Intestinal fatty acid binding protein regulates mitochondrion b-oxidation and cholesterol uptake

The role of intestinal fatty acid binding protein (IFABP) in lipid metabolism remains elusive. To address this issue, normal human intestinal epithelial cells (HIEC-6) were transfected with cDNA to overexpress I-FABP and compared with cells treated with empty pQCXIP vector. I-FABP overexpression stimulated mitochondrial [U-C]oleate oxidation to CO2 and acid-soluble metabolites via mechanisms including the upregulation of protein expression and the activity of carnitine palmitoyltransferase 1, a critical enzyme controlling the entry of fatty acid (FA) into mitochondria, and increased activity of 3-hydroxyacyl-CoA dehydrogenase, a mitochondrial b-oxidation enzyme. On the other hand, the gene and protein expression of the key enzymes FA synthase and acetylcoenzyme A carboxylase 2 was decreased, suggesting diminished lipogenesis. Furthermore, I-FABP overexpression caused a decline in [C]free cholesterol (CHOL) incorporation. Accordingly, a significant lessening was observed in the gene expression of Niemann Pick C1-Like 1, a mediator of CHOL uptake, along with an increase in the transcripts and protein content of ABCA1 and ABCG5/ABCG8, acting as CHOL efflux pumps. Furthermore, I-FABP overexpression resulted in increased levels of mRNA, protein mass, and activity of HMG-CoA reductase, the rate-limiting step in CHOL synthesis. Scrutiny of the nuclear receptors revealed augmented peroxisome proliferator-activated receptor a,g and reduced liver X receptor-a in HIEC-6 overexpressing IFABP. Finally, I-FABP overexpression did not influence acylcoenzyme A oxidase 1, which catalyzes the first rate-limiting step in peroxisomal FA b-oxidation. Overall, our data suggest that I-FABP may influence mitochondrial FA oxidation and CHOL transport by regulating gene expression and interaction with nuclear receptors.—Montoudis, A., E. Seidman, F. Boudreau, J-F. Beaulieu, D. Menard, M. Elchebly, G. Mailhot, A-T. Sane, M. Lambert, E. Delvin, and E. Levy. Intestinal fatty acid binding protein regulates mitochondrion b-oxidation and cholesterol uptake. J. Lipid Res. 2008. 49: 961–972. Supplementary key words I-FABP & HIEC-6 & cholesterol metabolism Fatty acid binding proteins (FABPs) display a high affinity for long-chain fatty acids (LCFA) and appear to function in the metabolism and intracellular transport of lipids (1). Two distinct FABPs are expressed in the small intestine: the liver (L)-FABP encoded by the FABP1 gene and the intestinal (I)-FABP encoded by the FABP2 gene (2). The function of these proteins is still under investigation. The presence of two structurally distinct, independently regulated FABPs in the intestine has led to the speculation that these proteins assume unique roles in intestinal fatty acid (FA) metabolism (2, 3). Because targeted gene disruption studies may shed light on the physiological importance of these proteins, Vassileva et al. (4) invalidated mouse FABP2 and studied the impact on intestinal lipid transport. The results of their experiments suggest that I-FABP knockout does not produce a detrimental effect on dietary fat absorption but may cause weight gain and hyperinsulinemia. Additionally, our observations using normal human intestinal epithelial cells (HIEC-6) overexpressing sizeable amounts of I-FABP demonstrated a negligible influence of this transporter on lipid synthesis, apolipoprotein biogenesis, and lipoprotein exocytosis (5). Conversely, examination of the human intestinal cell line Caco-2 overexpressing human I-FABP showed minimal effects on FA incorporation (6). Because no in vivo or in vitro function can be definitively put forward for I-FABP, a puzzling and interesting question pertaining to its specific role in the enterocyte persists. Findings from I-Fabp mice (4) and the Ala54Thr human mutation (7–9) suggest that I-FABP functions physiologically as a lipid-sensing component of energy homeostasis and not as a direct part of dietary FA absorption. Vassileva et al. (4) proposed that “I-FABP likely feeds information about dietary lipid status into mechanisms that Manuscript received 14 August 2007 and in revised form 11 January 2008. Published, JLR Papers in Press, January 30, 2008. DOI 10.1194/jlr.M700363-JLR200 1 To whom correspondence should be addressed. e-mail: [email protected] Copyright D 2008 by the American Society for Biochemistry and Molecular Biology, Inc. This article is available online at http://www.jlr.org Journal of Lipid Research Volume 49, 2008 961 by gest, on O cber 4, 2017 w w w .j.org D ow nladed fom universally control energy utilization, energy storage, and eventually body weight.” Therefore, we wonder whether I-FABP i) is involved in the movement of FA to specific subcellular compartments other than the endoplasmic reticulum; ii) influences b-oxidation in mitochondria; iii) intervenes in the metabolism of other lipid classes, such as cholesterol (CHOL); and iv) is associated with the alteration of transcription factor expression in the nucleus. These questions were addressed in HIEC-6 with forced expression of I-FABP. MATERIALS AND METHODS