Identification of a novel set of genes regulated by a unique liver X receptor-alpha -mediated transcription mechanism

Liver X receptors and skeleton: Current state-of-knowledge

The liver X receptors (LXR) is a nuclear receptor that acts as a prominent regulator of lipid homeostasis and inflammatory response. Its therapeutic effectiveness against various diseases like Alzheimer's disease and atherosclerosis has been investigated in detail. Emerging pieces of evidence now reveal that LXR is also a crucial modulator of bone remodeling. However, the molecular mechanisms underlying the pharmacological actions of LXR on the skeleton and its role in osteoporosis are poorly understood. Therefore, in the current review, we highlight LXR and its actions through different molecular pathways modulating skeletal homeostasis. The studies described in this review propound that LXR in association with estrogen, PTH, PPARγ, RXR hedgehog, and canonical Wnt signaling regulates osteoclastogenesis and bone resorption. It regulates RANKL-induced expression of c-Fos, NFATc1, and NF-κB involved in osteoclast differentiation. Additionally, several studies suggest suppression of RANKL-induced osteoclast differentiation by synthetic LXR ligands. Given the significance of modulation of LXR in various physiological and pathological settings, our findings indicate that therapeutic targeting of LXR might potentially prevent or treat osteoporosis and improve bone quality.

Sox6 as a new modulator of renin expression in the kidney

Juxtaglomerular (JG) cells, major sources of renin, differentiate from metanephric mesenchymal cells that give rise to JG cells or a subset of smooth muscle cells of the renal afferent arteriole. During periods of dehydration and salt deprivation, renal mesenchymal stromal cells (MSCs) differentiate from JG cells. JG cells undergo expansion and smooth muscle cells redifferentiate to express renin along the afferent arteriole. Gene expression profiling comparing resident renal MSCs with JG cells indicates that the transcription factor Sox6 is highly expressed in JG cells in the adult kidney. In vitro, loss of Sox6 expression reduces differentiation of renal MSCs to renin-producing cells. In vivo, Sox6 expression is upregulated after a low-Na⁺ diet and furosemide. Importantly, knockout of Sox6 in Ren1d+ cells halts the increase in renin-expressing cells normally seen during a low-Na⁺ diet and furosemide as well as the typical increase in renin. Furthermore, Sox6 ablation in renin-expressing cells halts the recruitment of smooth muscle cells along the afferent arteriole, which normally express renin under these conditions. These results support a previously undefined role for Sox6 in renin expression.

Adult renal mesenchymal stem cell-like cells contribute to juxtaglomerular cell recruitment

The renin-angiotensin-aldosterone system (RAAS) regulates BP and salt-volume homeostasis. Juxtaglomerular (JG) cells synthesize and release renin, which is the first and rate-limiting step in the RAAS. Intense pathologic stresses cause a dramatic increase in the number of renin-producing cells in the kidney, termed JG cell recruitment, but how this occurs is not fully understood. Here, we isolated renal CD44(+) mesenchymal stem cell (MSC)-like cells and found that they differentiated into JG-like renin-expressing cells both in vitro and in vivo. Sodium depletion and captopril led to activation and differentiation of these cells into renin-expressing cells in the adult kidney. In summary, CD44(+) MSC-like cells exist in the adult kidney and can differentiate into JG-like renin-producing cells under conditions that promote JG cell recruitment.

Nuclear receptors and their selective pharmacologic modulators

Nuclear receptors are ligand-activated transcription factors and include the receptors for steroid hormones, lipophilic vitamins, sterols, and bile acids. These receptors serve as targets for development of myriad drugs that target a range of disorders. Classically defined ligands that bind to the ligand-binding domain of nuclear receptors, whether they are endogenous or synthetic, either activate receptor activity (agonists) or block activation (antagonists) and due to the ability to alter activity of the receptors are often termed receptor "modulators." The complex pharmacology of nuclear receptors has provided a class of ligands distinct from these simple modulators where ligands display agonist/partial agonist/antagonist function in a tissue or gene selective manner. This class of ligands is defined as selective modulators. Here, we review the development and pharmacology of a range of selective nuclear receptor modulators.

