A novel oncoprotein Pirh2: rising from the shadow of MDM2

Pirh2-dependent DNA damage in neurons induced by the G-quadruplex ligand pyridostatin

Noncanonical base pairing between four guanines (G) within single-stranded G-rich sequences leads to formation of а G-quartet. Self-stacking of G-quartets results in a columnar four-stranded DNA structure known as the G-quadruplex (G4 or G4-DNA). In cancer cells, G4-DNA regulates multiple DNA-dependent processes, including transcription, replication, and telomere function. How G4s function in neurons is poorly understood. Here, we performed a genome-wide gene expression analysis (RNA-Seq) to identify genes modulated by a G4-DNA ligand, pyridostatin (PDS), in primary cultured neurons. PDS promotes stabilization of G4 structures, thus allowing us to define genes directly or indirectly responsive to G4 regulation. We found that 901 genes were differentially expressed in neurons treated with PDS out of a total of 18,745 genes with measured expression. Of these, 505 genes were downregulated and 396 genes were upregulated and included gene networks regulating p53 signaling, the immune response, learning and memory, and cellular senescence. Within the p53 network, the E3 ubiquitin ligase Pirh2 (Rchy1), a modulator of DNA damage responses, was upregulated by PDS. Ectopically overexpressing Pirh2 promoted the formation of DNA double-strand breaks, suggesting a new DNA damage mechanism in neurons that is regulated by G4 stabilization. Pirh2 downregulated DDX21, an RNA helicase that unfolds G4-RNA and R-loops. Finally, we demonstrated that Pirh2 increased G4-DNA levels in the neuronal nucleolus. Our data reveal the genes that are responsive to PDS treatment and suggest similar transcriptional regulation by endogenous G4-DNA ligands. They also connect G4-dependent regulation of transcription and DNA damage mechanisms in neuronal cells.

Recognition of an Ala-rich C-degron by the E3 ligase Pirh2

The ribosome-associated quality-control (RQC) pathway degrades aberrant nascent polypeptides arising from ribosome stalling during translation. In mammals, the E3 ligase Pirh2 mediates the degradation of aberrant nascent polypeptides by targeting the C-terminal polyalanine degrons (polyAla/C-degrons). Here, we present the crystal structure of Pirh2 bound to the polyAla/C-degron, which shows that the N-terminal domain and the RING domain of Pirh2 form a narrow groove encapsulating the alanine residues of the polyAla/C-degron. Affinity measurements in vitro and global protein stability assays in cells further demonstrate that Pirh2 recognizes a C-terminal A/S-X-A-A motif for substrate degradation. Taken together, our study provides the molecular basis underlying polyAla/C-degron recognition by Pirh2 and expands the substrate recognition spectrum of Pirh2.

Mechanism and Disease Association With a Ubiquitin Conjugating E2 Enzyme: UBE2L3

Ubiquitin conjugating enzyme E2 is an important component of the post-translational protein ubiquitination pathway, which mediates the transfer of activated ubiquitin to substrate proteins. UBE2L3, also called UBcH7, is one of many E2 ubiquitin conjugating enzymes that participate in the ubiquitination of many substrate proteins and regulate many signaling pathways, such as the NF-κB, GSK3β/p65, and DSB repair pathways. Studies on UBE2L3 have found that it has an abnormal expression in many diseases, mainly immune diseases, tumors and Parkinson's disease. It can also promote the occurrence and development of these diseases. Resultantly, UBE2L3 may become an important target for some diseases. Herein, we review the structure of UBE2L3, and its mechanism in diseases, as well as diseases related to UBE2L3 and discuss the related challenges.

The emerging roles of E3 ubiquitin ligases in ovarian cancer chemoresistance

Epithelial cancer of the ovary exhibits the highest mortality rate of all gynecological malignancies in women today, since the disease is often diagnosed in advanced stages. While the treatment of cancer with specific chemical agents or drugs is the favored treatment regimen, chemotherapy resistance greatly impedes successful ovarian cancer chemotherapy. Thus, chemoresistance becomes one of the most critical clinical issues confronted when treating patients with ovarian cancer. Convincing evidence hints that dysregulation of E3 ubiquitin ligases is a key factor in the development and maintenance of ovarian cancer chemoresistance. This review outlines recent advancement in our understanding of the emerging roles of E3 ubiquitin ligases in ovarian cancer chemoresistance. We also highlight currently available inhibitors targeting E3 ligase activities and discuss their potential for clinical applications in treating chemoresistant ovarian cancer patients.

