Absence of peroxisomes in mouse hepatocytes causes mitochondrial and ER abnormalities

Peroxisome deficiency in men causes severe pathology in several organs, particularly in the brain and liver, but it is still unknown how metabolic abnormalities trigger these defects. In the present study, a mouse model with hepatocyte-selective elimination of peroxisomes was generated by inbreeding Pex5-loxP and albumin-Cre mice to investigate the consequences of peroxisome deletion on the functioning of hepatocytes. Besides the absence of catalase-positive peroxisomes, multiple ultrastructural alterations were noticed, including hepatocyte hypertrophy and hyperplasia, smooth endoplasmic reticulum proliferation, and accumulation of lipid droplets and lysosomes. Most prominent was the abnormal structure of the inner mitochondrial membrane, which bore some similarities with changes observed in Zellweger patients. This was accompanied by severely reduced activities of complex I, III, and V and a collapse of the mitochondrial inner membrane potential. Surprisingly, these abnormalities provoked no significant disturbances of adenosine triphosphate (ATP) levels and redox state of the liver. However, a compensatory increase of glycolysis as an alternative source of ATP and mitochondrial proliferation were observed. No evidence of oxidative damage to proteins or lipids nor elevation of oxidative stress defence mechanisms were found. Altered expression of peroxisome proliferator-activated receptor alpha (PPAR-alpha) regulated genes indicated that PPAR-alpha is activated in the peroxisome-deficient cells. In conclusion, the absence of peroxisomes from mouse hepatocytes has an impact on several other subcellular compartments and metabolic pathways but is not detrimental to the function of the liver parenchyma. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Species-specific mechanisms for cholesterol 7alpha-hydroxylase (CYP7A1) regulation by drugs and bile acids

The gene encoding cholesterol 7alpha-hydroxylase (CYP7A1) is tightly regulated in order to control intrahepatic cholesterol and bile acid levels. Ligands of the xenobiotic-sensing pregnane X receptor inhibit CYP7A1 expression. To retrace the evolution of the molecular mechanisms underlying CYP7A1 inhibition, we used a chicken hepatoma cell system that retains the ability to be induced by phenobarbital and other drugs. Whereas bile acids regulate CYP7A1 via small heterodimer partner and liver receptor homolog-1, mRNA expression of these nuclear receptors is unchanged by xenobiotics. Instead, drugs repress chicken hepatic nuclear factor 4alpha (HNF4alpha) transcript levels concomitant with a reduction in CYP7A1 expression. Importantly, no reduction of HNF4alpha levels is found in mouse liver in vivo and in human primary hepatocyte cultures, respectively. Thus, besides the importance of HNF4alpha in CYP7A1 regulation in all species, birds and mammals use different signaling pathways to adjust CYP7A1 levels after exposure to xenobiotics.

Combined loss of orphan receptors PXR and CAR heightens sensitivity to toxic bile acids in mice

Efficient detoxification of bile acids is necessary to avoid pathological conditions such as cholestatic liver damage and colon cancer. The orphan nuclear receptors PXR and CAR have been proposed to play an important role in the detoxification of xeno- and endo-biotics by regulating the expression of detoxifying enzymes and transporters. In this report, we showed that the combined loss of PXR and CAR resulted in a significantly heightened sensitivity to bile acid toxicity in a sex-sensitive manner. A regimen of lithocholic acid treatment, which was tolerated by wild-type and PXR null mice, caused a marked accumulation of serum bile acids and histological liver damage as well as an increased hepatic lipid deposition in double knockout males. The increased sensitivity in males was associated with genotype-specific suppression of bile acid transporters and loss of bile acid-mediated downregulation of small heterodimer partner, whereas the transporter suppression was modest or absent in females. The double knockout mice also exhibited gene- and tissue-specific dysregulation of PXR and CAR target genes in response to PXR and CAR agonists. In conclusion, althoughthe cross-regulation of target genes by PXR and CAR has b een proposed, the current study represents in vivo evidence of the combined loss of both receptors causing a unique pattern of gene regulation that can be translated into physiological events such as sensitivity to toxic bile acids.

