Delayed tyrosine phosphorylation and nuclear expression of STAT1 following antigen receptor stimulation of B lymphocytes.

The regulation of the STAT1 alpha transcription factor was assessed during B cell activation induced by cross-linking the surface IgM Ag receptor. Surface Ig ligation or pharmacologic stimulation with PMA and ionomycin resulted in the delayed (2-3 h after stimulation) nuclear appearance of tyrosine-phosphorylated STAT1 alpha, in contrast to the rapid induction that follows cytokine treatment. Nuclear expression of phosphorylated STAT1 alpha was abrogated by co-incubation of anti-Ig-treated 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 recruited 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 inhibitable by CHX and rapamycin. The arrival of STAT1 alpha coincided with attenuation of anti-Ig-induced STAT-binding activity specific for the IRF-1 promoter site, and both rapamycin and CHX treatment counteracted the loss of this activity. Furthermore, basal transcription of the endogenous IRF-1 gene was decreased as a result of anti-Ig treatment, and this effect of anti-Ig was blocked by co-incubation with rapamycin. Thus, STAT1 alpha plays a dynamic role in the composition of IRF-1 promoter-specific DNA binding complexes stimulated by B cell Ag receptor ligation, and nuclear expression of phosphorylated STAT1 alpha is regulated in a unique fashion by Ag receptor engagement. In addition, surface Ig cross-linking imparts negative regulatory control of IRF-1 gene expression, possibly through activation of STAT1 alpha.