Interplay between cholesterol and drug metabolism

Cholesterol biosynthetic and metabolic pathways contain several branching points towards physiologically active molecules, such as coenzyme Q, vitamin D, glucocorticoid and steroid hormones, oxysterols, or bile acids. Sophisticated regulatory mechanisms are involved in maintenance of the homeostasis of not only cholesterol but also other cholesterogenic molecules. In addition to endogenous cues, cholesterol homeostasis needs to accommodate also to exogenous cues that are imported into the body, such as chemicals and medications. Steroid and nuclear receptors together with sterol regulatory element-binding protein (SREBP) mediate the fine tuning of biosynthetic and metabolic routes as well as transports of cholesterol and its derivatives. Similarly, drug/xenobiotic metabolism is the subject to the feedback regulation of cytochrome P450 enzymes and transporters. The regulatory mechanisms that maintain the homeostasis of cholesterogenic molecules and are involved in drug metabolism share similarities. Cholesterol and cholesterogenic compounds (bile acids, glucocorticoids, vitamin D, etc.) regulate the xenosensor signaling in drug-mediated induction of the major drug-metabolizing cytochrome P450 enzymes. The key cellular receptors, pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and glucocorticoid receptor (GR) provide a functional cross-talk between the pathways maintaining cholesterol homeostasis and controlling the expression of drug-metabolizing enzymes. These receptors serve as metabolic sensors, resulting in a coordinate regulation of cholesterogenic compounds metabolism and of the defense against xenobiotic and endobiotic toxicity. Herein we present a comprehensive review of functional interactions between cholesterol homeostasis and drug metabolism involving the main nuclear and steroid receptors.

Mesenchymal stem cells differentiate into renin-producing juxtaglomerular (JG)-like cells under the control of liver X receptor-alpha

Renin is a key enzyme for cardiovascular and renal homeostasis and is produced by highly specialized endocrine cells in the kidney, known as juxtaglomerular (JG) cells. The nature and origin of these cells remain as mysteries. Previously, we have shown that the nuclear hormone receptor liver X receptor-alpha (LXRalpha) is a major transcriptional regulator of the expression of renin, c-myc, and other genes involved with growth/differentiation. In this study we test the hypothesis that LXRalpha plays an important role not only in renin expression but also in renin-containing cell differentiation, specifically from the mesenchymal stem cell (MSC), which may be the origin of the JG cell. Indeed, our data demonstrated that LXRalpha activation by its ligands or cAMP stimulated renin gene expression in both murine and human MSCs. Furthermore, sustained cAMP stimulation of murine MSCs overexpressing LXRalpha led to their differentiation into JG-like cells expressing renin and alpha-smooth muscle actin. These MSC-derived JG-like cells contained renin in secretory granules and released active renin in response to cAMP. In conclusion, the activation of LXRalpha stimulates renin expression and induces MSCs differentiation into renin-secreting, JG-like cells. Our results suggest that the MSC may be the origin of the juxtaglomerular cell and provide insight into novel understanding of pathophysiology of the renin-angiotensin system.

ADD1/SREBP1c activates the PGC1-alpha promoter in brown adipocytes

Cold adaptation elicits a paradoxical simultaneous induction of fatty acid synthesis and beta-oxidation in brown adipose tissue. We show here that cold exposure coordinately induced liver X receptor alpha (LXRalpha), adipocyte determination and differentiation-dependent factor 1 (ADD1)/sterol regulatory element-binding protein-1c (SREBP1c) and peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC1alpha) in brown and inguinal white adipose tissues, but not in epididymal white adipose tissue. Using in vitro models of white and brown adipocytes we demonstrate that beta-adrenergic stimulation induced expression of LXRalpha, ADD1/SREBP1c and PGC1alpha in cells with a brown-like adipose phenotype. We demonstrate that ADD1/SREBP1c is a powerful inducer of PGC1alpha expression via a conserved E box in the proximal promoter and that beta-adrenergic stimulation led to recruitment of ADD1/SREBP1c to this E box. The ability of ADD1/SREBP1c to activate the PGC1alpha promoter exhibited a striking cell type dependency, suggesting that additional cell type-restricted factors contribute to ADD1/SREBP1c-mediated activation. In conclusion, our data demonstrate a novel role of ADD1/SREBP1c as a regulator of PGC1alpha expression in brown adipose tissue.

