Lipids up-regulate uncoupling protein 2 expression in rat hepatocytes.
OBJECTIVE
Hepatic steatosis reflects the accumulation of triglycerides and free fatty acids in hepatocytes. Although lipids and their metabolites are potentially hepatotoxic, the absence of overt injury in fatty livers suggests that adaptive responses to lipid accumulation occur. Fatty acids induce mitochondrial uncoupling proteins (UCP) 2 and 3 in muscle and fat, providing a mechanism to dispose of excessive fatty acids. Although hepatocytes do not normally express uncoupling proteins, UCP-2 is expressed in hepatocytes of genetically obese mice with fatty livers, suggesting that lipids also induce UCP-2 in hepatocytes.


METHODS
To test whether lipids up-regulate hepatocyte UCP-2, cultures of rat hepatocytes were treated with lipid emulsions, linoleic or oleic acid, and UCP-2 expression was evaluated by Northern blotting and immunocytochemistry. Because increased reactive oxygen species (ROS) production may contribute to lipid-related UCP-2 induction, the DNA-binding activity of the ROS-activated transcription factor, NF-kappaB, was measured, and the effects of tert-butyl hydroperoxide (TBHP) and glutathione (GSH) on UCP-2 induction were also assessed.


RESULTS
Lipid emulsions increased the DNA-binding activity of NF-kappaB and resulted in a dose- and time-dependent induction of UCP-2 transcripts in cultured hepatocytes; after 24 hours, UCP-2 messenger RNA levels were increased 4.5-fold, and increased UCP-2 protein was shown by immunocytochemistry. Consistent with the possibility that ROS generated intracellularly during lipid metabolism participates in UCP-2 induction, addition of the cell-impermeable antioxidant GSH did not alter lipid-related induction of UCP-2. Furthermore, TBHP, which is known to increase hepatocyte mitochondrial ROS production, also increased UCP-2 messenger RNA levels.


CONCLUSIONS
Lipids increase ROS and induce UCP-2 in hepatocytes. Thus, the liver may adapt to an excessive supply of lipid substrates by inducing UCP-2 to facilitate substrate disposal while constraining ROS production.
Cortez-Pinto
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Department of Medicine, The Johns Hopkins University Schools of Medicine and Public Health and Hygiene, Baltimore, Maryland, USA.
Zhi Lin
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H
Qi Yang
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Odwin Da Costa
S
S
Diehl
A M
AM
D000818:Animals; D015152:Blotting, Northern; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005217:Fat Emulsions, Intravenous / Q000494:pharmacology; D019084:Fluorescent Antibody Technique, Indirect; D015870:Gene Expression / Q000187:drug effects; D005978:Glutathione / Q000494:pharmacology; D007473:Ion Channels; D019787:Linoleic Acid / Q000494:pharmacology; D008055:Lipids / Q000502:physiology*; D008099:Liver / Q000378:metabolism*; D008297:Male; D026901:Membrane Transport Proteins; D024101:Mitochondrial Proteins; D016328:NF-kappa B / Q000378:metabolism; D019301:Oleic Acid / Q000494:pharmacology; D014176:Protein Biosynthesis; D011506:Proteins / Q000235:genetics; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D017382:Reactive Oxygen Species / Q000378:metabolism; D014475:Uncoupling Agents / Q000378:metabolism*; D000071246:Uncoupling Protein 2; D020122:tert-Butylhydroperoxide / Q000494:pharmacology
D005217:Fat Emulsions, Intravenous; D007473:Ion Channels; D008055:Lipids; D026901:Membrane Transport Proteins; D024101:Mitochondrial Proteins; D016328:NF-kappa B; D011506:Proteins; D012333:RNA, Messenger; D017382:Reactive Oxygen Species; C000604390:Ucp2 protein, mouse; C000604391:Ucp2 protein, rat; D014475:Uncoupling Agents; D000071246:Uncoupling Protein 2; D019301:Oleic Acid; D020122:tert-Butylhydroperoxide; D019787:Linoleic Acid; D005978:Glutathione