Expression of IκBα in the nucleus of human peripheral blood T lymphocytes

According to current models the inhibitory capacity of I kappa B alpha would be mediated through the retention of Rel/NF-kappa B proteins in the cytosol. However, I kappa B alpha has also been detected in the nucleus of cell lines and when overexpressed by transient transfection. To gain better insight into the potential role of nuclear I kappa B alpha in a physiological context we have analysed its presence in the nucleus of human peripheral blood T lymphocytes (PBL). We demonstrate the nuclear localization of I kappa B alpha in PBL by different techniques: Western blot, indirect immunofluorescence and electron microscopy. Low levels of nuclear I kappa B alpha were detected in resting cells whereas a superinduction was obtained after PMA activation. The nuclear pool of I kappa B alpha showed a higher stability than cytosolic I kappa B alpha and was partially independent of the resynthesis of the protein. Unexpectedly, the presence of nuclear I kappa B alpha did not inhibit NF-kappa B binding to DNA and this phenomenon was not due to the presence of I kappa B beta at the nuclear level. Immunoprecipitation experiments failed to demonstrate an association between nuclear I kappa B alpha and NF-kappa B proteins. Our results demonstrate that in resting and PMA-activated human PBL, I kappa B alpha is present in the nucleus in an apparently inactive form unable to disrupt NF-kappa B binding from DNA.