CYCLIN D3 SENSITIZES TUMOR-CELLS TO TUMOR NECROSIS FACTOR-INDUCED, C-MYC-DEPENDENT APOPTOSIS

c-Myc is an important mediator of apoptosis in cytokine- or serum-depr ived cells and sensitizes various cell types to tumor necrosis factor alpha (TNF) cytotoxicity, However, downstream mediators of c-Myc-depen dent apoptosis are largely unknown. In this study, we investigated whe ther one or more cyclins which, like c-Myc, are important regulators o f the cell cycle are involved in TNF-induced apoptosis downstream of c -Myc, Cyclin D3 and c-Myc levels in HeLa and fibrosarcoma cells correl ated with sensitivity of these cells to TNF-induced apoptosis, as both proteins were highly expressed in TNF-sensitive HeLa D98 cells and HT -1080 fibrosarcoma cells but not in their TNF-resistant counterparts, HeLa H21 and SS-HT-1080 cells, respectively, All other cyclins tested were equally expressed in all tumor cell lines. Reduction in the expre ssion of c-Myc by dexamethasone or inhibition of the transcriptional a ctivity of c-Myc by introduction of a dominant negative form of c-Myc into TNF-sensitive HeLa D98 cells strongly suppressed the expression o f cyclin D3 (but none of the other cyclins) and rendered the cells res istant to TNF-induced apoptosis, Conversely, introduction of the c-myc gene into TNF-resistant, c-Myc- and cyclin DJ-deficient HeLa H21 cell s resulted in enhanced cyclin D3 expression and TNF killing, When cycl in D3 expression in HeLa cells was altered by sense or antisense cycli n D3 cDNA, there was a concomitant alteration in their susceptibility to TNF-induced apoptosis without any change in c-Myc levels, Overall, our results show that cyclin D3 sensitizes tumor cells to TNF-induced apoptosis and indicate that the expression of c-Myc and expression of cyclin D3 in HeLa and in HT-1080 fibrosarcoma cells are closely linked .