DNA methyltransferase in normal and Dnmtn/Dnmtn mouse embryos

The mouse genome experiences a large decrease in net 5-methylcytosine between fertilization and implantation; de novo methylation brings 5-methylcytosine to adult somatic cell levels between implantation and gastrulation. Very little is known of the regulation of demethylation or de novo methylation. Levels of the one known form of DNA methyltransferase are very high in early embryos, but the enzyme is localized to the cytoplasm during most of preimplantation development. We show here that DNA methyltransferase is found exclusively in nuclei of the conceptus after implantation, and that nuclei of proximal decidual cells are free of detectable DNA methyltransferase. High levels of DNA methyltransferase were seen in all tissues, including the developing nervous system, of 9.5- to 12.5-day embryos. The large maternal stores of DNA methyltransferase become limiting prior to embryonic day 9.5, as shown by barely detectable immunostaining in 9.5-day embryos homozygous for a loss-of-function mutation (Dnmtn) in the DNA methyltransferase gene. These mutant embryos failed to develop past the 25-somite stage and showed evidence of developmental delay and some developmental asynchrony. Normal embryonic and extraembryonic tissues contained similar levels of DNA methyltransferase, even though severely reduced methylation levels and a loss of imprinting have previously been observed in extraembryonic tissues. These findings suggest that methylation patterns are not a simple function of the concentration of DNA methyltransferase, and that unidentified factors must be involved in the regulation of de novo methylation during early development of the mouse.

Structure/function of cytoplasmic vitamin A-binding proteins

Two cytoplasmic retinol-binding proteins, CRBP and CRBP II, and two cytoplasmic retinoic acid-binding proteins, CRABP-I and CRABP-II, have been well characterized. There has been significant progress in the structural analysis of these four proteins with X-ray crystallography, nuclear magnetic resonance, mutagenesis, and binding studies. In contrast, the cellular functions of these cytoplasmic vitamin A-binding proteins are less well understood. Since these proteins bind their respective ligands with high affinity, they are likely to influence retinoid signaling pathways. Analysis of retinoid metabolism in the presence or absence of these proteins provides support for the hypothesis that these proteins are involved in modulating intracellular retinoid metabolism. Molecular genetic approaches to alteration of the levels of these proteins in tissue culture cells and in whole animals have provided a powerful means toward defining the physiological roles of the cytoplasmic vitamin A-binding proteins in vivo.

Complementation of an Escherichia coli adhE mutant by the Entamoeba histolytica EhADH2 gene provides a method for the identification of new antiamebic drugs

The pathogenic protozoan parasite Entamoeba histolytica, the cause of amebic dysentery and amebic liver abscess, is an obligate anaerobe, and derives energy from the fermentation of glucose to ethanol with pyruvate and acetyl coenzyme A as intermediates. We have isolated EhADH2, a key enzyme in this pathway, that is a NAD+- and Fe2+-dependent bifunctional enzyme with acetaldehyde dehydrogenase and alcohol dehydrogenase activities. EhADH2 is the only known eukaryotic member of a newly defined family of prokaryotic multifunctional enzymes, which includes the Escherichia coli AdhE enzyme, an enzyme required for anaerobic growth of E. coli. Because of the critical role of EhADH2 in the amebic fermentation pathway and the lack of known eukaryotic homologues of the EhADH2 enzyme, EhADH2 represents a potential target for antiamebic chemotherapy. However, screening of compounds for antiamebic activity is hampered by the cost of large scale growth of Ent. histolytica, and difficulties in quantitating drug efficacy in vitro. To approach this problem, we expressed the EhADH2 gene in a mutant strain of E. coli carrying a deletion of the adhE gene. Expression of EhADH2 restored the ability of the mutant E. coli strain to grow under anaerobic conditions. By screening compounds for the ability to inhibit the anaerobic growth of the E. coli/EhADH2 strain, we have developed a rapid assay for identifying compounds with anti-EhADH2 activity. Using bacteria to bypass the need for parasite culture in the initial screening process for anti-parasitic agents could greatly simplify and reduce the cost of identifying new therapeutic agents effective against parasitic diseases.

