Astrocyte Elevated Gene-1 (AEG-1) Regulates Lipid Homeostasis

Astrocyte Elevated Gene-1/Metadherin (AEG-1/MTDH): A Promising Molecular Marker and Therapeutic Target for Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths. The 5-year survival rate has been estimated to be less than 20% while its incidence rates have more than tripled since the 1980s. Astrocyte elevated gene-1/Metadherin (AEG-1/MTDH) has been demonstrated to have an influential role in HCC progression and the development of an aggressive phenotype. AEG-1 has been shown to be upregulated in many cancers, including HCC. Studies have shown that it plays a crucial role in the proliferation, invasion and metastasis, and evasion of apoptosis in HCC. Its relationship with proteins and pathways, such as MYC, SND1, PI3K/AKT, and other signaling pathways demonstrates its pertinent role in oncogenic development and relevance as a biomarker and therapeutic target. Recent studies have shown that AEG-1 is present in tumor tissues, and the anti-AEG-1 antibody is detected in the blood of cancer patients, demonstrating its viability as a diagnostic/prognostic marker. This review paper shines light on recent findings regarding the molecular implications of AEG-1, with emphasis on its role of regulating metabolic dysfunction-associated steatohepatitis (MASH), a key predisposing factor for HCC, new treatment strategies targeting AEG-1, and challenges associated with analyzing this intriguing molecule.

Inhibition of Granule Cell Dispersion and Seizure Development by Astrocyte Elevated Gene-1 in a Mouse Model of Temporal Lobe Epilepsy

Although granule cell dispersion (GCD) in the hippocampus is known to be an important feature associated with epileptic seizures in temporal lobe epilepsy (TLE), the endogenous molecules that regulate GCD are largely unknown. In the present study, we have examined whether there is any change in AEG-1 expression in the hippocampus of a kainic acid (KA)-induced mouse model of TLE. In addition, we have investigated whether the modulation of astrocyte elevated gene-1 (AEG-1) expression in the dentate gyrus (DG) by intracranial injection of adeno-associated virus 1 (AAV1) influences pathological phenotypes such as GCD formation and seizure susceptibility in a KA-treated mouse. We have identified that the protein expression of AEG-1 is upregulated in the DG of a KA-induced mouse model of TLE. We further demonstrated that AEG-1 upregulation by AAV1 delivery in the DG-induced anticonvulsant activities such as the delay of seizure onset and inhibition of spontaneous recurrent seizures (SRS) through GCD suppression in the mouse model of TLE, while the inhibition of AEG-1 expression increased susceptibility to seizures. The present observations suggest that AEG-1 is a potent regulator of GCD formation and seizure development associated with TLE, and the significant induction of AEG-1 in the DG may have therapeutic potential against epilepsy.

Astrocyte Elevated Gene-1 Cys75 S-Palmitoylation by ZDHHC6 Regulates Its Biological Activity

Nonalcoholic fatty liver disease is a major risk factor for hepatocellular carcinoma (HCC). Astrocyte elevated gene-1/Metadherin (AEG-1/MTDH) augments lipid accumulation (steatosis), inflammation, and tumorigenesis, thereby promoting the whole spectrum of this disease process. Targeting AEG-1 is a potential interventional strategy for nonalcoholic steatohepatitis (NASH) and HCC. Thus, proper understanding of the regulation of this molecule is essential. We found that AEG-1 is palmitoylated at residue cysteine 75 (Cys75). Mutation of Cys75 to serine (Ser) completely abolished AEG-1 palmitoylation. We identified ZDHHC6 as a palmitoyltransferase catalyzing the process in HEK293T cells. To obtain insight into how palmitoylation regulates AEG-1 function, we generated knock-in mice by CRISPR/Cas9 in which Cys75 of AEG-1 was mutated to Ser (AEG-1-C75S). No developmental or anatomical abnormality was observed between AEG-1-wild type (AEG-1-WT) and AEG-1-C75S littermates. However, global gene expression analysis by RNA-sequencing unraveled that signaling pathways and upstream regulators, which contribute to cell proliferation, motility, inflammation, angiogenesis, and lipid accumulation, were activated in AEG-1-C75S hepatocytes compared to AEG-1-WT. These findings suggest that AEG-1-C75S functions as dominant positive and that palmitoylation restricts oncogenic and NASH-promoting functions of AEG-1. We thus identify a previously unknown regulatory mechanism of AEG-1, which might help design new therapeutic strategies for NASH and HCC.

