Placental proteins and steroid hormones in the soluble fraction of human syncytiotrophoblast plasma membrane

Control of placental glucose transfer

There is little evidence to suggest that the membrane transfer mechanism of the placenta for glucose becomes saturated until maternal blood glucose concentrations are quite high. Also, recent evidence suggests that the membrane transport system for glucose in the placenta is not stimulated by maternal or fetal insulin. Furthermore, there is no solid evidence that hormonal or non-hormonal factors function in vivo to limit membrane transport of glucose in the placenta. Therefore, the limited data which are available suggest that there are no specific mechanisms which acutely regulate placental membrane transport of glucose, and that this membrane transport mechanism operates to maximize maternal-to-fetal glucose transfer. The rate of maternal-to-fetal glucose transfer is a function of the transplacental concentration gradient. This gradient appears to be under the control of fetal insulin and placental lactogen. The available data suggest that both hormones act to increase this concentration gradient: insulin by decreasing fetal blood glucose, and placental lactogen by both decreasing fetal and increasing maternal blood glucose concentrations. Furthermore, high rates of glucose uptake by fetal erythrocytes tend to promote maintenance of this concentration gradient. Therefore, these influences of the maternal-fetal concentration gradient promote transplacental glucose flux to the fetus. As illustrated by the fetal complications associated with maternal hyperglycaemia, the cellular and organismic physiology of the fetus and placenta appears to maximize, rather than optimize, glucose availability to the fetus. It may be, however, that during normal pregnancy, maximal availability is optimal for fetal development.

Identification of a low molecular weight polypeptide of pregnant bovine uterine origin (LMW-UDF): influence on coagulation system in vitro. Part I

Acidified extracts of pregnant bovine uterus were found to contain a heparin-affinity polypeptide(s). The eluted peak was assayed in the Chandler loop method for whole blood and fibrin thrombus generation. Significant differences in the mean weight and size of whole blood thrombi (243.5 +/- 50 mg and 3.5 cm +/- 0.6 cm) occurred with 1 microgram of purified polypeptide when compared to control samples containing albumin (47.87 +/- .30 g and 0.8 cm +/- 0.3 cm). This activity was detected by the Chandler loop method with 100 ng to 10 micrograms of protein when analyzed in a dose-response fashion. When tested for fibrin thrombus formation this activity persisted. Experiments with 125I fibrinogen revealed no net increase or decrease of uptake into whole blood thrombi when purified polypeptide was added. The heparin-affinity polypeptide(s) possessed a molecular weight of approximately 6-4 kDa when examined by SDS-PAGE. We have therefore designated this polypeptide as low molecular weight-uterine derived factor (LMW-UDF).

Characterization of the human syncytiotrophoblast plasma membrane associated components

A purification procedure of synctiotrophoblast plasma membrane (STPM) has previously been described. We now report a further characterization of their associated components. By immunochemical analysis two groups of proteins were found. The first fraction solubilized by chaotrope treatment was made up with a complex of IgG, albumin, transferrin and alpha-fetoprotein. The second fraction, insolubilized by KCl-treatment but by detergents, did not react with antisera to human proteins; it contained human chorionic gonadotropin and placental lactogen. The amino acid, carbohydrate and lipid content of STPM were determined. High galactosyl- and sialyl-transferase activities were found in purified STPM. The role of these various components on the acceptance of the fetal allograft is discussed.

Binding of pregnancy-associated plasma protein-A (PAPP-A) to placental subfractions

Pregnancy-associated plasma protein-A (PAPP-A) has been shown to exert immunosuppressive effects both on complement and on lymphoblastogenesis. It was of interest to see whether this protein could bind to syncytiotrophoblast microvillous membranes since these represent the effective interface between fetal tissue and the immunocompetent mother. Placental subfractions were prepared according to established techniques. PAPP-A was purified from different sources (pregnancy serum and plasma, retroplacental serum, placental extracts) and labelled with radioactive iodine. It could be shown that radioactive PAPP-A, irrespective of its biological origin was primarily binding to brush border membrane preparations but that significant binding was also seen with plasma membrane preparations. The binding was specific since alpha 2-macroglobulin (a structurally related protein to PAPP-A) was unable to displace bound radioactive PAPP-A. Scatchard plot representation of the data indicated that the affinity of PAPP-A for its binding site was of the same order of magnitude as reported for the insulin or GnRH receptors of the trophoblast (Ka 2 10(7)-2.5 10(8) M-1). These results are in agreement with the immunohistochemical localization of PAPP-A on the trophoblast providing that the small binding to plasma membrane preparations is due to contamination.

Placental-type alkaline phosphatase in pregnancy and malignancy plasma: specific estimation using a monoclonal antibody in a solid phase enzyme immunoassay

A solid-phase enzyme immunoassay has been developed using a monoclonal antibody with specificity only for the placental-type alkaline phosphatase isoenzyme (Pl-ALP). This assay is uniquely sensitive, having a lower limit of detection of 0.1 U/l (approximately 0.07 micrograms/l) Pl-ALP. All plasma samples from healthy non-pregnant individuals had undetectable Pl-ALP levels whereas, in pregnancy, a wide range (0-400 U/l) of Pl-ALP levels was noted for 208 maternal plasma samples from 12 weeks gestation to term. There was no significant difference in circulating Pl-ALP levels between severe pre-eclamptic and normal pregnancy. Low levels of Pl-ALP (0.1-4.7 U/l) were found in 31 of 217 plasmas from human malignancies. Within these, 23 of 65 (35%) histologically-proven ovarian carcinoma patients were positive.

Antigens of the human syncytiotrophoblast microvillus plasma membrane

Further data are presented on the isolation and biochemical analysis of syncytiotrophoblast microvillus plasma membrane preparations from normal full-term human placentas. No evidence has been obtained that IgG associated with these membranes is maternal antibody to trophoblast antigen. An oncotrophoblast plasma membrane antigen was previously identified by immunofluorescence, and an indirect radioisotopic antibody assay has now been used to confirm the detection of a cell surface antigen common to breast carcinoma cells and normal trophoblast. The relation of trophoblast plasma membrane antigens to possible maternal immunization in pregnancy is discussed.

A cell membrane antigen expressed by both human breast carcinoma cells and normal human trophoblast

Immunohistological studies on frozen sections of human placentae and breast carcinoma tissue using heteroantisera raised against both trophoblast microvillous plasma membrane preparations isolated from normal placentae and irradiated MCF-7 breast carcinoma cells have demonstrated a cell membrane antigen expressed by both normal human trophoblast and human breast carcinoma cells. The heteroantisera used in this study had all been previously adsorbed with immobilized human term pregnancy serum, placental alkaline phosphatase and placental ferritin preparations, as well as with human peripheral blood leucocytes. The oncoplacental membrane antigen would appear not to be represented in other normal tissues, but is represented on Jar choriocarcinoma cells and to a lesser degree on AV3 amniotic cells. Adsorption experiments have demonstrated that this antigen may not be dominant within the range of antigenic specificities of heteroantisera raised against MCF-7 cells or isolated trophoblast microvillous plasma membrane preparations.