OVEREXPRESSION OF RELA IN TRANSGENIC MOUSE THYMOCYTES - SPECIFIC INCREASE IN LEVELS OF THE INHIBITOR PROTEIN I-KAPPA-B-ALPHA

RelA (p65) is one of the strongest activators of the Rel/NF-kappa B fa mily. As a first step to elucidate the mechanisms that regulate its ac tivity in vivo, we have generated transgenic mice overexpressing RelA in the thymus. Although the levels of RelA were significantly increase d in thymocytes of transgenic mice, the overall NF-kappa B-binding act ivity in unstimulated cells was not augmented compared with that in co ntrol thymocytes. This could be explained by the dramatic increase of endogenous I kappa B alpha levels observed in RelA-overexpressing cell s in both cytoplasmic and nuclear compartments. The ikba mRNA levels w ere not augmented by overexpressed RelA, but I kappa B alpha inhibitor was found to be stabilized through association with RelA. Although a fraction of RelA was associated with cytoplasmic p105, no changes in t he precursor levels were observed. Upon stimulation of RelA-overexpres sing thymocytes with phorbol 12-myristate 13-acetate and lectin (phyto hemagglutinin), different kappa B-binding complexes, including RelA ho modimers, were partially released from I kappa B alpha. Association of RelA with I kappa B alpha prevented complete degradation of the inhib itor. No effect of phorbol 12-myristate 13-acetate-lectin treatment wa s detected on RelA associated with p105. Our data indicate that cytopl asmic retention of overexpressed RelA by I kappa B alpha is the major in vive mechanism controlling the potential excess of NF-kappa B activ ity in long-term RelA-overexpressing thymocytes.