Chinese medicinal herbs as potential prodrugs for obesity

Obesity is a leading worldwide health threat with ever-growing prevalence, it promotes the incidence of various diseases, particularly cardiovascular disease, metabolic syndrome, diabetes, hypertension, and certain cancers. Traditional Chinese Medicine (TCM) has been used to control body weight and treat obesity for thousands of years, Chinese medicinal herbs provide a rich natural source of effective agents against obesity. However, some problems such as complex active ingredients, poor quality control, and unclear therapeutic mechanisms still need to be investigated and resolved. Prodrugs provide a path forward to overcome TCM deficiencies such as absorption, distribution, metabolism, excretion (ADME) properties, and toxicity. This article aimed to review the possible prodrugs from various medicinal plants that demonstrate beneficial effects on obesity and seek to offer insights on prodrug design as well as a solution to the global obesity issues.

Dissecting the Balance Between Metabolic and Oncogenic Functions of Astrocyte-Elevated Gene-1/Metadherin

Obesity is an enormous global health problem, and obesity-induced nonalcoholic steatohepatitis (NASH) is contributing to a rising incidence and mortality for hepatocellular carcinoma (HCC). Increase in de novo lipogenesis and decrease in fatty acid β-oxidation (FAO) underlie hepatic lipid accumulation in NASH. Astrocyte-elevated gene-1/metadherin (AEG-1) overexpression contributes to both NASH and HCC. AEG-1 harbors an LXXLL motif through which it blocks activation of peroxisome proliferator activated receptor α (PPARα), a key regulator of FAO. To better understand the role of LXXLL motif in mediating AEG-1 function, using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technology, we generated a mouse model (AEG-1-L24K/L25H) in which the LXXLL motif in AEG-1 was mutated to LXXKH. We observed increased activation of PPARα in AEG-1-L24K/L25H livers providing partial protection from high-fat diet-induced steatosis. Interestingly, even with equal gene dosage levels, compared with AEG-1-wild-type livers, AEG-1-L24K/L25H livers exhibited increase in levels of lipogenic enzymes, mitogenic activity and inflammation, which are attributes observed when AEG-1 is overexpressed. These findings indicate that while LXXLL motif favors steatotic activity of AEG-1, it keeps in check inflammatory and oncogenic functions, thus maintaining a homeostasis in AEG-1 function. AEG-1 is being increasingly appreciated as a viable target for ameliorating NASH and NASH-HCC, and as such, in-depth understanding of the functions and molecular attributes of this molecule is essential. Conclusion: The present study unravels the unique role of the LXXLL motif in mediating the balance between the metabolic and oncogenic functions of AEG-1.

MTDH Promotes Intestinal Inflammation by Positively Regulating TLR Signalling

Macrophages in the intestinal mucosa can rapidly engage Toll-like receptor [TLR]-mediated inflammatory responses to protect against pathogen invasion, but these same innate immune responses can also drive the induction of colitis. Our previous research revealed that metadherin [MTDH] is overexpressed in multiple cancers and plays vital roles in tumour progression. However, the role of MTDH in intestinal inflammation is largely unknown. In this study, we found the MTDH expression in colonic lamina propria [CLP] macrophages was positively correlated with inflammatory colitis severity. MTDH-/- mice were protected against the symptoms of dextran sodium sulphate [DSS]-induced colitis; however, adoptive transfer of MTDH wild-type [WT] monocytes partially restored the susceptibility of MTDH-/- mice to DSS-induced colitis. TLR stimulation was sufficient to induce the expression of MTDH, whereas the absence of MTDH was sufficient to suppress TLR-induced production of inflammatory cytokines by macrophages. From a mechanistic perspective, MTDH recruited TRAF6 to TAK1, leading to TRAF6-mediated TAK1 K63 ubiquitination and phosphorylation, ultimately facilitating TLR-induced NF-κB and MAPK signalling. Taken together, our results indicate that MTDH contributes to colitis development by promoting TLR-induced pro-inflammatory cytokine production in CLP macrophages and might represent a potential therapeutic approach for intestine inflammation intervention.

