Targeting of BMI-1 with PTC-209 shows potent anti-myeloma activity and impairs the tumour microenvironment

MicroRNA-128 acts as a suppressor in the progression of gastrointestinal stromal tumor by targeting B-lymphoma Mo-MLV insertion region 1

INTRODUCTION

The critical role of microRNA-128 (miR-128) in gastrointestinal-related diseases has been documented. In the current study, we tried to clarify the specific role miR-128 in gastrointestinal stromal tumor (GIST) and the underlying mechanism.

METHODS

Differentially expressed genes in GIST were identified following bioinformatics analysis. Then, expression patterns of miR-128 and B-lymphoma Mo-MLV insertion region 1 (BMI-1) in clinical tissue samples and cell lines were characterized, followed by validation of their correlation. GIST-T1 cells were selected and transfected with different mimic, inhibitor, or siRNA plasmids, after which the biological functions were assayed.

RESULTS

We identified low miR-128 and high BMI-1 expression in GIST tissues of 78 patients and 4 GIST cell lines. Ectopic expression of miR-128 or silencing of BMI-1 suppressed the malignant potentials of GIST-T1 cells. As a target of miR-128, BMI-1 re-expression could partly counteract the suppressive effect of miR-128 on the malignancy of GIST-T1 cells.

CONCLUSION

Our study provided evidence that miR-128-mediated silencing of BMI-1 could prevent malignant progression of GIST, highlighting a promising anti-tumor target for combating GIST.

CircularRNA Hsa_circ_0093335 promotes hepatocellular carcinoma progression via sponging miR-338-5p

Circular RNAs play an important role in the development of various malignancies, including hepatocellular carcinoma (HCC). Nevertheless, the role of Hsa_circ_0093335 (circ0093335) in HCC has not yet been explored. To investigate the biological effects and molecular mechanisms of circ0093335 on HCC. Circ0093335 expression was detected in HCC cells and clinical specimens using qRT-PCR. The association between circ0093335 expression and HCC patients' clinical characteristics was determined using SPSS. The role of circ0093335 in HCC was estimated by overexpression and knockdown experiments in vitro and in vivo. qRT-PCR, nucleoplasma separation assay, FISH assay, RIP, dual luciferase reporter assay and rescue assay were used to validate the regulatory effect of circ0093335 on miR-338-5p. The study findings showed that circ0093335 was upregulated in HCC. High circ0093335 expression was linked with the tumour-node-metastasis stage and microvascular tumour invasion. circ0093335 is greatly involved in HCC cell proliferation, aggressive ability and mouse tumour growth, according to many in vitro and in vivo tests. Mechanistically, circ0093335 downregulated miR-338-5p expression by sponging, consequently promoting HCC progression. Our research indicated that circ0093335 might be a target for HCC therapy since it promotes tumour progression by acting as a miR-338-5p 'sponge'.

Core-shell cisplatin/SiO2 nanocapsules combined with PTC-209 overcome chemotherapy-Acquired and intrinsic resistance in hepatocellular carcinoma

The primary cause of cisplatin resistance in liver cancer is reduced intracellular drug accumulation and altered DNA repair/apoptosis signaling. Existing strategies to reverse cisplatin resistance have limited efficacy, as they target individual factors. This study proposes a drug delivery system consisting of a cisplatin core, a silica shell with a tetra-sulfide bond, and a PEG-coated surface (Core/shell-PGCN). The system is designed to consume glutathione (GSH) and reduce cisplatin excretion from cells, thereby overcoming acquired cisplatin resistance. In addition, Core/shell-PGCN incorporates PTC-209 (Core/shell-PGCN@PTC-209), a Bmi1 inhibitor that suppresses liver cancer stem cells (CSC), to mitigate DNA repair/apoptosis signaling and reverse intrinsic cisplatin resistance. In vivo and in vitro results demonstrate that Core/shell-PGCN@PTC-209 can comprehensively regulate GSH and CSC, reverse intrinsic and acquired cisplatin resistance, and enhance the efficacy of cisplatin in treating liver cancer. This "inner cultivation, outer action" approach may offer a new strategy for reversing cisplatin resistance in liver cancer. STATEMENT OF SIGNIFICANCE: Cisplatin resistance is widely observed in liver cancer (HCC) chemotherapy, with two mechanisms identified: acquired and intrinsic. Most strategies aimed at overcoming cisplatin resistance focus on a single perspective. This study introduces a core-shell drug delivery system (DDS) combined with HCC stem cell inhibitors, which can effectively address cisplatin resistance in HCC by targeting both acquisition and internality. Specifically, the core-shell drug delivery system can impede cisplatin efflux by neutralizing the acquired resistance factor (GSH), thus overcoming acquired resistance. Additionally, HCC stem cell inhibitors can reverse intrinsic resistance by inhibiting HCC stem cells. Therefore, this study contributes to the application of DDS in combating drug resistance in HCC and enhances its potential for clinical implementation.

Polycomb group protein BMI1 protects neuroblastoma cells against DNA damage-induced apoptotic cell death

The overexpression of BMI1, a polycomb protein, correlates with cancer development and aggressiveness. We previously reported that MYCN-induced BMI1 positively regulated neuroblastoma (NB) cell proliferation via the transcriptional inhibition of tumor suppressors in NB cells. To assess the potential of BMI1 as a new target for NB therapy, we examined the effects of reductions in BMI1 on NB cells. BMI1 knockdown (KD) in NB cells significantly induced their differentiation for up to 7 days. BMI1 depletion significantly induced apoptotic NB cell death for up to 14 days along with the activation of p53, increases in p73, and induction of p53 family downstream molecules and pathways, even in p53 mutant cells. BMI1 depletion in vivo markedly suppressed NB xenograft tumor growth. BMI1 reductions activated ATM and increased γ-H2AX in NB cells. These DNA damage signals and apoptotic cell death were not canceled by the transduction of the polycomb group molecules EZH2 and RING1B. Furthermore, EZH2 and RING1B KD did not induce apoptotic NB cell death to the same extent as BMI1 KD. Collectively, these results suggest the potential of BMI1 as a target of molecular therapy for NB and confirmed, for the first time, the shared role of PcG proteins in the DNA damage response of NB cells.

