title: VLDL hydrolysis by hepatic lipase regulates PPARδ transcriptional responses.
abstract: BACKGROUND
  PPARs (α,γ,δ) 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.
  
  
  RESULTS
  Using PPAR ligand binding domain transactivation assays, we found that HL interacted with triglyceride-rich VLDL (>HDL≫LDL, IDL) to activate PPARδ preferentially over PPARα or PPARγ, an effect dependent on HL catalytic activity. In cell free ligand displacement assays, VLDL hydrolysis by HL activated PPARδ in a VLDL-concentration dependent manner. Extended further, VLDL stimulation of HL-expressing HUVECs and FAO hepatoma cells increased mRNA expression of canonical PPARδ 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 PPARδ activation. Global metabolite profiling identified HL/VLDL released fatty acids including oleic acid and palmitoleic acid that were capable of recapitulating PPARδ activation and ADRP gene regulation in vitro.
  
  
  CONCLUSIONS
  These data define a novel pathway involving HL hydrolysis of VLDL that activates PPARδ 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.
authors.affiliation: Division of Cardiovascular Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, United States of America.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Brown
authors.forename: Eric
authors.initials: E
authors.lastname: Oligino
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Rader
authors.forename: Alan
authors.initials: A
authors.lastname: Saghatelian
authors.forename: Jorge
authors.initials: J
authors.lastname: Plutzky
mesh_terms: D000818:Animals; D015153:Blotting, Western; D019556:COS Cells; D020134:Catalytic Domain / Q000235:genetics; D002460:Cell Line; D045744:Cell Line, Tumor; D002522:Chlorocebus aethiops; D005227:Fatty Acids / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D006868:Hydrolysis; D008049:Lipase / Q000235:genetics / Q000378:metabolism*; D008079:Lipoproteins, VLDL / Q000378:metabolism*; D008099:Liver / Q000201:enzymology / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008820:Mice, Obese; D009154:Mutation; D047494:PPAR delta / Q000235:genetics / Q000378:metabolism*; D000070780:Perilipin-2; D034622:RNA Interference; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014158:Transcription, Genetic
chemical_list: D005227:Fatty Acids; D008079:Lipoproteins, VLDL; D008565:Membrane Proteins; C000602982:PLIN2 protein, human; D047494:PPAR delta; D000070780:Perilipin-2; C000602983:Plin2 protein, mouse; D008049:Lipase