Expression of NAG-1, a Transforming Growth Factor-β Superfamily Member, by Troglitazone Requires the Early Growth Response Gene EGR-1

Abstract

Troglitazone (TGZ) and 15-deoxy-Δ12,14-prostaglandin J2 (PGJ2) are peroxisome proliferator-activated receptor-γ (PPARγ) ligands that have been shown to possess pro-apoptotic activity in human colon cancer. Although these compounds bind to PPARγ transcription factors as agonists, emerging evidence suggests that TGZ acts independently of PPARγ in many functions, including apoptosis. We previously reported that TGZ induces an early growth response transcription factor (EGR-1) by the ERK phosphorylation pathway rather than by the PPARγ pathway (Baek, S. J., Wilson, L. C., Hsi, L. C., and Eling, T. E. (2003) J. Biol. Chem. 278, 5845-5853). In this report, we show that the expression of the antitumorigenic and/or pro-apoptotic gene NAG-1 (nonsteroidal anti-inflammatory drug-activated gene-1) is induced by TGZ and correlates with EGR-1 induction. In cotransfection and gel shift assays, we show that EGR-1-binding sites are located within region -73 to -51 of the NAG-1 promoter and have an important role in the transactivation of TGZ-induced NAG-1 expression. In contrast, PGJ2 induced NAG-1 protein expression, but PJG 2 may not affect the same region that TGZ does in the NAG-1 promoter. The effect of PGJ2 is probably PPARγ-dependent because a PPARγ antagonist inhibited the PGJ2-induced expression of NAG-1. TGZ-induced NAG-1 expression was not inhibited by the PPARγ antagonist. The fact that TGZ-induced NAG-1 expression was accompanied by the biosynthesis of EGR-1 also suggests that EGR-1 plays a pivotal role in TGZ-induced NAG-1 expression. Our results suggest that EGR-1 induction is a unique property of TGZ, but is independent of PPARγ activation. The up-regulation of NAG-1 may provide a novel explanation for the antitumorigenic property of TGZ.