Anabelle Redonnet

High-fat diets affect the expression of nuclear retinoic acid receptor in rat liver.

The purpose of this study was to differentiate between the effects of the amount and the type of dietary lipids on the expression of the retinoic acid receptor (RAR), but also the peroxisome proliferator-activated receptor (PPAR) and the receptor of the 9-cis retinoic acid (retinoid X receptor (RXR)) in rat liver. Six groups of eight rats (5-weeks old) were fed during 4 weeks on the following diets: control 50 g vegetable oil/kg, high-fat diet 250 g vegetable oil/kg. These oils were either coconut oil (rich in saturated fatty acids) or olive oil (rich in monounsaturated fatty acids) or safflower oil (rich in polyunsaturated fatty acids, mainly as n-6). The three high-fat diets induced a significant decrease of the maximal binding capacity of RAR and of the abundance of RAR beta mRNA. Simultaneously, an increased expression of PPAR alpha mRNA was observed while no significant difference on abundance of RXR alpha mRNA was observed. The mechanisms involved are probably multiple, but one hypothesis is that a modification of the equilibrium between the nuclear receptors, resulting from an increased expression of PPAR, induces a decreased expression of RAR in rat liver.

Synergic effect of vitamin A and high-fat diet in adipose tissue development and nuclear receptor expression in young rats

In order to study the effects of dietary lipids and vitamin A on the development of adipose tissues, young rats were submitted for 8 d to a control or to two cafeteria diets with normal (Caf) or higher (Caf + ) vitamin A levels. Retinoid (retinoic acid receptor (RAR) a, RARg, retinoid X receptor(RXR) alpha) and fatty acid (PPARgamma) receptor mRNA was measured in the subcutaneous white adipose tissue (Swat) and in isolated mature adipocytes by RT-PCR. The stroma vascular fraction was cultured in vitro to test the capacities of the adipocyte precursors to proliferate and differentiate.The Caf diet enriched in vitamin A resulted in an increased adiposity, due to increased adipocyte hypertrophy. This was concomitant with a lower expression of RARa and RARg mRNA (234.6 and 238.6 %) and a higher expression of PPARgamma (+59 %) in the Swat and, to a less extent,in isolated adipocytes. Positive correlations were obtained between PPARgamma mRNA and Swat weights and between PPARgamma and RXRalpha mRNA. By contrast, RARgamma mRNA and Swat masses were negatively correlated. The adipocyte precursors from Caf + Swat proliferated more,in vitro, at the beginning of the culture. This difference progressively disappeared and was totally absent after 8 d of culture, but with a higher percentage of differentiated preadipocytes (+80.3 %) in the Caf + group. In conclusion, lipids and vitamin A act synergistically on the normal growth of the adipose tissue in young rats, concomitant with an imbalance in the pattern of the nuclear receptors. These changes influence the early normal development of the endogenous adipocyte precursors.

Retinoids and glucocorticoids target common genes in hippocampal HT22 cells.

Vitamin A metabolite retinoic acid (RA) plays a major role in the aging adult brain plasticity. Conversely, chronic excess of glucocorticoids (GC) elicits some deleterious effects in the hippocampus. We questioned here the involvement of RA and GC in the expression of target proteins in hippocampal neurons. We investigated proteins involved either in the signaling pathways [RA receptor β (RARβ) and glucocorticoid receptor (GR)] or in neuron differentiation and plasticity [tissue transglutaminase 2 (tTG) and brain‐derived neurotrophic factor (BDNF)] in a hippocampal cell line, HT22. We applied RA and/or dexamethasone (Dex) as activators of the pathways and investigated mRNA and protein expression of their receptors and of tTG and BDNF as well as tTG activity and BDNF secretion. Our results confirm the involvement of RA‐ and GC‐dependent pathways and their interaction in our neuronal cell model. First, both pathways regulate the transcription and expression of own and reciprocal receptors: RA and Dex increased RARβ and decreased GR expressions. Second, Dex reduces the expression of tTG when associated with RA despite stimulating its expression when used alone. Importantly, when they are combined, RA counteracts the deleterious effect of glucocorticoids on BDNF regulation and thus may improve neuronal plasticity under stress conditions. In conclusion, GC and RA both interact through regulations of the two receptors, RARβ and GR. Furthermore, they both act, synergistically or oppositely, on other target proteins critical for neuronal plasticity, tTG and BDNF.

