Mitochondrial BMI1 maintains bioenergetic homeostasis in cells

The transcription factor BMI1 increases hypoxic signaling in oral cavity epithelia

The tongue epithelium is maintained by a proliferative basal layer. This layer contains long-lived stem cells (SCs), which produce progeny cells that move up to the surface as they differentiate. B-lymphoma Mo-MLV insertion region 1 (BMI1), a protein in mammalian Polycomb Repressive Complex 1 (PRC1) and a biomarker of oral squamous cell carcinoma, is expressed in almost all basal epithelial SCs of the tongue, and single, Bmi1-labelled SCs give rise to cells in all epithelial layers. We previously developed a transgenic mouse model (KrTB) containing a doxycycline- (dox) controlled, Tet-responsive element system to selectively overexpress Bmi1 in the tongue basal epithelial SCs. Here, we used this model to assess BMI1 actions in tongue epithelia. Genome-wide transcriptomics revealed increased levels of transcripts involved in the cellular response to hypoxia in Bmi1-overexpressing (KrTB+DOX) oral epithelia even though these mice were not subjected to hypoxia conditions. Ectopic Bmi1 expression in tongue epithelia increased the levels of hypoxia inducible factor-1 alpha (HIF1α) and HIF1α targets linked to metabolic reprogramming during hypoxia. We used chromatin immunoprecipitation (ChIP) to demonstrate that Bmi1 associates with the promoters of HIF1A and HIF1A-activator RELA (p65) in tongue epithelia. We also detected increased SC proliferation and oxidative stress in Bmi1-overexpressing tongue epithelia. Finally, using a human oral keratinocyte line (OKF6-TERT1R), we showed that ectopic BMI1 overexpression decreases the oxygen consumption rate while increasing the extracellular acidification rate, indicative of elevated glycolysis. Thus, our data demonstrate that high BMI1 expression drives hypoxic signaling, including metabolic reprogramming, in normal oral cavity epithelia.

Chk2 Modulates Bmi1-Deficiency-Induced Renal Aging and Fibrosis via Oxidative Stress, DNA Damage, and p53/TGFβ1-Induced Epithelial-Mesenchymal Transition

Renal aging may lead to fibrosis and dysfunction, yet underlying mechanisms remain unclear. We explored whether deficiency of the Polycomb protein Bmi1 causes renal aging via DNA damage response (DDR) activation, inducing renal tubular epithelial cell (RTEC) senescence and epithelial-mesenchymal transition (EMT). Bmi1 knockout mice exhibited oxidative stress, DDR activation, RTEC senescence, senescence-associated secretory phenotype (SASP), and age-related fibrosis in kidneys. Bmi1 deficiency impaired renal structure and function, increasing serum creatinine/urea, reducing creatinine clearance, and decreasing cortical thickness and glomerular number. However, knockout of the serine-threonine kinase Chk2 alleviated these aging phenotypes. Transcriptomics identified transforming growth factor beta 1 (TGFβ1) upregulation in Bmi1-deficient RTECs, but TGFβ1 was downregulated upon Chk2 knockout. The tumor suppressor protein p53 transcriptionally activated TGFβ1, promoting EMT in RTECs. Bmi1 knockout or oxidative stress (induced with H2O2) increased TGFβ1 expression, and EMT in RTECs and was partly reversed by p53 inhibition. Together, Bmi1 deficiency causes oxidative stress and DDR-mediated RTEC senescence/SASP, thus activating p53 and TGFβ1 to induce EMT and age-related fibrosis. However, blocking DDR (via Chk2 knockout) or p53 ameliorates these changes. Our study reveals mechanisms whereby Bmi1 preserves renal structure and function during aging by suppressing DDR and p53/TGFβ1-mediated EMT. These pathways represent potential targets for detecting and attenuating age-related renal decline.

Mitochondrial metabolism: a predictive biomarker of radiotherapy efficacy and toxicity

INTRODUCTION

Radiotherapy is a mainstay of cancer treatment. Clinical studies revealed a heterogenous response to radiotherapy, from a complete response to even disease progression. To that end, finding the relative prognostic factors of disease outcomes and predictive factors of treatment efficacy and toxicity is essential. It has been demonstrated that radiation response depends on DNA damage response, cell cycle phase, oxygen concentration, and growth rate. Emerging evidence suggests that altered mitochondrial metabolism is associated with radioresistance.

