52 research outputs found
Influence of cell cycle on responses of MCF-7 cells to benzo[a]pyrene
<p>Abstract</p> <p>Background</p> <p>Benzo[a]pyrene (BaP) is a widespread environmental genotoxic carcinogen that damages DNA by forming adducts. This damage along with activation of the aryl hydrocarbon receptor (AHR) induces complex transcriptional responses in cells. To investigate whether human cells are more susceptible to BaP in a particular phase of the cell cycle, synchronised breast carcinoma MCF-7 cells were exposed to BaP. Cell cycle progression was analysed by flow cytometry, DNA adduct formation was assessed by <sup>32</sup>P-postlabeling analysis, microarrays of 44K human genome-wide oligos and RT-PCR were used to detect gene expression (mRNA) changes and Western blotting was performed to determine the expression of some proteins, including cytochrome P450 (CYP) 1A1 and CYP1B1, which are involved in BaP metabolism.</p> <p>Results</p> <p>Following BaP exposure, cells evaded <it>G1 </it>arrest and accumulated in <it>S</it>-phase. Higher levels of DNA damage occurred in <it>S</it>- and <it>G2/M</it>- compared with <it>G0/G1-</it>enriched cultures. Genes that were found to have altered expression included those involved in xenobiotic metabolism, apoptosis, cell cycle regulation and DNA repair. Gene ontology and pathway analysis showed the involvement of various signalling pathways in response to BaP exposure, such as the Catenin/Wnt pathway in <it>G1</it>, the ERK pathway in <it>G1 </it>and <it>S</it>, the Nrf2 pathway in <it>S </it>and <it>G2/M </it>and the Akt pathway in <it>G2/M</it>. An important finding was that higher levels of DNA damage in <it>S- </it>and <it>G2/M</it>-enriched cultures correlated with higher levels of <it>CYP1A1 </it>and <it>CYP1B1 </it>mRNA and proteins. Moreover, exposure of synchronised MCF-7 cells to BaP-7,8-diol-9,10-epoxide (BPDE), the ultimate carcinogenic metabolite of BaP, did not result in significant changes in DNA adduct levels at different phases of the cell cycle.</p> <p>Conclusions</p> <p>This study characterised the complex gene response to BaP in MCF-7 cells and revealed a strong correlation between the varying efficiency of BaP metabolism and DNA damage in different phases of the cell cycle. Our results suggest that growth kinetics within a target-cell population may be important determinants of susceptibility and response to a genotoxic agent.</p
Identification through microarray gene expression analysis of cellular responses to benzo(a)pyrene and its diol-epoxide that are dependent or independent of p53
Human colon carcinoma cells (HCT116) differing in p53 status were exposed to benzo(a)pyrene (BaP) or anti-benzo(a)pyrene-trans-7,8-dihydrodiol-9,10-epoxide (BPDE) and their gene expression responses compared by complementary DNA microarray technology. Exposure of cells to BPDE for up to 24 h resulted in gene expression profiles more distinguishable by duration of exposure than by p53 status, although a subset of genes were identified that had significantly different expression in p53 wild-type (WT) cells relative to p53-null cells. Apoptotic signalling genes were up-regulated in p53-WT cells but not in p53-null cells and, consistent with this, reduced viability and caspase activity were also p53 dependent. BPDE modulated cell cycle and histone genes in both cell lines and, in agreement with this, both cell lines accumulated in S phase. In p53-WT cells, G(2) arrest was also evident, which was associated with accumulation of CDKN1A. Regardless of p53 status, exposure to BaP for up to 48 h had subtle effects on gene transcription and had no influence on cell viability or cell cycle. Interestingly, DNA adduct formation after BaP, but not BPDE, exposure was p53 dependent with 10-fold lower levels detected in p53-null cells. Other cell lines were investigated for BaP-DNA adduct formation and in these the effect of p53 knockdown was also to reduce adduct formation. Taken together, these results give further insight into the role of p53 in the response of human cells to BaP and BPDE and suggest that loss of this tumour suppressor can influence the metabolic activation of BaP
Modification of Decay Constants of Superstring Axions: Effects of Flux Compactification and Axion Mixing
We study possibilities for lowering the decay constants of superstring
axions. In the heterotic Calabi-Yau compactification, a localized
model-dependent axion can appear at a nearly collapsing 2-cycle. The effect of
flux can be used for generating warp factor suppression of the axion decay
constant. We also point out that the hidden sector instanton potential much
higher than the QCD instanton potential picks up the larger effective axion
decay constant as that of the QCD axion. We show that this can be converted by
introducing many hidden-sector quarks so that the decay constant of the QCD
axion turns out to be much smaller than the string scale.Comment: 6 pages with 3 figures, revtex; figure added,section of axion mixing
modifie
High Energy QCD: Stringy Picture from Hidden Integrability
We discuss the stringy properties of high-energy QCD using its hidden
integrability in the Regge limit and on the light-cone. It is shown that
multi-colour QCD in the Regge limit belongs to the same universality class as
superconformal =2 SUSY YM with at the strong coupling
orbifold point. The analogy with integrable structure governing the low energy
sector of =2 SUSY gauge theories is used to develop the brane picture
for the Regge limit. In this picture the scattering process is described by a
single M2 brane wrapped around the spectral curve of the integrable spin chain
and unifying hadrons and reggeized gluons involved in the process. New
quasiclassical quantization conditions for the complex higher integrals of
motion are suggested which are consistent with the duality of the
multi-reggeon spectrum. The derivation of the anomalous dimensions of the
lowest twist operators is formulated in terms of the Riemann surfacesComment: 37 pages, 3 figure
Is Quantum Spacetime Foam Unstable?