Effect of vitamin A content in cafeteria diet on the expression of nuclear receptors in rat subcutaneous adipose tissue

The aim of this study was to determine the effects of cafeteria diet containing control or elevated level of vitamin A on the expression of nuclear receptors in adipose tissue. Male Wistar rats were submitted to 3 experimental diets during 8 weeks, a standard diet and two hyper-energetic, hyperlipidic “cafeteria” diets containing normal (Caf) or higher (Caf+) vitamin A level. During the experiment, body weights and energy intakes were measured. At the end of the experimental period, subcutaneous adipose tissue (Swat) and all the fat mass were removed and weighted. Nuclear receptors mRNA levels of RARα, RARγ, RXRα, PPARγ were measured in the Swat by a real-time semi-quantitative RT-PCR method. We observed that energy intake of Caf+ and Caf groups was significantly higher than that of the control group. Despite a higher increase of the energy intake in the Caf group compared to the Caf+ group, no significant difference was observed in the body weight gain of the Caf+ compared to the Caf group. The Caf+ and Caf diets led to a significant increase of adipose tissue in cafeteria groups as observed in the Swat depot. The mRNA levels of PPARγ and RXRα were significantly increased in the Caf+ group as compared to control group, with a significant positive correlation between these two parameters. Expressions of RARα and RARγ were not modified in experimental groups compared to controls. In conclusion, 8-week exposure to cafeteria diets with normal and higher levels of vitamin A led to an increase of adiposity in rats, associated, only in the group fed with the higher vitamin A level cafeteria diet, with an increase of PPARγ and RXRα expressions in subcutaneous adipose tissue.ResumenEl objetivo del presente trabajo consiste en determinar las consecuencias de un alto contenido en vitamina A en dieta de cafetería sobre la expresión de receptores nucleares en el tejido adiposo. Así, ratas macho Wistar se dividieron en tres grupos: Durante 8 semanas, el grupo control se alimentó con pienso estándar, mientras que los grupos tratados recibieron una dieta rica en grasa (dieta de cafetería) enriquecida (Caf+) o no (Caf) con vitamina A. El peso corporal y la ingesta se determinaron durante todo el experimento. Al final del tratamiento, se pesó el tejido adiposo subcutáneo (Swat) y las otras reservas de grasa. Los niveles de ARNm de los receptores nucleares RARα, RARγ, RXRα, PPARγ se determinaron en el Swat con un método semi-cuantitativo de RT-PCR en tiempo real. Las ingestas energéticas de los grupos Caf+ y Caf fueron significativamente mayores que las del grupo control. A pesar del aumento en la ingesta del grupo Caf respecto del Caf+, no se observaron diferencias significativas en el aumento de peso corporal entre ambos grupos. Además, las dietas de los grupos Caf+ y Caf provocaron un claro aumento del tamaño de las reservas de grasa, incluido el peso del Swat. Los niveles de ARNm de PPARγ y RXRα se incrementaron significativamente en el grupo Caf+ respecto del control, con correlación positiva entre ambos. En cambio, no se modificó la expresión de RARα y RARγ. En suma, 8 semanas de alimentación con dieta de cafetería con niveles normales o elevados de vitamina A dan lugar a aumento de la adiposidad en la rata, asociado con aumento de la expresión de PPARγ y RXRα en el tejido adiposo subcutáneo solo en el grupo que recibió suplemento de vitamina A.

Effect of a pharmacological activation of PPAR on the expression of RAR and TR in rat liver

It has recently been shown that high-fat diets induce the expression of peroxisome proliferator-activated receptor (PPAR) with a concomitant decrease in expression of retinoic acid (RAR) and triiodothyronine (TR) receptors in rat liver. The authors have suggested that PPAR activation may be responsible for these modifications in nuclear receptor expression. With the aim of gaining further insight into a possible relationship between the patterns of expression of these receptors, we have examined, using a pharmacological model, the effect of a strong and specific PPAR activation induced by bezafibrate, a peroxisome proliferator agent. Activation of PPAR was evaluated by quantifying PPARα mRNA and acyl-CoA oxidase mRNA. The expression of RAR and TR was determined by assaying the binding properties of these nuclear receptors and by quantifying the mRNA level of RARβ and TRα1,β1 isoforms. After a 10 day treatment of young rats, induction of PPAR (PPARα mRNA was increased by 40% [P< 0.05 and acyl-CoA oxidase mRNA by 411% [P<0.001]) and a concomitant decrease of RAR and TR expression (Maximal Binding Capacity was decreased by 21 and 26%, respectively [P<0.05]) in the liver was observed. RXRα mRNA expression was unchanged by treatment. Cross-talk between RAR, TR and PPAR signalling pathways may be implicated in the new patterns of nuclear receptor expression observed. The decreased expression of RAR and TR reported here could provide a novel element for the understanding of the link between PPAR and tumorigenesis in rat liver.ResumenRecientemente se ha señalado que las dietas ricas en grasa inducen la expresión del receptor activado por el pro1iferador de peroxisomas (PPAR) a la vez que disminuye la de los receptores del ácido retinoico (RAR) y de la triyodotironina (TR) en hígado de rata. Se sugiere que la activación de los PPAR puede ser responsab1e de las modificaciones en la expresión de estos receptores nucleares. Con objeto de aclarar la posible conexión entre los patrones de expresión de estos genes, se ha examinado en un modelo farmacológico el efecto del bezafibrato, agente estimulante de la proliferación de peroxisomas que activa específicamente al receptor nuclear PPAR. La activacion de este receptor se evalúa mediante la cuantificación de los niveles de mRNA de PPARα y acil CoA oxidasa. La expresión de RAR y TR se determina mediante ensayos de unión y por cuantificación de los niveles de mRNA de las isoformas RARβ y TRα1, β1 Después de 10 días de tratamiento con bezafibrato en ratas jóvenes, se observa en el hígado inducción de PPAR (los niveles de mRNA de PPAR aumentan significativamente en un 40% y los de Acil CoA oxidasa en un 411%) con disminución de la expresión de RAR y TR (la capacidad de unión máxima disminuye significativamente el 21 % y 26%, respectivamente). La expresión de RXRα no se modifica. Se concluye que diversas conexiones entre las rutas de señalización de RAR, TR, PPAR podrfan participar en la modulación de la expresión de los genes de los receptores nucleares. La disminución en la expresión de RAR y TR puede ayudar a entender la relación entre PPAR y tumorigénesis en hígado de rata.

