Dissertation
The consequences of regulation of hepatocyte identity by the unfolded protein response (UPR) during ER stress in the liver
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2022
DOI: 10.25820/etd.006823
Abstract
Endoplasmic reticulum (ER) stress is implicated in the etiologies of several chronic liver diseases. Upon ER stress, the unfolded protein response (UPR) is activated, and regulates transcription to maintain ER homeostasis, but the consequences of UPR activation for liver function are not well understood. Previous in-silico analysis from our and other groups has suggested that the UPR interferes with the gene regulatory network that maintains hepatocyte identity, possibly by interfering with hepatocyte nuclear factor 4α (Hnf4α/Hnf4a), a liver enriched transcription factor and a master regulator of hepatocyte differentiation. Yet it is unclear why the UPR, which has evolved to protect ER function principally by upregulating ER chaperones, would also suppress hepatocyte identity. We hypothesized that the purpose of UPR-mediated de-differentiation of hepatocytes is to protect against ER stress. We used parallel in vivo and in vitro approaches to test this hypothesis. In vivo, we challenged Hnf4a liver-specific knockout animals with the ER stress-inducing agent tunicamycin (TM). RNA-seq analysis indicated that a large majority of genes suppressed by ER stress were Hnf4 targets, showing that ER stress partially phenocopies hepatocyte loss of Hnf4a. Interestingly, gene set enrichment analysis showed a broad attenuation of UPR activation upon TM challenge in Hnf4a LKO animals, suggesting they resist ER stress. Supporting this idea, ER ultrastructure was less disrupted by TM in Hnf4aLKO animals than in wild-type littermates. These results suggested that loss of Hnf4α, which causes hepatocyte dedifferentiation, protects the ER from stress. In vitro, we tested the hypothesis that de-differentiation is protective against ER stress by exploiting the natural tendency of primary hepatocytes to dedifferentiate, comparing ER stress responsiveness in cells allowed to dedifferentiate or pharmacologically maintained in a five-chemical (5C) cocktail of differentiation-maintaining small molecules. Mirroring our results in vivo, dedifferentiated primary hepatocytes showed diminished UPR activation in response to ER stress compared to differentiated hepatocytes. Surprisingly, this result could not be explained by a diminished load of nascent ER client proteins in dedifferentiated cells, nor by altered ER chaperone expression. Rather, dedifferentiated cells showed attenuated metabolic production of NADPH, which we have shown previously to influence ER redox and stress sensitivity. Together, these data suggest that the UPR elicits hepatocyte de-differentiation as a protective mechanism for protecting against ER stress. They raise the possibility that the UPR compromises the organ's function (the liver) function in its attempt to protect the organelle (the ER).
Details
- Title: Subtitle
- The consequences of regulation of hepatocyte identity by the unfolded protein response (UPR) during ER stress in the liver
- Creators
- Anit Shah
- Contributors
- Thomas Rutkowski (Advisor)Robert Cornell (Committee Member)Matthew Potthoff (Committee Member)Ling Yang (Committee Member)Vitor Lira (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Biomedical Science (Cell and Developmental Biology)
- Date degree season
- Summer 2022
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.006823
- Number of pages
- xii, 153 pages
- Copyright
- Copyright 2022 Anit Shah
- Language
- English
- Description illustrations
- illustrations
- Description bibliographic
- Includes bibliographical references (pages 128-153).
- Public Abstract (ETD)
Metabolic diseases such as liver disease is progressive diseases and public health burdens. Out of many causative agents organellar stress called endoplasmic reticulum (ER) stress is one of them which when happens to disrupt the biological function of the liver. This disruption is one of the many reasons behind liver-related functional abnormalities. This organellar stress or ER stress can be caused by – foods, drugs, and pathogens. We know what an organelle will do to maintain homeostasis when it experiences ER stress, but we do not know how? So, we use genomic tools and techniques to address it. We delete a liver-specific protein-coding gene called Hnf4a and then elicit ER stress by using a drug. We found that this organelle (ER) protects itself from organellar (ER) stress by activating a cellular program that suppresses the hepatic identity of being a liver. We show that this mechanism of protection benefits the organelle but is harmful to the organ (liver).
- Academic Unit
- Biomedical Science Program
- Record Identifier
- 9984284950702771
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