Reconstitution and characterization of ATP-dependent bile acid transport in human and rat placenta

Models for Understanding Resistance to Chemotherapy in Liver Cancer

The lack of response to pharmacological treatment constitutes a substantial limitation in the handling of patients with primary liver cancers (PLCs). The existence of active mechanisms of chemoresistance (MOCs) in hepatocellular carcinoma, cholangiocarcinoma, and hepatoblastoma hampers the usefulness of chemotherapy. A better understanding of MOCs is needed to develop strategies able to overcome drug refractoriness in PLCs. With this aim, several experimental models are commonly used. These include in vitro cell-free assays using subcellular systems; studies with primary cell cultures; cancer cell lines or heterologous expression systems; multicellular models, such as spheroids and organoids; and a variety of in vivo models in rodents, such as subcutaneous and orthotopic tumor xenografts or chemically or genetically induced liver carcinogenesis. Novel methods to perform programmed genomic edition and more efficient techniques to isolate circulating microvesicles offer new opportunities for establishing useful experimental tools for understanding the resistance to chemotherapy in PLCs. In the present review, using three criteria for information organization: (1) level of research; (2) type of MOC; and (3) type of PLC, we have summarized the advantages and limitations of the armamentarium available in the field of pharmacological investigation of PLC chemoresistance.

Transcriptome Profiling of Placenta through Pregnancy Reveals Dysregulation of Bile Acids Transport and Detoxification Function

Placenta performs the function of several adult organs for the fetus during intrauterine life. Because of the dramatic physiological and metabolic changes during pregnancy and the strong association between maternal metabolism and placental function, the possibility that variation in gene expression patterns during pregnancy might be linked to fetal health warrants investigation. Here, next-generation RNA sequencing was used to investigate the expression profile, including mRNAs and long non-coding RNAs (lncRNAs) of placentas on day 60 of gestation (G60), day 90 of gestation (G90), and on the farrowing day (L0) in pregnant swine. Bioinformatics analysis of differentially expressed mRNAs and lncRNAs consistently showed dysregulation of bile acids transport and detoxification as pregnancy progress. We found the differentially expressed mRNAs, particularly bile salt export pump (ABCB11), organic anion-transporting polypeptide 1A2 (OATP1A2), carbonic anhydrase II (CA2), Na⁺-HCO₃⁻ cotransporter (NBC1), and hydroxysteroid sulfotransferases (SULT2A1) play an important role in bile acids transport and sulfation in placentas during pregnancy. We also found the potential regulation role of ALDBSSCG0000000220 and XLOC_1301271 on placental SULT2A1. These findings have uncovered a previously unclear function and its genetic basis for bile acids metabolism in developing placentas and have important implications for exploring the potential physiological and pathological pathway to improve fetal outcomes.

The effect of acetaminophen on the expression of BCRP in trophoblast cells impairs the placental barrier to bile acids during maternal cholestasis

Acetaminophen is used as first-choice drug for pain relief during pregnancy. Here we have investigated the effect of acetaminophen at subtoxic doses on the expression of ABC export pumps in trophoblast cells and its functional repercussion on the placental barrier during maternal cholestasis. The incubation of human choriocarcinoma cells (JAr, JEG-3 and BeWo) with acetaminophen for 48h resulted in no significant changes in the expression and/or activity of MDR1 and MRPs. In contrast, in JEG-3 cells, BCRP mRNA, protein, and transport activity were reduced. In rat placenta, collected at term, acetaminophen administration for the last three days of pregnancy resulted in enhanced mRNA, but not protein, levels of Mrp1 and Bcrp. In fact, a decrease in Bcrp protein was found. Using in situ perfused rat placenta, a reduction in the Bcrp-dependent fetal-to-maternal bile acid transport after treating the dams with acetaminophen was found. Complete biliary obstruction in pregnant rats induced a significant bile acid accumulation in fetal serum and tissues, which was further enhanced when the mothers were treated with acetaminophen. This drug induced increased ROS production in JEG-3 cells and decreased the total glutathione content in rat placenta. Moreover, the NRF2 pathway was activated in JEG-3 cells as shown by an increase in nuclear NRF2 levels and an up-regulation of NRF2 target genes, NQO1 and HMOX-1, which was not observed in rat placenta. In conclusion, acetaminophen induces in placenta oxidative stress and a down-regulation of BCRP/Bcrp, which may impair the placental barrier to bile acids during maternal cholestasis.

