF-1α and c-JUN. UBE2M is a dual E2 for targeted ubiquitylation and degradation of UBE2F, acting as a neddylation E2 to activate CUL3-Keap1 E3 under physiological conditions but as a ubiquitylation E2 for Parkin-DJ-1 E3 under stressed conditions. UBE2M-induced UBE2F degradation leads to CRL5 inactivation and subsequent NOXA accumulation to suppress the growth of lung cancer cells. Collectively, our study establishes a negative regulatory axis between two neddylation E2s with UBE2M ubiquitylating UBE2F, and two CRLs with CRL3 inactivating CRL5.

Investigating NanoLuc-EGFR engineered cell lines for real-time monitoring of EGFR protein dynamics in live cells.

Evaluating the steady-state protein level of the EGFR in live cells presents significant challenges compared to measuring its kinase activity. Traditional testing methods, such as immunoblotting, ELISA, and immunofluorescence assays, are generally restricted to fixed cells or cell lysates. Despite their utility, these methods are cumbersome and provide only intermittent snapshots of EGFR levels at specific time points. With emerging trends in drug development shifting toward engineering novel agents that promote protein degradation, rather than simply inhibiting kinase activity, a tool that enables real-time, quantitative detection of drug effects in live cells could catalyze advances in the field. Such an innovation would expedite the drug development process, enhancing the translation of research findings into effective, patient-centered therapies. The NanoLuc-EGFR cell line, created through CRISPR genome editing, allows for the continuous tracking and analysis of EGFR protein levels and their degradation within live cells. This approach provides quantitative monitoring of protein dynamics in real time, offering insights that go beyond absolute protein levels to include aspects such as protein stability and degradation rate. Using this cell line model, we observed that AT13387 and H84T BanLec induce EGFR degradation in A549-HiBiT cells, with the results confirmed by immunoblotting. In contrast, Erlotinib, Osimertinib, and Cetuximab inhibit EGFR phosphorylation without altering total EGFR levels, as validated by the HiBiT luciferase assay. The NanoLuc-EGFR cell line marks a significant advancement in understanding protein regulation and serves as an instrumental platform for investigating targeted therapies that modulate protein kinases, especially those that induce protein degradation.

FBXW7 Facilitates Nonhomologous End-Joining via K63-Linked Polyubiquitylation of XRCC4.

FBXW7 is a haploinsufficient tumor suppressor with loss-of-function mutations occurring in human cancers. FBXW7 inactivation causes genomic instability, but the mechanism remains elusive. Here we show that FBXW7 facilitates nonhomologous end-joining (NHEJ) repair and that FBXW7 depletion causes radiosensitization. In response to ionizing radiation, ATM phosphorylates FBXW7 at serine 26 to recruit it to DNA double-strand break (DSB) sites, whereas activated DNA-PKcs phosphorylates XRCC4 at serines 325/326, which promotes binding of XRCC4 to FBXW7. SCF(FBXW7) E3 ligase then promotes polyubiquitylation of XRCC4 at lysine 296 via lysine 63 linkage for enhanced association with the Ku70/80 complex to facilitate NHEJ repair. Consistent with these findings, a small-molecule inhibitor that abrogates XRCC4 polyubiquitylation reduces NHEJ repair. Our study demonstrates one mechanism by which FBXW7 contributes to genome integrity and implies that inactivated FBXW7 in human cancers could be a strategy for increasing the efficacy of radiotherapy.

LSD1 destabilizes FBXW7 and abrogates FBXW7 functions independent of its demethylase activity.

FBXW7 acts as a typical tumor suppressor, with loss-of-function alterations in human cancers, by promoting ubiquitylation and degradation of many oncoproteins. Lysine-specific demethylase 1 (LSD1) is a well-characterized histone demethylase. Whether LSD1 has demethylase-independent activity remains elusive. Here we report that LSD1 directly binds to FBXW7 to destabilize FBXW7 independent of its demethylase activity. Specifically, LSD1 is a pseudosubstrate of FBXW7 and LSD1-FBXW7 binding does not trigger LSD1 ubiquitylation, but instead promotes FBXW7 self-ubiquitylation by preventing FBXW7 dimerization. The self-ubiquitylated FBXW7 is subjected to degradation by proteasome as well as lysosome in a manner dependent on autophagy protein p62/SQSTM1. Biologically, LSD1 destabilizes FBXW7 to abrogate its functions in growth suppression, nonhomologous end-joining repair, and radioprotection. Collectively, our study revealed a previously unknown activity of LSD1, which likely contributes to its oncogenic function. Targeting LSD1 protein, not only its demethylase activity, might be a unique approach for LSD1-based drug discovery for anticancer application.

