NHR-49/HNF4 integrates regulation of fatty acid metabolism with a protective transcriptional response to oxidative stress and fasting

Endogenous and exogenous stresses elicit transcriptional responses that limit damage and promote cell/organismal survival. Like its mammalian counterparts, Hepatocyte Nuclear Factor 4 (HNF4) and peroxisome proliferator-activated receptor a (PPARa), Caenorhabditis elegans NHR-49 is a well-established regulator of lipid metabolism. Here, we reveal that NHR-49 is essential to activate a transcriptional response common to organic peroxide and fasting, which includes the pro-longevity gene fmo-2/flavin-containing monoxygenase. These NHR-49-dependent, stress-responsive genes are also upregulated in long-lived glp-1/notch receptor mutants, and two of them contribute to increased oxidative stress resistance in wild-type worms and long-lived glp-1 mutants. Similar to its role in lipid metabolism, NHR-49 requires the Mediator subunit mdt-15 to promote stress-induced gene expression. However, NHR-49 acts independently from the transcription factor hlh-30/TFEB that also promotes fmo-2 expression. Similarly, activation of the p38 MAPK, PMK-1, which is important for adaptation to a variety of stresses, is only important for peroxide-induced expression of a subset of NHR-49-dependent genes, including fmo-2. Notably, we find that organic peroxide increases NHR-49 protein levels, apparently by a post-transcriptional mechanism that does not require PMK-1 activation. Together these findings establish a new role for the HNF4/PPARa-related NHR-49 as a stress-activated regulator of cytoprotective gene expression.