Carcinogenic roles of MAFG-AS1 in human cancers

The MAF bZIP transcription factor G-antisense RNA 1 (MAFG-AS1) is located on chromosome 17. MAFG-AS1 was upregulated in 15 human cancers. MAFG-AS1 not only suppresses 16 miRNAs but also directly impacts 22 protein-coding genes' expression. Notably, abnormal MAFG-AS1 expression is connected to clinicopathological characteristics and a worse prognosis in a variety of cancers. Moreover, MAFG-AS1 takes its part in the tumorigenesis and progression of various human malignancies by suppressing apoptosis and promoting proliferation, migration, invasion, aerobic glycolysis, ferroptosis, angiogenesis, EMT, and metastasis. Besides, it can predict treatment effectiveness in ER + breast cancer, urothelial bladder carcinoma, and liver cancer by functioning as a trigger of resistance to tamoxifen, sorafenib, and cisplatin. This study systematically presents the functions of MAFG-AS1 in various cancers, as well as the findings of bioinformatics analyses of the MAFG-AS1, which should give clear advice for future research.

LncRNA MAFG-AS1 is involved in human cancer progression

Long noncoding RNAs (lncRNAs) refer to a type of non-protein-coding transcript of more than 200 nucleotides. LncRNAs play fundamental roles in disease development and progression, and lncRNAs are dysregulated in many pathophysiological processes. Thus, lncRNAs may have potential value in clinical applications. The lncRNA, MAF BZIP Transcription Factor G (MAFG)-AS1, is dysregulated in several cancer, including breast cancer, lung cancer, liver cancer, bladder cancer, colorectal cancer, gastric cancer, esophagus cancer, prostate cancer, pancreatic cancer, ovarian cancer, and glioma. Altered MAFG-AS1 levels are also associated with diverse clinical characteristics and patient outcomes. Mechanistically, MAFG-AS1 mediates a variety of cellular processes via the regulation of target gene expression. Therefore, the diagnostic, prognostic, and therapeutic aspects of MAFG-AS1 have been widely explored. In this review, we discuss the expression, major roles, and molecular mechanisms of MAFG-AS1, the relationship between MAFG-AS1 and clinical features of diseases, and the clinical applications of MAFG-AS1.

Stapled Peptides as Direct Inhibitors of Nrf2-sMAF Transcription Factors

Nuclear factor erythroid-related 2-factor 2 (Nrf2) is a transcription factor traditionally thought of as a cellular protector. However, in many cancers, Nrf2 is constitutively activated and correlated with therapeutic resistance. Nrf2 heterodimerizes with small musculoaponeurotic fibrosarcoma Maf (sMAF) transcription factors, allowing binding to the antioxidant responsive element (ARE) and induction of transcription of Nrf2 target genes. While transcription factors are historically challenging to target, stapled peptides have shown great promise for inhibiting these protein-protein interactions. Herein, we describe the first direct cell-permeable inhibitor of Nrf2/sMAF heterodimerization. N1S is a stapled peptide designed based on AlphaFold predictions of the interactions between Nrf2 and sMAF MafG. A cell-based reporter assay combined with in vitro biophysical assays demonstrates that N1S directly inhibits Nrf2/MafG heterodimerization. N1S treatment decreases the transcription of Nrf2-dependent genes and sensitizes Nrf2-dependent cancer cells to cisplatin. Overall, N1S is a promising lead for the sensitization of Nrf2-addicted cancers.

LncRNA MAFG-AS1-induced acute myeloid leukemia development via modulating miR-147b/HOXA9