The FBXW2-MSX2-SOX2 axis regulates stem cell property and drug resistance of cancer cells.

SOX2 is a key transcription factor that plays critical roles in maintaining stem cell property and conferring drug resistance. However, the underlying mechanisms by which SOX2 level is precisely regulated remain elusive. Here we report that MLN4924, also known as pevonedistat, a small-molecule inhibitor of neddylation currently in phase II clinical trials, down-regulates SOX2 expression via causing accumulation of MSX2, a known transcription repressor of SOX2 expression. Mechanistic characterization revealed that MSX2 is a substrate of FBXW2 E3 ligase. FBXW2 binds to MSX2 and promotes MSX2 ubiquitylation and degradation. Likewise, FBXW2 overexpression shortens the protein half-life of MSX2, whereas FBXW2 knockdown extends it. We further identified hypoxia as a stress condition that induces VRK2 kinase to facilitate MSX2-FBXW2 binding and FBXW2-mediated MSX2 ubiquitylation and degradation, leading to SOX2 induction via derepression. Biologically, expression of FBXW2 or SOX2 promotes tumor sphere formation, which is blocked by MSX2 expression. By down-regulating SOX2 through inactivation of FBXW2 E3 ligase, MLN4924 sensitizes breast cancer cells to tamoxifen in both in vitro and in vivo cancer cell models. Thus, a negative cascade of the FBXW2-MSX2-SOX2 axis was established, which regulates stem cell property and drug resistance. Finally, an inverse correlation of expression was found between FBXW2 and MSX2 in lung and breast cancer tissues. Collectively, our study revealed an anticancer mechanism of MLN4924. By inactivating FBXW2, MLN4924 caused MSX2 accumulation to repress SOX2 expression, leading to suppression of stem cell property and sensitization of breast cancer cells to tamoxifen.

Blockage of neddylation modification stimulates tumor sphere formation in vitro and stem cell differentiation and wound healing in vivo.

MLN4924, also known as pevonedistat, is the first-in-class inhibitor of NEDD8-activating enzyme, which blocks the entire neddylation modification of proteins. Previous preclinical studies and current clinical trials have been exclusively focused on its anticancer property. Unexpectedly, we show here, to our knowledge for the first time, that MLN4924, when applied at nanomolar concentrations, significantly stimulates in vitro tumor sphere formation and in vivo tumorigenesis and differentiation of human cancer cells and mouse embryonic stem cells. These stimulatory effects are attributable to (i) c-MYC accumulation via blocking its degradation and (ii) continued activation of EGFR (epidermal growth factor receptor) and its downstream pathways, including PI3K/AKT/mammalian target of rapamycin and RAS/RAF/MEK/ERK, via inducing EGFR dimerization. Finally, MLN4924 accelerates EGF-mediated skin wound healing in mouse and stimulates cell migration in an in vitro culture setting. Taking these data together, our study reveals that neddylation modification could regulate stem cell proliferation and differentiation and that a low dose of MLN4924 might have a therapeutic value for stem cell therapy and tissue regeneration.

Interleukin-6/signal transducer and activator of transcription 3 promotes prostate cancer resistance to androgen deprivation therapy via regulating pituitary tumor transforming gene 1 expression.