Critical Roles of Polycomb Repressive Complexes in Transcription and Cancer

Polycomp group (PcG) proteins are members of highly conserved multiprotein complexes, recognized as gene transcriptional repressors during development and shown to play a role in various physiological and pathological processes. PcG proteins consist of two Polycomb repressive complexes (PRCs) with different enzymatic activities: Polycomb repressive complexes 1 (PRC1), a ubiquitin ligase, and Polycomb repressive complexes 2 (PRC2), a histone methyltransferase. Traditionally, PRCs have been described to be associated with transcriptional repression of homeotic genes, as well as gene transcription activating effects. Particularly in cancer, PRCs have been found to misregulate gene expression, not only depending on the function of the whole PRCs, but also through their separate subunits. In this review, we focused especially on the recent findings in the transcriptional regulation of PRCs, the oncogenic and tumor-suppressive roles of PcG proteins, and the research progress of inhibitors targeting PRCs.

The Crucial Roles of Bmi-1 in Cancer: Implications in Pathogenesis, Metastasis, Drug Resistance, and Targeted Therapies

B-cell-specific Moloney murine leukemia virus integration region 1 (Bmi-1, also known as RNF51 or PCGF4) is one of the important members of the PcG gene family, and is involved in regulating cell proliferation, differentiation and senescence, and maintaining the self-renewal of stem cells. Many studies in recent years have emphasized the role of Bmi-1 in the occurrence and development of tumors. In fact, Bmi-1 has multiple functions in cancer biology and is closely related to many classical molecules, including Akt, c-MYC, Pten, etc. This review summarizes the regulatory mechanisms of Bmi-1 in multiple pathways, and the interaction of Bmi-1 with noncoding RNAs. In particular, we focus on the pathological processes of Bmi-1 in cancer, and explore the clinical relevance of Bmi-1 in cancer biomarkers and prognosis, as well as its implications for chemoresistance and radioresistance. In conclusion, we summarize the role of Bmi-1 in tumor progression, reveal the pathophysiological process and molecular mechanism of Bmi-1 in tumors, and provide useful information for tumor diagnosis, treatment, and prognosis.

Inhibition of BMI-1 Induces Apoptosis through Downregulation of DUB3-Mediated Mcl-1 Stabilization

BMI-1, a polycomb ring finger oncogene, is highly expressed in multiple cancer cells and is involved in cancer cell proliferation, invasion, and apoptosis. BMI-1 represents a cancer stemness marker that is associated with the regulation of stem cell self-renewal. In this study, pharmacological inhibition (PTC596) or knockdown (siRNA) of BMI-1 reduced cancer stem-like cells and enhanced cancer cell death. Mechanistically, the inhibition of BMI-1 induced the downregulation of Mcl-1 protein, but not Mcl-1 mRNA. PTC596 downregulated Mcl-1 protein expression at the post-translational level through the proteasome-ubiquitin system. PTC596 and BMI-1 siRNA induced downregulation of DUB3 deubiquitinase, which was strongly linked to Mcl-1 destabilization. Furthermore, overexpression of Mcl-1 or DUB3 inhibited apoptosis by PTC596. Taken together, our findings reveal that the inhibition of BMI-1 induces Mcl-1 destabilization through downregulation of DUB3, resulting in the induction of cancer cell death.

BMI1 regulates multiple myeloma-associated macrophage's pro-myeloma functions

Multiple myeloma (MM) is an aggressive malignancy characterized by terminally differentiated plasma cells accumulation in the bone marrow (BM). MM BM exhibits elevated MΦs (macrophages) numbers relative to healthy BM. Current evidence indicates that MM-MΦs (MM-associated macrophages) have pro-myeloma functions, and BM MM-MΦs numbers negatively correlate with patient survival. Here, we found that BMI1, a polycomb-group protein, modulates the pro-myeloma functions of MM-MΦs, which expressed higher BMI1 levels relative to normal MΦs. In the MM tumor microenvironment, hedgehog signaling in MΦs was activated by MM-derived sonic hedgehog, and BMI1 transcription subsequently activated by c-Myc. Relative to wild-type MM-MΦs, BMI1-KO (BMI1 knockout) MM-MΦs from BM cells of BMI1-KO mice exhibited reduced proliferation and suppressed expression of angiogenic factors. Additionally, BMI1-KO MM-MΦs lost their ability to protect MM cells from chemotherapy-induced cell death. In vivo analysis showed that relative to wild-type MM-MΦs, BMI1-KO MM-MΦs lost their pro-myeloma effects. Together, our data show that BMI1 mediates the pro-myeloma functions of MM-MΦs.

The Role of Polycomb Group Protein BMI1 in DNA Repair and Genomic Stability

The polycomb group (PcG) proteins are a class of transcriptional repressors that mediate gene silencing through histone post-translational modifications. They are involved in the maintenance of stem cell self-renewal and proliferation, processes that are often dysregulated in cancer. Apart from their canonical functions in epigenetic gene silencing, several studies have uncovered a function for PcG proteins in DNA damage signaling and repair. In particular, members of the poly-comb group complexes (PRC) 1 and 2 have been shown to recruit to sites of DNA damage and mediate DNA double-strand break repair. Here, we review current understanding of the PRCs and their roles in cancer development. We then focus on the PRC1 member BMI1, discussing the current state of knowledge of its role in DNA repair and genome integrity, and outline how it can be targeted pharmacologically.

