Dissertation
The impact of obesity-associated nitrosative stress on organelle function
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2022
DOI: 10.25820/etd.006543
Abstract
The global prevalence of obesity has increased significantly in recent decades. Obesity has been shown to be associated with many metabolic diseases, including type 2 diabetes, hypertension and cardiovascular diseases. At the cellular level, obesity promotes chronic inflammation and impairs organelle functions in different organs, consequentially leading to disturbance of metabolic homeostasis. However, in the obese condition, the molecular mechanism underlying the link between inflammation and abnormalities is incomplete. In my thesis study, we demonstrated the impact of obesity-associated nitrosative stress on the organelle function in different organs. Nitric oxide (NO) is one of the markers of inflammation. Normal level of NO is critical for physical process. However, in the obese condition, the intracellular NO level increases. We found that the protein S-nitrosylation (SNO) modification accompanied by the overproduction of NO is one of the main drivers that caused the obesity related organelle abnormalities. In both liver and brown adipose tissue (BAT), obesity elevates the SNO of proteins that are critical to maintain the proper tissue homeostasis. In the liver, local lysosomal inducible nitric oxide synthase (iNOS) expression and NO level are significantly elevated in the context of obesity, which can be attenuated by the NO scavenger. While overexpression of mTOR and the arginine transporter (SLC38A9) enhances the lysosome NO production and impairs hepatic autophagy. Furthermore, we demonstrated that deletion of iNOS improves the nitrosative stress and hepatic insulin sensitivity in the liver with diet induced obesity (DIO), which the protective effect is abolished by the deletion of transcription factor EB (TFEB) and autophagy related 7 (ATG7).
Previous studies including ours demonstrated that defective autophagy and a failure to initiate the adaptive unfolded protein response (UPR) in response to the endoplasmic reticulum (ER) stress contributes to obesity-associated metabolic dysfunction. However, how these two key stress responses integrate in the obese condition is unclear. Our RNA-seq analysis demonstrate that x-box binding protein 1 (XBP1, the key transcription factor regulates the adaptive unfolded protein responses) governs several lysosomal gene expressions. Our chromatin immunoprecipitation (ChIP) assay showed that XBP1 directly binds to the promoter of Tfeb to regulate the hepatic autophagy. Furthermore, our gain and loss of function of Tfeb in the mice reveal that sXbp1-TFEB axis has important role in regulating hepatic autophagy and systemic glucose homeostasis in the mice with diet-induced obesity.
In the BAT, the thermogenesis activity is tightly regulated by the cellular redox status. Obesity and acute cold exposure increased the general protein SNO level, including SNO of UCP1, the BAT specific thermogenesis protein. The cellular nitroso-redox status is controlled by the S-nitrosoglutathione reductase (alcohol dehydrogenase 5: ADH5). The deletion of ADH5 impairs cold-induced UCP1-dependent thermogenesis and worsens obesity-associated metabolic dysfunction. Mechanistically, we showed that the heat shock factor 1 (HSF1) binds to the promoter of Adh5 and administration of HFS1 activator in BAT increases the Adh5 expression and improves UCP1-mediated respiration.
Details
- Title: Subtitle
- The impact of obesity-associated nitrosative stress on organelle function
- Creators
- Zeyuan Zhang
- Contributors
- Ling Yang (Advisor)Botond Banfi (Committee Member)Huojun Cao (Committee Member)Matthew J Potthoff (Committee Member)Thomas Rutkowski (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Anatomy and Cell Biology
- Date degree season
- Spring 2022
- DOI
- 10.25820/etd.006543
- Publisher
- University of Iowa
- Number of pages
- xv, 147 pages
- Copyright
- Copyright 2022 Zeyuan Zhang
- Comment
This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: https://www.lib.uiowa.edu/sc/contact/.
- Language
- English
- Description illustrations
- illustrations (chiefly color)
- Description bibliographic
- Includes bibliographical references (pages 134-147).
- Public Abstract (ETD)
The prevalence of obesity has been increasing worldwide. Obesity is associated with the development of several metabolic diseases, such as hypertension, diabetes, heart-related diseases, and cancer. In addition, obesity can cause damage to cells of different organs, such as the liver and brown adipose tissue (BAT) through proteins structure modifications and alterations in normal cell stress responses.
In the liver, the intracellular nitric oxide (NO) level increases in diet-induced obesity (DIO) mice, primary due to the upregulated expression of inducible nitric oxide synthase (iNOS), the enzyme that produces NO. In my thesis work, we showed that the excess NO level is associated with the decreased lysosomal arginine, the substrate for iNOS, which impairs lysosomal enzyme function and autophagy (a process by which the cells degrade proteins and organelles). Together, all these alterations contribute to hepatic obesity-associated glucose intolerance in the liver.
In addition, obesity not only impairs hepatic autophagy, but also causes the failure of the adaptive unfolded protein response (UPR, which has the important role in recovering the function of the endoplasmic reticulum due to accumulated misfolded proteins). However, whether these two processes interact with each other remained unclear. Therefore, in this work we compared the genetic profile of the livers from wild type and Xbp1 (a key regulators of the UPR) liver specific knockout mice. We found that XBP1 regulates the expression of Tfeb, a key gene controling autophagy.
Finally, we found that DIO induces NO generation within BAT, resulting in reduction of the expression of the denitrosylase alcohol dehydrogenase 5 (ADH5; also named S-nitrosoglutathione reductase [GSNOR]) and impaired mitochondrial oxidation through protein S-nitrosylation (SNO) modification of the key mitochondrial protein, uncoupling protein 1. We also demonstrate that BAT ADH5 expression is controlled in part by heat shock factor 1, a key stress response element and transcription factor.
- Academic Unit
- Anatomy and Cell Biology; Craniofacial Anomalies Research Center
- Record Identifier
- 9984271454202771
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