SENSITIZATION OF TUMOR-CELLS TO TUMOR-NECROSIS-FACTOR ACTION BY THE PROTEIN-KINASE INHIBITOR STAUROSPORINE

Tumor necrosis factor (TNF), first described as a cytokine with tumor- necrotizing activity, is now known to be a pleiotropic molecule. The m olecular mechanisms responsible for the cytotoxic activity of TNF on m alignant cells are still largely unknown. In this study, we report tha t the protein kinase inhibitor staurosporine (56 to 1500 nM) increases about 500 times the in vitro cytotoxic activity of TNF for several mu rine and human tumor cell lines. Even some tumor cell lines which are resistant to TNF cytotoxicity could be sensitized to TNF killing by st aurosporine. In the 1,929 fibrosarcoma cell line, staurosporine also e nhanced the transcriptional activation of interleukin 6 synthesis by T NF (500-fold stimulation at 56 nM). At the biochemical level, staurosp orine increased the TNF-mediated activation of phospholipases C and D and the transcription factor NF-Kbeta in L929 cells. The TNF-sensitizi ng effect of staurosporine does not seem to be mediated by one of the currently known staurosporine-sensitive kinases, as various other inhi bitors which also inhibit one or more of these kinases were not synerg istic with TNF. Interestingly, staurosporine (1 mug) also enhanced the in vivo antitumor activity of TNF against a murine tumor model (L929 fibrosarcoma) in athymic nude mice (Swiss-nu/nu; s.c. treatment). Thes e results suggest that TNF responsiveness of tumor cells is regulated by a novel staurosporine-sensitive target and that the combination of TNF and staurosporine may open new strategies of tumor treatment.