Transcriptional regulation of apolipoprotein C-III gene expression by the orphan nuclear receptor RORalpha

Triglyceride-rich remnant lipoproteins are considered as major risk factors contributing to the pathogenesis of atherosclerosis. Because apolipoprotein (apo) C-III is a major determinant of plasma triglyceride and remnant lipoprotein metabolism, it is important to understand how the expression of this gene is regulated. In the present study, we identified the orphan nuclear receptor RORalpha1 as a regulator of human and mouse apo C-III gene expression. Plasma triglyceride and apo C-III protein concentrations in staggerer (sg/sg) mice, homozygous for a deletion in the RORalpha gene, were significantly lower than in wild type littermates. The lowered plasma apo C-III levels were associated with reduced apo C-III mRNA levels in liver and intestine of sg/sg mice. Transient transfection experiments in human hepatoma HepG2, human colonic CaCO2, and rabbit kidney RK13 cells demonstrated that overexpression of the human RORalpha1 isoform specifically increases human apo C-III promoter activity, indicating that RORalpha1 enhances human apo C-III gene transcription. RORalpha1 response elements were mapped by promoter deletion analysis and gel shift experiments to two AGGTCA half-sites located at positions -83/-78 (within the C3P site) and -23/-18 (downstream of the TATA box) in the human apo C-III promoter, with the -23/-18 site exhibiting the highest binding affinity. Transfection of site-directed mutated constructs in HepG2 cells indicated that the RORalpha1 effect is predominantly mediated by the -23/-18 site. This site is conserved in the mouse apo C-III gene promoter. Moreover, RORalpha binds to the equivalent mouse site and activates constructs containing three copies of the mouse site cloned in front of an heterologous promoter. Taken together, our data identify RORalpha as a transcriptional regulator of apo C-III gene expression, providing a novel, physiological role for RORalpha1 in the regulation of genes controlling triglyceride metabolism.

Peroxisome proliferator-activated receptor alpha activators improve insulin sensitivity and reduce adiposity

Fibrates and glitazones are two classes of drugs currently used in the treatment of dyslipidemia and insulin resistance (IR), respectively. Whereas glitazones are insulin sensitizers acting via activation of the peroxisome proliferator-activated receptor (PPAR) gamma subtype, fibrates exert their lipid-lowering activity via PPARalpha. To determine whether PPARalpha activators also improve insulin sensitivity, we measured the capacity of three PPARalpha-selective agonists, fenofibrate, ciprofibrate, and the new compound GW9578, in two rodent models of high fat diet-induced (C57BL/6 mice) or genetic (obese Zucker rats) IR. At doses yielding serum concentrations shown to activate selectively PPARalpha, these compounds markedly lowered hyperinsulinemia and, when present, hyperglycemia in both animal models. This effect relied on the improvement of insulin action on glucose utilization, as indicated by a lower insulin peak in response to intraperitoneal glucose in ciprofibrate-treated IR obese Zucker rats. In addition, fenofibrate treatment prevented high fat diet-induced increase of body weight and adipose tissue mass without influencing caloric intake. The specificity for PPARalpha activation in vivo was demonstrated by marked alterations in the expression of PPARalpha target genes, whereas PPARgamma target gene mRNA levels did not change in treated animals. These results indicate that compounds with a selective PPARalpha activation profile reduce insulin resistance without having adverse effects on body weight and adipose tissue mass in animal models of IR.

Modulation of rat liver apolipoprotein gene expression and serum lipid levels by tetradecylthioacetic acid (TTA) via PPARalpha activation

