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Overproduction of Apo CIII causes elevated plasma triglyceride levels in transgenic animals and is associated with hypertriglyceridemia in humans. The regulation of apo CIII production is likely to play an important role in controlling plasma triglyceride levels. As an initial step in determining the role of transcriptional regulation in the production of apo CIII and in triglyceride metabolism, we have begun to characterize the activity of specific transcriptional regulatory elements in the CIII promoter. In the current study, we have identified and characterized an NF-χB regulatory element located 150 nucleotides upstream from the transcriptional start site of the apo CIII gene. Purified NF-χB, as well as an NF-χB protein in HepG2 cell nuclear extracts, bound specifically to this sequence element. The hepatic protein was induced by phorbol ester (PMA), and reacted with antibodies to the p50 and p65 subunits of NF-χB. The NF-χB element conferred PMA and IL1-β inducible transcriptional activity to a heterologous promoter/reporter construct when transfected into HepG2 cells. Analysis of the full length CIII promoter demonstrated that the inducible activity of the NF-χB element was suppressed by sequences in the apo CIII enhancer element located approximately 500 nucleotides upstream of the NF-χB binding site. A deletion removing the enhancer restored the PMA inducible activity of the NF-χB binding site. These results indicate that apo CIII gene expression is regulated by NF-χB, and suggest that apo CIII production may be modulated by cellular signals, like inflammatory cytokines, that activate NF-kB.