Prostate cancer can progress from androgen dependence to androgen deprivation resistance with some unknown mechanisms. The current study aims to explore the possible role of pituitary tumor transforming gene1 (PTTG1) in castration-resistant prostate cancer (CRPC). Initially, we found that PTTG1 expression was significantly increased in androgen-independent prostate cancer cell lines PC3, DU145 and CRPC specimens compared with that in androgen-dependent prostate cancer cell line LNCaP and initial prostate cancer specimens. PTTG1 overexpression significantly enhanced the cell survival rate, clonality and tumorigenicity in LNCaP cells upon androgen-deprivation therapy (ADT). While knockdown of PTTG1 expression significantly elevated the sensitivity of DU145 cells to ADT. The effects of PTTG1 overexpression on LNCaP cells may be ascribed to the induced EMT and increased CD44+ CD24- cancer stem cell population. Furthermore, we detected that PTTG1 expression was regulated by interleukin-6 via activated signal transducer and activator of transcription 3 (STAT3) directly binding to the region -500 to +1 of PTTG1 promoter in LNCaP cells. In conclusion, our results elucidate that interleukin-6/STAT3 activation can increase PTTG1 expression and, consequently, promote the resistance to ADT in CRPC by inducing EMT and increasing the cancer stem cell population, suggesting that PTTG1 may be a novel therapeutic target for CRPC.

MicroRNA-34a Attenuates Metastasis and Chemoresistance of Bladder Cancer Cells by Targeting the TCF1/LEF1 Axis.

BACKGROUND/AIMS: Chemoresistance is largely responsible for relapses of bladder cancer during clinical therapy. However, the molecular mechanisms involved in the chemoresistance of bladder cancer are unclear. Growing evidence supports the theory that microRNAs (miRNAs) play an important role in chemotherapeutic drug resistance because they are downregulated in many malignancies that have been implicated in the regulation of diverse processes in cancer cells. More specifically, the extent and precise mechanism of the involvement of miR-34as in chemoresistance to epirubicin (EPI) in the treatment of bladder cancer remains unclear. METHODS: In this study, real-time quantitative polymerase chain reaction (PCR) was used to analyze the expression of miR-34a in bladder cancer cell line BIU87 and its EPI chemoresistant cell line BIU87/ADR. The miR-34a profiles in bladder cancer tissues were obtained from The Cancer Genome Atlas database. The effect of miR-34a on chemosensitivity was evaluated by cell viability assays, colony formation assays, and in vivo experimentation. Apoptosis and the cell cycle were examined by flow cytometry. A luciferase reporter assay was used to assess the target genes of miR-34a. Western blot and qPCR were used to analyze the expression of target proteins and downstream molecules. RESULTS: The downregulation of miR-34a in bladder cancer serves as an independent predictor of reduced patient survival. The CCK-8 assay showed that miR-34a overexpression resulted in increased sensitivity to EPI, while miR-34a downregulation resulted in chemoresistance to EPI in vitro. Moreover, it was found that miR-34a increased the sensitivity of BIU87/ADR cells to chemotherapy in vivo. The luciferase reporter assay ascertained that TCF1 and LEF1 are direct target genes of miR-34a. It was found that miR-34a increased chemosensitivity in BIU87/ADR cells by inhibiting the TCF1/LEF1 axis. CONCLUSIONS: The results of this study indicate that miR-34a contributes to the chemosensitivity of BIU87/ADR by inhibiting the TCF1/LEF1 axis. Consequently, miR-34a is a determinant of BIU87 chemosensitivity and may therefore serve as a potential therapeutic target in bladder cancer treatment.

Depletion of SAG/RBX2 E3 ubiquitin ligase suppresses prostate tumorigenesis via inactivation of the PI3K/AKT/mTOR axis.