The combination of the tubulin binding small molecule PTC596 and proteasome inhibitors suppresses the growth of myeloma cells

The novel small molecule PTC596 inhibits microtubule polymerization and its clinical development has been initiated for some solid cancers. We herein investigated the preclinical efficacy of PTC596 alone and in combination with proteasome inhibitors in the treatment of multiple myeloma (MM). PTC596 inhibited the proliferation of MM cell lines as well as primary MM samples in vitro, and this was confirmed with MM cell lines in vivo. PTC596 synergized with bortezomib or carfilzomib to inhibit the growth of MM cells in vitro. The combination treatment of PTC596 with bortezomib exerted synergistic effects in a xenograft model of human MM cell lines in immunodeficient mice and exhibited acceptable tolerability. Mechanistically, treatment with PTC596 induced cell cycle arrest at G2/M phase followed by apoptotic cell death, associated with the inhibition of microtubule polymerization. RNA sequence analysis also revealed that PTC596 and the combination with bortezomib affected the cell cycle and apoptosis in MM cells. Importantly, endoplasmic reticulum stress induced by bortezomib was enhanced by PTC596, providing an underlying mechanism of action of the combination therapy. Our results indicate that PTC596 alone and in combination with proteasome inhibition are potential novel therapeutic options to improve outcomes in patients with MM.

BMI1 in the heart: Novel functions beyond tumorigenesis

The BMI1 protein, a member of the PRC1 family, is a well recognised transcriptional suppressor and has the capability of maintaining the self-renewal and proliferation of tissue-specific stem cells. Numerous studies have established that BMI1 is highly expressed in a variety of malignant cancers and serves as a key regulator in the tumorigenesis process. However, our understanding of BMI1 in terminally differentiated organs, such as the heart, is relatively nascent. Importantly, emerging data support that, beyond the tumor, BMI1 is also expressed in the heart tissue and indeed exerts profound effects in various cardiac pathological conditions. This review gives a summary of the novel functions of BMI1 in the heart, including BMI1-positive cardiac stem cells and BMI1-mediated signaling pathways, which are involved in the response to various cardiac pathological stimuli. Besides, we summarize the recent progress of BMI1 in some novel and rapidly developing cardiovascular therapies. Furtherly, we highlight the properties of BMI1, a therapeutic target proved effective in cancer treatment, as a promising target to alleviate cardiovascular diseases.

Role of Polycomb Complexes in Normal and Malignant Plasma Cells

Plasma cells (PC) are the main effectors of adaptive immunity, responsible for producing antibodies to defend the body against pathogens. They are the result of a complex highly regulated cell differentiation process, taking place in several anatomical locations and involving unique genetic events. Pathologically, PC can undergo tumorigenesis and cause a group of diseases known as plasma cell dyscrasias, including multiple myeloma (MM). MM is a severe disease with poor prognosis that is characterized by the accumulation of malignant PC within the bone marrow, as well as high clinical and molecular heterogeneity. MM patients frequently develop resistance to treatment, leading to relapse. Polycomb group (PcG) proteins are epigenetic regulators involved in cell fate and carcinogenesis. The emerging roles of PcG in PC differentiation and myelomagenesis position them as potential therapeutic targets in MM. Here, we focus on the roles of PcG proteins in normal and malignant plasma cells, as well as their therapeutic implications.

The BCL-2 family members NOXA and BIM mediate fluorizoline-induced apoptosis in multiple myeloma cells

Fluorizoline is a new synthetic molecule that induces apoptosis by selectively targeting prohibitins. In this study, we have assessed the pro-apoptotic effect of fluorizoline in 3 different multiple myeloma cell lines and 12 primary samples obtained from treatment-naïve multiple myeloma patients. Fluorizoline induced apoptosis in both multiple myeloma cell lines and primary samples at concentrations in the low micromolar range. All primary samples were sensitive to fluorizoline. Moreover, fluorizoline increased the mRNA and protein levels of the pro-apoptotic BCL-2 family member NOXA both in cell lines and primary samples analyzed. Finally, NOXA-depletion by CRISPR/Cas9 in cells that do not express BIM conferred resistance to fluorizoline-induced apoptosis in multiple myeloma cells. These results suggest that targeting prohibitins could be a new therapeutic strategy for myeloma multiple.

BMI1-KLF4 axis deficiency improves responses to neoadjuvant concurrent chemoradiotherapy in patients with rectal cancer

PURPOSE

Neoadjuvant concurrent chemoradiotherapy (CCRT) is a gold standard treatment for patients with stage II/III rectal cancer. B-cell-specific Moloney murine leukemia virus insertion site 1 (BMI1) is a member of the polycomb group of proteins that are involved in regulating gene expression. High levels of BMI1 have been demonstrated to contribute to the malignant phenotypes of several cancers; however, its relevance in rectal cancer treated with CCRT is largely unknown.

METHODS AND MATERIALS

We used two patient cohorts to address the clinical relevance of BMI1 in human cancers. In addition, HT-29 and HCT-116 cells were chosen as our in vitro models to verify the role of BMI1 in cell response to ionizing radiation. Stemness-related proteins were analyzed by western blotting and cell survival was determined using clonogenic assays.

