DELAYED TYROSINE PHOSPHORYLATION AND NUCLEAR EXPRESSION OF STAT1 FOLLOWING ANTIGEN RECEPTOR STIMULATION OF LYMPHOCYTES

The regulation of the STAT1 alpha transcription factor was assessed du ring B cell activation induced by cross-linking the surface IgM Ag rec eptor, Surface Ig ligation or pharmacologic stimulation with PMA and i onomycin resulted in the delayed (2-3 h after stimulation) nuclear app earance of tyrosine-phosphorylated STAT1 alpha, in contrast to the rap id induction that follows cytokine treatment, Nuclear expression of ph osphorylated STAT1 alpha was abrogated by co-incubation of anti-lg-tre ated B cells with the protein synthesis inhibitor cycloheximide (CHX), with the protein kinase inhibitor H-7, or with the immunosuppressive drug rapamycin, Tyrosine-phosphorylated STAT1 alpha was found to be re cruited to the STAT binding site of the IFN regulatory factor-1 (IRF-1 ) gene promoter only after 2 to 3 h, and this association was also inh ibitable by CHX and rapamycin, The arrival of STAT1 alpha coincided wi th attenuation of anti-lg-induced STAT-binding activity specific for t he IRF-1 promoter site, and both rapamycin and CHX treatment counterac ted the loss of this activity, Furthermore, basal transcription of the endogenous IRF-1 gene was decreased as a result of anti-lg treatment, and this effect of anti-lg was blocked by co-incubation with rapamyci n, Thus, STAT1 alpha plays a dynamic role in the composition of IRF-1 promoter-specific DNA binding complexes stimulated by B cell Ag recept or ligation, and nuclear expression of phosphorylated STAT1 alpha is r egulated in a unique fashion by Ag receptor engagement, In addition, s urface Ig cross-linking imparts negative regulatory control of IRF-1 g ene expression, possibly through activation of STAT1 alpha.