Macrophage Liver X Receptor Is Required for Antiatherogenic Activity of LXR Agonists

Objective— Complications of atherosclerotic cardiovascular disease due to elevated blood cholesterol levels are the major cause of death in the Western world. The liver X receptors, LXRα and LXRβ (LXRs), are ligand-dependent transcription factors that act as cholesterol sensors and coordinately control transcription of genes involved in cholesterol and lipid homeostasis as well as macrophage inflammatory gene expression. LXRs regulate cholesterol balance through activation of ATP-binding cassette transporters that promote cholesterol transport and excretion from the liver, intestine, and macrophage. Although LXR agonists are known to delay progression of atherosclerosis in mouse models, their ability to abrogate preexisting cardiovascular disease by inducing regression and stabilization of established atherosclerotic lesions has not been addressed.

Methods and Results— We demonstrate that LXR agonist treatment increases ATP-binding cassette transporter expression within preexisting atherosclerotic lesions, resulting in regression of these lesions as well as remodeling from vulnerable to stable lesions and a reduction in macrophage content. Further, using macrophage-selective LXR-deficient mice created by bone marrow transplantation, we provide the first evidence that macrophage LXR expression is necessary for the atheroprotective actions of an LXR agonist.

Conclusions— These data substantiate that drugs targeting macrophage LXR activity may offer therapeutic benefit in the treatment of atherosclerotic cardiovascular disease.

Activation of liver X receptors and retinoid X receptors prevents bacterial-induced macrophage apoptosis

Microbe-macrophage interactions play a central role in the pathogenesis of many infections. The ability of some bacterial pathogens to induce macrophage apoptosis has been suggested to contribute to their ability to elude innate immune responses and successfully colonize the host. Here, we provide evidence that activation of liver X receptors (LXRs) and retinoid X receptors (RXRs) inhibits apoptotic responses of macrophages to macrophage colony-stimulating factor (M-CSF) withdrawal and several inducers of apoptosis. In addition, combined activation of LXR and RXR protected macrophages from apoptosis caused by infection with Bacillus anthracis, Escherichia coli, and Salmonella typhimurium. Expression-profiling studies demonstrated that LXR and RXR agonists induced the expression of antiapoptotic regulators, including AIM/CT2, Bcl-X(L), and Birc1a. Conversely, LXR and RXR agonists inhibited expression of proapoptotic regulators and effectors, including caspases 1, 4/11, 7, and 12; Fas ligand; and Dnase1l3. The combination of LXR and RXR agonists was more effective than either agonist alone at inhibiting apoptosis in response to various inducers of apoptosis, and it acted synergistically to induce expression of AIM/CT2. Inhibition of AIM/CT2 expression in response to LXR/RXR agonists partially reversed their antiapoptotic effects. These findings reveal unexpected roles of LXRs and RXRs in the control of macrophage survival and raise the possibility that LXR/RXR agonists may be exploited to enhance innate immunity to bacterial pathogens that induce apoptotic programs as a strategy for evading host responses.

Estrogen-related receptor alpha (ERRalpha) is a transcriptional regulator of apolipoprotein A-IV and controls lipid handling in the intestine

The estrogen-related receptor alpha (ERRalpha) is an orphan member of the superfamily of nuclear receptors involved in the control of energy metabolism. In particular, ERRalpha induces a high energy expenditure in the presence of the coactivator PGC-1alpha. However, ERRalpha knockout mice have reduced fat mass and are resistant to diet-induced obesity. ERRalpha is expressed in epithelial cells of the small intestine, and because the intestine is the first step in the energy chain, we investigated whether ERRalpha plays a function in dietary energy handling. Gene expression profiling in the intestine identified a subset of genes involved in oxidative phosphorylation that were down-regulated in the absence of ERRalpha. In support of the physiological role of ERRalpha in this pathway, isolated enterocytes from ERRalpha knockout mice display lower capacity for beta-oxidation. Microarray results also show altered expression of genes involved in dietary lipid digestion and absorption, such as pancreatic lipase-related protein 2 (PLRP2), fatty acid-binding protein 1 and 2 (L-FABP and I-FABP), and apolipoprotein A-IV (apoA-IV). In agreement, we found that ERRalpha-/- pups exhibit significant lipid malabsorption. We further show that the apoA-IV promoter is a direct target of ERRalpha and that its presence is required to maintain basal level but not feeding-induced regulation of the apoA-IV gene in mice. ERRalpha, in cooperation with PGC-1alpha, activates the apoA-IV promoter via interaction with the apoC-III enhancer in both human and mouse. Our results demonstrate that apoA-IV is a direct ERRalpha target gene and suggest a function for ERRalpha in intestinal fat transport, a crucial step in energy balance.