Prolonged treatment with the β3-adrenergic agonist CL 316243 induces adipose tissue remodeling in rat but not in guinea pig: 1) fat store depletion and desensitization of β-adrenergic responses

Abstractβ3-adrenergic agonists have been considered as potent antiobesity and antidiabetic agents mainly on the basis of their beneficial actions discovered twenty years ago in obese and diabetic rodents. The aim of this work was to verify whether prolonged treatment with a β3-adrenergic agonist known to stimulate lipid mobilisation, could promote desensitization of β-adrenergic responses. Wistar rats and guinea pigs were treated during one week with CL 316243 (CL, 1 mg/kg/d) by implanted osmotic minipumps. In control animals, β3-adrenergic agonists were lipolytic in rat but not in guinea pig adipocytes. CL-treatment did not alter body weight gain in both species, but reduced fat stores in rats. Lipolysis stimulation by forskolin was unmodified but responses to β1-, β2- and β3-agonists were reduced in visceral or subcutaneous white adipose tissues of CL-treated rats. Similarly, the β3-adrenergic-dependent impairment of insulin action on glucose transport and lipogenesis in rat adipocytes was diminished after CL-treatment. In rat adipocytes, [125I]ICYP binding and β3-adrenoceptor mRNA levels were reduced after sustained CL administration. These findings show that CL 316243 exerts β3-adrenergic lipolytic and antilipogenic effects in rat adipocytes. These actions, which are likely involved in the fat depletion observed in rat, also lead to the desensitization of all β-adrenergic responses. Therefore this desensitization, together with the lack of slimming action in guinea pig, seriously attenuates the usefulness of β3-agonists as antiobesity agents, and may explain why such agonists have not been conducted to a widespread clinical, use.ResumenLos agonistas β3-adrenérgicos son considerados potentes agentes anti-obesidad y antidiabéticos debido, fundamentalmente, a los efectos beneficiosos que producen en roedores obesos y diabeticos, descubiertos ya hace veinte años. El objetivo del presente estudio fue verificar si un tratamiento prolongado con agonists β3-adrenérgicos, conocidos estimulantes de la movilización lipídica, puede promover la desensibilización de las respuestas β-adrenérgicas. Para ello, se trataron ratas Wistar y cobayas con CL 316243 (CL, 1 mg/kg/d), administrado mediante el implante de minibombas osmóticas, durante una semana. En los adipocitos de ratas control, pero no en los de cobayas control, los agonistas β3-adrenérgicos produjeron efectos lipolíticos. El tratamiento con CL no modificó la ganancia de peso en ninguna de las dos especies, pero redujo los depósitos de grasa en ratas. En el tejido adiposo visceral y subcutáneo de las ratas tratadas con CL, la estimulación de la lipólisis por forskolina no se vió afectada, pero las respuestas a agonistas β1, β2, y β3 se redujeron. De manera análoga, el deterioro de la función insulínica, en lo que al transporte de glucosa y la lipogénesis se refiere, producido por los adrenérgicos β3 y que sólo se observa en los adipocitos de rata, disminuyó tras el tratamiento con CL. En los adipocitos de rata, la unión a [125I]ICYP y los niveles de ARNm del receptor adrenérgico β3 disminuyeron con la administración sostenida de CL. Estos resultados demuestran que el CL 316243 produce efectos lipolítico y antilipogénico únicamente en los adipocitos de rata. Estas acciones, muy probablemente relacionadas con la depleción de grasa observada en la rata, conducen a la desensibilización de todas las respuestas β-adrenérgicas. Esta desensibilización, junto con la ausencia de efecto adelgazante en cobayas, reduce la utilidad de los agonistas β3-adrenérgicos como agentes antiobesidad y podría explicar por qué no se han utilizado en la práctica clínica habitual como nuevos fármacos.

