A novel mechanism by which thiazolidinediones facilitate the proteasomal degradation of cyclin D1 in cancer cells

Authors:
Wei S, Yang HC, Chuang HC, Yang J, Kulp SK, Lu PJ, Lai MD, Chen CS
In:
Source: J Biol Chem
Publication Date: (2008)
Issue: epub: online
Research Area:
Cancer Research/Cell Biology
Cells used in publication:
LNCaP
Species: human
Tissue Origin: prostate
Platform:
Nucleofectorâ„¢ I/II/2b
Abstract
This study identifies a novel mechanism by which thiazolidinediones mediate cyclin D1 repression in prostate cancer cells. Based on the finding that the thiazolidinedione family of peroxisome proliferator-activated receptor gamma (PPARgamma) agonists mediated PPARgamma-independent cyclin D1 degradation, we developed a novel PPARgamma-inactive troglitazone derivative, STG28, with high potency in cyclin D1 ablation. STG28-mediated cyclin D1 degradation was preceded by Thr286 phosphorylation and nuclear export, which, however, were independent of glycogen synthase kinase 3beta. Mutational analysis further confirmed the pivotal role of Thr286 phosphorylation in STG28-induced nuclear export and proteolysis. Of several kinases examined, inhibition of IkappaB kinase alpha blocked STG28-mediated cytoplasmic sequestration and degradation of cyclin D1. Pull-down of ectopically expressed Cul1, the scaffold protein of the Skp-Cullin-F-box E3 ligase, in STG28-treated cells revealed an increased association of cyclin D1 with beta-TrCP, while no specific binding was noted with other F-box proteins examined, including Skp2, Fbw7, Fbx4, and Fbxw8. This finding represents the first evidence that cyclin D1 is targeted by beta-TrCP. Moreover, beta-TrCP expression was upregulated in response to STG28, and ectopic expression and siRNA-mediated knockdown of beta-TrCP enhanced and protected against STG28-facilitated cyclin D1 degradation, respectively. As cyclin D1 lacks the DSG destruction motif, mutational and modeling analyses indicate that cyclin D1 was targeted by beta-TrCP through an unconventional recognition site, 279EEVDLACpT286, reminiscent to that of Wee1. Moreover, we obtained evidence that this beta-TrCP-dependent degradation takes part in controlling cyclin D1 turnover when cancer cells undergo glucose starvation, which endows physiological relevance to this novel mechanism.