Rapamycin induces p53-independent apoptosis through the mitochondrial pathway in non-small cell lung cancer cells

The mammalian target of rapamycin (mTOR) is a key kinase acting downstreamof growth factor receptor PI3K and AKT signaling, leading to processes resultingin increased cell size and proliferation through translation control. Rapamycin,a specific inhibitor of mTOR, results predominately in G1 cell cycle arrest throughtranslation control and occasionally, cell type-dependent apoptosis by an unknownmechanism. In this study, we investigated the effect and mechanism of action ofrapamycin on non-small cell lung cancer (NSCLC) cell lines with p53 mutations.Cell proliferation was evaluated by modified MTT assay. The apoptotic effect ofrapamycin was measured by caspase-3 activation and flow cytometric analysis ofAnnexin V binding. The expression of Bcl-2 and the release of cytochrome c frommitochondria were evaluated by western blotting. We found that rapamycin inducedapoptosis in NSCLC cell lines with p53 mutations. Western blot analysis demonstratedthat rapamycin downregulates the expression levels of Bcl-2, which leads to increasedcytochrome c release from mitochondria and subsequent activation of caspase cascades.These findings suggest that rapamycin induces p53-independent apoptosis throughdownregulation of Bcl-2 and the mitochondrial pathway in NSCLC cell lines as anovel antitumor mechanism.