Excretion of biliary compounds during intrauterine life

In adults, the hepatobiliary system, together with the kidney, constitute the main routes for the elimination of several endogenous and xenobiotic compounds into bile and urine, respectively. However, during intrauterine life the biliary route of excretion for cholephilic compounds, such as bile acids and biliary pigments, is very poor. Although very early in pregnancy the fetal liver produces bile acids, bilirubin and biliverdin, these compounds cannot be efficiently eliminated by the fetal hepatobiliary system, owing to the immaturity of the excretory machinery in the fetal liver. Therefore, the potentially harmful accumulation of cholephilic compounds in the fetus is prevented by their elimination across the placenta. Owing to the presence of detoxifying enzymes and specific transport systems at different locations of the placental barrier, such as the endothelial cells of chorionic vessels and trophoblast cells, this organ plays an important role in the hepatobiliary-like function during intrauterine life. The relevance of this excretory function in normal fetal physiology is evident in situations where high concentrations of biliary compounds are accumulated in the mother. This may result in oxidative stress and apoptosis, mainly in the placenta and fetal liver, which might affect normal fetal development and challenge the fate of the pregnancy. The present article reviews current knowledge of the mechanisms underlying the hepatobiliary function of the fetal-placental unit and the repercussions of several pathological conditions on this tandem.

P-glycoprotein and breast cancer resistance protein expression in human placentae of various gestational ages

Placental efflux transporters such as P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) protect the developing fetus from exposure to potentially toxic xenobiotics. However, little is known about the expression of these transporters in human placentae of different gestational ages. Therefore, we quantified the expression of P-gp and BCRP in human placentae of different gestational ages. We also measured the expression of various nuclear regulatory factors such as the pregnane xenobiotic factor to determine whether their expression also changes with gestational age. Syncitial microvillous plasma membranes were isolated from human placentae of various gestational ages (60–90 days, 90–120 days, and full-term C-section placentae). P-gp and BCRP expression (protein) in these preparations were measured by Western blot analysis followed by an ELISA. Expression (mRNA) of P-gp, BCRP, and nuclear regulatory factors in the placentae were quantified by quantitative real-time PCR. P-gp expression (relative to that of alkaline phosphatase) was significantly ( P < 0.05) higher (44.8-fold as protein; 6.5-fold as mRNA) in early gestational age human placentae (60–90 days) vs. term placentae. In contrast, BCRP (protein and mRNA) and nuclear regulatory factors (mRNA) expression in placental tissue did not change significantly with gestational age. However, placental expression of P-gp and human chorionic gonadotropin-β (hCG-β) transcripts was highly correlated ( r = 0.73; P < 0.0001; Spearman rank correlation). Expression of P-gp, but not BCRP, decreases dramatically with gestational age in human placentae. This decrease in P-gp expression is not caused by a change in expression of nuclear receptor transcripts but appears to be related to hCG-β expression. The placental P-gp expression appears to be upregulated in early pregnancy to protect the fetus from xenobiotic toxicity at a time when it is most vulnerable to such toxicity.

Temporal expression profiles of organic anion transport proteins in placenta and fetal liver of the rat

Physiological cholestasis linked to immature hepatobiliary transport systems for organic anions occurs in rat and human neonates. In utero, the placenta facilitates vectorial transfer of certain fetal-derived solutes to the maternal circulation for elimination. We compared the ontogenesis of organic anion transporters in the placenta and the fetal liver of the rat to assess their relative abundance throughout gestation and to determine whether the placenta compensates for the late maturation of transporters in the developing liver. The mRNA of members of the organic anion transporting polypeptide (Oatp) superfamily, the multidrug resistance protein (Mrp) family, one organic anion transporter (OAT), and the bile acid carriers Na(+)-taurocholate cotransporting polypeptide (Ntcp) and bile salt export pump (Bsep) was quantified by real-time PCR. The most abundant placental transporters were Oatp4a1, whose mRNA increased 10-fold during gestation, and Mrp1. Mrp1 immunolocalized predominantly to epithelial cells of the endoplacental yolk sac, suggesting an excretory role that sequesters fetal-derived solutes in the yolk sac cavity, and faintly to the basal syncytiotrophoblast surface. The mRNA levels of Oatp2b1, Mrp3, and Bsep in the placenta exceeded those in the fetal liver until day 20 of gestation, suggesting that the fetus relies on placental clearance of substrates when expression in the developing liver is low. Mrp3 immunolocalized to the epithelium of the endoplacental yolk sac and less abundantly in the labyrinth zone and endothelium of the maternal arteries. The placental expression of Oatp1a1, Oatp1a4, Oatp1a5, Oatp1b2, Oat, Ntcp, Mrp2, and Mrp6 was low.