METHODS

This article provides a comprehensive evaluation of the role of mitochondria in radiotherapy efficacy and toxicity. In addition, it demonstrates how mitochondria might be involved in the famous 6Rs of radiobiology.

RESULTS

In terms of this idea, decreasing the mitochondrial metabolism of cancer cells may increase radiation response, and enhancing the mitochondrial metabolism of normal cells may reduce radiation toxicity. Enhancing the normal cells (including immune cells) mitochondrial metabolism can potentially improve the tumor response by enhancing immune reactivation. Future studies are invited to examine the impacts of mitochondrial metabolism on radiation efficacy and toxicity. Improving radiotherapy response with diminishing cancer cells' mitochondrial metabolism, and reducing radiotherapy toxicity with enhancing normal cells' mitochondrial metabolism.

The Vascular Niche for Adult Cardiac Progenitor Cells

Research on cardiac progenitor cell populations has generated expectations about their potential for cardiac regeneration capacity after acute myocardial infarction and during physiological aging; however, the endogenous capacity of the adult mammalian heart is limited. The modest efficacy of exogenous cell-based treatments can guide the development of new approaches that, alone or in combination, can be applied to boost clinical efficacy. The identification and manipulation of the adult stem cell environment, termed niche, will be critical for providing new evidence on adult stem cell populations and improving stem-cell-based therapies. Here, we review and discuss the state of our understanding of the interaction of adult cardiac progenitor cells with other cardiac cell populations, with a focus on the description of the B-CPC progenitor population (Bmi1+ cardiac progenitor cell), which is a strong candidate progenitor for all main cardiac cell lineages, both in the steady state and after cardiac damage. The set of all interactions should be able to define the vascular cardiac stem cell niche, which is associated with low oxidative stress domains in vasculature, and whose manipulation would offer new hope in the cardiac regeneration field.

BMI1 Silencing Induces Mitochondrial Dysfunction in Lung Epithelial Cells Exposed to Hyperoxia

Acute Lung Injury (ALI), characterized by bilateral pulmonary infiltrates that restrict gas exchange, leads to respiratory failure. It is caused by an innate immune response with white blood cell infiltration of the lungs, release of cytokines, an increase in reactive oxygen species (ROS), oxidative stress, and changes in mitochondrial function. Mitochondrial alterations, changes in respiration, ATP production and the unbalancing fusion and fission processes are key events in ALI pathogenesis and increase mitophagy. Research indicates that BMI1 (B cell-specific Moloney murine leukemia virus integration site 1), a protein of the Polycomb repressive complex 1, is a cell cycle and survival regulator that plays a role in mitochondrial function. BMI1-silenced cultured lung epithelial cells were exposed to hyperoxia to determine the role of BMI1 in mitochondrial metabolism. Its expression significantly decreases in human lung epithelial cells (H441) following hyperoxic insult, as determined by western blot, Qrt-PCR, and functional analysis. This decrease correlates with an increase in mitophagy proteins, PINK1, Parkin, and DJ1; an increase in the expression of tumor suppressor PTEN; changes in the expression of mitochondrial biomarkers; and decreases in the oxygen consumption rate (OCR) and tricarboxylic acid enzyme activity. Our bioinformatics analysis suggested that the BMI1 multifunctionality is determined by its high level of intrinsic disorder that defines the ability of this protein to bind to numerous cellular partners. These results demonstrate a close relationship between BMI1 expression and mitochondrial health in hyperoxia-induced acute lung injury (HALI) and indicate that BMI1 is a potential therapeutic target to treat ALI and Acute Respiratory Distress Syndrome.

Activity and Function in Human Cells of the Evolutionary Conserved Exonuclease Polynucleotide Phosphorylase

Polynucleotide phosphorylase (PNPase) is a phosphorolytic RNA exonuclease highly conserved throughout evolution. Human PNPase (hPNPase) is located in mitochondria and is essential for mitochondrial function and homeostasis. Not surprisingly, mutations in the PNPT1 gene, encoding hPNPase, cause serious diseases. hPNPase has been implicated in a plethora of processes taking place in different cell compartments and involving other proteins, some of which physically interact with hPNPase. This paper reviews hPNPase RNA binding and catalytic activity in relation with the protein structure and in comparison, with the activity of bacterial PNPases. The functions ascribed to hPNPase in different cell compartments are discussed, highlighting the gaps that still need to be filled to understand the physiological role of this ancient protein in human cells.

