Differentiation of Trophoblast Giant Cells and Their Metabolic Functions Are Dependent on Peroxisome Proliferator-Activated Receptor β/δ

Mutation of the nuclear receptor peroxisome proliferator-activated receptor β/δ (PPARβ/δ) severely affects placenta development, leading to embryonic death at embryonic day 9.5 (E9.5) to E10.5 of most, but not all, PPARβ/δ-null mutant embryos. While very little is known at present about the pathway governed by PPARβ/δ in the developing placenta, this paper demonstrates that the main alteration of the placenta of PPARβ/δ-null embryos is found in the giant cell layer. PPARβ/δ activity is in fact essential for the differentiation of the Rcho-1 cells in giant cells, as shown by the severe inhibition of differentiation once PPARβ/δ is silenced. Conversely, exposure of Rcho-1 cells to a PPARβ/δ agonist triggers a massive differentiation via increased expression of 3-phosphoinositide-dependent kinase 1 and integrin-linked kinase and subsequent phosphorylation of Akt. The links between PPARβ/δ activity in giant cells and its role on Akt activity are further strengthened by the remarkable pattern of phospho-Akt expression in vivo at E9.5, specifically in the nucleus of the giant cells. In addition to this phosphatidylinositol 3-kinase/Akt main pathway, PPARβ/δ also induced giant cell differentiation via increased expression of I-mfa, an inhibitor of Mash-2 activity. Finally, giant cell differentiation at E9.5 is accompanied by a PPARβ/δ-dependent accumulation of lipid droplets and an increased expression of the adipose differentiation-related protein (also called adipophilin), which may participate to lipid metabolism and/or steroidogenesis. Altogether, this important role of PPARβ/δ in placenta development and giant cell differentiation should be considered when contemplating the potency of PPARβ/δ agonist as therapeutic agents of broad application.