Recent references discovered that lncRNAs acted roles in malignant cancer development. However, the role of MAFG-AS1 in acute myeloid leukemia (AML) development remains unknown. MAFG-AS1 and miR-147b were determined in AML cells and specimens using qRT-PCR assay. Cell proliferation was detected by CCK-8 analysis and flow cytometry was carried out to measure cell cycle. Luciferase reporter analysis was done to determine the mechanism of MAFG-AS1 and miR-147b. We noted that MAFG-AS1 was overexpressed in AML cells and in serum and bone narrow from AML compared with normal controls specimen. Elevated expression of MAFG-AS1 increased cell growth, cycle and EMT in AML cell HL-60 cell. MAFG-AS1 sponged miR-147b expression in HL-60 cell. Moreover, miR-147b was downregulated in AML cells and in serum and bone narrow from AML compared with normal control specimen. miR-147b was negatively correlated with MAFG-AS1 in the serum and bone narrow of AML cases. We illustrated that HOXA9 was one target of miR-147b and ectopic expression of MAFG-AS1 enhanced HOXA9 expression HL-60 cell. Forced expression of MAFG-AS1 induced cell growth, cycle, and EMT via promoting HOXA9. These data illustrated that MAFG-AS1 acted as one oncogenic gene and accelerated AML progression via modulating miR-147b/HOXA9 axis.

LncRNA MAFG-AS1 Upregulates Polo-Like Kinase-1 by Sponging miR-505 to Promote Gastric Adenocarcinoma Cell Proliferation

Gastric cancer is a commonly diagnosed, often fatal malignancy and requires novel anticancer therapies and preventative approaches. This study described the involvement of MAFG-AS1, a lncRNA with important functions in cancer biology, in gastric adenocarcinoma (GA). Thirty-six male and forty-two female GA patients with an average age of 51.9 ± 5.7 years in the range of 35 to 68 years were enrolled. Paired gastric cancer (GC) and non-tumor tissues were collected from each patient. MAFG-AS1 expression was determined. RNA interaction prediction, dual luciferase reporter assay, RT-qPCR assay, Western blot, and CCK-8 assay were conducted. The results indicated that MAFG-AS1 was highly expressed in GA and closely correlated with poor survival. MAFG-AS1 interacted with miR-505, but MAFG-AS1 and miR-505 overexpression showed no significant effects on each other's expression. In addition, MAFG-AS1 increased the expression of PLK1, a miR-505 target. MAFG-AS1 and PLK1 overexpression increased GC cell proliferation rate. MiR-505 overexpression reduced the effects of MAFG-AS1 and PLK1 overexpression on cell proliferation. Therefore, MAFG-AS1 might upregulate PLK1 by sponging miR-505 to promote GA cell proliferation.

Opposite effects of the FXR agonist obeticholic acid on Mafg and Nrf2 mediate the development of acute liver injury in rodent models of cholestasis

The farnesoid-X-receptor (FXR) is validated target in the cholestatic disorders treatment. Obeticholic acid (OCA), the first in class of FXR agonist approved for clinical use, causes side effects including acute liver decompensation when administered to cirrhotic patients with primary biliary cholangitis at higher than recommended doses. The V-Maf avian-musculoaponeurotic-fibrosarcoma-oncogene-homolog-G (Mafg) and nuclear factor-erythroid-2-related-factor-2 (Nrf2) mediates some of the downstream effects of FXR. In the present study we have investigated the role of FXR/MafG/NRF2 pathway in the development of liver toxicity caused by OCA in rodent models of cholestasis. Cholestasis was induced by bile duct ligation (BDL) or administration of α-naphtyl-isothiocyanate (ANIT) to male Wistar rats and FXR^-/- and FXR^+/+ mice. Treating BDL and ANIT rats with OCA exacerbated the severity of cholestasis, hepatocytes injury and severely downregulated the expression of basolateral transporters. In mice, genetic ablation FXR or its pharmacological inhibition by 3-(naphthalen-2-yl)-5-(piperidin-4-yl)-1,2,4-oxadiazole rescued from negative regulation of MRP4 and protected against liver injury caused by ANIT. By RNAseq analysis we found that FXR antagonism effectively reversed the transcription of over 2100 genes modulated by OCA/ANIT treatment, including Mafg and Nrf2 and their target genes Cyp7a1, Cyp8b1, Mat1a, Mat2a, Gss. Genetic and pharmacological Mafg inhibition by liver delivery of siRNA antisense or S-adenosylmethionine effectively rescued from damage caused by ANIT/OCA. In contrast, Nrf2 induction by sulforaphane was protective. CONCLUSIONS: Liver injury caused by FXR agonism in cholestasis is FXR-dependent and is reversed by FXR and Mafg antagonism or Nrf2 induction.