The hepatobiliary-like excretory function of the placenta. A review

In the adult, several endogenous compounds, such as bile acids and biliary pigments, as well as many xenobiotics are mainly biotransformed and eliminated by the hepatobiliary system. However, because this function is immature in the foetus, this role is carried out by the placenta during the intrauterine life. This review describes current knowledge of the trophoblastic machinery responsible for this function, which includes transport and metabolic processes, similar in part to those existing in the mature liver. Because many of the studies reviewed here were conducted on human or rat near-term placentae, two aspects should be borne in mind: (i) although both types of placenta are haemochorial, profound species-specific differences at the structural, molecular and functional levels do exist, and (ii) the placenta is an organ undergoing continuous developmental changes, including its hepatobiliary-like excretory function.

Effect of ursodeoxycholic acid on the impairment induced by maternal cholestasis in the rat placenta-maternal liver tandem excretory pathway

We investigated the effects of ursodeoxycholic acid (UDCA; 60 microg/day/100 g b.wt.) on the impairment induced by maternal obstructive cholestasis during pregnancy (OCP) in the rat placenta-maternal liver tandem excretory pathway. A blunted catheter was implanted in the common bile duct on day 14 of pregnancy, and the tip was cut on day 21. [(14)C]Glycocholate (GC) was then administered through the umbilical artery of "in situ" perfused placenta (placental transfer test) or through the maternal jugular vein (biliary secretion test), and GC bile output was measured. OCP impaired both GC placental transfer and maternal biliary secretion. UDCA moderately improved the latter but had a more marked beneficial effect on GC placental transfer. Histological examination revealed trophoblast atrophy and structural alterations, e.g., loss of apical membrane microvilli in OCP placentas. Gene expression level was investigated by real-time quantitative reverse transcription-polymerase chain reaction and Western blot analysis. OCP reduced both placental lactogen II (a trophoblast-specific gene) mRNA and the functional amount of epithelial tissue, determined by transplacental diffusion of antipyrin. Using a rapid filtration technique, impairment in the ATP-dependent GC transport across trophoblast apical plasma membranes obtained from OCP placentas was found. UDCA partially prevented all these changes. The expression level of organic anion transporters Oatp1, Oatp2, and Oatp4, and multidrug resistance-associated proteins Mrp1, Mrp2, and Mrp3 in whole placenta were not affected or were moderately affected by OCP but greatly enhanced by UDCA. In summary, UDCA partially prevents deleterious effects of OCP on the rat placenta-maternal liver tandem excretory pathway, mainly by preserving trophoblast structure and function.

Excretion of foetal bilirubin by the rat placenta-maternal liver tandem

Using plasma membrane vesicles from human trophoblast, carrier-mediated transport of unconjugated bilirubin (UCBR) has been reported. In the present work, using the in situ perfused rat placenta-maternal liver tandem, the relevance of this pathway in vivo was investigated. After single-pass perfusion of rat placenta through the umbilical artery with 0.25 micromol [(3)H]-UCBR, approximately 15 per cent of it was taken up by the placenta, detected in maternal serum (>96 per cent was unconjugated) and subsequently secreted into maternal bile (approximately 15 per cent of administered dose; >88 per cent was glucuronidated bilirubin). Co-administration through the umbilical artery of 0.25 micromol [(3)H]-UCBR and 2.5 micromol unlabelled UCBR, bromosulfophthalein, cholic acid or biliverdin IXalpha, reduced [(3)H]-UCBR placenta uptake, and the amount of radioactivity found in the maternal serum and bile. Co-administration into maternal jugular vein of 0.1 micromol [(3)H]-UCBR-a dose 3-fold higher than that reaching the maternal compartment in placenta perfusion experiments-and 1.0 micromol bromosulfophthalein, cholic acid or biliverdin IXalpha, resulted in no marked inhibition of the amount of radioactivity bile output. When antipyrine and [(3)H]-UCBR were continuously co-infused to the mother, similar antipyrine concentrations in maternal and foetal serum were reached in approximately 15 min, while progressive increase in [(3)H]-bilirubin concentrations in maternal serum above 70 microM was accompanied by a very low transfer of this compound into foetal compartment where [(3)H]-bilirubin concentrations were always <10 microM. These results suggest that the transfer of UCBR across the rat placenta occurs, without biotransformation, via a foetal-to-maternal mainly unidirectional pathway that can be cis-inhibited by UCBR and other cholephilic organic anions.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Bile acid stress in the mother and baby unit

