Abstract
Reactive oxygen species (ROS) play important signaling roles in metazoans, but also cause significant molecular damage. Animals tightly control ROS levels using sophisticated defense mechanisms, yet the transcriptional pathways that induce ROS defense remain incompletely understood. In the nematode Caenorhabditis elegans, the transcription factor SKN-1 is considered a master regulator for detoxification and oxidative stress responses. Here, we show that MDT-15, a subunit of the conserved Mediator complex, is also required for oxidative stress responses in nematodes. Specifically, mdt-15 is required to express SKN-1 targets upon chemical and genetic increase in SKN-1 activity. mdt-15 is also required to express genes in SKN-1-dependent and SKN-1-independent fashions downstream of insulin/IGF-1 signaling and for the longevity of daf-2/insulin receptor mutants. At the molecular level, MDT-15 binds SKN-1 through a region distinct from the classical transcription-factor-binding KIX-domain. Moreover, mdt-15 is essential for the transcriptional response to and survival on the organic peroxide tert-butyl-hydroperoxide (tBOOH), a largely SKN-1-independent response. The MDT-15 interacting nuclear hormone receptor, NHR-64, is specifically required for tBOOH but not arsenite resistance, but NHR-64 is dispensable for the transcriptional response to tBOOH. Hence, NHR-64 and MDT-15’s mode of action remain elusive. Lastly, the role of MDT-15 in oxidative stress defense is functionally separable from its function in fatty acid metabolism, as exogenous polyunsaturated fatty acid complementation rescues developmental, but not stress sensitivity phenotypes of mdt-15 worms. Our findings reveal novel conserved players in the oxidative stress response and suggest a broad cytoprotective role for MDT-15.
Highlights
Reactive oxygen species (ROS) are ubiquitous molecules that occur as byproducts of aerobic metabolism
Tullet et al found several gst genes to be upregulated in daf-2(e1370) mutants in a skn-1-dependent fashion; we found that mdt-15 depletion significantly reduced these inductions (Fig. 3C, Fig. S6A). mdt-15 depletion affected SKN-1independent genes such as fat-5 and fat-6 (Taubert et al, 2006; Fig. 3C, Fig. S6A)
We found that mRNA induction of the direct DAF-16 target sod-3 was slightly reduced in daf-2(e1370) mutants grown on mdt-15 RNA interference (RNAi) (Fig. 3C)
Summary
Reactive oxygen species (ROS) are ubiquitous molecules that occur as byproducts of aerobic metabolism. ROS have important biological properties and activities: on one hand, they serve as signaling molecules in regulatory circuits; on the other hand, they can damage cellular macromolecules due to their reactive nature (Hekimi et al, 2011; Back et al, 2012). Proteins that protect against high ROS levels include catalases, superoxide dismutases (SODs), and glutathione S-transferases (GSTs) (Xu et al, 2005; Lindblom & Dodd, 2006). Many such cytoprotective genes are transcriptionally induced by oxidative stress, and numerous transcription factors are required to activate overlapping, but distinct gene sets upon oxidative stress (Ma, 2010). Defining the transcriptional regulatory networks that limit cellular damage is an important scientific question
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