Molecular cloning and characterization of a transmembrane surface antigen in human cells

The mouse mAb 6C6, raised against a plasma-membrane preparation from human breast-cancer cells, reacts with an antigen that appears to be overexpressed in human breast cancers and other human tumors. Here we describe the cDNA cloning and characterization of the antigen recognized by the 6C6 mAb. The isolated cDNA clone encodes a protein of 246 amino acids, with a predicted molecular mass of 27 991 Da. The protein contains three amino-terminal hydrophobic regions, which could represent transmembrane domains, and a hydrophilic carboxy-terminal region, which we show to be extracellular. The identity of the protein encoded by the cloned cDNA as the 6C6 antigen was confirmed by in vitro translation and immunoprecipitation experiments, and by transfection into cell lines that do not react with the 6C6 mAb, which resulted in the expression of a 28-kDa surface protein that was recognized by the antibody. The 6C6 antigen appears to be a type II transmembrane protein, with multiple membrane-spanning domains and a long extracellular non-glycosylated carboxy-terminal domain, to which the 6C6 epitope has been mapped. The overall structure of the protein and weak amino acid similarities with a family of multiple-transmembrane-spanning-domain proteins that includes some antigens (such as L6, CD63/ME491 and CO-029) that are overexpressed in tumor cells, suggest that the 6C6 antigen may belong to this family of proteins.

Germ-line passage is required for establishment of methylation and expression patterns of imprinted but not of nonimprinted genes

Embryonic stem (ES) cells homozygous for a disruption of the DNA (cytosine-5)-methyltransferase gene (Dnmt) proliferate normally with their DNA highly demethylated but die upon differentiation. Expression of the wild-type Dnmt cDNA in mutant male ES cells caused an increase in methylation of bulk DNA and of the Xist and Igf2 genes to normal levels, but did not restore the methylation of the imprinted genes H19 and Igf2r. These cells differentiated normally in vitro and contributed substantially to adult chimeras. While the Xist gene was not expressed in the remethylated male ES cells, no restoration of the normal expression profile was seen for H19, Igf2r, or Igf2. This indicates that ES cells can faithfully reestablish normal methylation and expression patterns of nonimprinted genes but lack the ability to restore those of imprinted genes. Full restoration of monoallelic methylation and expression was imposed on H19, Igf2, and Igf2r upon germ-line transmission. These results are consistent with the presence of distinct de novo DNA methyltransferase activities during oogenesis and spermatogenesis, which specifically recognize imprinted genes but are absent in the postimplantation embryo and in ES cells.

Pulmonary surfactant given prophylactically alleviates an asthma attack in guinea-pigs

BACKGROUND

Previous studies have indicated that the increased airway resistance that develops in asthma may partly be due to a surfactant dysfunction. If so, it might be possible to alleviate the acute signs following an allergen challenge by prophylactically instilling into the airways a well functioning pulmonary surfactant.

OBJECTIVE

The study was planned and enacted to test the above hypothesis.

METHODS

The lung function (airway resistance, tidal volume, minute ventilation, and dynamic compliance) of 22 immunized guinea-pigs was studied for 30 min following a challenge. Ten of the animals had received a tracheal instillation of 0.5 mL calf lung surfactant extract (CLSE, 35 mg/mL) prior to the challenge.

RESULTS

The animals receiving the dose of 17.5 mg surfactant were less affected by the challenge than were the controls. Only one of them died following the challenge, whereas four of the 12 controls succumbed. Lung function was significantly less affected among the nine surviving animals treated with surfactant prior to the challenge than among the eight surviving controls (P < 0.01) and also their blood gases (pCO2 and pO2) were less influenced (P < 0.05).

CONCLUSION

The study indicated that the symptoms developing after a challenge, which to some extent simulate those of asthma, can be alleviated by a prophylactic airway instillation of pulmonary surfactant.