Intrahepatic cholestasis of pregnancy (ICP) affects about 0.7% of deliveries in Britain. It is regarded as a benign condition for the mother but is associated with increased fetal mortality in late pregnancy and early delivery is advised. Ursodeoxycholic acid (UDCA) treatment is beneficial to the mother and does not appear to harm the fetus. ICP is often regarded as a disease of the maternal liver already made 'cholestatic' by high levels of circulating progesterone. We propose that ICP should be considered as a feto-maternal disease involving complex interactions between maternal and fetal bile acid metabolism across the placenta. During the late stages of gestation, when there is a rise in fetal and maternal bile acid levels, the placenta may fail to render potentially hepatotoxic bile acids water soluble and hence excretable. This might cause a vicious cycle leading to further cholestasis in the maternal liver already challenged by progesterone.

Evidence for carrier-mediated transport of unconjugated bilirubin across plasma membrane vesicles from human placental trophoblast

Unconjugated bilirubin (UCB) is currently believed to cross the placenta only by passive diffusion. To assess whether carrier-mediated transport might be involved, the uptake of [(3)H]-UCB by basal (bTPM) and apical (aTPM) plasma membrane vesicles from human placental trophoblast at term was investigated. In both types of vesicles, the uptake of [(3)H]-UCB into an osmotically sensitive space was temperature-dependent, independent of the presence of Na(+), and not affected by changes in membrane potential. The uptake of [(3)H]-UCB by aTPM, but not bTPM, was activated by ATP hydrolysis and inhibited by vanadate. Thus, the exact contribution of both inside out and right-side out bTPM to UCB uptake could not be distinguished, while only inverted aTPM were expected to carry out ATP-dependent UCB uptake. In bTPM and aTPM, uptake of free (unbound) [(3)H]-UCB (B(f)) consisted of a dominant, saturable, presumably carrier-mediated process and a diffusional component that became predominant only at B(f) near or above aqueous solubility limit for UCB (70 nM ). For bTPM, K(m)=7.2 nM; V(max)=9.8 pmol/20s/mg protein; and diffusion coefficient (K(D))=0.14 ml/20s/mg protein. For aTPM in the presence of 9.5m M ATP, K(m)=18 n M; V(max)=131 pmol/20s/mg protein; and K(D)=0.47 ml/20s/mg protein. The uptake of [(3)H]-UCB by bTPM was cis-inhibited by estrone-3-sulfate and estradiol-17 beta-glucuronide and trans-stimulated by unlabelled UCB and bromosulphopthalein. ATP-dependent UCB uptake by aTPM was cis-inhibited by doxorubicin, cholic acid, methotrexate and pronenecid. These findings suggest the presence of distinct transporters in the two domains of human placental trophoblast that could cooperate to transfer UCB from the foetus to the maternal circulation.

Transport of bile acids in hepatic and non-hepatic tissues

Bile acids are steroidal amphipathic molecules derived from the catabolism of cholesterol. They modulate bile flow and lipid secretion, are essential for the absorption of dietary fats and vitamins, and have been implicated in the regulation of all the key enzymes involved in cholesterol homeostasis. Bile acids recirculate through the liver, bile ducts, small intestine and portal vein to form an enterohepatic circuit. They exist as anions at physiological pH and, consequently, require a carrier for transport across the membranes of the enterohepatic tissues. Individual bile acid carriers have now been cloned from several species. Na(+)-dependent transporters that mediate uptake into hepatocytes and reabsorption from the intestine and biliary epithelium and an ATP-dependent transporter that pumps bile acids into bile comprise the classes of transporter that are specific for bile acids. In addition, at least four human and five rat genes that code for Na(+)-independent organic anion carriers with broad multi-substrate specificities that include bile acids have been discovered. Studies concerning the regulation of these carriers have permitted identification of molecular signals that dictate eventual changes in the uptake or excretion of bile acids, which in turn have profound physiological implications. This overview summarizes and compares all known bile acid transporters and highlights findings that have identified diseases linked to molecular defects in these carriers. Recent advances that have fostered a more complete appreciation for the elaborate disposition of bile acids in humans are emphasized.