MAFG-driven astrocytes promote CNS inflammation

Multiple sclerosis is a chronic inflammatory disease of the CNS1. Astrocytes contribute to the pathogenesis of multiple sclerosis2, but little is known about the heterogeneity of astrocytes and its regulation. Here we report the analysis of astrocytes in multiple sclerosis and its preclinical model experimental autoimmune encephalomyelitis (EAE) by single-cell RNA sequencing in combination with cell-specific Ribotag RNA profiling, assay for transposase-accessible chromatin with sequencing (ATAC-seq), chromatin immunoprecipitation with sequencing (ChIP-seq), genome-wide analysis of DNA methylation and in vivo CRISPR-Cas9-based genetic perturbations. We identified astrocytes in EAE and multiple sclerosis that were characterized by decreased expression of NRF2 and increased expression of MAFG, which cooperates with MAT2α to promote DNA methylation and represses antioxidant and anti-inflammatory transcriptional programs. Granulocyte-macrophage colony-stimulating factor (GM-CSF) signalling in astrocytes drives the expression of MAFG and MAT2α and pro-inflammatory transcriptional modules, contributing to CNS pathology in EAE and, potentially, multiple sclerosis. Our results identify candidate therapeutic targets in multiple sclerosis.

Free heme regulates placenta growth factor through NRF2-antioxidant response signaling

Free heme activates erythroblasts to express and secrete Placenta Growth Factor (PlGF), an angiogenic peptide of the VEGF family. High circulating levels of PlGF have been associated in experimental animals and in patients with sickle cell disease with echocardiographic markers of pulmonary hypertension, a life-limiting complication associated with more intense hemolysis. We now show that the mechanism of heme regulation of PlGF requires the contribution of the key antioxidant response regulator NRF2. Mimicking the effect of heme, the NRF2 agonist sulforaphane stimulates the PlGF transcript level nearly 30-fold in cultured human erythroblastoid cells. Heme and sulforaphane also induce transcripts for NRF2 itself, its partners MAFF and MAFG, and its competitor BACH1. Furthermore, heme induction of the PlGF transcript is significantly diminished by the NRF2 inhibitor brusatol and by siRNA knockdown of the NRF2 and/or MAFG transcription factors. Chromatin immunoprecipitation experiments show that heme induces NRF2 to bind directly to the PlGF promoter region. In complementary in vivo experiments, mice injected with heme show a significant increase in their plasma PlGF protein as early as 3 h after treatment. Our results reveal an important mechanism of PlGF regulation, adding to the growing literature that supports the pivotal importance of the NRF2 axis in the pathobiology of sickle cell disease.

MAFG is a potential therapeutic target to restore chemosensitivity in cisplatin-resistant cancer cells by increasing reactive oxygen species

Adjuvant chemotherapy for solid tumors based on platinum-derived compounds such as cisplatin is the treatment of choice in most cases. Cisplatin triggers signaling pathways that lead to cell death, but it also induces changes in tumor cells that modify the therapeutic response, thereby leading to cisplatin resistance. We have recently reported that microRNA-7 is silenced by DNA methylation and is involved in the resistance to platinum in cancer cells through the action of the musculoaponeurotic fibrosarcoma oncogene family, protein G (MAFG). In the present study, we first confirm the miR-7 epigenetic regulation of MAFG in 44 normal- and/or tumor-paired samples in non-small-cell lung cancer (NSCLC). We also provide translational evidence of the role of MAFG and the clinical outcome in NSCLC by the interrogation of two extensive in silico databases of 2019 patients. Moreover, we propose that MAFG-mediated resistance could be conferred due to lower reactive oxygen species production after cisplatin exposure. We developed specifically selected aptamers against MAFG, with high sensitivity to detect the protein at a nuclear level probed by aptacytochemistry and histochemistry analyses. The inhibition of MAFG activity through the action of the specific aptamer apMAFG6F increased the levels of reactive oxygen species production and the sensitivity to cisplatin. We report first the specific nuclear identification of MAFG as a novel detection method for diagnosis in NSCLC, and then we report that MAFG modulates the redox response and confers cell protection against free radicals generated after platinum administration, thus also being a promising therapeutic target.

