Cytoprotective responses and lipid metabolism are tied to longevity. Genes in both pathways are differentially regulated in several long-lived mutants, and many regulators of these genes are key effectors of longevity. One such regulator is the transcriptional coregulator MDT-15, a subunit of the Mediator complex. MDT-15 regulates lipid metabolism and oxidative stress response genes, and is required for normal lifespan. In the oxidative stress response, MDT-15 binds the transcription factor SKN-1 and regulates SKN-1-dependent genes. However, not all oxidative stress response genes are SKN-1-dependent. For example, the tert-butyl hydroperoxide (tBOOH) response is largely SKN-1-independent. We previously showed that MDT-15 is required for the tBOOH response; however, the transcription factor that regulates this response is unknown.
The MDT-15 KIX domain interacts with multiple transcription factors. We found that a gain-of-function (GOF) mutation of mdt-15 located near the KIX specifically induces SKN-1-independent tBOOH-responsive genes. This implies that the tBOOH response is regulated by a transcription factor that binds the KIX domain of MDT-15. We conducted an RNAi screen to find transcription factors required to induce a promoter::GFP fusion of fmo-2, a tBOOH-responsive gene. We tested transcription factors that bind the KIX domain of MDT-15, as determined by published screens. Of the candidates screened, only the nuclear receptor NHR-49 was required for fmo-2p::GFP induction on tBOOH. NHR-49 is required to express genes involved in lipid metabolism, including β-oxidation and fatty acid desaturation, but has not been implicated in oxidative stress responses. Using qPCR, we show that nhr-49 null mutants fail to fully induce other tBOOH-responsive genes in addition to fmo-2. Conversely, nhr-49 GOF mutants show upregulation of many tBOOH-responsive genes even without tBOOH. Interestingly, we previously found that nhr-49 nulls are hypersensitive to tBOOH; therefore, the regulation of tBOOH-responsive genes by NHR-49 also has functional consequences. We are currently testing whether any known NHR-49 partners, e.g. NHR-80 or NHR-13, are also required for the tBOOH response.
In conclusion, we describe a new role for NHR-49 as a regulator of the oxidative stress response. NHR-49 has recently emerged as an important regulator of longevity, e.g. in germline-less glp-1 mutants. This was thought to relate to nhr-49’s role in lipid metabolism. Our work suggests an alternative possibility, i.e. that NHR-49’s role in the oxidative stress response contributes to its action in longevity pathways.
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