Inverse correlation between Naa10p and Pirh2 expression and the combined prognostic value in oral squamous cell carcinoma patients

BACKGROUND

This study aims to explore the associations between N-α-acetyltransferase 10 protein (Naa10p) and p53-induced protein with a RING-H2 domain (Pirh2) expression and clinicopathological characteristics in oral squamous cell carcinoma (OSCC).

METHODS

Immunohistochemistry was performed to detect Naa10p and Pirh2 levels containing 118 OSCC specimens, and additional analyses were used to determine correlations between Naa10p and Pirh2 expressions, generate survival curves, and perform univariate and multivariate statistical analyses. Further, quantitative real-time PCR (qRT-PCR) and western blot were employed to examine Naa10p and Pirh2 expression level in OSCC patients' samples. We further validated the result using RNAseq data from The Cancer Genome Atlas (TCGA) and mRNA array data from GSE31056 and GSE30784.

RESULTS

Naa10p and Pirh2 are overexpression, and the protein level of Naa10p was negatively correlated with that of Pirh2 in OSCC tissues. Multivariate Cox proportional hazard regression analysis showed that positive Naa10p expression and negative Pirh2 expression were both independent good prognostic factors for OSCC patients. Furthermore, the Naa10p-positive/Pirh2-negative group has the best prognosis among all OSCC patients. Results from qRT-PCR showed the higher expression level of Naa10 and lower expression level of Pirh2 in tumor tissues than adjacent normal tissues. TCGA database and data from GSE31056 and GSE30784 showed the similar result. The correlation analysis showed that the mRNA level of Naa10 was negatively correlated that of Pirh2.

CONCLUSION

The expression of Naa10p is negatively correlated with that of Pirh2, and positive Naa10p and negative Pirh2 might be independent biomarkers for better OSCC prognoses.

Regulation of p27 by ubiquitin ligases and its pathological significance in human lung carcinomas

Down-regulation of cyclin-dependent kinase inhibitor protein p27, due to enhanced degradation, is frequently observed in various cancers. The ubiquitin ligases that mediate this degradation have been identified as S-phase kinase-associated protein-2 (Skp2), Kip1 ubiquitylation-promoting complex (KPC), and p53-inducible protein with RING-H2 domain (Pirh2) as well. We investigated the correlation among expression of these 3 ligases and p27 status in surgical specimens of human lung carcinomas by immunohistochemical analysis. Among 93 cases, expressions of p27, Skp2, KPC, and Pirh2 were found in 89.2%, 59.1%, 59.1%, and 67.7%, respectively. Down-regulation of p27 in cancer cells was frequently observed in adenocarcinoma (AC) and squamous cell carcinoma (SCC), but not in small cell carcinoma (SmCC). Overexpression of ubiquitin ligases was variously observed among histological types: Skp2 was more frequently observed in SCC and SmCC, KPC in SCC and Pirh2 in AC, followed by SCC. Several novel findings were obtained: (i) cytoplasmic p27 was observed in 8.6%, most frequently in SCC (13.3%), and correlated with nodal metastasis (P=.0044), (ii) significant inverse correlation between nuclear p27 and Pirh2 expression was observed by statistical analysis and at the cellular level, and (iii) cytoplasmic Pirh2 and total (cytoplasmic and/or nuclear) Pirh2 were significantly correlated with the nodal status (P=.0225, 0.0314), the pathological stage (P=.0213, 0.0475) and recurrence-free survival (P=.0194, 0.0482, respectively) in AC. Altogether, our data suggests that p27 and its cognate ubiquitin ligases are specifically involved in the clinical profiles, and thus, molecular targeting of these ubiquitin ligases, in particular, Pirh2, may have therapeutic value for human lung carcinomas.