Astrocyte elevated gene-1 (AEG-1): A key driver of hepatocellular carcinoma (HCC)

An array of human cancers, including hepatocellular carcinoma (HCC), overexpress the oncogene Astrocyte elevated gene-1 (AEG-1). It is now firmly established that AEG-1 is a key driver of carcinogenesis, and enhanced expression of AEG-1 is a marker of poor prognosis in cancer patients. In-depth studies have revealed that AEG-1 positively regulates different hallmarks of HCC progression including growth and proliferation, angiogenesis, invasion, migration, metastasis and resistance to therapeutic intervention. By interacting with a plethora of proteins as well as mRNAs, AEG-1 regulates gene expression at transcriptional, post-transcriptional, and translational levels, and modulates numerous pro-tumorigenic and tumor-suppressive signal transduction pathways. Even though extensive research over the last two decades using various in vitro and in vivo models has established the pivotal role of AEG-1 in HCC, effective targeting of AEG-1 as a therapeutic intervention for HCC is yet to be achieved in the clinic. Targeted delivery of AEG-1 small interfering ribonucleic acid (siRNA) has demonstrated desired therapeutic effects in mouse models of HCC. Peptidomimetic inhibitors based on protein-protein interaction studies has also been developed recently. Continuous unraveling of novel mechanisms in the regulation of HCC by AEG-1 will generate valuable knowledge facilitating development of specific AEG-1 inhibitory strategies. The present review describes the current status of AEG-1 in HCC gleaned from patient-focused and bench-top studies as well as transgenic and knockout mouse models. We also address the challenges that need to be overcome and discuss future perspectives on this exciting molecule to transform it from bench to bedside.

Multifunctional Role of Astrocyte Elevated Gene-1 (AEG-1) in Cancer: Focus on Drug Resistance

Simple Summary

Chemotherapy is a major mode of treatment for cancers. However, cancer cells adapt to survive in stressful conditions and in many cases, they are inherently resistant to chemotherapy. Additionally, after initial response to chemotherapy, the surviving cancer cells acquire new alterations making them chemoresistant. Genes that help adapt the cancer cells to cope with stress often contribute to chemoresistance and one such gene is Astrocyte elevated gene-1 (AEG-1). AEG-1 levels are increased in all cancers studied to date and AEG-1 contributes to the development of highly aggressive, metastatic cancers. In this review, we provide a comprehensive description of the mechanism by which AEG-1 augments tumor development with special focus on its ability to regulate chemoresistance. We also discuss potential ways to inhibit AEG-1 to overcome chemoresistance.

Abstract

Cancer development results from the acquisition of numerous genetic and epigenetic alterations in cancer cells themselves, as well as continuous changes in their microenvironment. The plasticity of cancer cells allows them to continuously adapt to selective pressures brought forth by exogenous environmental stresses, the internal milieu of the tumor and cancer treatment itself. Resistance to treatment, either inherent or acquired after the commencement of treatment, is a major obstacle an oncologist confronts in an endeavor to efficiently manage the disease. Resistance to chemotherapy, chemoresistance, is an important hallmark of aggressive cancers, and driver oncogene-induced signaling pathways and molecular abnormalities create the platform for chemoresistance. The oncogene Astrocyte elevated gene-1/Metadherin (AEG-1/MTDH) is overexpressed in a diverse array of cancers, and its overexpression promotes all the hallmarks of cancer, such as proliferation, invasion, metastasis, angiogenesis and chemoresistance. The present review provides a comprehensive description of the molecular mechanism by which AEG-1 promotes tumorigenesis, with a special emphasis on its ability to regulate chemoresistance.

