Expression and localization of intestinal 15 kDa protein in the rat

Evidence for the involvement of FXR signaling in ovarian granulosa cell function

Farnesoid X receptor (FXR) is mainly present in enterohepatic tissues and regulates cholesterol, lipid, and glucose homeostasis in coordination with target genes such as SHP and FABP6. Although FXR has been revealed to be expressed in reproductive tissues, FXR function and expression levels in the ovary remain unknown. In this study, we investigated FXR expression in mouse ovaries and its target genes in ovarian granulosa cells. In situ hybridization and immunohistochemical staining showed that FXR was mainly distributed in secondary and tertiary follicles. The agonist-induced activation of FXR in cultured granulosa cells induced the expression of SHP and FABP6, while siRNA targeting of FXR decreased CYP19a1 and HSD17b1 expression. Upon examination of the roles of SHP and FABP6 in granulosa cells, we found that SHP overexpression significantly decreased StAR, CYP11a1, and HSD3b gene expression. In addition, siRNA targeting of FABP6 decreased CYP19a1 and HSD17b1 expression, while FABP6 overexpression increased CYP19a1 expression. In conclusion, the present study demonstrates the presence of FXR signaling in the ovary and reveals that FXR signaling may have a role in function of granulosa cells.

Division of Chinese soft-shelled turtle intestine with molecular markers is slightly different from the morphological and histological observation

The Chinese soft-shelled turtle (Pelodiscus sinensis) is a commercially important species in Asian countries. Knowledge of its nutritional requirements and physiology is essential for determining the appropriate content of the feed for this animal. However, the lack of functional characterization of the intestine of this turtle limits the understanding of its absorption and utilization of nutritional materials. To solve this problem, this work utilized anatomical and histological methods to characterize 9 segments sampled along the anterior-posterior axis of the intestine. Furthermore, 9 genes, which have been well documented in the intestine division of mammals and fish, were employed to functionally characterize the 9 sampled segments. Our results suggest that regions covering from the starting site to S3 (position at 29.9% of the total length from the starting of the intestine) are the equivalent of mammalian dedumonen, and those covering S4 (40.2%) and S5 (65.4%), posterior to S8 (92.7%), are the equivalent of the mammalian ileum and the large intestine, respectively. As to the region spaning S6 (81.3%) and S7 (87.3%), its functional equivalent (small intestine or large intestine) may be variable and depends on the functional genes. This molecular characterization in relation to the division of the intestine of Chinese soft-shelled turtle may contribute to the understanding of the nutritional physiology of the turtle, and promote Chinese soft-shelled turtle production.

Evolution of the duplicated intracellular lipid-binding protein genes of teleost fishes

Increasing organismal complexity during the evolution of life has been attributed to the duplication of genes and entire genomes. More recently, theoretical models have been proposed that postulate the fate of duplicated genes, among them the duplication-degeneration-complementation (DDC) model. In the DDC model, the common fate of a duplicated gene is lost from the genome owing to nonfunctionalization. Duplicated genes are retained in the genome either by subfunctionalization, where the functions of the ancestral gene are sub-divided between the sister duplicate genes, or by neofunctionalization, where one of the duplicate genes acquires a new function. Both processes occur either by loss or gain of regulatory elements in the promoters of duplicated genes. Here, we review the genomic organization, evolution, and transcriptional regulation of the multigene family of intracellular lipid-binding protein (iLBP) genes from teleost fishes. Teleost fishes possess many copies of iLBP genes owing to a whole genome duplication (WGD) early in the teleost fish radiation. Moreover, the retention of duplicated iLBP genes is substantially higher than the retention of all other genes duplicated in the teleost genome. The fatty acid-binding protein genes, a subfamily of the iLBP multigene family in zebrafish, are differentially regulated by peroxisome proliferator-activated receptor (PPAR) isoforms, which may account for the retention of iLBP genes in the zebrafish genome by the process of subfunctionalization of cis-acting regulatory elements in iLBP gene promoters.

