The nuclear receptor liver X receptor (LXR) ␣, an important regulator of cholesterol and bile acid metabolism, was analyzed after insulin stimulation in liver in vitro and in vivo. A time-and dose-dependent increase in LXR␣ steady-state mRNA level was seen after insulin stimulation of primary rat hepatocytes in culture. A maximal induction of 10-fold was obtained when hepatocytes were exposed to 400 nM insulin for 24 h. Cycloheximide, a potent inhibitor of protein synthesis, prevented induction of LXR␣ mRNA expression by insulin, indicating that the induction is dependent on de novo synthesis of proteins. Stabilization studies using actinomycin D indicated that insulin stimulation increased the half-life of LXR␣ transcripts in cultured primary hepatocytes. Complementary studies where rats and mice were injected with insulin induced LXR␣ mRNA levels and confirmed our in vitro studies. Furthermore, deletion of both the LXR␣ and LXR␤ genes (double knockout) in mice markedly suppressed insulin-mediated induction of an entire class of enzymes involved in both fatty acid and cholesterol metabolism. The discovery of insulin regulation of LXR in hepatic tissue as well as gene targeting studies in mice provide strong evidence that LXRs plays a central role not only in cholesterol homeostasis, but also in fatty acid metabolism. Furthermore, LXRs appear to be important insulin-mediating factors in regulation of lipogenesis.Insulin plays a major role in the regulation of carbohydrate and lipid metabolism in the liver, adipose tissue, and muscle. Hepatic fatty acid oxidation, lipogenesis, and glycerolipid synthesis are subject to regulation by insulin (for review, see Ref.

Fatty acids and the peroxisomal proliferator, 3-tetradecylthioacetic acid (TTA) stimulate transcription of peroxisomal P-oxidation enzymes. Recently, we have shown that their actions are markedly modulated by dexamethasone and insulin which show synergistic and inhibitory effects, respectively. In this study, we describe the regulation of the peroxisomal proliferator-activated receptor (PPAR), a member of the steroid-hormone-receptor superfamily, in a similar manner by hormones and fatty acids, supporting the hypothesis that PPAR may act as a ligand-activated transcription factor. Northem-blot analysis of steady-state mRNA levels revealed three different specific transcripts for PPAR of 10.2, 4.6 and 1.8 kb, and the former two being regulated in hepatic tissue, hepatocytes and hepatoma cells. Dexamethasone produced a pronounced overall stimulatory effect (15.3-fold) in rat hepatocytes, while insulin blocked this action completely. Minor inductions of PPAR mRNA (up to twofold induction) were observed when different fatty acids were administrated alone. However, in combination with dexamethasone, additive or synergistic actions, mounting to 24-fold stimulation, were observed, while insulin always exerted an over-riding down-regulatory effect. In non-fasting rats receiving dexamethasone, elevation of serum insulin, a slight increase in serum free fatty acids accompanied by PPAR mRNA level increases of 2.4-fold and stimulation of liver peroxisomal acyl-CoA oxidase mRNA were observed. Our results suggest that PPAR mRNA expression is under strict hormonal control and that the fatty acids and hormones affect PPAR mRNA levels in a manner analogous to the regulation of the peroxisomal P-oxidation enzymes. The PPAR gene-regulating unit apparently contains hormone-response elements (HRE) for dexamethasone and insulin, which are thus functionally important for PPAR transcription in liver cells, making a significant enhancement or inhibition of the physiological actions of fatty acids possible.We have previously shown that normal fatty acids and the peroxisomal proliferator 3-tetradecylthioacetic acid (TTA) separately, and especially in combination with dexamethasone, have a strong stimulatory effect on peroxisomal /?-oxidation enzyme activities and mRNA and protein levels in hepatoma cells and hepatocytes in culture [l -41. All three peroxisomal P-oxidation enzymes were induced by fatty acids in a distally progressive manner, but these actions could be obliterated by insulin [2, 31. Since the intracellular mediator(s) for these effects idare unknown, we raised the question as to whether or not the peroxisomal-proliferator-activated receptor (PPAR) [5] could be involved and, if so, wether or not it is regulated by fatty acid, TTA and the two hormones,Correspondence to H. Nebb Sgrensen, Institute of Medical Biochemistry, University of Oslo, P. 0. Box 1112, Blindern, N-0317 Oslo, NorwayAbbreviations. PPAR, peroxisomal proliferator-activated receptor; PPRE, PPAR-responsive element; TTA, tetradecylthioacetic acid; RAR, retinoic...

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LXR alpha (liver X receptor, also called RLD-1) is a nuclear receptor, highly expressed in tissues that play a role in lipid homeostasis. In this report we show that fatty acids are positive regulators of LXR alpha gene expression and we investigate the molecular mechanisms underlying this regulation. In cultured rat hepatoma and primary hepatocyte cells, fatty acids and the sulfur-substituted fatty acid analog, tetradecylthioacetic acid, robustly induce LXR alpha (up to 3.5- and 7-fold, respectively) but not LXR beta (also called OR-1) mRNA steady state levels, with unsaturated fatty acids being more effective than saturated fatty acids. RNA stability and nuclear run-on studies demonstrate that changes in the transcription rate of the LXR alpha gene account for the major part of the induction of LXR alpha mRNA levels. A similar induction of protein level was also seen after treatment of primary hepatocytes with the same fatty acids. Consistent with such a transcriptional effect, transient transfection studies with a luciferase reporter gene, driven by 1.5 kb of the 5'-flanking region of the mouse (m)LXR alpha gene, show a peroxisome proliferator-activated receptor-alpha-dependent increase in luciferase activity upon treatment with tetradecylthioacetic acid and the synthetic peroxisome proliferator-activated receptor-alpha activator, Wy 14.643, suggesting that the mLXR alpha 5'-flanking region contains the necessary sequence elements for fatty acid responsiveness. In addition, in vivo LXR alpha expression was induced by fatty acids, consistent with the in vitro cell culture data. These observations demonstrate that LXR alpha expression is controlled by fatty acid signaling pathways and suggest an important cross-talk between fatty acid and cholesterol regulation of lipid metabolism.

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