Biomedical subjects

M A Nikiforov

Publications and source records attributed to M A Nikiforov.

12 recordsLinked to original sources

PubMed2006

Transcriptional activation by the Myc oncoprotein.

The Myc transcription factor functions as a downstream effector of most mitogenic signals. Myc is synthesized rapidly in response to extracellular mitogenic signals, and blocking Myc induction abolishes or at least severely attenuates any mitogenic response. Furthermore, ectopic Myc expression can often bypass the requirement for extracellular signals for entry into S phase. Thus, the Myc transcription factor is both necessary and in many ways sufficient to promote the growth of diverse cell types. Given this potent biological activity, it is not surprising that mutations in the myc gene are among the most frequent in human and animal cancers. Understanding the molecular basis of Myc function has been a central issue in the fields of cancer biology and signal transduction for 20 years.

Adaptor Proteins, Signal Transducing↗

PubMed2000

Complementation of Myc-dependent cell proliferation by cDNA expression library screening.

The targeted knockout of the c-myc gene from rat fibroblasts leads to a stable defect in cell proliferation. We used complex cDNA libraries expressed from retroviral vectors and an efficient sorting procedure to rapidly select for cDNAs that can restore the growth rate of c-myc deficient cells. All of the biologically active cDNAs contained either c-myc or N-myc, suggesting that no other cellular genes can effectively bypass the requirement for c-myc in fibroblast proliferation. This approach provides a powerful screening method for cell cycle changes in genetically defined systems.

Animals↗

PubMed2000

A novel chromodomain protein, pdd3p, associates with internal eliminated sequences during macronuclear development in Tetrahymena thermophila.

Conversion of the germ line micronuclear genome into the genome of a somatic macronucleus in Tetrahymena thermophila requires several DNA rearrangement processes. These include (i) excision and subsequent elimination of several thousand internal eliminated sequences (IESs) scattered throughout the micronuclear genome and (ii) breakage of the micronuclear chromosomes into hundreds of DNA fragments, followed by de novo telomere addition to their ends. Chromosome breakage sequences (Cbs) that determine the sites of breakage and short regions of DNA adjacent to them are also eliminated. Both processes occur concomitantly in the developing macronucleus. Two stage-specific protein factors involved in germ line DNA elimination have been described previously. Pdd1p and Pdd2p (for programmed DNA degradation) physically associate with internal eliminated sequences in transient electron-dense structures in the developing macronucleus. Here, we report the purification, sequence analysis, and characterization of Pdd3p, a novel developmentally regulated, chromodomain-containing polypeptide. Pdd3p colocalizes with Pdd1p in the peripheral regions of DNA elimination structures, but is also found more internally. DNA cross-linked and immunoprecipitated with Pdd1p- or Pdd3p-specific antibodies is enriched in IESs, but not Cbs, suggesting that different protein factors are involved in elimination of these two groups of sequences.

Amino Acid Sequence↗

PubMed1999

Excision of micronuclear-specific DNA requires parental expression of pdd2p and occurs independently from DNA replication in Tetrahymena thermophila.

Elimination of germ-line DNA segments is an essential step in the somatic development of many organisms and in the terminal differentiation of several specialized cell types. In binuclear ciliates, including Tetrahymena thermophila, DNA elimination occurs during the conversion of the germ-line micronuclear genome into the somatic genome of the new macronucleus. Little is known about molecular determinants and regulatory mechanisms involved in this process. Pdd2p is one of a small set of Tetrahymena polypeptides whose time of synthesis, nuclear localization, and physical association with sequences destined for elimination suggest an involvement in the DNA elimination process. In this study, we report that loss of parental expression of Pdd2p leads to the perturbation of several DNA rearrangement processes in developing zygotic macronuclei, including excision of internal eliminated sequences, excision of chromosome breakage sequences, and endoreplication of the new macronuclear genome and eventually results in lethality of the progeny. We demonstrate that excision and elimination of micronuclear-specific DNA occurs independently of endoreplication of the new macronuclear genome that takes place during the same period of time. Thus, our data indicate that parental expression of Pdd2p is required for successful DNA elimination and development of somatic nuclei.

Animals↗

PubMed1999

Parental expression of the chromodomain protein Pdd1p is required for completion of programmed DNA elimination and nuclear differentiation.

Thousands of DNA elimination events occur during somatic differentiation of many ciliated protozoa. In Tetrahymena, the eliminated DNA aggregates into submacronuclear structures containing the protein Pdd1p, a member of the chromodomain family. We disrupted somatic copies of PDD1, eliminating parental expression of the gene early in the sexual phase of the life cycle. Even though zygotic expression, from the undisrupted germline PDD1 copy, is activated before DNA elimination normally occurs, the somatic knockout cells suffer defects in DNA elimination, genome endoduplication, and nuclear resorption, and eventually die, demonstrating that PDD1 is essential and suggesting Pdd1p is directly involved in establishing a chromatin structure required for DNA elimination.

Animals↗

PubMed1999

A nuclear protein involved in apoptotic-like DNA degradation in Stylonychia: implications for similar mechanisms in differentiating and starved cells.

