Abstract
The integration of metabolic signals required for the regulation of hepatic lipid homeostasis is complex. Previously, we showed that mice lacking expression of the mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) have increased fatty acid oxidation and are protected from the development of hepatic steatosis. Here, we show that leptin receptor-deficient (db/db) mice lacking MKP-1 are also resistant to the development of hepatic steatosis. Microarray analyses of livers from db/db mice lacking MKP-1 showed suppression of peroxisome proliferator-activated receptor γ (PPARγ) target genes. We identified the fat-specific protein 27 (Fsp27), which promotes PPARγ-mediated hepatic steatosis, as repressed in livers of both db/db and high fat diet-fed mice lacking MKP-1. Hepatocytes from MKP-1-deficient mice exhibited reduced PPARγ-induced lipid droplet formation. Mechanistically, loss of MKP-1 inhibited PPARγ function by increasing MAPK-dependent phosphorylation on PPARγ at its inhibitory residue of serine 112. These results demonstrate that in addition to inhibiting hepatic fatty acid oxidation, MKP-1 promotes hepatic lipogenic gene expression through PPARγ. Hence, MKP-1 plays an important role in MAPK-mediated control of hepatic lipid homeostasis.
Highlights
The conclusion that MAPK phosphatases (MKPs)-1 promotes hepatic steatosis is based upon the observation that db/db;mkp-1Ϫ/Ϫ mice were resistant to the accumulation of triglycerides in the liver accompanied by a commensurate protection from the increase in liver mass associated with this genetic model of obesity
The levels of triglyceride content in db/db;mkp-1Ϫ/Ϫ mice were significantly lower in 16-week-old mice, strongly suggesting that loss of MKP-1 inhibited rather than delayed the development of hepatic steatosis
We suggested that the ability of MKP-1-deficient mice to resist the development of hepatic steatosis was due, at least in part, to the fact that p38 mitogen-activated protein kinase (MAPK)-dependent phosphorylation on peroxisome proliferator-activated receptor (PPAR)␣ was enhanced resulting in increased ligand-induced activation of hepatic -oxidation [6]
Summary
Primary Cell Cultures and Tissue Analysis—Primary hepatocytes were isolated by collagenase digestion [17] and cultured as described previously [13]. Primary hepatocytes were loaded overnight with complete Williams Medium E (Invitrogen) containing 500 M oleic acid conjugated to 0.5% free fatty acid-free BSA (Sigma Aldrich) and 1 Ci of [9,103H]oleic acid (PerkinElmer Life Science). Export, -oxidation, and triglyceride turnover assays were performed as described [19]. For imaging and gene expression studies, primary hepatocytes were treated overnight with dimethyl sulfoxide, 100 M rosiglitazone, or 1 mM oleic acid (Cayman Chemical) conjugated to 1% free fatty acid-free BSA. Cells were processed for RNA or fixed with 4% paraformaldehyde, stained with BODIPY 493/503 (Invitrogen), and mounted in DAPIcontaining medium (Vector Laboratories). Triglyceride turnover rates were compared by linear regression with Graph Pad Prism Software
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