Regulation of antioxidant functions during salinity
stress
Aldehydes are toxic carbonyl compounds which can be created in the body
as a byproduct of lipid peroxidation and a variety of other metabolic
processes (Laskar & Younus, 2019), and are converted into non-harmful
compounds by ALDH. In one study, lipid peroxidation increased
significantly by 48h after initially decreasing following acute transfer
to high salinity in the croaker fish Pseudosciaena crocea ,
suggesting that antioxidant pools were reduced and that the system was
overwhelmed over time (Zeng, Ai, Wang, Zhang, & Wu, 2017). Other
regulated cellular functions align well with known responses to salinity
stress including additional solute transporters in network 2, oxidative
phosphorylation in network 4, and DNA protection in network 5. These
findings agree with the notion that oxidative stress is a secondary,
non-specific consequence of osmotic and other environmental stresses,
resulting from upregulation of oxidative metabolism and the associated
increase in oxygen radical leakage (Kültz, 2020). ALDH3 is up-regulated
in response to hydrocarbon exposure in mice (Lindros et al., 1998), and
found to be highly responsive to phenylurea herbicides in teleost fish
(Marlatt & Martyniuk, 2017). The specific isoform of UGT significantly
up-regulated in this data set, UGT2C1-X2 (FC=4.03, p=.030) , was found
to be transcriptionally up-regulated in Cynoglossus semilaevisflatfish in response to the environmental pollutant Perfluorooctane
sulfonate (Zhang, Sun, Chen, Zhang, & Cai, 2020).
Many of the other proteins found in network 3 have detoxification/
antioxidant functions, including the aldo-keto reductases and the
significantly regulated quinoid dihydropteridine reductase (QDPR)
(FC=9.84, p=.039). QDPR was shown to induce production of major
antioxidant enzymes including glutathione peroxidase 3 and superoxide
dismutase 1 in cell culture (Gu et al., 2017). Like ALDH and DHRS
(significant but not found in STRING network 3), aldo/keto reductases
are another system for metabolizing toxic aldehydes (Laskar & Younus,
2019) but they were non-significantly down-regulated in BW acclimated
tilapia. Therefore, our data suggest that ALDH3 and QDPR are more
important for kidney function in BW acclimated tilapia than the
aldo/keto reductases.
Many of the proteins in STRING network 1 are also involved in protecting
against oxidative stress, including thioredoxin related proteins and
several glutathione S-transferase (GST) isoforms. Evidence from rats
exposed to polycyclic hydrocarbons indicates co-induction of ALDH3 along
with GST in liver tissue, which points to common enhancers or
transcription factors (Lindros et al., 1998). Three different GST
isoforms are connected in network 1 to another highly regulated protein,
elongation factor 1-delta (FC=4.48, p=.0025). EF 1-delta was found to be
regulated in several studies of fish exposed to pollutants (Jeffries,
Brander, Britton, Fangue, & Connon, 2015; Williams, Gensberg, Minchin,
& Chipman, 2003) and putative disease agents (Lü et al., 2014), as well
as hypo-osmotic stress in shrimp (Liu et al., 2016).