Nuclear envelope–localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis
Ji-Yeon Shin, … , Henry N. Ginsberg, Howard J. Worman
Ji-Yeon Shin, … , Henry N. Ginsberg, Howard J. Worman
Published November 1, 2019; First published August 13, 2019
Citation Information: J Clin Invest. 2019;129(11):4885-4900. https://doi.org/10.1172/JCI129769.
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Research Article Cell biology Metabolism

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

Abstract

Deciphering novel pathways that regulate liver lipid content has profound implications for understanding the pathophysiology of nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). 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) causes 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. Mice from both of the mutant lines studied 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 NAFLD.

Authors

Ji-Yeon Shin, Antonio Hernandez-Ono, Tatyana Fedotova, Cecilia Östlund, Michael J. Lee, Sarah B. Gibeley, Chun-Chi Liang, William T. Dauer, Henry N. Ginsberg, Howard J. Worman

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Figure 4

Intranuclear and increased cytoplasmic lipid droplets and reduced apoB100 secretion in hepatocytes isolated from adult mice with “acute” depletion of LAP1.

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Intranuclear and increased cytoplasmic lipid droplets and reduced apoB10...
(A) Immunoblot of hepatocyte protein lysates from Tor1aip1fl/fl mice injected with AAV-LacZ (control) or AAV-Cre probed with antibodies against LAP1. Hepatocytes were isolated 3 weeks and 8 weeks after virus injection. Each lane represents a primary culture of hepatocytes from 1 mouse. Migrations of molecular mass standards are indicated at the left and migrations of LAP1A/LAP1B and LAP1C at the right. The arrow points to the nonspecific band. An immunoblot for actin is shown as a loading control. (B) Confocal micrographs of isolated hepatocytes from Tor1aip1fl/fl mice injected with AAV-LacZ or AAV-Cre. Lipids were stained with BODIPY (green) and nuclei with DAPI (blue). The far-right panel is a zoomed image of the dashed-line square region. White arrowheads indicate intranuclear lipid droplets. Scale bar: 10 μm (zoom, 10 μm). (C) Autoradiogram of SDS-polyacrylamide gel showing newly synthesized 35S-methionine–labeled proteins in cell lysates and media fractions of primary hepatocytes cultures from Tor1aip1fl/fl mice injected with AAV-LacZ or AAV-Cre. Migrations of 35S-labeled apoB100 and apoB48 are indicated. (D) Bands corresponding to apoB100 and apoB48 were cut from the gel shown in C, and radioactivity was quantified by scintillation counting. The left panel shows results from the media fraction and the right panel from the cell fraction. Results were normalized to the mean values from AAV-LacZ–injected mice and set to 100% (n = 3 different hepatocyte cultures from 1 mouse of each group. *P < 0.05 and **P < 0.01, by Student’s t test. In D, the values for individual mice are shown, with longer horizontal bars indicating the mean and vertical bars indicating the SEM. Mice were 4 months old at the time of virus injection.