The fatty acid binding protein 6 gene (Fabp6) is expressed in murine granulosa cells and is involved in ovulatory response to superstimulation

The fatty acid binding protein 6 (Fabp6) is commonly regarded as a bile acid binding protein found in the distal portion of the small intestine and has been shown to be important in maintaining bile acid homeostasis. Previous studies have also reported the presence of Fabp6 in human, rat and fish ovaries, but the significance of Fabp6 in this organ is largely unknown. Therefore, we surveyed murine ovaries for Fabp6 gene expression and evaluated its role in ovarian function using mice with whole body Fabp6 deficiency. Here we show that the Fabp6 gene is expressed in granulosa and luteal cells of the mouse ovary. Treatment with gonadotropins stimulated Fabp6 gene expression in large antral follicles. The ovulation rate in response to superovulatory treatment in Fabp6-deficient mice was markedly decreased compared to wildtype (C57BL/6) mice. The results of this study suggest that expression of Fabp6 gene in granulosa cells serves an important and previously unrecognized function in fertility.

Expression of selected genes involved in steroidogenesis in the course of enucleation-induced rat adrenal regeneration

The enucleation-induced (EI) rapid proliferation of adrenocortical cells is followed by their differentiation, the degree of which may be characterized by the expression of genes directly and indirectly involved in steroid hormone biosynthesis. In this study, out of 30,000 transcripts of genes identified by means of Affymetrix Rat Gene 1.1 ST Array, we aimed to select genes (either up- or downregulated) involved in steroidogenesis in the course of enucleation-induced adrenal regeneration. On day 1, we found 32 genes with altered expression levels, 15 were upregulated and 17 were downregulated [i.e., 3β-hydroxysteroid dehydrogenase (Hsd3β), nuclear receptor subfamily 0, group B, member 1 (Nr0b1), cytochrome P450 aldosterone synthase (Cyp11b2) and sterol O-acyltransferase 1 (Soat1)]. On day 15, the expression of only 2 genes was increased and that of 3 was decreased. The investigated genes were clustered according to an hierarchical clustering algorithm and 4 clusters were obtained. Quantitative PCR (qPCR) confirmed the much lower mRNA expression levels of steroidogenic acute regulatory protein (StAR) during the regeneration process compared to the control, while the cholesterol side-chain cleavage enzyme (cholesterol desmolase; Cyp11a1) and Hsd3β genes presented similar expression profiles throughout the entire regeneration process. Cyp11b2 mRNA levels remained very low during the whole regeneration period. Fatty acid binding protein 6 (Fabp6) was markedly upregulated, whereas hormone-sensitive lipase (Lipe) was downregulated. The expression of Soat1 was lowest on regeneration day 1 and, subsequently, its expression increased from there on, reaching levels higher than the control. Dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1 (Dax-1) mRNA levels were lowest on day 1 of the experiment; however, throughout the entire experimental period, there were no statistically significant differences observed. After the initial decrease in steroidogenic factor 1 (Sf-1) mRNA levels observed on the 1st day of the experiment, a marked upregulation in its expression was observed from there on. Data from the current study strongly suggest the role of Fabp6, Lipe and Soat1 in supplying substrates of regenerating adrenocortical cells for steroid synthesis. Our results indicate that during the first days of adrenal regeneration, intense synthesis of cholesterol may occur, which is then followed by its conversion into cholesteryl esters. Moreover, our data demonstrated that in enucleation-induced regeneration, the restoration of genes involved in glucocorticoid synthesis is notably shorter than that of those involved in aldosterone synthesis.

RUNX2 transcription factor regulates gene expression in luteinizing granulosa cells of rat ovaries