BACKGROUND: SAG (Sensitive to Apoptosis Gene), also known as RBX2, ROC2 or RNF7, is a RING component of CRL (Cullin-RING ligase), required for its activity. Our recent study showed that SAG/RBX2 co-operated with Kras to promote lung tumorigenesis, but antagonized Kras to inhibit skin tumorigenesis, suggesting a tissue/context dependent function of Sag. However, it is totally unknown whether and how Sag would play in prostate tumorigenesis, triggered by Pten loss. METHODS: Sag and Pten double conditional knockout mice were generated and prostate specific deletion of Sag and Pten was achieved by PB4-Cre, and their effect on prostate tumorigenesis was evaluated by H&E staining. The methods of immunohistochemistry (IHC) staining and Western blotting were utilized to examine expression of various proteins in prostate cancer tissues or cell lines. The effect of SAG knockdown in proliferation, survival and migration was evaluated in two prostate cancer cell lines. The poly-ubiquitylation of PHLPP1 and DEPTOR was evaluated by both in vivo and in vitro ubiquitylation assays. RESULTS: SAG is overexpressed progressively from early-to-late stage of human prostate cancer with the highest expression seen in metastatic lesion. Sag deletion inhibits prostate tumorigenesis triggered by Pten loss in a mouse model as a result of suppressed proliferation. SAG knockdown in human prostate cancer cells inhibits a) proliferation in monolayer and soft agar, b) clonogenic survival, and c) migration. SAG is an E3 ligase that promotes ubiquitylation and degradation of PHLPP1 and DEPTOR, leading to activation of the PI3K/AKT/mTOR axis, whereas SAG knockdown caused their accumulation. Importantly, growth suppression triggered by SAG knockdown was partially rescued by simultaneous knockdown of PHLPP1 or DEPTOR, suggesting their causal role. Accumulation of Phlpp1 and Deptor with corresponding inactivation of Akt/mTOR was also detected in Sag-null prostate cancer tissues. CONCLUSIONS: Sag is an oncogenic cooperator of Pten-loss for prostate tumorigenesis. Targeting SAG E3 ligase may, therefore, have therapeutic value for the treatment of prostate cancer associated with Pten loss.

The UBE2F-CRL5ASB11-DIRAS2 axis is an oncogene and tumor suppressor cascade in pancreatic cancer cells.

UBE2F, a neddylation E2, neddylates CUL5 to activate cullin-RING ligase-5, upon coupling with neddylation E3 RBX2/SAG. Whether and how UBE2F controls pancreatic tumorigenesis is previously unknown. Here, we showed that UBE2F is essential for the growth of human pancreatic cancer cells with KRAS mutation. In the mouse KrasG12D pancreatic ductal adenocarcinoma (PDAC) model, Ube2f deletion suppresses cerulein-induced pancreatitis, and progression of acinar-to-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia. Mechanistically, Ube2f deletion inactivates the Mapk-c-Myc signals via blocking ubiquitylation of Diras2, a substrate of CRL5Asb11 E3 ligase. Biologically, DIRAS2 suppresses growth and survival of human pancreatic cancer cells harboring mutant KRAS, and Diras2 deletion largely rescues the phenotypes induced by Ube2f deletion. Collectively, Ube2f or Diras2 plays a tumor-promoting or tumor-suppressive role in the mouse KrasG12D PDAC model, respectively. The UBE2F-CRL5ASB11 axis could serve as a valid target for pancreatic cancer, whereas the levels of UBE2F or DIRAS2 may serve as prognostic biomarkers for PDAC patients.

A molecularly engineered lectin destroys EGFR and inhibits the growth of non-small cell lung cancer.

Survival rates for non-small cell lung cancer (NSCLC) remain low despite the advent of novel therapeutics. Tyrosine kinase inhibitors (TKIs) targeting mutant epidermal growth factor receptor (EGFR) in NSCLC have significantly improved mortality but are plagued with challenges--they can only be used in the small fraction of patients who have susceptible driver mutations, and resistance inevitably develops. Aberrant glycosylation on the surface of cancer cells is an attractive therapeutic target as these abnormal glycosylation patterns are typically specific to cancer cells and are not present on healthy cells. H84T BanLec (H84T), a lectin previously engineered by our group to separate its antiviral activity from its mitogenicity, exhibits precision binding of high mannose, an abnormal glycan present on the surface of many cancer cells, including NSCLC. Here, we show that H84T binds to and inhibits the growth of diverse NSCLC cell lines by inducing lysosomal degradation of EGFR and leading to cancer cell death through autophagy. This is a mechanism distinct from EGFR TKIs and is independent of EGFR mutation status; H84T inhibited proliferation of both cell lines expressing wild type EGFR and those expressing mutant EGFR that is resistant to all TKIs. Further, H84T binds strongly to multiple and diverse clinical samples of both pulmonary adenocarcinoma and squamous cell carcinoma. H84T is thus a promising potential therapeutic in NSCLC, with the ability to circumvent the challenges currently faced by EGFR TKIs.

Sag/Rbx2 Partial Inactivation Sensitizes Mice to Radiation and Radiation-Induced Tumorigenesis1.