Expression of members of the multidrug resistance protein family in human term placenta

The placenta serves, in part, as a barrier to exclude noxious substances from the fetus. In humans, a single-layered syncytium of polarized trophoblast cells and the fetal capillary endothelium separate the maternal and fetal circulations. P-glycoprotein is present in the syncytiotrophoblast throughout gestation, consistent with a protective role that limits exposure of the fetus to hydrophobic and cationic xenobiotics. We have examined whether members of the multidrug resistance protein (MRP) family are expressed in term placenta. After screening a placenta cDNA library, partial clones of MRP1, MRP2, and MRP3 were identified. Immunofluorescence and immunoblotting studies demonstrated that MRP2 was localized to the apical syncytiotrophoblast membrane. MRP1 and MRP3 were predominantly expressed in blood vessel endothelia with some evidence for expression in the apical syncytiotrophoblast. ATP-dependent transport of the anionic substrates dinitrophenyl-glutathione and estradiol-17-beta-glucuronide was also demonstrated in apical syncytiotrophoblast membranes. Given the cellular distribution of these transporters, we hypothesize that MRP isoforms serve to protect fetal blood from entry of organic anions and to promote the excretion of glutathione/glucuronide metabolites in the maternal circulation.

Effect of maternal cholestasis on bile acid transfer across the rat placenta-maternal liver tandem

Cholestasis of pregnancy induces alterations in bile acid transport by human trophoblast plasma membrane (TPM) vesicles. We investigated whether maternal cholestasis affects the overall ability of the rat placenta to carry out vectorial bile acid transfer from the fetus to the mother. Complete obstructive cholestasis (OCP) was maintained during the last week of pregnancy and released at term (day 21), before experiments were performed. In situ single-pass perfusion of one placenta per rat with 250 nmol [(14)C]glycocholic acid (GC) revealed an impaired uptake in OCP rats (2.28 vs. 5.53 nmol in control rats). Approximately 100% of GC taken up by control placentas was secreted in maternal bile over 120 minutes (5.38 nmol), whereas this was only 61% (1.40 nmol) of the GC taken up by OCP placentas. When 5 nmol GC was administered through the jugular vein no significant difference between both groups in total GC bile output was found. The efficiency (V(max)/K(M)) of adenosine triphosphate (ATP)-dependent GC transport by vesicles from the maternal side of TPM was decreased (-41%) in OCP. Moreover, histological examination of the placentas suggested a reduction in the amount of functional trophoblast in the OCP group. This was consistent with a lower antipyrine diffusion across the placenta in these animals. In sum, our results indicate that maternal cholestasis affects the ability of the placenta to efficiently carry out bile acid transfer from fetal to maternal blood. Changes in both the structure and the functionality of the chorionic tissue may account for this impairment.

Intrahepatic cholestasis of pregnancy

Intrahepatic cholestasis of pregnancy is one of the primary disorders of the liver that adversely affects maternal well-being and fetal outcome. Early identification of this condition, careful interdisciplinary monitoring, and prompt delivery at fetal maturity can improve outcomes in the mother and child. Although the cause is unclear, IHCP probably arises from a genetic predisposition for increased sensitivity to estrogens and progestogens and altered membrane composition and expression of bile ducts, hepatocytes, and canalicular transport systems. As a result, the elevations in maternal levels of bile acids and their molar ratios seen in healthy pregnancy rise further in IHCP patients. Also, as the normal fetal-to-maternal transfer of bile acids across the trophoblast is impaired, the excess bile acids with abnormal profiles accumulate and are toxic to the fetus. The management of IHCP is dictated by the increased risks of fetal distress, spontaneous preterm delivery, and sudden death, as well as by alleviating pruritus in the mother. These risks to the fetus rise progressively to delivery, regardless of serum levels of bile acids and ALT. Close monitoring of these markers is essential but does not prevent sudden fetal distress and death. Provision should be made to induce labor as soon as fetal lung maturity has been established. Ursodeoxycholic acid is the only therapy that has proven effective, albeit in small studies, in alleviating pruritus and restoring towards normal the abnormal profiles of bile acids and sulfated steroids in serum and other body fluids. Ursodeoxycholic acid seems to have no obvious adverse effects on the fetus, but experience is insufficient to draw conclusions regarding teratogenicity and prevention of adverse outcomes.