E3 ubiquitin ligase Pirh2 enhances tumorigenic properties of human non-small cell lung carcinoma cells

The product of RCHY1 human gene, Pirh2, is a RING-finger containing E3 ligase that modifies p53 with ubiquitin residues resulting in its subsequent degradation in proteasomes. Transcription of RCHY1 is regulated by p53 itself thus forming a negative regulatory feedback loop. Functionally, by eliminating p53, Pirh2 facilitates tumorigenesis. However, the role of Pirh2 in cancer cells lacking p53 is yet not well understood. Therefore, we decided to elucidate the role of Pirh2 in p53-negative human non-small cell lung carcinoma cells, H1299. We found that ectopic expression of Pirh2 enhanced cell proliferation, resistance to doxorubicin, and increased migration potential. Ablation of Pirh2 by specific shRNA reversed these phenotypes. Mechanistically, Pirh2 increased mRNA and protein levels of the c-Myc oncogene. The bioinformatics data indicate that co-expression of both c-Myc and Pirh2 strongly correlated with poor survival of lung cancer patients. Collectively, our results suggest that Pirh2 can be considered as a potential pharmacological target for developing anticancer therapies to treat p53-negative cancers.

Downregulated PIRH2 Can Decrease the Proliferation of Breast Cancer Cells

BACKGROUND AND AIMS

We undertook this study to investigate the influence of PIRH2 (p53-induced RING-H2) protein on the proliferation and cell cycle of breast cancer cell lines.

METHODS

PIRH2 expression was detected by Western blot analysis, immunohistochemistry (IHC) and Kaplan-Meier curve analysis. Cell proliferation was assessed by cell counting kit-8 (CCK-8). Cell cycle control was analyzed by flow cytometry.

RESULTS

PIRH2 was up-regulated in breast cancer tissues and cell lines and up-regulated PIRH2 was highly associated with tumor size, grade, ER, and Ki-67. Moreover, Kaplan-Meier curve showed that up-regulated PIRH2 was related to the poor overall survival of patients with breast carcinoma. When the expression of PIRH2 was inhibited by siRNA transfection, cell proliferation was reduced. In addition, the number of G0/G1 phase cells was increased, but G2/M cells were not affected significantly.

CONCLUSION

Decrease of PIRH2 expression in the breast cancer cell line MDA-MB-231 resulted in reduced tumor cell growth via the inhibition of cell proliferation and the interruption of cell cycle transition.

Molecular Analysis of the HOXA2-Dependent Degradation of RCHY1

The homeodomain transcription factor Hoxa2 interacts with the RING-finger type E3 ubiquitin ligase RCHY1 and induces its proteasomal degradation. In this work, we dissected this non-transcriptional activity of Hoxa2 at the molecular level. The Hoxa2-mediated decay of RCHY1 involves both the 19S and 20S proteasome complexes. It relies on both the Hoxa2 homeodomain and C-terminal moiety although no single deletion in the Hoxa2 sequence could disrupt the RCHY1 interaction. That the Hoxa2 homeodomain alone could mediate RCHY1 binding is consistent with the shared ability all the Hox proteins we tested to interact with RCHY1. Nonetheless, the ability to induce RCHY1 degradation although critically relying on the homeodomain is not common to all Hox proteins. This identifies the homeodomain as necessary but not sufficient for what appears to be an almost generic Hox protein activity. Finally we provide evidence that the Hoxa2-induced degradation of RCHY1 is evolutionarily conserved among vertebrates. These data therefore support the hypothesis that the molecular and functional interaction between Hox proteins and RCHY1 is an ancestral Hox property.

Human Pirh2 is a novel inhibitor of prototype foamy virus replication

Prototype foamy virus (PFV) is a member of the unconventional and nonpathogenic retroviruses. PFV causes lifelong chronic infections, which are partially attributable to a number of host cell factors that restrict viral replication. Herein, we identified human p53-induced RING-H2 protein (Pirh2) as a novel inhibitor of prototype foamy virus. Overexpression of Pirh2 decreased the replication of PFV, whereas knockdown of Pirh2 with specific siRNA increased PFV replication. Dual-luciferase assays and coimmunoprecipitation demonstrated that Pirh2 negatively influences the Tas-dependent transcriptional activation of the PFV long terminal repeat (LTR) and internal promoter (IP) by interacting with the transactivator Tas and down-regulating its expression. In addition, the viral inhibitory function of Pirh2 is N-terminal and RING domain dependent. Together, these results indicated that Pirh2 suppresses PFV replication by negatively impacting its transactivator Tas and the transcription of two viral promoters, which may contribute to the latency of PFV infection.

