Autophagy mediates proteolysis of NPM1 and HEXIM1 and sensitivity to BET inhibition in AML cells

Unlocking the secrets of aging: Epigenetic reader BRD4 as the target to combatting aging-related diseases

BACKGROUND

Aging, a complex and profound journey, leads us through a labyrinth of physiological and pathological transformations, rendering us increasingly susceptible to aging-related diseases. Emerging investigations have unveiled the function of bromodomain containing protein 4 (BRD4) in manipulating the aging process and driving the emergence and progression of aging-related diseases.

AIM OF REVIEW

This review aims to offer a comprehensive outline of BRD4's functions involved in the aging process, and potential mechanisms through which BRD4 governs the initiation and progression of various aging-related diseases.

KEY SCIENTIFIC CONCEPTS OF REVIEW

BRD4 has a fundamental role in regulating the cell cycle, apoptosis, cellular senescence, the senescence-associated secretory phenotype (SASP), senolysis, autophagy, and mitochondrial function, which are involved in the aging process. Several studies have indicated that BRD4 governs the initiation and progression of various aging-related diseases, including Alzheimer's disease, ischemic cerebrovascular diseases, hypertension, atherosclerosis, heart failure, aging-related pulmonary fibrosis, and intervertebral disc degeneration (IVDD). Thus, the evidence from this review supports that BRD4 could be a promising target for managing various aging-related diseases, while further investigation is warranted to gain a thorough understanding of BRD4's role in these diseases.

The role of autophagy in acute myeloid leukemia development

INTRODUCTION

Autophagy is a highly conservative self-degradative process. It aims at elimination-impaired proteins and cellular organelles. Previous research confirmed the autophagy role in cancer pathogenesis.

AREAS COVERED

This article discusses the role of autophagy in the development of AML. Autophagy seems to be a 'double-sword' mechanism, hence, either its suppression or induction could promote neoplasm growth. This mechanism could also be the aim of the 'molecular targeted therapy.' Chemo- and radiotherapy induce cellular stress in neoplasm cells with subsequent autophagy suppression. Simultaneously, it is claimed that the autophagy suppression increases chemosensitivity 'in neoplastic cells. Some agents, like bortezomib, in turn could promote autophagy process, e.g. in AML (acute myeloid leukemia). However, currently there are not many studies focusing on the role of autophagy in patients suffering for AML. In this review, we summarize the research done so far on the role of autophagy in the development of AML.

EXPERT OPINION

The analysis of autophagy genes expression profiling in AML could be a relevant factor in the diagnostic process and treatment 'individualization.' Autophagy modulation seems to be a relevant target in the oncological therapy - it could limit disease progression and increase the effectiveness of treatment.

Nucleophosmin 1: from its pathogenic role to a tantalizing therapeutic target in acute myeloid leukemia

Nucleophosmin 1 (NPM1, also known as B23) is a multifunctional protein involved in a variety of cellular processes, including ribosomal maturation, centrosome replication, maintenance of genomic stability, cell cycle control, and apoptosis. NPM1 is the most commonly mutated gene in adult acute myeloid leukemia (AML) and is present in approximately 40% of all AML cases. The underlying mechanisms of mutant NPM1 (NPM1^mut) in leukemogenesis remain unclear. This review summarizes the structure and physiological function of NPM1, mechanisms underlying the pathogenesis of NPM1-mutated AML, and the potential role of NPM1 as a therapeutic target. It is reported that dysfunctional NPM1 might cause AML pathogenesis via its role as a protein chaperone, inhibiting differentiation of leukemia stem cells and regulation of non-coding RNAs. Besides conventional chemotherapies, NPM1 is a promising therapeutic target against AML that warrants further investigation. NPM1-based therapeutic strategies include inducing nucleolar relocalisation of NPM1 mutants, interfering with NPM1 oligomerization, and NPM1 as an immune response target.

BET Inhibition Induces HEXIM1- and RAD51-Dependent Conflicts between Transcription and Replication

BET bromodomain proteins are required for oncogenic transcription activities, and BET inhibitors have been rapidly advanced into clinical trials. Understanding the effects of BET inhibition on processes such as DNA replication will be important for future clinical applications. Here, we show that BET inhibition, and specifically inhibition of BRD4, causes replication stress through a rapid overall increase in RNA synthesis. We provide evidence that BET inhibition acts by releasing P-TEFb from its inhibitor HEXIM1, promoting interference between transcription and replication. Unusually, these transcription-replication conflicts do not activate the ATM/ATR-dependent DNA damage response but recruit the homologous recombination factor RAD51. Both HEXIM1 and RAD51 promote BET inhibitor-induced fork slowing but also prevent a DNA damage response. Our data suggest that BET inhibitors slow replication through concerted action of transcription and recombination machineries and shed light on the importance of replication stress in the action of this class of experimental cancer drugs.

BET Inhibition Suppresses S100A8 and S100A9 Expression in Acute Myeloid Leukemia Cells and Synergises with Daunorubicin in Causing Cell Death

S100A8 and S100A9 are both members of the S100 family and have been shown to play roles in myeloid differentiation, autophagy, apoptosis, and chemotherapy resistance. In this study we demonstrate that the BET-bromodomain inhibitor JQ1 causes rapid suppression of S100A8 and S100A9 mRNA and protein in a reversible manner. In addition, we show that JQ1 synergises with daunorubicin in causing AML cell death. Daunorubicin alone causes a dose- and time-dependent increase in S100A8 and S100A9 protein levels in AML cell lines which is overcome by cotreatment with JQ1. This suggests that JQ1 synergises with daunorubicin in causing apoptosis via suppression of S100A8 and S100A9 levels.

