Mad1 function in cell proliferation and transcriptional repression is antagonized by cyclin E/CDK2

MAD1 deficiency accelerates hepatocellular proliferation via suppressing TGF-β signaling

Numerous researches have reported on the regulatory network of liver regeneration induced by partial hepatectomy (PH). However, information on key molecules and/or signaling pathways regulating the termination stage of liver regeneration remains limited. In this study, we identify hepatic mitotic arrest deficient 1 (MAD1) as a crucial regulator of transforming growth factor β (TGF-β) in the hepatocyte to repress liver regeneration. MAD1 has a low expression level at the rapid proliferation phase but significantly increases at the termination phase of liver regeneration. We show that MAD1 deficiency accelerates hepatocyte proliferation and enhances mitochondrial biogenesis and respiratory. Mechanistically, MAD1 deficiency in hepatocytes enhances mitochondrial function and promotes hepatocyte proliferation by suppressing TGF-β signaling. Our study reveals MAD1 as a novel suppressor of hepatocyte proliferation, which may provide a new therapeutic target for the recovery of liver function after liver transplant and partial hepatectomy.

Foxo3 circular RNA retards cell cycle progression via forming ternary complexes with p21 and CDK2

Most RNAs generated by the human genome have no protein-coding ability and are termed non-coding RNAs. Among these include circular RNAs, which include exonic circular RNAs (circRNA), mainly found in the cytoplasm, and intronic RNAs (ciRNA), predominantly detected in the nucleus. The biological functions of circular RNAs remain largely unknown, although ciRNAs have been reported to promote gene transcription, while circRNAs may function as microRNA sponges. We demonstrate that the circular RNA circ-Foxo3 was highly expressed in non-cancer cells and were associated with cell cycle progression. Silencing endogenous circ-Foxo3 promoted cell proliferation. Ectopic expression of circ-Foxo3 repressed cell cycle progression by binding to the cell cycle proteins cyclin-dependent kinase 2 (also known as cell division protein kinase 2 or CDK2) and cyclin-dependent kinase inhibitor 1 (or p21), resulting in the formation of a ternary complex. Normally, CDK2 interacts with cyclin A and cyclin E to facilitate cell cycle entry, while p21works to inhibit these interactions and arrest cell cycle progression. The formation of this circ-Foxo3-p21-CDK2 ternary complex arrested the function of CDK2 and blocked cell cycle progression.

Mitotic arrest deficient-like 1 is correlated with poor prognosis in small-cell lung cancer after surgical resection

Mitotic arrest deficient-like 1 (MAD1L1) whose dysfunction is associated with chromosomal instability plays a pathogenic role in a few human cancers. However, the status of MAD1L1 expression in small-cell lung cancer (SCLC) remains unknown. Immunohistochemistry was used to determine the expression of MAD1L1 protein in 32 lymph node metastasis (LN-M) tissues and 88 primary SCLCs compared with 32 adjacent noncancerous tissues. The associations of MAD1L1 protein expression with the clinicopathologic features and clinical outcomes in patients with SCLC were analyzed. The ratio of MAD1L1 positive expression was higher in primary SCLC tissues (39.8 %) and LN-M tissues (46.9 %) compared with adjacent noncancerous tissues (9.4 %). MAD1L1 positive expression was associated with tumor-node-metastasis (TNM) stage (P = 0.003), International Association for the Study of Lung Cancer (IASLC) stage (P = 0.004), tumor size (P = 0.015), lymph node metastasis (P = 0.014), and recurrence (P < 0.001). Multivariate analysis suggested that MAD1L1 positive expression was an independent factor for overall survival (hazard ratio (HR) 2.002; 95 % confidence interval (CI) 1.065-3.763; P = 0.031) and recurrence-free survival (HR 2.263; 95 % CI 1.197-4.276; P = 0.012). To sum up, MAD1L1 positive expression may be associated with tumour progression and metastasis in SCLCs and may thus serve as a new biomarker for prognosis in these patients.

