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Nuclear envelope-localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis.

ABSTRACT: Deciphering novel pathways regulating liver lipid content has profound implications for understanding the pathophysiology of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Recent evidence suggests that the nuclear envelope is a site of regulation of lipid metabolism but there is limited appreciation of the responsible mechanisms and molecular components within this organelle. We showed that conditional hepatocyte deletion of the inner nuclear membrane protein lamina-associated polypeptide 1 (LAP1) caused defective VLDL secretion and steatosis, including intranuclear lipid accumulation. LAP1 binds to and activates torsinA, an AAA+ ATPase that resides in the perinuclear space and continuous main ER. Deletion of torsinA from mouse hepatocytes caused even greater reductions in VLDL secretion and profound steatosis. Both of these mutant mouse lines developed hepatic steatosis and subsequent steatohepatitis on a regular chow diet in the absence of whole-body insulin resistance or obesity. Our results establish an essential role for the nuclear envelope-localized torsinA-LAP1 complex in hepatic VLDL secretion and suggest that the torsinA pathway participates in the pathophysiology of nonalcoholic fatty liver disease.

SUBMITTER: Shin JY

PROVIDER: S-EPMC6819140 | biostudies-literature | 2019 Aug

REPOSITORIES: biostudies-literature

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Nuclear envelope-localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis.

Shin Ji-Yeon JY Hernandez-Ono Antonio A Fedotova Tatyana T Östlund Cecilia C Lee Michael J MJ Gibeley Sarah B SB Liang Chun-Chi CC Dauer William T WT Ginsberg Henry N HN Worman Howard J HJ

The Journal of clinical investigation 20190813 11

Deciphering novel pathways regulating liver lipid content has profound implications for understanding the pathophysiology of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Recent evidence suggests that the nuclear envelope is a site of regulation of lipid metabolism but there is limited appreciation of the responsible mechanisms and molecular components within this organelle. We showed that conditional hepatocyte deletion of the inner nuclear membrane protein lamina-associate ...[more]

PMID: 31408437

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Project description:BACKGROUND:Non-alcoholic fatty liver disease (NAFLD) is one of the most common liver diseases worldwide, although its pathogenesis remains to be elucidated. A recent study revealed that hepatic Niemann-Pick C1-Like 1 (NPC1L1), a cholesterol re-absorber from bile to the liver expressed on the bile canalicular membrane, is an exacerbation factor of NAFLD. Indeed, transgenic mice with hepatic expression of human NPC1L1 under a liver-specific promoter (L1-Tg mice) developed steatosis with a high-fat diet (HFD) containing cholesterol within a few weeks. However, the mechanism underlying diet-induced hepatic NPC1L1-mediated lipid accumulation is poorly defined. METHODS:To achieve a deeper understanding of steatosis development in L1-Tg mice, the biochemical features of hepatic NPC1L1-mediated steatosis were investigated. Hemizygous L1-Tg mice and wild-type littermate controls fed a HFD or control-fat diet were used. At the indicated time points, the livers were evaluated for cholesterol and triglyceride (TG) contents as well as mRNA levels of hepatic genes involved in the maintenance of lipid homeostasis. The hepatic ability to secrete very low-density lipoprotein (VLDL)-TG was also investigated. RESULTS:Unlike the livers of wild-type mice that have little expression of hepatic Npc1l1, the livers of L1-Tg mice displayed time-dependent changes that indicated steatosis formation. In steatosis, there were three different stages of development: mild accumulation of hepatic cholesterol and TG (early stage), acceleration of hepatic TG accumulation (middle stage), and further accumulation of hepatic cholesterol (late stage). In the early stage, between WT and L1-Tg mice fed a HFD for 2?weeks, there were no significant differences in the hepatic expression of Ppar?, Acox1, Fat/Cd36, Srebf1, and Srebf2; however, the hepatic ability to secrete VLDL-TG decreased in L1-Tg mice (P?<?0.05). Furthermore, this decrease was completely prevented by administration of ezetimibe, an NPC1L1-selective inhibitor. CONCLUSION:Hepatic NPC1L1 exacerbates diet-induced steatosis, which was accompanied by decreased hepatic ability of VLDL-TG secretion. The obtained results provide a deeper understanding of L1-Tg mice as a promising NAFLD animal model that is able to re-absorb biliary-secreted cholesterol similar to humans. Furthermore, this work supports further studies of the pathophysiological impact of re-absorbed biliary cholesterol on the regulation of hepatic lipid homeostasis.