The orphan nuclear receptor constitutive active/androstane receptor is essential for liver tumor promotion by phenobarbital in mice

Hepatocellular carcinoma (HCC) is known to progress through a step often called tumor promotion. Phenobarbital (PB) is the prototype of nongenotoxic cacinogens that promote HCC in rodents. The molecular target of PB to elicit the promotion has been the subject of intense investigations over the last 30 years since it was discovered. The nuclear receptor constitutive active/androstane receptor (CAR) is activated by PB as well as by various other xenobiotics such as therapeutic drugs and environmental pollutants. CAR activation results in the transcriptional induction of numerous hepatic genes including those that encode xenobiotic-metabolizing enzymes such as a set of cytochrome P450s. In addition to PB, many CAR activators are nongenotoxic carcinogens, but the role of CAR in liver tumor promotion remains unexplored. Using Car(-/-) mice, we have here examined tumor promotion by chronic treatment with PB in drinking water after tumor initiation with a single dose of the genotoxic carcinogen diethylnitrosamine. None of the Car(-/-) mice developed either eosinophilic foci or advanced liver tumors, whereas all Car(+/+) mice developed HCC and/or adenoma by 39 weeks. The results indicate that CAR is the molecular target of promotion by PB and that activation of this receptor is an essential requirement for liver tumor development.

Genome-wide expression profiling; a panel of mouse tissues discloses novel biological functions of liver X receptors in adrenals

The liver X receptors alpha and beta (LXRalpha and LXRbeta ) are members of the nuclear receptor superfamily of proteins which are highly expressed in metabolically active tissues. They regulate gene expression of critical genes involved in cholesterol catabolism and transport, lipid and triglyceride biosynthesis and carbohydrate metabolism in response to distinct oxysterols and intermediates in the cholesterol metabolic pathway. The biological roles of the LXRs in tissues other than liver, intestine and adipose tissue are poorly elucidated. In this study we used global gene-expression profiling analysis to detect differences in expression patterns in several tissues from mice fed an LXR agonist or vehicle. Our results show that LXR plays an important role in the kidney, lung, adrenals, brain, testis and heart where several putative LXR target genes were found. The effects of the LXRs were further analysed in adrenals where treatment with an LXR agonist induced expression of adrenocorticotrophic hormone receptor, suppressed expression of uncoupling protein (UCP)-1 and UCP-3 as well as several glycolytic enzymes and led to increased serum corticosterone levels. These results indicate novel biological roles of the LXR including regulation of energy metabolism, glycolysis and steroidogenesis in the adrenals via alteration of expression profiles of putative target genes.

Regulation of phase I and phase II steroid metabolism enzymes by PPAR alpha activators

Peroxisome proliferators (PP) are a large class of structurally diverse chemicals that mediate their effects in the liver mainly through the peroxisome proliferator-activated receptor alpha (PPARalpha). Exposure to some PP results in alterations of steroid levels that may be mechanistically linked to adverse effects in reproductive organs. We hypothesized that changes in steroid levels after PP exposure are due to alterations in the levels of P450 enzymes that hydroxylate testosterone and estrogen. In testosterone hydroxylase assays, exposure to the PP, WY-14,643 (WY), gemfibrozil or di-n-butyl phthalate (DBP) led to compound-specific increases in 6beta and 16beta-testosterone and androstenedione hydroxylase activities and decreases in 16alpha, 2alpha-hydroxylase activities by all three PP. The decreases in 16alpha and 2alpha-testosterone hydroxylase activity can be attributed to a 2alpha and 16alpha- testosterone hydroxylase, CYP2C11, which we previously showed was dramatically down-regulated in these same tissues (Corton et al., 1998; Mol. Pharmacol. 54, 463-473). To explain the increases in 6beta- and 16beta-testosterone hydroxylase activities, we examined the expression of P450 family members known to carry out these functions. Alterations in the 6beta-testosterone hydroxylases CYP3A1, CYP3A2 and the 16beta-testosterone hydroxylase, CYP2B1 were observed after exposure to some PP. The male-specific estrogen sulfotransferase was down-regulated in rat liver after exposure to all PP. The mouse 6beta-testosterone hydroxylase, Cyp3a11 was down-regulated by WY in wild-type but not PPARalpha-null mice. In contrast, DEHP increased Cyp3a11 in both wild-type and PPARalpha-null mice. These studies demonstrate that PP alter the expression and activity of a number of enzymes which regulate levels of sex steroids. The changes in these enzymes may help explain why exposure to some PP leads to adverse effects in endocrine tissues that produce or are the targets of sex hormones.

