Regulation of interferon-alpha responsiveness by the duration of Janus kinase activity

Daudi B lymphoblastoid cells are highly sensitive to the anti-growth and anti-viral effects of interferon (IFN). Unlike many cell lines, these cells show prolonged transcription of IFN-stimulated genes following treatment with IFN-alpha. This prolonged response correlated with the continued presence of the activated transcription factor, IFN-stimulated gene factor 3 (ISGF3). Pulse-chase labeling experiments indicated that the half-life of the phosphorylation of signal transducers and activators 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 led to rapid inhibition of the response, indicating that tyrosine kinase activity was continuously required. Catalytic activity of Jak1 and Tyk2 kinases remained elevated for prolonged periods following stimulation. Continuous presence of IFN-alpha was necessary for maintaining prolonged activation of ISGF3 and of Janus kinases, an activity that was blocked by antibodies to IFN-alpha or by cycloheximide. Conditioned medium of IFN-alpha-stimulated cells was capable of stimulating STAT activation in naive cells. Taken together, these results suggest that the response to IFN-alpha is controlled by the duration of stimulated Janus kinase activity over the background of constitutive dephosphorylation and that this response can be sustained by autocrine secretion of IFN-alpha.