PRMT5 regulates Golgi apparatus structure through methylation of the golgin GM130

Selective delivery of imaging probes and therapeutics to the endoplasmic reticulum or Golgi apparatus: Current strategies and beyond

To maximize therapeutic effects and minimize unwanted effects, the interest in drug targeting to the endoplasmic reticulum (ER) or Golgi apparatus (GA) has been recently growing because two organelles are distributing hubs of cellular building/signaling components (e.g., proteins, lipids, Ca2+) to other organelles and the plasma membrane. Their structural or functional damages induce organelle stress (i.e., ER or GA stress), and their aggravation is strongly related to diseases (e.g., cancers, liver diseases, brain diseases). Many efforts have been developed to image (patho)physiological functions (e.g., oxidative stress, protein/lipid-related processing) and characteristics (e.g., pH, temperature, biothiols, reactive oxygen species) in the target organelles and to deliver drugs for organelle disruption using organelle-targeting moieties. Therefore, this review will overview the structure, (patho)physiological functions/characteristics, and related diseases of the organelles of interest. Future direction on ER or GA targeting will be discussed by understanding current strategies and investigations on targeting, imaging/sensing, and therapeutic strategies.

Protein arginine methyltransferases (PRMTs): Orchestrators of cancer pathogenesis, immunotherapy dynamics, and drug resistance

Protein Arginine Methyltransferases (PRMTs) are a family of enzymes regulating protein arginine methylation, which is a post-translational modification crucial for various cellular processes. Recent studies have highlighted the mechanistic role of PRMTs in cancer pathogenesis, immunotherapy, and drug resistance. PRMTs are involved in diverse oncogenic processes, including cell proliferation, apoptosis, and metastasis. They exert their effects by methylation of histones, transcription factors, and other regulatory proteins, resulting in altered gene expression patterns. PRMT-mediated histone methylation can lead to aberrant chromatin remodeling and epigenetic changes that drive oncogenesis. Additionally, PRMTs can directly interact with key signaling pathways involved in cancer progression, such as the PI3K/Akt and MAPK pathways, thereby modulating cell survival and proliferation. In the context of cancer immunotherapy, PRMTs have emerged as critical regulators of immune responses. They modulate immune checkpoint molecules, including programmed cell death protein 1 (PD-1), through arginine methylation. Drug resistance is a significant challenge in cancer treatment, and PRMTs have been implicated in this phenomenon. PRMTs can contribute to drug resistance through multiple mechanisms, including the epigenetic regulation of drug efflux pumps, altered DNA damage repair, and modulation of cell survival pathways. In conclusion, PRMTs play critical roles in cancer pathogenesis, immunotherapy, and drug resistance. In this overview, we have endeavored to illuminate the mechanistic intricacies of PRMT-mediated processes. Shedding light on these aspects will offer valuable insights into the fundamental biology of cancer and establish PRMTs as promising therapeutic targets.

PRMT5 as a Potential Therapeutic Target in MYC-Amplified Medulloblastoma

MYC amplification or overexpression is most common in Group 3 medulloblastomas and is positively associated with poor clinical outcomes. Recently, protein arginine methyltransferase 5 (PRMT5) overexpression has been shown to be associated with tumorigenic MYC functions in cancers, particularly in brain cancers such as glioblastoma and medulloblastoma. PRMT5 regulates oncogenes, including MYC, that are often deregulated in medulloblastomas. However, the role of PRMT5-mediated post-translational modification in the stabilization of these oncoproteins remains poorly understood. The potential impact of PRMT5 inhibition on MYC makes it an attractive target in various cancers. PRMT5 inhibitors are a promising class of anti-cancer drugs demonstrating preclinical and preliminary clinical efficacies. Here, we review the publicly available preclinical and clinical studies on PRMT5 targeting using small molecule inhibitors and discuss the prospects of using them in medulloblastoma therapy.

Cdk1 protects against oxygen-glucose deprivation and reperfusion-induced Golgi fragmentation and apoptosis through mediating GM130 phosphorylation

Increasing evidence has indicated that the Golgi apparatus (GA) is involved in the development of cerebral ischemia-reperfusion (IR) injury. Finding effective neuroprotective agents targeting GA has become a priority in the treatment of ischemic stroke. GM130, a key structural protein present on the cis-face of the GA, maintains its structure through its phosphorylation and dephosphorylation. However, the molecular mechanisms by which GM130 regulates IR-induced neuronal apoptosis are not well elucidated. Mouse neuroblastoma Neuro2a (N2A) cells were subjected to oxygen-glucose deprivation and reperfusion (OGDR) insult. Cell proliferation and apoptosis were determined using MTT assay, TUNEL staining, and flow cytometry. GA morphology was detected by immunocytochemical staining and immunofluorescence microscopy. GA-related protein and mRNA levels were detected by WB and qPCR, respectively. Treatment with Purvalanol A, an effective Cdk1 inhibitor, and transfection of Cdk1-shRNA were carried out to inhibit OGDR-induced Cdk1 elevation. The results demonstrated that OGDR induced Golgi fragmentation, neuronal apoptosis, GM130 phosphorylation, and p115 cleavage in N2A cells. Cdk1 elevation after OGDR was closely correlated with GM130 phosphorylation, not p115. Inhibition of Cdk1 significantly attenuated OGDR-induced Golgi fragmentation and cell apoptosis. Cdk1 interacted with GM130 and decreased its phosphorylation on the serine 25 site in N2A cells exposed to OGDR. The present findings reveal that Cdk1 protects against IR-induced GA fragmentation and apoptosis, likely through the mediation of GM130 phosphorylation. This neuroprotective potential of Cdk1 against IR insult and the underlying mechanism will pave the way for potential clinical applications targeting the GA organelle for cerebral IR-related disorders.

The crescent-like Golgi ribbon is shaped by the Ajuba/PRMT5/Aurora-A complex-modified HURP

BACKGROUND

Golgi apparatus (GA) is assembled as a crescent-like ribbon in mammalian cells under immunofluorescence microscope without knowing the shaping mechanisms. It is estimated that roughly 1/5 of the genes encoding kinases or phosphatases in human genome participate in the assembly of Golgi ribbon, reflecting protein modifications play major roles in building Golgi ribbon.

METHODS

To explore how Golgi ribbon is shaped as a crescent-like structure under the guidance of protein modifications, we identified a protein complex containing the scaffold proteins Ajuba, two known GA regulators including the protein kinase Aurora-A and the protein arginine methyltransferase PRMT5, and the common substrate of Aurora-A and PRMT5, HURP. Mutual modifications and activation of PRMT5 and Aurora-A in the complex leads to methylation and in turn phosphorylation of HURP, thereby producing HURP p725. The HURP p725 localizes to GA vicinity and its distribution pattern looks like GA morphology. Correlation study of the HURP p725 statuses and GA structure, site-directed mutagenesis and knockdown-rescue experiments were employed to identify the modified HURP as a key regulator assembling GA as a crescent ribbon.

RESULTS

The cells containing no or extended distribution of HURP p725 have dispersed GA membranes or longer GA. Knockdown of HURP fragmentized GA and HURP wild type could, while its phosphorylation deficiency mutant 725A could not, restore crescent Golgi ribbon in HURP depleted cells, collectively indicating a crescent GA-constructing activity of HURP p725. HURP p725 is transported, by GA membrane-associated ARF1, Dynein and its cargo adaptor Golgin-160, to cell center where HURP p725 forms crescent fibers, binds and stabilizes Golgi assembly factors (GAFs) including TRIP11, GRASP65 and GM130, thereby dictating the formation of crescent Golgi ribbon at nuclear periphery.

