Fatty Acid Binding Protein-4 is expressed in the mouse placental labyrinth, yet is dispensable for placental triglyceride accumulation and fetal growth
Introduction
The trans-placental maternal-to-fetal transport of nutrients is essential for normal growth and development of the eutherian embryo. Fetal growth restriction that results from fetal hypo-nutrition has been associated with intrauterine fetal death, neonatal and childhood morbidity, and the adult metabolic syndrome with its consequences [1]. Among nutrients that support intrauterine growth, fatty acids are germane, particularly during the second half of pregnancy, when fetal growth accelerates and fat accretion increases exponentially [2], [3], [4], [5]. Fatty acids are also essential for organ development–for instance, in the brain, retina, heart and lungs [6], [7], [8], [9], [10], [11]–where they serve as a source of calories and membrane building blocks and also as precursors for the production of eicosanoids, phospholipids, and their derivatives [12], [13].
The major fraction of fatty acids that circulate in the maternal blood is covalently bound by triglycerides and lipoproteins, with a smaller fraction circulating as free fatty acids bound by albumin, VLDL, or chylomicron. Fatty acids are released for placental uptake by the action of triglyceride hydrolases and transported across the trophoblastic microvillous membrane, with kinetics that depend on biochemical characteristics such as chain length, lipid solubility, and affinity to carrier proteins [14], [15], [16], [17], [18], [19]. Within cells, cytoplasmic fatty acids are bound by fatty acid binding proteins (FABPs). These small (132 aa) proteins mediate intracellular fatty acid transport to lipid droplets for storage, to mitochondria or peroxisomes for oxidation and energy production, to the endoplasmic reticulum for membrane synthesis and cell signaling, and to the nucleus for regulation of gene expression [20].
We have previously determined that FABP1, FABP3, FABP4, FABP5, and FABPpm are the dominant FABP forms expressed in human placental trophoblasts [21], [22]. Importantly, we found that hypoxia selectively upregulated the expression of FABP1, FABP3 and FABP4 and that fatty acids enhance the expression of FABP4. These changes were associated with increased lipid droplet and triglyceride accumulation in primary human trophoblasts (PHT) and were attenuated by inhibition of FABP4 [21], [22]. Coupled with the known function of FABP4 (also known as adipocyte FABP4, AFABP4, adipocyte protein 2, and aP2) as a fatty acid chaperone [23], [24], [25], our findings led us to propose the hypothesis that FABP4 is a key regulator of trophoblastic lipid transport and accumulation. To assess the function of FABP4 in vivo, we sought to determine the expression of FABP4 in the mouse placenta, and determine the impact of Fabp4 ablation in mice on feto-placental development and triglyceride accumulation.
Section snippets
Animals and breeding
Our experiments were performed using either wild type (wt) C57Bl/6 mice, obtained from Jackson Laboratory, or C57Bl/6 mice that harbor a targeted mutation in the Fabp4 gene, which were provided by Dr. G. Hotamisligil (Harvard University) [26]. Our experimental protocols were approved by the Institutional Animal Care and Use Committee of the University of Pittsburgh. We performed timed breeding by pairing heterozygous males and females overnight, with the morning after mating labeled as
Results
To gain insight into the role of placental FABP4 in vivo, we initially sought to examine the expression of FABP4 in placentas of wild type C57Bl/6 mice. Using immunohistochemistry (Fig. 1), we confirmed that FABP4 is highly expressed in the maternal decidua, as previously shown [29]. Importantly, within the placenta proper, we found that FABP4 is primarily expressed in the labyrinthine layer and not in the junctional zone (Fig. 1, Fig. 2J). Using immunofluorescent co-staining for the
Discussion
We previously found that FABP4 is expressed in human placental trophoblasts. We also found that exposure of PHT cells to either fatty acids or hypoxia in vitro enhanced the expression of FABP4 and that pharmacological or siRNA-mediated inhibition of FABP4 markedly reduced the accumulation of triglycerides in trophoblasts [21], [22]. Notably, FABP4 was previously shown to be highly expressed in the mouse decidua as early as E5, with a major increase in expression on E7–8 [29], yet the expression
Conflict of interest
None declared.
Financial support
This project was supported by R01-HD045675, P01-HD069316, and R01-ES011597 (to YS).
Disclosure
The authors have nothing to disclose.
Acknowledgments
We thank Dr. Dr. Gokhan Hotamisligil (Harvard School of Public Health) for generously providing the Fabp4 heterozygous mice. We thank Judy Ziegler and Sun Huijie for technical assistance, Lori Rideout for assistance in manuscript preparation, and Bruce Campbell for editing. The project was supported by NIH grants R01-HD045675, P01-HD069316, and R01-ES011597 (to YS).
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