TPK inhibitors differentially affect IFN-γ activities

The effect of various tyrosine protein kinase inhibitors on processes involved in the antiproliferative effect of interferon-γ on WISH cells was studied. Following 24 hr treatment interferon-γ inhibited thymidine incorporation into DNA and thymidine kinase activity but no significant effect on cell number was observed. The isoflavonoid, genistein, which is a specific inhibitor of tyrosine protein kinase, reversed the inhibition in thymidine incorporation caused by the cytokine in a dose dependent manner. Prunetin, a member of the same group, did not significantly antagonize this effect. Nα-tosyl-L-lysyl-chloromethane, a serine protease inhibitor which also serves as a tyrosine protein kinase inhibitor, partially reversed the effect of interferon-γ at a concentration of 100 μM. The bioflavonoid, quercetin, a non-specific tyrosine protein kinase inhibitor, at a concentration of 30 μM completely abolished the action of interferon-γ on thymidine incorporation. Genistein completely reversed the inhibition of thymidine kinase exerted by interferon, while quercetin had only a slight effect. However, the drugs could not antagonize the antiproliferative effect of interferon following 48 hr incubation, as measured by reduction of cell number. The results indicate that tyrosine protein kinase may play a role in the effects of interferon on thymidine methabolism and thymidine kinase activity. The differential effects of the inhibitors on thymidine metabolism and cell proliferation could support dissociation between the effect of interferon-γ on these processes. Alternatively, this dissociation of effects could point to the limited use of inhibitors in clarifying modes of action as described.