Chemosensitization of pancreatic cancer by inhibition of the 26S proteasome

Background. Pancreatic cancer is extremely resistant to the induction of apoptosis by chemotherapies; agents that regulate sensitivity to apoptosis may lead to chemosensitization of pancreatic cancer. Materials and methods. MIA-PaCa-2 human pancreatic cancer cells were treated in vitro with the 26S-proteasome inhibitor PS-341. Levels of the apoptosis-regulating proteins (BCL-2, BAK, and BAX) were determined by Western blotting. The effect of PS-341 on BCL-2 gene transcription was examined using a BCL-2 promoter/luciferase reporter construct. The chemosensitizing effect of PS-341 was determined by measurement of the cytotoxic effect of gemcitabine in the presence of PS-341 (10-1000 nM) using the MTT assay. A corresponding in vivo experiment using tumor xenografts in athymic mice was also performed. Results. PS-341 decreased BCL-2, without effect on BAX or BAK. The downregulation of BCL-2 by PS-341 appears to be transcriptionally mediated. PS-341 induced apoptosis at high does (1000 nM) and increased the cytotoxicity of gemcitabine at low doses (10-100 nM). Xenograft growth was inhibited 59% by gemcitabine; the addition of PS-341 increased growth inhibition to 75%. Conclusions. Inhibition of the 26S proteasome disrupts the cellular content of key regulators of cell cycle progression and apoptotic control leading to increased sensitivity to standard chemotherapeutic agents, such as gemcitabine, in pancreatic cancer. Combination therapy may lead to better response rates. (C) 2001 Academic Press.