Thimerosal stimulates Ca2+ flux through inositol 1,4,5-trisphosphate receptor type 1, but not type 3, via modulation of an isoform-specific Ca2+-dependent intramolecular interaction

Thiol-reactive agents such as thimerosal have been shown to modulate the Ca2+-flux properties of IP3 (inositol 1,4,5-trisphosphate) receptor (IP3R) via an as yet unidentified mechanism [Parys, Missiaen, De Smedt, Droogmans and Casteels (1993) Pflügers Arch. 424, 516-522; Kaplin, Ferris, Voglmaier and Snyder (1994) J. Biol. Chem. 269, 28972-28978; Missiaen, Taylor and Berridge (1992) J. Physiol. (Cambridge, U.K.) 455, 623-640; Missiaen, Parys, Sienaert, Maes, Kunzelmann, Takahashi, Tanzawa and De Smedt (1998) J. Biol. Chem. 273, 8983-8986]. In the present study, we show that thimerosal potentiated IICR (IP3-induced Ca2+ release) and IP3-binding activity of IP3R1, expressed in triple IP3R-knockout R23-11 cells derived from DT40 chicken B lymphoma cells, but not of IP3R3 or [D1-225]-IP3R1, which lacks the N-terminal suppressor domain. Using a 45Ca2+-flux technique in permeabilized A7r5 smooth-muscle cells, we have shown that Ca2+ shifted the stimulatory effect of thimerosal on IICR to lower concentrations of thimerosal and thereby increased the extent of Ca2+ release. This suggests that Ca2+ and thimerosal synergetically regulate IP3R1. Glutathione S-transferase pull-down experiments elucidated an interaction between amino acids 1-225 (suppressor domain) and amino acids 226-604 (IP3-binding core) of IP3R1, and this interaction was strengthened by both Ca2+ and thimerosal. In contrast, calmodulin and sCaBP-1 (short Ca2+-binding protein-1), both having binding sites in the 1-225 region, weakened the interaction. This interaction was not found for IP3R3, in agreement with the lack of functional stimulation of this isoform by thimerosal. The interaction between the IP3-binding and transmembrane domains (amino acids 1-604 and 2170-2749 respectively) was not affected by thimerosal and Ca2+, but it was significantly inhibited by IP3 and adenophostin A. Our results demonstrate that thimerosal and Ca2+ induce isoform-specific conformational changes in the N-terminal part of IP3R1, leading to the formation of a highly IP3-sensitive Ca2+-release channel.

Regulation of CYP3A4 by the bile acid receptor FXR

CYP3A4, the most abundant cytochrome P450 in human liver, is responsible for the metabolism of numerous xenobiotics and endobiotics. CYP3A4 expression is highly variable and is induced by numerous compounds of exogenous and endogenous origin, including elevated concentrations of secondary bile acids via the pregnane X receptor (PXR). We show that physiological concentrations of the primary bile acid chenodeoxycholic acid regulate the expression of CYP3A4 via the bile acid receptor FXR. Experiments performed in vitro in different cell culture systems, gel-mobility shift assays and experiments performed in vivo in transgenic mice lacking FXR or PXR and treated with the synthetic FXR agonist GW4064 were undertaken to study the implication of FXR in the regulation of CYP3A. Our data provide evidence for the presence of two functional FXR recognition sites located in a 345-bp element within the 5'-flanking region of CYP3A4. Mutational analysis of these sites and experiments in transgenic mice lacking FXR or PXR support the relevance of FXR activation for CYP3A regulation. Thus, whereas elevated concentrations of precursors of bile acids and secondary bile acids induce CYP3A via PXR, primary bile acids can modulate the expression of CYP3A via FXR. These findings may explain elevated CYP3A expression in cholestasis and part of the variability of drug responsiveness and toxicity between individuals.