BMI1 Drives Steroidogenesis Through Epigenetically Repressing the p38 MAPK Pathway

Testosterone biosynthesis progressively decreases in aging males primarily as a result of functional changes to Leydig cells. Despite this, the mechanisms underlying steroidogenesis remain largely unclear. Using gene knock-out approaches, we and others have recently identified Bmi1 as an anti-aging gene. Herein, we investigate the role of BMI1 in steroidogenesis using mouse MLTC-1 and primary Leydig cells. We show that BMI1 can positively regulate testosterone production. Mechanistically, in addition to its known role in antioxidant activity, we also report that p38 mitogen-activated protein kinase (MAPK) signaling is activated, and testosterone levels reduced, in BMI1-deficient cells; however, the silencing of the p38 MAPK pathway restores testosterone production. Furthermore, we reveal that BMI1 directly binds to the promoter region of Map3k3, an upstream activator of p38, thereby modulating its chromatin status and repressing its expression. Consequently, this results in the inhibition of the p38 MAPK pathway and the promotion of steroidogenesis. Our study uncovered a novel epigenetic mechanism in steroidogenesis involving BMI1-mediated gene silencing and provides potential therapeutic targets for the treatment of hypogonadism.

Bmi-1 alleviates adventitial fibroblast senescence by eliminating ROS in pulmonary hypertension

Objectives

Pulmonary hypertension (PH) is a life-threatening progressive disease with high mortality in the elderly. However, the pathogenesis of PH has not been fully understood and there is no effective therapy to reverse the disease process. This study aims to determine whether cellular senescence is involved in the development of PH.

Methods

The rat PH model was established by intraperitoneal injection of monocrotaline and evaluated by pulmonary arteriole wall thickness and right ventricular hypertrophy index. Human lung fibroblasts (HLFs) were treated with CoCl₂ or hypoxia to induce cellular senescence in vitro. SA-β-gal staining and the changes of senescent markers were used to examine cellular senescence. The molecular mechanism of cellular senescence was further explored by detecting reactive oxygen species (ROS) levels and culturing cells with a conditioned medium.

Results

We revealed the cellular senescence of pulmonary adventitial fibroblasts in vivo in the rat PH model. The expression of Bmi-1, an important regulator of senescence, was decreased in the lungs of PH rats and localized in adventitial fibroblasts. The in vitro experiments showed that p16 expression was increased while Bmi-1 expression was decreased after CoCl₂ treatment in HLFs. Mechanistically, Bmi-1 could alleviate CoCl₂-induced HLFs senescence by eliminating ROS which further promoted the proliferation of pulmonary artery smooth muscle cells by paracrine mode of action of HLFs.

Conclusion

Bmi-1 alleviates the cellular senescence of pulmonary fibroblasts in PH, which expands the pathogenesis of PH and provides a theoretical basis for targeting senescent cells in the treatment of PH.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12890-021-01439-0.

The p53 Pathway and Metabolism: The Tree That Hides the Forest

Simple Summary

The p53 pathway is a major tumor suppressor pathway that prevents the propagation of abnormal cells by regulating DNA repair, cell cycle progression, cell death, or senescence. The multiple cellular processes regulated by p53 were more recently extended to the control of metabolism, and many studies support the notion that perturbations of p53-associated metabolic activities are linked to cancer development. Converging lines of evidence support the notion that, in addition to p53, other key components of this molecular cascade are also important regulators of metabolism. Here, we illustrate the underestimated complexity of the metabolic network controlled by the p53 pathway and show how its perturbation contributes to human diseases including cancer, aging, and metabolic diseases.

Abstract

The p53 pathway is functionally inactivated in most, if not all, human cancers. The p53 protein is a central effector of numerous stress-related molecular cascades. p53 controls a safeguard mechanism that prevents accumulation of abnormal cells and their transformation by regulating DNA repair, cell cycle progression, cell death, or senescence. The multiple cellular processes regulated by p53 were more recently extended to the control of metabolism and many studies support the notion that perturbations of p53-associated metabolic activities are linked to cancer development, as well as to other pathophysiological conditions including aging, type II diabetes, and liver disease. Although much less documented than p53 metabolic activities, converging lines of evidence indicate that other key components of this tumor suppressor pathway are also involved in cellular metabolism through p53-dependent as well as p53-independent mechanisms. Thus, at least from a metabolic standpoint, the p53 pathway must be considered as a non-linear pathway, but the complex metabolic network controlled by these p53 regulators and the mechanisms by which their activities are coordinated with p53 metabolic functions remain poorly understood. In this review, we highlight some of the metabolic pathways controlled by several central components of the p53 pathway and their role in tissue homeostasis, metabolic diseases, and cancer.

