The unfolded protein response (UPR) is activated by protein misfolding stress in the endoplasmic reticulum (ER), and it culminates in the transcriptional upregulation of ER chaperones, degradation factors, and other genes that help the organelle properly fold or degrade client proteins. However, nutritional flux can elicit ER stress in metabolic tissues such as the liver, and the UPR can in turn regulate metabolic pathways. A number of diseases are associated with both altered metabolism and chronic ER stress, most notably including obesity and its complications. Therefore, it is important to understand how the UPR regulates metabolism and how metabolic flux influences the ER protein processing capacity. Data presented in this proposal show that ER stress in the liver leads to transcriptional suppression of fatty acid oxidation, and that inhibition of fatty acid oxidation alters the oxidizing environment of the ER and protects hepatocytes from stress. These findings suggest that the regulation of fatty acid oxidation by the UPR represents a novel pathway by which the response protects ER function during stress. Yet very little is known about the mechanisms by which the UPR represses rather than activates transcription. Nor is it known how flux through metabolic pathways influences the ER protein folding and processing capacity. Thus, the objective of this work is to understand how ER stress regulates fatty acid oxidation, how fatty acid oxidation regulates ER function, and how these pathways interact during feeding and fasting. The work in this proposal tests the central hypothesis that ER stress leads to direct transcriptional suppression of fatty acid oxidation through the UPR-regulated transcription factor CHOP, and that this in turn alters the oxidative protein folding capacity of the ER to alleviate stress. This hypothesis will be tested by three complementary aims.
The first aim will elucidate the gene regulatory network by which the UPR regulates fatty acid oxidation. The role of CHOP in this network will be tested, as will the interactions of CHOP with other C/EBP-family transcription factors and the impact of CHOP action on the master regulators of fatty acid oxidation. In the second aim, the ability of the ER to efficiently import, fold, modify, oxidize, transport, and degrade client proteins will be systematically examined when fatty acid oxidation is manipulated, and the roles of NADPH and glutathione redox in linking fatty acid oxidation to ER function will be tested.
The third aim will determine how the relationship between fatty acid oxidation and ER protein processing contributes to the regulation of metabolic activity and ER stress during feeding and fasting. The contribution of CHOP to the suppression of fatty acid oxidation during feeding and its enhancement during fasting will be tested, as will the effects of manipulating fatty acid oxidation, NADPH generation, and glutathione oxidation on ER function. Together, these aims will form a cross-disciplinary approach linking ER protein folding, UPR signaling, lipid metabolism, and redox homeostasis.
A number of very common human diseases are associated with both dysregulated lipid metabolism and chronic endoplasmic reticulum (ER) stress, including obesity and the constellation of symptoms included under the umbrella of the Metabolic Syndrome, which affect approximately 50 million Americans. Our recent work has demonstrated that ER stress in the liver represses lipid catabolism, and this repression in turn fundamentally alters the functionality of the ER. Thus, by understanding how ER stress and lipid catabolism are linked, and how this relationship contributes to metabolic regulation during feeding and fasting, this work will help to elucidate the role of ER stress signaling in the lipid dysregulation that accompanies metabolic disease.
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