RESULTS

BMI1 overexpression was found to significantly correlate with advanced pre-treatment nodal status (N1-N2; p < 0.001), post-treatment tumor stage (T1-T2; p = 0.015), inferior tumor regression grade (p = 0.001), and also an independent prognosis factor in 172 rectal cancer patients receiving CCRT. Serial cell-based functional examination indicated that BMI1 deficiency sensitized cells to radiation treatment by modulating the gene expression of Kruppel-like factor 4 (KLF4) and enhanced radiosensitivity in microsatellite stable (MSS) colorectal cancers. Overexpression of KLF4 partially overcame BMI1-deficiency-mediated γ-H2AX expression after ionizing radiation exposure. Consistent with in vitro data, an analysis of an additional 30 rectal cancer tissue specimens revealed a positive correlation between BMI1 and KLF4 (p = 0.02).

CONCLUSION

Higher levels of BMI1 are associated with poor therapeutic response and adverse outcomes in rectal cancer patients receiving CCRT.

Epigenetic Regulation of Bmi1 by Ubiquitination and Proteasomal Degradation Inhibit Bcl-2 in Acute Myeloid Leukemia

Bmi1 is associated with advanced prognosis of acute myeloid leukemia (AML), and polyethylenimine (PEI)-stabilized Bmi1 siRNA-entrapped human serum albumin (HSA) nanocarriers (PEI@HSANCs) were used to protect siRNA from degradation and also to control epigenetic regulation-based AML therapy. The nanoform increased the transfection efficiency of Bmi1 siRNA through caveolae-mediated endocytosis and enhanced Bax translocation into the mitochondria. It enhanced the caspase 3-mediated apoptosis through the Bax activation and Bcl-2 inhibition. The molecular analysis reveals the downregulation of polycomb proteins, Bmi1 and EzH2, along with inhibition of H3K27me3 and H2AK119ub1. The signaling cascade revealed downregulation of Bmi1 through ubiquitin-mediated degradation and is reversed by a proteasome inhibitor. Further mechanistic studies established a crucial role of transcription factor, C-Myb and Bmi1, as its direct targets for maintenance and progression of AML. Chromatin immunoprecipitation (ChIP) assay confirmed Bmi1 as a direct target of C-Myb as it binds to promoter sequence of Bmi1 between -235 to +43 and -111 to +43. The in vivo studies performed in the AML xenograft model evidence a decrease in the population of leukemic stem cells marker (CD45⁺) and an increase in the myeloid differentiating marker expression (CD11b⁺) in the bone marrow after the Bmi1 siRNA nanoconjugated therapy. Activation of apoptotic pathways and withdrawal of epigenetic repression through a ubiquitin proteasomal pathway potentiating a novel antileukemic therapy were established.

B lymphoma Moloney murine leukemia virus insertion region 1: An oncogenic mediator in prostate cancer

B lymphoma Moloney murine leukemia virus insertion region 1 (BMI1), a core member of polycomb repressive complex 1 (PRC1), has been intensely investigated in the field of cancer epigenetics for decades. Widely known as a critical regulator in cellular physiology, BMI1 is essential in self-renewal and differentiation in different lineages of stem cells. BMI1 also plays a significant role in cancer etiology for its involvement in pathological progress such as epithelial–mesenchymal transition (EMT) and cancer stem cell maintenance, propagation, and differentiation. Importantly, overexpression of BMI1 is predictive for drug resistance, tumor recurrence, and eventual therapy failure of various cancer subtypes, which renders the pharmacological targeting at BMI1 as a novel and promising therapeutic approach. The study on prostate cancer, a prevalent hormone-related cancer among men, has promoted enormous research advancements in cancer genetics and epigenetics. This review summarizes the role of BMI1 as an oncogenic and epigenetic regulator in tumor initiation, progression, and relapse of prostate cancer.

The anti-mitotic agents PTC-028 and PTC596 display potent activity in pre-clinical models of multiple myeloma but challenge the role of BMI-1 as an essential tumour gene

Future progress in the treatment of multiple myeloma (MM) requires both the characterisation of key drivers of the disease and novel, innovative approaches to tackle these vulnerabilities. The present study focussed on the pre-clinical evaluation of a novel drug class, BMI-1 modulators, in MM. We demonstrate potent activity of PTC-028 and PTC596 in a comprehensive set of in vitro and in vivo models, including models of drug resistance and stromal support. Treatment of MM cells with PTC-028 and PTC596 downregulated BMI-1 protein levels, which was found to correlate with drug activity. Surprisingly, BMI-1 was dispensable for the activity of BMI-1 modulators and MM cell growth. Our data rather point to mitotic arrest accompanied by myeloid cell leukaemia-1 (MCL-1) loss as key anti-MM mechanisms and reveal impaired MYC and AKT signalling activity due to BMI-1 modulator treatment. Moreover, we observed a complete eradication of MM after PTC596 treatment in the 5TGM.1 in vivo model and define epigenetic compounds and B cell leukaemia/lymphoma 2 homology domain 3 (BH3) mimetics as promising combination partners. These results bring into question the postulated role of BMI-1 as an essential MM gene and confirm BMI-1 modulators as potent anti-mitotic agents with encouraging pre-clinical activity that supports their rapid translation into clinical trials.

PTC-209 Anti-Cancer Effects Involved the Inhibition of STAT3 Phosphorylation

Introduction: Lung, breast, and colorectal cancers are the leading causes of cancer-related deaths despite many therapeutic options, including targeted therapy and immunotherapies.

Methods: Here, we investigated the impact of PTC-209, a small-molecule Bmi-1 inhibitor, on human cancer cell viability alone and in combination with anticancer drugs, namely, cisplatin, oxaliplatin, 5-fluorouracil, camptothecin, and Frondoside-A and its impact on cellular migration and colony growth in vitro and on tumor growth in ovo.