CONCLUSIONS

The Ajuba/PRMT5/Aurora-A complex integrates the signals of protein methylation and phosphorylation to HURP, and the HURP p725 organizes GA by stabilizing and recruiting GAFs to its crescent-like structure, therefore shaping GA as a crescent ribbon. Therefore, the HURP p725 fiber serves a template to construct GA according to its shape. Video Abstract.

Targeting Arginine Methyltransferase PRMT5 for Cancer Therapy: Updated Progress and Novel Strategies

As a predominant type II protein arginine methyltransferase, PRMT5 plays critical roles in various normal cellular processes by catalyzing the mono- and symmetrical dimethylation of a wide range of histone and nonhistone substrates. Clinical studies have revealed that high expression of PRMT5 is observed in different solid tumors and hematological malignancies and is closely associated with cancer initiation and progression. Accordingly, PRMT5 is becoming a promising anticancer target and has received great attention in both the pharmaceutical industry and the academic community. In this Perspective, we comprehensively summarize recent advances in the development of first-generation PRMT5 enzymatic inhibitors and highlight novel strategies targeting PRMT5 in the past 5 years. We also discuss the challenges and opportunities of PRMT5 inhibition, with the aim of shedding light on future PRMT5 drug discovery.

Fluorescence-resonance-energy-transfer-based assay to estimate modulation of TDP1 activity through arginine methylation

Tyrosyl DNA phosphodiesterase (TDP1) is a DNA repair enzyme that hydrolyzes the phosphotyrosyl linkage between 3'-DNA-protein crosslinks such as stalled topoisomerase 1 cleavage complexes (Top1cc). Here, we present a fluorescence-resonance-energy-transfer-(FRET) based assay to estimate modulation of TDP1 activity through arginine methylation. We describe steps for TDP1 expression and purification and estimating TDP1 activity using fluorescence-quenched probes mimicking Top1cc. We then detail data analysis of real-time TDP1 activity and screening of TDP1-selective inhibitors. For complete details on the use and execution of this protocol, please refer to Bhattacharjee et al. (2022).¹.

A New Insight into MYC Action: Control of RNA Polymerase II Methylation and Transcription Termination

MYC oncoprotein deregulation is a common catastrophic event in human cancer and limiting its activity restrains tumor development and maintenance, as clearly shown via Omomyc, an MYC-interfering 90 amino acid mini-protein. MYC is a multifunctional transcription factor that regulates many aspects of transcription by RNA polymerase II (RNAPII), such as transcription activation, pause release, and elongation. MYC directly associates with Protein Arginine Methyltransferase 5 (PRMT5), a protein that methylates a variety of targets, including RNAPII at the arginine residue R1810 (R1810me2s), crucial for proper transcription termination and splicing of transcripts. Therefore, we asked whether MYC controls termination as well, by affecting R1810me2S. We show that MYC overexpression strongly increases R1810me2s, while Omomyc, an MYC shRNA, or a PRMT5 inhibitor and siRNA counteract this phenomenon. Omomyc also impairs Serine 2 phosphorylation in the RNAPII carboxyterminal domain, a modification that sustains transcription elongation. ChIP-seq experiments show that Omomyc replaces MYC and reshapes RNAPII distribution, increasing occupancy at promoter and termination sites. It is unclear how this may affect gene expression. Transcriptomic analysis shows that transcripts pivotal to key signaling pathways are both up- or down-regulated by Omomyc, whereas genes directly controlled by MYC and belonging to a specific signature are strongly down-regulated. Overall, our data point to an MYC/PRMT5/RNAPII axis that controls termination via RNAPII symmetrical dimethylation and contributes to rewiring the expression of genes altered by MYC overexpression in cancer cells. It remains to be clarified which role this may have in tumor development.

Generation of GM130 Conditional Knockout Mouse

The Golgi apparatus is a common and highly dynamic organelle in eukaryotic cells. It plays an important role in secretory trafficking and cargo modifications. Increasing evidence suggests that structural changes and functional disorders of the Golgi apparatus are involved in many human diseases, but whether Golgi dysfunction is a causal factor in regard to the progression of these diseases remains unknown. GM130 has been postulated to play roles in Golgi stack formation and vesicular transport based on studies on cultured cells and in vitro reconstitutions. To define the role of GM130 in animal, a GM130 knockout mouse has recently been created. Based on the principle of homologous recombination, the GM130 conditional knockout mouse model was established through gene targeting, stem cell screening, and blastocyst injection. Such model has been successfully applied for studies of physiological functions of GM130 and Golgi apparatus at the cellular and animal levels.

Combination RSL3 Treatment Sensitizes Ferroptosis- and EGFR-Inhibition-Resistant HNSCCs to Cetuximab

Head and neck squamous cell carcinomas (HNSCCs) are a type of cancer originating in the mucosal epithelium of the mouth, pharynx, and larynx, the sixth most common cancer in the world. However, there is no effective treatment for HNSCCs. More than 90% of HNSCCs overexpress epidermal growth factor receptors (EGFRs). Although small molecule inhibitors and monoclonal antibodies have been developed to target EGFRs, few EGFR-targeted therapeutics are approved for clinical use. Ferroptosis is a new kind of programmed death induced by the iron catalyzed excessive peroxidation of polyunsaturated fatty acids. A growing body of evidence suggests that ferroptosis plays a pivotal role in inhibiting the tumor process. However, whether and how ferroptosis-inducers (FINs) play roles in hindering HNSCCs are unclear. In this study, we analyzed the sensitivity of different HNSCCs to ferroptosis-inducers. We found that only tongue squamous cell carcinoma cells and laryngeal squamous cell carcinoma cells, but not nasopharyngeal carcinoma cells, actively respond to ferroptosis-inducers. The different sensitivities of HNSCC cells to ferroptosis induction may be attributed to the expression of KRAS and ferritin heavy chain (FTH1) since a high level of FTH1 is associated with the poor prognostic survival of HNSCCs, but knocked down FTH1 can promote HNSCC cell death. Excitingly, the ferroptosis-inducer RSL3 plays a synthetic role with EGFR monoclonal antibody Cetuximab to inhibit the survival of nasopharyngeal carcinoma cells (CNE-2), which are insensitive to both ferroptosis induction and EGFR inhibition due to a high level of FTH1 and a low level of EGFR, respectively. Our findings prove that FTH1 plays a vital role in ferroptosis resistance in HNSCCs and also provide clues to target HNSCCs resistant to ferroptosis induction and/or EGFR inhibition.

How Protein Methylation Regulates Steroid Receptor Function

Steroid receptors (SRs) are members of the nuclear hormonal receptor family, many of which are transcription factors regulated by ligand binding. SRs regulate various human physiological functions essential for maintenance of vital biological pathways, including development, reproduction, and metabolic homeostasis. In addition, aberrant expression of SRs or dysregulation of their signaling has been observed in a wide variety of pathologies. SR activity is tightly and finely controlled by post-translational modifications (PTMs) targeting the receptors and/or their coregulators. Whereas major attention has been focused on phosphorylation, growing evidence shows that methylation is also an important regulator of SRs. Interestingly, the protein methyltransferases depositing methyl marks are involved in many functions, from development to adult life. They have also been associated with pathologies such as inflammation, as well as cardiovascular and neuronal disorders, and cancer. This article provides an overview of SR methylation/demethylation events, along with their functional effects and biological consequences. An in-depth understanding of the landscape of these methylation events could provide new information on SR regulation in physiology, as well as promising perspectives for the development of new therapeutic strategies, illustrated by the specific inhibitors of protein methyltransferases that are currently available.