Functional interactions among members of the MAX and MLX transcriptional network during oncogenesis

The transcription factor MYC and its related family members MYCN and MYCL have been implicated in the etiology of a wide spectrum of human cancers. Compared to other oncoproteins, such as RAS or SRC, MYC is unique because its protein coding region is rarely mutated. Instead, MYC's oncogenic properties are unleashed by regulatory mutations leading to unconstrained high levels of expression. Under both normal and pathological conditions MYC regulates multiple aspects of cellular physiology including proliferation, differentiation, apoptosis, growth and metabolism by controlling the expression of thousands of genes. How a single transcription factor exerts such broad effects remains a fascinating puzzle. Notably, MYC is part of a network of bHLHLZ proteins centered on the MYC heterodimeric partner MAX and its counterpart, the MAX-like protein MLX. This network includes MXD1-4, MNT, MGA, MONDOA and MONDOB proteins. With some exceptions, MXD proteins have been functionally linked to cell cycle arrest and differentiation, while MONDO proteins control cellular metabolism. Although the temporal expression patterns of many of these proteins can differ markedly they are frequently expressed simultaneously in the same cellular context, and potentially bind to the same, or similar DNA consensus sequence. Here we review the activities and interactions among these proteins and propose that the broad spectrum of phenotypes elicited by MYC deregulation is intimately connected to the functions and regulation of the other network members. Furthermore, we provide a meta-analysis of TCGA data suggesting that the coordinate regulation of the network is important in MYC driven tumorigenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Transcriptional consequences of schizophrenia candidate miR-137 manipulation in human neural progenitor cells

MIR137, transcribed as the microRNA miR-137, is one of the leading candidate schizophrenia susceptibility genes to arise from large genome-wide association studies (GWAS) of the disorder. Recent data suggest that miR-137 modulates the expression of other schizophrenia susceptibility genes. Although bioinformatic resources are available with which to predict genes regulated by individual microRNA, there has been a lack of empirical data on genome-wide gene expression changes following miR-137 manipulation. We have therefore performed a genome-wide assessment of transcriptional changes in a human neural progenitor cell line after miR-137 over-expression and inhibition in order to elucidate molecular pathways by which genetic perturbation of miR-137 could promote susceptibility to schizophrenia. Bioinformatically-predicted miR-137 targets showed a small but highly significant down-regulation following miR-137 over-expression. Genes that were significantly down-regulated in association with miR-137 over-expression were enriched for involvement in neuronal differentiation. Differentially expressed genes that were confirmed by qPCR included others at genome-wide significant risk loci for schizophrenia (MAD1L1 and DPYD) and BDNF. These data point to molecular pathways through which genetic variation at the MIR137 locus could confer risk for schizophrenia.

Basic concepts of epigenetics: impact of environmental signals on gene expression

Through epigenetic modifications, specific long-term phenotypic consequences can arise from environmental influence on slowly evolving genomic DNA. Heritable epigenetic information regulates nucleosomal arrangement around DNA and determines patterns of gene silencing or active transcription. One of the greatest challenges in the study of epigenetics as it relates to disease is the enormous diversity of proteins, histone modifications and DNA methylation patterns associated with each unique maladaptive phenotype. This is further complicated by a limitless combination of environmental cues that could alter the epigenome of specific cell types, tissues, organs and systems. In addition, complexities arise from the interpretation of studies describing analogous but not identical processes in flies, plants, worms, yeast, ciliated protozoans, tumor cells and mammals. This review integrates fundamental basic concepts of epigenetics with specific focus on how the epigenetic machinery interacts and operates in continuity to silence or activate gene expression. Topics covered include the connection between DNA methylation, methyl-CpG-binding proteins, transcriptional repression complexes, histone residues, histone modifications that mediate gene repression or relaxation, histone core variant stability, H1 histone linker flexibility, FACT complex, nucleosomal remodeling complexes, HP1 and nuclear lamins.

