The capability of platelet-activating factor (PAF) to induce transcription factor activation was examined. In stably transfected Chinese hamster ovary cells expressing the PAF receptor (CHO-PAFR), PAF stimulation resulted in the nuclear expression of a DNA binding activity with specificity to the κB sequence. The p50 and p65 proteins, constituents of the prototypic nuclear factor κB (NF-κB), were identified as components of the DNA protein complexes by antipeptide antibodies in gel supershift as well as UV cross- linking experiments. PAF induced an initial decrease and subsequent increase of cytoplasmic IκBα levels, accompanied by up-regulation of the IκBα messenger RNA, a feature of NF-κB activation. PAF-induced κB binding activity was detected within 15 min after agonist stimulation, peaked at 30- 40 min, and remained detectable by 2.5 h. SR 27417, a PAF receptor antagonist, blocked PAF-induced κB binding activity but not that induced by tumor necrosis factor-α (TNFα). Cholera toxin treatment markedly reduced PAF-induced κB binding activity, whereas pertussis toxin had no significant inhibitory effect. Neither of the two toxins affected the κB binding activity induced by TNFα in the same cells. In addition to the CHO-PAFR cells, PAF stimulated κB binding activity in the routine P388D macrophage and the human ASK.0 B cell lines that express endogenous PAF receptors. These results imply a potential role of PAF in the regulation of gene expression through a G protein-coupled transcription factor activation pathway.