The Role of GM130 in Nervous System Diseases

Golgi matrix protein 130 (GM130) is a Golgi-shaping protein located on the cis surface of the Golgi apparatus (GA). It is one of the most studied Golgin proteins so far. Its biological functions are involved in many aspects of life processes, including mitosis, autophagy, apoptosis, cell polarity, and directed migration at the cellular level, as well as intracellular lipid and protein transport, microtubule formation and assembly, lysosome function maintenance, and glycosylation modification. Mutation inactivation or loss of expression of GM130 has been detected in patients with different diseases. GM130 plays an important role in the development of the nervous system, but the studies on it are limited. This article reviewed the current research progress of GM130 in nervous system diseases. It summarized the physiological functions of GM130 in the occurrence and development of Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), microcephaly (MCPH), sepsis associated encephalopathy (SAE), and Ataxia, aiming to provide ideas for the further study of GM130 in nervous system disease detection and treatment.

PRMT5: An Emerging Target for Pancreatic Adenocarcinoma

Simple Summary

The burden of pancreatic ductal adenocarcinoma (PDAC) increases with rising incidence, yet 5-year overall survival remains poor at 17%. Routine comprehensive genomic profiling of PDAC only finds 2.5% of patients who may benefit and receive matched targeted therapy. Protein arginine methyltransferase 5 (PRMT5) as an anti-cancer target has gained significant interest in recent years and high levels of PRMT5 protein are associated with worse survival outcomes across multiple cancer types. Inhibition of PRMT5 in pre-clinical models can lead to cancer growth inhibition. However, PRMT5 is involved in multiple cellular processes, thus determining its mechanism of action is challenging. While past reviews on PRMT5 have focused on its role in diverse cellular processes and past research studies have focused mainly on haematological malignancies and glioblastoma, this review provides an overview of the possible biological mechanisms of action of PRMT5 inhibition and its potential as a treatment in pancreatic cancer.

Abstract

The overall survival of pancreatic ductal adenocarcinoma (PDAC) remains poor and its incidence is rising. Targetable mutations in PDAC are rare, thus novel therapeutic approaches are needed. Protein arginine methyltransferase 5 (PRMT5) overexpression is associated with worse survival and inhibition of PRMT5 results in decreased cancer growth across multiple cancers, including PDAC. Emerging evidence also suggests that altered RNA processing is a driver in PDAC tumorigenesis and creates a partial dependency on this process. PRMT5 inhibition induces altered splicing and this vulnerability can be exploited as a novel therapeutic approach. Three possible biological pathways underpinning the action of PRMT5 inhibitors are discussed; c-Myc regulation appears central to its action in the PDAC setting. Whilst homozygous MTAP deletion and symmetrical dimethylation levels are associated with increased sensitivity to PRMT5 inhibition, neither measure robustly predicts its growth inhibitory response. The immunomodulatory effect of PRMT5 inhibitors on the tumour microenvironment will also be discussed, based on emerging evidence that PDAC stroma has a significant bearing on disease behaviour and response to therapy. Lastly, with the above caveats in mind, current knowledge gaps and the implications and rationales for PRMT5 inhibitor development in PDAC will be explored.

The PRMT5/HURP axis retards Golgi repositioning by stabilizing acetyl-tubulin and Golgi apparatus during cell migration

The Golgi apparatus (GA) translocates to the cell leading end during directional migration, thereby determining cell polarity and transporting essential factors to the migration apparatus. The study provides mechanistic insights into how GA repositioning (GR) is regulated. We show that the methyltransferase PRMT5 methylates the microtubule regulator HURP at R122. The HURP methylation mimicking mutant 122F impairs GR and cell migration. Mechanistic studies revealed that HURP 122F or endogenous methylated HURP, that is, HURP m122, interacts with acetyl-tubulin. Overexpression of HURP 122F stabilizes the bundling pattern of acetyl-tubulin by decreasing the sensitivity of the latter to a microtubule disrupting agent nocodazole. HURP 122F also rigidifies GA via desensitizing the organelle to several GA disrupting chemicals. Similarly, the acetyl-tubulin mimicking mutant 40Q or tubulin acetyltransferase αTAT1 can rigidify GA, impair GR, and retard cell migration. Reversal of HURP 122F-induced GA rigidification, by knocking down GA assembly factors such as GRASP65 or GM130, attenuates 122F-triggered GR and cell migration. Remarkably, PRMT5 is found downregulated and the level of HURP m122 is decreased during the early hours of wound healing-based cell migration, collectively implying that the PRMT5-HURP-acetyl-tubulin axis plays the role of brake, preventing GR and cell migration before cells reach empty space.

PRMT5-mediated arginine methylation activates AKT kinase to govern tumorigenesis

AKT is involved in a number of key cellular processes including cell proliferation, apoptosis and metabolism. Hyperactivation of AKT is associated with many pathological conditions, particularly cancers. Emerging evidence indicates that arginine methylation is involved in modulating AKT signaling pathway. However, whether and how arginine methylation directly regulates AKT kinase activity remain unknown. Here we report that protein arginine methyltransferase 5 (PRMT5), but not other PRMTs, promotes AKT activation by catalyzing symmetric dimethylation of AKT1 at arginine 391 (R391). Mechanistically, AKT1-R391 methylation cooperates with phosphatidylinositol 3,4,5 trisphosphate (PIP3) to relieve the pleckstrin homology (PH)-in conformation, leading to AKT1 membrane translocation and subsequent activation by phosphoinositide-dependent kinase-1 (PDK1) and the mechanistic target of rapamycin complex 2 (mTORC2). As a result, deficiency in AKT1-R391 methylation significantly suppresses AKT1 kinase activity and tumorigenesis. Lastly, we show that PRMT5 inhibitor synergizes with AKT inhibitor or chemotherapeutic drugs to enhance cell death. Altogether, our study suggests that R391 methylation is an important step for AKT activation and its oncogenic function.

Arginine methylation: the promise of a 'silver bullet' for brain tumours?

Despite intense research efforts, our pharmaceutical repertoire against high-grade brain tumours has not been able to increase patient survival for a decade and life expectancy remains at less than 16 months after diagnosis, on average. Inhibitors of protein arginine methyltransferases (PRMTs) have been developed and investigated over the past 15 years and have now entered oncology clinical trials, including for brain tumours. This review collates recent advances in the understanding of the role of PRMTs and arginine methylation in brain tumours. We provide an up-to-date literature review on the mechanisms for PRMT regulation. These include endogenous modulators such as alternative splicing, miRNA, post-translational modifications and PRMT-protein interactions, and synthetic inhibitors. We discuss the relevance of PRMTs in brain tumours with a particular focus on PRMT1, -2, -5 and -8. Finally, we include a future perspective where we discuss possible routes for further research on arginine methylation and on the use of PRMT inhibitors in the context of brain tumours.

The uncharacterized protein FAM47E interacts with PRMT5 and regulates its functions

The uncharacterized protein FAM47E interacts with and stabilizes the protein arginine methyltransferase PRMT5, regulating its epigenetic functions thereby modulating target gene expression.