Results: We demonstrate that PTC-209 causes a concentration- and time-dependent decrease in the cellular viability of lung cancer cells (LNM35 and A549), breast cancer cells (MDA-MB-231 and T47D), and colon cancer cells (HT-29, HCT8/S11, and HCT-116). Similarly, treatment with PTC-209 significantly decreased the growth of LNM35, A549, MDA-MB-231, and HT-29 clones and colonies in vitro and LNM35 and A549 tumor growth in the in ovo tumor xenograft model. PTC-209 at the non-toxic concentrations significantly reduced the migration of lung (LNM35 and A549) and breast (MDA-MB-231) cancer cells. Moreover, we show that PTC-209, at a concentration of 1 μM, enhances the anti-cancer effects of Frondoside-A in lung, breast, and colon cancer cells, as well as the effect camptothecin in breast cancer cells and the effect of cisplatin in lung cancer cells in vitro. However, PTC-209 failed to enhance the anti-cancer effects of oxaliplatin and 5-fluorouracil in colon cancer cells. Treatment of lung, breast, and colon cancer cells with PTC-209 (1 and 2.5 μM) for 48 h showed no caspase-3 activation, but a decrease in the cell number below the seeding level suggests that PTC-209 reduces cellular viability probably through inhibition of cell proliferation and induction of cell death via a caspase-3–independent mechanism. Molecular mechanism analysis revealed that PTC-209 significantly inhibited the STAT3 phosphorylation by decreasing the expression level of gp130 as early as 30 min post-treatment.

Conclusion: Our findings identify PTC-209 as a promising anticancer agent for the treatment of solid tumors either alone and/or in combination with the standard cytotoxic drugs cisplatin and camptothecin and the natural product Frondoside-A.

Acute myeloid leukemia driven by the CALM-AF10 fusion gene is dependent on BMI1

A subset of acute myeloid and lymphoid leukemia cases harbor a t(10;11)(p13;q14) translocation resulting in the CALM-AF10 fusion gene. Standard chemotherapeutic strategies are often ineffective in treating patients with CALM-AF10 fusions. Hence, there is an urgent need to identify molecular pathways dysregulated in CALM-AF10-positive leukemias which may lay the foundation for novel targeted therapies. Here we demonstrate that the Polycomb Repressive Complex 1 gene BMI1 is consistently overexpressed in adult and pediatric CALM-AF10-positive leukemias. We demonstrate that genetic Bmi1 depletion abrogates CALM-AF10-mediated transformation of murine hematopoietic stem and progenitor cells (HSPCs). Furthermore, CALM-AF10-positive murine and human AML cells are sensitive to the small-molecule BMI1 inhibitor PTC-209 as well as to PTC-596, a compound in clinical development that has been shown to result in downstream degradation of BMI1 protein. PTC-596 significantly prolongs survival of mice injected with a human CALM-AF10 cell line in a xenograft assay. In summary, these results validate BMI1 as a bona fide candidate for therapeutic targeting in AML with CALM-AF10 rearrangements.

Epigenetic-Based Mechanisms of Osteoblast Suppression in Multiple Myeloma Bone Disease

Multiple myeloma (MM) bone disease is characterized by the development of osteolytic lesions, which cause severe complications affecting the morbidity, mortality, and treatment of myeloma patients. Myeloma tumors seeded within the bone microenvironment promote hyperactivation of osteoclasts and suppression of osteoblast differentiation. Because of this prolonged suppression of bone marrow stromal cells’ (BMSCs) differentiation into functioning osteoblasts, bone lesions in patients persist even in the absence of active disease. Current antiresorptive therapy provides insufficient bone anabolic effects to reliably repair MM lesions. It has become widely accepted that myeloma‐exposed BMSCs have an altered phenotype with pro‐inflammatory, immune‐modulatory, anti‐osteogenic, and pro‐adipogenic properties. In this review, we focus on the role of epigenetic‐based modalities in the establishment and maintenance of myeloma‐induced suppression of osteogenic commitment of BMSCs. We will focus on recent studies demonstrating the involvement of chromatin‐modifying enzymes in transcriptional repression of osteogenic genes in MM‐BMSCs. We will further address the epigenetic plasticity in the differentiation commitment of osteoprogenitor cells and assess the involvement of chromatin modifiers in MSC‐lineage switching from osteogenic to adipogenic in the context of the inflammatory myeloma microenvironment. Lastly, we will discuss the potential of employing small molecule epigenetic inhibitors currently used in the MM research as therapeutics and bone anabolic agents in the prevention or repair of osteolytic lesions in MM. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Synthesis of Cyanoenone-Modified Diterpenoid Analogs as Novel Bmi-1-Mediated Antitumor Agents

Bmi-1 is overexpressed in colorectal cancer (CRC) and served as a novel therapeutic target for the treatment of CRC. A series of novel cyanoenone-modified diterpenoid analogs was synthesized and investigated for their antiproliferative activity against CRC cells. The results showed that most of these compounds exhibited potent antiproliferative and Bmi-1 inhibitory activity. Among them, the most active compound 33 (SH498) showed more potent antiproliferative activity than the positive control compound PTC-209. These synthetic diterpenoid analogs were less toxic for normal human fibroblasts (HAF) than for CRC cells. Especially 33, its selectivity index (SI) between HAF and tumor cells was 7.3-13.1, which was much better than PTC-209. The polycomb repressive complex 1 (PRC1) complex, transwell migration, colony formation, cancer stem cell proliferation, and apoptosis assays of 33 were performed on CRC cell lines. The in vivo antitumor effect of 33 was also observed in HCT116 tumor-bearing mice.

Polycomb complexes in normal and malignant hematopoiesis

Di Carlo et al. discuss how the regulation/dysregulation of Polycomb group proteins contributes to hematopoiesis and hematological disorders.

