INVOLVEMENT OF PROTEIN-KINASE-C DURING TAXOL-INDUCED ACTIVATION OF MURINE PERITONEAL-MACROPHAGES

Taxol has been known to block cell division by stabilizing microtubule s with promising anticancer activity. However, taxol has distinct cell cycle-independent effects. Recently, this novel drug has been shown t o provide a second signal for murine macrophage activation to tumorici dal activity via L-arginine-dependent nitric oxide (NO)synthesis. To i nvestigate the mechanism of taxol-induced NO synthesis, we evaluated t he ability of protein kinse C (PKC) inhibitors such as staurosporine ( STSN) or polymyxin B to block taxol-induced effects. Taxol alone had o nly a small effect, whereas taxol in combination with rIFN-gamma marke dly increased NO synthesis in a dose-dependent manner. STSN and polymy xin B decreased NO synthesis, which had been induced by IFN-gamma plus taxol. Furthermore, prolonged incubation of the cells with phorbol es ter, which down-regulates PKC activity, abolished synergistic cooperat ive effect of taxol with rIFN-gamma on NO synthesis. Synergy between I FN-gamma and taxol was mainly dependent on taxol-induced TNF-alpha sec retion because not only the increase of inducible NO synthase (iNOS) g ene expression by rIFN-gamma plus taxol was associated with the increa sed expression of TNF-alpha gene but also taxol-induced NO production was decreased by the treatment of anti-murine TNF-alpha neutralizing A bs. STSN and polymyxin B potently inhibited taxol-induced TNF-alpha se cretion and TNF-alpha gene expression as well as iNOS gene expression by rIFN-gamma plus taxol. However, rIFN-gamma plus TNF-alpha-induced N O synthesis was not blocked by STSN or polymyxin B. This result indica tes that TNF-alpha-induced signaling for induction of NO synthesis is not dependent on PKC activation, and further suggests that the point a t which TNF-alpha acts on the NO synthesis from rIFN-gamma-primed macr ophages lies next to the point of PKC activation. In conclusion, the p resent results strongly suggest that the capacity of taxol to increase NO synthesis from rIFN-gamma-primed macrophages is the result of taxo l-induced TNF-alpha secretion via the signal transduction pathway of P KC activation.