Abstract
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The cellular outcomes of chemical
exposure are as much about the
cellular response to the chemical as it is an effect of the chemical. We are growing in our understanding
of the genotoxic interaction between chemistry and biology. For example,
recent data has revealed the biological basis for mutation induction
curves for a methylating chemical, which has been shown to be dependent
on the repair capacity of the cells. However, this is just one end
point in the toxicity pathway from chemical exposure to cell death.
Much remains to be known in order for us to predict how cells will
respond to a certain dose. Methylating agents, a subset of alkylating
agents, are of particular interest, because of the variety of adverse
genetic end points that can result, not only at increasing doses,
but also over time. For instance, methylating agents are mutagenic,
their potency, for this end point, is determined by the cellular repair
capacity of an enzyme called methylguanine DNA-methyltransferase (MGMT)
and its ability to repair the induceed methyl adducts. However, methyl
adducts can become clastogenic. Erroneous biological processing will
convert mutagenic adducts to clastogenic events in the form of double
strand breaks (DSBs). How the cell responds to DSBs is via a cascade
of protein kinases, which is called the DNA damage response (DDR),
which will determine if the damage is repaired effectively, via homologous
recombination, or with errors, via nonhomologous end joining, or whether
the cell dies via apoptosis or enters senescence. The fate of cells
may be determined by the extent of damage and the resulting strength
of DDR signaling. Therefore, thresholds of damage may exist that determine
cell fate. Such thresholds would be dependent on each of the repair
and response mechanisms that these methyl adducts stimulate. The molecular
mechanism of how methyl adducts kill cells is still to be fully resolved.
If we are able to quantify each of these thresholds of damage for
a given cell, then we can ascertain, of the many adducts that are
induced, what proportion of them are mutagenic, what proportion are
clastogenic, and how many of these clastogenic events are toxic. This
review examines the possibility of dose and damage thresholds for
methylating agents, from the perspective of the underlying evolutionary
mechanisms that may be accountable.
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