Étude du rôle du general receptor for phosphoinositides 1 (GRP1) dans l'adipogenèse
Many studies have shown that peroxisome-proliferator-activated receptor [gamma] (PPAR[gamma]) plays an important role in adipose tissue formation by activating genes implicated in adipogenesis. PPAR[gamma] heterodimerizes with retinoid X receptor [alpha] (RXR[alpha]), in the presence of ligand, on PPAR response elements (PPREs) in the promoter of target genes involved in adipocyte differentiation. General receptor for phosphoinositides 1 (GRP1) is a corepressor of thyroid hormone receptors (TRs), a nuclear receptor like PPAR[gamma]. GRP1 decreases TRs' transcriptional activity by lowering dimerisation on DNA. Since PPARs and TRs have important structural similarities and that GRP1 interacts with PPARs in vitro, we hypothesized that GRP1 could be a coregulator of PPARs and, thus be implicated in adipogenesis. To better understand GRP1's effect on PPAR[gamma]2, transcriptional activity assays have been done and show that increasing concentrations of GRP1 decrease the transcriptional activity of PPAR[gamma]2. We also studied GRP1 expression by Western blots of total protein extracts from 3T3-L1 cells at different times during differentiation: GRP1 is present in 3T3-L1 preadipocytes and its expression decreases during adipogenesis. According to those results, GRP1 may be a PPAR[gamma] corepressor. After those observations, GRP1 effects on adipogenesis were studied by modulating its expression with lentiviral particles. Interestingly, GRP1 knock-down before inducing 3T3-L1 differentiation, almost abrogates adipogenesis and adipocytes markers, PPAR[gamma] and aP2, while its overexpression increases lipid storage without affecting PPAR[gamma] expression. On the opposite, GRP1 modulation after differentiation induction shows that expression knock-down slightly promotes adipogenesis by increasing PPAR[gamma], aP2 and lipid accumulation and that overexpression weakly decreases lipid storage. Our results suggest that GRP1 implication during adipogenesis occurs at two distinct and precise moments. It seems to be a key factor in the early stages of adipocyte differentiation and to be implicated as a PPAR[gamma] transcriptional activity modulator as a corepressor. Future experiments will help detail modulation of protein expression and underlying mechanisms to better understand the role of GRP1 in adipogenesis and, eventually, comorbidities linked to obesity like cardiovascular diseases and type 2 diabetes mellitus.