T cell responses in calcineurin A alpha-deficient mice

We have created embryonic stem (ES) cells and mice lacking the predominant isoform (alpha) of the calcineurin A subunit (CNA alpha) to study the role of this serine/threonine phosphatase in the immune system. T and B cell maturation appeared to be normal in CNA alpha -/- mice. CNA alpha -/- T cells responded normally to mitogenic stimulation (i.e., PMA plus ionomycin, concanavalin A, and anti-CD3 epsilon antibody). However, CNA alpha -/- mice generated defective antigen-specific T cell responses in vivo. Mice produced from CNA alpha -/- ES cells injected into RAG-2-deficient blastocysts had a similar defective T cell response, indicating that CNA alpha is required for T cell function per se, rather than for an activity of other cell types involved in the immune response. CNA alpha -/- T cells remained sensitive to both cyclosporin A and FK506, suggesting that CNA beta or another CNA-like molecule can mediate the action of these immunosuppressive drugs. CNA alpha -/- mice provide an animal model for dissecting the physiologic functions of calcineurin as well as the effects of FK506 and CsA.

DNA methylation specifies chromosomal localization of MeCP2

MeCP2 is a chromosomal protein that is concentrated in the centromeric heterochromatin of mouse cells. In vitro, the protein binds preferentially to DNA containing a single symmetrically methylated CpG. To find out whether the heterochromatic localization of MeCP2 depended on DNA methylation, we transiently expressed MeCP2-LacZ fusion proteins in cultured cells. Intact protein was targeted to heterochromatin in wild-type cells but was inefficiently localized in mutant cells with low levels of genomic DNA methylation. Deletions within MeCP2 showed that localization to heterochromatin required the 85-amino-acid methyl-CpG binding domain but not the remainder of the protein. Thus MeCP2 is a methyl-CpG-binding protein in vivo and is likely to be a major mediator of downstream consequences of DNA methylation.

Nuclear magnetic resonance studies demonstrate differences in the interaction of retinoic acid with two highly homologous cellular retinoic acid binding proteins

Cellular retinoic acid binding protein-I (CRABP-I) and cellular retinoic acid binding protein-II (CRABP-II) are highly homologous, 15 kDa proteins which bind all-trans-retinoic acid. In the adult, CRABP-II is expressed predominately in the epidermis, while CRAPB-I is expressed in a variety of tissues. To obtain structural information which could aid the design of more selective ligands, isotope-directed NMR methods were employed to observe the CRABP-bound conformation of 13C-labeled retinoic acid and to identify its contact points with neighboring amino acids. Analysis of HMQC, HMQC-TOCSY, and 13C-TOCSY-REVINEPT on CRABP-bound (2,3,6,7,8,9,10,11,19-13C)- and (1,4,5,8,9,16, 17,18,19-13C)-all trans-retinoic acid allowed the unambiguous assignment of all labeled protons and their attached 13C resonances. The volumes of 16 olefinic proton-methyl NOE cross-peaks measured from 30-ms 13C-(omega 2)-filtered 1H NOESY experiments were used to determine the conformations about the 6-, 8-, and 10-single bonds of the retinoic acid polyene chain. These spectra show qualitatively distinct NOE patterns for the two CRABPs. Measured cross-peak volumes for CRABP-II bound retinoic acid were well predicted by a single, static conformational having a 6-s torsion angle of -60 degrees skewed from a cis conformation. In contrast, for CRABP-I no single, static conformation was able to match the pattern of cross-peaks, suggesting motion about the 6-s bond. The measured cross-peaks were best described by 8-s and 10-s torsion angles of 180 degrees +/- 30 degrees, a trans configuration, for both proteins. The pattern of intermolecular NOESY cross-peaks between 13C-labeled protons in the ring portion of retinoic acid and protein protons were different between CRABP-I and CRABP-II. These differences coincide well with nearby amino acid substitutions in the recently reported X-ray structures of crystalline CRABP-I and CRABP-II and may assist rational design of selective ligands.

