Further evidence for the presence of two facilitative glucose transporter isoforms in the brush border membrane of the syncytiotrophoblast of the human full term placenta

Fetomaternal Expression of Glucose Transporters (GLUTs)—Biochemical, Cellular and Clinical Aspects

Several types of specialized glucose transporters (GLUTs) provide constant glucose transport from the maternal circulation to the developing fetus through the placental barrier from the early stages of pregnancy. GLUT1 is a prominent protein isoform that regulates placental glucose transfer via glucose-facilitated diffusion. The GLUT1 membrane protein density and permeability of the syncytial basal membrane (BM) are the main factors limiting the rate of glucose diffusion in the fetomaternal compartment in physiological conditions. Besides GLUT1, the GLUT3 and GLUT4 isoforms are widely expressed across the human placenta. Numerous medical conditions and molecules, such as hormones, adipokines, and xenobiotics, alter the GLUT’s mRNA and protein expression. Diabetes upregulates the BM GLUT’s density and promotes fetomaternal glucose transport, leading to excessive fetal growth. However, most studies have found no between-group differences in GLUTs’ placental expression in macrosomic and normal control pregnancies. The fetomaternal GLUTs expression may also be influenced by several other conditions, such as chronic hypoxia, preeclampsia, and intrahepatic cholestasis of pregnancy.

Proton gradient-dependent transport of valproic acid in human placental brush-border membrane vesicles

PURPOSE

To investigate the transport mechanism of valproic acid across the human placenta, we used human placental brush-border membrane vesicles and compared them with that of lactic acid.

METHODS

Transport of [3H]valproic acid and [14C]lactic acid was measured by using human placental brush-border membrane vesicles.

RESULTS

The uptakes of [3H]valproic acid and [14C]lactic acid into brush-border membrane vesicles were greatly stimulated at acidic extravesicular pH. The uptakes of [3H]valproic acid and [14C]lactic acid were inhibited by various fatty acids, p-chloromercuribenzene sulfonate, alpha-cyano-4-hydroxycinnamate, and FCCP. A kinetic analysis showed that it was saturable, with Michaelis constants (Kt) of 1.04 +/- 0.41 mM and 1.71 +/- 0.33 mM for [3H]valproic acid and [14C]lactic acid, respectively. Furthermore, lactic acid competitively inhibited [3H]valproic acid uptake and vice versa.

CONCLUSION

These results suggest that the transport of valproic acid across the microvillous membrane of human placenta is mediated by a proton-linked transport system that also transports lactic acid. However, some inhibitors differentially inhibited the uptakes of [3H]valproic acid and [14C]lactic acid, suggesting that other transport systems may also contribute to the elevated fetal blood concentration of valproic acid in gravida.

Insulin homeostasis in the extremely low birth weight infant

The need to improve the nutritional status of extremely low birth weight infants has resulted in a higher incidence of problems related to glucose intolerance. The inability of the newborn to inhibit gluconeogenesis in response to a glucose infusion has been postulated as an important determinant of the hyperglycemia observed in extremely low birth weight infants. The 2 proposed mechanisms to explain this finding include inappropriate secretion of insulin by the pancreas and decrease sensitivity of the liver to the gluco-regulatory effect of insulin. The capacity of extremely low birth weight infants to oxidize glucose at higher rates, and the positive effect that insulin may have in glucose utilization and tolerance, support the use of insulin in the prevention and treatment of hyperglycemia. Continuous infusion of insulin appears to be safe for the treatment of hyperglycemia, based on the available studies. However, the effectiveness of insulin treatment needs to be critically tested further before it can be implemented in routine clinical practice.

Uptake mechanism of valproic acid in human placental choriocarcinoma cell line (BeWo)

Valproic acid is an anticonvulsant widely used for the treatment of epilepsy. However, valproic acid is known to show fetal toxicity, including teratogenicity. In the present study, to elucidate the mechanisms of valproic acid transport across the blood-placental barrier, we carried out transcellular transport and uptake experiments with human placental choriocarcinoma epithelial cells (BeWo cells) in culture. The permeability coefficient of [3H]valproic acid in BeWo cells for the apical-to-basolateral flux was greater than that for the opposite flux, suggesting a higher unidirectional transport in the fetal direction. The uptake of [3H]valproic acid from the apical side was temperature-dependent and enhanced under acidic pH. In the presence of 50 microM carbonyl cyanide p-trifluoromethoxylhydrazone, the uptake of [3H]valproic acid was significantly reduced. A metabolic inhibitor, 10 mM sodium azide, also significantly reduced the uptake of [3H]valproic acid. Therefore, valproic acid is actively transported in a pH-dependent manner on the brush-border membrane of BeWo cells. Kinetic analysis of valproic acid uptake revealed the involvement of a non-saturable component and a saturable component. The Michaelis constant for the saturable transport (K(t)) was smaller under acidic pH, suggesting a proton-linked active transport mechanism for valproic acid in BeWo cells. In the inhibitory experiments, some short-chain fatty acids, such as acetic acid, lactic acid, propanoic acid and butyric acid, and medium-chain fatty acids, such as hexanoic acid and octanoic acid, inhibited the uptake of [3H]valproic acid. The uptake of [3H]valproic acid was also significantly decreased in the presence of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, salicylic acid and furosemide, which are well-known inhibitors of the anion exchange system. Moreover, p-aminohippuric acid significantly reduced the uptake of [3H]valproic acid. These results suggest that an active transport mechanism for valproic acid exists on the brush-border membrane of placental trophoblast cells and operates in a proton-linked manner.

Human placental transport of vinblastine, vincristine, digoxin and progesterone: contribution of P-glycoprotein

To elucidate the role of P-glycoprotein in human placenta, we examined its expression in placenta, and the transcellular transport and uptake of P-glycoprotein substrates in cultured human placental choriocarcinoma epithelial cells (BeWo cells). The uptake of [(3)H]vinblastine and [(3)H]vincristine into BeWo cells was increased in the presence of a metabolic inhibitor, sodium azide. The basolateral-to-apical transcellular transport of [(3)H]vinblastine, [(3)H]vincristine and [(3)H]digoxin was greater than the apical-to-basolateral transcellular transport. In the presence of cyclosporin A, the basolateral-to-apical transcellular transport of [(3)H]vinblastine, [(3)H]vincristine and [(3)H]digoxin was significantly increased, and the apical-to-basolateral transcellular transport was decreased. The uptake of [(3)H]vinblastine, [(3)H]vincristine and [(3)H]digoxin into BeWo cells was significantly enhanced in the presence of several inhibitors, such as verapamil or mouse monoclonal antibody anti-P-glycoprotein MX-MDR (MRK16) as well as cyclosporin A. Although progesterone significantly enhanced the uptake of [(3)H]vinblastine, [(3)H]vincristine and [(3)H]digoxin into BeWo cells, the uptake of [(3)H]progesterone was not affected by these inhibitors. Immunoblot analysis revealed that P-glycoprotein with a molecular weight of 172 kDa was expressed in BeWo cells and isolated trophoblast cells. Furthermore, P-glycoprotein was detected in human placental brush-border membrane vesicles, but not in human placental basolateral membrane vesicles. In conclusion, these data suggest that P-glycoprotein is expressed on the brush-border membrane (maternal side) of human placental trophoblast cells. P-Glycoprotein is considered to regulate the transfer of several substances including vinblastine, vincristine and digoxin from mother to fetus, and to protect the fetus from toxic substances.