Lipids up-regulate uncoupling protein 2 expression in rat hepatocytes

Background & Aims: Hepatic steatosis reflects the accumulation of triglycer ides and free fatty acids in hepatocytes. Although lipids and their metabol ites are potentially hepatotoxic, the absence of overt injury in fatty live rs suggests that adaptive responses to lipid accumulation occur. Fatty acid s induce mitochondrial uncoupling proteins (UCP) 2 and 3 in muscle and fat, providing a mechanism to dispose of excessive fatty acids. Although hepato cytes do not normally express uncoupling proteins, UCP-2 is expressed in he patocytes of genetically obese mice with fatty livers, suggesting that lipi ds also induce UCP-2 in hepatocytes. Methods: To test whether lipids up-reg ulate hepatocyte UCP-2, cultures of rat hepatocytes were treated with lipid emulsions, linoleic or oleic acid, and UCP-2 expression was evaluated by N orthern 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-kapp a B, was measured, and the effects of tert-butyl hydroperoxide (TBHP) and g lutathione (GSH) on UCP-2 induction were also assessed. Results: Lipid emul sions increased the DNA-binding activity of NF-kappa B and resulted in a do se- and time-dependent induction of UCP-2 transcripts in cultured hepatocyt es; 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 antioxida nt GSH did not alter lipid-related induction of UCP-2. Furthermore, TBHP, w hich is known to increase hepatocyte mitochondrial ROS production, also inc reased UCP-2 messenger RNA levels. Conclusions: Lipids increase ROS and ind uce UCP-2 in hepatocytes. Thus, the liver may adapt to an excessive supply of lipid substrates by inducing UCP-2 to facilitate substrate disposal whil e constraining ROS production.