Loss of H2B monoubiquitination is associated with poor-differentiation and enhanced malignancy of lung adenocarcinoma

Structure of the human Bre1 complex bound to the nucleosome

Histone H2B monoubiquitination (at Lys120 in humans) regulates transcription elongation and DNA repair. In humans, H2B monoubiquitination is catalyzed by the heterodimeric Bre1 complex composed of Bre1A/RNF20 and Bre1B/RNF40. The Bre1 proteins generally function as tumor suppressors, while in certain cancers, they facilitate cancer cell proliferation. To obtain structural insights of H2BK120 ubiquitination and its regulation, we report the cryo-electron microscopy structure of the human Bre1 complex bound to the nucleosome. The two RING domains of Bre1A and Bre1B recognize the acidic patch and the nucleosomal DNA phosphates around SHL 6.0-6.5, which are ideally located to recruit the E2 enzyme and ubiquitin for H2BK120-specific ubiquitination. Mutational experiments suggest that the two RING domains bind in two orientations and that ubiquitination occurs when Bre1A binds to the acidic patch. Our results provide insights into the H2BK120-specific ubiquitination by the Bre1 proteins and suggest that H2B monoubiquitination can be regulated by nuclesomal DNA flexibility.

Inhibition of USP7 upregulates USP22 and activates its downstream cancer-related signaling pathways in human cancer cells

Deubiquitinases (DUBs) play important roles in various human cancers and targeting DUBs is considered as a novel anticancer therapeutic strategy. Overexpression of ubiquitin specific protease 7 and 22 (USP7 and USP22) are associated with malignancy, therapy resistance, and poor prognosis in many cancers. Although both DUBs are involved in the regulation of similar genes and signaling pathways, such as histone H2B monoubiquitination (H2Bub1), c-Myc, FOXP3, and p53, the interdependence of USP22 and USP7 expression has never been described. In the study, we found that targeting USP7 via either siRNA-mediated knockdown or pharmaceutical inhibitors dramatically upregulates USP22 in cancer cells. Mechanistically, the elevated USP22 occurs through a transcriptional pathway, possibly due to desuppression of the transcriptional activity of SP1 via promoting its degradation upon USP7 inhibition. Importantly, increased USP22 expression leads to significant activation of downstream signal pathways including H2Bub1 and c-Myc, which may potentially enhance cancer malignancy and counteract the anticancer efficacy of USP7 inhibition. Importantly, targeting USP7 further suppresses the in vitro proliferation of USP22-knockout (USP22-Ko) A549 and H1299 lung cancer cells and induces a stronger activation of p53 tumor suppressor signaling pathway. In addition, USP22-Ko cancer cells are more sensitive to a combination of cisplatin and USP7 inhibitor. USP7 inhibitor treatment further suppresses in vivo angiogenesis and tumor growth and induced more apoptosis in USP22-Ko cancer xenografts. Taken together, our findings demonstrate that USP7 inhibition can dramatically upregulate USP22 in cancer cells; and targeting USP7 and USP22 may represent a more effective approach for targeted cancer therapy, which warrants further study. Video Abstract.

SET-mediated epigenetic dysregulation of p53 impairs trichloroethylene-induced DNA damage response

Trichloroethylene (TCE) was a widely used industrial solvent, and now has become a major environmental pollutant. Exposure to TCE has been found to result in significant damage to the liver, leading to hepatic toxicity. In our previous study, we discovered that a histone chaperon called SET plays a crucial role in mediating the DNA damage and apoptosis caused by TCE in hepatic cells. However, the precise function of SET in the response to DNA damage is still not fully understood. In this study, we evaluated TCE-induced DNA damage of hepatic L-02 cells with SET-knockdown, then analyzed alterations of H3K79me3 and p53 in hepatic cells and carcinogenic mice livers. Results suggested that SET interferes with DNA response via mediating down-regulation of p53 and partially suppressing H3K79me3 under treatment of TCE. To further verify the regulatory cascade, H3K79me3 was reduced and p53 was knocked down in L-02 cells respectively, and extent of DNA damage was evaluated. Reduced H3K79me3 was found leading to down-regulation of p53 which further exacerbated TCE-induced DNA injury. These findings demonstrated that SET-H3K79me3-p53 served as an epigenetic regulatory axis involved in TCE-induced DNA damage response.

