Radiosensitization of Cancer Cells by Inactivation of Cullin-RING E3 Ubiquitin Ligases

Targeting NEDD8-activating enzyme for cancer therapy: developments, clinical trials, challenges and future research directions

NEDDylation, a post-translational modification through three-step enzymatic cascades, plays crucial roles in the regulation of diverse biological processes. NEDD8-activating enzyme (NAE) as the only activation enzyme in the NEDDylation modification has become an attractive target to develop anticancer drugs. To date, numerous inhibitors or agonists targeting NAE have been developed. Among them, covalent NAE inhibitors such as MLN4924 and TAS4464 currently entered into clinical trials for cancer therapy, particularly for hematological tumors. This review explains the relationships between NEDDylation and cancers, structural characteristics of NAE and multistep mechanisms of NEDD8 activation by NAE. In addition, the potential approaches to discover NAE inhibitors and detailed pharmacological mechanisms of NAE inhibitors in the clinical stage are explored in depth. Importantly, we reasonably investigate the challenges of NAE inhibitors for cancer therapy and possible development directions of NAE-targeting drugs in the future.

Smurf2 inhibition enhances chemotherapy and radiation sensitivity in non-small-cell lung cancer

Lung cancer has been the most common cancer worldwide for several decades. The outcomes of patients with locally advanced lung cancer remain dismal, and only a minority of patients survive more than 5 years. However, tumor therapeutic resistance mechanisms are poorly studied. Identification of therapeutic resistance pathways in lung cancer in order to increase the sensitivity of lung tumor cells to therapeutic agents is a crucial but challenging need. To identify novel genes that modulate the response to platinum-based therapy, we performed a genome-wide high-throughput ribonucleic acid interference (RNAi) screen via transfection of human lung cancer (PC9) cells with a viral short hairpin RNA (shRNA) library. We further validated a potential target via 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and clonogenic survival assays on PC9 and A549 lung tumor cells transfected with small interfering RNAs (siRNAs) to successfully downregulate protein expression and then treated with increasing doses of cisplatin or X-ray radiation. We determined protein expression by immunohistochemistry (IHC) after chemoradiotherapy and analyzed gene expression-based survival outcomes in two cohorts of human non-small-cell lung cancer (NSCLC) patients. The screen identified several targets involved in epithelial-to-mesenchymal transition (EMT), including Smurf1, Smurf2, YAP1, and CEBPZ, and glycolytic pathway proteins, including PFKFB3. Furthermore, we found that the small molecule proteasome inhibitor bortezomib significantly downregulated Smurf2 in lung cancer cells. The addition of bortezomib in combination with cisplatin and radiation therapy in PC9 and A549 cells led to an increase in deoxyribonucleic acid (DNA) double-strand breaks with increased numbers of γ-H2AX-positive cells and upregulation of apoptosis. Finally, we found that Smurf2 protein expression was upregulated in situ after treatment with cisplatin and radiation therapy in a relevant cohort of patients with stage III NSCLC. Additionally, Smurf2 gene expression was the strongest predictor of survival in patients with squamous NSCLC after chemotherapy or chemoradiotherapy. We successfully identified and validated Smurf2 as both a common modulator of resistance and an actionable target in lung cancer. These results suggest the urgent need to investigate clinical Smurf2 inhibition via bortezomib in combination with cisplatin and radiation for patients with locally advanced NSCLC.

Cullin-RING Ligases as Promising Targets for Gastric Carcinoma Treatment

Gastric carcinoma has serious morbidity and mortality, which seriously threats human health. The studies on gastrointestinal cell biology have shown that the ubiquitination modification that occurs after protein translation plays an essential role in the pathogenesis of gastric carcinoma. Protein ubiquitination is catalyzed by E3 ubiquitin ligase and can regulate various substrate proteins in different cellular pathways. Cullin-RING E3 ligase (CRLs) is a representative of the E3 ubiquitin ligase family, which requires cullin (CUL) neddylation modification for activation to regulate homeostasis of ~20% of cellular proteins. The substrate molecules regulated by CRLs are often involved in many cell progressions such as cell cycle progression, cell apoptosis, DNA damage and repair. Given that CRLs play an important role in modulation of biological activities, so targeting a certain CULs member neddylation may be an attractive strategy for selectively controlling the cellular proteins levels to achieve the goal of cancer treatment. In this review, we will discuss the roles of CULs and Ring protein in gastric carcinoma and summarize the current neddylation modulators for gastric carcinoma treatment.