Epigenetic mechanisms are crucial for sustaining cell type–specific transcription programs. Among the distinct factors, Polycomb group (PcG) proteins are major negative regulators of gene expression in mammals. These proteins play key roles in regulating the proliferation, self-renewal, and differentiation of stem cells. During hematopoietic differentiation, many PcG proteins are fundamental for proper lineage commitment, as highlighted by the fact that a lack of distinct PcG proteins results in embryonic lethality accompanied by differentiation biases. Correspondingly, proteins of these complexes are frequently dysregulated in hematological diseases. In this review, we present an overview of the role of PcG proteins in normal and malignant hematopoiesis, focusing on the compositional complexity of PcG complexes, and we briefly discuss the ongoing clinical trials for drugs targeting these factors.

Targeting EZH2 in Multiple Myeloma-Multifaceted Anti-Tumor Activity

The enhancer of zeste homolog 2 (EZH2) is the enzymatic subunit of the polycomb repressive complex 2 (PRC2) that exerts important functions during normal development as well as disease. PRC2 through EZH2 tri-methylates histone H3 lysine tail residue 27 (H3K27me3), a modification associated with repression of gene expression programs related to stem cell self-renewal, cell cycle, cell differentiation, and cellular transformation. EZH2 is deregulated and subjected to gain of function or loss of function mutations, and hence functions as an oncogene or tumor suppressor gene in a context-dependent manner. The development of highly selective inhibitors against the histone methyltransferase activity of EZH2 has also contributed to insight into the role of EZH2 and PRC2 in tumorigenesis, and their potential as therapeutic targets in cancer. EZH2 can function as an oncogene in multiple myeloma (MM) by repressing tumor suppressor genes that control apoptosis, cell cycle control and adhesion properties. Taken together these findings have raised the possibility that EZH2 inhibitors could be a useful therapeutic modality in MM alone or in combination with other targeted agents in MM. Therefore, we review the current knowledge on the regulation of EZH2 and its biological impact in MM, the anti-myeloma activity of EZH2 inhibitors and their potential as a targeted therapy in MM.

Targeting of BMI-1 with PTC-209 inhibits glioblastoma development

Glioblastoma multiforme (GBM) is the most common and aggressive brain tumor and refractory to existing therapies. The oncogene BMI-1, a member of Polycomb Repressive Complex 1 (PRC1) plays essential roles in various human cancers and becomes an attractive therapeutic target. Here we showed that BMI-1 is highly expressed in GBM and especially enriched in glioblastoma stem cells (GSCs). Then we comprehensively investigated the anti-GBM effects of PTC-209, a novel specific inhibitor of BMI-1. We found that PTC-209 efficiently downregulates BMI-1 expression and the histone H2AK119ub1 levels at microM concentrations. In vitro, PTC-209 effectively inhibits glioblastoma cell proliferation and migration, and GSC self-renewal. Transcriptomic analyses of TCGA datasets of glioblastoma and PTC-209-treated GBM cells demonstrate that PTC-209 reverses the altered transcriptional program associated with BMI-1 overexpression. And Chromatin Immunoprecipitation assay confirms that the derepressed tumor suppressor genes belong to BMI-1 targets and the enrichment levels of H2AK119ub1 at their promoters is decreased upon PTC-209 treatment. Strikingly, the glioblastoma growth is significantly attenuated by PTC-209 in a murine orthotopic xenograft model. Therefore our study provides proof-of-concept for inhibitors targeting BMI-1 in potential applications as an anti-GBM therapy.

Downregulation of Bmi1 in breast cancer stem cells suppresses tumor growth and proliferation

Targeting cancer stem cells during initial treatment is important to reduce incidence of recurrent disease. Bmi1 has been associated with cancer stem cell self-renewal and aggressive disease. The purpose of this study was to determine the effects of downregulation of Bmi1 in breast cancer stem cells in order to target and eliminate the stem cell population in the tumor mass. Bmi1 was downregulated using two approaches in the mouse breast cancer stem cell line FMMC 419II—a small molecule inhibitor (PTC 209) and stable transfection with a Bmi1 shRNA plasmid. The functional effect of Bmi1 downregulation was tested in vitro and in vivo. Each approach led to decreased Bmi1 expression that correlated with an inhibition of cancer stem cell properties in vitro including cell cycle arrest and reduced mammosphere forming potential, and a decrease in tumor mass in vivo after either intra-tumoral or systemic nanoparticle-targeted delivery of anti-Bmi1. These results show that inhibiting Bmi1 expression in breast cancer stem cells could be important for the complete elimination of tumor and potentially preventing disease relapse.

Synthesis and Characterization of Novel BMI1 Inhibitors Targeting Cellular Self-Renewal in Hepatocellular Carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) represents one of the most lethal cancers worldwide due to therapy resistance and disease recurrence. Tumor relapse following treatment could be driven by the persistence of liver cancer stem-like cells (CSCs). The protein BMI1 is a member of the polycomb epigenetic factors governing cellular self-renewal, proliferation, and stemness maintenance. BMI1 expression also correlates with poor patient survival in various cancer types.

OBJECTIVE

We aimed to elucidate the extent to which BMI1 can be used as a potential therapeutic target for CSC eradication in HCC.

METHODS

We have recently participated in characterizing the first known pharmacological small molecule inhibitor of BMI1. Here, we synthesized a panel of novel BMI1 inhibitors and examined their ability to alter cellular growth and eliminate cancer progenitor/stem-like cells in HCC with different p53 backgrounds.