Role of adipogenic and thermogenic genes in susceptibility or resistance to develop diet-induced obesity in rats

The aim of the present work was to assess whether changes in adipose tissue gene expression related with adipogenesis and/or thermogenesis could be involved in the mechanism conferring susceptibility or resistance to develop obesity in high-fat fed outbreed rats. For this purpose, male Wistar rats were fed with standard laboratory diet (control group) or high fat diet. After 15 days, two groups of rats with significant differences on body weight gain in response to the high fat diet were characterized and identified as diet-induced obesity (DIO) and diet resistant (DR) rats. A significant increase in visceral white adipose tissue (WAT) PRARγ and aP2 (p<0.05) mRNA levels associated to a decrease in RARγ expression (p<0.05) was observed in DIO rats, suggesting an increase of adipogenesis. Furthermore, our data showed a marked increase in brown adipose tissue (BAT) of UCP1 mRNA in DIO animals (p<0.01) (without affecting PGC-1α gene expression), whereas no changes were found in WAT UCP2 gene expression. All these data suggest that the variations found in the expression pattern of PPARγ, aP2 and RARγ by high-fat diet could be involved, at least in part, in the differences in body weight gain and adiposity observed between DR and DIO animals. The compensatory adaptations through the increase in energy expenditure by changes on the expression levels of UCP1 seem not to be enough to avoid the obesity onset in the DIO group.ResumenEI objeto del presente trabajo fue determinar si cambios en la expresión de genes del tejido adiposo relacionados con la adipogénesis y la termogénesis pudieran estar implicados en la resistencia o susceptibilidad al desarrollo de obesidad en ratas alimentadas con dieta hipergrasa. Tras 15 días de ingesta de la dieta rica en grasa (dieta de cafetería), se identificó un grupo susceptible (DIO) y otro resistente (DR) al desarrollo de obesidad. El incremento significativo observado en la expresión de PPARγ y aP2 (p<0.05), junto con el descenso en la expresión de RARγ (p<0.05) en el tejido adiposo blaco de ratas DIO, sugiere un incremento del proceso adipogénico en estos animales. Además, nuestros datos mostraron aumento en los niveles de ARNm de UCP1 (sin afectar a la expresión del gen PGC-1α) en el tejido adiposo marrón en los animales DIO, mientras que no se observaron cambios en la expresión de UCP2 en tejido adiposo blanco. Todos estos datos sugieren que los cambios encontrados en la expresión de PPARγ, aP2 y RARγ podrían estar implicados en las diferencias en la ganancia del peso corporal y la adiposidad observadas entre los grupos DIO y DR. Los mecanismos compensadores puestos en marcha mediante el incremento de los niveles de expresión de UCP1 no parecen ser suficientes para evitar la instauración de la obesidad en este grupo de animales.

RARγ and TRβ expressions are decreased in PBMC and SWAT of obese subjects in weight gain

In order to evaluate the expression of nuclear receptors at the peripheral level in obese subjects, messenger RNA (mRNA) levels of different isoforms of retinoic acid receptor (RAR), triiodothyronine (TR), and peroxisome proliferator-activated receptor (PPAR) were determined and compared in peripheral mononuclear blood cells (PBMC) and subcutaneous white adipose tissue (SWAT). Twelve lean subjects and 68 obese subjects divided into weight gain (WG), weight-stable (WS), and weight loss (WL) groups were studied. Nuclear receptor mRNA levels were assessed in PBMC and SWAT using a quantitative real-time reverse transcription polymerase chain reaction method. mRNA levels of RARγ were significantly lower in PBMC of obese subjects (WG −19%, WS −30%, and WL −24.7%) as in SWAT of WG (−50%). Lower mRNA levels of TRβ were observed in PBMC and SWAT of WG (−50.7% and −28%, respectively) just as for TRα in PBMC of WG (−19%). In contrast, retinoid X receptors α (RXRα) and RARα mRNA levels were higher in PBMC of obese subjects (+53% and +54.5% in WG, +56% and +67% in WS, and +68% and +49.7% in WL, respectively), while expression of RXRα was lower in SWAT of WG (−24.5%). As for PPARγ, its mRNA level was significantly higher in PBMC of WG subjects (+34%) while its expression was not modified in SWAT, contrary to the PPARγ2 isoform which was significantly higher. These data show that in both adipose tissue and blood compartment of obese subjects, expressions of RARγ and TRβ were downregulated. Thus, we suggest that the expression in PBMC of obese subjects may constitute new cellular indicators of nuclear receptor retinoid and thyroid status.