Biomedical subjects

Nikita Popov

Publications and source records attributed to Nikita Popov.

6 recordsLinked to original sources

PubMed2005

Mnt transcriptional repressor is functionally regulated during cell cycle progression.

The Myc/Max/Mad network of transcription factors regulates cell proliferation, differentiation, and transformation. Similar to other proteins of the network, Mnt forms heterodimers with Max and binds CACGTG E-Box elements. Transcriptional repression by Mnt is mediated through association with mSin3, and deletion of the mSin3-interacting domain (SID) converts Mnt to a transcriptional activator. Mnt is coexpressed with Myc in proliferating cells and has been suggested to be a modulator of Myc function. We report that Mnt is expressed both in growth-arrested and proliferating mouse fibroblasts and is phosphorylated when resting cells are induced to re-enter the cell cycle. Importantly, the interaction between Mnt and mSin3 is disrupted upon serum stimulation resulting in decreased Mnt-associated HDAC activity. Furthermore, we demonstrate that Mnt binds and recruits mSin3 to the Myc target gene cyclin D2 in quiescent mouse fibroblasts. Interference with Mnt expression by RNAi resulted in upregulation of cyclin D2 expression in growth-arrested fibroblasts, supporting the view that Mnt represses cyclin D2 transcription in quiescent cells. Our data suggest a model in which phosphorylation of Mnt at cell cycle entry results in disruption of Mnt-mSin3-HDAC1 interaction, which allows induction of Myc target genes by release of Mnt-mediated transcriptional repression.

Animals↗

PubMed2005

The ubiquitin ligase HectH9 regulates transcriptional activation by Myc and is essential for tumor cell proliferation.

The Myc oncoprotein forms a binary activating complex with its partner protein, Max, and a ternary repressive complex that, in addition to Max, contains the zinc finger protein Miz1. Here we show that the E3 ubiquitin ligase HectH9 ubiquitinates Myc in vivo and in vitro, forming a lysine 63-linked polyubiquitin chain. Miz1 inhibits this ubiquitination. HectH9-mediated ubiquitination of Myc is required for transactivation of multiple target genes, recruitment of the coactivator p300, and induction of cell proliferation by Myc. HectH9 is overexpressed in multiple human tumors and is essential for proliferation of a subset of tumor cells. Our results suggest that site-specific ubiquitination regulates the switch between an activating and a repressive state of the Myc protein, and they suggest a strategy to interfere with Myc function in vivo.

Amino Acid Sequence↗

PubMed2004

Expression and DNA-binding activity of MYCN/Max and Mnt/Max during induced differentiation of human neuroblastoma cells.

Amplification of MYCN is one of the most important prognostic markers for neuroblastoma and is correlated with rapid tumor progression and poor prognosis. MYCN belongs to the Myc/Max/Mad/Mnt network of proteins that regulate proliferation, apoptosis, and differentiation. It is well established that MYCN is downregulated during induced differentiation of neuroblastoma cells carrying an amplified MYCN gene, but very little is known about other components of the network, i.e., the Max, Mad, and Mnt proteins, during this process. In this study we show that Mad and Mnt expression was only modestly regulated in differentiating SK-N-BE(2) neuroblastoma cells, while MYCN was rapidly downregulated. This downregulation was reflected in a decreased MYCN/Max DNA-binding activity while the Mnt/Max binding did not change during differentiation. In parallel experiments we also analyzed the Myc/Max/Mad expression and DNA binding capacity during induced differentiation in the MYCN single copy neuroblastoma cell line SH-SY5Y. In this cell line only modest changes in expression of the components of the MYCN/Max/Mad/Mnt network was detected, but since the cell line expresses relatively low levels of MYCN and c-Myc, these changes might be of functional significance. Cell cycle analyses of SK-N-BE(2) demonstrated an increase in the G1-phase fraction after RA-treatment. These data show that the decreased MYCN expression and MYCN DNA-binding is correlated with retarded cell cycle progression. Furthermore, when Mad1 or Mnt was overexpressed in SK-N-BE(2) cells they retained the capacity to differentiate, underscoring the notion that MYCN downregulation, and not changes in Mad/Mnt expression, is essential for neuroblastoma cell differentiation.

