In human pregnancy, trophoblast cells play an essential role in embryo implantation and placental development. These cells differentiate according to one of two distinct pathways. In the extravillous pathway, cytotrophoblasts (CT) proliferate, differentiate into an invasive phenotype, and penetrate into the maternal decidua and myometrium [
1,
2]. In the villous pathway, mononuclear CT fuse to form a specialized multinuclear syncytium called syncytiotrophoblast (ST) on the outer layer of placental villi [
1]. ST formation plays an important role in human placentation. This process might be affected in some pathological pregnancy situations. For example, altered ST formation was observed in human preeclampsia [
2].
The ST layer is the site of many placental functions necessary for foetal growth and development, including nutrient, gas exchanges, and synthesis of steroid and peptide hormones [
2]. Characteristics related to trophoblast differentiation include the production of hormones like human chorionic gonadotropin (hCG), human placental lactogen, and leptin [
3]. However, morphological changes, which involve fusion of CT to form the ST layer represent a hallmark of this differentiation. Studies have highlighted the impact of adhesion molecules such as cadherins in trophoblast differentiation. Among these, E-cadherin is localized at the membrane of the isolated CT and disappears when the CT fuse into ST [
4,
5]. Very recently, studies have demonstrated the role of former envelope viral proteins derived from human endogenous retrovirus (HERVs) in trophoblast cell fusion, of which syncytin-1 [
6] and syncytin-2 [
7] seem to be of high importance. Moreover, syncytin-2 mRNA and protein are particularly expressed in the ST [
7,
8].
Different
in vitro studies have shown that the villous CT differentiation could be modulated by hormones and by soluble factors. For example, epidermal growth factor (EGF) [
9], 17β-estradiol [
10], granulocyte macrophage-colony stimulating factor (GM-CSF) [
11], glucocorticoids [
12], and hCG [
13] induce differentiation, whereas tumor necrosis factor α (TNFα) [
2,
14] and tumor growth factor β1 (TGFβ1) [
15] impair this process. Adipokines such as leptin and adiponectin have recently been shown to affect the reproductive system through central effects on the hypothalamus and/or peripheral effects on the ovary, endometrium, or directly on the embryo and placenta developments [
16‐
21]. Indeed, leptin is specifically expressed in the ST [
18], and is considered as a new placental hormone [
18,
22]. Adiponectin is a cytokine, predominantly produced by adipose tissue, and present at high concentrations in human circulation (5-15 μg/ml) [
23]. This adipokine is described as an insulin sensitizing hormone [
24‐
26], and has been shown to have anti-inflammatory, anti-angiogenic, anti-atherosclerotic and anti-proliferative roles in various cell types [
25]. Adiponectin is a 30 kDa protein that is assembled into an array of complexes composed of adiponectin multimers. Adiponectin subunits assemble into trimers called low molecular weight complexes (LMW), hewamers or middle molecular weight forms (MMW), or more elaborate high molecular weight complexes (HMW) composed of 9 hewamers. The HMW form is predominant in human circulation [
27]. Two specific adiponectin receptors, AdipoR1 and AdipoR2 have been identified [
28]. Both receptors contain seven transmembrane domains but are structurally and functionally distinct from G-protein coupled receptors. AdipoR1 and AdipoR2 are both expressed in human endometrium and placenta [
19,
29,
30]. However, adiponectin is only produced by endometrial cells at the foetal-maternal interface [
19]. An additional receptor for adiponectin, T-cadherin, has recently been described [
31] but is not expressed in human trophoblast [
30]. AdipoR1 and AdipoR2 activate different signal transduction pathways such as the AMPK, PKA, PI3K and P38/P42/P44 MAPK pathways [
16,
25,
28,
32]. Recently, we have shown that adiponectin exerts anti-proliferative effects on trophoblastic cell lines (JEG-3 and BeWo) and also on human trophoblasts [
30]. Moreover, it has been shown that adiponectin serum concentrations are deregulated in some placental pathologies as gestational diabetes mellitus [
33], and preeclampsia [
34,
35]. However, to date, there are no data concerning the direct impact of adiponectin in trophoblast differentiation.
To study adiponectin effects on trophoblast differentiation, the widely used trophoblast differentiation model BeWo choriocarcinoma cell line was chosen [
36,
37]. These cells have indeed a high degree of similarity to normal placental trophoblasts and can morphologically and functionally differentiate
in vitro into ST. In particular, BeWo differentiation can be strongly induced by cAMP analogs or forskolin, an adenylate cyclase activator [
37,
38]. Thus, the effects of adiponectin on differentiation in both BeWo cells and in villous cytotrophoblasts were tested by measuring hCG secretion and expression of various differentiation markers (leptin, syncytin-2 and E-cadherin) to evaluate the associated morphological and biochemical changes.