Ring box protein-1 is associated with a poor prognosis and tumor progression in esophageal cancer

Ring box protein-1 (RBX1) is an essential component of the S-phase kinase-associated protein, Cullin and F-box containing ubiquitin ligases. Overexpression of RBX1 has been reported in several cancer types; however, little is known regarding the prognostic value and role of RBX1 in esophageal cancer. The present study examined 120 patients with esophageal cancer (EC) who underwent curative esophagectomy and 61 patients with EC who underwent neoadjuvant combination chemotherapy with docetaxel, cisplatin and 5-fluorouracil (5-FU; DCF) using immunohistochemistry. All specimens were classified into two groups according to the percentage of RBX1-positive tumor cells. In addition, the impact of RBX1 expression on cancer cell proliferation was analyzed in vitro using a small interfering RNA silencing technique. RBX1 expression levels showed significant differences according to tumor size (P<0.001), tumor depth (P=0.002), lymph node metastasis (P=0.004), pathological stage (P=0.001), lymphatic invasion (P=0.001) and venous invasion (P=0.001). The overall survival (OS) rate in the RBX1 high expression group was significantly lower compared with that in the low group (P=0.004). Multivariate analysis demonstrated that RBX1 status was an independent prognostic factor. RBX1 gene silencing inhibited the proliferation of human EC cells and enhanced the antitumor effect of 5-FU. Among patients who underwent neoadjuvant DCF therapy, the RBX1 high expression group had a significantly lower OS rate compared with that of the RBX1-low group (P<0.001). In conclusion, RBX1 has notable prognostic value, and RBX1 may serve an important function in the tumor progression of EC.

Radiosensitization by the investigational NEDD8-activating enzyme inhibitor MLN4924 (pevonedistat) in hormone-resistant prostate cancer cells

Salvage radiotherapy (SRT) is the first-line treatment for prostate cancer patients with biochemical recurrence following radical prostatectomy, and new specific radiosensitizers are in urgent need to enhance SRT effect. MLN4924 (also known as Pevonedistat), a specific inhibitor of NEDD8-activating enzyme, has recently entered phase I/II clinical trials in several malignancies. By inhibiting cullin neddylation, MLN4924 inactivates Cullin-RING ligases (CRL), which have been validated as an attractive radiosensitizing target. In our study, we demonstrate that MLN4924 can be used as a potent radiosensitizer in hormone-resistant prostate cancer cells. We found that MLN4924 inhibited cullin neddylation and sensitized prostate cancer cells to irradiation (IR). Mechanistically, MLN4924 enhanced IR-induced G2 cell-cycle arrest, by inducing accumulation of WEE1/p21/p27, three well-known CRL substrates. Importantly, siRNA knockdown of WEE1/p21/p27 partially abrogated MLN4924-induced G2 cell-cycle arrest, indicating a causal role of WEE1/p21/p27 in MLN4924-induced radiosensitization. Further mechanistic studies revealed that induction of DNA damage and apoptosis also contributed to MLN4924 radiosensitization in hormone-resistant prostate cancer cells. Our findings lay the foundation for future application of MLN4924 as a potential radiosensitizer in hormone refractory prostate cancer (HRPC).

MLN4924 protects against bleomycin-induced pulmonary fibrosis by inhibiting the early inflammatory process

Pulmonary fibrosis is a complex pathological process characterized by massive destruction of the structure of lung tissues and aggravated pulmonary function impairment. The underlying mechanisms of pulmonary fibrosis are incompletely understood and therefore limited treatment options are available currently. Here, we report that MLN4924, an NEDD8 activation enzyme (NAE) activity-inhibiting molecule, blocks the maintenance and progression of established pulmonary fibrosis. We found that MLN4924 acts against bleomycin-induced pulmonary fibrosis mainly at the early inflammatory stage. Pharmacologically targeting the neddylation of Cullin-Ring E3 ligase (CRL) by MLN4924, significantly abrogated NF-κB responses, suppressed MAPK activity, and reduced secretion of TNF-α-elicited pro-inflammatory cytokines and MCP1-induced chemokines. MLN4924 inhibited pro-inflammatory responses while maintaining or increasing the production of the anti-inflammatory mediators such as anti-inflammatory interleukins (ILs) following bleomycin administration, which is closely correlated to its blocking NF-κB-mediated signaling. Consistently, our studies identified MLN4924 as a promising therapeutic drug for pulmonary fibrosis and suggested a potential role of MLN4924 that fine tunes the MAPK signaling pathway controlling the inflammatory reactions at the early stages of pulmonary fibrosis. In addition, our findings may broaden the potential practical application of MLN4924 as an effective therapeutic strategy against other inflammation-associated diseases.