Interaction of laminin with Entamoeba histolytica cysteine proteinases and its effect on amebic pathogenesis

The Entamoeba histolytica 27-kDa cysteine proteinases exhibit striking binding specificities for immobilized laminin over other components of the extracellular matrix, such as collagen and fibronectin. Inactivation of the proteinase with the active-site inhibitor L-trans-epoxysuccinyl-leucylamido(4-guanidino)butane abolishes laminin binding by the enzyme, and conversely, laminin inhibits cleavage of a fluorogenic dipeptide substrate of the amebic cysteine proteinase, suggesting that the substrate binding pocket of the enzyme is involved in the binding of laminin. The addition of laminin but not fibronectin or collagen to E. histolytica trophozoites significantly reduces amebic liver abscess formation in severe combined immunodeficient mice, further supporting the hypothesis that E. histolytica cysteine proteinases play an important role in amebic pathogenesis. The specific interaction of amebic proteinases with laminin may be exploited in designing new inhibitors of these enzymes.

Loss of methylation activates Xist in somatic but not in embryonic cells

The mouse Xist gene, which is expressed only from the inactive X chromosome, is thought to play a role in the initiation of X inactivation. The 5' end of this gene is fully methylated on the active X chromosome and completely demethylated on the inactive X chromosome, suggesting that DNA methylation may be involved in controlling allele-specific transcription of this gene. To directly investigate the importance of DNA methylation in the control of Xist expression, we have examined its methylation patterns and expression in ES cells and embryos that are deficient in DNA methyltransferase activity. We report here that demethylation of the Xist locus in male mutant embryos induces Xist expression, thus establishing a direct link between demethylation and expression of the Xist gene in the postgastrulation embryo. The transcriptional activity of Xist in undifferentiated ES cells, however, appears to be independent of its methylation status. These results suggest that methylation may only become essential for Xist repression after ES cells have differentiated or after the embryo has undergone gastrulation.

Conformationally defined 6-s-trans-retinoic acid analogs. 2. Selective agonists for nuclear receptor binding and transcriptional activity

We recently demonstrated in animal models that a new conformationally defined RA isomer (Vaezi et al. J. Med. Chem. 1994, 37, 4499-4507) was as effective as RA in the prevention of skin papillomas but was less toxic. In order to provide more details concerning this improved action, we report here the preparation of a homologous conformationally defined 6-s-trans-retinoid (1) and investigate its ability to interact with proteins and to activate gene expression. Four configurational isomers of 1 were evaluated in binding assays for cellular retinoic acid binding protein, CRABP (isolated from chick skin); CRABP-I and CRABP-II (cloned from mouse); nuclear retinoic acid receptors (RARs); and nuclear retinoid X receptors (RXRs). In each assay the all-E-isomer of this retinoid had an activity that was comparable to that of (all-E)-RA. However, the 9Z-isomer was at least 200-fold less active than (all-E)-RA in binding to different RARs, while it was only 6-20 times less active than (9Z)-RA in binding to different RXRs. In an in vivo transient transfection assay, the all-E-isomer activated a reporter gene containing a retinoic acid response element (RARE) with efficiency similar to (all-E)-RA when expression vectors for either RAR alpha, RAR beta, RAR gamma alone or RAR alpha together with RXR alpha were cotransfected. In contrast, the 9Z-isomer was much less active than (9Z)-RA in the same assay systems. However, (9Z)-1 efficiently enhanced the DNA binding and transactivational activity of RXR alpha homodimers. Taken together, these studies demonstrate that the all-E- and 9Z-isomers of this retinoid are selective and potent agonists of RAR and RXR binding and activation.

[Influence of dihydroetorphine hydrochloride and tramadol on labor pain and umbilical blood gas]