Liver X receptors alpha and beta regulate renin expression in vivo

The renin-angiotensin-aldosterone system controls blood pressure and salt-volume homeostasis. Renin, which is the first enzymatic step of the cascade, is critically regulated at the transcriptional level. In the present study, we investigated the role of liver X receptor alpha (LXR(alpha)) and LXR(beta) in the regulation of renin. In vitro, both LXRs could bind to a noncanonical responsive element in the renin promoter and regulated renin transcription. While LXR(alpha) functioned as a cAMP-activated factor, LXR(beta) was inversely affected by cAMP. In vivo, LXRs colocalized in juxtaglomerular cells, in which LXR(alpha) was specifically enriched, and interacted with the renin promoter. In mouse models, renin-angiotensin activation was associated with increased binding of LXR(alpha) to the responsive element. Moreover, acute administration of LXR agonists was followed by upregulation of renin transcription. In LXR(alpha) mice, the elevation of renin triggered by adrenergic stimulation was abolished. Untreated LXR(beta) mice exhibited reduced kidney renin mRNA levels compared with controls. LXR(alpha)LXR(beta) mice showed a combined phenotype of lower basal renin and blunted adrenergic response. In conclusion, we show herein that LXR(alpha) and LXR(beta) regulate renin expression in vivo by directly interacting with the renin promoter and that the cAMP/LXR(alpha) signaling pathway is required for the adrenergic control of the renin-angiotensin system.

Profile of changes in gene expression in cultured hippocampal neurones evoked by the GABAB receptor agonist baclofen

Metabotropic gamma-aminobutyric acid receptors (GABA(B)Rs) play a critical role in inhibitory synaptic transmission in the hippocampus. However, little is known about a possible long-term effect requiring transcriptional changes. Here, using microarray technology and RT-PCR of RNA from cultured rat embryonic hippocampal neurones, we report the profile of genes that are up- or downregulated by activation of GABA(B)Rs by baclofen but are not changed by baclofen in the presence of the GABA(B)R antagonist CGP-55845A. Our data show, for the first time, regulation of transcription of defined mRNAs after specific GABA(B) receptor activation. The identified genes can be grouped into those encoding signal transduction, endocytosis/trafficking, and structural classes of proteins. For example, butyrylcholinesterase, brain-derived neurotrophic factor, and COPS5 (Jab1) genes were upregulated, whereas Rab8 interacting protein and Rho GTPase-activating protein 4 were downregulated. These results provide important baseline genomic data for future studies aimed at investigating the long-term effects of GABA(B)R activation in neurones such as their roles in neuronal growth, pathway formation and stabilization, and synaptic plasticity.

Profile of changes in gene expression in cultured hippocampal neurones evoked by the GABAB receptor agonist baclofen

Metabotropic gamma-aminobutyric acid receptors (GABA(B)Rs) play a critical role in inhibitory synaptic transmission in the hippocampus. However, little is known about a possible long-term effect requiring transcriptional changes. Here, using microarray technology and RT-PCR of RNA from cultured rat embryonic hippocampal neurones, we report the profile of genes that are up- or downregulated by activation of GABA(B)Rs by baclofen but are not changed by baclofen in the presence of the GABA(B)R antagonist CGP-55845A. Our data show, for the first time, regulation of transcription of defined mRNAs after specific GABA(B) receptor activation. The identified genes can be grouped into those encoding signal transduction, endocytosis/trafficking, and structural classes of proteins. For example, butyrylcholinesterase, brain-derived neurotrophic factor, and COPS5 (Jab1) genes were upregulated, whereas Rab8 interacting protein and Rho GTPase-activating protein 4 were downregulated. These results provide important baseline genomic data for future studies aimed at investigating the long-term effects of GABA(B)R activation in neurones such as their roles in neuronal growth, pathway formation and stabilization, and synaptic plasticity.

