Suppression of NF-κB and GSK-3β is involved in colon cancer cell growth inhibition by the PPAR agonist troglitazone

Peroxisome proliferator-activated receptor (PPAR)-gamma agonists such as troglitazone, pioglitazone and thiazolidine have been shown to induce apoptosis in human colon cancer cells. The molecular mechanism of PPAR-gamma agonist-induced apoptosis of colon cancer cells, however, is not clear. Glycogen synthase kinase-3 beta (GSK-3 beta) is an indispensable element for the activation of nuclear factor-kappa B (NF-kappa B) which plays a critical role in the mediation of survival signals in cancer cells. To investigate the mechanisms of PPAR gamma agonist-induced apoptosis of colon cancer cells, we examined the effect of troglitazone (0-16 mu M) on the activation of GSK-3 beta and NF-kappa B. Our study showed that the inhibitory effect of troglitazone on colon cancer cell growth was associated with inhibition of NF-kappa B activity and GSK-3 beta expression in a dose-dependent manner. Cells were arrested in G(0)/G(1) phase followed by the induction of apoptosis after treatment of troglitazone with concomitant decrease in the expression of the G(0)/G(1) phase regulatory proteins; Cdk2, Cdk4, cyclin B1, D1, and E as well as in the anti-apoptosis protein Bcl-2 along with an increase in the expression of the pro-apoptosis-associated proteins; Caspase-3, Caspase-9 and Bax. Transient transfection of GSK-3 beta recovered troglitazone-induced cell growth inhibition and NF-kappa B inactivation. In contrast, co-treatment of troglitazone with a GSK-3 beta inhibitor (AR-a014418) or siRNA against GSK-3 beta, significantly augmented the inhibitory effect of troglitazone on the NF-kappa B activity, the cancer cell growth and on the expression of G(0)/G(1) phase regulatory proteins and pro-apoptosis regulatory proteins. These results suggest that the PPAR gamma agonist, troglitazone, inhibits colon cancer cell growth via inactivation of NF-kappa B by suppressing GSK-3 beta activity. (C) 2010 Elsevier Ireland Ltd. All rights reserved.