Regulation of transcriptional activity of PPARγ by lipin1 in adipogenesis and mature adipocytes

Abstract

Peroxisome proliferator-activated receptorγ2 (PPARγ2) is a key transcription factor in adipogenesis. In absence of ligand, PPARγ2 interacts with corepressors, acting as repressor to its target genes. PPARγ2 ligands replace the corepressors with coactivators, converting PPARγ2 to transcriptional activator. However, it is controversial whether the endogenous ligands regulate PPARγ activity in adipocytes. The several candidates, such as free fatty acid and PGJ2, were suggested as endogenous PPARγ ligands, but their actions on PPARγ in adipocytes are not certain. For these reasons, it is important to address how PPARγ acts as transcriptional activator in absence of precise ligands, maintaining the high expression levels of adipocyte-specific genes in adipocytes. Present study shows that lipin1 can activate PPARγ activity even in absence of ligands, by replacing the corepressors with coactivators like artificial PPARγ ligands, such as rosiglitazone. The activation domain in lipin1 resides in residue 217 to 399, where there is no conserved sequence and no homology with other lipin isoforms. This activation domain plays a critical role only in PPARγ activation, but not in PPARα activation. Activation domain of lipin1 is stimulated by p300, and SRC-1 but not by PGC-1α and P/CAF. The physical interaction with PPARγ2 occurs at C-terminal region from residue 825 to 926. LXXIL motif in lipin1 is not important in interaction with PPARγ2 and its activation, while PPARα absolutely requires this motif for transcriptional activity. Yeast two-hybrid system revealed that lipin1 strongly interacts with homeodomain-interacting protein kinase 2 (HIPK2). HIPK2 is induced at late stage during adipocyte differentiation in a similar pattern to that of lipin1. Present study proved that HIPK2 has an important role in adipogenesis by showing the effects on adipogenesis of overexpression or knocking-down of HIPK2. HIPK2 drastically up-regulates the activity of lipin1α as a coactivator of PPARγ2. In summary, lipin1 and HIPK2 is key modulator to activate PPARγ activity in absence or presence of ligands in adipocytes.