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

Abstract

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-(4-hydroxy-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.In addition to maintaining the basal transcription of housekeeping genes, increasing evidence indicates that the transcription factor Sp1 also plays an important role in regulating the expression of a host of key effectors of signaling pathways governing cell cycle progression, cell proliferation, angiogenesis, apoptosis, and metastasis (Wierstra, 2008). These target proteins include receptor tyrosine kinases and their growth factor ligands, cyclin-dependent kinase inhibitors, c-Myc, Mdm2, Mcl-1, survivin, XIAP, Fas ligand, PUMA, and death receptor 5 (Wierstra, 2008). Moreover, Sp1

AKT1 Inhibits Epithelial-to-Mesenchymal Transition in Breast Cancer through Phosphorylation-Dependent Twist1 Degradation

“…4C) and HeLa cells (Supplementary Fig. S4C) by ciglitazone (CG), troglitazone (TG), or resveratrol (26,27) reduced Twist1 expression, supporting that β-TrCP upregulation mediates Twist1 degradation. A double thymidine (Fig.…”

Interplay of Posttranslational Modifications in Sp1 Mediates Sp1 Stability during Cell Cycle Progression

“…However, studies have demonstrated that reduced O-glycosylation of Sp1 was associated with increased proteasome susceptibility and that a E3 ligase SCF (β-transducin repeat-containing protein) targeted Sp1 for proteasomal degradation in response to glucose starvation. 54,55 Therefore, different ubiquitin E3 ligases may modulate Sp1 stability under the different physical conditions; the detail mechanism needs further address. It is also interesting to note that no upshift bands were found in sumoylated Sp1 during mitosis (Fig.…”

Targeting the Oncogenic E3 Ligase Skp2 in Prostate and Breast Cancer Cells with a Novel Energy Restriction-Mimetic Agent

“…ΔF-β-TrCP and ΔNT-β-TrCP were generated as previously described [13]. The resulting fragments were ligated into the pMyc4-CMV14 expression vector.…”