Ninety primigravide were randomly allocated into three groups at the beginning of active phase of labor. Dihydroetorphine hydrochloride (DHE) was administered to group A (n = 30), tramadol to group B (n = 30), and group C (n = 30) was a blank control. Various parameters about analgesia effects, progress of labour and fetal and neonatal well-being were investigated and also umbilical artery and vein blood gases analyzing were carried out in all cases. As a result, the effective rate of pain relief in group A was 67% and in group B 63% (P > 0.05). The average time of onset of action in DHE group was 16.5 +/- 2.9 minutes which was significantly shorter than 26.1 +/- 5.4 minutes in tramadol group (P = 0.0001). There were higher rate (36.7%) of operative intervention (forceps and vaginally or cesarean section) and a higher average amount of postpartum hemorrhage in group A, as compared with the control group, but no significant difference was shown between group B and group C. No difference was found in other parameters (among the three groups duration of labour, Apgar scores of neonates, cord blood gases, etc.). The conclusion is that, both DHE and tramadol may have a good effect of pain relief in labour. The time of onset of action in DHE group is significantly shorter than that in tramadol group. Neither analgesics cause circulation and respiratory depression in the mother and neonates, but DHE may have influence on uterine contraction. To achieve an excellent pain relief by higher dosage of DHE and tramadol need further randomized investigation.

Suppression of intestinal neoplasia by DNA hypomethylation

We have used a combination of genetics and pharmacology to assess the effects of reduced DNA methyltransferase activity on ApcMin-induced intestinal neoplasia in mice. A reduction in the DNA methyltransferase activity in Min mice due to heterozygosity of the DNA methyltransferase gene, in conjunction with a weekly dose of the DNA methyltransferase inhibitor 5-aza-deoxycytidine, reduced the average number of intestinal adenomas from 113 in the control mice to only 2 polyps in the treated heterozygotes. Hence, DNA methyltransferase activity contributes substantially to tumor development in this mouse model of intestinal neoplasia. Our results argue against an oncogenic effect of DNA hypomethylation. Moreover, they are consistent with a role for DNA methyltransferase in the generation of the C to T transitions seen at high frequency in human colorectal tumors.

[Effect of bushen shengxue recipe on EPO gene expression of chronic renal insufficiency anemia in rats]

The rats were fed with adenine to establish the chronic renal insufficiency anemia model, and the relationship between renal functional impairment and reticulocytes, hemoglobin and erythropoietin (EPO) synthesis was observed to explore the mechanism of Bushen Shengxue drugs in improving the hemopoietic function. The results showed that the Bushen Shengxue drugs could significantly lower the serum creatinine and blood urea nitrogen of renal insufficiency rats, and through the promotion of EPO gene expression, the animals' anemic status was improved.

Role of the Entamoeba histolytica cysteine proteinase in amebic liver abscess formation in severe combined immunodeficient mice

Evidence from in vitro studies suggest that the Entamoeba histolytica cysteine proteinase plays a role in the tissue lysis and cytopathic effects seen in invasive amebiasis. We used affinity-purified antibodies against a recombinant E. histolytica cysteine proteinase to demonstrate that the proteinase is present extracellularly in amebic liver abscesses in mice with severe combined immunodeficiency (SCID mice). Treatment of E. histolytica trophozoites with specific cysteine proteinase inhibitor E-64 blocked or greatly reduced liver abscess formation at 48 h in SCID mice. Our study suggests an important role for a functional cysteine proteinase in amebic liver abscess formation.

Oral immunization with the dodecapeptide repeat of the serine-rich Entamoeba histolytica protein (SREHP) fused to the cholera toxin B subunit induces a mucosal and systemic anti-SREHP antibody response

The intestinal protozoan parasite Entamoeba histolytica causes amebic dysentery, a major cause of morbidity worldwide. The induction of a mucosal antibody response capable of blocking amebic adhesion to intestinal cells could represent an approach to preventing E. histolytica infection and disease. Here we describe the expression of a chimeric protein containing an immunogenic dodecapeptide derived from the serine-rich E. histolytica protein (SREHP), fused to the cholera toxin B subunit (CtxB). The CtxB-SREHP-12 chimeric protein was purified from Escherichia coli lysates and retained the critical GM1 ganglioside-binding activity of the CtxB moiety. Mice fed the CtxB-SREHP-12 fusion protein along with a subclinical dose of cholera toxin developed mucosal immunoglobulin A and immunoglobulin G and systemic antibody responses that recognized recombinant and native SREHP. Our study confirms the feasibility of inducing mucosal immune responses to immunogenic peptides by their genetic fusion to the CtxB subunit and identifies the CtxB-SREHP-12 chimeric protein as a candidate oral vaccine to prevent E. histolytica infection.

