Polar expression and phosphorylation of human leptin receptor isoforms in paired, syncytial, microvillous and basal membranes from human term placenta

Leptin affects system A amino acid transport activity in the human placenta: evidence for STAT3 dependent mechanisms

BACKGROUND

Amino acids are important nutrients during fetal development, and the activity of placental amino acid transporters is crucial in the regulation of fetal growth. Leptin, an adipocyte- and placenta-derived hormone, has been proposed to act as a peripheral signal in reproduction in humans. Leptin is elevated during pregnancy and elevated further in pathologic pregnancies such as preeclampsia. However, the role of leptin in placental function has not been fully elucidated. We hypothesize that leptin plays a role in the regulation of placental amino acid transport by activation of the JAK-STAT pathway.

METHODS

Placental amino acid transport, specifically system A transport was studied in placental villous fragments using the amino acid analog, methylaminoisobutyric acid (MeAIB). Specific inhibitors of the JAK-STAT signal transduction pathway were used to further elucidate their role in leptin-mediated effects on amino acid transport activity. Western blotting was performed to identify STAT3 phosphorylation as a measure of leptin receptor activation.

RESULTS

Leptin significantly increased system A amino acid transporter activity by 22-42% after 1h of incubation. Leptin activated JAK-STAT signaling pathway as evidenced by STAT3 phosphorylation, and inhibition of STAT3 or JAK2 resulted in 36-45% reduction in system A amino acid transporter activity. Furthermore, blocking endogenously produced leptin also decreased system A transport by 45% comparable to STAT3 inhibition.

CONCLUSIONS

These data demonstrate that leptin stimulates system A by JAK-STAT dependent pathway in placental villous fragments. Our findings support the autocrine/paracrine role of leptin in regulating amino acid transport in the human placenta.

Expression of placental leptin and leptin receptors in preeclampsia

This study investigated the expression profile of placental leptin and leptin receptor isoforms in preeclampsia, using placental tissue from normal pregnancies that were matched in gestational age and birth weight as controls. A total of 29 cases of preeclampsia were studied by immunohistochemistry, including 16 severe and 13 mild preeclampsia cases. Reverse transcriptase-polymerase chain reaction (RT-PCR) was further performed using RNA extracted from frozen tissue (10 severe preeclampsia, 10 mild preeclampsia, and 20 normal third trimester placentas). In all tissue sections, immunostaining signal was shown in the cytoplasmic compartment of the trophoblastic cells. Both the severe and mild preeclampsia groups showed significantly higher immunostaining for leptin compared with normal controls (p < 0.05), but there was no significant difference between the severe and mild preeclampsia groups (p > 0.05). There was no significant difference in immunostaining for leptin receptor between both severe and mild preeclampsia compared with controls (p > 0.05). RT-PCR showed significantly higher levels of mRNA transcripts of leptin in severe preeclampsia (p < 0.05), but not mild preeclampsia (p > 0.05), compared with normal controls. No significant difference in expression of all the receptor isoforms was demonstrated between both severe and mild preeclampsia groups compared with controls (p > 0.05). In conclusion, we confirmed an up-regulated expression of leptin in placental tissue in preeclampsia. However, there was no difference in the expression of all leptin receptor isoforms in placental tissue between preeclamptic and normal pregnancies. The leptin signal probably does not play a major primary role in the pathogenesis of preeclampsia.

Expression of leptin and leptin receptors in gestational trophoblastic diseases

OBJECTIVES

To investigate the expression profile of leptin and leptin receptors in gestational trophoblastic diseases (GTDs).

METHODS

Using immunohistochemical staining on archival paraffin-embedded tissue sections, we studied the expression of leptin and leptin receptor in hydatidiform moles, with gestational age-matched normal first-trimester placenta used as control. A total of 38 cases of hydatidiform moles were studied, including 20 complete moles (CHMs) and 18 partial moles (PHMs). Among them, 10 cases of the CHM group and 8 cases of the PHM group subsequently developed residual trophoblastic disease (RTD). In addition, two cases of choriocarcinoma and three cases of placental site trophoblastic tumor (PSTT) were also studied. Reverse transcriptase-polymerase chain reaction (RT-PCR) was further performed using RNA extracted from frozen tissue (five CHMs, four PHMs and nine normal first-trimester placenta) to study the expression of leptin and individual leptin receptor isoforms at the transcription level.

RESULTS

In all tissue sections, immunostaining signal was shown in the cytoplasmic compartment of cytotrophoblasts and syncytiotrophoblasts, with much stronger staining in the former. Significantly higher immunostaining intensity was shown for both leptin (P < 0.05) and leptin receptor (P < 0.001) in both CHMs and PHMs compared to normal first-trimester placenta. There was no significant difference between those cases subsequently developing RTD and those which did not (P > 0.05). In the choriocarcinoma and PSTT cases, intense immunostaining was found in the tumor cells. RT-PCR revealed that the expression of leptin and all leptin receptor isoforms were significantly higher in both CHMs and PHMs than in normal placenta (P < 0.05).

CONCLUSIONS

There is up-regulated expression of leptin and leptin receptor in GTDs. However, there is no obvious correlation with the development of RTD. The exact role played by leptin and its receptors in the pathogenesis of GTDs awaits further investigations.