The MDM2-p53 pathway revisited

The p53 tumor suppressor is a key transcription factor regulating cellular pathways such as DNA repair, cell cycle, apoptosis, angiogenesis, and senescence. It acts as an important defense mechanism against cancer onset and progression, and is negatively regulated by interaction with the oncoprotein MDM2. In human cancers, the TP53 gene is frequently mutated or deleted, or the wild-type p53 function is inhibited by high levels of MDM2, leading to downregulation of tumor suppressive p53 pathways. Thus, the inhibition of MDM2-p53 interaction presents an appealing therapeutic strategy for the treatment of cancer. However, recent studies have revealed the MDM2-p53 interaction to be more complex involving multiple levels of regulation by numerous cellular proteins and epigenetic mechanisms, making it imperative to reexamine this intricate interplay from a holistic viewpoint. This review aims to highlight the multifaceted network of molecules regulating the MDM2-p53 axis to better understand the pathway and exploit it for anticancer therapy.

Natural product MDM2 inhibitors: anticancer activity and mechanisms of action

The mdm2 oncogene has recently been suggested to be a valuable target for cancer therapy and prevention. Overexpression of mdm2 is often seen in various human cancers and correlates with high-grade, late-stage, and more treatment-resistant tumors. The MDM2-p53 auto-regulatory loop has been extensively investigated and is an attractive cancer target, which indeed has been the main focus of anti-MDM2 drug discovery. Much effort has been expended in the development of small molecule MDM2 antagonists targeting the MDM2-p53 interaction, and a few of these have advanced into clinical trials. However, MDM2 exerts its oncogenic activity through both p53-dependent and -independent mechanisms. Recently, there is an increasing interest in identifying natural MDM2 inhibitors; some of them have been shown to decrease MDM2 expression and activity in vitro and in vivo. These identified natural MDM2 inhibitors include a plethora of diverse chemical frameworks, ranging from flavonoids, steroids, and sesquiterpenes to alkaloids. In addition to a brief review of synthetic MDM2 inhibitors, this review focuses on natural product MDM2 inhibitors, summarizing their biological activities in vitro and in vivo and the underlying molecular mechanisms of action, targeting MDM2 itself, regulators of MDM2, and/or the MDM2-p53 interaction. These MDM2 inhibitors can be used alone or in combination with conventional treatments, improving the prospects for cancer therapy and prevention. Their complex and unique molecular architectures may provide a stimulus for developing synthetic analogs in the future.

WWP1 E3 ligase targets LATS1 for ubiquitin-mediated degradation in breast cancer cells

The Large Tumor Suppressor 1 (LATS1) is a serine/threonine kinase and tumor suppressor found down-regulated in various human cancers. LATS1 has recently been identified as a central player of the emerging Hippo signaling pathway, which plays important roles in organ size control, tumorigenesis, and stem cell differentiation and renewal, etc. Although mounting evidence supports a role of LATS1 in tumor suppression and tumorigenesis, how LATS1 is regulated at the molecular level is not fully understood. Recently several positive regulators of LATS1 (Mst1/2, MOB1, Kibra, etc) have been identified but how LATS1 is negatively regulated is still largely unknown. We have recently identified Itch, a member of the NEDD4-like family E3 ubiquitin ligases, as a novel negative regulator of LATS1. However, whether other ubiquitin ligases modulate LATS1 stability and function is unclear. By screening many E3 ligases of the NEDD4-like family using over-expression and short-interference RNA knockdown approaches, we have identified WWP1 E3 ligase as another novel negative regulator of LATS1. We have provided in vitro and in vivo evidence that WWP1 is essential for LATS1 stability and negatively regulate LATS1 by promoting LATS1 degradation through polyubiquitination and the 26S proteasome pathway. Importantly, we also showed that degradation of LATS1 is critical in mediating WWP1-induced increased cell proliferation in breast cancer cells. Since WWP1 is an oncogene and LATS1 is a tumor suppressor gene in breast cancer, our studies provide a promising therapeutic strategy in which developed drugs targeting WWP1 cause activation of LATS1 in suppressing breast cancer cell growth.