The LH surge promotes terminal differentiation of follicular cells to become luteal cells. RUNX2 has been shown to play an important role in cell differentiation, but the regulation of Runx2 expression and its function in the ovary remain to be determined. The present study examined 1) the expression profile of Runx2 and its partner CBFbeta during the periovulatory period, 2) regulatory mechanisms of Runx2 expression, and 3) its potential function in the ovary. Runx2 expression was induced in periovulatory granulosa cells of human and rodent ovaries. RUNX2 and core binding factor-beta (CBFbeta) proteins in nuclear extracts and RUNX2 binding to a consensus binding sequence increased after human chorionic gonadotropin (hCG) administration. This in vivo up-regulation of Runx2 expression was recapitulated in vitro in preovulatory granulosa cells by stimulation with hCG. The hCG-induced Runx2 expression was reduced by antiprogestin (RU486) and EGF-receptor tyrosine kinase inhibitor (AG1478), indicating the involvement of EGF-signaling and progesterone-mediated pathways. We also found that in the C/EBPbeta knockout mouse ovary, Runx2 expression was reduced, indicating C/EBPbeta-mediated expression. Next, the function of RUNX2 was investigated by suppressing Runx2 expression by small interfering RNA in vitro. Runx2 knockdown resulted in reduced levels of mRNA for Rgc32, Ptgds, Fabp6, Mmp13, and Abcb1a genes. Chromatin immunoprecipitation analysis demonstrated the binding of RUNX2 in the promoter region of these genes, suggesting that these genes are direct downstream targets of RUNX2. Collectively, the present data indicate that the LH surge-induced RUNX2 is involved in various aspects of luteal function by directly regulating the expression of diverse luteal genes.

Molecular cloning and tissue-specific expression of intestinal-type fatty acid binding protein in porcine

The intestinal fatty acid-binding protein (I-FABP) shows binding specificity for long-chain fatty acids and is proposed to be involved in the uptake of dietary fatty acids and their intracellular transport. In this study, the full-length cDNA of I-FABP was cloned from pig intestine by homology cloning approach combined with 3' and 5' RACE. Sequence analysis and bioinformatics study showed that this cDNA contained 614 nucleotides, with a 399 bp open reading frame (ORF) flanked by a 43 bp 5' UTR and a 172 bp 3' UTR. The encoded 132 amino acids of pig I-FABP with a molecular weight of approximately 15 kDa shared a high sequence identity of 68%-85% with those of other species. In addition, the phylogenetical analysis also indicated that the pig I-FABP was in the same branch with those of other species. The tissue-specific expression of pig I-FABP was measured by Northern hybridization and semi-quantitative RT-PCR. The results demonstrated that pig I-FABP mRNA was extensively present in various tissues, but I-FABP transcript of approximately 620 bp was more abundant in intestine than in other tissues.

Spatio-temporal distribution of fatty acid-binding protein 6 (fabp6) gene transcripts in the developing and adult zebrafish (Danio rerio)

We have determined the structure of the fatty acid-binding protein 6 (fabp6) gene and the tissue-specific distribution of its transcripts in embryos, larvae and adult zebrafish (Danio rerio). Like most members of the vertebrate FABP multigene family, the zebrafish fabp6 gene contains four exons separated by three introns. The coding region of the gene and expressed sequence tags code for a polypeptide of 131 amino acids (14 kDa, pI 6.59). The putative zebrafish Fabp6 protein shared greatest sequence identity with human FABP6 (55.3%) compared to other orthologous mammalian FABPs and paralogous zebrafish Fabps. Phylogenetic analysis showed that the zebrafish Fabp6 formed a distinct clade with the mammalian FABP6s. The zebrafish fabp6 gene was assigned to linkage group (chromosome) 21 by radiation hybrid mapping. Conserved gene synteny was evident between the zebrafish fabp6 gene on chromosome 21 and the FABP6/Fabp6 genes on human chromosome 5, rat chromosome 10 and mouse chromosome 11. Zebrafish fabp6 transcripts were first detected in the distal region of the intestine of embryos at 72 h postfertilization. This spatial distribution remained constant to 7-day-old larvae, the last stage assayed during larval development. In adult zebrafish, fabp6 transcripts were detected by RT-PCR in RNA extracted from liver, heart, intestine, ovary and kidney (most likely adrenal tissue), but not in RNA from skin, brain, gill, eye or muscle. In situ hybridization of a fabp6 riboprobe to adult zebrafish sections revealed intense hybridization signals in the adrenal homolog of the kidney and the distal region of the intestine, and to a lesser extent in ovary and liver, a transcript distribution that is similar, but not identical, to that seen for the mammalian FABP6/Fabp6 gene.

