Search PubMed⌕ Search

Biomedical subjects

S K Dhar

Publications and source records attributed to S K Dhar.

At least 19 recordsLinked to original sources
PubMed2006

RelB regulates manganese superoxide dismutase gene and resistance to ionizing radiation of prostate cancer cells.

The relationship between NF-kappaB and resistance to radiation treatment in many tumor cell types has been generally well recognized. However, which members of the NF-kappaB family contribute to radiation resistance is unclear. In the present study, we demonstrate that RelB plays an important radioprotective role in aggressive prostate cancer cells, in part by the induction of antioxidant and antiapoptotic manganese superoxide dismutase (MnSOD) gene. RelB is both constitutively present and is inducible by radiation in aggressive prostate cancer cells. Using ectopically expressed dominant negative inhibitor, p100 mutant, and the siRNA approach, we demonstrate that selective inhibition of RelB significantly decreases the levels of MnSOD resulting in a significant increase in the sensitivity of prostate cancer cells to radiation treatment. These results demonstrate that RelB plays an important role in redox regulation of the cell and protects aggressive prostate cancer cells against radiation-induced cell death. Thus, inhibition of RelB could be a novel mechanism to radiosensitize prostate cancer.

Enzyme Induction↗
PubMed2001

Replication from oriP of Epstein-Barr virus requires human ORC and is inhibited by geminin.

A hypomorphic mutation made in the ORC2 gene of a human cancer cell line through homologous recombination decreased Orc2 protein levels by 90%. The G1 phase of the cell cycle was prolonged, but there was no effect on the utilization of either the c-Myc or beta-globin cellular origins of replication. Cells carrying this mutation failed to support the replication of a plasmid bearing the oriP replicator of Epstein Barr virus (EBV), and this defect was rescued by reintroduction of Orc2. Orc2 specifically associates with oriP in cells, most likely through its interaction with EBNA1. Geminin, an inhibitor of the mammalian replication initiation complex, inhibits replication from oriP. Therefore, ORC and the human replication initiation apparatus is required for replication from a viral origin of replication.

Alleles↗
PubMed2001

Architecture of the human origin recognition complex.

All the human homologs of the six subunits of Saccharomyces cerevisiae origin recognition complex have been reported so far. However, not much has been reported on the nature and the characteristics of the human origin recognition complex. In an attempt to purify recombinant human ORC from insect cells infected with baculoviruses expressing HsORC subunits, we found that human ORC2, -3, -4, and -5 form a core complex. HsORC1 and HsORC6 subunits did not enter into this core complex, suggesting that the interaction of these two subunits with the core ORC2-5 complex is extremely labile. We found that the C-terminal region of ORC2 interacts directly with the N-terminal region of ORC3. The C-terminal region of ORC3 was, however, necessary to bring ORC4 and ORC5 into the core complex. A fragment containing the N-terminal 200 residues of ORC3 (ORC3N) competitively inhibited the ORC2-ORC3 interaction. Overexpression of this fragment in U2OS cells blocked the cells in G(1), providing the first evidence that a mammalian ORC subunit is important for the G(1)-S transition in mammalian cells.

Animals↗
PubMed2001

Rvb1p and Rvb2p are essential components of a chromatin remodeling complex that regulates transcription of over 5% of yeast genes.

Eukaryotic Rvb1p and Rvb2p are two highly conserved proteins related to the helicase subset of the AAA+ family of ATPases. Conditional mutants in both genes show rapid changes in the transcription of over 5% of yeast genes, with a similar number of genes being repressed and activated. Both Rvb1p and Rvb2p are required for maintaining the induced state of many inducible promoters. ATP binding and hydrolysis by Rvb1p and Rvb2p is individually essential in vivo, and the two proteins are associated with each other in a high molecular weight complex that shows ATP-dependent chromatin remodeling activity in vitro. Our findings show that Rvb1p and Rvb2p are essential components of a chromatin remodeling complex and determine genes regulated by the complex.

