Identification of a composite steroid hormone response element on the human placental lactogen promoter

Nuclear retinoid receptors and pregnancy: placental transfer, functions, and pharmacological aspects

Animal models of vitamin A (retinol) deficiency have highlighted its crucial role in reproduction and placentation, whereas an excess of retinoids (structurally or functionally related entities) can cause toxic and teratogenic effects in the embryo and foetus, especially in the first trimester of human pregnancy. Knock-out experimental strategies-targeting retinoid nuclear receptors RARs and RXRs have confirmed that the effects of vitamin A are mediated by retinoic acid (especially all-trans retinoic acid) and that this vitamin is essential for the developmental process. All these data show that the vitamin A pathway and metabolism are as important for the well-being of the foetus, as they are for that of the adult. Accordingly, during this last decade, extensive research on retinoid metabolism has yielded detailed knowledge on all the actors in this pathway, spurring the development of antagonists and agonists for therapeutic and research applications. Natural and synthetic retinoids are currently used in clinical practice, most often on the skin for the treatment of acne, and as anti-oncogenic agents in acute promyelocytic leukaemia. However, because of the toxicity and teratogenicity of retinoids during pregnancy, their pharmacological use needs a sound knowledge of their metabolism, molecular aspects, placental transfer, and action.

The expression of nuclear retinoid receptors in human implantation

Vitamin A and retinoids play an important role during development. They affect morphogenesis, cell growth and differentiation by interacting with two types of receptor, the RARs and the RXRs. Despite the well known established teratogenic effects of retinoids during human pregnancy, little is known about the effect of retinoids on human placental development. We studied the possible involvement of retinoids during the implantation process by investigating the spatial distribution of retinoid receptors in the human implantation site by in situ hybridization and immunohistochemistry. For in situ hybridization, we used digoxigenin-labelled antisense riboprobes. Immunochemical staining was performed with specific antibodies against the various retinoid receptors and a streptavidin-alkaline phosphatase immunostaining kit. We found that only two types of receptors were expressed at the implantation site: RARalpha and RXRalpha. Both types of receptors were present in the proliferative intermediate trophoblast, the invasive extravillous trophoblast and decidual cells. Both receptors were also present in the villous cytotrophoblasts. The presence of this retinoid receptor in the cytotrophoblasts suggests a key role for all-trans retinoic acid and/or 9-cis retinoic acid in the development of human placenta.

Activator protein-2 regulates human placental lactogen gene expression

DNase I footprint analysis of the human placental lactogen-A (hPL-A) promoter using nuclear extracts from purified human trophoblast cells and BeWo choriocarcinoma cells revealed five protected regions within the proximal 325 bp. Two of the protected regions, FP4 (-289--267) and FP5 (-167--154), are homologous to regions on the human growth hormone (hGH) promoter that bind transcription factors AP-2 and/or NFI. Competitive gel shift assays and supershift assays indicated that FP4 forms complexes with activator protein-2 (AP-2) and nuclear factor I (NFI), while FP5 forms a complex with AP-2 alone. In transient transfection studies in human trophoblast cells, hPL promoter constructs containing point mutations in the AP-2 binding sites of FP4 and/or FP5 were 60-80% less active than plasmids containing the wild-type promoter. A mutation in the NFI binding site of FP4, however, had little effect on promoter activity in these cells. Overexpression of AP-2 in HepG2 cells co-transfected with the wild-type hPL promoter resulted in a significant increase in promoter activity. Taken together, these findings suggest a critical role for AP-2 in the regulation of hPL gene expression.

Pre-beta-HDL stimulates placental lactogen release from human trophoblast cells

To examine whether pre-beta-high-density lipoprotein (HDL) may be involved in regulation of human placental lactogen (hPL) release, pre-beta-HDL was isolated from term pregnancy serum, and the effect of purified pre-beta-HDL on hPL release from trophoblast cells was examined after 1 h of exposure. Pre-beta-HDL stimulated a dose-dependent increase in hPL release with half-maximal stimulation at a dose of 300-400 microgram/ml, which is within the normal physiological range during pregnancy. Analysis of pre-beta-HDL and alpha-HDL in serum from pregnant women at different stages of gestation (determined by Western blot analysis) indicated that the pre-beta-HDL-to-alpha-HDL ratio increased linearly after the 10th week of gestation (r = 0.88, P < 0.001), reaching a maximum sixfold greater than that of nonpregnant women. The increase in serum pre-beta-HDL during pregnancy paralleled that of plasma hPL concentrations (r = 0.93, P < 0.001). Two-dimensional electrophoresis indicated that the increase in pre-beta-HDL was due primarily to an increase in pre-beta1-HDL and pre-beta2-HDL, two of the three forms of pre-beta-HDL present in blood. These results suggest a role for pre-beta-HDL in the regulation of hPL expression during pregnancy.