Hélène Poirier

Differential involvement of peroxisome-proliferator-activated receptors alpha and delta in fibrate and fatty-acid-mediated inductions of the gene encoding liver fatty-acid-binding protein in the liver and the small intestine.

Liver fatty-acid-binding protein (L-FABP) is a cytoplasmic polypeptide that binds with strong affinity especially to long-chain fatty acids (LCFAs). It is highly expressed in both the liver and small intestine, where it is thought to have an essential role in the control of the cellular fatty acid (FA) flux. Because expression of the gene encoding L-FABP is increased by both fibrate hypolipidaemic drugs and LCFAs, it seems to be under the control of transcription factors, termed peroxisome-proliferator-activated receptors (PPARs), activated by fibrate or FAs. However, the precise molecular mechanism by which these regulations take place remain to be fully substantiated. Using transfection assays, we found that the different PPAR subtypes (alpha, gamma and delta) are able to mediate the up-regulation by FAs of the gene encoding L-FABP in vitro. Through analysis of LCFA- and fibrate-mediated effects on L-FABP mRNA levels in wild-type and PPARalpha-null mice, we have found that PPARalpha in the intestine does not constitute a dominant regulator of L-FABP gene expression, in contrast with what is known in the liver. Only the PPARdelta/alpha agonist GW2433 is able to up-regulate the gene encoding L-FABP in the intestine of PPARalpha-null mice. These findings demonstrate that PPARdelta can act as a fibrate/FA-activated receptor in tissues in which it is highly expressed and that L-FABP is a PPARdelta target gene in the small intestine. We propose that PPARdelta contributes to metabolic adaptation of the small intestine to changes in the lipid content of the diet.

New insights into the fatty acid-binding protein (FABP) family in the small intestine

The fatty acid-binding protein (FABP) superfamily is constituted by 14–15 kDa soluble proteins which bind with a high affinity either long-chain fatty acids (LCFAs), bile acids (BAs) or retinoids. In the small intestine, three different FABP isoforms exhibiting a high affinity for LCFAs and/or BAs are expressed: the intestinal and the liver-type (I-FABP and L-FABP) and the ileal bile acid-binding protein (I-BABP). Despite of extensive investigations, their respective physiological function(s) are not clearly established. In contrast to the I-FABP, L-FABP and I-BABP share several common structural features (shape, size and volume of the hydrophobic pocket). Moreover, L-FABP and I-BABP genes are also specifically regulated by their respective preferential ligands through a very similar molecular mechanism. Although, they exhibit differences in their binding specificities and location along the small intestine supporting a specialization, it is likely that L-FABP and I-BABP genes exert the same type of basic function(s) in the enterocyte, in contrast to I-FABP.

Luminal Lipid Regulates CD36 Levels and Downstream Signaling to Stimulate Chylomicron Synthesis

The membrane glycoprotein CD36 binds nanomolar concentrations of long chain fatty acids (LCFA) and is highly expressed on the luminal surface of enterocytes. CD36 deficiency reduces chylomicron production through unknown mechanisms. In this report, we provide novel insights into some of the underlying mechanisms. Our in vivo data demonstrate that CD36 gene deletion in mice does not affect LCFA uptake and subsequent esterification into triglycerides by the intestinal mucosa exposed to the micellar LCFA concentrations prevailing in the intestine. In rodents, the CD36 protein disappears early from the luminal side of intestinal villi during the postprandial period, but only when the diet contains lipids. This drop is significant 1 h after a lipid supply and associates with ubiquitination of CD36. Using CHO cells expressing CD36, it is shown that the digestion products LCFA and diglycerides trigger CD36 ubiquitination. In vivo treatment with the proteasome inhibitor MG132 prevents the lipid-mediated degradation of CD36. In vivo and ex vivo, CD36 is shown to be required for lipid activation of ERK1/2, which associates with an increase of the key chylomicron synthesis proteins, apolipoprotein B48 and microsomal triglyceride transfer protein. Therefore, intestinal CD36, possibly through ERK1/2-mediated signaling, is involved in the adaptation of enterocyte metabolism to the postprandial lipid challenge by promoting the production of large triglyceride-rich lipoproteins that are rapidly cleared in the blood. This suggests that CD36 may be a therapeutic target for reducing the postprandial hypertriglyceridemia and associated cardiovascular risks.

9-cis-Retinoic acid enhances fatty acid-induced expression of the liver fatty acid-binding protein gene

The role of retinoic acids (RA) on liver fatty acid‐binding protein (L‐FABP) expression was investigated in the well differentiated FAO rat hepatoma cell line. 9‐cis‐Retinoic acid (9‐cis‐RA) specifically enhanced L‐FABP mRNA levels in a time‐ and dose‐dependent manner. The higher induction was found 6 h after addition of 10−6 M 9‐cis‐RA in the medium. RA also enhanced further both L‐FABP mRNA levels and cytosolic L‐FABP protein content induced by oleic acid. The retinoid X receptor (RXR) and the peroxisome proliferator‐activated receptor (PPAR), which are known to be activated, respectively, by 9‐cis‐RA and long chain fatty acid (LCFA), co‐operated to bind specifically the peroxisome proliferator‐responsive element (PPRE) found upstream of the L‐FABP gene. Our result suggest that the PPAR‐RXR complex is the molecular target by which 9‐cis‐RA and LCFA regulate the L‐FABP gene.