3-Thia fatty acids are modified fatty acids that promote hepatic peroxisome proliferation and decrease serum triacylglycerol, cholesterol and free fatty acid levels in rats. In vivo administration of tetradecylthioacetic acid (TTA) to rats led to a significant decrease in liver apolipoproteins apoA-I, A-II, A-IV, and C-III mRNA levels, and to an increase of liver acyl-CoA oxidase (ACO), carnitine palmitoyltransferase-II, and 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMG-CoA synthase) mRNA levels and activities. By contrast, no significant changes of lipoprotein lipase (LPL) mRNA levels were detected in rat epididymal adipose tissue. Liver carnitine palmitoyltransferase-I, apoB, apoE, and LDL receptor mRNA levels were not significantly affected. When tested in vitro, TTA increased rat ACO and carnitine palmitoyltransferase-I mRNA levels in primary rat hepatocytes and also LPL mRNA levels in 3T3-L1 preadipocytes. TTA also enhanced the transcriptional activity of chimeras containing the DNA binding domain of the yeast transcription factor Gal4 fused to the ligand binding domain of either human PPARalpha or human PPARgamma. The effect depended on the concentration tested and the cell type. In conclusion, our data suggest that in vitro, TTA activates both PPARalpha and PPARgamma, but the latter with much lower affinity. TTA affects serum lipid levels in vivo in rats by acting mainly on the liver via PPARalpha where it decreases the liver expression of genes involved in vascular lipid transport and increases the expression of genes involved in intracellular fatty acid metabolism. -Raspé, E., L. Madsen, A-M. Lefebvre, I. Leitersdorf, L. Gelman, J. Peinado-Onsurbe, J. Dallongeville, J-C. Fruchart, R. Berge, and B. Staels. Modulation of rat liver apolipoprotein gene expression and serum lipid levels by tetradecylthioacetic acid (TTA) via PPARalpha activation.

p300 interacts with the N- and C-terminal part of PPARgamma2 in a ligand-independent and -dependent manner, respectively

The nuclear peroxisome proliferator-activated receptor gamma (PPARgamma) activates the transcription of multiple genes involved in intra- and extracellular lipid metabolism. Several cofactors are crucial for the stimulation or the silencing of nuclear receptor transcriptional activities. The two homologous cofactors p300 and CREB-binding protein (CBP) have been shown to co-activate the ligand-dependent transcriptional activities of several nuclear receptors as well as the ligand-independent transcriptional activity of the androgen receptor. We show here that the interaction between p300/CBP and PPARgamma is complex and involves multiple domains in each protein. p300/CBP not only bind in a ligand-dependent manner to the DEF region of PPARgamma but also bind directly in a ligand-independent manner to a region in the AB domain localized between residue 31 to 99. In transfection experiments, p300/CBP could thereby enhance the transcriptional activities of both the activating function (AF)-1 and AF-2 domains. p300/CBP displays itself at least two docking sites for PPARgamma located in its N terminus (between residues 1 and 113 for CBP) and in the middle of the protein (between residues 1099 and 1460).

The organization, promoter analysis, and expression of the human PPARgamma gene

PPARgamma is a member of the PPAR subfamily of nuclear receptors. In this work, the structure of the human PPARgamma cDNA and gene was determined, and its promoters and tissue-specific expression were functionally characterized. Similar to the mouse, two PPAR isoforms, PPARgamma1 and PPARgamma2, were detected in man. The relative expression of human PPARgamma was studied by a newly developed and sensitive reverse transcriptase-competitive polymerase chain reaction method, which allowed us to distinguish between PPARgamma1 and gamma2 mRNA. In all tissues analyzed, PPARgamma2 was much less abundant than PPARgamma1. Adipose tissue and large intestine have the highest levels of PPARgamma mRNA; kidney, liver, and small intestine have intermediate levels; whereas PPARgamma is barely detectable in muscle. This high level expression of PPARgamma in colon warrants further study in view of the well established role of fatty acid and arachidonic acid derivatives in colonic disease. Similarly as mouse PPARgammas, the human PPARgammas are activated by thiazolidinediones and prostaglandin J and bind with high affinity to a PPRE. The human PPARgamma gene has nine exons and extends over more than 100 kilobases of genomic DNA. Alternate transcription start sites and alternate splicing generate the PPARgamma1 and PPARgamma2 mRNAs, which differ at their 5'-ends. PPARgamma1 is encoded by eight exons, and PPARgamma2 is encoded by seven exons. The 5'-untranslated sequence of PPARgamma1 is comprised of exons A1 and A2, whereas that of PPARgamma2 plus the additional PPARgamma2-specific N-terminal amino acids are encoded by exon B, located between exons A2 and A1. The remaining six exons, termed 1 to 6, are common to the PPARgamma1 and gamma2. Knowledge of the gene structure will allow screening for PPARgamma mutations in humans with metabolic disorders, whereas knowledge of its expression pattern and factors regulating its expression could be of major importance in understanding its biology.

