Regulated Production of a Peroxisome Proliferator-activated Receptor-γ Ligand during an Early Phase of Adipocyte Differentiation in 3T3-L1 Adipocytes

Peroxisome proliferator-activated receptor-γ (PPARγ) is a nuclear hormone receptor that is critical for adipogenesis and insulin sensitivity. Ligands for PPARγ include some polyunsaturated fatty acids and prostanoids and the synthetic high affinity antidiabetic agents thiazolidinediones. However, the identity of a biologically relevant endogenous PPARγ ligand is unknown, and limited insight exists into the factors that may regulate production of endogenous PPARγ ligands during adipocyte development. To address this question, we created a line of 3T3-L1 preadipocytes that carry a β-galactosidase-based PPARγ ligand-sensing vector system. In this system, induction of adipogenesis resulted in elevated β-galactosidase activity that signifies activation of PPARγ via its ligand-binding domain (LBD) and suggests generation and/or accumulation of a ligand moiety. The putative endogenous ligand appeared early in adipogenesis in response to increases in cAMP, accumulated in the medium, and dissipated later in adipogenesis. Organically extracted and high pressure liquid chromatography-fractionated conditioned media from differentiating cells, but not from mature adipocytes, were enriched in this activity. One or more components within the organic extract activated PPARγ through interaction with its LBD, induced lipid accumulation in 3T3-L1 cells as efficiently as the differentiation mixture, and competed for binding of rosiglitazone to the LBD of PPARγ. The active species appears to be different from other PPARγ ligands identified previously. Our findings suggest that a novel biologically relevant PPARγ ligand is transiently produced in 3T3-L1 cells during adipogenesis.