Regulation of gene expression by fatty acids: Special reference to fatty acid-binding protein (FABP)

During the last years, the direct involvement of lipidic nutrients in the regulation of genes has been established. Fatty acids may induce or repress the transcription rate of several genes involved in both lipid and carbohydrate metabolisms. Gene up-regulation has been found in various tissues including liver, adipose tissue and small intestine. It is only triggered by saturated and unsaturated long-chain fatty acids or their CoA-derivatives. In contrast, gene down-regulation appears to be restricted to the liver. This negative effect is exerted only by polyunsaturated fatty acids. Long-chain fatty acids are able to regulate the expression of two different genes oppositely in the same cell type. The molecular mechanism of these fatty acid-mediated effects remains unclear. The involvement of members of the peroxisome proliferator-activated receptor is discussed.

Adipose tissue adaptive response to trans-10,cis-12–conjugated linoleic acid engages alternatively activated M2 macrophages

In mice, nutritional supplementation with the trans‐10,cis‐12 isomer of linoleic acid (t10,c12‐CLA) promotes lipoatrophy, hyperinsulinemia, and macrophage infiltration in white adipose tissue (WAT). We explored the dynamics of these interrelated responses over 2 consecu‐five 7 d periods of t10,c12‐CLA administration and withdrawal. t10,c12‐CLA down‐regulated lipogenic and lipolytic gene expression and increased collagen deposition, but with no evidence of cross‐linking. An abundant CD45+ cell infiltrate, comprising prominently CD206+CD11c‐ macrophages, was found in WAT in association with an anti‐inflammatory gene signature. Infiltration of natural killer (NK) and dendritic cells contributed to WATs innate immune response to t10,d12‐CLA. Less abundant adaptive immune cells colonized WAT, including B, NK T, γδ T, and αβ T cells. By contrast, T‐regulatory cell abundance was not affected. Interruption of treatment allowed recovery of WAT mass and normalization of insulinemia, coincident with regain of WAT homeostasis owing to a coordinated reversion of genic, structural, and immune deregulations. These data revealed a striking resilience of WAT after a short‐term metabolic injury induced by t10,d2‐CLA which relies on alternatively activated M2 macrophage engagement. In addition, the temporal links between variations in WAT alterations and insulinemia upon t10,c12‐CLA manipulation strengthen the view that WAT dysfunctional status is critically involved in altered glucose homeostasis.—Pini, M., Touch, S., Poirier, H., Dalmas, E., Niot, I., Rouault, C., Druart, C., Delzenne, N., Clément, K., André, S., Guerre‐Millo, M. Adipose tissue adaptive response to trans‐10,cis‐12‐conjugated linoleic acid engages alternatively activated M2 macrophages. FASEB J. 30, 241‐251 (2016). www.fasebj.org

INTESTINAL LIPID ESTERIFICATION AND AGING IN MICE AND RATS

The effects of aging on lipid absorption, particularly on fatty acid glycerophospholipid and triacyl-glycerol esterification, were investigated in 2.5-, 12- and 24-month-old mice and rats. Two intestinal mucosa microsomal enzymes, involved in the dietary fatty acid absorption, were assayed: acylCoA:2-monoacylglycerol acyltransferase and acylCoA:1-lysophosphatidylcholine acyltransferase. In both mice and rats, the activities of both enzymes varied with the nature of the acyl-CoA. Indeed acylCoa:2-monoacylglycerol acyltransferase activities were significantly higher with oleoyl-CoA and linoleoyl-CoA than with palmitoyl-CoA and arachidonoyl-CoA, while acylCoA:1-lysophosphatidylcholine acyltransferase activities were highest with arachidonoyl-CoA. AcylCoA:2-monoacylglycerol acyltransferase activity did not decrease significantly with aging in mice or rats, whatever the acyl-CoA used. In contrast, acylCoA:1-lysophosphatidylcholine acyltransferase activity in the 24-month-old rats was significantly lower (-47%) than in 2.5-month-old rats, with oleoyl-CoA, linoleoyl-CoA and arachidonoyl-CoA. Simultaneously we observed that less glycerophospholipid esterification of oleic and linoleic acid occurs in older rats than in 2.5-month-old rats.

P278 Un régime enrichi en isomères conjugués de l’acide linoléique (CLA) induit une inflammation massive et réversible du tissu adipeux chez la souris

Introduction Chez la souris, un regime enrichi en isomeres conjugues de l’acide linoleique (CLA) provoque une lipodystrophie majeure. Une etude recente (Poirier et al., Diabetes, 2006, 55 : 1634) revele l’accumulation rapide de macrophages dans le tissu adipeux blanc de souris traitees par l’isomere actif, t10, c12-CLA, apres 7 jours de gavage. Nous avons utilise ce modele pour caracteriser la cinetique d’infiltration du tissu adipeux par les differents types de cellules de l’immunite innee et adaptative. Materiels et Methodes Des souris C57Bl/6 ont ete soumises a un regime enrichi en CLA pendant 2, 10 semaines ou 10 semaines suivies de 4 semaines de regime normal. Les cellules immunitaires ont ete detectees par immunohistochimie dans le tissu adipeux retroperitoneal et inguinal. Resultats Un infiltrat de lymphocytes T (CD3 +) et de cellules NK (Ly49 +) apparait des 2 semaines de regime et diminue a 10 semaines. Des macrophages (F4/80 +) et des mastocytes (Tryptase +) sont presents a 2 semaines et s’accumulent massivement a 10 semaines. Le developpement d’une fibrose importante est mis en evidence par coloration au trichome de Masson. A l’arret du regime, la reconstitution rapide des reserves de graisses s’accompagne d’une diminution de tous les types de cellules de l’immunite et de la fibrose. Conclusion En conclusion, l’administration d’un regime enrichi en CLA chez la souris represente un modele d’inflammation aigue et expeditif du tissu adipeux, qui est surprenant par sa reversibilite et qui reproduit certains aspects de la pathologie d’organe associee a l’obesite, malgre l’absence d’hypertrophie adipocytaire.