Identification of the thyroid Na+/I- cotransporter as a potential autoantigen in thyroid autoimmune disease

The thyroid gland is the target of several autoimmune diseases. Specific thyroid proteins have been identified as autoantigens associated with these diseases (e.g. thyroperoxidase, thyroglobulin and the thyrotrophin (TSH) receptor). In this paper, we report that the serum of a patient suffering from Hashimoto's thyroiditis, autoimmune gastritis and rheumatoid arthritis was able to inhibit the chronic TSH-induced I- uptake of dog thyrocytes in culture, even at a 1:1000-fold dilution, without affecting their 86Rb+ uptake. This blocking activity is rare as 147 sera (from patients positive for antibodies to the thyroid microsomes and the gastric parietal cell antigen, patients with Sjögren's syndrome, patients with a high titre of microsomal antibodies and low or negative for antibodies to thyroperoxidase, and patients with a high titre of microsomal antibodies and frank hypothyroidism) were negative when tested for their ability to inhibit I- uptake. Subsequently we tested 20 murine monoclonal antibodies previously obtained by immunizing mice with a crude human thyroid membrane preparation, which were all negative when tested against thyroglobulin and thyroperoxidase. One of the monoclonal antibodies displayed a 50% inhibition of the chronic TSH-induced 125I- uptake of dog thyrocytes without affecting the 86Rb+ uptake of the cells. Immunoglobulins purified from the ascite fluid by affinity chromatography on a protein A cellulose column had the same characteristics. Taken together, the data suggest that thyroidal 125I- uptake can be inhibited by antibodies, that autoantibodies in the patient's serum are most probably responsible for the observed inhibition and therefore that the Na+/I- cotransporter is probably an autoantigen.

Tonic modulation of dog thyrocyte H2O2 generation and I- uptake by thyrotropin through the cyclic adenosine 3',5'-monophosphate cascade

The dog thyrocyte I- trapping activity and the expression of the genes coding for dog thyrocyte thyroglobulin or thyroid peroxidase are enhanced by TSH through the cAMP cascade and reduced by mitogens such as epidermal growth factor (EGF) or 12-O-tetradecanoylphorbol 13-acetate (TPA). In this work, we investigated whether H2O2 generation (a limiting step of thyroid hormone synthesis) is modulated by chronic treatment of the thyrocyte with TSH or mitogens such as EGF or TPA. We observed that both basal and carbachol- or ionomycin-stimulated H2O2 generation by the dog thyrocyte were concentration and time dependently enhanced by prolonged (12- to 72-h) exposure to TSH. This effect was reproduced by agents that increase the dog thyrocyte cAMP level or that mimic this increase. It was abolished when protein or RNA synthesis was inhibited. By contrast, EGF and TPA concentration and time dependently antagonized the effect of TSH. In addition, chronic exposure to EGF reduced both basal and carbachol- or ionomycin-stimulated H2O2 generation. The effect of TPA was reproduced by another protein kinase-C activating phorbol ester, phorbol dibutyrate, but not by beta-phorbol, an inactive phorbol ester. Modulation of dog thyrocyte H2O2 generation by chronic exposure to TSH or to the mitogens EGF and TPA was totally parallel to the modulation of their 125I- uptake. Taken together our results suggest that H2O2 generation (or at least one of its constituents) is a differentiation characteristic of the dog thyrocyte under tonic control of TSH through the cAMP cascade as iodide transport, thyroid peroxidase, and thyroglobulin.