Nuclear receptor LXRalpha is involved in cAMP-mediated human renin gene expression

The cAMP-signaling pathway plays a crucial role in the regulation of the renin gene, but the mechanism involved remains poorly understood. We have focused our studies of renin gene regulation on the unique cAMP responsive element (huREN/CNRE, -135 to -107) in the human renin promoter. We have cloned a protein that binds to this unique CNRE and demonstrated that this protein is liver X receptor-alpha (LXRalpha), a transcriptional factor of the nuclear receptor family. Transient expression of LXRalpha in human renin-producing Calu-6 cells increased cAMP inducibility of human renin promoter. Similarly, LXRalpha-stably transfected Calu-6 cells exhibited increased cAMP inducibility of renin promoter as well as the endogenous renin gene. Site-directed mutation of huREN/CNRE, which disrupted LXRalpha binding, decreased cAMP-induced transcriptional activity of human renin promoter. Furthermore, we demonstrated that the binding of LXRalpha derived from human juxtaglomerular cells, the main production site of renin in the kidney, to the huREN/CNRE in vivo. These results suggest that LXRalpha plays an important role in the cAMP-mediated regulation of human renin gene transcription by binding to CNRE.

Fibulins: physiological and disease perspectives

The fibulins are a family of proteins that are associated with basement membranes and elastic extracellular matrix fibres. This review summarizes findings from studies of animal models of fibulin deficiency, human fibulin gene mutations, human tumours and injury models that have advanced our understanding of the normal and pathological roles of members of this formerly obscure family.

Putative metabolic effects of the liver X receptor (LXR)

The nuclear receptors liver X receptor (LXR)alpha and LXRbeta are sensors of cholesterol metabolism and lipid biosynthesis. They have recently been found to be regulators of inflammatory cytokines, suppressors of hepatic glucose production, and involved in different cell-signaling pathways. LXRalpha is a target gene of the peroxisome proliferator-activated receptor-gamma, a target of drugs used in treating elevated levels of glucose seen in diabetes. Furthermore, insulin induces LXRalpha in hepatocytes, resulting in increased expression of lipogenic enzymes and suppression of key enzymes in gluconeogenesis, including PEPCK. LXR seems to have an important role in the regulation of glucocorticoid action and a role in the overall energy homeostasis suggested by its putative regulatory effect on leptin and uncoupling protein 1. The physiological roles of LXR indicate that it is an interesting potential target for drug treatment of diabetes.

Crystal structure of the heterodimeric complex of LXRalpha and RXRbeta ligand-binding domains in a fully agonistic conformation

The nuclear receptor heterodimers of liver X receptor (LXR) and retinoid X receptor (RXR) are key transcriptional regulators of genes involved in lipid homeostasis and inflammation. We report the crystal structure of the ligand-binding domains (LBDs) of LXRalpha and RXRbeta complexed to the synthetic LXR agonist T-0901317 and the RXR agonist methoprene acid (Protein Data Base entry 1UHL). Both LBDs are in agonist conformation with GRIP-1 peptides bound at the coactivator binding sites. T-0901317 occupies the center of the LXR ligand-binding pocket and its hydroxyl head group interacts with H421 and W443, residues identified by mutational analysis as critical for ligand-induced transcriptional activation by T-0901317 and various endogenous oxysterols. The topography of the pocket suggests a common anchoring of these oxysterols via their 22-, 24- or 27-hydroxyl group to H421 and W443. Polyunsaturated fatty acids act as LXR antagonists and an E267A mutation was found to enhance their transcriptional inhibition. The present structure provides a powerful tool for the design of novel modulators that can be used to characterize further the physiological functions of the LXR-RXR heterodimer.