Thiazolidinediones Mimic Glucose Starvation in Facilitating Sp1 Degradation through the Up-Regulation of β-Transducin Repeat-Containing Protein

This study investigated the mechanism by which the transcription factor Sp1 is degraded in prostate cancer cells. We recently developed a thiazolidinedione derivative, (Z)-5-(4hydroxy-3-trifluoromethylbenzylidene)-3-(1-methylcyclohexyl)thiazolidine-2,4-dione (OSU-CG12), that induces Sp1 degradation in a manner paralleling that of glucose starvation. Based on our finding that thiazolidinediones suppress -catenin and cyclin D1 by up-regulating the E3 ligase SCF -TrCP , we hypothesized that -transducin repeat-containing protein (-TrCP) targets Sp1 for proteasomal degradation in response to glucose starvation or OSU-CG12. Here we show that either treatment of LNCaP cells increased specific binding of Sp1 with -TrCP. This direct binding was confirmed by in vitro pull-down analysis with bacterially expressed -TrCP. Although ectopic expression of -TrCP enhanced the ability of OSU-CG12 to facilitate Sp1 degradation, suppression of endogenous -TrCP function by a dominant-negative mutant or small interfering RNA-mediated knockdown blocked OSU-CG12-facilitated Sp1 ubiquitination and/or degradation. Sp1 contains a C-terminal conventional DSG destruction box ( 727 DSGAGS 732 ) that mediates -TrCP recognition and encompasses a glycogen synthase kinase 3 (GSK3) phosphorylation motif (SXXXS). Pharmacological and molecular genetic approaches and mutational analyses indicate that extracellular signal-regulated kinasemediated phosphorylation of Thr739 and GSK3-mediated phosphorylation of Ser728 and Ser732 were critical for Sp1 degradation. The ability of OSU-CG12 to mimic glucose starvation to activate -TrCP-mediated Sp1 degradation has translational potential to foster novel strategies for cancer therapy.