Adenosine Triphosphatases↗
PubMed2001

Longitudinal assessment of neuropsychological functioning, psychiatric status, functional disability and employment status in chronic fatigue syndrome.

The longitudinal course of subjective and objective neuropsychological functioning, psychological functioning, disability level, and employment status in chronic fatigue syndrome (CFS) was examined. The relations among several key outcomes at follow-up, as well as the baseline characteristics that predict change (e.g., improvement), were also evaluated. The study sample consisted of 35 individuals who met the 1988 and 1994 CFS case definition criteria of the Centers for Disease Control (CDC) at intake. Participants were evaluated a mean of 41.9 (SEM = 1.7) months following their initial visit (range = 24-63 months). Results indicated that objective and subjective attention abilities, mood, level of fatigue, and disability improve over time in individuals with CFS. Moreover, improvements in these areas were found to be interrelated at follow-up. Finally, psychiatric status, age, and between-test duration were significant predictors of outcome. Overall, the prognosis for CFS appears to be poor, as the majority of participants remained functionally impaired over time and were unemployed at follow-up, despite the noted improvements.

Adult↗
PubMed2000

Inhibition of eukaryotic DNA replication by geminin binding to Cdt1.

In all eukaryotic organisms, inappropriate firing of replication origins during the G2 phase of the cell cycle is suppressed by cyclin-dependent kinases. Multicellular eukaryotes contain a second putative inhibitor of re-replication called geminin. Geminin is believed to block binding of the mini-chromosome maintenance (MCM) complex to origins of replication, but the mechanism of this inhibition is unclear. Here we show that geminin interacts tightly with Cdt1, a recently identified replication initiation factor necessary for MCM loading. The inhibition of DNA replication by geminin that is observed in cell-free DNA replication extracts is reversed by the addition of excess Cdt1. In the normal cell cycle, Cdt1 is present only in G1 and S, whereas geminin is present in S and G2 phases of the cell cycle. Together, these results suggest that geminin inhibits inappropriate origin firing by targeting Cdt1.

Amino Acid Sequence↗
PubMed2000

Subsets of human origin recognition complex (ORC) subunits are expressed in non-proliferating cells and associate with non-ORC proteins.

The origin recognition complex (ORC) in yeast is a complex of six tightly associated subunits essential for the initiation of DNA replication. Human ORC subunits are nuclear in proliferating cells and in proliferative tissues like the testis, consistent with a role of human ORC in DNA replication. Orc2, Orc3, and Orc5 also are detected in non-proliferating cells like cardiac myocytes, adrenal cortical cells, and neurons, suggesting an additional role of these proteins in non-proliferating cells. Although Orc2-5 co-immunoprecipitate with each other under mild extraction conditions, a holo complex of the subunits is difficult to detect. When extracted under more stringent extraction conditions, several of the subunits co-immunoprecipitate with stoichiometric amounts of other unidentified proteins but not with any of the known ORC subunits. The variation in abundance of individual ORC subunits (relative to each other) in several tissues, expression of some subunits in non-proliferating tissues, and the absence of a stoichiometric complex of all the subunits in cell extracts indicate that subunits of human ORC in somatic cells might have activities independent of their role as a six subunit complex involved in replication initiation. Finally, all ORC subunits remain consistently nuclear, and Orc2 is consistently phosphorylated through all stages of the cell cycle, whereas Orc1 is selectively phosphorylated in mitosis.

Blotting, Northern↗
PubMed2000

Identification and characterization of the human ORC6 homolog.

A new protein was cloned and identified as the sixth member of the human origin recognition complex (ORC). The newly identified 30-kDa protein hsORC6 is 28% identical and 49% similar to ORC6p from Drosophila melanogaster, which is consistent with the identities and similarities found among the other ORC members reported in the two species. The human ORC6 gene is located on chromosome 16q12. ORC6 protein level did not change through the cell cycle. Like ORC1, ORC6 did not co-immunoprecipitate with other ORC subunits but was localized in the nucleus along with the other ORC subunits. Several cellular proteins co-immunoprecipitated with ORC6, including a 65-kDa protein that was hyperphosphorylated in G(1) and dephosphorylated in mitosis. Therefore, unlike the tight stoichiometric association of six yeast ORC subunits in one holo-complex, only a small fraction of human ORC1 and ORC6 is likely to be associated with a subcomplex of ORC2, 3, 4, and 5, suggesting differences in the architecture and regulation of human ORC.