Protein arginine methyltransferase 5 (PRMT5) symmetrically dimethylates arginine residues in various proteins affecting diverse cellular processes such as transcriptional regulation, splicing, DNA repair, differentiation, and cell cycle. Elevated levels of PRMT5 are observed in several types of cancers and are associated with poor clinical outcomes, making PRMT5 an important diagnostic marker and/or therapeutic target for cancers. Here, using yeast two-hybrid screening, followed by immunoprecipitation and pull-down assays, we identify a previously uncharacterized protein, FAM47E, as an interaction partner of PRMT5. We report that FAM47E regulates steady-state levels of PRMT5 by affecting its stability through inhibition of its proteasomal degradation. Importantly, FAM47E enhances the chromatin association and histone methylation activity of PRMT5. The PRMT5–FAM47E interaction affects the regulation of PRMT5 target genes expression and colony-forming capacity of the cells. Taken together, we identify FAM47E as a protein regulator of PRMT5, which promotes the functions of this versatile enzyme. These findings imply that disruption of PRMT5–FAM47E interaction by small molecules might be an alternative strategy to attenuate the oncogenic function(s) of PRMT5.

NBS1 interacts with Notch signaling in neuronal homeostasis

NBS1 is a critical component of the MRN (MRE11/RAD50/NBS1) complex, which regulates ATM- and ATR-mediated DNA damage response (DDR) pathways. Mutations in NBS1 cause the human genomic instability syndrome Nijmegen Breakage Syndrome (NBS), of which neuronal deficits, including microcephaly and intellectual disability, are classical hallmarks. Given its function in the DDR to ensure proper proliferation and prevent death of replicating cells, NBS1 is essential for life. Here we show that, unexpectedly, Nbs1 deletion is dispensable for postmitotic neurons, but compromises their arborization and migration due to dysregulated Notch signaling. We find that Nbs1 interacts with NICD-RBPJ, the effector of Notch signaling, and inhibits Notch activity. Genetic ablation or pharmaceutical inhibition of Notch signaling rescues the maturation and migration defects of Nbs1-deficient neurons in vitro and in vivo. Upregulation of Notch by Nbs1 deletion is independent of the key DDR downstream effector p53 and inactivation of each MRN component produces a different pattern of Notch activity and distinct neuronal defects. These data indicate that neuronal defects and aberrant Notch activity in Nbs1-deficient cells are unlikely to be a direct consequence of loss of MRN-mediated DDR function. This study discloses a novel function of NBS1 in crosstalk with the Notch pathway in neuron development.

The Golgi Apparatus May Be a Potential Therapeutic Target for Apoptosis-Related Neurological Diseases

Increasing evidence shows that, in addition to the classical function of protein processing and transport, the Golgi apparatus (GA) is also involved in apoptosis, one of the most common forms of cell death. The structure and the function of the GA is damaged during apoptosis. However, the specific effect of the GA on the apoptosis process is unclear; it may be involved in initiating or promoting apoptosis, or it may inhibit apoptosis. Golgi-related apoptosis is associated with a variety of neurological diseases including glioma, Alzheimer’s disease (AD), Parkinson’s disease (PD), and ischemic stroke. This review summarizes the changes and the possible mechanisms of Golgi structure and function during apoptosis. In addition, we also explore the possible mechanisms by which the GA regulates apoptosis and summarize the potential relationship between the Golgi and certain neurological diseases from the perspective of apoptosis. Elucidation of the interaction between the GA and apoptosis broadens our understanding of the pathological mechanisms of neurological diseases and provides new research directions for the treatment of these diseases. Therefore, we propose that the GA may be a potential therapeutic target for apoptosis-related neurological diseases.

ATR is essential for preservation of cell mechanics and nuclear integrity during interstitial migration

ATR responds to mechanical stress at the nuclear envelope and mediates envelope-associated repair of aberrant topological DNA states. By combining microscopy, electron microscopic analysis, biophysical and in vivo models, we report that ATR-defective cells exhibit altered nuclear plasticity and YAP delocalization. When subjected to mechanical stress or undergoing interstitial migration, ATR-defective nuclei collapse accumulating nuclear envelope ruptures and perinuclear cGAS, which indicate loss of nuclear envelope integrity, and aberrant perinuclear chromatin status. ATR-defective cells also are defective in neuronal migration during development and in metastatic dissemination from circulating tumor cells. Our findings indicate that ATR ensures mechanical coupling of the cytoskeleton to the nuclear envelope and accompanying regulation of envelope-chromosome association. Thus the repertoire of ATR-regulated biological processes extends well beyond its canonical role in triggering biochemical implementation of the DNA damage response.

The nucleus is a mechanically stiff organelle of the cell and the DNA damage response protein ATR can localize to the nuclear envelope upon mechanical stress. Here, the authors show that ATR may contribute to the integrity of the nuclear envelope and may play a role in cell migration.

PRMT5-dependent transcriptional repression of c-Myc target genes promotes gastric cancer progression

The proto-oncogene c-Myc regulates multiple biological processes mainly through selectively activating gene expression. However, the mechanisms underlying c-Myc-mediated gene repression in the context of cancer remain less clear. This study aimed to clarify the role of PRMT5 in the transcriptional repression of c-Myc target genes in gastric cancer.

Methods: Immunohistochemistry was used to evaluate the expression of PRMT5, c-Myc and target genes in gastric cancer patients. PRMT5 and c-Myc interaction was assessed by immunofluorescence, co-immunoprecipitation and GST pull-down assays. Bioinformatics analysis, immunoblotting, real-time PCR, chromatin immunoprecipitation, and rescue experiments were used to evaluate the mechanism.

Results: We found that c-Myc directly interacts with protein arginine methyltransferase 5 (PRMT5) to transcriptionally repress the expression of a cohort of genes, including PTEN, CDKN2C (p18^INK4C), CDKN1A (p21^CIP1/WAF1), CDKN1C (p57^KIP2) and p63, to promote gastric cancer cell growth. Specifically, we found that PRMT5 was required to promote gastric cancer cell growth in vitro and in vivo, and for transcriptional repression of this cohort of genes, which was dependent on its methyltransferase activity. Consistently, the promoters of this gene cohort were enriched for both PRMT5-mediated symmetric di-methylation of histone H4 on Arg 3 (H4R3me2s) and c-Myc, and c-Myc depletion also upregulated their expression. H4R3me2s also colocalized with the c-Myc-binding E-box motif (CANNTG) on these genes. We show that PRMT5 directly binds to c-Myc, and this binding is required for transcriptional repression of the target genes. Both c-Myc and PRMT5 expression levels were upregulated in primary human gastric cancer tissues, and their expression levels inversely correlated with clinical outcomes.

Conclusions: Taken together, our study reveals a novel mechanism by which PRMT5-dependent transcriptional repression of c-Myc target genes is required for gastric cancer progression, and provides a potential new strategy for therapeutic targeting of gastric cancer.

PRMT5 in gene regulation and hematologic malignancies

Arginine methylation is a common posttranslational modification that governs important cellular processes and impacts development, cell growth, proliferation, and differentiation. Arginine methylation is catalyzed by protein arginine methyltransferases (PRMTs), which are classified as type I and type II enzymes responsible for the formation of asymmetric and symmetric dimethylarginine, respectively. PRMT5 is the main type II enzyme that catalyzes symmetric dimethylarginine of histone proteins to induce gene silencing by generating repressive histone marks, including H2AR3me2s, H3R8me2s, and H4R3me2s. PRMT5 can also methylate nonhistone proteins such as the transcription factors p53, E2F1 and p65. Modifications of these proteins by PRMT5 are involved in diverse cellular processes, including transcription, translation, DNA repair, RNA processing, and metabolism. A growing literature demonstrates that PRMT5 expression is upregulated in hematologic malignancies, including leukemia and lymphoma, where PRMT5 regulates gene expression to promote cancer cell proliferation. Targeting PRMT5 by specific inhibitors has emerged as a potential therapeutic strategy to treat these diseases.

