Potentiation of paclitaxel cytotoxicity in lung and esophageal cancer cells by pharmacologic inhibition of the phosphoinositide 3-kinase/protein kinase B (Akt)-mediated signaling pathway

Background: Constitutive activation of the phosphoinositide 3-kinase/protein kinase B survival signal transduction pathway influences the intrinsic chemoresistance of cancer cells. This study evaluates the effect of LY294002, a pharmacologic inhibitor of phosphoinositide 3-kinase, on the sensitivity of lung and esophageal cancer cells to paclitaxel (Taxol) in vitro. Materials and Methods: Cell viability and apoptosis of cancer cells treated with paclitaxel + LY294002 combinations were quantitated by methyl-thiazol-diphenyltetrazolium and terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling-based ApoBrdU assays, respectively. The effect of LY294002-mediated phosphoinositide 3-kinase inhibition on protein kinase B (Akt) activation and nuclear factor-kappaB signaling was determined. by Western blot analysis. Nuclear factor-kappaB transcription activity in cultured cancer cells either at baseline or after treatments with LY294002 or BAY11-0782 (a pharmacologic inhibitor of nuclear factor-kappaB) was determined by the nuclear factor-kappaB-Luciferase reporter system. Results: A 4- to more than 20-fold reduction of paclitaxel IC50 values was observed in cancer cells treated with paclitaxel + LY294002 combinations. This was paralleled with synergistic induction of apoptosis. LY294002 treatment caused a significant dose-dependent inhibition of protein kinase B (Akt) activation and suppression of nuclear factor-kappaB transcriptional activity that was accompanied by elevation of IkappaB the intrinsic inhibitor of nuclear factor-kappaB, and concomitant reduction of nuclear factor-kappaB-regulated antiapoptotic proteins cIAP1, cIAP2, and BclXL. Direct inhibition of nuclear factor-kappaB activity by BAY11-0782 also resulted in profound enhancement of paclitaxel sensitivity and paclitaxel-mediated induction of apoptosis in lung and esophageal cancer cells. Conclusion: LY294002-mediated inhibition of the phosphoinositide 3-kinase/protein kinase B-dependent survival pathway with secondary suppression of nuclear factor-kappaB transcriptional activity was associated with enhancement of paclitaxel cytotoxicity in lung and esophageal cancer cells. Direct inhibition of nuclear factor-kappaB by BAY11-0782 also sensitized these cancer cells to paclitaxel, indicating that nuclear factor-kappaB may be the crucial intermediary step connecting phosphomositide 3-kinase/protein kinase B (Akt) to the intrinsic susceptibility of cancer cells to chemotherapeutic agents.