Peroxisome proliferator-activated receptors gamma (PPARgamma) ligands have been shown to inhibit the growth of non-small cell lung cancer (NSCLC) cells. However, the mechanisms underlying this effect remain incompletely elucidated.

Cell proliferation and apoptosis were measured by cell viability, MTT and caspase3/7 activity assays. Phosphorylation/protein expression and gene silence/overexpression of AMPKalpha, phosphoinositide-dependent protein kinase 1 (PDK1), Egr-1 and PPARgamma were measured by Western blot and siRNA/transfection assays. Dual-Luciferase Reporter Kit was used to measure the PPAR response elements (PPRE) reporter and PDK1 promoter activities, and ChIP assay was used to detect the Egr-1 protein binding to the DNA site in the PDK1 gene promoter.

We found that ciglitazone, one synthetic PPARgamma ligand, inhibited growth and induced apoptosis of NSCLC cells through decreased expression of PDK1, which was not blocked by GW9662 (a specific PPARgamma antagonist). Overexpression of PDK1 overcame the effect of ciglitazone on cell growth and caspase 3/7 activity. Ciglitazone increased the phosphorylation of AMPKalpha and c-Jun N-terminal kinase (JNK), and the inhibitor of AMPK (compound C), but not JNK (SP600125), reversed the effect of ciglitazone on PDK1 protein expression. Ciglitazone reduced PDK1 gene promoter activity, which was not observed in cells exposed to compound C, but not silenced of PPARgamma siRNA. Combination of ciglitazone and metformin further reduced PDK1 expression and promoter activity. Furthermore, we showed that ciglitazone induced the protein expression of Egr-1, which was not observed in cells silencing of AMPKalpha. Moreover, silencing of Egr-1 abrogated the effect of ciglitazone on PDK1 promoter activity and cell growth. On the contrary, overexpression of Egr-1 enhanced the effect of ciglitazone on PDK1 gene promoter activity. ChIP assays demonstrated that ciglitazone induced Egr-1 bind to the specific DNA binding site in the PDK1 gene promoter.

Collectively, our results demonstrate that ciglitazone inhibits PDK1 expression through AMPKalpha-mediated induction of Egr-1 and Egr-1 binding to the specific DNA site in the PDK1 gene promoter, which is independent of PPARgamma. Activation of AMPKalpha by metformin enhances the effect of ciglitazone. In turn, this leads to inhibition of NSCLC cell proliferation.