Maternal cholestasis does not affect the ontogenic pattern of expression of the Na+/taurocholate cotransporting polypeptide (ntcp) in the fetal and neonatal rat liver

To assess the effects of cholestasis during pregnancy on fetal and neonatal mRNA expression, protein mass, and function of the Na+/taurocholate cotransporting polypeptide (Ntcp), common bile duct ligation (BDL) was performed in pregnant rats on day 14 of pregnancy (maternal cholestasis [MC] group), and livers were harvested at days 20 and 21 of fetal life, as well as at days 4, 7, 14, 21, and 28 after birth. Sham-operated rats and their litters were used as controls. Ntcp steady-state mRNA levels, protein mass, and function were determined by Northern blotting, immunoblotting, and taurocholate (TC) transport studies in isolated short-term cultured hepatocytes, respectively. In addition, protein mass and function of the organic anion transporting polypeptide (Oatp1), another sinusoidal bile acid transporter, were studied at 4 weeks of age. The majority of pregnant cholestatic rats (94%) were able to carry pregnancy to term. Body and liver weights of the offspring from the MC group were lower than those from sham-operated animals at all postnatal time points. Ntcp steady-state mRNA levels, protein mass, and function were unaffected by MC. The ontogenic pattern of expression was identical in offspring from MC and controls with detection of the Ntcp mRNA at day 21 of fetal life. There was a significant increase in mRNA postnatally, reaching adult levels by 7 days of age. Protein mass and function of Ntcp as well as of Oatp1 were similar in offspring from MC and control groups at 4 weeks of age. In conclusion, maternal obstructive cholestasis during the last third of pregnancy does not affect the fetal/neonatal expression of the basolateral bile acid transporters, Ntcp and Oatp1. This suggests that the impaired bile acid excretion described in this experimental model is not related to altered uptake of bile acids in the affected neonate.

Beneficial effect of ursodeoxycholic acid on alterations induced by cholestasis of pregnancy in bile acid transport across the human placenta

BACKGROUND/AIMS

The existence of impairment in bile acid transport across the placenta during intrahepatic cholestasis of pregnancy and the effect of ursodeoxycholic acid treatment (1 g/day) were investigated.

METHODS

Kinetic parameters were calculated from experiments carried out on membrane vesicles obtained from basal (TPMb, fetal-facing) and apical (TPMa, maternal-facing) trophoblast plasma membranes. Bile acid uptake was measured using varying concentrations of [14C]-glycocholate and a rapid filtration technique.

RESULTS

The maximal velocity of transport (Vmax), the apparent affinity constant (Kt) and the efficiency (Ef) of transport (Vmax/Kt) of the anion:bile acid exchanger located at the TPMb were reduced in intrahepatic cholestasis of pregnancy. Ursodeoxycholic acid induced a reversal of Vmax, Kt and Ef to normal values. Owing to the 3-fold increase in Vmax, with no change in Kt, intrahepatic cholestasis of pregnancy induced an enhancement in Ef of ATP-independent bile acid transport across TPMa. Both Vmax and Ef were restored to normal values by ursodeoxycholic acid. Finally, in ATP-dependent bile acid transport across TPMa, a reduction in the Ef due to an increase in Vmax together with a more pronounced increase in Kt was found. This impairment was also reversed by ursodeoxycholic acid.

CONCLUSIONS

These results suggest that placenta bile acid transport systems are impaired in intrahepatic cholestasis of pregnancy. Moreover, together with the confirmed beneficial effect for intrahepatic cholestasis of pregnancy patients, such as the relief of pruritus and the improvement in biochemical markers of cholestasis, ursodeoxycholic acid treatment restores the ability of the placenta to carry out vectorial bile acid transfer.