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.

Cystathionine beta synthase regulates mitochondrial dynamics and function in endothelial cells

Mutations in the human cystathionine beta synthase (CBS) gene are known to cause endothelial dysfunction responsible for cardiovascular and neurovascular diseases. CBS is the predominant hydrogen sulfide (H2 S)-producing enzyme in endothelial cells (ECs). Recently, H2 S was shown to attenuate ROS and improve mitochondrial function. Mitochondria are metabolic organelles that actively transform their ultrastructure to mediate their function. Therefore, we questioned whether perturbation of CBS/H2 S activity could drive mitochondrial dysfunction via mitochondrial dynamics in ECs. Here we demonstrate that silencing CBS induces mitochondria fragmentation, attenuates efficient oxidative phosphorylation, and decreases EC function. Mechanistically, CBS silencing significantly elevates ROS production, thereby leading to reduced mitofusin 2 (MFN2) expression, decouple endoplasmic reticulum-mitochondria contacts, increased mitochondria fission, enhanced receptor-mediated mitophagy, and increased EC death. These defects were significantly rescued by the treatment of H2 S donors. Taken together our data highlights a novel signaling axis that mechanistically links CBS with mitochondrial function and ER-mitochondrial tethering and could be considered as a new therapeutic approach for the intervention of EC dysfunction-related pathologies.

BMI1 promotes steroidogenesis through maintaining redox homeostasis in mouse MLTC-1 and primary Leydig cells

In males, aging is accompanied by decline in serum testosterone levels due to impairment of testicular Leydig cells. The polycomb protein BMI1 has recently been identified as an anti-aging factor. In our previous study, BMI1 null mice showed decreased serum testosterone and Leydig cell population, excessive oxidative stress and p16/p19 signaling activation. However, a cause-and-effect relationship between phenotypes and pathways was not investigated. Here, we used the rescue approach to study the role of oxidative stress or p16/p19 in BMI1-mediated steroidogenesis. Our results revealed that treatment with antioxidant NAC, but not down-regulation of p16/p19, largely rescued cell senescence, DNA damage and steroidogenesis in BMI1-deficient mouse MLTC-1 and primary Leydig cells. Collectively, our study demonstrates that BMI1 orchestrates steroidogenesis mainly through maintaining redox homeostasis, and thus, BMI1 may be a novel and potential therapeutic target for treatment of hypogonadism.

Doxycycline-induced exogenous Bmi-1 expression enhances tumor formation in a murine model of oral squamous cell carcinoma

B Cell-Specific Moloney Murine Leukemia Virus Integration Site 1 (Bmi-1, Bmi1), an epigenetic protein, is necessary for normal stem cell self-renewal in adult animals and for cancer stem cell (CSC) functions in adult animals. To elucidate the functions of Bmi-1 in the oral cavity we created a transgenic mouse line (KrTBmi-1) that expresses ectopic, Flag-tagged Bmi-1 in tongue basal epithelial stem cells only upon doxycycline (DOX) treatment. Genome wide transcriptomics and Ingenuity Pathway Analysis identified several pathways altered by exogenous Bmi-1 expression in the normal tongue epithelium, including EIF2 signaling (P value = 1.58 x 10^-49), mTOR signaling (P value = 2.45 x 10^-12), oxidative phosphorylation (P = 6.61 x 10^-3) and glutathione redox reactions I (P = 1.74 x 10^-2). Overall, our data indicate that ectopic Bmi-1 expression has an impact on normal tongue epithelial homeostasis. We then assessed the KrTBmi-1 mice in the 4-nitroquinoline 1-oxide (4-NQO) model of oral carcinogenesis. We found that 80% of mice expressing exogenous Bmi-1 (+DOX, +4-NQO KrTBmi-1; N = 10) developed tumors classified as grade 3 or higher, compared to 60% and 40% of mice expressing just endogenous Bmi-1 (+DOX, +4-NQO Kr and -DOX, +4-NQO KrTBmi-1 groups, respectively; N = 10/group; P value = <0.0001); and 30% of mice expressing ectopic Bmi-1 mice developed 20 or more lesions compared to 10% of mice expressing only endogenous Bmi-1 (P = .009). This demonstrates that exogenous Bmi-1 expression increases the susceptibility of mice to 4-NQO-induced oral carcinogenesis, strengthening the evidence for Bmi-1 as a therapeutic target in human oral squamous cell carcinoma.