A portrayal of E3 ubiquitin ligases and deubiquitylases in cancer

E3 ubiquitin ligases and deubiquitylating enzymes (DUBs) are the key components of ubiquitin proteasome system which plays a critical role in cellular protein homeostasis. Any shortcoming in their biological roles can lead to various diseases including cancer. The dynamic interplay between ubiquitylation and deubiquitylation determines the level and activity of several proteins including p53, which is crucial for cellular stress response and tumor suppression pathways. In this review, we describe the different types of E3 ubiquitin ligases including those targeting tumor suppressor p53, SCF ligases and RING type ligases and accentuate on biological functions of few important E3 ligases in the cellular regulatory networks. Tumor suppressor p53 level is tightly regulated by multiple E3 ligases including Mdm2, COP1, Pirh2, etc. SCF ubiquitin ligase complexes are key regulators of cell cycle and signal transduction. BRCA1 and VHL RING type ligases function as tumor suppressors and play an important role in DNA repair and hypoxia response respectively. Further, we discuss the biological consequences of deregulation of the E3 ligases and the implications for cancer development. We also describe deubiquitylases which reverse the process of ubiquitylation and regulate diverse cellular pathways including metabolism, cell cycle control and chromatin remodelling. As the E3 ubiquitin ligases and DUBs work in a substrate specific manner, an improved understanding of them can lead to better therapeutics for cancer.

High expression of P53-induced Ring-h2 protein is associated with poor prognosis in clear cell renal cell carcinoma

PURPOSE

This study was carried out to examine P53-induced Ring-h2 protein (Pirh2) expression and investigate its clinical and prognostic significance in patients with clear cell renal cell carcinoma (ccRCC).

METHODS

Pirh2 mRNA and protein expressions were detected by quantitative reverse-transcription polymerase chain reaction (Q-RT PCR) and Western blotting in 35 frozen renal cancer tissue specimens and 35 adjacent normal renal tissue specimens of the same patients. Pirh2 protein expression was assessed by immunohistochemical analysis in 92 paraffin-embedded specimens of human ccRCC and 30 specimens of adjacent normal renal tissue. Correlations between Pirh2 and clinicopathologic features and prognosis were analyzed statistically.

RESULTS

Pirh2 mRNA and protein levels in ccRCC samples were increased significantly as compared with the adjacent normal renal tissues (P < 0.001). Pirh2 mRNA overexpression correlated with high stage and grade of the renal cancer (P < 0.001 and P < 0.001 respectively). Pirh2 protein expression was negative in most normal renal tissue specimens (23/30) but positive in 52.2% (48/92) of ccRCC specimens (P = 0.006). Pirh2 protein expression correlated with tumor grade and stage (P < 0.001 and P < 0.001 respectively). The median follow-up interval was 42.0 months. Overexpression of Pirh2 protein in ccRCC was significantly associated with shorter overall survival and recurrence-free survival (P = 0.001 and P = 0.003, respectively). Multivariate analysis showed that Pirh2 expression was an independent prognostic factor for ccRCC patients (P = 0.037).

CONCLUSIONS

Pirh2 was up-regulated in ccRCC at both transcriptional and translational levels compared with normal renal tissues, suggesting that Pirh2 may be a potential prognostic marker for ccRCC.

The p73 tumor suppressor is targeted by Pirh2 RING finger E3 ubiquitin ligase for the proteasome-dependent degradation

The p73 gene, a homologue of the p53 tumor suppressor, is expressed as TA and ΔN isoforms. TAp73 has similar activity as p53 and functions as a tumor suppressor whereas ΔNp73 has both pro- and anti-survival functions. While p73 is rarely mutated in spontaneous tumors, the expression status of p73 is linked to the sensitivity of tumor cells to chemotherapy and prognosis for many types of human cancer. Thus, uncovering its regulators in tumors is of great interest. Here, we found that Pirh2, a RING finger E3 ubiquitin ligase, promotes the proteasome-dependent degradation of p73. Specifically, we showed that knockdown of Pirh2 up-regulates, whereas ectopic expression of Pirh2 down-regulates, expression of endogenous and exogenous p73. In addition, Pirh2 physically associates with and promotes TAp73 polyubiquitination both in vivo and in vitro. Moreover, we found that p73 can be degraded by both 20 S and 26 S proteasomes. Finally, we showed that Pirh2 knockdown leads to growth suppression in a TAp73-dependent manner. Taken together, our findings indicate that Pirh2 promotes the proteasomal turnover of TAp73, and thus targeting Pirh2 to restore TAp73-mediated growth suppression in p53-deficient tumors may be developed as a novel anti-cancer strategy.