Poly(ADP-ribose) polymerase-1 modulates Nrf2-dependent transcription

The basic leucine zipper transcription factor Nrf2 has emerged as a master regulator of intracellular redox homeostasis by controlling the expression of a battery of redox-balancing antioxidants and phase II detoxification enzymes. Under oxidative stress conditions, Nrf2 is induced at the protein level through redox-sensitive modifications on critical cysteine residues in Keap1, a component of an E3 ubiquitin ligase complex that targets Nrf2 for proteasomal degradation. Poly(ADP-ribose) polymerase-1 (PARP-1) is historically known to function in DNA damage detection and repair; however, recently PARP-1 has been shown to play an important role in other biochemical activities, such as DNA methylation and imprinting, insulator activity, chromosome organization, and transcriptional regulation. The exact role of PARP-1 in transcription modulation and the underlying mechanisms remain poorly defined. In this study, we report that PARP-1 forms complexes with the antioxidant response element (ARE) within the promoter region of Nrf2 target genes and upregulates the transcriptional activity of Nrf2. Interestingly, PARP-1 neither physically interacts with Nrf2 nor promotes the expression of Nrf2. In addition, PARP-1 does not target Nrf2 for poly(ADP-ribosyl)ation. Instead, PARP-1 interacts directly with small Maf proteins and the ARE of Nrf2 target genes, which augments ARE-specific DNA-binding of Nrf2 and enhances the transcription of Nrf2 target genes. Collectively, these results suggest that PARP-1 serves as a transcriptional coactivator, upregulating the transcriptional activity of Nrf2 by enhancing the interaction among Nrf2, MafG, and the ARE.

Characterization of Nrf1b, a novel isoform of the nuclear factor-erythroid-2 related transcription factor-1 that activates antioxidant response element-regulated genes

Nuclear factor E2-related factor 1 (Nrf1) is a basic leucine zipper transcription factor that plays an important role in the activation of cytoprotective genes through the antioxidant response elements. The previously characterized long isoform of Nrf1 (Nrf1a) is targeted to the endoplasmic reticulum and accumulates in the nucleus in response to activating signals. Here we characterized a novel Nrf1 protein isoform (Nrf1b) generated through an alternative promoter and first exon that lacks the ER targeting domain of Nrf1a. The 5′-flanking region of Nrf1b directed high levels of luciferase reporter expression in cells. RT-PCR and Western blotting showed Nrf1b is widely expressed in various cell lines and mouse tissues. Immunoblot analysis of subcellular fractions and imaging of green fluorescence protein (GFP)-tagged Nrf1b demonstrate Nrf1b is constitutively localized to the nucleus. Nrf1b can activate GAL4-dependent transcription when fused to the heterologous GAL4 DNA-binding domain. Gel-shift and coimmunoprecipitation experiments demonstrate that Nrf1b forms a complex with MafG, and expression of Nrf1b activates the expression of antioxidant response element containing reporters and genes in cells. These results suggest Nrf1b is targeted to the nucleus where it activates ARE-driven genes and may play a role in modulating antioxidant response elements.

Nrf2-MafG heterodimers contribute globally to antioxidant and metabolic networks

NF-E2-related factor 2 (Nrf2) is a key transcription factor that is critical for cellular defense against oxidative and xenobiotic insults. Nrf2 heterodimerizes with small Maf (sMaf) proteins and binds to antioxidant response elements (AREs) to activate a battery of cytoprotective genes. However, it remains unclear to what extent the Nrf2–sMaf heterodimers contribute to ARE-dependent gene regulation on a genome-wide scale. We performed chromatin immunoprecipitation coupled with high-throughput sequencing and identified the binding sites of Nrf2 and MafG throughout the genome. Compared to sites occupied by Nrf2 alone, many sites co-occupied by Nrf2 and MafG exhibit high enrichment and are located in species-conserved genomic regions. The ARE motifs were significantly enriched among the recovered Nrf2–MafG-binding sites but not among the Nrf2-binding sites that did not display MafG binding. The majority of the Nrf2-regulated cytoprotective genes were found in the vicinity of Nrf2–MafG-binding sites. Additionally, sequences that regulate glucose metabolism and several amino acid transporters were identified as Nrf2–MafG target genes, suggesting diverse roles for the Nrf2–MafG heterodimer in stress response. These data clearly support the notion that Nrf2–sMaf heterodimers are complexes that regulate batteries of genes involved in various aspects of cytoprotective and metabolic functions through associated AREs.