H2Bub1 loss is an early contributor to clear cell ovarian cancer progression

Epigenetic aberrations, including posttranslational modifications of core histones, are major contributors to cancer. Here, we define the status of histone H2B monoubiquitylation (H2Bub1) in clear cell ovarian carcinoma (CCOC), low-grade serous carcinoma, and endometrioid carcinomas. We report that clear cell carcinomas exhibited profound loss, with nearly all cases showing low or negative H2Bub1 expression. Moreover, we found that H2Bub1 loss occurred in endometriosis and atypical endometriosis, which are established precursors to CCOCs. To examine whether dysregulation of a specific E3 ligase contributes to the loss of H2Bub1, we explored expression of ring finger protein 40 (RNF40), ARID1A, and UBR7 in the same case cohort. Loss of RNF40 was significantly and profoundly correlated with loss of H2Bub1. Using genome-wide DNA methylation profiles of 230 patients with CCOC, we identified hypermethylation of RNF40 in CCOC as a likely mechanism underlying the loss of H2Bub1. Finally, we demonstrated that H2Bub1 depletion promoted cell proliferation and clonogenicity in an endometriosis cell line. Collectively, our results indicate that H2Bub1 plays a tumor-suppressive role in CCOCs and that its loss contributes to disease progression.

Epigenetics alternation in lung fibrosis and lung cancer

Respiratory disease including interstitial lung diseases (ILDs) and lung cancer is a group of devastating diseases that linked with increased morbidity and healthcare burden. However, respiratory diseases cannot be fully explained by the alternation of genetic information. Genetic studies described that epigenetic mechanisms also participate to transmit genetic information. Recently, many studies demonstrated the role of altered epigenetic modification in the pathogenesis of lung cancer and pulmonary fibrosis. Due to lacking effective medication, the underlying pathophysiological processes and causal relationships of lung diseases with epigenetic mechanisms still need to be better understood. Our present review provided a systematic revision of current knowledge concerning diverse epigenetic aberrations in major lung diseases, with special emphasis on DNA methylation, histone modifications, lncRNAs profiles, telomere patterns, as well as chromatin-remodelling complexes. We believed that a new target therapy for lung disease based on findings of the involved epigenetic pathway is a promising future direction.

Monoubiquitination in Homeostasis and Cancer

Monoubiquitination is a post-translational modification (PTM), through which a single ubiquitin molecule is covalently conjugated to a lysine residue of the target protein. Monoubiquitination regulates the activity, subcellular localization, protein-protein interactions, or endocytosis of the substrate. In doing so, monoubiquitination is implicated in diverse cellular processes, including gene transcription, endocytosis, signal transduction, cell death, and DNA damage repair, which in turn regulate cell-cycle progression, survival, proliferation, and stress response. In this review, we summarize the functions of monoubiquitination and discuss how this PTM modulates homeostasis and cancer.

Novel Angiogenic Regulators and Anti-Angiogenesis Drugs Targeting Angiogenesis Signaling Pathways: Perspectives for Targeting Angiogenesis in Lung Cancer

Lung cancer growth is dependent on angiogenesis. In recent years, angiogenesis inhibitors have attracted more and more attention as potential lung cancer treatments. Current anti-angiogenic drugs targeting VEGF or receptor tyrosine kinases mainly inhibit tumor growth by reducing angiogenesis and blocking the energy supply of lung cancer cells. However, these drugs have limited efficiency, raising concerns about limited scope of action and mechanisms of patient resistance to existing drugs. Therefore, current basic research on angiogenic regulators has focused more on screening carcinogenic/anticancer genes, miRNAs, lncRNAs, proteins and other biomolecules capable of regulating the expression of specific targets in angiogenesis signaling pathways. In addition, new uses for existing drugs and new drug delivery systems have received increasing attention. In our article, we analyze the application status and research hotspots of angiogenesis inhibitors in lung cancer treatment as a reference for subsequent mechanistic research and drug development.