Lymphocyte development and function in the absence of retinoic acid-related orphan receptor alpha

The orphan nuclear receptor, retinoid acid-related orphan receptor (ROR)alpha, is essential for the development of cerebellar Purkinje cells and bone tissue. RORalpha may also play a critical role in lymphocyte development and function because staggerer mice, a natural mutant strain with a disrupted expression of RORalpha, have reduced thymic and splenic cellularity. In this report, we analyzed the role of RORalpha in lymphocyte development by examining lymphoid compartments in RORalpha(-/-) mice and Rag-2(-/-) mice reconstituted with RORalpha(-/-) bone marrow. We found that T and B cell development was severely defective in RORalpha(-/-) mice, but not in Rag-2(-/-)/RORalpha(-/-) chimeric mice. We also analyzed cellular and humoral immune responses in Rag-2(-/-)/RORalpha(-/-) chimeric mice. Our results show that serum IgG levels were elevated in Rag-2(-/-)/RORalpha(-/-) chimeric mice after immunization with a T-dependent Ag compared with control chimeras. IFN-gamma production by RORalpha(-/-) CD8(+) T cells after TCR stimulation was also increased. Furthermore, RORalpha(-/-) mast cells and macrophages produced an increased amount of TNF-alpha and IL-6 upon activation. These results indicate that RORalpha indirectly regulates lymphocyte development by providing an appropriate microenvironment and controls immune responses by negatively regulating cytokine production in innate immune cells and lymphocytes.

PXR and the regulation of apoA1 and HDL-cholesterol in rodents

Orphan nuclear receptors (ONRs) have been implicated in the regulation of lipids. Several clinical studies conducted either prospectively or epidemiologically have pointed to a link between the regulation of hepatic CYP enzymes and HDL-cholesterol (HDL-C) and/or apolipoprotein A1 (apoA1). The treatment of rats with a series of imidazole inducers of CYP3A yielded correlations between in vitro CYP3A activity measured as erythromycin demethylase activity and plasma HDL-C and hepatic apoA1 mRNA. Similarly, a correlation was established between in vivo CYP3A activity, measured as ethosuximide clearance, and plasma HDL-C and hepatic apoA1 mRNA. The treatment of wild-type (WT) mice with PXR agonists elicited increases in serum HDL-C and serum apoA1 levels. On the other hand, the treatment of PXR-knockout mice (PXR-KOs) with the same PXR agonists failed to elicit increases in either serum HDL-C or serum apoA1 levels. Superposition of the structures of three imidazoles known to be active CYP3A inducers in rats with the human PXR pharmacophore demonstrated a partial fit and predicted EC(50) values typical of weak-moderate hPXR inducers in humans. These imidazoles have been shown to increase apoA1 and HDL-C in rats and mice. Taken together, these data suggest that PXR plays an important role in the regulation of apoA1 and HDL-C in rodents.

Unbuckling lipodystrophy from insulin resistance and hypertension

Lipodystrophy and insulin resistance are the core features of human PPARgamma deficiency states. Metabolic complications in PPARgamma deficiency, such as hypertension, have been considered to be secondary to insulin resistance. However, a new mouse model that expresses the analog of a human PPARG mutation displays minimal lipodystrophy and insulin resistance but rather severe hypertension. Furthermore, the mutant protein appears to directly modulate the renin-angiotensin system in adipose tissue, providing evidence of the pleiotropic effects of PPARgamma.

PPARalpha governs glycerol metabolism

Glycerol, a product of adipose tissue lipolysis, is an important substrate for hepatic glucose synthesis. However, little is known about the regulation of hepatic glycerol metabolism. Here we show that several genes involved in the hepatic metabolism of glycerol, i.e., cytosolic and mitochondrial glycerol 3-phosphate dehydrogenase (GPDH), glycerol kinase, and glycerol transporters aquaporin 3 and 9, are upregulated by fasting in wild-type mice but not in mice lacking PPARalpha. Furthermore, expression of these genes was induced by the PPARalpha agonist Wy14643 in wild-type but not PPARalpha-null mice. In adipocytes, which express high levels of PPARgamma, expression of cytosolic GPDH was enhanced by PPARgamma and beta/delta agonists, while expression was decreased in PPARgamma(+/-) and PPARbeta/delta(-/-) mice. Transactivation, gel shift, and chromatin immunoprecipitation experiments demonstrated that cytosolic GPDH is a direct PPAR target gene. In line with a stimulating role of PPARalpha in hepatic glycerol utilization, administration of synthetic PPARalpha agonists in mice and humans decreased plasma glycerol. Finally, hepatic glucose production was decreased in PPARalpha-null mice simultaneously fasted and exposed to Wy14643, suggesting that the stimulatory effect of PPARalpha on gluconeogenic gene expression was translated at the functional level. Overall, these data indicate that PPARalpha directly governs glycerol metabolism in liver, whereas PPARgamma regulates glycerol metabolism in adipose tissue.