Regulation of gene transcription by the oncoprotein MYC

The proteins of the MYC/MAX/MAD network are central regulators of many key processes associated with basic cell physiology. These include the regulation of protein biosynthesis, energy metabolism, proliferation, and apoptosis. Molecularly the MYC/MAX/MAD network achieves these broad activities by controlling the expression of many target genes, which are primarily responsible for the diverse physiological consequences elicited by the network. The MYC proteins of the network possess oncogenic activity and their functional deregulation is associated with the majority of human tumors. Over the last years we have witnessed the accumulation of a considerable number of molecular observations that suggest many different biochemical means and tools by which MYC controls gene expression. We will summarize the more recent findings and discuss how these different building blocks might come together to explain how MYC regulates gene transcription. We note that despite the many molecular details known, we do not have an integrated view of how MYC uses the different tools, neither in a spatial nor in a temporal order.

MAD1 and its life as a MYC antagonist: an update

The MYC/MAX/MAD network is of central importance for controlling cell physiology. The network is compiled of transcriptional regulators that form different heterodimers, which can either activate or repress the expression of target genes. Thus these proteins function as a molecular switch to control gene expression. MAD1, a member of this network, acts as a transcriptional repressor. It interacts with MAX to form the OFF position of the switch, antagonizing MYC/MAX complexes that define the ON position. MAD1 regulates cell proliferation and apoptosis through a number of target genes. In addition recent evidence indicates that the expression and activity of MAD1 are regulated at multiple levels. Here the recent developments are summarized, in comparison to MYC, of our understanding how the expression of the MAD1 gene and protein are controlled and what the functional consequences and downstream effectors of MAD1 are, which relay its activity as a transcriptional regulator.

Phosphorylation by Cdk2 is required for Myc to repress Ras-induced senescence in cotransformation

The MYC and RAS oncogenes are frequently activated in cancer and, together, are sufficient to transform rodent cells. The basis for this cooperativity remains unclear. We found that although Ras interfered with Myc-induced apoptosis, Myc repressed Ras-induced senescence, together abrogating two main barriers of tumorigenesis. Inhibition of cellular senescence required phosphorylation of Myc at Ser-62 by cyclin E/cyclin-dependent kinase (Cdk) 2. Cdk2 interacted with Myc at promoters, where it affected Myc-dependent regulation of genes, including Bmi-1, p16, p21, and hTERT, which encode proteins known to control senescence. Repression of senescence by Myc was abrogated by the Cdk inhibitor p27Kip1, which is induced by antiproliferative signals like IFN-gamma or by pharmacological inhibitors of Cdk2 but not by inhibitors of other Cdks. In contrast, a phospho-mimicking Myc-S62D mutant was resistant to these manipulations. Inhibition of cyclin E/Cdk2 reversed the senescence-associated gene expression pattern imposed by Myc/cyclin E/Cdk2. This indicates a role of Cdk2 as a transcriptional cofactor and activator of the antisenescence function of Myc and provides mechanistic insight into the Myc-p27Kip1 antagonism. Finally, our findings highlight that pharmacological inhibition of Cdk2 activity is a potential therapeutical principle for cancer therapy, in particular for tumors with activated Myc or Ras.

The regulation of SIRT2 function by cyclin-dependent kinases affects cell motility