Ring finger 20/ring finger 40/WW domain-containing adaptor with coiled-coil complex interacts with p53 to regulate gene transcription in DNA damage response

p53 is one of the most important tumor suppressor genes, and its primary function is to act as a transcriptional activator to control cell cycle arrest, DNA repair and cellular metabolism by recognizing and binding to specific DNA sequences. Defects in the ring finger (RNF)20/RNF40/WW domain-containing adaptor with coiled-coil (WAC) complex, one of the histone H2B ubiquitination E3 ligases, have been reported to be a key factor in oncogenesis, cancer cell migration and invasion. Histone H2B mono-ubiquitination has been demonstrated to be essential for maintaining the functionality of the p53 tumor suppressor protein. The aim of the present study was to identify any sites in the p53 DNA-binding domain (DBD) specific to the RNF20/RNF40/WAC complex that may be involved in the gene regulation in DNA damage response. The results demonstrated that p53 and the RNF20/RNF40/WAC complex interacted with each other, and the coiled-coil regions in RNF20, RNF40 and WAC were identified to directly interact with p53. The R282 site in the p53 DBD, one of the frequent missense mutations associated with p53 mutation-dependent cancer, was demonstrated to be the key binding site for the RNF20/RNF40/WAC complex. Furthermore, knockout of RNF20/RNF40 suppressed the expression levels of p53 and its target genes in HCT116 cells compared with those in wild-type HCT116 cells. Consistent with these results, the R282W mutation in p53 inhibited the expression levels of p53 and its downstream genes by inactivating the interaction between p53 and RNF20/RNF40 compared with those in wild-type HCT116 cells. In conclusion, the results of the present study revealed the molecular mechanism of the interaction between the RNF20/RNF40/WAC complex and p53, and demonstrated that these proteins regulated gene transcription in the DNA damage response.

Role of H2B mono-ubiquitination in the initiation and progression of cancer

Numerous epigenetic alterations are observed in cancer cells, and dysregulation of mono-ubiquitination of histone H2B (H2Bub1) has often been linked to tumorigenesis. H2Bub1 is a dynamic post-translational histone modification associated with transcriptional elongation and DNA damage response. Histone H2B monoubiquitination occurs in the site of lysine 120, written predominantly by E3 ubiquitin ligases RNF20/RNF40 and deubiquitinated by ubiquitin specific peptidase 22 (USP22). RNF20/40 is often altered in the primary tumors including colorectal cancer, breast cancer, ovarian cancer, prostate cancer, and lung cancer, and the loss of H2Bub1 is usually associated with poor prognosis in tumor patients. The purpose of this review is to summarize the current knowledge of H2Bub1 in transcription, DNA damage response and primary tumors. This review also provides novel options for exploiting the potential therapeutic target H2Bub1 in personalized cancer therapy.

Histone Monoubiquitination in Chromatin Remodelling: Focus on the Histone H2B Interactome and Cancer

Chromatin remodelling is a major mechanism by which cells control fundamental processes including gene expression, the DNA damage response (DDR) and ensuring the genomic plasticity required by stem cells to enable differentiation. The post-translational modification of histone H2B resulting in addition of a single ubiquitin, in humans at lysine 120 (K120; H2Bub1) and in yeast at K123, has key roles in transcriptional elongation associated with the RNA polymerase II-associated factor 1 complex (PAF1C) and in the DDR. H2Bub1 itself has been described as having tumour suppressive roles and a number of cancer-related proteins and/or complexes are recognised as part of the H2Bub1 interactome. These include the RING finger E3 ubiquitin ligases RNF20, RNF40 and BRCA1, the guardian of the genome p53, the PAF1C member CDC73, subunits of the switch/sucrose non-fermenting (SWI/SNF) chromatin remodelling complex and histone methyltransferase complexes DOT1L and COMPASS, as well as multiple deubiquitinases including USP22 and USP44. While globally depleted in many primary human malignancies, including breast, lung and colorectal cancer, H2Bub1 is selectively enriched at the coding region of certain highly expressed genes, including at p53 target genes in response to DNA damage, functioning to exercise transcriptional control of these loci. This review draws together extensive literature to cement a significant role for H2Bub1 in a range of human malignancies and discusses the interplay between key cancer-related proteins and H2Bub1-associated chromatin remodelling.

