Karine Gauthier

Regulation of Lipoprotein Lipase by the Oxysterol Receptors, LXRα and LXRβ

Lipoprotein lipase (LPL) is a key enzyme for lipoprotein metabolism and is responsible for hydrolysis of triglycerides in circulating lipoproteins, releasing free fatty acids to peripheral tissues. In liver, LPL is also believed to promote uptake of high density lipoprotein (HDL)-cholesterol and thereby facilitate reverse cholesterol transport. In this study we show that the Lpl gene is a direct target of the oxysterol liver X receptor, LXRα. Mice fed diets containing high cholesterol or an LXR-selective agonist exhibited a significant increase in LPL expression in the liver and macrophages, but not in other tissues (e.g. adipose and muscle). Studies inLxr-deficient mice confirmed that this response was dependent more on the presence of LXRα than LXRβ. Analysis of theLpl gene revealed the presence of a functional DR4 LXR response element in the intronic region between exons 1 and 2. This response element directly binds rexinoid receptor (RXR)/LXR heterodimers and is sufficient for rexinoid- and LXR agonist-induced transcription of the Lpl gene. Together, these studies further distinguish the roles of LXRα and β and support a growing body of evidence that LXRs function as key regulators of lipid metabolism and are anti-atherogenic.

Identification of bile acid precursors as endogenous ligands for the nuclear xenobiotic pregnane X receptor

Sterol 27-hydroxylase (CYP27A1) is required for bile acid synthesis by both the classical and alternate pathways. Cyp27a1−/− mice exhibit a dramatic increase in the activity of cytochrome P450 3A (CYP3A), which catalyzes side-chain hydroxylations of bile acid intermediates, thereby facilitating their excretion in the bile and urine. We examine the role of the nuclear xenobiotic receptor PXR (pregnane X receptor) in this process. We demonstrate that expression of Cyp3a11 and other established PXR target genes is increased in the Cyp27a1−/− mice. WhenCyp27a1−/− mice are fed a diet containing either cholic acid or chenodeoxycholic acid, expression of CYP7A1, which catalyzes the rate-limiting step in bile acid biosynthesis, is strongly suppressed. In parallel, the induction of Cyp3a11 observed in these mice is reversed, suggesting that bile acid intermediates serve as PXR activators. In support of this hypothesis, three potentially toxic sterols (7α-hydroxy-4-cholesten-3-one, 5β-cholestan-3α,7α,12α-triol, and 4-cholesten-3-one), including two that are known to accumulate in Cyp27a1−/− mice, are efficacious activators of mouse PXR. All three compounds are more potent activators of mouse PXR than of human PXR, which may explain in part why humans who lack functional CYP27A1 do not display a corresponding increase in CYP3A activity and are stricken with the disease cerebrotendinous xanthomatosis. Taken together, these results reveal the existence of a feedforward regulatory loop by which potentially toxic bile acid intermediates activate PXR and induce their own metabolism. In addition, this study demonstrates that animal models with alterations in gene expression can be used to identify endogenous ligands for orphan nuclear receptors.

Functional Interference between Thyroid Hormone Receptor α (TRα) and Natural Truncated TRΔα Isoforms in the Control of Intestine Development

ABSTRACT Thyroid hormone is known to participate in the control of intestine maturation at weaning. Its action is mediated by the thyroid hormone nuclear receptors, encoded by the TRα andTRβ genes. Since previous studies have shown thatTRβ plays a minor role in the gut, we focused here our analysis on the TRα gene. The TRα locus generates the TRα1 receptor together with the splicing variant TRα2 and the truncated products TRΔα1 and TRΔα2, which all lack an intact ligand binding domain. The TRΔα isoforms are transcribed from an internal promoter located in intron 7, and their distribution is restricted to a few tissues including those of the intestine. In order to define the functions of the different isoforms encoded by theTRα locus in the intestinal mucosa, we produced mice either lacking all known TRα products or harboring a mutation which inactivates the intronic promoter. We performed a detailed analysis of the intestinal phenotypes in these mice and compared it to that of the previously described TRα−/− mice, in which TRα isoforms are abolished but the TRΔα isoforms remain. This comparative analysis leads us to the following conclusions: (i) the TRα1 receptor mediates the T3-dependent functions in the intestine at weaning time and (ii) the TRΔα products negatively control the responsiveness of the epithelial cells to T3. Moreover, we show that TRΔα proteins can interfere with the transcription of the intestine-specific homeobox genes cdx1 and cdx2and that their activity is regulated by TRα1. Altogether these data demonstrate that cooperation of TRα and TRΔα products is essential to ensure the normal postnatal development of the intestine and that mutations in the TRα locus can generate different phenotypes caused by the disruption of the equilibrium between these products.

