Identification and characterization of a novel interaction between peroxisome-proliferator activated receptor gamma (PPARγ) and human mesoderm induction-early response 1 and potential implications for adipogenesis

1222 PPARγ is a nuclear hormone receptor and master regulator of lipid metabolism and adipogenesis. It is also the target for the thiazolidinediones, a class of drugs used in the treatment of type 2 diabetes. PPARγ regulates these processes through the recruitment of a diverse set of transcriptional coregulators (both coactivators and corepressors) in a tissue and time specific manner. This study focused on characterizing the interaction between PPARγ and human mesoderm induction early response 1 (hMI-ER1), a transcription factor that has been shown to interact with other nuclear hormone receptors and regulate target gene expression. Glutathione-S-transferase pull-down assays have revealed that PPARγ interacts with both the α and β forms of hMI-ER1, specifically through the SANT domain located near the C-termini of both protein isoforms. Coimmunoprecipitations in HEK-293 (human embryonic kidney cells) confirmed that this interaction occurs in vivo . Treatment with ligand (troglitazone) had no effect on the ability of PPAR to bind hMI-ER1 indicating that the interaction is ligand-independent. A reporter assay using luciferase regulated by the PPAR response element (PPRE) demonstrated that hMI-ER1α and β cause a 2-fold activation of PPAR-driven transcriptional activity and this was similar to the 3-fold activation observed with a known PPARγ coactivator (PPARγ coactivator 1-alpha, PGC1-α). This activation was also ligand-independent. Thus, hMI-ER1 interacts with PPAR in a ligand independent manner through its SANT domain, and causes activation of PPARγ-mediated transcriptional activity. We have recently discovered that hMI-ER1 expression is regulated in 3T3-L1 preadipocytes during their differentiation into adipocytes. Future work will determine the role of hMI-ER1 in adipogenesis using the well-established 3T3-L1 differentiation system.