Control of the dog thyrocyte plasma membrane iodide permeability by the Ca(2+)-phosphatidylinositol and adenosine 3',5'-monophosphate cascades

Protein iodination by the dog thyrocyte (a marker of thyroid hormone synthesis) is stimulated by the Ca(2+)-phosphatidylinositol and cAMP cascades. We have shown previously that H2O2 generation, a limiting step of thyroid hormone synthesis, is modulated by these two cascades. In this work, we show that the I- release from preloaded thyrocytes is also activated by agents activating the Ca(2+)-phosphatidylinositol cascade and by Ca2+ ionophores, especially in synergy with 12-O-tetradecanoylphorbol 13-acetate, a potent activator of protein kinase-C. The effect of carbachol is reduced when the extracellular Ca2+ is depleted. Thus, both arms of the Ca(2+)-phosphatidylinositol cascade, Ca2+ and diacylglycerol, acutely and synergistically activate dog thyrocyte I- release. This I- release was also accelerated by acute and chronic exposure to TSH, forskolin, or (BU)2cAMP. The chronic stimulation of I- release by TSH exposure was diminished by chronic epidermal growth factor treatment (which dedifferentiates the thyrocytes). In addition, the chronic stimulation of I- release by forskolin was not affected by withdrawal of the agent up to 4 h before the experiment, in contrast to the acute effect of forskolin, which vanished within 16 min after forskolin withdrawal. These results suggest that the chronic stimulation of I- release by TSH or forskolin involves a stable mechanism. The I- transport system causing the release of I- from the dog thyrocyte is almost insensitive to inhibition by NaClO4 and KSCN. Hence, the iodide release cannot be due to the action of the basolateral Na+/I- cotransporter. In addition, we show that I- release was less sensitive than I- uptake to the inhibition by dysidenin, a marine toxin isolated from the sponge, Dysidea herbacea, known to inhibit I- uptake by dog thyroid slices. In summary, this work suggests that in a well defined model of the thyroid, the dog thyrocyte in primary culture, an I- transport system distinct from the basolateral Na+/I- cotransporter, is responsible for the observed I- release. The complex modulation of this transport system, involving at least the Ca(2+)-phosphatidylinositol and cAMP cascades, parallels the regulation of protein iodination, which itself reflects thyroid hormone synthesis.

Lack of correlation between the activation of the Ca(2+)-phosphatidylinositol cascade and the regulation of DNA synthesis in the dog thymocyte

Changes in the [Ca2+]i and/or activation of phospholipase C are thought to participate in the control by several growth factors of the mammalian cell proliferation. It has even been claimed that activation of the Ca(2+)-phosphatidylinositol cascade is sufficient to elicit cell proliferation [Jackson et al. (1988) Nature 335, 437-440; Julius et al. (1989) Science 244, 1057-1062]. In this work, we have evaluated the control of DNA synthesis by this cascade in a differentiated epithelial cell model: the dog thyrocyte in primary culture. We first observed that potent activators of the dog thyrocyte (2+)-phosphatidylinositol cascade such as carbachol or bradykinin failed to promote the onset of DNA synthesis in these cells. Moreover, carbachol inhibited the mitogenic effect of thyroid stimulating hormone (TSH) and of epidermal growth factor (EGF). The mitogenic effect of EGF was also reduced by bradykinin. Nevertheless, carbachol enhanced the expression of the protooncogenes c-fos and c-myc mRNAs. The time course of this enhancement was identical to the time course for the induction of c-fos and c-myc mRNAs by phorbol esters or EGF. On the other hand, in most experiments, TSH and EGF were able to trigger the onset of dog thyrocyte DNA synthesis without affecting their intracellular free Ca2+ concentration [Ca2+]i, 45Ca2+ efflux, or inositol phosphate generation. In several experiments, TSH increased the dog thyrocyte 45Ca2+ release and promoted a rise in the [Ca2+]i or the inositol phosphate accumulation but these effects were weak. In contrast to the effect of carbachol, the TSH effects on the [Ca2+]i and the 45Ca2+ efflux appeared slowly, were sustained, and were extremely sensitive to extracellular Ca2+ depletion. They were observed at hormone concentrations higher than the concentration achieving maximal stimulation of DNA synthesis. Similarly, in a few experiments, a slight increase in the [Ca2+]i or in the inositol trisphosphate generation were provoked by EGF. However, these modifications were not associated with an increased mitogenic potency of EGF. Finally, in all experiments, fetal calf serum slightly accelerated the dog thyrocyte 45Ca2+ efflux and increased their inositol phosphate generation.(ABSTRACT TRUNCATED AT 400 WORDS)