RESULTS

Among various molecules examined, RU-A1 particularly downregulated BMI1 expression, impaired cell viability, reduced cell migration, and sensitized HCC cells to 5-fluorouracil (5-FU) in vitro. Notably, long-term analysis of HCC survival showed that, unlike chemotherapy, RU-A1 effectively reduced CSC content, even as monotherapy. BMI1 inhibition with RU-A1 diminished the number of stem-like cells in vitro more efficiently than the model compound C-209, as demonstrated by clonogenic assays and impairment of CSC marker expression. Furthermore, xenograft assays in zebrafish showed that RU-A1 abrogated tumor growth in vivo.

CONCLUSIONS

This study demonstrates the ability to identify agents with the propensity for targeting CSCs in HCC that could be explored as novel treatments in the clinical setting.

Maternal embryonic leucine zipper kinase is a novel target for proliferation-associated high-risk myeloma

Treatment of high-risk patients is a major challenge in multiple myeloma. This is especially true for patients assigned to the gene expression profiling-defined proliferation subgroup. Although recent efforts have identified some key players of proliferative myeloma, genetic interactions and players that can be targeted with clinically effective drugs have to be identified in order to overcome the poor prognosis of these patients. We therefore examined maternal embryonic leucine zipper kinase (MELK) for its implications in hyper-proliferative myeloma and analyzed the activity of the MELK inhibitor OTSSP167 both in vitro and in vivoMELK was found to be significantly overexpressed in the proliferative subgroup of myeloma. This finding translated into poor overall survival in patients with high vs low MELK expression. Enrichment analysis of upregulated genes in myeloma cells of MELK^high patients confirmed the strong implications in myeloma cell proliferation. Targeting MELK with OTSSP167 impaired the growth and survival of myeloma cells, thereby affecting central survival factors such as MCL-1 and IRF4 This activity was also observed in the 5TGM.1 murine model of myeloma. OTSSP167 reduced bone marrow infiltration and serum paraprotein levels in a dose-dependent manner. In addition, we revealed a strong link between MELK and other proliferation-associated high-risk genes (PLK-1, EZH2, FOXM1, DEPDC1) and MELK inhibition also impaired the expression of those genes. We therefore conclude that MELK is an essential component of a proliferative gene signature and that pharmacological inhibition of MELK represents an attractive novel approach to overcome the poor prognosis of high-risk patients with a proliferative expression pattern.

The polycomb group protein BMI-1 inhibitor PTC-209 is a potent anti-myeloma agent alone or in combination with epigenetic inhibitors targeting EZH2 and the BET bromodomains

Multiple myeloma (MM) is a tumor of plasmablasts/plasma cells (PCs) characterized by the expansion of malignant PCs with complex genetic aberrations in the bone marrow (BM). Recent reports, by us and others, have highlighted the polycomb group (PcG) proteins as potential targets for therapy in MM. The PcG protein BMI-1 of the polycomb repressive complex 1 (PRC1) has been reported to be overexpressed and to possess oncogenic functions in MM. Herein, we report on the anti-myeloma effects of the BMI-1 inhibitor PTC-209 and demonstrate that PTC-209 is a potent anti-myeloma agent in vitro using MM cell lines and primary MM cells. We show that PTC-209 reduces the viability of MM cells via induction of apoptosis and reveal that the anti-MM actions of PTC-209 are mediated by on-target effects i.e. downregulation of BMI-1 protein and the associated repressive histone mark H2AK119ub, leaving other PRC1 subunits such as CBX-7 and the catalytic subunit RING1B unaffected. Importantly, we demonstrate that PTC-209 exhibits synergistic and additive anti-myeloma activity when combined with other epigenetic inhibitors targeting EZH2 and BET bromodomains. Collectively, these data qualify BMI-1 as a candidate for targeted therapy in MM alone or in combinations with epigenetic inhibitors directed to PRC2/EZH2 or BET bromodomains.

Epigenetics in multiple myeloma: From mechanisms to therapy

Multiple myeloma (MM) is a tumor of antibody producing plasmablasts/plasma cells that resides within the bone marrow (BM). In addition to the well-established role of genetic lesions and tumor-microenvironment interactions in the development of MM, deregulated epigenetic mechanisms are emerging as important in MM pathogenesis. Recently, MM sequencing and expression projects have revealed that mutations and copy number variations as well as deregulation in the expression of epigenetic modifiers are characteristic features of MM. In the past decade, several studies have suggested epigenetic mechanisms via DNA methylation, histone modifications and non-coding RNAs as important contributing factors in MM with impacts on disease initiation, progression, clonal heterogeneity and response to treatment. Herein we review the present view and knowledge that has accumulated over the past decades on the role of epigenetics in MM, with focus on the interplay between epigenetic mechanisms and the potential use of epigenetic inhibitors as future treatment modalities for MM.

Bispecific antibody suppresses osteosarcoma aggressiveness through regulation of NF-κB signaling pathway

Osteosarcoma is one of the most lethal malignancies, and the prognosis remains dismal due to the paucity of effective therapeutic targets. Bmi-1 and TRIM-14 are associated with the initiation and progression of osteosarcoma, which could promote angiogenesis, invasion, and apoptotic resistance in bone cancer tissue. In this study, we constructed a bispecific antibody of BsAbBmi/TRIM targeting Bmi-1 and TRIM-14 and investigated the therapeutic value in bone carcinoma cells and xenograft mice. Our results showed that Bmi-1 and TRIM-14 expression levels were markedly upregulated correlated with nuclear factor-κB nuclear translocation in bone cancer cells and clinical carcinoma tissues. Results have demonstrated that overexpression of Bmi-1 and TRIM-14 promoted growth, proliferation, aggressiveness, and apoptosis resistance of osteosarcoma cells. BsAbBmi/TRIM administration significantly inhibited nuclear factor-κB expression derived by matrix metalloproteinase-9 promoter. BsAbBmi/TRIM administration inhibited growth of osteosarcoma cells and downregulated Bmi-1 and TRIM-14 expression levels. Data also demonstrated that migration and invasion of osteosarcoma cells were also inhibited by BsAbBmi/TRIM. In addition, results illustrated that BsAbBmi/TRIM inhibited tumor growth and tumorigenicity by blockaded sensor expression in nuclear factor-κB signal pathway. Furthermore, in vivo study showed that BsAbBmi/TRIM treatment markedly inhibited the tumorigenicity and growth of osteosarcoma cells compared to either AbBmi-1 or AbTRIM-14 treatment. Notably, survival of xenograft mice was prolonged by BsAbBmi/TRIM treatment compared to either AbBmi-1 or AbTRIM-14 treatment. In conclusion, these results provided new evidence that BsAbBmi/TRIM inhibited the progression of osteosarcoma, which suggest that BsAbBmi/TRIM may be a novel anti-cancer agent for osteosarcoma therapy.