Oxysterol nuclear receptor LXRβ regulates cholesterol homeostasis and contractile function in mouse uterus

The uterus is an organ where lipid distribution plays a critical role for its function. Here we show that nuclear receptor for oxysterols LXRβ prevents accumulation of cholesteryl esters in mouse myometrium by controlling expression of genes involved in cholesterol efflux and storage (abca1 and abcg1). Upon treatment with an LXR agonist that mimics activation by oxysterols, expression of these target genes was increased in wild-type mice, whereas under basal conditions, lxrα;β-/- mice exhibited a marked decrease in abcg1 accumulation. This change resulted in a phenotype of cholesteryl ester accumulation. Besides, a defect of contractile activity induced by oxytocin or PGF2α was observed in mice lacking LXRβ. These results imply that LXRβ provides a safety valve to limit cholesteryl ester levels as a basal protective mechanism in the uterus against cholesterol accumulation and is necessary for a correct induction of contractions.

Interference of a Mutant Thyroid Hormone Receptor α1 with Hepatic Glucose Metabolism

Mice expressing the mutant thyroid hormone receptor TRalpha1R384C, which has a 10-fold reduced affinity to the ligand T(3), exhibit hypermetabolism due to an overactivation of the sympathetic nervous system. To define the consequences in the liver, we analyzed hepatic metabolism and the regulation of liver genes in the mutant mice. Our results showed that hepatic phosphoenolpyruvate-carboxykinase was up-regulated and pyruvate kinase mRNA down-regulated, contrary to what observed after T(3) treatment. In contrast, mice expressing a mutant TRalpha1L400R specifically in the liver did not show a dysregulation of these genes; however, when the TRalpha1L400R was expressed ubiquitously, the hepatic phenotype differed from TRalpha1R384C animals, suggesting that the localization of the mutation plays an important role for its consequences on glucose metabolism. Furthermore, we observed that glycogen stores were completely depleted in TRalpha1R384C animals, despite increased gluconeogenesis and decreased glycolysis. Exposure of the mutant mice to high maternal levels of thyroid hormone during fetal development leads to a normal liver phenotype in the adult. Our results show how genetic and maternal factors interact to determine the metabolic setpoint of the offspring and indicate an important role for maternal thyroid hormone in the susceptibility to metabolic disorders in adulthood.

TRα inhibits arterial renin-angiotensin system expression and prevents cholesterol accumulation in vascular smooth muscle cells

OBJECTIVESnThe tissue renin-angiotensin system (tRAS) plays a key role in the maintenance of cellular homeostasis but is also implicated in atherosclerosis. Thyroid hormone (TH) contributes, via genomic effects, to control of tRAS gene expression in the arterial wall and vascular smooth muscle cells (VSMCs). We investigated the specific functions of TH receptors-α and -β (TRα and TRβ) on tRAS gene expression in the aorta and VSMCs, and the potential protective effect of TRα against atherosclerosis.nnnMATERIAL AND METHODSnUsing aorta and cultured aortic VSMCs from TRα and TRβ deficient mice, tRAS gene expression was analyzed by determining mRNA levels on real-time PCR. Gene regulation under cholesterol loading mimicking atherosclerosis conditions was also examined in VSMCs in vitro.nnnRESULTSnTRα deletion significantly increased expression of angiotensinogen (AGT) and angiotensin II receptor type 1 subtype a (AT1Ra) at transcriptional level in aorta, a tissue with high TRα expression level. TRα activity thus seems to be required for maintenance of physiological levels of AGTand AT1Raexpression in the arterial wall. In addition, during cholesterol loading, TRα deletion significantly increased cholesterol content in VSMCs, with a weaker decrease in AGTexpression.nnnCONCLUSIONnTRα seems to have an inhibitory impact on AGTand AT1Raexpression, and loss of TRα function in TRα0/0 mice increases tRAS expression in the aortic wall. More importantly, TRα deletion significantly increases VSMC cholesterol content. Our results are consistent with a protective role of TRα against atherosclerosis.