Establishment and characterization of monoclonal and polyclonal antibodies against human intestinal fatty acid-binding protein (I-FABP) using synthetic regional peptides and recombinant I-FABP

We have succeeded in raising highly specific anti-human intestinal fatty acid-binding protein (I-FABP) monoclonal antibodies by immunizing animals with three synthetic regional peptides, i.e., the amino terminal (RP-1: N-acetylated 1-19-cysteine), middle portion (RP-2: cysteinyl-91-107) and carboxylic terminal (RP-3: cysteinyl-121-131) regions of human I-FABP, and the whole I-FABP molecule as antigens. We also raised a polyclonal antibody by immunizing with a recombinant (r) I-FABP. To ascertain the specificity of these antibodies for human I-FABP, the immunological reactivity of each was examined by a binding assay using rI-FABP, partially purified native I-FABP and related proteins such as liver-type (L)-FABP, heart-type (H)-FABP, as well as the regional peptides as reactants, and by Western blot analysis. In addition, the expression and distribution of I-FABP in the human gastrointestinal tract were investigated by an immunohistochemical technique using a carboxylic terminal region-specific monoclonal antibody, 8F9, and a polyclonal antibody, DN-R2. Our results indicated that both the monoclonal and polyclonal antibodies established in this study were highly specific for I-FABP, but not for L-FABP and H-FABP. Especially, the monoclonal antibodies raised against the regional peptides, showed regional specificity for the I-FABP molecule. Immunoreactivity of I-FABP was demonstrated in the mucosal epithelium of the jejunum and ileum by immunohistochemical staining, and the immunoreactivity was based on the presence of the whole I-FABP molecule but not the presence of any precursors or degradation products containing a carboxylic terminal fragment. It is concluded that some of these monoclonal and polyclonal antibodies, such as 8F9, 4205, and DN-R2, will be suitable for use in research on the immunochemistry and clinical chemistry of I-FABP because those antibodies can recognize both types of native and denatured I-FABP. In order to detect I-FABP in blood samples, it is essential to use this type of antibody, reactive to native type of I-FABP. It is anticipated that, in the near future, such a method for measuring I-FABP will be developed as a useful tool for diagnosing intestinal ischemia by using some of these antibodies.

FXR regulates organic solute transporters alpha and beta in the adrenal gland, kidney, and intestine

Expression of the farnesoid X receptor (FXR; NR1H4) is limited to the liver, intestine, kidney, and adrenal gland. However, the role of FXR in the latter two organs is unknown. In the current study, we performed microarray analysis using RNA from H295R cells infected with constitutively active FXR. Several putative FXR target genes were identified, including the organic solute transporters alpha and beta (OSTalpha and OSTbeta). Electromobility shift assays and promoter-reporter studies identified functional farnesoid X receptor response elements (FXREs) in the promoters of both human genes. These FXREs are conserved in both mouse genes. Treatment of wild-type mice with 3-(2,6-dichlorophenyl)-4-(3'-carboxy-2-chloro-stilben-4-yl)-oxymethyl-5-isopropyl-isoxazole (GW4064), a synthetic FXR agonist, induced OSTalpha and OSTbeta mRNAs in the intestine and kidney. Both mRNAs were also induced when wild-type, but not FXR-deficient (FXR-/-), adrenals were cultured in the presence of GW4064. OSTalpha and OSTbeta mRNA levels were also induced in the adrenals and kidneys of wild-type, but not FXR-/-, mice after the increase of plasma bile acids in response to the hepatotoxin alpha-naphthylisothiocyanate. Finally, overexpression of human OSTalpha and OSTbeta facilitated the uptake of conjugated chenodeoxycholate and the activation of FXR target genes. These results demonstrate that OSTalpha and OSTbeta are novel FXR target genes that are expressed in the adrenal gland, kidney, and intestine.

