Bag-1 consists in humans of four isoforms generated from the same RNA by alternative translation. Overexpression of single Bag-1 isoforms has identified Bag-1 as a negative regulator of action of many proteins including the glucocorticoid receptor (GR). Here we have analysed the ability of Bag-1 to regulate the transrepression function of the GR. Silencing Bag-1 expression only marginally affects the transrepression action of the GR but decreased the action of the transcription factor NF-kappa B. Furthermore phosphorylation and degradation of the inhibitor protein I kappa B alpha and nuclear accumulation of p65 and p50 NF-kappa B proteins in response to phorbol ester was attenuated following Bag-1 depletion in HeLa cells. Reconstitution of Bag-1 in depleted cells partially restored I kappa B alpha and NF-kappa B activation. Knock-down of Bag-1 expression also did not significantly alter GR-mediated transactivation but affected the basal transcription of some of the target genes. Thus Bag-1 proteins function as regulators of the action of selective transcription factors. (C) 2010 Elsevier Inc. All rights reserved.