The Scope of Astrocyte Elevated Gene-1/Metadherin (AEG-1/MTDH) in Cancer Clinicopathology: A Review

Since its initial cloning in 2002, a plethora of studies in a vast number of cancer indications, has strongly established AEG-1 as a bona fide oncogene. In all types of cancer cells, overexpression and knockdown studies have demonstrated that AEG-1 performs a seminal role in regulating proliferation, invasion, angiogenesis, metastasis and chemoresistance, the defining cancer hallmarks, by a variety of mechanisms, including protein-protein interactions activating diverse oncogenic pathways, RNA-binding promoting translation and regulation of inflammation, lipid metabolism and tumor microenvironment. These findings have been strongly buttressed by demonstration of increased tumorigenesis in tissue-specific AEG-1 transgenic mouse models, and profound resistance of multiple types of cancer development and progression in total and conditional AEG-1 knockout mouse models. Additionally, clinicopathologic correlations of AEG-1 expression in a diverse array of cancers establishing AEG-1 as an independent biomarker for highly aggressive, chemoresistance metastatic disease with poor prognosis have provided a solid foundation to the mechanistic and mouse model studies. In this review a comprehensive analysis of the current and up-to-date literature is provided to delineate the clinical significance of AEG-1 in cancer highlighting the commonality of the findings and the discrepancies and discussing the implications of these observations.

Lipopolysaccharide-induced expression of astrocyte elevated gene-1 promotes degeneration and inflammation of chondrocytes via activation of nuclear factor-κB signaling

Osteoarthritis is an inflammatory disease characterized by joint degeneration and inflammation. Astrocyte elevated gene-1 (AEG-1) has been suggested as a novel inflammation-related factor in the pathological processes of various inflammatory diseases. To date, little is known about the role of AEG-1 in osteoarthritis. The aim of the present study was to explore the potential role of AEG-1 in the regulation of lipopolysaccharide-induced apoptosis and inflammation of chondrocytes. The results showed that AEG-1 expression was significantly upregulated in chondrocytes following exposure to lipopolysaccharide. Knockdown of AEG-1 increased the survival and decreased the expression of matrix metalloproteinases in chondrocytes treated with lipopolysaccharide. Moreover, silencing of AEG-1 restricted the lipopolysaccharide-induced production of proinflammatory cytokines. In contrast, AEG-1 overexpression caused opposite effects. Notably, we found that AEG-1 inhibition blocked the lipopolysaccharide-induced activation of nuclear factor-κB signaling through impeding the nuclear translocation of nuclear factor-κB p65 subunit. Additionally, inhibition of nuclear factor-κB partially reversed the AEG-1-mediated promotion of lipopolysaccharide-induced inflammatory injury in chondrocytes. In conclusion, our results demonstrate that inhibition of AEG-1 expression attenuates lipopolysaccharide-induced degeneration and inflammation of chondrocytes through suppressing the activation of nuclear factor-κB signaling. This work therefore highlights a potential role of AEG-1 in the pathogenesis of osteoarthritis, and indicates its potential as a therapeutic target.

Important candidate genes for abdominal fat content identified by linkage disequilibrium and fixation index information