Curcumin ameliorates PRMT5-MEP50 arginine methyltransferase expression by decreasing the Sp1 and NF-YA transcription factors in the A549 and MCF-7 cells

The protein arginine methyltransferase 5 (PRMT5) and its catalytic partner methylosome protein MEP50 (WDR77) catalyse the mono- and symmetric di-methylation of selective arginines in various histones and non-histone target proteins. It has emerged as a crucial epigenetic regulator in cell proliferation and differentiation; which also reported to be overexpressed in many forms of cancers in humans. In this study, we aimed to assess the modulations in the expression of this enzyme upon exposure to the well-studied natural compound from the spice turmeric, curcumin. We exposed the lung and breast cancer cell lines (A549 and MCF-7) to curcumin (2 and 20 μM) and observed a highly significant inhibitory effect on the expression of both PRMT5 and MEP50. The level of symmetrical dimethylarginine (SDMA) in multiple proteins, and more specifically, the H4R3me2s mark (which predominates in GC-rich motifs in nucleosomal DNA) was also diminished significantly. We also found that curcumin significantly reduced the level and enrichment of the transcription factors Sp1 and NF-YA which shares their binding sites within the GC-rich region of the PRMT5 proximal promoter. Furthermore, the involvement of both PKC-p38-ERK-cFos and AKT-mTOR signalling was observed in reducing the Sp1 and NF-YA expression by curcumin. Therefore, we propose curcumin decreased the expression of PRMT5 in these cells by affecting at least these two transcription factors. Altogether, we report a new molecular target of curcumin and further elucidation of this proposed mechanism through which curcumin affects the PRMT5-MEP50 methyltransferase expression might be explored for its therapeutic application.

PRMT5-mediated H4R3sme2 Confers Cell Differentiation in Pediatric B-cell Precursor Acute Lymphoblastic Leukemia

PURPOSE

Little is known about the function of histone arginine methylation in acute lymphoblastic leukemia (ALL). The objective was to evaluate whether protein arginine methyltransferase 5 (PRMT5) plays a role in pediatric ALL and to determine the possible mechanism of epigenetic regulation.

EXPERIMENTAL DESIGN

We used bone marrow samples from patients with pediatric ALL, the Nalm6 cell line, mature B-cell lines, and mouse xenograft models to evaluate the function of PRMT5 in ALL tumorigenesis.

RESULTS

This study showed that PRMT5 and the symmetric dimethylation of H4R3 (H4R3sme2) were upregulated in most initially diagnosed (n = 15; 100%) and relapsed (n = 4; 75%) bone marrow leukemia cells from patients with pediatric B-cell precursor ALL (BCP-ALL) and were decreased when the disease was in remission (n = 15; 6.7%). Downregulation of H4R3sme2 by PRMT5 silencing induced BCP-ALL cell differentiation from the pre-B to immature B stage, whereas overexpressed PRMT5 with enhanced H4R3sme2 promoted human mature B cells to dedifferentiate back to the pre-B II/immature B stages in vitro. High PRMT5 expression enhanced the proportion of CD43⁺/B220⁺/sIgM^- B leukocytes in recipient mice. CLC and CTSB were identified as potential target genes of PRMT5 in BCP-ALL cells and were inhibited by H4R3sme2 in gene promoters.

CONCLUSIONS

We demonstrate that enhanced PRMT5 promotes BCP-ALL leukemogenesis partially by the dysregulation of B-cell lineage differentiation. H4R3sme2 and PRMT5 may serve as potential sensitive biomarkers of pediatric BCP-ALL. Suppression of the activation of PRMT5 may offer a promising therapeutic strategy against pediatric BCP-ALL.

Golgi Structure and Function in Health, Stress, and Diseases

The Golgi apparatus is a central intracellular membrane-bound organelle with key functions in trafficking, processing, and sorting of newly synthesized membrane and secretory proteins and lipids. To best perform these functions, Golgi membranes form a unique stacked structure. The Golgi structure is dynamic but tightly regulated; it undergoes rapid disassembly and reassembly during the cell cycle of mammalian cells and is disrupted under certain stress and pathological conditions. In the past decade, significant amount of effort has been made to reveal the molecular mechanisms that regulate the Golgi membrane architecture and function. Here we review the major discoveries in the mechanisms of Golgi structure formation, regulation, and alteration in relation to its functions in physiological and pathological conditions to further our understanding of Golgi structure and function in health and diseases.

Selected Golgi-Localized Proteins and Carcinogenesis: What Do We Know?

The role of the Golgi apparatus in carcinogenesis still remains unclear. A number of structural and functional cis-, medial-, and trans-Golgi proteins as well as a complexity of metabolic pathways which they mediate may indicate a central role of the Golgi apparatus in the development and progression of cancer. Pleiotropy of cellular function of the Golgi apparatus makes it a "metabolic heart" or a relay station of a cell, which combines multiple signaling pathways involved in carcinogenesis. Therefore, any damage to or structural abnormality of the Golgi apparatus, causing its fragmentation and/or biochemical dysregulation, results in an up- or downregulation of signaling pathways and may in turn promote tumor progression, as well as local nodal and distant metastases. Three alternative or parallel models of spatial and functional Golgi organization within tumor cells were proposed: (1) compacted Golgi structure, (2) normal Golgi structure with its increased activity, and (3) the Golgi fragmentation with ministacks formation. Regardless of the assumed model, the increased activity of oncogenesis initiators and promoters with inhibition of suppressor proteins results in an increased cell motility and migration, increased angiogenesis, significantly activated trafficking kinetics, proliferation, EMT induction, decreased susceptibility to apoptosis-inducing factors, and modulating immune response to tumor cell antigens. Eventually, this will lead to the increased metastatic potential of cancer cells and an increased risk of lymph node and distant metastases. This chapter provided an overview of the current state of knowledge of selected Golgi proteins, their role in cytophysiology as well as potential involvement in tumorigenesis.

Protein arginine methyltransferase 5 (PRMT5) dysregulation in cancer

Protein arginine methyltransferases (PRMTs) are known for their ability to catalyze methylation of specific arginine residues in a wide variety of cellular proteins, which are involved in a plethora of processes including signal transduction, transcription, and more recently DNA recombination. All members of the PRMT family can be grouped into three main classes depending on the type of methylation they catalyze. Type I PRMTs induce monomethylation and asymmetric dimethylation, while type II PRMTs catalyze monomethylation and symmetric dimethylation of specific arginine residues. In contrast, type III PRMTs carry out only monomethylation of arginine residues. In this review, we will focus on PRMT5, a type II PRMT essential for viability and normal development, which has been shown to be overexpressed in a wide variety of cancer cell types, owing it to the crucial role it plays in controlling key growth regulatory pathways. Furthermore, the role of PRMT5 in regulating expression and stability of key transcription factors that control normal stem cell function as well as cancer stem cell renewal will be discussed. We will review recent work that shows that through its ability to methylate various cellular proteins, PRMT5 functions as a master epigenetic regulator essential for growth and development, and we will highlight studies that have examined its dysregulation and the effects of its inhibition on cancer cell growth.

