RESUMO
Tumor necrosis factor (TNF)-α promotes tumor development under chronic inflammation. Because TNF also activates caspase-8, selective inhibition of TNF-induced extrinsic apoptosis would be required for inflammation-associated tumor growth. In a mouse model of inflammation-associated colon carcinogenesis, we found nuclear expression of ß-catenin in tumors of wild-type, but not mutant, mice that were made resistant to TNF-induced apoptosis by a germline mutation blocking caspase cleavage of the retinoblastoma (RB) protein, despite similar frequencies of ß-catenin exon-3 mutations in these two genetic backgrounds. TNF-induced apoptosis was also attenuated in human colon cancer cell lines with genetically activated ß-catenin. However, we found that HCT116 cells, which contain an activated allele of ß-catenin but do not express nuclear ß-catenin, were sensitive to TNF-induced apoptosis. In HCT116 cells, TNF stimulated efficient RB cleavage that preceded chromatin condensation. In contrast, TNF did not induce RB cleavage in colon cancer cells expressing nuclear ß-catenin and these cells could be sensitized to basal and/or TNF-induced apoptosis by the knockdown of ß-catenin or RB. In the apoptosis-resistant colon cancer cells, knockdown of ß-catenin led to a reduction in the RB protein without affecting RB mRNA. Furthermore, ectopic expression of the caspase-resistant, but not the wild-type, RB re-established resistance to TNF-induced caspase activation in colon cancer cells without ß-catenin. Together, these results suggest that nuclear ß-catenin-dependent RB stabilization suppresses TNF-induced apoptosis in caspase-8-positive colon cancer cells.