Protective effect of endogenous PPARgamma against acute gastric mucosal lesions associated with ischemia-reperfusion

Acute gastric mucosal lesions (AGMLs) are an important cause of gastrointestinal bleeding. Herein, we demonstrate that peroxisome proliferator-activated receptor-gamma (PPARgamma), a member of a nuclear receptor family, functions as an endogenous anti-inflammatory pathway in a murine model of AGML induced by ischemia-reperfusion (I/R). Treatment with specific PPARgamma ligands such as BRL-49653, pioglitazone, or troglitazone was examined in a model of AGML induced by I/R. PPARgamma-deficient and wild-type mice were also examined for their response to I/R in stomach. Specific PPARgamma ligands exhibited dramatic and rapid protection against AGML formation associated with I/R in mice in a dose-dependent manner. In contrast, the AGML induced by I/R in PPARgamma-deficient mice was more severe than that observed in wild-type mice. Administration of the PPARgamma ligand significantly inhibited the upregulation of TNF-alpha, ICAM-1, inducible nitric oxide synthase, apoptosis, and nitrotyrosine formation induced by I/R in the stomach. These data indicate that an endogenous pathway associated with PPARgamma plays an important role in the pathogenesis of I/R-associated injury in the stomach.

Meclizine is an agonist ligand for mouse constitutive androstane receptor (CAR) and an inverse agonist for human CAR

The constitutive androstane receptor (CAR, NR1I3) is a key regulator of xenobiotic and endobiotic metabolism. The ligand-binding domains of murine (m) and human (h) CAR are divergent relative to other nuclear hormone receptors, resulting in species-specific differences in xenobiotic responses. Here we identify the widely used antiemetic meclizine (Antivert; Bonine) as both an agonist ligand for mCAR and an inverse agonist for hCAR. Meclizine increases mCAR transactivation in a dose-dependent manner. Like the mCAR agonist 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, meclizine stimulates binding of steroid receptor coactivator 1 to the murine receptor in vitro. Meclizine administration to mice increases expression of CAR target genes in a CAR-dependent manner. In contrast, meclizine suppresses hCAR transactivation and inhibits the phenobarbital-induced expression of the CAR target genes, cytochrome p450 monooxygenase (CYP)2B10, CYP3A11, and CYP1A2, in primary hepatocytes derived from mice expressing hCAR, but not mCAR. The inhibitory effect of meclizine also suppresses acetaminophen-induced liver toxicity in humanized CAR mice. These results demonstrate that a single compound can induce opposite xenobiotic responses via orthologous receptors in rodents and humans.

Alterations in the distribution and orexigenic effects of dexamethasone in CAR-null mice

The constitutive androstane receptor (CAR, NR1I3) has emerged as an important regulator of drug metabolism. CAR responds to a wide spectrum of xenobiotics by inducing expression of cytochrome P450 (CYP) enzymes and a number of other proteins responsible for drug metabolism in the liver. The xenosensor function of CAR overlaps with that of the pregnane X receptor (PXR), another xenobiotic receptor that belongs to the nuclear hormone superfamily. We observed that injection of dexamethasone (Dex), a ligand for the glucocorticoid receptor (GR) and PXR but not CAR, results in an unexpected twofold increase in the stomach weight of CAR-null animals relative to wild-type animals. Here, we show that CAR knockout mice have elevated levels of Dex in the brain, resulting in a more rapid and robust increase in the hypothalamic expression of the GR-responsive target genes encoding neuropeptide Y (NPY) and neuropeptide Y receptor subtype 1 (NPY-R1). As expected, this is accompanied by a higher increase in the food intake of the CAR-null animals. The data described here highlight the complexity of the overlapping functions of CAR and PXR.