The histone H2B ubiquitin ligase RNF40 is required for HER2-driven mammary tumorigenesis

The HER2-positive breast cancer subtype (HER2⁺-BC) displays a particularly aggressive behavior. Anti-HER2 therapies have significantly improved the survival of patients with HER2⁺-BC. However, a large number of patients become refractory to current targeted therapies, necessitating the development of new treatment strategies. Epigenetic regulators are commonly misregulated in cancer and represent attractive molecular therapeutic targets. Monoubiquitination of histone 2B (H2Bub1) by the heterodimeric ubiquitin ligase complex RNF20/RNF40 has been described to have tumor suppressor functions and loss of H2Bub1 has been associated with cancer progression. In this study, we utilized human tumor samples, cell culture models, and a mammary carcinoma mouse model with tissue-specific Rnf40 deletion and identified an unexpected tumor-supportive role of RNF40 in HER2⁺-BC. We demonstrate that RNF40-driven H2B monoubiquitination is essential for transcriptional activation of RHO/ROCK/LIMK pathway components and proper actin-cytoskeleton dynamics through a trans-histone crosstalk with histone 3 lysine 4 trimethylation (H3K4me3). Collectively, this work demonstrates a previously unknown essential role of RNF40 in HER2⁺-BC, revealing the H2B monoubiquitination axis as a possible tumor context-dependent therapeutic target in breast cancer.

Cetuximab-Triptolide Conjugate Suppresses the Growth of EGFR-Overexpressing Lung Cancers through Targeting RNA Polymerase II

To overcome poor pharmacokinetics and toxicity of triptolide (TPL), a natural compound that exhibits potent anticancer activities, we developed a novel antibody-drug conjugate (ADC) to specifically deliver TPL to epidermal growth factor receptor (EGFR)-overexpressing non-small cell lung cancer (NSCLC) and others. The ADC (Cet-TPL) is made by conjugation of TPL to lysine residues of cetuximab (Cet), a clinically available anti-EGFR monoclonal antibody. Studies of antitumor efficacy demonstrated that Cet-TPL drastically suppressed in vitro proliferation and in vivo growth of these EGFR-overexpressing cancers, including NSCLC A549 and H1299 cells and two patient-derived xenografts, and head and neck squamous carcinoma UM-SCC6 cell, while it did not inhibit the proliferation and growth of NSCLC H520 that rarely expresses EGFR. Furthermore, immunofluorescence analysis revealed that Cet-TPL was effectively internalized and transported into lysosomes of EGFR-overexpressing cells. Cet-TPL effectively led to degradation of RNA polymerase II (Pol II) and demethylation of histone H3 lysines, and significantly induced apoptosis in these EGFR-overexpressing cancers. Compared with TPL, Cet, or their combination, Cet-TPL displayed higher target-specific cytotoxicity against EGFR-expressing cancers and much lower in vivo toxicity. In addition, Cet-TPL efficiently suppressed the activated EGFR pathway in UM-SCC6 cancer cells. Taken together, Cet-TPL represents a potent targeting therapeutic agent against EGFR-overexpressing NSCLC and others.