MLN4924 therapy as a novel approach in cancer treatment modalities

MLN4924 is an investigational and a newly discovered small molecule that is a potent and selective inhibitor of the NEDD8 (Neural precursor cell-Expressed Developmentally down-regulated 8) Activating Enzyme (NAE), a pivotal regulator of the Cullin Ring Ligases E3 (CRL), which has been implicated recently in DNA damage. MLN4924 effectively inhibits tumour cell growth by inducing all three common types of death, namely apoptosis, autophagy and senescence and it was also reported that the formation of capillary vessels was significantly suppressed by MLN4924.In this review, we are going to highlight the molecular mechanism of MLN4924 in cancer therapy and its pros and cons in cancer therapy.

MLN4924, a Novel NEDD8-activating enzyme inhibitor, exhibits antitumor activity and enhances cisplatin-induced cytotoxicity in human cervical carcinoma: in vitro and in vivo study

MLN4924, an inhibitor of NEDD8 activating enzyme (NAE), has been reported to have activity against various malignancies. Here, we investigated the antitumor properties of MLN4924 and MLN4924 in combination with cisplatin on human cervical carcinoma (CC) in vitro and in vivo. Two human CC cell lines, ME-180 and HeLa, were used in this study. The cytotoxic effects of MLN4924 and/or cisplatin were measured by cell viability (MTT), proliferation (BrdU incorporation), apoptosis (flow cytometry with annexin V-FITC labeling), and the expression of cell apoptosis-related proteins (Western blotting). In vivo efficacy was determined in Nu/Nu nude mice with ME-180 and HeLa xenografts. The results showed that MLN4924 elicited viability inhibition, anti-proliferation and apoptosis in human CC cells, accompanied by activations of apoptosis-related molecules and Bid, Bcl-2 phosphorylation interruption, and interference with cell cycle regulators. Moreover, MLN4924 caused an endoplasmic reticulum stress response (caspase-4, ATF-4 and CHOP activations) and expression of other cellular stress molecules (JNK and c-Jun activations). Additionally, MLN4924 suppressed growth of CC xenografts in nude mice. Furthermore, we demonstrated that MLN4924 potentiated cisplatin-induced cytotoxicity in CC cells with activation of caspases. Consistently with this, MLN4924 significantly enhanced cisplatin-induced growth inhibition of CC xenografts. Together, these findings suggest that MLN4924 alone or in combination with cisplatin is of value in treating human CCs.

Upregulated expression of ubiquitin-conjugating enzyme E2Q1 (UBE2Q1) is associated with enhanced cell proliferation and poor prognosis in human hapatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer in the world. Ubiquitin-proteasome system has been shown to play a pivotal role in the pathophysiology of HCC and other malignancies. UBE2Q1 is a putative E2 ubiquitin conjugating enzyme, and may be involved in the regulation of cancer-related proteins. In this study, we investigated the expression pattern of UBE2Q1 in HCC cell lines and human HCC specimens, and its potential clinical and biological significance in HCC. Western blot and immunohistochemical analyses revealed that UBE2Q1 was significantly upregulated in HCC tumorous tissues compared with the adjacent noncancerous ones. Next, univariate and multivariate survival analyses were performed to determine the prognostic significance of UBE2Q1 in HCC. The results showed that upregulated expression of UBE2Q1 was positively correlated with high histological grades of HCC and predicted poor prognosis. In addition, the expression of UBE2Q1 was progressively increased in serum-refed HCC cells. UBE2Q1 depletion by small interfering RNA inhibited cell proliferation and led to G1 phase arrest in HepG2 and BEL-7404 cells. Furthermore, we showed that cells transfected with UBE2Q1-targeting siRNA resulted in significant increase in the levels of p53, p21 in HepG2 and BEL-7404 cells. These data imply that UBE2Q1 is upregulated in liver cancer cell lines and tumorous samples and may play a role in the development of HCC.