Studies of the FABP family: a retrospective

Following my research on the role played by soluble proteins in their function as hydrophobic ligand carriers acting through squalene epoxidase, Dr Odani and I started to work together on low molecular lipid binding proteins. As a result of this collaboration, in 1982 we managed to determine the complete primary structure of Z-protein in rat liver. This was the first report ever to give the complete amino acid sequence of a fatty acid binding protein (FABP). This gave momentum to further such research, and now extensive exploration has been carried out on a whole family of homologous intracellular hydrophobic ligand binding proteins, the product of the expression of an ancient gene family in numerous organisms. Takahashi et al. have determined the primary structures of mammalian FABP family protein in liver, intestine, heart, kidney, and skin through amino acid sequencing as well as through determination of the cDNA sequence. Out of all my research on the FABP family, I believe, my initial study on FABP in liver, my work on kidney FABP, heart type FABP and my discovery of an I-15P (BAPB) and I-FABP application as a diagnostic marker stand out in particular.

Ileal Bile Acid-binding Protein, Functionally Associated with the Farnesoid X Receptor or the Ileal Bile Acid Transporter, Regulates Bile Acid Activity in the Small Intestine

Bile acids secreted in the small intestine are reabsorbed in the ileum where they activate the nuclear farnesoid X receptor (FXR), which in turn stimulates expression of the ileal bile acid-binding protein (I-BABP). We first hypothesized that I-BABP may negatively regulate the FXR activity by competing for the ligands, bile acids. Reporter assays using stable HEK293 cell lines expressing I-BABP revealed that I-BABP enhances rather than attenuates FXR activity. In these cells I-BABP localizes predominantly in the cytosol and partially in the nucleus, a distribution that does not shift in response to FXR expression. In vitro binding assays reveal that recombinant I-BABP is able to bind 35S-labeled FXR and that chenodeoxycholic acid (CDCA) stimulates this interaction modestly. When FLAG-tagged FXR was expressed in stable cells, the FXR.I-BABP complex in the nuclear extracts was more efficiently immunoprecipitable with anti-FLAG antibodies in the presence of CDCA. These results indicate that I-BABP stimulates FXR activity through a mutual interaction augmented by bile acids. When stable cells were transfected with an expression plasmid of the ileal bile acid transporter 14(IBAT) essential for the reabsorption of conjugated bile acids, the C-labeled conjugated bile acid, glycocholic acid, was more efficiently imported via IBAT in the presence than absence of I-BABP, whereas no change was observed in 14C-labeled CDCA uptake, which is independent of IBAT. Immunofluorescent staining analysis revealed that these two proteins co-localize in the vicinity of the plasma membrane in stable cells. Taken together, the current data provide the first evidence that I-BABP is functionally associated with FXR and IBAT in the nucleus and on the membrane, respectively, stimulating FXR transcriptional activity and the conjugated bile acid uptake mediated by IBAT in the ileum.

Functional conserved elements mediate intestinal-type fatty acid binding protein (I-FABP) expression in the gut epithelia of zebrafish larvae

Intestinal-type fatty acid binding protein (I-FABP) plays an important role in the intracellular binding and trafficking of long chain fatty acids in the intestine. The aim of this study, therefore, was to elucidate the regulation and spatiotemporal expression of the I-FABP gene during zebrafish larval development. We performed in vivo reporter-gene analysis in zebrafish by using a transient and transgenic approach. Green fluorescent protein-reporter analyses revealed that the proximal 192-bp region of the I-FABP promoter is sufficient to direct intestine-specific expression during zebrafish larval development. Functional dissection of a 41-bp region within this 192-bp promoter revealed that one C/EBP and two GATA-like binding sites, along with a novel 15-bp element within it are required for I-FABP gene expression in vivo. In addition, the six consensus sites (CCACATCAGCATGAA) in the 15-bp element are critical for I-FABP gene regulation in the zebrafish gut epithelia. Comparison analyses of the orthologous 15-bp element from mammalian I-FABP genes suggests that these mammalian elements are functionally equivalent to the zebrafish 15 element. These results provide the first in vivo evidence that these binding sites (C/EBP and GATA) and the novel 15-bp element contribute to intestine-specific gene expression and that they are functionally conserved across vertebrate evolution.