PPARgamma insufficiency enhances osteogenesis through osteoblast formation from bone marrow progenitors

Based on the fact that aging is associated with a reciprocal decrease of osteogenesis and an increase of adipogenesis in bone marrow and that osteoblasts and adipocytes share a common progenitor, this study investigated the role of PPARgamma, a key regulator of adipocyte differentiation, in bone metabolism. Homozygous PPARgamma-deficient ES cells failed to differentiate into adipocytes, but spontaneously differentiated into osteoblasts, and these were restored by reintroduction of the PPARgamma gene. Heterozygous PPARgamma-deficient mice exhibited high bone mass with increased osteoblastogenesis, but normal osteoblast and osteoclast functions, and this effect was not mediated by insulin or leptin. The osteogenic effect of PPARgamma haploinsufficiency became prominent with aging but was not changed upon ovariectomy. The PPARgamma haploinsufficiency was confirmed to enhance osteoblastogenesis in the bone marrow cell culture but did not affect the cultures of differentiated osteoblasts or osteoclast-lineage cells. This study demonstrates a PPARgamma-dependent regulation of bone metabolism in vivo, in that PPARgamma insufficiency increases bone mass by stimulating osteoblastogenesis from bone marrow progenitors.

Loss of chaotic trabecular structure in OPG-deficient juvenile Paget's disease patients indicates a chaogenic role for OPG in nonlinear pattern formation of trabecular bone

The RANK-RANKL-OPG system of osteoclast regulation may play a key role in determining chaotic structure in trabecular bone. Iliac trabecular bone from juvenile Paget's disease patients deficient in functional OPG shows parallel, anisotropic structure instead of normal chaotic structure. Evidence from experimental systems suggests that RANK-RANKL-OPG controls key nonlinear "chaogenic" parameters, such as friction, forcing frequency, feedback, and boundary forcing. The RANK-RANKL-osteoprotegerin (OPG) system of osteoclast regulation may play a key role in determining chaotic structure in trabecular bone. Iliac trabecular bone from juvenile Paget's disease (JPD) patients deficient in functional OPG shows parallel, anisotropic structure instead of normal chaotic structure. Evidence from experimental systems suggests that RANK-RANKL-OPG controls key nonlinear "chaogenic" parameters, such as friction, forcing frequency, feedback, and boundary forcing. The Belousov-Zhabotinsky reaction-diffusion system, the catalytic oxidation of CO on platinum surfaces, and thermal diffusion in liquid helium allow visualization of nonlinear emergent patterns such as labyrinthine structures, turbulence, and cellular structures, all of which bear some resemblance to trabecular bone. In JPD, the gene for OPG (TNFRSF11B) is subject to an inactivating mutation, leading to increased resorption and accelerated remodeling. Histomorphometric images of iliac crest trabecular bone from teenagers suffering from JPD show a highly unusual array of parallel, regular trabecular plates, instead of the typical chaotic, fractal patterns of normal trabecular bone. Loss of OPG function is associated with a change from chaotic to regular structure, suggesting that the RANK-RANKL-OPG system is controlling key nonlinear "chaogenic" parameters. Looking at trabecular bone from the perspective of nonlinear pattern formation may help understand other phenomena, such as the marked dependence of trabecular bone's architectural and mechanical quality on remodeling rate independent of the trabecular bone mass.

Agonist-induced Ca2+ entry determined by inositol 1,4,5-trisphosphate recognition

It has been considered that Ca2+ release is the causal trigger for Ca2+ entry after receptor activation. In DT40 B cells devoid of inositol 1,4,5-trisphosphate receptors (IP3R), the lack of Ca2+ entry in response to receptor activation is attributed to the absence of Ca2+ release. We reveal in this article that IP3R recognition of IP3 determines agonist-induced Ca2+ entry (ACE), independent of its Ca2+ release activity. In DT40 IP3R(-/-) cells, endogenous ACE can be rescued with type 1 IP3R mutants (both a DeltaC-terminal truncation mutant and a D2550A pore mutant), which are defective in Ca2+ release channel activity. Thus, in response to B cell receptor activation, ACE is restored in an IP3R-dependent manner without Ca2+ store release. Conversely, ACE cannot be rescued with mutant IP3Rs lacking IP3 binding (both the Delta90-110 and R265Q IP3-binding site mutants). We conclude that an IP3-dependent conformational change in the IP3R, not endoplasmic reticulum Ca2+ pool release, triggers ACE.