Epigenetic regulation of the Warburg effect by H2B monoubiquitination

Cancer cells reprogram their energy metabolic system from the mitochondrial oxidative phosphorylation (OXPHOS) pathway to a glucose-dependent aerobic glycolysis pathway. This metabolic reprogramming phenomenon is known as the Warburg effect, a significant hallmark of cancer. However, the detailed mechanisms underlying this event or triggering this reprogramming remain largely unclear. Here, we found that histone H2B monoubiquitination (H2Bub1) negatively regulates the Warburg effect and tumorigenesis in human lung cancer cells (H1299 and A549 cell lines) likely through controlling the expression of multiple mitochondrial respiratory genes, which are essential for OXPHOS. Moreover, our work also suggested that pyruvate kinase M2 (PKM2), the rate-limiting enzyme of glycolysis, can directly interact with H2B in vivo and in vitro and negatively regulate the level of H2Bub1. The inhibition of cell proliferation and nude mice xenograft of human lung cancer cells induced by PKM2 knockdown can be partially rescued through lowering H2Bub1 levels, which indicates that the oncogenic function of PKM2 is achieved, at least partially, through the control of H2Bub1. Furthermore, PKM2 and H2Bub1 levels are negatively correlated in cancer specimens. Therefore, these findings not only provide a novel mechanism triggering the Warburg effect that is mediated through an epigenetic pathway (H2Bub1) but also reveal a novel metabolic regulator (PKM2) for the epigenetic mark H2Bub1. Thus, the PKM2-H2Bub1 axis may become a promising cancer therapeutic target.

The Functional Effects of Key Driver KRAS Mutations on Gene Expression in Lung Cancer

Lung cancer is a common malignant cancer. Kirsten rat sarcoma oncogene (KRAS) mutations have been considered as a key driver for lung cancers. KRAS p.G12C mutations were most predominant in NSCLC which was comprised about 11–16% of lung adenocarcinomas (p.G12C accounts for 45–50% of mutant KRAS). But it is still not clear how the KRAS mutation triggers lung cancers. To study the molecular mechanisms of KRAS mutation in lung cancer. We analyzed the gene expression profiles of 156 KRAS mutation samples and other negative samples with two stage feature selection approach: (1) minimal Redundancy Maximal Relevance (mRMR) and (2) Incremental Feature Selection (IFS). At last, 41 predictive genes for KRAS mutation were identified and a KRAS mutation predictor was constructed. Its leave one out cross validation MCC was 0.879. Our results were helpful for understanding the roles of KRAS mutation in lung cancer.

Ubiquitin-specific protease 22 is critical to in vivo angiogenesis, growth and metastasis of non-small cell lung cancer

BACKGROUND

Loss of monoubiquitination of histone H2B (H2Bub1) was found to be associated with poor differentiation, cancer stemness, and enhanced malignancy of non-small cell lung cancer (NSCLC). Herein, we investigated the biological significance and therapeutic implications of ubiquitin-specific protease 22 (USP22), an H2Bub1 deubiquitinase, in non-small cell lung cancer (NSCLC).

METHODS

USP22 expression and its clinical relevance were assessed in NSCLC patients. The effects of USP22 knockout on sensitivity to cisplatin and irradiation, and growth, metastasis of NSCLC xenografts, and survival of cancer-bearing mice were investigated. The underlying mechanisms of targeting USP22 were explored.

RESULTS

Overexpression of USP22 was observed in 49.0% (99/202) of NSCLC tissues; higher USP22 immunostaining was found to be associated with enhanced angiogenesis and recurrence of NSCLC. Notably, USP22 knockout dramatically suppressed in vitro proliferation, colony formation; and angiogenesis, growth, metastasis of A549 and H1299 in mouse xenograft model, and significantly prolonged survival of metastatic cancer-bearing mice. Furthermore, USP22 knockout significantly impaired non-homologous DNA damage repair capacity, enhanced cisplatin and irradiation-induced apoptosis in these cells. In terms of underlying mechanisms, RNA sequencing and gene ontology enrichment analysis demonstrated that USP22 knockout significantly suppressed angiogenesis, proliferation, EMT, RAS, c-Myc pathways, concurrently enhanced oxidative phosphorylation and tight junction pathways in A549 and H1299 NSCLC cells. Immunoblot analysis confirmed that USP22 knockout upregulated E-cadherin, p16; reduced ALDH1A3, Cyclin E1, c-Myc, and attenuated activation of AKT and ERK pathways in these cells.

CONCLUSIONS

Our findings suggest USP22 plays critical roles in the malignancy and progression of NSCLC and provide rationales for targeting USP22, which induces broad anti-cancer activities, as a novel therapeutic strategy for NSCLC patient.

