REGULATION OF INTERFERON-ALPHA RESPONSIVENESS BY THE DURATION OF JANUS KINASE-ACTIVITY

Daudi B lymphoblastoid cells are highly sensitive to the anti-growth a nd anti-viral effects of interferon (IFN). Unlike many cell lines, the se cells show prolonged transcription of IFN-stimulated genes followin g treatment with IFN-alpha. This prolonged response correlated with th e continued presence of the activated transcription factor, IFN-stimul ated gene factor 3 (ISGF3). Pulse-chase labeling experiments indicated that the half life of the phosphorylation of signal transducers and a ctivators of transcription (Stat)1 and Stat2 was short (<2 h) although the turnover of the proteins themselves was slow (>24 h), indicative of a constitutive phosphatase activity. The administration of protein- tyrosine kinase inhibitors at any time point during IFN stimulation le d to rapid inhibition of the response, indicating that tyrosine kinase activity was continuously required. Catalytic activity of Jak1 and Ty k2 kinases remained elevated for prolonged periods following stimulati on. Continuous presence of IFN-alpha was necessary for maintaining pro longed activation of ISGF3 and of Janus kinases, an activity that was blocked by antibodies to IFN-alpha or by cycloheximide. Conditioned me dium of IFN-alpha-stimulated cells was capable of stimulating STAT act ivation in naive cells. Taken together, these results suggest that the response to IFN-alpha is controlled by the duration of stimulated Jan us kinase activity over the background of constitutive dephosphorylati on and that this response can be sustained by autocrine secretion of I FN-alpha.