Ciliates are unicellular eukaryotic organisms containing two types of nuclei: macronuclei and micronuclei. After the sexual pathway takes place, a new macronucleus is formed from a zygote nucleus, whereas the old macronucleus is degraded and resorbed. In the course of macronuclear differentiation, polytene chromosomes are synthesized that become degraded again after some hours. Most of the DNA is eliminated, and the remaining DNA is fragmented into small DNA molecules that are amplified to a high copy number in the new macronucleus. The protein Pdd1p (programmed DNA degradation protein 1) from Tetrahymena has been shown to be present in macronuclear anlagen in the DNA degradation stage and also in the old macronuclei, which are resorbed during the formation of the new macronucleus. In this study the identification and localization of a Pdd1p homologous protein in Stylonychia (Spdd1p) is described. Spdd1p is localized in the precursor nuclei in the DNA elimination stage and in the old macronuclei during their degradation, but also in macronuclei and micronuclei of starved cells. In all of these nuclei, apoptotic-like DNA breakdown was detected. These data suggest that Spdd1p is a general factor involved in programmed DNA degradation in Stylonychia.

Animals↗

PubMed1998

Cell transforming genes and tumor progression: in vivo unified secondary phenotypic cell changes.

We have earlier shown that Syrian hamster cells spontaneously transformed in vitro during in vivo progression, acquire in 1 step, along with highly increased tumorigenicity, 2 new properties characterizing the [H2O2CA + tPGE(S)] phenotype, i.e., a high H2O2 catabolizing (antioxidant) activity and the ability to release PGE2 upon contact with NK cells. In contrast, RSV-SR-(v-src)-transformed cells acquire the [H2O2CA + PGE(S)] phenotype and high tumorigenicity during in vitro transformation, i.e., without preliminary in vivo selection. In the present study, the possible influence of different transforming genes on the rates of subsequent in vivo tumor progression was studied using cells in vitro transformed by SV40, BAV-3, or transduced by activated genes Ha-ras, p53, myc and bcl-2. The expression of the [H2O2CA + PGE(S)] phenotype, the extent of tumorigenic and spontaneous metastasizing activities were examined before and during in vivo cells selection in s.c. growing tumors. Our results demonstrate that: (1) after in vitro transformation all cell lines (except v-src) were negative for the expression of [H2O2CA + PGE(S)] phenotype and remained equally low-tumorigenic; (2) independently of the types of genes initially transforming the cells, in vivo tumor progression was consistently leading to the replacement of parental cells by cells expressing the [H2O2CA + PGE(S)] phenotype, to 30-200 times increased tumorigenicity and less frequently to metastasizing; (3) the time necessary for selection of cells expressing this phenotype was the same (about 180 days in vivo) for all transformants, except bcl-2; the latter reaching similar values after a significant delay. Thus, common secondary src-like phenotypic cell changes, regardless of initially cell transforming genes are necessarily selected during tumor progression in vivo.

Animals↗

PubMed1997

Suppression of apoptosis by bcl-2 does not prevent p53-mediated control of experimental metastasis and anchorage dependence.

Mutations in the p53 tumor suppressor gene are frequently associated with the metastatic stage of tumor progression. Inactivation of p53 was shown to promote metastasis under experimental conditions. To determine the p53 functions that are involved in the control of tumor metastasis, we compared properties of three types of transformed mouse fibroblasts: with intact p53, with p53-mediated apoptosis suppressed by bcl-2 and with p53 inactivated by dominant negative mutants. Although expression of bcl-2 blocked apoptosis in detached cells and increased tumor cell survival in the blood circulation, it was insufficient to affect the ability of p53 to cause cell cycle arrest in detached cells and suppress experimental metastasis. For the suppression of metastasis complete inactivation of p53 was required. We conclude that the apoptotic function of p53 is dispensable for the p53-dependent suppression of experimental metastasis that is presumably achieved by controlling anchorage dependence. These data provide a possible explanation to dramatic differences in values of bcl-2 and mutant p53 as prognostic markers in human cancer.

Animals↗

PubMed1996

p53 modulation of anchorage independent growth and experimental metastasis.

Death in circulation is one of the natural barriers preventing dissemination of tumor cells and formation of metastases. One of the negative factors acting in circulation is the loss of cell contact with natural substrate which can be imitated in vitro by the incubation of cells in suspension or in semi-solid media. Normal mouse fibroblasts (MEFs) stay viable in suspension and undergo p53-independent G1 growth arrest. Transformation with Ela and ras oncogenes leads to the abrogation of this arrest and to the p53-dependent apoptosis occurring in G1 phase of the cell cycle. Suppression of apoptosis by p53 gene knock-out, transduction of dominant negative p53 mutant or bcl-2 prevents death in suspension and greatly induces frequency of colony formation in semi-solid media. The ability of cells to undergo apoptosis does not correlate with their tumorigenicity in nude mice but does correlate with their ability to survive in lungs of intravenously injected mice and to form experimental metastases. We suggest that abrogation of a p53-mediated apoptosis facilitates experimental metastasis by promoting survival of tumor cells in circulation.

