BACKGROUND. Proteosome inhibition is associated with induction of the proapoptotic BH3 protein NOXA. Therefore, we hypothesize that the combination of the irreversible proteosome inhibitor, carfilzomib, will cooperatively enhance apoptosis induction by the BH3 mimetic drug, ABT-263. Given that nononcogenic addiction to proteosome activity has been reported in KRAS mutant cells, we utilized isogenic colorectal cancer (CRC) cell lines in which the endogenous wild-type (wt) or mutant alleles had been inactivated through targeted homologous recombination. Since proteasome blockade results in proteotoxic stress, we determined whether an autophagy inhibitor can enhance chemosensitivity.

EXPERIMENTAL METHODS. Isogenic KRAS mutant (MUT) CRC cells (HCT116, DLD1; obtained from B. Vogelstein, JHU) were incubated with carfilzomib (25,50nM) alone or combined with ABT-263 (1 µM) and apoptosis was measured by caspase cleavage and annexin V+ labeling. Synergy was measured by calculation of the combination index. Effect of carfilzomib on Bcl-2 family proteins was determined by immunoblotting. Interaction of NOXA with MCL-1 was examined by immunoprecipitation. Knockdown of NOXA was achieved by lentiviral shRNA. Cells were incubated with drugs alone or combined with chloroquine, and autophagy (LC3I/II conversion) and apoptosis were analyzed.

RESULTS. Carfilzomib was shown to potently and synergistically enhance ABT-263-induced apoptosis (2-fold) in both KRAS MUT and wild type (WT) cell lines, with the effect being modestly higher in wild type cells, as shown by annexin V+ staining and caspase cleavage. Carfilzomib induced NOXA expression in both KRAS MUT and WT cells, and reduced MCL-1 expression. Induced NOXA was shown to bind and sequester MCL-1 proteins. Knockdown of NOXA attenuated drug-induced apoptosis. Autophagy inhibition by chloroquine accumulated LC3II, but only modestly increased carfilzomib-induced cell death.

CONCLUSION. Irreversible proteasome inhibition by carfilzomib synergistically enhanced ABT-263-induced cell death that is mediated by NOXA upregulation and its sequestration/neutralization of MCL-1. The finding of similar efficacy in KRAS MUT vs WT cells suggests that this combination may represent a novel therapeutic strategy in this tumor subset.

Citation Format: Shengbing Huang, Koichi Okamoto, Frank A. Sinicrope. Carfilzomib synergistically enhances ABT-263-induced apoptosis due to NOXA induction in KRAS wild type and mutant colorectal cancer cell lines. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 337. doi:10.1158/1538-7445.AM2014-337