The serine-rich Entamoeba histolytica protein is a phosphorylated membrane protein containing O-linked terminal N-acetylglucosamine residues

Previously, we described the isolation of a cDNA clone and the gene encoding a protective antigen of the protozoan parasite Entamoeba histolytica, the serine-rich Entamoeba histolytica protein (SREHP). The derived amino acid sequence of the SREHP cDNA clone was remarkable for a high serine content (52/233 amino acids), a putative signal sequence, multiple hydrophilic dodecapeptide and octapeptide tandem repeats, and a hydrophobic C-terminal putative membrane-spanning region. Here, we show that SREHP is modified by the addition of phosphate at serine residues, O-linked terminal N-acetylglucosamine residues, and by acylation. When the SREHP gene is expressed in baculovirus transformed Sf-9 cells, the product is also phosphorylated and glycosylated and is localized to the plasma membrane of the insect cells. The native SREHP molecule also serves as a potent chemoattractant for amebic trophozoites. The data presented here suggest that SREHP is a unique membrane protein with phosphorylation and glycosylation patterns usually associated with nuclear or cytoplasmic proteins.

Microbial metabolites of ophiobolin A and antimicrobial evaluation of ophiobolins

Ophiobolin A [1], 3-anhydroophiobolin A [2], ophiobolin B [3], and ophiobolin L [4] were isolated from fermentation broths of Cochliobolus heterostrophus. Preliminary screening showed that a number of organisms were capable of metabolizing the sesterterpene ophiobolin A [1]. Large-scale transformations of ophiobolin A [1] with Polyangium cellulosum produced 6 and 7 while Pseudomonas aeruginosa produced 8. Resting-cell preparations of Penicillium patulum afforded 9 and 10. The structures of these metabolites were established by spectroscopic methods and by comparison of the spectral data with those of the starting material. The antimicrobial activity of the ophiobolins was also evaluated.

Antifungal evaluation of pseudolaric acid B, a major constituent of Pseudolarix kaempferi

Pseudolaric acid B [1] was isolated and identified as the main antifungal constituent of Pseudolarix kaempferi using bioassay-directed fractionation. Pseudolaric acid B was active against Trichophyton mentagrophytes, Torulopsis petrophilum, Microsporum gypseum, and Candida spp., while its methylated or hydrolyzed derivatives were not active against these same organisms. The minimum inhibitory concentrations and minimum fungicidal concentrations of pseudolaric acid B [1] against Candida and Torulopsis species were comparable with those of amphotericin B. The in vivo activity of pseudolaric acid B was evaluated in a murine model of disseminated candidiasis. Pseudolaric acid B [1] reduced the number of recovered colony-forming units significantly at different dosages. Infected mice treated intravenously with pseudolaric acid B [1] also had a longer survival time than those treated with vehicle alone.

Toxicity of 5-aza-2'-deoxycytidine to mammalian cells is mediated primarily by covalent trapping of DNA methyltransferase rather than DNA demethylation

The deoxycytidine analog 5-aza-2'-deoxycytidine (5-azadCyd) has been widely used as a DNA methylation inhibitor to experimentally induce gene expression and cellular differentiation. Prior to the availability of mutant mice with altered DNA methyltransferase levels, treatment of cells with drugs has been the only means to experimentally manipulate the level of genomic DNA methylation in mammalian cells. Substitution of DNA with 5-azadCyd leads to covalent trapping of the enzyme, thereby depleting the cells of enzyme activity and resulting in DNA demethylation. 5-AzadCyd or 5-azacytidine treatment causes multiple changes in treated cells, including activation of silent genes, decondensation of chromatin, and induction of cellular differentiation, all of which are believed to be consequences of drug-induced demethylation. 5-AzadCyd is highly toxic in cultured cells and animals and is utilized as a potent antitumor agent for treatment of certain human cancers. It has been postulated that the toxicity of the drug in mammalian cells is also due to its inhibition of DNA methylation. The chemistry of the methylation reaction is consistent, however, with an alternative mechanism: the cytotoxic effect of 5-azadCyd may be directly mediated through the covalent binding of DNA methyltransferase to 5-azadCyd-substituted DNA. We have tested this possibility by using embryonic stem cells and mice with reduced levels of DNA methyltransferase due to a targeted mutation of the gene. When exposed to 5-azadCyd mutant embryonic stem cells or embryos were significantly more resistant to the toxic effects of the drug than wild-type cells and embryos, respectively. These results strongly suggest that the cellular DNA methyltransferase itself, rather than the secondary demethylation of genomic DNA, is the primary mediator of 5-azadCyd cytotoxicity. In light of our results, some conclusions from previous studies using 5-azadCyd in order to experimentally manipulate cellular methylation levels may have to be reassessed. Also, our data make clear predictions for cancer treatment: tumor cells with elevated DNA methyltransferase levels would be expected to be susceptible to treatment with 5-azadCyd, whereas tumors with reduced levels of the enzyme would be resistant.

