Troglitazone, a Peroxisome Proliferator-activated Receptor γ (PPARγ) Ligand, Selectively Induces the Early Growth Response-1 Gene Independently of PPARγ

Troglitazone (TGZ) is a peroxisome proliferator-activated receptor γ (PPARγ) ligand that has pro-apoptotic activity in human colon cancer. Although TGZ binds to PPARγ transcription factors as an agonist, emerging evidence suggests that TGZ acts independently of PPARγ in many functions, including apoptosis. Early growth response-1 (Egr-1) transcription factor has been linked to apoptosis and shown to be activated by extracellular signal-regulated kinase (ERK). We investigated whether TGZ-induced apoptosis may be related to Egr-1 induction, because TGZ has been known to induce ERK activity. Our results show that Egr-1 is induced dramatically by TGZ but not by other PPARγ ligands. TGZ affects Egr-1 induction at least by two mechanisms; TGZ increases Egr-1 promoter activity by 2-fold and prolongs Egr-1 mRNA stability by 3-fold. Inhibition of ERK phosphorylation in HCT-116 cells abolishes the Egr-1 induction by TGZ, suggesting its ERK-dependent manner. Further, the TGZ-induced Egr-1 expression results in increased promoter activity using a reporter system containing four copies of Egr-1 binding sites, and TGZ induces Egr-1 binding activity to Egr-1 consensus sites as assessed by gel shift assay. In addition, TGZ induces ERK-dependent phosphorylation of PPARγ, resulting in the down-regulation of PPARγ activity. The fact that TGZ-induced apoptosis is accompanied by the biosynthesis of Egr-1 suggests that Egr-1 plays a pivotal role in TGZ-induced apoptosis in HCT-116 cells. Our results suggest that Egr-1 induction is a unique property of TGZ compared with other PPARγ ligands and is independent of PPARγ activation. Thus, the up-regulation of Egr-1 may provide an explanation for the anti-tumorigenic properties of TGZ.