SAG (sensitive to apoptosis gene)/RBX2 (RING box-2), is the second family member of RING component of cullin-RING ligase (CRL) complex required for its enzymatic activity. Using total or conditional Sag knockout mouse models, we previously showed that Sag plays an essential role in embryonic development, apoptosis, vasculogenesis, angiogenesis and tumorigenesis. We also found that Sag-null ES cells are more sensitive to radiation. In this study, we generated the SagΔ/flneo mice with partial Sag inactivation due to deletion in one allele (Δ allele), and disrupted expression in the another (by a neo cassette). Compared to wild-type, SagΔ/fl-neo mice are more sensitive to a lethal dose of radiation with significantly shortened life span, resulting from an increased tissue damage with reduced proliferation and increased apoptosis in the intestines. Similar observations were made when SagΔ/fl-neo mice received a high dose of radiation directly delivered to the abdomen with reduced proliferation and prolonged DNA damage repair. Mechanistically, we found accumulations of Sag substrates, p21 and p27, explaining the proliferation defect. Finally, we found that SagΔ/fl-neo mice are more prone to tumorigenesis induced by a low dose of radiation with shortened life-span and increased incidence of lymphoma. Collectively, our study demonstrates that Sag protects mice from radiation-induced tissue damages and tumorigenesis.

RBX1/ROC1 disruption results in early embryonic lethality due to proliferation failure, partially rescued by simultaneous loss of p27.

RBX1 (RING box protein-1) or ROC1 (regulator of cullins-1) is the RING component of SCF (Skp1, Cullins, F-box proteins) E3 ubiquitin ligases, which regulate diverse cellular processes by targeting various substrates for degradation. However, the in vivo physiological function of RBX1 remains uncharacterized. Here, we show that a gene trap disruption of mouse Rbx1 causes embryonic lethality at embryonic day (E)7.5, mainly due to a failure in proliferation; p27, a cyclin dependent kinase inhibitor, normally undetectable in the early embryos, accumulates at high levels in the absence of Rbx1. Although mice heterozygous for the Rbx1 gene trap appear viable and fertile without obvious abnormalities, the Rbx1(+/Gt) MEFs do show retarded growth with G1 arrest and p27 accumulation. Simultaneous loss of p27 extended the life span of Rbx1(Gt/Gt) embryos from E6.5 to E9.5, indicating that p27-mediated cell cycle inhibition contributes to the early embryonic lethality in the Rbx1-deficient embryos. Our study demonstrates that the in vivo physiological function of RBX1 is to ensure cell proliferation by preventing p27 accumulation during the early stage of embryonic development.

The Sag-Shoc2 axis regulates conversion of mPanINs to cystic lesions in Kras pancreatic tumor model.

SAG/RBX2 is an E3 ligase, whereas SHOC2 is a RAS-RAF positive regulator. In this study, we address how Sag-Shoc2 crosstalk regulates pancreatic tumorigenesis induced by KrasG12D. Sag deletion increases the size of pancreas and causes the conversion of murine pancreatic intraepithelial neoplasms (mPanINs) to neoplastic cystic lesions with a mechanism involving Shoc2 accumulation, suggesting that Sag determines the pathological process via targeting Shoc2. Shoc2 deletion significantly inhibits pancreas growth, mPanIN formation, and acinar cell transdifferentiation, indicating that Shoc2 is essential for KrasG12D-induced pancreatic tumorigenesis. Likewise, in a primary acinar 3D culture, Sag deletion inhibits acinar-to-ductal transdifferentiation, while Shoc2 deletion significantly reduces the duct-like structures. Mechanistically, SAG is an E3 ligase that targets SHOC2 for degradation to affect both Mapk and mTorc1 pathways. Shoc2 deletion completely rescues the phenotype of neoplastic cystic lesions induced by Sag deletion, indicating physiological relevance of the Sag-Shoc2 crosstalk. Thus, the Sag-Shoc2 axis specifies the pancreatic tumor types induced by KrasG12D.

Transgenic expression of Sag/Rbx2 E3 causes early stage tumor promotion, late stage cytogenesis and acinar loss in the Kras-PDAC model.