Predictive base substitution rules that determine the binding and transcriptional specificity of Maf recognition elements

Small Maf transcription factors possess a basic region-leucine zipper motif through which they form homodimers or heterodimers with CNC and Bach proteins. Different combinations of small Maf and CNC/Bach protein dimers bind to cis-acting DNA elements, collectively referred to as Maf-recognition elements (MAREs), to either activate or repress transcription. As MAREs defined by function are often divergent from the consensus sequence, we speculated that sequence variations in the MAREs form the basis for selective Maf:Maf or Maf:CNC dimer binding. To test this hypothesis, we analyzed the binding of Maf-containing dimers to variant sequences of the MARE using bacterially expressed MafG and Nrf2 proteins and a surface plasmon resonance-microarray imaging technique. We found that base substitutions in the MAREs actually determined their binding preference for different dimers. In fact, we were able to categorize MAREs into five groups: MafG homodimer-orientd MAREs (Groups I and II), ambivalent MAREs (Group III), MafG:Nrf2 heterodimer-orientd MAREs (Group IV), and silent MAREs (Group V). This study thus manifests that a clear set of rules pertaining to the cis-acting element determine whether a given MARE preferentially associates with MafG homodimer or with MafG:Nrf2 heterodimer.

MafG sumoylation is required for active transcriptional repression

A straightforward mechanism for eliciting transcriptional repression would be to simply block the DNA binding site for activators. Such passive repression is often mediated by transcription factors that lack an intrinsic repressor activity. MafG is a bidirectional regulator of transcription, a repressor in its homodimeric state but an activator when heterodimerized with p45. Here, we report that MafG is conjugated to SUMO-2/3 in vivo. To clarify the possible physiological role(s) for sumoylation in regulating MafG activity, we evaluated mutant and wild-type MafG in transgenic mice and cultured cells. Whereas sumoylation-deficient MafG activated p45-dependent transcription normally and did not affect heterodimer activity, repression by the sumoylation-deficient MafG mutant was severely compromised in vivo. Furthermore, the SUMO-dependent repression activity of MafG was sensitive to histone deacetylase inhibition. Thus, repression by MafG is not achieved through simple passive repression by competing for the activator binding site but requires sumoylation, which then mediates transcriptional repression through recruitment of a repressor complex containing histone deacetylase activity.

Neuronal expression of nuclear transcription factor MafG in the rat medulla oblongata after baroreceptor stimulation

The medulla oblongata is the site of central baroreceptive neurons in mammals. These neurons express specific basic-leucine zipper transcription factors (bZIP) after baroreceptor stimulation. Previously we showed that activation of baroreceptors induced expression of nuclear transcription factors c-Fos and FosB in central baroreceptive neurons. Here we studied the effects of baroreceptor stimulation on induction of MafG, a member of small Maf protein family that functions as dimeric partners for various bZIP transcription factors by forming transcription-regulating complexes, in the rat medulla oblongata. To determine whether gene expression of MafG is induced by stimulation of arterial baroreceptors, we examined the expression of its mRNA by semi-quantitative reverse transcription-PCR method and its gene product by immunohistochemistry. We found that the number of MafG transcripts increased significantly in the medulla oblongata after baroreceptor stimulation. MafG-immunoreactive neurons were distributed in the nucleus tractus solitarii, the dorsal motor nucleus of the vagus nerve, the ambiguous nucleus and the ventrolateral medulla. The numbers of MafG-immunoreactive neurons in these nuclei were significantly greater in test rats than in saline-injected control rats. We also found approximately 20% of MafG-immunoreactive neurons coexpress FosB after baroreceptor stimulation. Our results suggest that MafG cooperates with FosB to play critical roles as an immediate early gene in the signal transduction of cardiovascular regulation mediated by baroreceptive signals in the medulla oblongata.