Ubiquitination involved enzymes and cancer

Ubiquitination is a post-translational modification process that regulates multiple cell functions. It also plays important roles in the development of cancer. Mechanistically, ubiquitination is a complex process that is comprised of a series of events involving ubiquitin-activating enzymes, ubiquitin-conjugating enzymes and ubiquitin ligases. In general, covalent attachment of ubiquitin to the target proteins marks them for degradation. Dysregulation of the ubiquitination process may cause carcinogenesis. In this review, we summarize recent developments in understanding the relationship between ubiquitination enzymes and carcinogenesis.

Improving the efficacy of chemoradiation with targeted agents

Chemoradiation is the standard therapy for the majority of inoperable, locally advanced cancers. Although there is a need to improve chemoradiation efficacy, normal-tissue toxicity limits our ability to give additional chemotherapy or higher doses of radiation. Thus, there is excitement about the addition of molecularly targeted agents, which tend to be less toxic than chemotherapy, to chemoradiation regimens. Unfortunately, initial empiric attempts have not been successful. This review will focus on the evidence that supports rational combinations of targeted agents with chemoradiation, with an emphasis on agents that target the DNA damage response and radiation-induced membrane signaling.

Ubiquitin E3 ligase CRL4(CDT2/DCAF2) as a potential chemotherapeutic target for ovarian surface epithelial cancer

Cullin-RING ubiquitin ligases (CRLs) are the largest family of E3 ligases and require cullin neddylation for their activation. The NEDD8-activating enzyme inhibitor MLN4924 reportedly blocked cullin neddylation and inactivated CRLs, which resulted in apoptosis induction and tumor suppression. However, CRL roles in ovarian cancer cell survival and the ovarian tumor repressing effects of MLN4924 are unknown. We show here that CRL4 components are highly expressed in human epithelial ovarian cancer tissues. MLN4924-induced DNA damage, cell cycle arrest, and apoptosis in ovarian cancer cells in a time- and dose-dependent manner. In addition, MLN4924 sensitized ovarian cancer cells to other chemotherapeutic drug treatments. Depletion of CRL4 components Roc1/2, Cul4a, and DDB1 had inhibitory effects on ovarian cancer cells similar to MLN4924 treatment, which suggested that CRL4 inhibition contributed to the chemotherapeutic effect of MLN4924 in ovarian cancers. We also investigated for key CRL4 substrate adaptors required for ovarian cancer cells. Depleting Vprbp/Dcaf1 did not significantly affect ovarian cancer cell growth, even though it was expressed by ovarian cancer tissues. However, depleting Cdt2/Dcaf2 mimicked the pharmacological effects of MLN4924 and caused the accumulation of its substrate, CDT1, both in vitro and in vivo. MLN4924-induced DNA damage and apoptosis were partially rescued by Cdt1 depletion, suggesting that CRL4(CDT2) repression and CDT1 accumulation were key biochemical events contributing to the genotoxic effects of MLN4924 in ovarian cancer cells. Taken together, these results indicate that CRL4(CDT2) is a potential drug target in ovarian cancers and that MLN4924 may be an effective anticancer agent for targeted ovarian cancer therapy.

Functional characterization of SAG/RBX2/ROC2/RNF7, an antioxidant protein and an E3 ubiquitin ligase

SAG (Sensitive to Apoptosis Gene), also known as RBX2 (RING box protein 2), ROC2 (Regulator of Cullins 2), or RNF7 (RING Finger Protein 7), was originally cloned in our laboratory as a redox inducible antioxidant protein and later characterized as the second member of the RBX/ROC RING component of the SCF (SKP1-CUL-F-box Proteins) E3 ubiquitin ligase. When acting alone, SAG scavenges oxygen radicals by forming inter- and intra-molecular disulfide bonds, whereas by forming a complex with other components of the SCF E3 ligase, SAG promotes ubiquitination and degradation of a number of protein substrates, including c-JUN, DEPTOR, HIF-1α, IκBα, NF1, NOXA, p27, and procaspase-3, thus regulating various signaling pathways and biological processes. Specifically, SAG protects cells from apoptosis, confers radioresistance, and plays an essential and non-redundant role in mouse embryogenesis and vasculogenesis. Furthermore, stress-inducible SAG is overexpressed in a number of human cancers and SAG overexpression correlates with poor patient prognosis. Finally, SAG transgenic expression in epidermis causes an early stage inhibition, but later stage promotion, of skin tumorigenesis triggered by DMBA/TPA. Given its major role in promoting targeted degradation of tumor suppressive proteins, leading to apoptosis suppression and accelerated tumorigenesis, SAG E3 ligase appears to be an attractive anticancer target.