Lipid homeostasis is essential for a maximal ER stress response

  1. Gilberto Garcia
  2. Hanlin Zhang
  3. Sophia Moreno
  4. C Kimberly Tsui
  5. Brant Michael Webster
  6. Ryo Higuchi-Sanabria
  7. Andrew Dillin

  • Curated by eLife

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    eLife assessment

    This valuable study addresses the anticipated but poorly understood interconnections between ER proteostasis and lipid metabolism. The authors discovered key metabolic enzymes required for integration of ER stress and lipid synthesis and followed up with several direct experiments that provide solid evidence for a broad conservation of the described interactions.

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Abstract

Changes in lipid metabolism are associated with aging and age-related diseases, including proteopathies. The endoplasmic reticulum (ER) is uniquely a major hub for protein and lipid synthesis, making its function essential for both protein and lipid homeostasis. However, it is less clear how lipid metabolism and protein quality may impact each other. Here, we identified let-767 , a putative hydroxysteroid dehydrogenase in Caenorhabditis elegans , as an essential gene for both lipid and ER protein homeostasis. Knockdown of let-767 reduces lipid stores, alters ER morphology in a lipid-dependent manner, and blocks induction of the Unfolded Protein Response of the ER (UPR ER ). Interestingly, a global reduction in lipogenic pathways restores UPR ER induction in animals with reduced let-767 . Specifically, we find that supplementation of 3-oxoacyl, the predicted metabolite directly upstream of let-767 , is sufficient to block induction of the UPR ER . This study highlights a novel interaction through which changes in lipid metabolism can alter a cell’s response to protein-induced stress.

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  1. Version published to 10.7554/elife.83884 on eLife
  2. eLife

    eLife assessment

    This valuable study addresses the anticipated but poorly understood interconnections between ER proteostasis and lipid metabolism. The authors discovered key metabolic enzymes required for integration of ER stress and lipid synthesis and followed up with several direct experiments that provide solid evidence for a broad conservation of the described interactions.

  3. eLife

    Reviewer #1 (Public Review):

    The underlying principle of the experimental system described here is to test potential candidate genes that intersect with the proteotoxic-induced UPR by screening an siRNA pool that diminishes the UPR transcription reporter activated by sec-11 RNAi-mediated ER proteotoxic stress. The authors specifically focused on genes reported to play roles in LD biology, instead of general lipid synthesis genes. Systematic evaluation of the LD genes with respect to the induction of the UPR provides important insights into the overall functions and mechanisms of the UPR.

    Using this set-up, the authors identified the hydroxysteroid dehydrogenase gene let-767/HSD17B12. Subsequent analyses revealed that let-767-mediated signaling is a key component that establishes the orchestration of both ER lipid and protein homeostasis and ER organismal functions, including ER lipid storage and ER structural changes. In addition, the authors found that acs-1i, knockdown of a gene involved in metabolism of lipids such as LCFA and mmBCFA, also diminished UPRE-GFP levels induced by sec-11i, albeit to a lesser extent than let-767i. Supplementation of lipid metabolites such as LCFA and mmBCFA recovered not only the sec-11-induced UPRE-GFP reporter phenotypes in acs-1i worms, but also the ER size and morphology and the LD and body sizes.

    In contrast, the UPRE-GFP reporter phenotype in let-767i worms was not recovered by exogenously added LCFA or mmBCFA, although it was recovered by spb-1 RNAi, knockdown of a major lipogenic enzyme/pathway. The system established by the authors allowed them to quantitatively dissect the involvement of the Ire1-Xbp1 splicing UPR signaling branch. Finally, the authors demonstrated similar effects in mammalian tissue culture cells, suggesting conservation of the mechanisms.

    The conclusions of this manuscript are generally in agreement with the data and the authors' interpretations are reasonable. However, at this point, the work remains descriptive and does not provide a mechanistic understanding. Overall contributions/advances towards providing new insights into how the UPR pathway is wired with respect to lipid-associated perturbations remain somewhat limited.

  4. eLife

    Reviewer #2 (Public Review):

    This paper is an interesting and novel addition to our understanding of the link between ER stress and lipid homeostasis. Utilizing a genetic screen to determine modulators of the UPRER, Garcia, G., et al., determine C. elegans cannot activate the UPRER as strongly with knockdown of the putative hydroxysteroid dehydrogenase let-767. Additionally, let-767 knockdown results in smaller lipid droplets and changes to ER morphology. Both lipid droplet size and ER morphology size can be restored with supplementation of lipids, while the defect to UPRER activation persists. The authors elegantly show that one impact of let-767 knockdown on UPR is downstream of XBP1 splicing. The authors then go on to show that in mammalian cells, the lipid precursor 3-oxoacyl-CoA can cause a similar reduction to UPRER activation to that seen in C. elegans with let-767 knockdown. Some limitations of this study are that let-767 exact role in lipid metabolism is not well understood and it is unclear what the impact of let-767 knockdown in C. elegans has on lipid composition. It is also unclear mechanistically how let-767 is able to effect UPRER, as the authors show one potential mechanism is by blocking activation of the UPR downstream of XBP1 splicing. While the authors demonstrate that high levels of 3-oxoacyl-CoA can cause a reduction in the UPR response in mammalian cells, this finding is not recapitulated in C. elegans, nor does the study determine whether this compound accumulates in a let-767 knockdown.

  5. eLife

    Reviewer #3 (Public Review):

    Here, the authors identify and characterize the role of C. elegans putative hydroxysteroid dehydrogenase gene let-767 to be essential for both lipid and endoplasmic reticulum (ER) homeostasis. They demonstrated that plays a role in lipid storage, maintaining ER morphology and that the lack of let-767 inhibits the unfolded protein response (UPR) upon proteotoxic stress, presumably by the accumulation of the predicted metabolite directly upstream of LET-767, 3-oxoacyl.

    Strengths of the manuscript
    The complementary data in human cell line huh-7 that support the authors findings in C. elegans. The ablation of let-767 in C. elegans render the animal incapable of mounting a UPR response upon proteotoxic stress (tunicamycin). Similarly, supplementing the media of huh-7 cells with LET-767 precursor, 3-oxoacyl, attenuates the UPR activation by tunicamycin.

    Overall, the experiments are well designed and in logical order throughout the manuscript.

    Weakness of the manuscript
    The biggest weakness of this manuscript is the difficulty to appreciate the differences reported by the authors from the images provided. Providing images of higher quality or highlighting the differences to note within the figure panels will make the interpretation of data easier.

    Additionally, many of the reported data are from biological duplicates. The lack of additional biological replicate might undermine the authors' findings.

  6. Version published to 10.1101/2022.10.27.513991v1 on bioRxiv