Histone arginine methylation by Prmt5 is required for lung branching morphogenesis through repression of BMP signaling

Branching morphogenesis is essential for the successful development of a functional lung to accomplish its gas exchange function. Although many studies have highlighted requirements for the bone morphogenetic protein (BMP) signaling pathway during branching morphogenesis, little is known about how BMP signaling is regulated. Here, we report that the protein arginine methyltransferase 5 (Prmt5) and symmetric dimethylation at histone H4 arginine 3 (H4R3sme2) directly associate with chromatin of Bmp4 to suppress its transcription. Inactivation of Prmt5 in the lung epithelium results in halted branching morphogenesis, altered epithelial cell differentiation and neonatal lethality. These defects are accompanied by increased apoptosis and reduced proliferation of lung epithelium, as a consequence of elevated canonical BMP-Smad1/5/9 signaling. Inhibition of BMP signaling by Noggin rescues the lung branching defects of Prmt5 mutant in vitro Taken together, our results identify a novel mechanism through which Prmt5-mediated histone arginine methylation represses canonical BMP signaling to regulate lung branching morphogenesis.

LLY-283, a Potent and Selective Inhibitor of Arginine Methyltransferase 5, PRMT5, with Antitumor Activity

Protein arginine methyltransferase 5 (PRMT5) is a type II arginine methyltransferase that catalyzes the formation of symmetric dimethylarginine in a number of nuclear and cytoplasmic proteins. Although the cellular functions of PRMT5 have not been fully unraveled, it has been implicated in a number of cellular processes like RNA processing, signal transduction, and transcriptional regulation. PRMT5 is ubiquitously expressed in most tissues and its expression has been shown to be elevated in several cancers including breast cancer, gastric cancer, glioblastoma, and lymphoma. Here, we describe the identification and characterization of a novel and selective PRMT5 inhibitor with potent in vitro and in vivo activity. Compound 1 (also called LLY-283) inhibited PRMT5 enzymatic activity in vitro and in cells with IC₅₀ of 22 ± 3 and 25 ± 1 nM, respectively, while its diastereomer, compound 2 (also called LLY-284), was much less active. Compound 1 also showed antitumor activity in mouse xenografts when dosed orally and can serve as an excellent probe molecule for understanding the biological function of PRMT5 in normal and cancer cells.

The E3 ubiquitin ligase APC/CCdh1 degrades MCPH1 after MCPH1-βTrCP2-Cdc25A-mediated mitotic entry to ensure neurogenesis

Mutations of microcephalin (MCPH1) can cause the neurodevelopmental disorder primary microcephaly type 1. We previously showed that MCPH1 deletion in neural stem cells results in early mitotic entry that distracts cell division mode, leading to exhaustion of the progenitor pool. Here, we show that MCPH1 interacts with and promotes the E3 ligase βTrCP2 to degrade Cdc25A independent of DNA damage. Overexpression of βTrCP2 or the knockdown of Cdc25A remedies the high mitotic index and rescues the premature differentiation of Mcph1-deficient neuroprogenitors in vivo MCPH1 itself is degraded by APC/C^Cdh1, but not APC/C^Cdc20, in late mitosis and G1 phase. Forced MCPH1 expression causes cell death, underlining the importance of MCPH1 turnover after mitosis. Ectopic expression of Cdh1 leads to premature differentiation of neuroprogenitors, mimicking differentiation defects of Mcph1-knockout neuroprogenitors. The homeostasis of MCPH1 in association with the ubiquitin-proteasome system ensures mitotic entry independent of cell cycle checkpoint. This study provides a mechanistic understanding of how MCPH1 controls neural stem cell fate and brain development.

Selective small-chemical inhibitors of protein arginine methyltransferase 5 with anti-lung cancer activity

Protein arginine methyltransferase 5 (PRMT5) plays critical roles in a wide variety of biological processes, including tumorigenesis. By screening a library of small chemical compounds, we identified eight compounds that selectively inhibit the PRMT5 enzymatic activity, with IC₅₀ values ranging from 0.1 to 6 μM. Molecular docking simulation and site-directed mutagenesis indicated that identified compounds target the substrate-binding site in PRMT5. Treatment of lung cancer cells with identified inhibitors led to inhibition of the symmetrical arginine methylation of SmD3 and histones and the cellular proliferation. Oral administration of the inhibitor demonstrated antitumor activity in a lung tumor xenograft model. Thus, identified PRMT5-specific small-molecule inhibitors would help elucidate the biological roles of PRMT5 and serve as lead compounds for future drug development.

Loss of the golgin GM130 causes Golgi disruption, Purkinje neuron loss, and ataxia in mice

The Golgi apparatus lies at the heart of the secretory pathway where it is required for secretory trafficking and cargo modification. Disruption of Golgi architecture and function has been widely observed in neurodegenerative disease, but whether Golgi dysfunction is causal with regard to the neurodegenerative process, or is simply a manifestation of neuronal death, remains unclear. Here we report that targeted loss of the golgin GM130 leads to a profound neurological phenotype in mice. Global KO of mouse GM130 results in developmental delay, severe ataxia, and postnatal death. We further show that selective deletion of GM130 in neurons causes fragmentation and defective positioning of the Golgi apparatus, impaired secretory trafficking, and dendritic atrophy in Purkinje cells. These cellular defects manifest as reduced cerebellar size and Purkinje cell number, leading to ataxia. Purkinje cell loss and ataxia first appear during postnatal development but progressively worsen with age. Our data therefore indicate that targeted disruption of the mammalian Golgi apparatus and secretory traffic results in neuronal degeneration in vivo, supporting the view that Golgi dysfunction can play a causative role in neurodegeneration.

Targeting methyltransferase PRMT5 eliminates leukemia stem cells in chronic myelogenous leukemia

Imatinib-insensitive leukemia stem cells (LSCs) are believed to be responsible for resistance to BCR-ABL tyrosine kinase inhibitors and relapse of chronic myelogenous leukemia (CML). Identifying therapeutic targets to eradicate CML LSCs may be a strategy to cure CML. In the present study, we discovered a positive feedback loop between BCR-ABL and protein arginine methyltransferase 5 (PRMT5) in CML cells. Overexpression of PRMT5 was observed in human CML LSCs. Silencing PRMT5 with shRNA or blocking PRMT5 methyltransferase activity with the small-molecule inhibitor PJ-68 reduced survival, serial replating capacity, and long-term culture-initiating cells (LTC-ICs) in LSCs from CML patients. Further, PRMT5 knockdown or PJ-68 treatment dramatically prolonged survival in a murine model of retroviral BCR-ABL-driven CML and impaired the in vivo self-renewal capacity of transplanted CML LSCs. PJ-68 also inhibited long-term engraftment of human CML CD34+ cells in immunodeficient mice. Moreover, inhibition of PRMT5 abrogated the Wnt/β-catenin pathway in CML CD34+ cells by depleting dishevelled homolog 3 (DVL3). This study suggests that epigenetic methylation modification on histone protein arginine residues is a regulatory mechanism to control self-renewal of LSCs and indicates that PRMT5 may represent a potential therapeutic target against LSCs.

Protein arginine methyltransferase 5 regulates multiple signaling pathways to promote lung cancer cell proliferation

BACKGROUND

Protein arginine methyltransferase 5 (PRMT5) catalyzes the formation of symmetrical dimethylation of arginine residues in proteins. WD repeat domain 77 (WDR77), also known as p44, MEP50, or WD45, forms a stoichiometric complex with PRMT5. The PRMT5/p44 complex is required for cellular proliferation of lung and prostate epithelial cells during earlier stages of development and is re-activated during prostate and lung tumorigenesis. The molecular mechanisms by which PRMT5 and p44 promote cellular proliferation are unknown.

METHODS

Expression of PRMT5 and p44 in lung and prostate cancer cells was silenced and their target genes were identified. The regulation of target genes was validated in various cancer cells during lung development and tumorigenesis. Altered expression of target genes was achieved by ectopic cDNA expression and shRNA-mediated silencing.

