Rosiglitazone, an Agonist of PPARγ, Inhibits Non-Small Cell Carcinoma Cell Proliferation In Part through Activation of Tumor Sclerosis Complex-2

PPARγ ligands inhibit the proliferation of non-small cell lung carcinoma (NSCLC) cells in vitro. The mechanisms responsible for this effect remain incompletely elucidated, but PPARγ ligands appear to inhibit the mammalian target of rapamycin (mTOR) pathway. We set out to test the hypothesis that PPARγ ligands activate tuberous sclerosis complex-2 (TSC2), a tumor suppressor gene that inhibits mTOR signaling. We found that the PPARγ ligand rosiglitazone stimulated the phosphorylation of TSC2 at serine-1254, but not threonine-1462. However, an antagonist of PPARγ and PPARγ siRNA did not inhibit these effects. Rosiglitazone also increased the phosphorylation of p38 MAPK, but inhibitors of p38 MAPK and its downstream signal MK2 had no effect on rosiglitazone-induced activation of TSC2. Activation of TSC2 resulted in downregulation of phosphorylated p70S6K, a downstream target of mTOR. A TSC2 siRNA induced p70S6K phosphorylation at baseline and inhibited p70S6K downregulation by rosiglitazone. When compared to a control siRNA in a thymidine incorporation assay, the TSC2 siRNA reduced the growth inhibitory effect of rosiglitazone by fifty percent. These observations suggest that rosiglitazone inhibits NSCLC growth partially through phosphorylation of TSC2 via PPARγ-independent pathways.