N-end rule pathway inhibitor sensitizes cancer cells to antineoplastic agents by regulating XIAP and RAD21 protein expression

Targeting mammalian N-end rule pathway for cancer therapy

Regulated protein degradation plays a crucial role in maintaining proteostasis along with protein refolding and compartmentalisation which collectively control biological functions. The N-end rule pathway is a major ubiquitin-dependent protein degradation system. The short-lived protein substrates containing destabilizing amino acid residues (N-degrons) are recognized by E3 ubiquitin ligases containing UBR box domains (N-recognin) for degradation. The dysregulated pathway fails to maintain the metabolic stability of the substrate proteins which leads to diseases. The mammalian substrates of this pathway are involved in many hallmarks of cancer such as resisting cell death, evading growth suppression, chromosomal instability, angiogenesis, and deregulation of cellular metabolism. Besides, mutations in E3 N-recognin have been detected in human cancers. In this review, we discuss the mammalian N-end rule pathway components, functions, and mechanism of degradation of substrates, and their implications in cancer pathogenesis. We also discuss the impact of pharmacological and genetic inhibition of this pathway component on cancer cells and chemoresistance. We further highlight how this pathway can be manipulated for selective protein degradation; for instance, using PROTAC technique. The challenges and future perspectives to utilize this pathway as a drug target for cancer therapy are also discussed.

UBR1 promotes anaplastic thyroid carcinoma progression via stabilizing YAP through monoubiquitylation

Anaplastic thyroid carcinoma (ATC) is a highly aggressive human malignancy without effective treatment. Yes-associated protein (YAP) is a critical effector of the Hippo pathway, which is essential in thyroid carcinogenesis. However, the underlying mechanisms of aberrant YAP expression in ATC are not completely understood. Ubiquitylation-related enzyme siRNA screening identified the ubiquitin protein ligase E3 component n-recognin 1 (UBR1) as a stabilizer of YAP in ATC cells. UBR1 deficiency reduced YAP protein levels and its target gene expression. UBR1 directly interacted with YAP and promoted its monoubiquitylation, competitively suppressing its polyubiquitylation and resulting in extended protein half-life. UBR1 depletion reduced ATC cell proliferation and migration in vitro. Xenograft tumor studies also suggested that UBR1 knockdown suppressed ATC cell growth in vivo. Furthermore, exogenous YAP expression partially reversed the inhibitive effects of UBR1 depletion on ATC cell proliferation and migration. Our studies demonstrated that UBR1 directly interacts with YAP and stabilized it in a monoubiquitylation-dependent manner, consequently promoting ATC tumorigenesis, suggesting that UBR1 might be a potentially therapeutic target for ATC treatment.

N-Terminal-Dependent Protein Degradation and Targeting Cancer Cells

Intracellular protein degradation is mediated selectively by the Ubiquitin-Proteasome System (UPS) and autophagic-lysosomal system in mammalian cells. Many cellular and physiological processes, such as cell division, cell differentiation, and cellular demise, are fine-tuned via the UPS-mediated protein degradation. Notably, impairment of UPS contributes to human disorders, including cancer and neurodegeneration. The proteasome- dependent N-degron pathways mediate the degradation of proteins through their destabilizing aminoterminal residues. Recent advances unveiled that targeting N-degron proteolytic pathways can aid in sensitizing some cancer cells to chemotherapeutic agents. Furthermore, interestingly, exploiting the N-degron feature, the simplest degradation signal in mammals, and fusing it to a ligand specific for Estrogen-Related Receptor alpha (ERRa) has demonstrated its utility in ERRa knockdown, via N-terminal dependent degradation, and also its efficiency in the inhibition of growth of breast cancer cells. These recent advances uncover the therapeutic implications of targeting and exploiting N-degron proteolytic pathways to curb growth and migration of cancer cells.

MFAP2 Promotes the Proliferation of Cancer Cells and Is Associated With a Poor Prognosis in Hepatocellular Carcinoma

Backgrounds:

Microfibril-associated protein 2 (MFAP2) is an extracellular matrix protein that regulates the function of microfibrils by interacting with fibrillin. MFAP2 has been reported to play an important role in metabolic diseases and has been shown to be significantly overexpressed in head and neck squamous cell carcinoma and Hepatocellular carcinoma (HCC). However, the molecular function and prognostic value of MFAP2 have never been reported in HCC or other tumors.

Methods:

In the present study, expression characteristics of MFAP2 in HCC, its influence on the development of HCC, as well as its function and potential mechanism in HCC were verified by Quantitative reverse transcription-polymerase chain reaction, bioinformatics data mining and in vitro cell experiments.

Results:

MFAP2 was prominently high-expressed in HCC and associated with cancer stages. HCC patients with higher MFAP2 expression displayed lower overall survival (OS) and disease-specific survival(DSS), while there was no significant difference in recurrence-free survival (RFS). In vitro experiments showed that downregulation of MFAP2 inhibited proliferation, migration level of HCC cells. Transcription factors, DNA methyltransferases, immune factors may interact with MFAP2 mRNA to promote tumor progression in HCC.

Conclusion:

These findings suggest that MFAP2 may play a key role in the development of HCC. Therefore, MFAP2 may be a valuable prognostic marker and an effective anticancer target in HCC.

UBR-box containing protein, UBR5, is over-expressed in human lung adenocarcinoma and is a potential therapeutic target

BACKGROUND

N-end rule ubiquitination pathway is known to be disrupted in many diseases, including cancer. UBR5, an E3 ubiquitin ligase, is mutated and/or overexpressed in human lung cancer cells suggesting its pathological role in cancer.

METHODS

We determined expression of UBR5 protein in multiple lung cancer cell lines and human patient samples. Using immunoprecipitation followed by mass spectrometry we determined the UBR5 interacting proteins. The impact of loss of UBR5 for lung adenocarcinoma cell lines was analyzed using cell viability, clonogenic assays and in vivo xenograft models in nude mice. Additional Western blot analysis was performed to assess the loss of UBR5 on downstream signaling. Statistical analysis was done by one-way ANOVA for in vitro studies and Wilcoxon paired t-test for in vivo tumor volumes.

RESULTS

We show variability of UBR5 expression levels in lung adenocarcinoma cell lines and in primary human patient samples. To gain better insight into the role that UBR5 may play in lung cancer progression we performed unbiased interactome analyses for UBR5. Data indicate that UBR5 has a wide range of interacting protein partners that are known to be involved in critical cellular processes such as DNA damage, proliferation and cell cycle regulation. We have demonstrated that shRNA-mediated loss of UBR5 decreases cell viability and clonogenic potential of lung adenocarcinoma cell lines. In addition, we found decreased levels of activated AKT signaling after the loss of UBR5 in lung adenocarcinoma cell lines using multiple means of UBR5 knockdown/knockout. Furthermore, we demonstrated that loss of UBR5 in lung adenocarcinoma cells results in significant reduction of tumor volume in nude mice.

CONCLUSIONS

These findings demonstrate that deregulation of the N-end rule ubiquitination pathway plays a crucial role in the etiology of some human cancers, and blocking this pathway via UBR5-specific inhibitors, may represent a unique therapeutic target for human cancers.