RESULTS

PRMT5 and p44 regulate expression of a specific set of genes encoding growth and anti-growth factors, including receptor tyrosine kinases and antiproliferative proteins. Genes whose expression was suppressed by PRMT5 and p44 encoded anti-growth factors and inhibited cell growth when ectopically expressed. In contrast, genes whose expression was enhanced by PRMT5 and p44 encoded growth factors and increased cell growth when expressed. Altered expression of target genes is associated with re-activation of PRMT5 and p44 during lung tumorigenesis.

CONCLUSIONS

Our data provide the molecular basis by which PRMT5 and p44 regulate cell growth and lay a foundation for further investigation of their role in lung tumor initiation.

The arginine methyltransferase PRMT5 regulates CIITA-dependent MHC II transcription

Class II major histocompatibility complex (MHC II) dependent antigen presentation serves as a key step in mammalian adaptive immunity and host defense. In antigen presenting cells (e.g., macrophages), MHC II transcription can be activated by interferon gamma (IFN-γ) and mediated by class II transactivator (CIITA). The underlying epigenetic mechanism, however, is not completely understood. Here we report that following IFN-γ stimulation, symmetrically dimethylated histone H3 arginine 2 (H3R2Me2s) accumulated on the MHC II promoter along with CIITA. IFN-γ augmented expression, nuclear translocation, and promoter binding of the protein arginine methyltransferase PRMT5 in macrophages. Over-expression of PRMT5 potentiated IFN-γ induced activation of MHC II transcription in an enzyme activity-dependent manner. In contrast, PRMT5 silencing or inhibition of PRMT5 activity by methylthioadenosine (MTA) suppressed MHC II transactivation by IFN-γ. CIITA interacted with and recruited PRMT5 to the MHC II promoter and mediated the synergy between PRMT5 and ASH2/WDR5 to activate MHC II transcription. PRMT5 expression was down-regulated in senescent and H2O2-treated macrophages rendering ineffectual induction of MHC II transcription by IFN-γ. Taken together, our data reveal a pathophysiologically relevant role for PRMT5 in MHC II transactivation in macrophages.

Toxoplasma gondii Arginine Methyltransferase 1 (PRMT1) Is Necessary for Centrosome Dynamics during Tachyzoite Cell Division

The arginine methyltransferase family (PRMT) has been implicated in a variety of cellular processes, including signal transduction, epigenetic regulation, and DNA repair pathways. PRMT1 is thought to be responsible for the majority of PRMT activity in Toxoplasma gondii, but its exact function is unknown. To further define the biological function of the PRMT family, we generated T. gondii mutants lacking PRMT1 (Δprmt1) by deletion of the PRMT1 gene. Δprmt1 parasites exhibit morphological defects during cell division and grow slowly, and this phenotype reverses in the Δprmt::PRMT1mRFP complemented strain. Tagged PRMT1 localizes primarily in the cytoplasm with enrichment at the pericentriolar material, and the strain lacking PRMT1 is unable to segregate progeny accurately. Unlike wild-type and complemented parasites, Δprmt1 parasites have abnormal daughter buds, perturbed centrosome stoichiometry, and loss of synchronous replication. Whole-genome expression profiling demonstrated differences in expression of cell-cycle-regulated genes in the Δprmt1 strain relative to the complemented Δprmt1::PRMT1mRFP and parental wild-type strains, but these changes do not correlate with a specific block in cell cycle. Although PRMT1’s primary biological function was previously proposed to be methylation of histones, our studies suggest that PRMT1 plays an important role within the centrosome to ensure the proper replication of the parasite.

Apicomplexan parasites include several important pathogens, including Toxoplasma gondii, a major cause of opportunistic infections and congenital birth defects. These parasites divide using a unique form of cell division called endodyogeny that is different from those of most eukaryotes. PRMT1 is a conserved arginine methyltransferase that was thought to regulate gene expression of T. gondii by modifying histone methylation. Using genetic techniques, we show that disruption of PRMT1 affects the parasite’s ability to perform accurate cell division. Our studies reveal an unexpected role for arginine methylation in centrosome biology and regulation of parasite replication.

Myc and Omomyc functionally associate with the Protein Arginine Methyltransferase 5 (PRMT5) in glioblastoma cells

The c-Myc protein is dysregulated in many human cancers and its function has not been fully elucitated yet. The c-Myc inhibitor Omomyc displays potent anticancer properties in animal models. It perturbs the c-Myc protein network, impairs c-Myc binding to the E-boxes, retaining transrepressive properties and inducing histone deacetylation. Here we have employed Omomyc to further analyse c-Myc activity at the epigenetic level. We show that both Myc and Omomyc stimulate histone H4 symmetric dimethylation of arginine (R) 3 (H4R3me2s), in human glioblastoma and HEK293T cells. Consistently, both associated with protein Arginine Methyltransferase 5 (PRMT5)—the catalyst of the reaction—and its co-factor Methylosome Protein 50 (MEP50). Confocal experiments showed that Omomyc co-localized with c-Myc, PRMT5 and H4R3me2s-enriched chromatin domains. Finally, interfering with PRMT5 activity impaired target gene activation by Myc whereas it restrained Omomyc-dependent repression. The identification of a histone-modifying complex associated with Omomyc represents the first demonstration of an active role of this miniprotein in modifying chromatin structure and adds new information regarding its action on c-Myc targets. More importantly, the observation that c-Myc may recruit PRMT5-MEP50, inducing H4R3 symmetric di-methylation, suggests previously unpredictable roles for c-Myc in gene expression regulation and new potential targets for therapy.

Differential expression, distinct localization and opposite effect on Golgi structure and cell differentiation by a novel splice variant of human PRMT5

Alternative splicing contributes greatly to the proteomic diversity of metazoans. Protein arginine methyltransferase 5 (PRMT5) methylates arginines of Golgi components and other factors exerting diverse effects on cell growth/differentiation, but the underlying molecular basis for its subcellular distribution and diverse roles has not been fully understood. Here we show the detailed properties of an evolutionarily emerged splice variant of human PRMT5 (PRMT5S) that is distinct from the original isoform (PRMT5L). The isoforms are differentially expressed among mammalian cells and tissues. The PRMT5S is distributed all over the cell but PRMT5L mainly colocalizes with Giantin, a Golgi marker. PRMT5 knockdown led to an enlarged Giantin pattern, which was prevented by the expression of either isoform. Rescuing PRMT5S also increased the percentage of cells with an interphase Giantin pattern compacted at one end of the nucleus, consistent with its cell cycle-arresting effect, while rescuing PRMT5L increased that of the mitotic Giantin patterns of dynamically fragmented structures. Moreover, the isoforms are differentially expressed during neuronal or dendritic cell differentiation, and their ectopic expression showed an opposite effect on dendritic cell differentiation. Furthermore, besides their differential regulation of gene expression, both isoforms also similarly regulate over a thousand genes particularly those involved in apoptosis and differentiation. Taking these properties together, we propose that their differential expression and subcellular localization contribute to spatial and temporal regulation of arginine methylation and gene expression to exert different effects. The novel PRMT5S likely contributes to the observed diverse effects of PRMT5 in cells.

Golgi post-translational modifications and associated diseases

For non specialists, Golgi is a very well known subcellular compartment involved in secretion and correct targeting of soluble and transmembrane proteins. Nevertheless, Golgi is also specifically involved in many different and diverse post-translational modifications. Through its diverse functions, Golgi is not only able to modify secreted and transmembrane proteins but also cytoplasmic proteins. The Golgi apparatus research field is so broad that an exhaustive review of this organelle is not doable here. The goal of this review is to cover the main post-translational modifications occurring at the Golgi level and present the identified associated diseases.