Amino Acid Sequence↗
PubMed2000

Distinct variation pattern in the env of macrophage-tropic simian immunodeficiency virus in vivo demonstrated by denaturing gradient gel electrophoresis.

Genetic variations, occurring during the infection in the AIDS animal model using a molecular clone virus, often consist of single nucleotide substitutions distributed sparsely in the viral genome. A highly sensitive method was developed to detect such mutations, consisting of denaturing gradient gel electrophoresis (DGGE) and direct sequencing. The genetic variation in the env of a macrophage tropic simian immunodeficiency virus (SIV) in a macaque infected chronically was examined by this technique and compared with that of a T cell tropic SIV in an animal infected chronically. A whole env sequence was amplified by 11 sets of PCR for DNA ranging from 300 to 420 bp. The amplified DNA was subjected to mutational screening by DGGE and to subsequent direct sequencing. Imaging analysis of separated DNA bands by DGGE provided information on the proportion of each variant DNA. The validity of this technique was confirmed by sequencing of variant DNAs cloned by limiting dilutions. Thus, this technique is suitable for analysis of genetic variations in the AIDS animal model using a molecular clone virus.

Animals↗
PubMed1996

Replication initiates at multiple dispersed sites in the ribosomal DNA plasmid of the protozoan parasite Entamoeba histolytica.

In the protozoan parasite Entamoeba histolytica (which causes amoebiasis in humans), the rRNA genes (rDNA) in the nucleus are carried on an extrachromosomal circular plasmid. For strain HM-1:IMSS, the size of the rDNA plasmid is 24.5 kb, and 200 copies per genome are present. Each circle contains two rRNA transcription units as inverted repeats separated by upstream and downstream spacers. We have studied the replication of this molecule by neutral/neutral two-dimensional gel electrophoresis and by electron microscopy. All restriction fragments analyzed by two-dimensional gel electrophoresis gave signals corresponding to simple Y's and bubbles. This showed that replication initiated in this plasmid at multiple, dispersed locations spread throughout the plasmid. On the basis of the intensity of the bubble arcs, initiations from the rRNA transcription units seemed to occur more frequently than those from intergenic spacers. Multiple, dispersed initiation sites were also seen in the rDNA plasmid of strain HK-9 when it was analyzed by two-dimensional gel electrophoresis. Electron microscopic visualization of replicating plasmid molecules in strain HM-1:IMISS showed multiple replication bubbles in the same molecule. The location of bubbles on the rDNA circle was mapped by digesting with PvuI or BsaHI, which linearize the molecule, and with SacII, which cuts the circle twice. The distance of the bubbles from one end of the molecule was measured by electron microscopy. The data corroborated those from two-dimensional gels and showed that replication bubbles were distributed throughout the molecule and that they appeared more frequently in rRNA transcription units. The same interpretation was drawn from electron microscopic analysis of the HK-9 plasmid. Direct demonstration of more than one bubble in the same molecule is clear evidence that replication of this plasmid initiates at multiple sites. Potential replication origins are distributed throughout the plasmid. Such a mechanism is not known to operate in any naturally occurring prokaryotic or eukaryotic plasmid.

Amebiasis↗
PubMed1995

Anti-fertility activity and hormonal profile of trans-anethole in rats.

Trans-anethole was studied for antifertility activity in rats at dose levels of 50 mg, 70 mg and 80 mg/kg po. Dose-dependent activity was observed, a 100% anti-implantation activity being achieved at 80 mg/kg, po. The compound showed a significant estrogenic activity and did not possess anti-estrogenic, progestational, anti-progestational, androgenic or anti-androgenic activities. In an earlier study, the compound was found to be safe, its LD50 being more than 3000 mg/kg, po in mice.