Animals↗

PubMed1996

Mechanisms of unusually high antioxidant activity of RSV-SR-transformed cells and of its suppression by activated p21ras.

We have previously demonstrated that hamster embryo fibroblasts (HEFs) transformed by Rous Sarcoma virus, Schmidt-Ruppin strain (RSV-SR) are highly resistant to damage by H202 (H2O2R), (in contrast to HEFs transformed spontaneously, or by bovine adenovirus and SV40), while N-ras transfection of RSV-SR transformants leads to suppression of pp6Ov-scr and of H2O2R. In this study we have examined (1) mechanisms of antioxidant activity (AOA) of HEFs transformed by these agents and (2) the possible role of the v-src gene in unusually high AOA of RSV-SR transformants and of activated ras oncogenes in its suppression. All transformants exhibit increased catalase and glutathione peroxidase (GP) activities, while SOD, glutathione and glutathione reductase (GR) were reduced. As compared with other transformants, the significantly higher catalase and the low SOD activities were characteristic of RSV-SR-transformants, while an increase in GP was observed in all types of transformants. Correspondingly, RSV-SR-transformants showed an extremely high H202-catabolizing activity (H2O2CA) and no lipid peroxidation chain reaction (LPCR). N-ras-induced suppression of pp60v-scr of RSV-SR-transformed HEFs coincided with the suppression of catalase, GP, H202 and H202CA. However, suppression of catalase and GP was also observed in N-ras- and Ha-ras-transfected, spontaneously transformed HEFs. Thus, extremely high catalase activity and suppression of LPCR are apparently the main mechanisms of the unusually high H202R of RSV-SR transformants, while its suppression by activated ras oncogenes may also take place in some transformants, free of v-src activity.

Animals↗

PubMed1994

ART-CH: a VL30 in chickens?

The complete sequence of ART-CH, a recently found chicken retrotransposon (A. V. Gudkov, E. A. Komarova, M. A. Nikiforov, and T. E. Zaitsevskaya, J. Virol. 66:1726-1736, 1992), was characterized. ART-CH has the structure of a 3,300-bp-long provirus, including two 388-bp long terminal repeats (LTRs) (U3, 245 bp; R region, 17 bp; and U5, 126 bp), a tRNA(Trp)-binding site, and a polypurine tract, similar to avian leukosis viruses. At least some of the approximately 50 genomic copies of ART-CH are transcribed into polyadenylated RNA, which is initiated and terminated at the expected sites within the LTRs. In contrast to the regulatory sequences involved in proviral expression and replication, the internal regions of ART-CH seem to be completely defective. Several short regions of homology with avian leukosis virus genes, most of which encode gag-related sequences, were found among different reading frames of ART-CH, which are not organized like regular retroviral genes. Both sequence analysis and restriction fragment length polymorphism analysis revealed a high degree of sequence (97% homology) and structural similarity among members of the ART-CH family, indicating their common origin and recent penetration into chicken DNA. ART-CH sequences were detected in mouse cells infected with Rous sarcoma virus produced by an ART-CH-expressing Rous sarcoma. These data are consistent with the hypothesis that ART-CH belongs to a class of defective retrotransposons whose replication strategy requires the use of helper viruses. They might originate from an avian leukosis virus-related retrovirus which completely lost its coding capacities as a result of multiple mutations and deletions. These features apparently group ART-CH with the VL30 retrotransposons of rodents.

Amino Acid Sequence↗

PubMed1992

ART-CH, a new chicken retroviruslike element.

A 3' region of a previously unknown retroviruslike element named ART-CH (avian retrotransposon from chicken genome) was obtained in the course of polymerase chain reaction-mediated cloning of avian leukosis virus long terminal repeats (LTRs) from DNAs of infected chicken cells. About 50 copies of ART-CH are present in the genome of chickens of different breeds. ART-CH is not found in DNA of quails, ducks, turkeys, or several other birds tested. The ART-CH element is about 3 kb in size, including 388 bp LTRs. The major class of ART-CH-specific RNA, also 3 kb in size, is detected in various organs of chickens. An ART-CH polypurine tract, a tRNA(Trp)-binding site, regions around the TATA box and polyadenylation signal, and the beginning of the putative gag gene strongly resemble the corresponding regions of avian leukosis viruses and EAV, the two described classes of chicken retroviruses. An open reading frame capable of encoding a polypeptide with a putative transmembrane domain is located upstream of the right ART-CH LTR. This sequence, as well as the U3 and U5 regions of the ART-CH LTR, has no obvious similarities with the corresponding parts of other known vertebrate retroviruses and retrotransposons. A short sequence upstream of the right LTR of ART-CH is very similar to sequences which flank the 3' ends of the oncogenes v-src, v-myc, v-fps, and v-crk in four different recombinant avian retroviruses and which are absent from the genomes of other studied avian retroviruses. Thus, ART-CH is a new endogenous chicken provirus that may participate in the formation of recombinant oncogenic retroviruses.

Amino Acid Sequence↗