SAG (Sensitive to Apoptosis Gene), also known as RBX2 or ROC2, is a RING component of CRL (Cullin-RING ligase), required for its activity. Our previous studies showed that Sag/Rbx2 co-operated with Kras or Pten loss to promote tumorigenesis in the lung and prostate, respectively, but antagonized Kras to inhibit skin tumorigenesis, suggesting a tissue/context dependent function of Sag. The role of SAG in KRAS-induced pancreatic tumorigenesis is unknown. In this study, we mined a cancer database and found that SAG is overexpressed in pancreatic cancer tissues and correlates with decreased patient survival. Whether Sag overexpression plays a causal role in pancreatic tumorigenesis is unknown. Here, we reported the generation of Sag transgenic mouse model alone (CS), or in combination with KrasG12D, driven by p48-Cre (KCS mice) for pancreatic specific Sag expression. Sag transgenic expression alone has no phenotypical abnormality, but in combination with KrasG12D promotes ADM (acinar-to-ductal metaplasia) conversion in vitro and mPanIN1 formation in vivo at the early stage, and impairs pancreatic functions at the late stage, as evidenced by poor glucose tolerance and significantly reduced α-Amylase activity, and induction of cytogenesis and acinar cell loss, eventually leading to atrophic pancreata and shortened mouse life-span. Mechanistically, Sag transgenic expression altered several key signaling pathways, particularly inactivation of mTORC1 signaling due to Deptor accumulation, and activation of the antioxidant Nrf2-Nqo1 axis. Thus, Sag plays a stage dependent promotion (early) and fate-changing (late) role during Kras-pancreatic tumorigenesis, likely via regulating its key substrates, which control growth-related signal transduction pathways.

Neddylation inactivation represses androgen receptor transcription and inhibits growth, survival and invasion of prostate cancer cells.

Androgen receptor (AR) and its constitutively active variants (AR-Vs) have been extensively implicated in the progression and recurrence of prostate cancer, making them attractive targets in the treatment of this disease. Whether and how neddylation modification regulates AR, and the therapeutic implications of this potential regulation, are relatively unexplored areas of investigation. Here we report that neddylation inactivation by the pharmacological inhibitor MLN4924 or Lenti-shRNA-based genetic knockdown of neddylation activating enzyme (NAE) selectively suppressed growth and survival of prostate cancer cells with minor, if any, effect on normal prostate epithelial cells. MLN4924 also significantly suppressed the invasive capacity of prostate cancer cells. Furthermore, compared to monotherapy, the combination of MLN4924 with AR antagonist or castration significantly enhanced growth suppression of prostate cancer cells in vitro, and tumor growth in an in vivo xenograft model. Mechanistically, MLN4924 repressed the transcription of AR/AR-V7 and its downstream targets, and blocked MMP2 and MMP9 expression. Taken together, our study reveals that the neddylation pathway positively regulates AR/AR-V7 transcription, and that the neddylation inhibitor MLN4924 has therapeutic potential for the treatment of aggressive prostate cancers.

SAG/ROC2/Rbx2 is a novel activator protein-1 target that promotes c-Jun degradation and inhibits 12-O-tetradecanoylphorbol-13-acetate-induced neoplastic transformation.

SAG (sensitive to apoptosis gene) was first identified as a stress-responsive protein that, when overexpressed, inhibited apoptosis both in vitro and in vivo. SAG was later found to be the second family member of ROC1 or Rbx1, a RING component of SCF and DCX E3 ubiquitin ligases. We report here that SAG/ROC2/Rbx2 is a novel transcriptional target of activator protein-1 (AP-1). AP-1 bound both in vitro and in vivo to two consensus binding sites in a 1.3-kb region of the mouse SAG promoter. The SAG promoter activity, as measured by luciferase reporter assay, was dependent on these sites. Consistently, endogenous SAG is induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) with an induction time course following the c-Jun induction in both mouse epidermal JB6-Cl.41 and human 293 cells. TPA-mediated SAG induction was significantly reduced in JB6-Cl.41 cells overexpressing a dominant-negative c-Jun, indicating a requirement of c-Jun/AP-1. On the other hand, SAG seemed to modulate the c-Jun levels. When overexpressed, SAG remarkably reduced both basal and TPA-induced c-Jun levels, whereas SAG small interfering RNA (siRNA) silencing increased substantially the levels of both basal and TPA-induced c-Jun. Consistently, SAG siRNA silencing reduced c-Jun polyubiquitination and blocked c-Jun degradation induced by Fbw7, an F-box protein of SCF E3 ubiquitin ligase. Finally, SAG overexpression inhibited, whereas SAG siRNA silencing enhanced, respectively, the TPA-induced neoplastic transformation in JB6-Cl.41 preneoplastic model. Thus, AP-1/SAG establishes an autofeedback loop, in which on induction by AP-1, SAG promotes c-Jun ubiquitination and degradation, thus inhibiting tumor-promoting activity of AP-1.