LKB1 deficiency promotes proliferation and invasion of glioblastoma through activation of mTOR and focal adhesion kinase signaling pathways

Liver kinase B1 (LKB1), a serine/threonine kinase, is frequently inactivated in several types of human cancers. To date, inactivation of LKB1 tumor suppressor has rarely been reported in glioblastoma. In this study, we investigated LKB1 status, biological significance, and therapeutic implications in glioblastoma. Loss of LKB1 immunostaining was identified in 8.6% (5/58), while decrease of LKB1 immunostaining was found in 29.3% (17/58) of glioblastoma tissues. Notably, mining TCGA database of LKB1 expression in glioblastoma revealed that lower mRNA level of LKB1 was associated with shorter survival in glioblastoma. We found that knockdown of LKB1 significantly promoted in vitro proliferation, adhesion, invasion, and metformin-induced apoptosis, and simultaneously enhanced activation of ERK and mammalian-target of rapamycin (mTOR) signaling pathways in LKB1-compenent U87 and T98 glioblastoma cells. Moreover, global transcriptional profiling revealed that adhesion and cytoskeletal proteins such as Vinculin, Talin and signaling pathways including focal adhesion kinase (FAK), extracellular martrix (ECM) receptor interaction, and cellular motility were significantly enriched in U87 and T98 glioblastoma cells upon LKB1 knockdown. Additionally, we demonstrated that the enhanced activation of FAK by LKB1 knockdown was dependent on differentially expressed cytoskeletal proteins in these glioblastoma cells. Importantly, we further found that mTOR1 inhibitor rapamycin dominantly inhibited in vitro cellular proliferation, while FAK inhibitor PF-573288 drastically decreased invasion of LKB1-attenuated glioblastoma cells. Therefore, downregulation of LKB1 may contribute to the pathogenesis and malignancy of glioblastoma and may have potential implications for stratification and treatment of glioblastoma patients.

Epigenetic regulation of ferroptosis by H2B monoubiquitination and p53

Monoubiquitination of histone H2B on lysine 120 (H2Bub1) is an epigenetic mark generally associated with transcriptional activation, yet the global functions of H2Bub1 remain poorly understood. Ferroptosis is a form of non-apoptotic cell death characterized by the iron-dependent overproduction of lipid hydroperoxides, which can be inhibited by the antioxidant activity of the solute carrier family member 11 (SLC7A11/xCT), a component of the cystine/glutamate antiporter. Whether nuclear events participate in the regulation of ferroptosis is largely unknown. Here, we show that the levels of H2Bub1 are decreased during erastin-induced ferroptosis and that loss of H2Bub1 increases the cellular sensitivity to ferroptosis. H2Bub1 epigenetically activates the expression of SLC7A11. Additionally, we show that the tumor suppressor p53 negatively regulates H2Bub1 levels independently of p53's transcription factor activity by promoting the nuclear translocation of the deubiquitinase USP7. Moreover, our studies reveal that p53 decreases H2Bub1 occupancy on the SLC7A11 gene regulatory region and represses the expression of SLC7A11 during erastin treatment. These data not only suggest a noncanonical role of p53 in chromatin regulation but also link p53 to ferroptosis via an H2Bub1-mediated epigenetic pathway. Overall, our work uncovers a previously unappreciated epigenetic mechanism for the regulation of ferroptosis.

Writing Histone Monoubiquitination in Human Malignancy-The Role of RING Finger E3 Ubiquitin Ligases

There is growing evidence highlighting the importance of monoubiquitination as part of the histone code. Monoubiquitination, the covalent attachment of a single ubiquitin molecule at specific lysines of histone tails, has been associated with transcriptional elongation and the DNA damage response. Sites function as scaffolds or docking platforms for proteins involved in transcription or DNA repair; however, not all sites are equal, with some sites resulting in actively transcribed chromatin and others associated with gene silencing. All events are written by E3 ubiquitin ligases, predominantly of the RING (really interesting new gene) finger type. One of the most well-studied events is monoubiquitination of histone H2B at lysine 120 (H2Bub1), written predominantly by the RING finger complex RNF20-RNF40 and generally associated with active transcription. Monoubiquitination of histone H2A at lysine 119 (H2AK119ub1) is also well-studied, its E3 ubiquitin ligase constituting part of the Polycomb Repressor Complex 1 (PRC1), RING1B-BMI1, associated with transcriptional silencing. Both modifications are activated as part of the DNA damage response. Histone monoubiquitination is a key epigenomic event shaping the chromatin landscape of malignancy and influencing how cells respond to DNA damage. This review discusses a number of these sites and the E3 RING finger ubiquitin ligases that write them.

