Rac1, but not Rac1B, stimulates RelB-mediated gene transcription in colorectal cancer cells

Abstract

Increased NF-κB-mediated transcription has been extensively linked to tumorigenesis and can be stimulated by deregulated Rac1 signaling. For example, the overexpression of Rac1b, a highly activated splicing variant of Rac1 with increased expression in colorectal tumors, stimulates NF-κB-mediated G1/S progression and cell survival, and was shown to promote cell transformation and epithelial-mesenchymal transition. Here we show evidence of further complexity between Rac1b and Rac1 signaling toward NF-κB in colorectal cells. Consistent with data from other cell types we demonstrate that both Rac1 and Rac1b stimulate transcriptional activation from reporter genes driven by NF-κB motifs or the cyclin D1 promoter in an IκBα- and reactive oxygen species-dependent manner. However, we found that in colorectal cells Rac1, but not Rac1b, induces nuclear translocation of RelB and p52, activates transcription from a RelB-specific reporter, and can be isolated in a complex with endogenous RelB and its inhibitor NF-κB2/p100. In addition, Rac1 colocalizes at the plasmamembrane with RelB, p100, and cullin-1, a core subunit of the E3 ubiquitin ligase that marks p100 for proteolytic processing to p52. Interestingly, this Rac1-specific pathway is not mediated via the production of reactive oxygen species. These data provide evidence that both Rac1 and Rac1b activate the canonical RelA-IκBα pathway, whereas Rac1 further stimulates NF-κB by inducing the RelB-NF-κB2/p100 pathway. The RelB pathway was reported to down-regulate canonical NF-κB activation during the inflammatory response, suggesting that increased levels of Rac1b in colorectal tumors may promote tumorigenesis by stimulating canonical NF-κB signaling while circumventing a negative feedback from the RelB pathway. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.