Selection for rapid growth in chickens has always been accompanied by increased fat deposition and excessive fat deposition, especially abdominal fat, cannot only decrease feed efficiency but also cause many diseases. Finding the candidate genes associated with abdominal fat deposition is essential for breeding. To identify these candidate genes, we applied linkage disequilibrium and selection signature analysis using chicken 60 k single nucleotide polymorphism (SNP) chips in two broiler lines divergently selected for abdominal fat content for 11 generations. After quality control, 46,033 SNPs were left for analysis. Using these SNPs, we found that r2 was 0.06 to 0.14 in the lean line and 0.07 to 0.13 in the fat line for all 28 chromosomes (except GGA16). Pairwise SNP distances <25 kb showed a mean r2 = 0.33 in the lean line and r2 = 0.32 in the fat line. The fixation index (FST) analysis was carried out and 46 SNPs with the top 0.1% of the FST value was detected as the loci with selection signatures. Besides FST, hapFLK was also used to detect selection signatures for abdominal fat content. A total of 11 genes, including transient receptor potential cation channel subfamily C member 4, estrogen related receptor gamma, fibroblast growth factor 13, G-protein-signaling modulator 2, RAR related orphan receptor A, phospholipase A2 group X, mitochondrial ribosomal protein L28, metadherin, calcitonin receptor like receptor, serine/threonine kinase 39, and nuclear factor I A, were detected as the important candidate genes for abdominal fat deposition based on their basic functions. The results of the present study may benefit the understanding of genetic mechanism of abdominal fat deposition in chicken.

Oncoprotein AEG-1 is an endoplasmic reticulum RNA-binding protein whose interactome is enriched in organelle resident protein-encoding mRNAs

Astrocyte elevated gene-1 (AEG-1), an oncogene whose overexpression promotes tumor cell proliferation, angiogenesis, invasion, and enhanced chemoresistance, is thought to function primarily as a scaffolding protein, regulating PI3K/Akt and Wnt/β-catenin signaling pathways. Here we report that AEG-1 is an endoplasmic reticulum (ER) resident integral membrane RNA-binding protein (RBP). Examination of the AEG-1 RNA interactome by HITS-CLIP and PAR-CLIP methodologies revealed a high enrichment for endomembrane organelle-encoding transcripts, most prominently those encoding ER resident proteins, and within this cohort, for integral membrane protein-encoding RNAs. Cluster mapping of the AEG-1/RNA interaction sites demonstrated a normalized rank order interaction of coding sequence >5' untranslated region, with 3' untranslated region interactions only weakly represented. Intriguingly, AEG-1/membrane protein mRNA interaction sites clustered downstream from encoded transmembrane domains, suggestive of a role in membrane protein biogenesis. Secretory and cytosolic protein-encoding mRNAs were also represented in the AEG-1 RNA interactome, with the latter category notably enriched in genes functioning in mRNA localization, translational regulation, and RNA quality control. Bioinformatic analyses of RNA-binding motifs and predicted secondary structure characteristics indicate that AEG-1 lacks established RNA-binding sites though shares the property of high intrinsic disorder commonly seen in RBPs. These data implicate AEG-1 in the localization and regulation of secretory and membrane protein-encoding mRNAs and provide a framework for understanding AEG-1 function in health and disease.

A micro-RNA expression signature for human NAFLD progression

BACKGROUND

The spectrum of nonalcoholic fatty liver disease (NAFLD) describes disease conditions deteriorating from nonalcoholic fatty liver (NAFL) to nonalcoholic steatohepatitis (NASH) to cirrhosis (CIR) to hepatocellular carcinoma (HCC). From a molecular and biochemical perspective, our understanding of the etiology of this disease is limited by the broad spectrum of disease presentations, the lack of a thorough understanding of the factors contributing to disease susceptibility, and ethical concerns related to repeat sampling of the liver. To better understand the factors associated with disease progression, we investigated by next-generation RNA sequencing the altered expression of microRNAs (miRNAs) in liver biopsies of class III obese subjects (body mass index ≥40 kg/m(2)) biopsied at the time of elective bariatric surgery.

METHODS

Clinical characteristics and unbiased RNA expression profiles for 233 miRs, 313 transfer RNAs (tRNAs), and 392 miscellaneous small RNAs (snoRNAs, snRNAs, rRNAs) were compared among 36 liver biopsy specimens stratified by disease severity.

RESULTS

The abundances of 3 miRNAs that were found to be differentially regulated (miR-301a-3p and miR-34a-5p increased and miR-375 decreased) with disease progression were validated by RT-PCR. No tRNAs or miscellaneous RNAs were found to be associated with disease severity. Similar patterns of increased miR-301a and decreased miR-375 expression were observed in 134 hepatocellular carcinoma (HCC) samples deposited in The Cancer Genome Atlas (TCGA).