Sequencing and chromosomal localization of Fabp6 and an intronless Fabp6 segment in the rat

The fatty acid binding protein 6 gene (Fabp6) codes for ileal lipid binding protein. After sequencing of rat Fabp6, the gene was localized in a radiation hybrid (RH) map on chromosome 10. An intronless Fabp6 segment was found in four related rat inbred strains (SHR; SHRSP; WKY; and OKA), but not in 62 other rat inbred strains. The intronless Fabp6 segment, which might be a pseudogene of Fabp6, was localized on rat chromosome 15.

Structural biology and function of solute transporters: implications for identifying and designing substrates

Solute carrier (SLC) proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made in characterizing the peptide transporter (PepT1) and the apical sodium dependent bile acid transporter (ASBT) that are important for both their native transporter function as well as targets to increase absorption and act as therapeutic targets. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of SLC function will be led by integrated in vitro and in silico approaches.

Synthesis of [3,4-(13)c(2)]-enriched bile salts as NMR probes of protein-ligand interactions

Synthetic methodology that allows for incorporation of isotopic carbon at the C-3 and C-4 positions of bile salts is reported. Three [3,4-(13)C(2)]-enriched bile salts were synthesized from either deoxycholic or lithocholic acid. The steroid 3alpha-OH group was oxidized and the A-ring was converted into the Delta(4)-3-ketone. The C-24 carboxylic acid was next converted into the carbonate group and selectively reduced to the alcohol in the presence of the A-ring enone. Following protection of the 24-OH group, the Delta(4)-3-ketone was converted into the A-ring enol lactone. Condensation of the enol lactone with [1,2-(13)C(2)]-enriched acetyl chloride and subsequent Robinson annulation afforded a [3,4-(13)C(2)]-enriched Delta(4)-3-ketone that was subsequently converted back into a 3alpha-hydroxy-5beta-reduced bile steroid. C-7 hydroxylation, when necessary, was achieved via conversion of the Delta(4)-3-ketone into the corresponding Delta(4,6)-dien-3-one, epoxidation of the Delta(6)-double bond, and hydrogenolysis/hydrogenation of the 5,6-epoxy enone system. The [3,4-(13)C(2)]-enriched bile salts were subsequently complexed to human ileal bile acid binding protein (I-BABP), and (1)H-(13)C HSQC spectra were recorded to show the utility of the compounds for investigating the interactions of bile acids with I-BABP.

Cellular retinol-binding protein type II (CRBPII) in adult zebrafish (Danio rerio). cDNA sequence, tissue-specific expression and gene linkage analysis

We have determined the nucleotide sequence of a zebrafish cDNA clone that codes for a cellular retinol-binding protein type II (CRBPII). Radiation hybrid mapping revealed that the zebrafish and human CRBPII genes are located in syntenic groups. In situ hybridization and emulsion autoradiography localized the CRBPII mRNA to the intestine and the liver of adult zebrafish. CRBPII and intestinal fatty acid binding protein (I-FABP) mRNA was colocalized to the same regions along the anterior-posterior gradient of the zebrafish intestine. Similarly, CRBPII and I-FABP mRNA are colocalized in mammalian and chicken intestine. CRBPII mRNA, but not I-FABP mRNA, was detected in adult zebrafish liver which is in contrast to mammals where liver CRBPII mRNA levels are high during development but rapidly decrease to very low or undetectable levels following birth. CRBPII and I-FABP gene expression appears therefore to be co-ordinately regulated in the zebrafish intestine as has been suggested for mammals and chicken, but CRBPII gene expression is markedly different in the liver of adult zebrafish compared to the livers of mammals. As such, retinol metabolism in zebrafish may differ from that of mammals and require continued production of CRBPII in adult liver. The primary sequence of the coding regions of fish and mammalian CRBPII genes, their relative chromosomal location in syntenic groups and possibly portions of the control regions involved in regulation of CRBPII gene expression in the intestine appear therefore to have been conserved for more than 400 million years.

