Enhancement of radiosensitivity by proteasome inhibition: Implications fora role of NF-kappa B

Purpose: NF-kappaB is activated by tumor necrosis factor, certain chemother apeutic agents, and ionizing radiation, leading to inhibition of apoptosis, NF-kappaB activation is regulated by phosphorylation of I kappaB inhibitor molecules that are subsequently targeted for degradation by the ubiquitin- proteasome pathway. PS-341 is a specific and selective inhibitor of the pro teasome that inhibits NF-kappaB activation and enhances cytotoxic effects o f chemotherapy in vitro and in vivo, The objective of this study was to det ermine if proteasome inhibition leads to enhanced radiation sensitivity. Methods and Materials: Inhibition of NF-kappaB activation in colorectal can cer cells was performed by treatment of LOVO cells with PS-341 or infection with an adenovirus encoding I kappaB super-repressor, a selective NF-kappa B inhibitor. Cells were irradiated at 0, 2, 4, 6, 8, and 10 Gy with or with out inhibition of NF-kappaB, NF-kappaB activation was determined by electro phoretic mobility gel shift assay, and apoptosis was evaluated using the TU NEL assay, Growth and clonogenic survival data were obtained to assess effe cts of treatment on radiosensitization, In vitro results were tested in viv o using a LOVO xenograft model, Results: NF-kappaB activation was induced by radiation and inhibited by pre treatment with either PS-341 or I kappaB alpha super-repressor in all cell lines. Inhibition of radiation-induced NF-kappaB activation resulted in inc reased apoptosis and decreased cell growth and clonogenic survival, A 7-41% increase in radiosensitivity was observed for cells treated with PS-341 or I kappaB alpha, An 84% reduction in initial tumor volume was obtained in L OVO xenografts receiving radiation and PS-341, Conclusions: Inhibition of NF-kappaB activation increases radiation-induced apoptosis and enhances radiosensitivity in colorectal cancer cells in vitr o and in vivo. Results are encouraging for the use of PS-341 as a radiosens itizing agent in the treatment of colorectal cancer. (C) 2001 Elsevier Scie nce Inc.