Measurement of subnanomolar retinoic acid binding affinities for cellular retinoic acid binding proteins by fluorometric titration

Cellular retinoic acid binding protein I (CRABP-I) and cellular retinoic acid binding protein II (CRABP-II) are small, cytoplasmic proteins which bind all-trans-retinoic acid with high affinity. Both of these proteins belong to a family of intracellular proteins which bind amphiphilic lipids, including fatty acids, bile salts, and retinoids. Because CRABP-I and -II exhibit different tissue distributions and differential transcriptional regulation, they are proposed to serve different functions. The binding properties of mouse CRABP-I and -II purified from Escherichia coli were examined to further understand their role in intracellular retinoic acid processing. Fluorescence titrations were performed using nanomolar protein concentrations, near the obtained dissociation constants, and analyzed by direct mathematical fitting to raw data, in order to extend the range and accuracy of binding constant determination. The apparent dissociation constants, K'd, of mouse CRABP-I and CRABP-II binding all-trans-retinoic acid were determined to be 0.4 +/- 0.3 nM and 2 +/- 1 nM respectively, stronger binding than previously reported. The K'd of mCRABP-I and mCRABP-II complexing with acitretin, a pharmacologically active synthetic retinoid used in the treatment of psoriasis, was 3 +/- 1 nM and 15 +/- 11 nM. Both CRABPs bound 9-cis-retinoic acid with a K'd of roughly 200 nM, and neither exhibited significant binding of 13-cis-retinoic acid.

Entamoeba histolytica interactions with polarized human intestinal Caco-2 epithelial cells

To model the initial pathogenic effects of Entamoeba histolytica trophozoites on intestinal epithelial cells, the interactions of E. histolytica HM1-IMSS trophozoites with polarized human intestinal Caco-2 cell monolayers grown on permeabilized filters were examined. Trophozoites, when incubated with the apical surface of the monolayers at 37 degrees C, induced a rapid decrease in transepithelial resistance over 15 to 60 min. The transmonolayer resistance response was not associated with changes in short-circuit current but was associated with an increase in mannitol flux, suggesting that the drop in resistance reflected a nonselective increase in epithelial permeability rather than stimulation of electrogenic ion transport. This response preceded the earliest detection of morphologic disruption of monolayer integrity by light or electron microscopy. Apical injury to the monolayer was detected by ultrastructural studies which revealed a loss of brush border in regions of contact between epithelial cells and amebas and by chromium release assays where a small increase in the apical release of 51Cr from the monolayer (6% over background) was observed. The transmonolayer resistance response was inhibited when the temperature was reduced to 4 degrees C and by addition of cytochalasin D (1 microgram/ml) to the medium at concentrations that did not directly affect transmonolayer resistance. Application of amebic lysates or medium conditioned by coincubation of amebas with Caco-2 monolayers failed to lower transmonolayer resistance, suggesting that this effect was not mediated by soluble amebic cytotoxins. Polarized Caco-2 monolayers grown on permeable filters provide a useful model for studying the initial interactions of E. histolytica trophozoites with intestinal epithelial cells.