A ROLE OF THE INTERLEUKIN-1 RECEPTOR IN THE SYNERGISTIC ANTITUMOR EFFECTS OF HUMAN INTERLEUKIN 1 ALPHA AND ETOPOSIDE AGAINST HUMAN-MELANOMA CELLS

To investigate the possibility that anticancer drugs combined with cytokines may show increased activity, human tumor cells were treated with combinations of human recombinant interleukin 1-alpha (rIL-1-alpha) and etoposide (VP-16). The cytotoxicity of these combinations was evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay using rIL-1-alpha-sensitive A375-C6 melanoma cells and A375-C5 cells, a clonal variant line that is resistant to IL-1-alpha. Data were analyzed for synergism by the median effect principle of T-C. Chou and P. Talalay (J. Biol. Chem., 252: 6438-6442, 1977). At a dose ratio of VP-16 to rIL-1-alpha of 12 nM: 1 unit/ml in either simultaneous or sequential exposure (VP-16 first), the calculated combination index values indicated synergistic cytotoxicity toward both A375-C6 cells and A375-C5 cells. IL-1-alpha treatment 24 h prior to VP-16 exposure had no advantage over simultaneous treatment. Surface IL-1-alpha receptors on both A375-C6 and A375-C5 cells were measured using I-125-radiolabeled rIL-1-alpha binding; A375-C6 cells had 701 +/- 128 (SD) receptor molecules/cell and A375-C5 cells only had 58 +/- 33 receptor molecules/cell. The dissociation constants for IL-1-alpha were similar in both cell types (19 +/- 6 pM for A375-C6 and 17 +/- 2 pM for A375-C5). The specific binding of rIL-1-alpha to the surface IL-1-alpha receptors of both sensitive and resistant cells was significantly increased in a dose-dependent fashion by the prior treatment with VP-16 (1.75-fold on A375-C6 cells and 3.5-fold on A375-C5 cells). VP-16 also enhanced the internalization of receptor-bound rIL-1-alpha, suggesting that a possible mechanism of the synergistic cytotoxicity of rIL-1-alpha and VP-16 might be related to the modulation of rIL-1-alpha receptors by VP-16, resulting in increased internalization of rIL-1-alpha.