INTERFERON-GAMMA PRIMING EFFECTS IN THE ACTIVATION AND DEACTIVATION OF ISGF3 IN K562 CELLS

ISGF3 is a major protein factor which mediates the transcriptional act ivation of interferon-inducible genes. ISGF3 is composed of two subuni ts, ISGF3gamma and ISGF3alpha, which are stimulated by interferon-gamm a (IFN-gamma) and interferon-alpha (IFN-alpha), respectively. In this paper, the effect of pretreatment of IFN-gamma on the response of K562 cells to IFN-alpha, termed ''gamma-priming,'' is examined. Using tech niques of gene transfection and mobility shift assay, we studied the g amma-priming effects on the kinetics of appearance and disappearance o f (i) the protein product of a luciferase reporter gene driven by an I FN-inducible promoter and (ii) the binding of ISGF3 to the interferon- stimulated response element. We found that exposure of cells to IFN-ga mma prior to IFN-alpha greatly increased both the levels and the rate of ISGF3 binding activity during the early phase of IFN-alpha treatmen t and caused the amount of ISGF3 bound to the interferon-stimulated re sponse element to decrease more rapidly with time during the later pha se. Such effects were reflected also on the kinetics of expression of the interferon-inducible luciferase gene induced by IFN-alpha. In orde r to understand the basis of these gamma-priming effects, we use an in vitro reconstitution method to examine the kinetics of the subcompone nts of ISGF3 in response to different IFN treatments in K562 cells. It was found that gamma-priming not only increased the levels of ISGF3ga mma, but it also stimulated both the rates of rise and fall of the act ivated alpha-component of ISGF3. A molecular model is proposed to expl ain these findings.