Heparin induces the expression of hepatic triglyceride lipase in a human hepatoma (HepG2) cell line.
Hepatic triglyceride lipase (H-TGL) is a key lipolytic enzyme in the metabolism of human plasma high density lipoproteins. The enzyme is bound to glycosaminoglycans on endothelial cells in the liver and is immediately released into the circulation after heparin administration. In addition to releasing H-TGL, heparin-like glycosaminoglycans have also been shown to suppress hepatocyte proliferation and to alter tissue-specific gene expression. In the present study, the effects of heparin exposure on the secretion of H-TGL were examined in a human parenchymal hepatoma (HepG2) cell line. The addition of heparin to serum-supplemented medium induced the secretion of H-TGL in a time- and concentration-dependent manner. At 5.4 micrograms/ml heparin, H-TGL levels, as determined by triacyglycerol hydrolase activity, increased 7-fold after a 44-h incubation. Heparin exposure decreased intracellular H-TGL activity from 21.3 to 4.8 nmol of oleic acids released/h/10(8) cells and increased enzyme activity in the medium from 16.2 to 165.3 nmol of oleic acids released/h/10(8) cells. The heparin-induced secretion of H-TGL was associated with increased levels of H-TGL-specific mRNA. The addition of actinomycin D or cycloheximide reversed the heparin-induced increase in H-TGL activity and mRNA. Heparin treatment did not increase the level of actin mRNA suggesting that elevated H-TGL mRNA is due to enhanced tissue-specific expression of H-TGL. Expression of apolipoprotein E, another protein involved in lipoprotein metabolism, also showed induced levels of mRNA by heparin but to a lesser extent than that for H-TGL. We conclude that heparin stimulates the de novo synthesis of H-TGL in liver parenchymal cells in vivo by influencing both transcriptional and post-transcriptional events.
Busch
S J
SJ
Merrell Dow Research Institute, Cincinnati, Ohio 45215.
Martin
G A
GA
Barnhart
R L
RL
Jackson
R L
RL
D006528:Carcinoma, Hepatocellular / Q000201:enzymology* / Q000378:metabolism; D002460:Cell Line; D002474:Cell-Free System; D003470:Culture Media; D004790:Enzyme Induction / Q000187:drug effects; D006493:Heparin / Q000494:pharmacology*; D006801:Humans; D008049:Lipase / Q000096:biosynthesis* / Q000235:genetics / Q000502:physiology; D008066:Lipolysis / Q000187:drug effects; D008113:Liver Neoplasms / Q000201:enzymology* / Q000378:metabolism; D014176:Protein Biosynthesis / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis; D014158:Transcription, Genetic / Q000187:drug effects
D003470:Culture Media; D012333:RNA, Messenger; D006493:Heparin; D008049:Lipase