Fetal excretion of the fluorescent bile acid derivative cholylglycylamido-fluorescein (FITC-GC) by the rat placenta-maternal liver tandem

Bile acid transfer from the fetus to maternal bile was studied using in situ perfused rat placenta on day 21 of gestation and a fluorescent derivative of glycocholate (GC): cholylglycylamido-fluorescein (FITC-GC). Single-pass perfusion of the placenta with 0.25 mumol FITC-GC via the umbilical artery over 5 min was followed by the output of 6 per cent of this amount in maternal bile collected over the ensuing 120 min. This amount was reduced (-35 per cent) by simultaneous administration of 2.5 mumol GC through the jugular vein of the mother. This inhibition was stronger (-73 per cent) when 2.5 mumol GC was co-infused with FITC-GC through the umbilical artery. These results suggested that FITC-GC was, at least in part, transported by bile acid carriers across both the liver and the placenta. Using isolated perfused rat livers obtained from female virgin or 21-day pregnant rats, a slight increase in the residence time of FITC-GC in the liver of pregnant rats was found. However, no change in the ability of the liver to take up FITC-GC was observed. By contrast, when FITC-GC was injected into the left jugular vein of anaesthetized pregnant rats, a delayed plasma disappearance of this compound was seen, which may have been due in part to the existence of a transient and reversible FITC-GC exchange with the placental-fetal compartment. The maximal rate of FITC-GC output into bile after FITC-GC administration (1 mumol/100 g body weight) to pregnant rats was approximately 0.2 mumol/min, while maximal FITC-GC bile output was approximately 1 nmol/min when this compound was given through the umbilical artery (2.5 mumol). Therefore, the rate of FITC-GC output into bile was considered to reflect the rate of transfer across the placenta. Using this approach no saturation but rather a linear regression (slope = 1.1 microliters/min, p < 0.05) was found between placental transfer and placental perfusate concentrations in the 10-1000-mumol/l FITC-GC range. In summary, the in situ perfused rat placenta is a useful model to study the fetal excretion of cholephilic compounds, and transfer across the trophoblast would be the limiting step in the excretion of fetal bile acids by the placenta-maternal liver tandem.

Age-related changes in the biliary bile acid composition of bovids

The biliary bile acid composition of 12 tribes of bovids (66 species, 168 animals) was determined by high-performance liquid chromatography and mass spectrometry. In adult animals, the biliary bile acids were conjugated with taurine or glycine and consisted mostly (> 90%) of three bile acids: cholic acid (CA), chenodeoxycholic acid (CDCA), and deoxycholic acid (DCA). Biliary bile acid composition did not vary among species, and was identical in male and female bovids. Within each species, there were consistent changes in biliary bile acid composition with age. Three steady-state stages could be distinguished: (1) the fetal stage, when bile acid input is from placental transfer from the mother as well as biosynthesis (from cholesterol) by the newborn liver (45 ± 12% CA; 50 ± 11% CDCA; 5 ± 4% DCA (mean ± SD)); (2) the infant stage, when bile acid input is solely from biosynthesis by the infant liver (80 ± 6% CA; 20 ± 6% CDCA; 0.5 ± 0.7% DCA); and (3) the adult stage, when bile acid input is not only from biosynthesis by the adult liver but also from intestinal absorption of DCA, formed by bacterial 7-dehydroxylation of CA (75 ± 12% CA; 6 ± 7% CDCA; 19 ± 9% DCA). The transition from the infant stage to the adult stage, indicating the development of an anerobic cecum, occurred before weaning. These three stages of biliary bile acid composition are likely to be present in other placental vertebrates, including most primates, in whom a cecum containing an anerobic flora develops after birth; the functional implications of these changes are discussed.

Glutamine transport in human and rat placenta

Glutamine plays an important role in fetal nutrition. This study explored the transport of [3H]glutamine into apical and basal predominant membrane vesicles derived from rat and human placenta. Na+-dependent glutamine transport was present in both apical and basal predominant vesicles derived from 20- and, to a lesser degree, 14-day gestation rat placenta. Amino-acid transport systems A, ASC-like, B(o,+) (in apical membrane vesicles) and, perhaps, y+L were involved in Na+-dependent glutamine transport. Na+-dependent glutamine uptake into human placental microvillus and basolateral membrane vesicles also occurred via several distinct transport activities. Glutamine transport via system N was not detected in either rat or human placental preparations. Na+-dependent glutamine transport in the rat was more pronounced in basal as compared to apical membrane vesicles. Conversely, in the human preparations, activity was significantly higher in microvillus as compared to basolateral membrane vesicles. It is concluded that Na+-dependent glutamine transport occurs through a variety of transport agencies in both the rat and human placenta. Transport varies with ontogeny and between species.