Inactivation of SAG or ROC1 E3 Ligase Inhibits Growth and Survival of Renal Cell Carcinoma Cells: Effect of BIM.

SAG (Sensitive to Apoptosis Gene) and ROC1 (Regulator of Cullin-1) are two family members of the RING component of CRL (Cullin RING ligase). Both members are essential for growth and survival of several types of human cancer cells; their role in renal cell carcinoma (RCC), however, remains elusive. Here we reported that compared to adjacent normal tissues, both SAG and ROC1 are overexpressed in RCC, which is positively correlated with poor patient survival, particularly for SAG. Depletion of SAG or ROC1 inhibited growth and survival of RCC cells by inducing G2/M arrest, senescence, and apoptosis likely due to accumulation of WEE1, p21, p27, NOXA, and BIM. Interestingly, simultaneous BIM knockdown in RCC cells partially rescues growth suppression triggered by depletion of SAG, but not ROC1, suggesting a differential role of BIM. Collectively, our study provides the proof-of-concept evidence that RING components of CRL are attractive candidates for targeted therapy of RCC.

FBXW2 suppresses migration and invasion of lung cancer cells via promoting ß-catenin ubiquitylation and degradation.

FBXW2 inhibits proliferation of lung cancer cells by targeting SKP2 for degradation. Whether and how FBXW2 regulates tumor invasion and metastasis is previously unknown. Here, we report that FBXW2 is an E3 ligase for ß-catenin. FBXW2 binds to ß-catenin upon EGF-AKT1-mediated phosphorylation on Ser552, and promotes its ubiquitylation and degradation. FBXW2 overexpression reduces ß-catenin levels and protein half-life, whereas FBXW2 knockdown increases ß-catenin levels, protein half-life and transcriptional activity. Functionally, FBXW2 overexpression inhibits migration and invasion by blocking transactivation of MMPs driven by ß-catenin, whereas FXBW2 knockdown promotes migration, invasion and metastasis both in vitro and in vivo lung cancer models. In human lung cancer specimens, while FBXW2 levels are inversely correlated with ß-catenin levels and lymph-node metastasis, lower FBXW2 coupled with higher ß-catenin, predict a worse patient survival. Collectively, our study demonstrates that FBXW2 inhibits tumor migration, invasion and metastasis in lung cancer cells by targeting ß-catenin for degradation.

Neddylation inhibitor MLN4924 suppresses cilia formation by modulating AKT1.

The primary cilium is a microtubule-based sensory organelle. The molecular mechanism that regulates ciliary dynamics remains elusive. Here, we report an unexpected finding that MLN4924, a small molecule inhibitor of NEDD8-activating enzyme (NAE), blocks primary ciliary formation by inhibiting synthesis/assembly and promoting disassembly. This is mainly mediated by MLN4924-induced phosphorylation of AKT1 at Ser473 under serum-starved, ciliary-promoting conditions. Indeed, pharmaceutical inhibition (by MK2206) or genetic depletion (via siRNA) of AKT1 rescues MLN4924 effect, indicating its causal role. Interestingly, pAKT1-Ser473 activity regulates both ciliary synthesis/assembly and disassembly in a MLN4924 dependent manner, whereas pAKT-Thr308 determines the ciliary length in MLN4924-independent but VHL-dependent manner. Finally, MLN4924 inhibits mouse hair regrowth, a process requires ciliogenesis. Collectively, our study demonstrates an unexpected role of a neddylation inhibitor in regulation of ciliogenesis via AKT1, and provides a proof-of-concept for potential utility of MLN4924 in the treatment of human diseases associated with abnormal ciliogenesis.