ATP, bradykinin, TRH and TSH activate the Ca(2+)-phosphatidylinositol cascade of human thyrocytes in primary culture

We have recently shown that adenosine triphosphate (ATP), bradykinin and thyrotropin-releasing hormone (THR) increase the ([Ca2+]i) of human thyrocytes in primary culture. We show here that these agents also stimulate the generation of [3H]-inositol monophosphate (IP1), inositol bisphosphate (IP2) and inositol trisphosphate (IP3). The stimulation of IP3 generation followed two distinct kinetics: it was sustained when the cells were triggered with ATP and transient when the response was elicited by TRH or bradykinin. In addition, we have shown that under the appropriate experimental conditions, high thyroid-stimulating hormone (TSH) concentrations were also able to stimulate human thyrocyte IP1, IP2 and IP3 accumulation and to increase their [Ca2+]i. These data suggest that ATP, bradykinin, TRH and high TSH concentrations activate the Ca(2+)-phosphatidylinositol cascade of human thyrocytes. Since this cascade plays a crucial role in the control of protein iodination, ATP, TRH and bradykinin could be important regulators of thyroid hormone synthesis in human thyrocytes.

Control of the intracellular Ca(2+)-concentration and the inositol phosphate accumulation in dog thyrocyte primary culture: evidence for different kinetics of Ca(2+)-phosphatidylinositol cascade activation and for involvement in the regulation of H2O2 production

Carbachol, through a muscarinic receptor, thyrotropin-releasing hormone (TRH), prostaglandin F2 alpha (PGF2 alpha), bradykinin, and adenosine triphosphate (ATP) increased the apparent [Ca2+]i (intracellular free Ca2(+)-concentration) of dog thyrocytes in primary culture. The [Ca2+]i measured by the Quin-2 technique rose immediately after the addition of the agonists and reached a maximal value after less than 30 seconds. Afterwards, the [Ca2+]i declined to a plateau higher than the basal level when the cells were triggered with carbachol. By contrast, in most experiments with PGF2 alpha and in the case of bradykinin, TRH, and ATP, the [Ca2+]i returned to the basal value. If the extracellular Ca2+ was chelated by excess of EGTA, the addition of all agents caused a sharp reduced transient rise in the [Ca2+]i followed by a decline of the [Ca2+]i often below the basal level (especially in the case of carbachol). It is suggested that the first transient phase of these responses is due at least in part to the mobilisation of Ca2+ from intracellular stores whereas the second sustained phase of the response to carbachol mainly originates from an increased Ca2+ influx into the thyrocytes. Carbachol, bradykinin, TRH, PGF2 alpha, and ATP also increased generation of inositol phosphates in dog thyrocytes. This effect was sustained when the cells were triggered with carbachol and was more transient with bradykinin, TRH, PGF2 alpha, or ATP. All these agents and the phorbdester TPA as well as forskolin enhanced to various extent the thyrocyte H2O2 generation. This enhancement was severely reduced in the absence of extracellular Ca2+ and was mimicked by Ca2+ ionophores in the presence of extracellular Ca2+ especially in synergy with protein kinase C activators. These data suggest that the dog thyrocyte H2O2 generation, the limiting step of the thyroid hormone synthesis, is modulated by carbachol, TRH, PGF2 alpha, bradykinin, and ATP through their action on the Ca2(+)-phosphatidylinositol cascade.

