Ubiquitin/proteasome-dependent degradation of D-type cyclins is linked to tumor necrosis factor-induced cell cycle arrest

Tumor necrosis factor-alpha (TNF) is well known for its cytotoxic effect on malignant cells. Its role in cell cycle control is relatively less known. In this study, we found that TNF induced G(1) arrest of TF-1 and MV4-11 cells while simultaneously causing apoptosis. Treatment of the cells with TNF for 48 h caused cell cycle arrest, accompanied by dephosphorylation of pRb and reduction in D-type cyclin expression. The down-regulation of the D-type cyclins resulted in approximately 50-80% decrease of the cyclin-dependent kinase activities. Cells treated with calpain-dependent inhibitor ALLN and apoptosis inhibitor zVAD-FMK suppressed degradation of IkappaBalpha and activation of caspase 3, respectively. However, treatment of cells with these two inhibitors was not able to prevent TNF-induced down-regulation of the D-type cyclins. In contrast, proteasome inhibitor MG-132 and lactacystin blocked both TNF-induced degradation of IkappaBalpha and down-regulation of D-type cyclins. These data suggest that down-regulation of D-type cyclins by TNF may be proteasome-proteolysis dependent. Additional support for this conclusion was obtained from experiments showing an increase of proteasome activity in TNF-treated cells and in vitro degradation of cyclin D3 by 26 S proteasome.