GRP78 expression inhibits insulin and ER stress–induced SREBP-1c activation and reduces hepatic steatosis in mice
Hélène L. Kammoun, … , Pascal Ferré, Fabienne Foufelle
Hélène L. Kammoun, … , Pascal Ferré, Fabienne Foufelle
Published April 13, 2009
Citation Information: J Clin Invest. 2009;119(5):1201-1215. https://doi.org/10.1172/JCI37007.
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Research Article Metabolism

GRP78 expression inhibits insulin and ER stress–induced SREBP-1c activation and reduces hepatic steatosis in mice

Abstract

Hepatic steatosis is present in insulin-resistant obese rodents and is concomitant with active lipogenesis. Hepatic lipogenesis depends on the insulin-induced activation of the transcription factor SREBP-1c. Despite prevailing insulin resistance, SREBP-1c is activated in the livers of genetically and diet-induced obese rodents. Recent studies have reported the presence of an ER stress response in the livers of obese ob/ob mice. To assess whether ER stress promotes SREBP-1c activation and thus contributes to lipogenesis, we overexpressed the chaperone glucose-regulated protein 78 (GRP78) in the livers of ob/ob mice using an adenoviral vector. GRP78 overexpression reduced ER stress markers and inhibited SREBP-1c cleavage and the expression of SREBP-1c and SREBP-2 target genes. Furthermore, hepatic triglyceride and cholesterol contents were reduced, and insulin sensitivity improved, in GRP78-injected mice. These metabolic improvements were likely mediated by restoration of IRS-2 expression and tyrosine phosphorylation. Interestingly, GRP78 overexpression also inhibited insulin-induced SREBP-1c cleavage in cultured primary hepatocytes. These findings demonstrate that GRP78 inhibits both insulin-dependent and ER stress–dependent SREBP-1c proteolytic cleavage and explain the role of ER stress in hepatic steatosis in obese rodents.

Authors

Hélène L. Kammoun, Hervé Chabanon, Isabelle Hainault, Serge Luquet, Christophe Magnan, Tatsuro Koike, Pascal Ferré, Fabienne Foufelle

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Figure 12

In vitro effects of GRP78 overexpression on insulin-induced SREBP-1c cleavage in rat hepatocytes and analysis of SREBP-1c complex and GRP78 interaction in mouse livers.

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In vitro effects of GRP78 overexpression on insulin-induced SREBP-1c cle...
(A) Immunoblot of the IR precursor and IR-β in total lysates of hepatocytes infected for 24 h with Ad GRP78 or Ad β-gal. The blot was hybridized with a GRP78 antibody to verify the overexpression of the transgene. (B) After a 24-h period of infection with Ad GRP78 or Ad β-gal, cultured hepatocytes were changed to a fresh M199 medium containing 10 μM TO-901317 for 6 h, then treated for 1 h with 300 nM thapsigargin or 100 nM insulin. Shown is immunoblot analysis of microsomal SREBP-1c precursor and nuclear SREBP-1c. (C) Microsomal membranes were isolated from the livers of fed ob/+ and ob/ob mice. Detergent-solubilized membranes were subjected to immunoprecipitation with polyclonal H160 anti–SREBP-1 antibody. Immunoprecipitated proteins were probed by Western blot with anti-GRP78 and anti–SREBP-1 or anti-calnexin as a nonrelevant microsomal antibody. An aliquot of microsomal proteins before immunoprecipitation from a pooled preparation of ob/+ mice livers was run on the gel in parallel (Input). Control immunoprecipitation with an irrelevant antibody, Glut1, was made on microsomal proteins from a pooled preparation of ob/+ mouse livers. The quantified ratio of GRP78 to SREBP-1 precursor is shown below. ***P < 0.001 versus ob/+.