Ubenimex enhances Brd4 inhibition by suppressing HEXIM1 autophagic degradation and suppressing the Akt pathway in glioma cells

Inhibition of Brd4 by JQ1 treatment showed potential in the treatment of glioma, however, some cases showed low sensitivity of JQ1. In addition, the pre-clinical analysis showed its limitation by demonstrating that transient treatment with JQ1 leads to aggressive tumor development. Thus, an improved understanding of the mechanisms underlying JQ1 is urgently required to design strategies to improve its efficiency, as well as overcome its limitation. HEXIM1 has been confirmed to have an important role in regulating JQ1 sensitivity. In our study, ubenimex, a classical anti-cancer drug showed potential in regulating the JQ1 sensitivity of glioma cells using the WST-1 proliferation assay. Further studies demonstrated that ubenimex inhibited autophagy and downregulated the autophagic degradation of HEXIM1. The role of HEXIM1 in regulating JQ1 sensitivity was verified by the HEXIM1 knockdown. Since ubenimex was verified as an Akt inhibitor, we further studied the role of Akt inhibition in regulating JQ1 sensitivity and migration of glioma cells. Data showed that ubenimex improved the efficiency of JQ1 treatment and suppressed migration both in the in vitro and in vivo xenografts models. The Akt agonist attenuated these effects, pointing to the role of Akt inhibition in JQ1 sensitivity and suppressed migration. Our findings suggest the potential of ubenimex adjuvant treatment to enhance JQ1 efficiency and attenuate parts of its side effect (enhancing tumor aggressive) by regulating the autophagic degradation of HEXIM1 and Akt inhibition.

Multiplexed Proteome Dynamics Profiling Reveals Mechanisms Controlling Protein Homeostasis

Protein degradation plays important roles in biological processes and is tightly regulated. Further, targeted proteolysis is an emerging research tool and therapeutic strategy. However, proteome-wide technologies to investigate the causes and consequences of protein degradation in biological systems are lacking. We developed “multiplexed proteome dynamics profiling” (mPDP), a mass-spectrometry-based approach combining dynamic-SILAC labeling with isobaric mass tagging for multiplexed analysis of protein degradation and synthesis. In three proof-of-concept studies, we uncover different responses induced by the bromodomain inhibitor JQ1 versus a JQ1 proteolysis targeting chimera; we elucidate distinct modes of action of estrogen receptor modulators; and we comprehensively classify HSP90 clients based on their requirement for HSP90 constitutively or during synthesis, demonstrating that constitutive HSP90 clients have lower thermal stability than non-clients, have higher affinity for the chaperone, vary between cell types, and change upon external stimuli. These findings highlight the potential of mPDP to identify dynamically controlled degradation mechanisms in cellular systems.

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Multiplexed proteome dynamics profiling, mPDP, measures changes in proteostasis

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JQ1-PROTAC degrades a key mRNA export factor and blocks protein synthesis

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Raloxifene induces TMEM97 degradation dysregulating cholesterol homeostasis

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Characterization of proteins dependent on HSP90 constitutively or during synthesis

Brd4 regulates the expression of essential autophagy genes and Keap1 in AML cells

We have recently reported that activation of Brd4 is associated with the presence of autophagy in NPMc+ and MLL AML cells. In order to determine the mechanisms underlying this relationship, we have examined the role of Brd4 in regulating the expression of several genes that are central to the process of autophagy. We found that Brd4 binds to the promoters of ATG 3, 7 and CEBPβ, and expression of these genes is markedly reduced by inhibitors of Brd4, as well as by Brd4-shRNA and depletion of CEBPβ. Inhibitors of Brd4 also dramatically suppress the transcription of Keap1, thereby increasing the expression of anti-oxidant genes through the Nrf2 pathway and reducing the cytotoxicity induced by Brd4 inhibitors. Elimination of ATG3 or KEAP1 expression using CRISPR-cas9 mediated genomic editing markedly reduced autophagy. We conclude that Brd4 plays a significant role in autophagy activation through the direct transcriptional regulation of genes essential for it, as well as through the Keap1-Nrf2 axis in NPMc+ and MLL-fusion AML cells.

Discovery of a Small-Molecule Bromodomain-Containing Protein 4 (BRD4) Inhibitor That Induces AMP-Activated Protein Kinase-Modulated Autophagy-Associated Cell Death in Breast Cancer

Upon the basis of The Cancer Genome Atlas (TCGA) data set, we identified that several autophagy-related proteins such as AMP-activated protein kinase (AMPK) were remarkably downregulated in breast cancer. Combined with coimmunoprecipitation assay, we demonstrated that BRD4 might interact with AMPK. After analyses of the pharmacophore and WPF interaction optimization, we designed a small-molecule inhibitor of BRD4, 9f (FL-411) which was validated by cocrystal structure with BD1 of BRD4. Subsequently, 9f was discovered to induce ATG5-dependent autophagy-associated cell death (ACD) by blocking BRD4-AMPK interaction and thus activating AMPK-mTOR-ULK1-modulated autophagic pathway in breast cancer cells. Interestingly, the iTRAQ-based proteomics analyses revealed that 9f induced ACD pathways involved in HMGB1, VDAC1/2, and eEF2. Moreover, 9f displayed a therapeutic potential on both breast cancer xenograft mouse and zebrafish models. Together, these results demonstrate that a novel small-molecule inhibitor of BRD4 induces BRD4-AMPK-modulated ACD in breast cancer, which may provide a candidate drug for future cancer therapy.