Evolutionary emergence of a novel splice variant with an opposite effect on the cell cycle

Alternative splicing contributes greatly to the diversification of mammalian proteomes, but the molecular basis for the evolutionary emergence of splice variants remains poorly understood. We have recently found a novel class of splicing regulatory elements between the polypyrimidine tract (Py) and 3' AG (REPA) at intron ends in many human genes, including the multifunctional PRMT5 (for protein arginine methyltransferase 5) gene. The PRMT5 element is comprised of two G tracts that arise in most mammals and accompany significant exon skipping in human transcripts. The G tracts inhibit splicing by recruiting heterogeneous nuclear ribonucleoprotein (hnRNP) H and F (H/F) to reduce U2AF65 binding to the Py, causing exon skipping. The resulting novel shorter variant PRMT5S exhibits a histone H4R3 methylation effect similar to that seen with the original longer PRMT5L isoform but exhibits a distinct localization and preferential control of critical genes for cell cycle arrest at interphase in comparison to PRMT5L. This report thus provides a molecular mechanism for the evolutionary emergence of a novel splice variant with an opposite function in a fundamental cell process. The presence of REPA elements in a large group of genes implies their wider impact on different cellular processes for increased protein diversity in humans.

Proliferative role of TRAF4 in breast cancer by upregulating PRMT5 nuclear expression

In this study, we examined protein arginine methyltransferase 5 (PRMT5) and tumor necrosis factor receptor-associated 4 (TRAF4) expression in breast cancer to find the interaction mechanism between the two. We examined TRAF4 and PRMT5 expression by immunohistochemistry and found that their expression is positively correlated in breast cancer. Besides, PRMT5 expression was significantly associated with histological type and tumor size (p < 0.05). PRMT5 nuclear expression was significantly associated with HER2 expression (p < 0.05). PRMT5 and TRAF4 were both overexpressed in breast cancer tissues and cells, and we found that PRMT5 binds to the zinc finger structures in TRAF4 by coimmunoprecipitation and Western blotting. We also tested the potential regulatory effect between TRAF4 and PRMT5. TRAF4 upregulated PRMT5 expression, which occurred predominantly in the nucleus, on which TRAF4 promotion of cell proliferation in breast cancer is mainly dependent. PRMT5 may play an important role in activation of the NF-κB signaling pathway.

Protein arginine methyltransferase 5 (PRMT5) is a novel coactivator of constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR) plays a key role in the expression of xenobiotic/steroid and drug metabolizing enzymes and their transporters. In this study, we demonstrated that protein arginine methyltransferase 5 (PRMT5) is a novel CAR-interacting protein. Furthermore, the PRMT-dependent induction of a CAR reporter gene, which was independent of methyltransferase activity, was enhanced in the presence of steroid receptor coactivator 1 (SRC1), peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) or DEAD box DNA/RNA helicase DP97. Using tetracycline inducible-hCAR system in HepG2 cells, we showed that knockdown of PRMT5 with small interfering RNA suppressed tetracycline -induced mRNA expression of CYP2B6 but not of CYP2C9 or CYP3A4. PRMT5 enhanced phenobarbital-mediated transactivation of a phenobarbital-responsive enhancer module (PBREM)-driven reporter gene in co-operation with PGC-1α in rat primary hepatocytes. Based on these findings, we suggest PRMT5 to be a gene (or promoter)-selective coactivator of CAR by mediating the formation of complexes between hCAR and appropriate coactivators.

JMJD3 promotes SAHF formation in senescent WI38 cells by triggering an interplay between demethylation and phosphorylation of RB protein

Primary human fibroblasts undergoing oncogene-induced or replicative senescence are known to form senescence-associated heterochromatin foci (SAHF), which can stabilize the state of senescence. The retinoblastoma (RB) protein has an important role in SAHF; cells that lack active RB pathway fail to form SAHF. It has been known that the posttranslational modifications of RB, for example, phosphorylation, regulate its function. To date, whether methylation of RB impacts on the SAHF formation is unknown. Here we report that JMJD3, a histone demethylase catalyzing the tri-methylation of H3K27 (H3K27me3), can demethylate the non-histone protein RB at the lysine810 residue (K810), which is a target of the methyltransferase Set7/9. We detected a significant upregulation of JMJD3 during cellular senescence and SAHF formation in WI38 cells induced by H-RasV(12), and we found that ectopic expression of JMJD3 promoted cellular senescence and SAHF formation in WI38 cells. Furthermore, during the process of SAHF assembly, JMJD3 was transported to the cytoplasm and interacted with RB through its demethylase domain JmjC. Significantly, our data demonstrated that the JMJD3-mediated demethylation of RB at K810 impeded the interaction of RB with the protein kinase CDK4 and resulted in reduced level of phosphorylation of RB at Serine807/811 (S807/811), implicating an important role of the interplay between the demethylation and phosphorylation of RB in SAHF assembly. This study highlights the role of JMJD3 as a novel inducer of SAHF formation through demethylating RB and provides new insights into the mechanisms of cellular senescence and SAHF assembly.

Methylosome protein 50 promotes androgen- and estrogen-independent tumorigenesis

Methylosome protein 50 (MEP50) is a component of methylosome where MEP50 binds protein substrates and activates the oncogenic protein arginine methyl transferase 5 (PRMT5). MEP50 is also a coactivator for androgen receptor (AR) and estrogen receptor (ER), and transforms cells in the presence of androgen or estrogen. To extend the understanding of how MEP50 transforms cells, we investigated whether MEP50 could transform cells independent of AR and ER, and clarified whether PRMT5 could contribute to the MEP50-caused tumor formation. Microarray and Western blot analyses revealed the association of MEP50 with many human cancers including lung cancer. Knockdown of MEP50 retarded cell growth and migration in selected lung cancer cell lines, which expressed very low level of AR and ER and were insensitive to inhibitors of AR and ER. Moreover, overexpression of Myc-MEP50 enhanced cell transforming activities of 293T cells which are known lack of expression of AR and ER. Mechanistic analyses showed that MEP50 controlled G2 progression, upregulated cyclin-dependent kinase 1(CDK1)/cyclin B1, and activated the survival cascade Phosphoinositide 3-kinase (PI3K)/AKT. MEP50 promoted cell migration, and activated the cell migration pathways such as Ras-related C3 botulinum toxin substrate 1 (Rac1)/vasodilator-stimulated phosphoprotein (VASP), and forkhead box protein A2 (FOXA2)/slug/cadherin cascades. Further analyses revealed that MEP50 activated the survival factor PI3K through PRMT5-catalyzed dimethylation of PI3K. Collectively, it is concluded that MEP50 can transform cells independent of AR and ER, and PRMT5 has partial contribution to that process.

Protein arginine methyltransferases (PRMTs): role in chromatin organization

The mammalian genome encodes eleven protein arginine methyltransferases (PRMTs) that are involved in the transfer of a methyl group from S-adenosylmethionine (AdoMet) to the guanidino nitrogen of arginine. The substrates for these enzymes range from histones to several nuclear and cytoplasmic proteins. Methylation of histones by PRMTs can block the docking site for other reader/effector molecules and thus this modification can interfere with histone code orchestration. Several members of the PRMTs have roles in chromatin organization and function. Although PRMT aberrant expression is correlated with several diseases including cancer, the underlying mechanisms are still obscure in most cases.