The p50-p50 NF-κB complex as a stimulus-specific repressor of gene activation

The transcription factor NF- Kappa B can be activated in different forms, including transcriptional activating and repressing forms. Intestinal epithelial cells have been found to modulate the relative levels of the p65-p50 and p50-p50 NF- Kappa B complexes in a number of instances, and here we show that this ratio was altered in response to dietary fiber (wheat bran) and carcinogen exposure (azoxymethane). The influence of these complexes on gene regulation was examined in more detail in cell culture models. The colon-derived HT-29 cell line likewise activated both p65-p50 and p50-p50 NF- Kappa B complexes: TNF- alpha triggered a strong, sustained p65-p50 activation with lower relative levels of p50-p50, whereas IL-1 beta transiently activated p65-p50 with higher relative levels of p50-p50. Transfection experiments with an NF- Kappa B reporter plasmid indicated that p50 was a repressor in HT-29 cells. Increased expression of the p50-p50 dimer by an adenovirus showed that the p50-p50 dimer suppressed IL-1 beta activation of endogenous genes more than 5-fold (TNF- alpha , Cox-2 and IL-8), whereas gene activation by TNF- alpha was not significantly affected. DNA binding analyses showed a number of strong p50-p50 binding sites on these promoters. The selective p50-p50 suppression of IL-1 beta gene activation corresponded to the transient nature of p65-p50 activation induced by IL-1 beta (in both HT-29 and Caco-2 cells). Our findings demonstrate a novel gene regulatory mechanism for the NF- Kappa B p50-p50 complex: a signal-specific transcriptional repression that appears to selectively inhibit stimuli that transiently activate p65-p50 complexes. (Mol Cell Biochem 265: 171-183, 2004)