Peroxisome proliferator-activated receptor γ-independent ablation of cyclin D1 by thiazolidinediones and their derivatives in breast cancer cells

In light of the clinical relevance of targeting cyclin D1 in breast cancer, we have investigated the mechanism underlying the effect of the peroxisome proliferator-activated receptor-gamma (PPAR gamma) agonists troglitazone and ciglitazone on cyclin D1 repression. We obtain evidence that the ability of high doses of troglitazone and ciglitazone to repress cyclin D1 is independent of PPAR gamma activation. PPAR gamma-inactive troglitazone and ciglitazone analogs 5-[4-(6-hydroxy-2,5,7,8-tetramethyl-chroman-2yl-methoxy)-benzylidene]-2,4-thiazolidinedione (Delta 2-TG) and 5-[4-(1-methyl-cyclohexylmethoxy)-benzylidene]-thiazolidine2,4-dione are able to facilitate cyclin D1 ablation with potency similar to that of troglitazone and ciglitazone in MCF-7 cells. Reverse transcription-polymerase chain reaction shows that the mRNA level of cyclin D1 remains unaltered in drug-treated cells, indicating the repression is mediated at the post-transcriptional level. Moreover, the ablative effect of these agents is specific to cyclin D1, in that the expression levels of many other cyclins and cyclin-dependent kinases examined remain unchanged after drug treatment. Our data indicate that troglitazone- and Delta 2-TG-induced cyclin D1 repression is mediated via proteasome-facilitated proteolysis because it is inhibited by different proteasome inhibitors, including N-carbobenzoxy-L-leucinyl-L-leucinyl-L-norleucinal (MG132), lactacystin, and epoxomicin, and is preceded by increased ubiquitination. The dissociation of these two pharmacological activities (i.e., PPAR gamma activation and cyclin D1 ablation) provides a molecular basis to use Delta 2-TG as a scaffold to develop a novel class of cyclin D1-ablative agents. Therefore, a series of Delta 2-TG derivatives have been synthesized. Among them, 5-[4-(6-allyoxy-2,5,7,8-tetramethyl-chroman-2-yl-methoxy)-benzylidene]-2,4-thiazolidinedione represents a structurally optimized agent with potency that is an order of magnitude higher than that of Delta 2-TG in cyclin D1 repression and MCF-7 cell growth inhibition.