Abortion, Induced↗
PubMed1995

Anti-implantation activity of 2 derivatives of o-hydroxy naphthaquinones in rats.

AIM: To search for female contraceptive agents. METHODS: 2-Hydroxy-3-methyl-1, 4-naphthoquinone monosemicarbazone (HMNQS) and 2-hydroxy-1, 4-naphthoquinone monothiosemicarbazone (HNQTS) were screened for anti-implantation activity in rats. RESULTS: Both compounds showed a dose-dependent activity, and HNQTS was more potent. An 100% anti-implantation activity was observed with HNQTS 150 mg kg-1 ig. Its LD50 was found to be > 2 g kg-1 ig in mice. CONCLUSION: HNQTS was more potent than HMNQS for anti-implantation activity in rats.

Animals↗
PubMed1994

Nucleotide sequence organisation and analysis of the nuclear ribosomal DNA circle of the protozoan parasite Entamoeba histolytica.

We have sequenced the extrachromsomal ribosomal DNA (rDNA) circle of the human protozoan parasite Entamoeba histolytica HM-1:IMSS and present here the complete sequence organisation of the 24.5-kb molecule. Each circle contains two 5.9-kb rDNA transcription units organised as inverted repeats. The regions downstream (3543 bp) and upstream (9216 bp) of the rDNAs contain various families of short tandem repeats. Some of the upstream repeats share extensive sequence homology with the downstream repeats. In addition to the rDNAs themselves, the rDNA circle appears to code for only one other transcript which is 0.7 kb in size as seen in Northern blots. From DNA sequence analysis, no open reading frame could be assigned to the transcript. Extrachromosomal rDNA circles also exist in other E. histolytica strains. Restriction enzyme maps of rDNA circles were constructed from E. histolytica strains 200:NIH, HK-9 and Rahman; and Entamoeba moshkovskii strain Laredo. Striking differences were observed in the organisation of some of them, e.g. the HK-9, Rahman and Laredo circles contained only one rDNA unit and lacked the 0.7-kb transcript sequence. The short repeat sequences upstream and downstream of rDNAs were present in HK-9 and Rahman but absent in Laredo. Circles with one rDNA unit may be derived from those with two units by homologous recombination at direct repeat sequences located upstream and downstream of the two rDNAs.

Animals↗
PubMed1993

Nature of heme metabolizing enzymes in a mutant rat with hyperbilirubinuria.

Eisai hyperbilirubinuria rat (EHBR) is a mutant strain which is originated from Sprague Dawley rats. The activities of heme metabolizing enzymes in EHBR were compared with those of Sprague Dawley rats as the control. The activity of delta-aminolevulinic acid (delta-ALA) synthetase, the initial and rate-limiting enzyme in heme biosynthesis, was significantly higher in the liver of EHBR. However, no significant difference in the activity of heme oxygenase, the first and rate-limiting enzyme in heme degradation, was observed in EHBR as compared to control rats. DDC, a porphyrinogenic agent, caused marked and prolonged increase in the activity of delta-ALA synthetase in EHBR. However, the contents of microsomal total heme and cytochrome P-450 were significantly decreased in EHBR 24 hrs after DDC injection. The activity of heme oxygenase by CdCl2, a inducer of heme oxygenase, was considerably higher in EHBR than in control rats. These results indicate some specific hypersensitivity for heme metabolizing enzymes in EHBR.

Animals↗
PubMed1990

Toxicity studies with potassium embelate, a new analgesic compound.

Potassium embelate, 2,5-dihydroxy, 3-undecyl-1, 4-benzoquinone, from Embelia ribes Burm. was subjected to toxicity evaluation which included subacute, chronic, reproductive toxicity testing and teratological investigations in laboratory animals (mice, rats and monkeys). The results did not indicate adverse effects suggesting that potassium embelate is a safe compound.

Abnormalities, Drug-Induced↗