A very simple wormhole geometry is considered as a model of a mode of
topological fluctutation in Planck-scale spacetime foam. Quantum dynamics of
the hole reduces to quantum mechanics of one variable, throat radius, and
admits a WKB analysis. The hole is quantum-mechanically unstable: It has no
bound states. Wormhole wave functions must eventually leak to large radii. This
suggests that stability considerations along these lines may place strong
constraints on the nature and even the existence of spacetime foam.Comment: 15 page
Quantum Dynamics of Lorentzian Spacetime Foam
A simple spacetime wormhole, which evolves classically from zero throat
radius to a maximum value and recontracts, can be regarded as one possible mode
of fluctuation in the microscopic ``spacetime foam'' first suggested by
Wheeler. The dynamics of a particularly simple version of such a wormhole can
be reduced to that of a single quantity, its throat radius; this wormhole thus
provides a ``minisuperspace model'' for a structure in Lorentzian-signature
foam. The classical equation of motion for the wormhole throat is obtained from
the Einstein field equations and a suitable equation of state for the matter at
the throat. Analysis of the quantum behavior of the hole then proceeds from an
action corresponding to that equation of motion. The action obtained simply by
calculating the scalar curvature of the hole spacetime yields a model with
features like those of the relativistic free particle. In particular the
Hamiltonian is nonlocal, and for the wormhole cannot even be given as a
differential operator in closed form. Nonetheless the general solution of the
Schr\"odinger equation for wormhole wave functions, i.e., the wave-function
propagator, can be expressed as a path integral. Too complicated to perform
exactly, this can yet be evaluated via a WKB approximation. The result
indicates that the wormhole, classically stable, is quantum-mechanically
unstable: A Feynman-Kac decomposition of the WKB propagator yields no spectrum
of bound states. Though an initially localized wormhole wave function may
oscillate for many classical expansion/recontraction periods, it must
eventually leak to large radius values. The possibility of such a mode unstable
against growth, combined withComment: 37 pages, 93-
Hagedorn Inflation: Open Strings on Branes Can Drive Inflation
We demonstrate an inflationary solution to the cosmological horizon problem
during the Hagedorn regime in the early universe. Here the observable universe
is confined to three spatial dimensions (a three-brane) embedded in higher
dimensions. The only ingredients required are open strings on D-branes at
temperatures close to the string scale. No potential is required. Winding modes
of the strings provide a negative pressure that can drive inflation of our
observable universe. Hence the mere existence of open strings on branes in the
early hot phase of the universe drives Hagedorn inflation, which can be either
power law or exponential. We note the amusing fact that, in the case of
stationary extra dimensions, inflationary expansion takes place only for branes
of three or less dimensions.Comment: Talk given by Katherine Frees
de Sitter invariance of the dS graviton vacuum
The two-point function of linearized gravitons on de Sitter space is infrared
divergent in the standard transverse traceless synchronous gauge defined by
cosmological coordinates (also called conformal or Poincare coordinates).
We show that this divergence can be removed by adding a linearized
diffeomorphism to each mode function; i.e., by an explicit change of gauge. It
follows that the graviton vacuum state is well-defined and de Sitter invariant
in agreement with various earlier arguments.Comment: 14 pages, 1 figur
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