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.

Role of RNF20 in cancer development and progression - a comprehensive review

Evolving strategies to counter cancer initiation and progression rely on the identification of novel therapeutic targets that exploit the aberrant genetic changes driving oncogenesis. Several chromatin associated enzymes have been shown to influence post-translational modification (PTM) in DNA, histones, and non-histone proteins. Any deregulation of this core group of enzymes often leads to cancer development. Ubiquitylation of histone H2B in mammalian cells was identified over three decades ago. An exciting really interesting new gene (RING) family of E3 ubiquitin ligases, known as RNF20 and RNF40, monoubiquitinates histone H2A at K119 or H2B at K120, is known to function in transcriptional elongation, DNA double-strand break (DSB) repair processes, maintenance of chromatin differentiation, and exerting tumor suppressor activity. RNF20 is somatically altered in breast, lung, prostate cancer, clear cell renal cell carcinoma (ccRCC), and mixed lineage leukemia, and its reduced expression is a key factor in initiating genome instability; and it also functions as one of the significant driving factors of oncogenesis. Loss of RNF20/40 and H2B monoubiquitination (H2Bub1) is found in several cancers and is linked to an aggressive phenotype, and is also an indicator of poor prognosis. In this review, we summarized the current knowledge of RNF20 in chronic inflammation-driven cancers, DNA DSBs, and apoptosis, and its impact on chromatin structure beyond the single nucleosome level.

Targeting USP22 Suppresses Tumorigenicity and Enhances Cisplatin Sensitivity Through ALDH1A3 Downregulation in Cancer-Initiating Cells from Lung Adenocarcinoma

Loss of monoubiquitination of histone H2B (H2Bub1) was found to be associated with poor-differentiation and enhanced malignancy of lung adenocarcinoma. This study investigated the association and impact of the ubiquitin-specific peptidase 22 (USP22), an H2Bub1 deubiquitinase, on stem cell-like characteristics and cisplatin resistance in cancer-initiating cells (CIC) from primary lung adenocarcinoma. CICs were isolated, enriched, and characterized from patient-derived cancer tissues using both in vitro tumorsphere formation and in vivo xenograft assays. USP22 was determined to be predominantly expressed in CICs, a subpopulation of cells with high expression of the stem cell biomarkers, CD133 and CD44. The expression of USP22 in CICs is markedly reduced upon FBS/retinoic acid-induced differentiation. Moreover, knockdown of USP22 significantly suppressed tumorsphere formation and xenograft growth in NOD-SCID gamma (NSG) mice. Notably, USP22 and aldehyde dehydrogenase (ALDH) activity were elevated in tumorsphere cells that survived cisplatin treatment, whereas knockdown of USP22 significantly sensitizes tumorsphere cells to cisplatin. Interestingly, ALDH1A3, a predominant ALDH isozyme implicated in enhancing cisplatin resistance in lung adenocarcinoma, is significantly downregulated upon knockdown of USP22 in tumorsphere cells. Furthermore, knockdown of ALDH1A3 significantly sensitizes tumorsphere cells to cisplatin. Combined, these data demonstrate that USP22, predominantly expressed in CD133⁺ CICs, plays a critical role in tumorigenicity and cisplatin resistance in lung adenocarcinoma.Implications: Targeting USP22 represents a potential therapeutic approach to suppress CICs in lung adenocarcinoma partially through downregulation of ALDH1A3 expression. Mol Cancer Res; 16(7); 1161-71. ©2018 AACR.