Basic Helix-Loop-Helix Leucine Zipper Transcriptio↗

PubMed2003

The Mad and Myc basic domains are functionally equivalent.

The Myc/Max/Mad family of transcription factors plays a fundamental role in the regulation of cell proliferation, oncogenic transformation, and cell differentiation. However, it remains unclear whether different heterodimers, such as Myc/Max and Mad/Max, recognize the same or different target genes in vivo. We show by chromatin immunoprecipitation that Myc target genes are also recognized by Mad1 in differentiated HL60 cells. We also substituted the complete basic region of Myc for the corresponding region of Mad. Wild-type c-Myc was then compared with c-Myc(Mad-BR) in oncogenic transformation, regulation of cell proliferation, induction of apoptosis, activation of chromosomal gene expression, and direct binding to chromosomal sites by chromatin immunoprecipitation. We find that the wild-type c-Myc and c-Myc/MadBR proteins have indistinguishable biological activity and target gene recognition in vivo. These data are consistent with a model in which Myc and Mad regulate a common set of target genes.

Amino Acid Sequence↗

PubMed2002

The histone deacetylase inhibitor trichostatin A derepresses the telomerase reverse transcriptase (hTERT) gene in human cells.

Activation of telomerase, essential for cellular immortalization and transformation, requires the induction of its catalytic component, telomerase reverse transcriptase (hTERT). However, biochemical and genetic mechanisms for the control of hTERT expression remain undefined. In the present study, we demonstrate that the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) induces hyperacetylation of histones at the hTERT proximal promoter, directly transactivates the hTERT gene in normal human telomerase-negative cells, and upregulates hTERT expression in telomerase-positive tumor cells. Overexpression of HDAC1 leads to repression of the hTERT promoter activity. TSA-mediated activation of the hTERT promoter is abolished by the mutation of Sp1 sites at the proximal promoter, suggesting that the effect of TSA is regulated through Sp1 motifs. We also show a physical interaction of Sp1 with HDAC1 and the presence of HDAC1 at the hTERT promoter region. Moreover, hyperacetylation of histones at the hTERT promoter is associated with the natural up-regulation of hTERT expression that occurs in activated T lymphocytes. Taken together, histone acetylation/deacetylation may be a common underlying feature to hTERT transactivation/repression in human normal and malignant cells.

Acetylation↗

PubMed2002

Repression of in vivo growth of Myc/Ras transformed tumor cells by Mad1.

The Myc/Max/Mad network of transcriptional regulatory proteins plays an essential role in cell proliferation, growth, apoptosis, and differentiation. Whereas Myc proteins affect cell cycle progression positively, Mad proteins are negative regulators of cell proliferation. It has been shown in several in vitro systems that Mad proteins antagonize c-Myc functions. In this report we describe the inhibition of tumor cell outgrowth in vivo by Mad1 expression. Transformed cell lines were generated by co-transfection of c-myc, c-H-ras, and a chimeric mad1ER construct into primary rat embryo cells (MRMad1ER cells). Activation of Mad1 by 4-Hydroxy-Tamoxifen (OHT) resulted in abrogation of telomerase activity, reduced cloning efficiency, and decreased proportion of cells in S phase. Injection of MRMad1ER cells into syngenic rats induced aggressively growing tumors after a short latency period. This tumor growth was inhibited by OHT-treatment of animals, with the extent of inhibition correlating with the amount of OHT injected. No effect of OHT on tumor growth was observed with similarly transformed Myc/Ras cell lines which did not express Mad1ER. These data demonstrate that Mad1 is able to suppress Myc/Ras-mediated transformation under in vivo conditions.

Animals↗