Modeling of active transport systems

Transport proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made to characterize the P-glycoprotein efflux pump, the peptide transporter (PepT1) and the apical sodium-dependent transporter (ASBT) which are important not only for their native transporter function but also as drug targets to increase absorption and bioactivity. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of transporter function will be led by integrated in vitro and in silico approaches.

The mammalian fatty acid-binding protein multigene family: molecular and genetic insights into function

Intracellular fatty acid-binding proteins associate with fatty acids and other hydrophobic biomolecules in an internal cavity, providing for solubilization and metabolic trafficking. Analyses of their in vivo function by molecular and genetic techniques reveal specific function(s) that fatty acid-binding proteins perform with respect to fatty acid uptake, oxidation and overall metabolic homeostasis.

cDNA sequence and tissue-specific expression of a basic liver-type fatty acid binding protein in adult zebrafish (Danio rerio)

We have determined the nucleotide sequence for a cDNA clone derived from zebrafish (Danio rerio) that codes for a fatty acid binding protein (FABP). Amino acid sequence similarity of the zebrafish FABP was highest to FABPs isolated from the livers of catfish, chicken, salamander and iguana. The open-reading frame of the zebrafish FABP cDNA codes for a protein of 14.0 kDa with a calculated isoelectric point of 8.8. The zebrafish liver-type FABP (L-FABP) cDNA hybridized to single restriction fragments of total zebrafish genomic DNA digested with the restriction endonucleases HaeIII or EcoRI suggesting that a single copy of the L-FABP gene is present in the zebrafish genome. Northern blot analysis demonstrated that the zebrafish L-FABP cDNA hybridized to a transcript of 700 nucleotides in total zebrafish RNA. In situ hybridization and emulsion autoradiography revealed that the L-FABP was expressed exclusively in the liver of the adult zebrafish. Based on amino acid sequence similarity, the isoelectric point and its tissue-specific pattern of expression, we conclude that this zebrafish FABP belongs to the basic liver-type FABPs only found, thus far, in non-mammalian vertebrates.

Hormonal regulation of expression of ileal bile acid binding protein in suckling rats

Ileal bile acid binding protein (IBABP) is a cytosolic protein believed to be involved in the absorption of conjugated bile acids. In rodents this protein and its mRNA have been shown to increase markedly during the third postnatal week. Because this period of ontogeny is characterized by increasing circulating concentrations of glucocorticoids and thyroxine, the goal of our study was to investigate the role of these hormones in IBABP expression in the developing rat. Administration of various doses of dexamethasone (Dex) during the second postnatal week caused a robust induction of IBABP mRNA and protein. Plateau levels of IBABP mRNA occurred at a Dex dose of 0.1 microg/g body wt, which is within the physiological range. IBABP mRNA was not appreciably induced until 24 h after treatment, suggesting that glucocorticoids influence IBABP either through a delayed primary or a secondary response mechanism. The regional pattern of IBABP mRNA elicited by Dex mimicked that seen during normal development, with appearance in distal ileum preceding proximal ileum. Thyroxine injections did not result in a significant increase of IBABP mRNA, and synergism between Dex and thyroxine was not observed. Taken together, our data suggest that maturation of IBABP expression is influenced by glucocorticoids but not by thyroxine.

Nucleotide sequence of a cDNA clone coding for an intestinal-type fatty acid binding protein and its tissue-specific expression in zebrafish (Danio rerio)

We have cloned a cDNA from zebrafish (Danio rerio) that contains an open-reading frame of 132 amino acids coding for a fatty acid binding protein (FABP) of approximately 15 kDa. Multiple sequence alignment revealed extensive amino acid identity between this zebrafish FABP and intestinal-like FABPs (I-FABP) from other species. The zebrafish I-FABP cDNA hybridized to single restriction fragments of total zebrafish genomic DNA digested with the restriction endonucleases PstI Bg/II or EcoRI suggesting that a single copy of the I-FABP gene is present in the zebrafish genome. An oligonucleotide probe complementary to the zebrafish I-FABP mRNA hybridized to an mRNA of approximately 800 bases in Northern blot analysis. In situ hybridization revealed that the I-FABP mRNA was expressed exclusively in the intestine of the adult zebrafish.