CONCLUSIONS

Our analytical results suggest that NAFLD severity is associated with a specific pattern of altered hepatic microRNA expression that may drive the hallmark of this disorder: altered lipid and carbohydrate metabolism. The three identified miRNAs can potentially be used as biomarkers to access the severity of NAFLD. The persistence of this miRNA expression pattern in an external validation cohort of HCC samples suggests that specific microRNA expression patterns may permit and/or sustain NAFLD development to HCC.

AEG-1/MTDH/LYRIC: A Promiscuous Protein Partner Critical in Cancer, Obesity, and CNS Diseases

Since its original discovery in 2002, AEG-1/MTDH/LYRIC has emerged as a primary regulator of several diseases including cancer, inflammatory diseases, and neurodegenerative diseases. AEG-1/MTDH/LYRIC has emerged as a key contributory molecule in almost every aspect of cancer progression, including uncontrolled cell growth, evasion of apoptosis, increased cell migration and invasion, angiogenesis, chemoresistance, and metastasis. Additionally, recent studies highlight a seminal role of AEG-1/MTDH/LYRIC in neurodegenerative diseases and obesity. By interacting with multiple protein partners, AEG-1/MTDH/LYRIC plays multifaceted roles in the pathogenesis of a wide variety of diseases. This review discusses the current state of understanding of AEG-1/MTDH/LYRIC regulation and function in cancer and other diseases with a focus on its association/interaction with several pivotal protein partners.

Molecular analysis of acute and chronic reactive astrocytes in the pilocarpine model of temporal lobe epilepsy

Astroglia, the most abundant glial cells in the mammalian central nervous system (CNS), are considered an emerging key player in seizure induction and progression. Although astrocytes undergo reactive gliosis in temporal lobe epilepsy (TLE) with dramatic morphological and molecular changes, specific astrocyte targets/molecular pathways that contribute to the induction and progression of seizure remain largely unknown. By combining translating ribosomal affinity purification (TRAP) with the pilocarpine model of TLE in BAC aldh1l1 TRAP mice, we profiled translating mRNAs from hippocampal or cortical astrocytes at different phases (3days, 30days, and 60days post-pilocarpine injections) of pilocarpine-induced epilepsy models. Our results found that hippocampal (but not cortical) astrocytes undergo early and unique molecular changes at 3days post-pilocarpine injections. These changes indicate a potentially primary pathogenic role of hippocampal astrocytes in seizure induction and progression and provide new insights about the involvement of specific astrocytic pathways/targets in epilepsy. In particular, we validated expression changes of ocrl and aeg1 in pilocarpine models. Follow-up studies on these genes may reveal new roles of hippocampal astrocytes in TLE.

The role of astrocytes in the hypothalamic response and adaptation to metabolic signals

The hypothalamus is crucial in the regulation of homeostatic functions in mammals, with the disruption of hypothalamic circuits contributing to chronic conditions such as obesity, diabetes mellitus, hypertension, and infertility. Metabolic signals and hormonal inputs drive functional and morphological changes in the hypothalamus in attempt to maintain metabolic homeostasis. However, the dramatic increase in the incidence of obesity and its secondary complications, such as type 2 diabetes, have evidenced the need to better understand how this system functions and how it can go awry. Growing evidence points to a critical role of astrocytes in orchestrating the hypothalamic response to metabolic cues by participating in processes of synaptic transmission, synaptic plasticity and nutrient sensing. These glial cells express receptors for important metabolic signals, such as the anorexigenic hormone leptin, and determine the type and quantity of nutrients reaching their neighboring neurons. Understanding the mechanisms by which astrocytes participate in hypothalamic adaptations to changes in dietary and metabolic signals is fundamental for understanding the neuroendocrine control of metabolism and key in the search for adequate treatments of metabolic diseases.