Differential roles of STAT1 and STAT1 in fludarabine-induced cell cycle arrest and apoptosis in human B cells

Signal transducer and activator of transcription 1 (STAT1), a transcription factor known to participate in antiviral responses, acts as a tumor suppressor inhibiting cell growth and promoting apoptosis. To study the role of STAT1 in DNA damage–induced apoptosis in B lymphocytes, its active form, STAT1 , was specifically inhibited by the overexpression of STAT1 , the STAT1 truncated inhibitory isoform. An episomal vector with a tetracycline-inducible bidirectional promoter was created to induce the expression of 2 proteins, STAT1 and enhanced green fluorescence protein (EGFP). The same vector was used to overexpress STAT1 as a control. Expression of STAT1 inhibited the phosphorylation, the DNA-binding activity, and the transcriptional activity of STAT1 , as well as the expression of STAT1 target genes such as p21WAF1/ CIP1, TAP1, IRF1, and PKR. Inhibiting STAT1 by STAT1 increased the growth rate of transfected cells and their resistance to fludarabine-induced apoptosis and cell cycle arrest. Overexpressing STAT1 reversed the negative regulation of Mdm2 expression observed after treatment with interferon-gamma (IFN), which activates STAT1, or with fludarabine. Nuclear translocation of p53 after fludarabine treatment was decreased when STAT1 was overexpressed, and it was increased when STAT1 was induced. Oligonucleotide pull-down experiments showed a physical STAT1/p53 interaction. Our results show that imbalance between the antiproliferative/proapoptotic isoform STAT1 and the proliferative isoform STAT1 is likely to play a crucial role in the regulation of proliferation and apoptosis and that STAT1 may regulate p53 activity and sensitize B cells to fludarabine-induced apoptosis. (Blood. 2004;104:2475-2483)