Topological photoaffinity labeling of the rabbit ileal Na+/bile-salt-cotransport system

For the investigation of the topology of the rabbit ileal Na+/bile-salt-cotransport system, composed of a 93-kDa integral membrane protein and a peripheral 14-kDa bile-acid-binding protein (ILBP), we have synthesized photolabile dimeric bile-salt-transport inhibitors (photoblockers), G1-X-G2, where two bile acid moieties (G1 and G2) are tethered together via a spacer, X, and where one of the two bile acid moieties carries a photoactivatable group. These photoblockers specifically interact with the ileal Na+/bile-salt-cotransport system as demonstrated by a concentration-dependent inhibition of [3H]cholyltaurine uptake by rabbit ileal brush-border membrane vesicles and by inhibition of photolabeling of the 93-kDa and 14-kDa bile-salt-binding proteins by 7,7-azo and 3,3-azo derivatives of cholyltaurine. Ileal bile-salt uptake was specifically inhibited by the photoblockers, which were not taken up themselves by the small intestine as demonstrated by in vivo ileal perfusion. Dependent on the photoblocker used several polypeptides in the molecular-mass range of 14-130 kDa were labeled. The cytoplasmically attached 14-kDa ILBP was significantly labeled only by inhibitors that are photoactivatable in bile acid moiety G1, suggesting that during binding and translocation of a bile-salt molecule by the ileal bile-salt-transport system the steroid nucleus gets access to the cytoplasmic site of the ileal brush-border membrane first. Photoaffinity labeling in the frozen state with the transportable 3,3-azo and 7,7-azo derivatives of cholyltaurine revealed a time-dependent increase in the extent of labeling of the 14-kDa and 93-kDa proteins, suggesting a labeling of these proteins from the cytoplasmic site of the ileal brush-border membrane. By photoaffinity labeling in the frozen state with the various photoblockers time-dependent changes in the extent of photoaffinity labeling of bile-salt-binding proteins were observed, demonstrating the possibility of topological analysis of the rabbit ileal Na+/bile-salt-cotransport system.

Tissue-specific regulation of the expression of rat intestinal bile acid-binding protein

A lipid-binding protein identical to the rat intestinal bile acid-binding protein, termed I-15P, was expressed in steroid hormone-producing tissues such as ovary and adrenal gland, but not testis. In immature rats, I-15P was expressed in intestine but not in ovaries. The expression of I-15P in the ovaries of immature rats was induced to the level in immature rats by gonadotropin treatment. This suggests that the expression of I-15P in the ovaries is controlled by the ovarian cycle. The present results indicate that the expression of I-15P is developmentally and hormonally controlled in a tissue-specific manner.

Molecular cloning, expression, and characterization of a human intestinal 15-kDa protein

We have isolated a cDNA encoding a human intestinal 15-kDa protein (I-15P) from a human ileal lambda gt 11 cDNA library, using a full-length rat I-15P cDNA. One clone encompassed 571 nucleotides and encoded a 128-amino-acid protein with a calculated molecular mass of 14355 Da. The deduced amino acid sequence of human I-15P showed high similarity to the rat counterpart (78%), mouse ileal lipid-binding protein (80%) and porcine gastrotropin (75%). It also exhibited 36% similarity to human liver fatty-acid-binding protein (L-FABP). Northern blot analysis of human I-15P revealed a single transcript only in ileum, however, the reverse-transcription/PCR demonstrated expression in ovary and placenta, but it was much lower than in ileum. Transformation of Escherichia coli with the I-15P cDNA resulted in the efficient expression of a protein that was identical to the ileal cytosolic I-15P. In vitro binding studies revealed that the bacterially expressed recombinant I-15P showed much lower affinities for palmitate and oleate than L-FABP. However, it showed similar affinity for taurocholate, compared with a control, BSA. Comparison of the structural features of human I-15P and human L-FABP suggested that loss of a long alpha-helix region and hydrophobic profile of I-15P may be attributable to a unique ligand-binding specificity of I-15P.