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

Increased NF- kappa B- 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- kappa B- mediated G(1)/ 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- kappa B 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- kappa B motifs or the cyclin D1 promoter in an I kappa B alpha- 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- kappa B2/ 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- I kappa B alpha pathway, whereas Rac1 further stimulates NF- kappa B by inducing the RelB- NF- kappa B2/ p100 pathway. The RelB pathway was reported to down- regulate canonical NF- kappa B activation during the inflammatory response, suggesting that increased levels of Rac1b in colorectal tumors may promote tumorigenesis by stimulating canonical NF- kappa B signaling while circumventing a negative feedback from the RelB pathway.