Bmi1-Progenitor Cell Ablation Impairs the Angiogenic Response to Myocardial Infarction

Objective- Cardiac progenitor cells reside in the heart in adulthood, although their physiological relevance remains unknown. Here, we demonstrate that after myocardial infarction, adult Bmi1⁺ (B lymphoma Mo-MLV insertion region 1 homolog [PCGF4]) cardiac cells are a key progenitor-like population in cardiac neovascularization during ventricular remodeling. Approach and Results- These cells, which have a strong in vivo differentiation bias, are a mixture of endothelial- and mesenchymal-related cells with in vitro spontaneous endothelial cell differentiation capacity. Genetic lineage tracing analysis showed that heart-resident Bmi1⁺ progenitor cells proliferate after acute myocardial infarction and differentiate to generate de novo cardiac vasculature. In a mouse model of induced myocardial infarction, genetic ablation of these cells substantially deteriorated both heart angiogenesis and the ejection fraction, resulting in an ischemic-dilated cardiac phenotype. Conclusions- These findings imply that endothelial-related Bmi1⁺ progenitor cells are necessary for injury-induced neovascularization in adult mouse heart and highlight these cells as a suitable therapeutic target for preventing dysfunctional left ventricular remodeling after injury.

Increased expression of Sonic hedgehog restores diabetic endothelial progenitor cells and improves cardiac repair after acute myocardial infarction in diabetic mice

Damaged endothelial progenitor cells (EPCs) are associated with poor prognosis in diabetic myocardial infarction (DMI). Our previous studies revealed that an impaired Sonic hedgehog (Shh) pathway contributes to insufficient function in diabetic EPCs; however, the roles of the Shh pathway in diabetic EPC apoptosis under basal and hypoxic/ischemic conditions remain unknown. Therefore, the present study investigated whether Shh revitalized diabetic EPCs and consequently improved the deteriorative status of DMI. For this purpose, streptozotocin injection was used in male C57/BL6 mice to induce type-1 diabetes, and diabetic EPCs were isolated from the bone marrow. Apoptosis, cell function, and protein expression were investigated in EPCs in vitro. Mouse hearts were injected with adenovirus Shh-modified diabetic EPCs (DM-EPC^Shh) or control DM-EPC^Null immediately after coronary artery ligation in vivo. Cardiac function, capillary numbers, fibrosis, and cell apoptosis were then detected. First, the in vitro results demonstrated that the apoptosis of diabetic EPCs was reduced following treatment with Shh protein for 24 h, under normal or hypoxic conditions. BMI1 proto-oncogene (Bmi1), an antiapoptotic protein found in several cells, was reduced in diabetic EPCs under normal or hypoxic conditions, but was upregulated after Shh protein stimulation. When Bmi1-siRNA was administered, the antiapoptotic effect of Shh protein was significantly reversed. In addition, p53, a Bmi1-targeted gene, was demonstrated to mediate the antiapoptotic effect of the Shh/Bmi1 pathway in diabetic EPCs. The Shh/Bmi1/p53 axis also enhanced the diabetic EPC function. In vivo, Shh-modified diabetic EPCs exhibited increased EPC retention and decreased apoptosis at 3 days post-DMI. At 14 days post-DMI, these cells presented enhanced capillary density, reduced myocardial fibrosis and improved cardiac function. In conclusion, the present results demonstrated that the Shh pathway restored diabetic EPCs through the Shh/Bmi1/p53 axis, suppressed myocardial apoptosis and improved myocardial angiogenesis, thus reducing cardiac fibrosis and finally restoring myocardial repair and cardiac function in DMI. Thus, the Shh pathway may serve as a potential target for autologous cell therapy in diabetic myocardial ischemia.