Identification of BMI1 Promoter Inhibitors from Beaumontia murtonii and Eugenia operculata

B-Cell-specific Moloney murine leukemia virus insertion region 1 (BMI1) is a core component of the polycomb repressive complex 1 (PRC1). Abnormal expression of BMI1 is associated with a number of human malignances and cancer stem cells (CSCs), which cause chemotherapy resistance. Therefore, small molecules that inhibit BMI1 expression are potential candidates for cancer therapy. In this study, a cell-based reporter gene assay was developed that allowed BMI1 promoter activity to be measured in 293T human embryonic kidney cells based on luciferase expression levels. Using this screening assay, the methanol-soluble extracts of Beaumontia murtonii and Eugenia operculata were selected as leads. Bioassay-guided fractionation of the extracts led to the isolation of three known cardenolides (1-3) and one new compound (4) from B. murtonii and two known triterpenoids (5 and 6) and one new compound (7) from E. operculata. These seven compounds inhibited BMI1 promoter activity (IC₅₀ range 0.093-23.0 μM), and the most active compound, wallichoside (1), was further evaluated. Western blot analysis revealed that wallichoside (1) decreases BMI1 protein levels in HCT116 human colon carcinoma cells, and flow cytometry analysis showed that it significantly reduced levels of the CSC biomarker epithelial cell adhesion molecule. Wallichoside (1) also inhibited sphere formation of Huh7 human hepatocellular carcinoma cells, indicating that it diminished the self-renewal capability of CSCs.

Silencing BMI1 radiosensitizes human breast cancer cells by inducing DNA damage and autophagy

Overexpression of BMI1 in human cancer cells, a member of the polycomb group of repressive complexes, correlates with advanced stage of disease, aggressive clinico-pathological behavior, poor prognosis, and resistance to radiation and chemotherapy. Studies have shown that experimental reduction of BMI1 protein level in tumor cells results in inhibition of cell proliferation, induction of apoptosis and/or senescence, and increased susceptibility to cytotoxic agents and radiation therapy. Although a role for BMI1 in cancer progression and its importance as a molecular target for cancer therapy has been established, information on the impact of silencing BMI1 in triple-negative breast cancer (TNBC) and its consequence on radiotherapy have not been well studied. Therefore, in the present study we investigated the potential therapeutic benefit of radiation therapy in BMI1-silenced breast cancer cells and studied the mechanism(s) of radiosensitization. Human MDA-MB-231 and SUM159PT breast cancer cells that were either stably transfected with a lentiviral vector expressing BMI1 shRNA (shBMI1) or control shRNA (shControl) or transient transfection with a BMI1-specific siRNA were used. Silencing of BMI1 resulted in marked reduction in BMI1 both at the mRNA and protein level that was accompanied by a significant reduction in cell migration compared to control cells. Further, BMI1 knockdown produced a marked enhancement of DNA damage as evidenced by Comet Assay and γH2AX foci, resulting in a dose-dependent radiosensitization effect. Molecular studies revealed modulation of protein expression that is associated with the DNA damage response (DDR) and autophagy pathways. Our results demonstrate that BMI1 is an important therapeutic target in breast cancer and suppression of BMI1 produces radiation sensitivity. Further, combining BMI1-targeted therapeutics with radiation might benefit patients diagnosed with TNBC.

The novel BMI-1 inhibitor PTC596 downregulates MCL-1 and induces p53-independent mitochondrial apoptosis in acute myeloid leukemia progenitor cells

Disease recurrence is the major problem in the treatment of acute myeloid leukemia (AML). Relapse is driven by leukemia stem cells, a chemoresistant subpopulation capable of re-establishing disease. Patients with p53 mutant AML are at an extremely high risk of relapse. B-cell-specific Moloney murine leukemia virus integration site 1 (BMI-1) is required for the self-renewal and maintenance of AML stem cells. Here we studied the effects of a novel small molecule inhibitor of BMI-1, PTC596, in AML cells. Treatment with PTC596 reduced MCL-1 expression and triggered several molecular events consistent with induction of mitochondrial apoptosis: loss of mitochondrial membrane potential, BAX conformational change, caspase-3 cleavage and phosphatidylserine externalization. PTC596 induced apoptosis in a p53-independent manner. PTC596 induced apoptosis along with the reduction of MCL-1 and phosphorylated AKT in patient-derived CD34⁺CD38^low/− stem/progenitor cells. Mouse xenograft models demonstrated in vivo anti-leukemia activity of PTC596, which inhibited leukemia cell growth in vivo while sparing normal hematopoietic cells. Our results indicate that PTC596 deserves further evaluation in clinical trials for refractory or relapsed AML patients, especially for those with unfavorable complex karyotype or therapy-related AML that are frequently associated with p53 mutations.