Thyrotropin activates both the cyclic AMP and the PIP2 cascades in CHO cells expressing the human cDNA of TSH receptor

The effect of thyrotropin (TSH) on cyclic AMP accumulation, phosphatidylinositol bisphosphate (PIP2) hydrolysis and [Ca2+]i rise has been studied in CHO cells stably transfected with human TSH receptor (hTSHR) cDNA. In human thyroid slices, TSH activates these two intracellular cascades with a higher affinity for the adenylate cyclase activation (from 0.1 to 1 mU/ml TSH) than for phospholipase C activation (from 1 to 10 mU/ml TSH). The CHO cells transfected with the recently cloned cDNA of human TSH receptor respond in the same way to TSH. They respond between 0.1 and 1 mU/ml TSH for cyclic AMP accumulation and between 1 and 10 mU/ml TSH for inositol monophosphate (IP1) increase. In these same cells, TSH 10 mU/ml, but not forskolin (10 microM), or dibutyryl cyclic AMP (100 microM), clearly enhances intracellular calcium concentration [( Ca2+]i). Our results demonstrate unequivocally that a single transcription unit has the potential to encode receptor molecules coupled to both cascades.

Adenosine triphosphate, bradykinin, and thyrotropin-releasing hormone regulate the intracellular Ca2+ concentration and the 45Ca2+ efflux of human thyrocytes in primary culture

The hormonal stimulation of phospholipase C and the consequent activation of the Ca2+-phosphatidylinositol cascade in eukaryotic cells is associated with modifications of the [Ca2+]i (intracellular Ca2+ concentration) which modulates cellular functions. In this study, these modifications were investigated in primary cultures of human thyroid cells. The mean apparent basal [Ca2+]i of human thyrocytes measured using the intracellularly trapped fluorescent indicator Quin-2 was found to be 89 +/- 16 nM (n = 49). ATP and, to a lesser extent, ADP, but not AMP or adenosine, elicited a concentration-dependent biphasic rise in human thyrocytes [Ca2+]i and increased their 45Ca2+ efflux. The first transient phase of the [Ca2+]i rise induced by ATP was resistant to extracellular Ca2+ depletion, whereas the second sustained phase was abolished in these conditions. This suggests that although the first phase of this response involves a release of Ca2+ from intracellular stores, the second phase requires extracellular Ca2+ influx. The response of human thyrocytes to analogs of ATP is compatible with a P2-purinergic effect of ATP on these cells. Bradykinin and TRH affected the human thyrocyte [Ca2+]i and 45Ca2+ efflux similarly to ATP. The human thyrocyte [Ca2+]i and the 45Ca2+ efflux were not modified by carbachol, a nonhydrolyzable analog of acetylcholine. The present results suggest the presence of P2-purinergic receptors to ATP and of receptors to TRH and bradykinin on human follicular thyroid cells. They also confirm that the Ca2+-phosphatidylinositol cascade is present in these cells and suggest that this cascade is modulated by ATP, TRH, and bradykinin. As this cascade is involved in the regulation of protein iodination, and therefore of thyroid hormones synthesis, these agents might have an important role in the regulation of the thyroid function.

Carbamylcholine, TRH, PGF2 alpha and fluoride enhance free intracellular Ca++ and Ca++ translocation in dog thyroid cells

Effects on Ca++ translocation and [Ca++]i were studied in dog thyroïd cell monolayers using both 45Ca++ efflux and the indicator quin-2. Carbamylcholine, a non hydrolysable analog of acetylcholine, through muscarinic receptors, and to a lesser extent TRH and PGF2 alpha increased both these parameters. [Ca++]i increased by 171, 100 and 75% respectively over a basal level of 66 +/- 17 nM (mean +/- SD). The response to carbamylcholine was biphasic. A transient increase in [Ca++]i was followed by a more sustained phase where the [Ca++]i was slightly higher than the basal level. Only the first phase was insensitive to extracellular Ca++ depletion. This phase is probably due to a release of Ca++ from an intracellular store. NaF also induced a sustained rise in [Ca++]i dependent on extracellular Ca++ and affected 45Ca++ efflux. Our data provide direct evidence of an implication of intracellular Ca++ in the response of dog thyroïd cells to all these agents.