Age-related oxidative stress confines damage-responsive Bmi1+ cells to perivascular regions in the murine adult heart

Adult progenitor cells reside in specialized microenvironments which maintain their undifferentiated cell state and trigger regenerative responses following injury. Although these environments are well described in several tissues, the cellular components that comprise the cardiac environment where progenitor cells are located remain unknown. Here we use Bmi1^CreERT and Bmi1^GFP mice as genetic tools to trace cardiac damage-responsive cells throughout the mouse lifespan. In adolescent mice, Bmi1⁺ damage-responsive cells are broadly distributed throughout the myocardium. In adult mice, however, Bmi1⁺ cells are confined predominately in perivascular areas with low levels of reactive oxygen species (ROS) and their number decline in an age-dependent manner. In vitro co-culture experiments with endothelial cells supported a regulatory role of the endothelium in damage-responsive cell behavior. Accordingly, in vivo genetic decrease of ROS levels in adult heart disengaged Bmi1⁺ cells from the cardiovascular network, recapitulating an adolescent-like Bmi1 expression profile. Thus, we identify cardiac perivascular regions as low-stress microenvironments that favor the maintenance of adult damage-responsive cells.

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Bmi1⁺ cardiac damage-responsive cells are sheltered in areas with low ROS levels.

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Aging-related oxidative damage confines cardiac Bmi1⁺ cells to perivascular regions.

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Microvasculature-derived signals regulate adult Bmi1⁺ damage-responsive cell behavior.

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Genetic ROS levels manipulation modifies the percentage and identity of Bmi1⁺ cells.

Redox-dependent BMI1 activity drives in vivo adult cardiac progenitor cell differentiation

Accumulation of reactive oxygen species (ROS) is associated with several cardiovascular pathologies and with cell cycle exit by neonanatal cardiomyocytes, a key limiting factor in the regenerative capacity of the adult mammalian heart. The polycomb complex component BMI1 is linked to adult progenitors and is an important partner in DNA repair and redox regulation. We found that high BMI1 expression is associated with an adult Sca1⁺ cardiac progenitor sub-population with low ROS levels. In homeostasis, BMI1 repressed cell fate genes, including a cardiogenic differentiation program. Oxidative damage nonetheless modified BMI1 activity in vivo by derepressing canonical target genes in favor of their antioxidant and anticlastogenic functions. This redox-mediated mechanism is not restricted to damage situations, however, and we report ROS-associated differentiation of cardiac progenitors in steady state. These findings demonstrate how redox status influences the cardiac progenitor response, and identify redox-mediated BMI1 regulation with implications in maintenance of cellular identity in vivo.

BMI1, a new target of CK2α

BACKGROUND

The polycomb group protein, BMI1 plays important roles in chromatin modification, stem cell function, DNA damage repair and mitochondrial bioenergetics. Such diverse cellular functions of BMI1 could be, in part, due to post-translational modifications, especially phosphorylation. To date, AKT has been reported as a kinase that by site specific phosphorylation of BMI1 modulates its oncogenic functions.

METHODS

Immunoprecipitation in conjunction with kinase assay and mass spectrometry was used to determine association with and site specific phosphorylation of BMI1 by CK2α. Functional implications of the BMI1/CK2α axis was examined in cancer cells utilizing siRNA and exogenous gene expression followed by biochemical and phenotypic studies. Correlations between expression of CK2α and BMI1 were determined from cell lines and formalin fixed paraffin embedded tissues representing the normal fallopian tube epithelium and high grade serous ovarian cancer samples.

RESULTS

Here we report that CK2α, a nuclear serine threonine kinase, phosphorylates BMI1 at Serine 110 as determined by in-vitro/ex-vivo kinase assay and mass spectrometry. In ovarian cancer cell lines, expression of CK2α correlated with the phospho-species, as well as basal BMI1 levels. Preventing phosphorylation of BMI1 at Serine 110 significantly decreased half-life and stability of the protein. Additionally, re-expression of the phosphorylatable but not non-phosphorylatable BMI1 rescued clonal growth in endogenous BMI1 silenced cancer cells leading us to speculate that CK2α-mediated phosphorylation stabilizes BMI1 and promotes its oncogenic function. Clinically, compared to normal fallopian tube epithelial tissues, the expression of both BMI1 and CK2α were significantly higher in tumor tissues obtained from high-grade serous ovarian cancer patients. Among tumor samples, the expression of BMI1 and CK2α positively correlated (Spearman coefficient = 0.62, P = 0.0021) with each other.

CONCLUSION

Taken together, our findings establish an important regulatory role of CK2α on BMI1 phosphorylation and stability and implicate the CK2α/BMI1 axis in ovarian cancer.