Project description:BackgroundPPARs (?,?,?) are a family of ligand-activated transcription factors that regulate energy balance, including lipid metabolism. Despite these critical functions, the integration between specific pathways of lipid metabolism and distinct PPAR responses remains obscure. Previous work has revealed that lipolytic pathways can activate PPARs. Whether hepatic lipase (HL), an enzyme that regulates VLDL and HDL catabolism, participates in PPAR responses is unknown.Methods/principal findingsUsing PPAR ligand binding domain transactivation assays, we found that HL interacted with triglyceride-rich VLDL (>HDL?LDL, IDL) to activate PPAR? preferentially over PPAR? or PPAR?, an effect dependent on HL catalytic activity. In cell free ligand displacement assays, VLDL hydrolysis by HL activated PPAR? in a VLDL-concentration dependent manner. Extended further, VLDL stimulation of HL-expressing HUVECs and FAO hepatoma cells increased mRNA expression of canonical PPAR? target genes, including adipocyte differentiation related protein (ADRP), angiopoietin like protein 4 and pyruvate dehydrogenase kinase-4. HL/VLDL regulated ADRP through a PPRE in the promoter region of this gene. In vivo, adenoviral-mediated hepatic HL expression in C57BL/6 mice increased hepatic ADRP mRNA levels by 30%. In ob/ob mice, a model with higher triglycerides than C57BL/6 mice, HL overexpression increased ADRP expression by 70%, demonstrating the importance of triglyceride substrate for HL-mediated PPAR? activation. Global metabolite profiling identified HL/VLDL released fatty acids including oleic acid and palmitoleic acid that were capable of recapitulating PPAR? activation and ADRP gene regulation in vitro.ConclusionsThese data define a novel pathway involving HL hydrolysis of VLDL that activates PPAR? through generation of specific monounsaturated fatty acids. These data also demonstrate how integrating cell biology with metabolomic approaches provides insight into specific lipid mediators and pathways of lipid metabolism that regulate transcription.

Project description:BACKGROUND: DYT1 dystonia is an autosomal dominant neurological condition caused by a mutation that removes a single glutamic acid residue (?E) from the torsinA (torA) AAA+ protein. TorA appears to possess a nuclear envelope (NE) localized activity that requires Lamina-Associated-Polypeptide 1 (LAP1), which is an inner nuclear membrane localized torA-binding partner. Although hypoactive, the DYT1 dystonia torA-?E isoform often concentrates in the NE, suggesting that torA-?E also interacts with an NE-localized binding partner. RESULTS: We confirm that NE-localized torA-?E does not co-immunoprecipitate with LAP1, and find that torA-?E continues to concentrate in the NE of cells that lack LAP1. Instead, we find that variability in torA-?E localization correlates with the presence of the SUN-domain and Nesprin proteins that assemble into the LINC complex. We also find that siRNA depletion of SUN1, but not other LINC complex components, removes torA-?E from the NE. In contrast, the LAP1-dependent NE-accumulation of an ATP-locked torA mutant is unaffected by loss of LINC complex proteins. This SUN1 dependent torA-?E localization requires the torA membrane association domain, as well as a putative substrate-interaction residue, Y147, neither of which are required for torA interaction with LAP1. We also find that mutation of these motifs, or depletion of SUN1, decreases the amount of torA-WT that colocalizes with NE markers, indicating that each also underlies a normal NE-localized torA binding interaction. CONCLUSIONS: These data suggest that the disease causing ?E mutation promotes an association between torA and SUN1 that is distinct to the interaction between LAP1 and ATP-bound torA. This evidence for two NE-localized binding partners suggests that torA may act on multiple substrates and/or possesses regulatory co-factor partners. In addition, finding that the DYT1 mutation causes abnormal association with SUN1 implicates LINC complex dysfunction in DYT1 dystonia pathogenesis, and suggests a gain-of-function activity contributes to this dominantly inherited disease.

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Nuclear envelope-localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis.

Publications

Nuclear envelope-localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis.

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