Cyclin-dependent kinases (Cdks) fulfill key functions in many cellular processes, including cell cycle progression and cytoskeletal dynamics. A limited number of Cdk substrates have been identified with few demonstrated to be regulated by Cdk-dependent phosphorylation. We identify on protein expression arrays novel cyclin E–Cdk2 substrates, including SIRT2, a member of the Sirtuin family of NAD⁺-dependent deacetylases that targets α-tubulin. We define Ser-331 as the site phosphorylated by cyclin E–Cdk2, cyclin A–Cdk2, and p35–Cdk5 both in vitro and in cells. Importantly, phosphorylation at Ser-331 inhibits the catalytic activity of SIRT2. Gain- and loss-of-function studies demonstrate that SIRT2 interfered with cell adhesion and cell migration. In postmitotic hippocampal neurons, neurite outgrowth and growth cone collapse are inhibited by SIRT2. The effects provoked by SIRT2, but not those of a nonphosphorylatable mutant, are antagonized by Cdk-dependent phosphorylation. Collectively, our findings identify a posttranslational mechanism that controls SIRT2 function, and they provide evidence for a novel regulatory circuitry involving Cdks, SIRT2, and microtubules.

The Mad side of the Max network: antagonizing the function of Myc and more

A significant body of evidence has been accumulated that demonstrates decisive roles of members of the Myc/Max/Mad network in the control of various aspects of cell behavior, including proliferation, differentiation, and apoptosis. The components of this network serve as transcriptional regulators. Mad family members, including Mad1, Mxi1, Mad3, Mad4, Mnt, and Mga, function in part as antagonists of Myc oncoproteins. At the molecular level this antagonism is reflected by the different cofactor/chromatin remodeling complexes that are recruited by Myc and Mad family members. One important function of the latter is their ability to repress gene transcription. In this review we summarize the current view of how this repression is achieved and what the consequences of Mad action are for cell behavior. In addition, we point out some of the many aspects that have not been clarified and thus leave us with a rather incomplete picture of the functions, both molecular and at the cellular level, of Mad family members.

Gain of a region on 7p22.3, containing MAD1L1, is the most frequent event in small-cell lung cancer cell lines

Small-cell lung cancer (SCLC) is a highly aggressive lung neoplasm, which accounts for 20% of yearly lung cancer cases. The lack of knowledge of the progenitor cell type for SCLC precludes the definition of a normal gene expression profile and has hampered the identification of gene expression changes, while the low resolution of conventional genomic screens such as comparative genomic hybridization (CGH) and loss of heterozygosity analysis limit our ability to fine-map genetic alterations. The recent advent of whole genome tiling path array CGH enables profiling of segmental DNA copy number gains and losses at a resolution 100 times that of conventional methods. Here we report the analysis of 14 SCLC cell lines and six matched normal B-lymphocyte lines. We detected 7p22.3 copy number gain in 13 of the 14 SCLC lines and 0 of the 6 matched normal lines. In 4 of the 14 cell lines, this gain is present as a 350 kbp gene specific copy number gain centered at MAD1L1 (the human homologue of the yeast gene MAD1). Fluorescence in situ hybridization validated the array CGH finding. Intriguingly, MAD1L1 has been implicated to have tumor-suppressing functions. Our data suggest a more complex role for this gene, as MAD1L1 is the most frequent copy number gain in SCLC cell lines.

Gene profiling the response to kainic acid induced seizures

Kainic acid activates non-N-methyl-d-aspartate (NMDA) glutamate receptors where it increases synaptic activity resulting in seizures, neurodegeneration, and remodeling. We performed microarray analysis on rat hippocampal tissue following kainic acid treatment in order to study the signaling mechanisms underlying these diverse processes in an attempt to increase our current understanding of mechanisms contributing to such fundamental processes as neuronal protection and neuronal plasticity. The kainic acid-treated rats used in our array experiments demonstrated severe seizure behavior that was also accompanied by neuronal degeneration which is suggested by fluoro-jade B staining and anti-caspase-3 immunohistochemistry. The gene profile revealed 36 novel kainic acid regulated genes along with additional genes previously reported. The functional roles of these novel genes are discussed. These genes mainly have roles in transcription and to a lesser extent have roles in cell death, extracellular matrix remodeling, cell cycle progression, neuroprotection, angiogenesis, and synaptic signaling. Gene regulation was confirmed via quantitative real time polymerase chain reaction and in situ hybridization.