Synergistic Effects Induced
by a Low Dose of Diesel
Particulate Extract and Ultraviolet‑A in <i>Caenorhabditis
elegans</i>: DNA Damage-Triggered Germ Cell Apoptosis
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Abstract
Diesel exhaust has been classified
as a potential carcinogen and
is associated with various health effects. A previous study showed
that the doses for manifesting the mutagenetic effects of diesel exhaust
could be reduced when coexposed with ultraviolet-A (UVA) in a cellular
system. However, the mechanisms underlying synergistic effects remain
to be clarified, especially in an <i>in vivo</i> system.
In the present study, using <i>Caenorhabditis elegans</i> (<i>C. elegans</i>) as an <i>in vivo</i> system
we studied the synergistic effects of diesel particulate extract (DPE)
plus UVA, and the underlying mechanisms were dissected genetically
using related mutants. Our results demonstrated that though coexposure
of wild type worms at young adult stage to low doses of DPE (20 μg/mL)
plus UVA (0.2, 0.5, and 1.0 J/cm<sup>2</sup>) did not affect worm
development (mitotic germ cells and brood size), it resulted in a
significant induction of germ cell death. Using the strain of <i>hus-1::gfp</i>, distinct foci of HUS-1::GFP was observed in
proliferating germ cells, indicating the DNA damage after worms were
treated with DPE plus UVA. Moreover, the induction of germ cell death
by DPE plus UVA was alleviated in single-gene loss-of-function mutations
of core apoptotic, checkpoint HUS-1, CEP-1/p53, and MAPK dependent
signaling pathways. Using a reactive oxygen species (ROS) probe, it
was found that the production of ROS in worms coexposed to DPE plus
UVA increased in a time-dependent manner. In addition, employing a
singlet oxygen (<sup>1</sup>O<sub>2</sub>) trapping probe, 2,2,6,6-tetramethyl-4-piperidone,
coupled with electron spin resonance analysis, we demonstrated the
increased <sup>1</sup>O<sub>2</sub> production in worms coexposed
to DPE plus UVA. These results indicated that UVA could enhance the
apoptotic induction of DPE at low doses through a DNA damage-triggered
pathway and that the production of ROS, especially <sup>1</sup>O<sub>2</sub>, played a pivotal role in initiating the synergistic process