Networking activity of PPARs during development and in adult metabolic homeostasis
As they mediate intracellular hormone action, nuclear receptors play a crucial multi-faceted role in coordinating growth during development, and homeostasis at adult stage. Among them, the peroxisome-proliferator activated receptors (PPARs) act as fatty acids sensors, responding to dietary as well as to endogenous challenges. Accordingly, they have an integrative role in controlling the expression of genes regulating the storage, mobilization, and/or utilization of lipids. Using various molecular, cellular, and animal approaches, our studies are aimed at understanding how PPARs are integrated in the main pathways that shape the organism during development on the one hand and maintain systemic homeostasis on the other hand.
We were among the first to generate PPAR mutant mice. Following a clinician-type of approach, our activities have been centered on revealing and understanding at the molecular levels the phenotypic expressions of PPAR mutations, taking them as leads to explore the physiopathological significance and novel therapeutic advances that PPARs carry.
Our studies during development show that both PPARβ and PPARγ are required for placenta development but each has specific activities: on the differentiation of trophoblast giant cells, via activation of the PI3K pathway and inhibition of Id2 for PPARβ, and on vascular development via controlling the expression of angiogenic factors for PPARγ. Further studies of the role of PPARβ and PPARγ in development were impeded by a partially and fully penetrant lethality of PPARβ-/- and PPARγ-/- embryos, respectively, at embryonic day 10.5. However, we have now generated fully viable mutant embryos and live pups, through an epiblastic selective PPAR deletion. It demonstrates that the cause of embryonic lethality in PPAR mutants is mainly due to the placental defects and gives us new tols for exploring the role of PPARγ in late development and in adult tissues.
In the adult animals, the gut is a very interesting organ, combining critical metabolic functions and a tightly regulated and highly active cell renewal capacity, from few adult stem cells to highly differentiated enterocytes, Paneth, Goblet, and enterodendocrine cells. In this tissue, we demonstrated that decrease of Indian Hedgehog via PPARβ ensures the final maturation of Paneth cell precursors whereas PPARγ is rather involved in controlling inflammation processes. We are now further exploring the importance of PPARs in response to challenges, either metabolic, infectious, or physical damages, with the purpose of bringing our mechanistic approaches in mouse models closer to human pathologies.
The role of PPARs as mediating the activity of some endocrine disruptors was an important focus of our recent activity. To understand how endocrine disruptors behave at the molecular levels in the living cells, we used a combination of Fluorescence Recovery After Photobleaching (FRAP), Fluorescence Correlation Spectroscopy (FCS) and Fluorescence Resonance Energy Transfer (FRET). We first demonstrated that PPARs readily heterodimerize with retinoid X receptor (RXR) and exhibit a ligand-induced reduction of mobility, probably due to enhanced interactions with cofactors and/or chromatin. We also demonstrated that coregulator recruitment (and not DNA binding) plays a crucial role in receptor mobility, suggesting that transcriptional complexes are formed prior to promoter binding. This allowed us to demonstrate that, in the living cells, the pollutant monoethylhexyl-
phthalate (MEHP) directly binds to the ligand binding domain of PPAR and drives the recruitement of a specific subset of cofactors. We further analyzed the consequences in vivo of such activities and showed that MEHP protects mice from diet-induced obesity via a PPARα-dependent activation of hepatic fatty acid catabolism. This is accompanied by, and possibly due to, the up-regulation of the anti-obesity factor FGF21 hepatic expression. However, both effects are reversed in PPARα-humanized mice, underlining the importance of PPARα species-specific activities and questioning the impact of phthalates in human metabolic homeostasis.