Influence of hydroxylation and conjugation in cross-inhibition of bile acid transport across the human trophoblast basal membrane

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.

Ontogenic development-associated changes in the expression of genes involved in rat bile acid homeostasis

Ontogenic changes in the rat bile acid (BA) pool, measured enzymatically and by GC-MS, and expression of enzymes (5alpha-reductase, 5beta-reductase, and cytochrome P450 enzymes Cyp7a1, Cyp8b1, Cyp27 and Cyp3a11), transporters [bile salt export pump, sodium taurocholate-cotransporting polypeptide, apical sodium-dependent bile acid transporter, and organic solute transporter alpha/beta (Ostalpha/Ostbeta)], and nuclear receptors [fetoprotein transcription factor (Ftf), farnesoid X receptor (Fxr), small heterodimer partner (Shp), and hepatic nuclear factor 4alpha (HNF-4alpha)], determined by quantitative PCR, were investigated. The absolute size of the BA pool increased progressively up to adulthood, whereas the complexity of its composition was high in fetuses, decreased after birth, increased again progressively up to adulthood, and decreased in aged animals. Allo-cholic acid only appeared early in development, in spite of low 5alpha-reductase expression. The relative size of the BA pool, corrected by liver weight, was maintained from 1 week after birth, except at weaning, when a transient peak accompanied by Shp downregulation and Cyp7a1 upregulation was observed. An imposed weaning delay of 1 week had no effect on the time course of the BA pool size but decreased the proportion of chenodeoxycholic and alpha-muricholic acids, whereas the proportion of cholic acid was increased, probably as a result of Cyp8b1 upregulation. In conclusion, changes in the expression of genes involved in BA homeostasis may play a role in physiological adaptations to digestive functions during the rat life span.

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.

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.

Characteristics of L-lactic acid transport in basal membrane vesicles of human placental syncytiotrophoblast

The characteristics of L-lactic acid transport across the trophoblast basal membrane were investigated and compared with those across the brush-border membrane by using membrane vesicles isolated from human placenta. The uptake of L-[(14)C]lactic acid into basal membrane vesicles was Na(+) independent, and an uphill transport was observed in the presence of a pH gradient ([H(+)](out) > [H(+)](in)). L-[(14)C]lactic acid uptake exhibited saturation kinetics with a K(m) value of 5.89 +/- 0.68 mM in the presence of a pH gradient. p-Chloromercuribenzenesulfonate and alpha-cyano-4-hydroxycinnamate inhibited the initial uptake, whereas phloretin or 4,4'-diisothiocyanostilbene-2,2'-disulfonate did not. Mono- and dicarboxylic acids suppressed the initial uptake. In conclusion, L-lactic acid transport in the basal membrane is H(+) dependent and Na(+) independent, as is also the case for the brush-border membrane transport, and its characteristics resemble those of monocarboxylic acid transporters. However, there were several differences in the effects of inhibitors between basal and brush-border membrane vesicles, suggesting that the transporter(s) involved in L-lactic acid transport in the basal membrane of placental trophoblast may differ from those in the brush-border membrane.

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.

Uptake of dehydroepiandrosterone-3-sulfate by isolated trophoblasts from human term placenta, JEG-3, BeWo, Jar, BHK cells, and BHK cells transfected with human sterylsulfatase-cDNA

The human placenta lacks the enzyme 17alpha-hydroxylase/17-20-lyase, and is thus unable to convert cholesterol into estrogens. Therefore estrogen synthesis of trophoblast cells depends on the supply of precursors such as dehydroepiandrosterone-3-sulfate (DHEA-S) and 16alpha-hydroxy-dehydroepiandrosterone-3-sulfate by maternal and fetal blood. To investigate the cellular internalisation of these anionic hydrophilic precursors, the uptake of [(3)H]-/[(35)S]-DHEA-S and [(3)H]-taurocholate by isolated cytotrophoblasts, cells of choriocarcinoma cell lines (JEG-3, BeWo, Jar), BHK and BHK cells transfected with human sterylsulfatase-cDNA (BHK-STS cells) was studied. Furthermore, the activity of sterylsulfatase of these cells in suspension and in corresponding cell homogenate was measured. During the first 5 min of incubation with [(3)H]-DHEA-S or [(35)S]-DHEA-S, radioactivity of cytotrophoblasts increased significantly, while radioactivity of JEG-3, Jar, BHK and BHK-STS cells did not increase. Radioactivity of BeWo cells increased slightly. For all cell types, there was no significant difference for uptake of either substrate. During incubation with [(3)H]-taurocholate, radioactivity of cytotrophoblasts did not increase. Sterylsulfatase activity of cytotrophoblast homogenate was significantly lower than that of cytotrophoblast suspension. Sterylsulfatase activity of BHK-STS, JEG-3 or BeWo cell homogenate was significantly higher than that of the corresponding cell suspension. In BHK and Jar cells sterylsulfatase activity was not detectable. Cytotrophoblasts take up DHEA-S without prior hydrolysis. BHK, BHK-STS, JEG-3, and Jar cells do not take up and BeWo cells slowly take up DHEA-S. In cytotrophoblasts extracellular DHEA-S rapidly gains access to intracellular sterylsulfatase, while in choriocarcinoma and BHK-STS cells access of DHEA-S to sterylsulfatase is limited. Our results indicate, that uptake by cytotrophoblasts is mediated by a carrier which is not expressed in choriocarcinoma or BHK cells and which is different from the known taurocholate-transporting organic anion transporting polypetides.

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.

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

Bile acid (BA) transport across the human microvillus maternal-facing trophoblast plasma membrane (mTPM) has been recently reported to be stimulated by the presence of ATP [Marin, Bravo, El-Mir and Serrano (1993) J. Hepatol. 18, S41]. Reconstitution of BA transport activity in proteoliposomes from human mTPM is reported in this paper. Typical characteristics of BA transport in native mTPM vesicles, including a requirement for ATP hydrolysis and inhibition by other BA species, were preserved in proteoliposome preparations. BA transport into 20- and 14-day-gestation rat mTPM vesicles was also stimulated by the presence of ATP as noted in human mTPM and in the rat liver canalicular membrane. Besides this functional similarity, these ATP-dependent carriers may share structural characteristics, as demonstrated by studies using an antibody (100 Ab) raised against the 100 kDa BA carrier of the canalicular membrane from rat liver which recognized proteins in both human and rat brush-border trophoblast membranes.