Rac1 inhibition negatively regulates transcriptional activity of the amyloid precursor protein gene

Rac1, a member of the Rho family GTPases, participates in a variety of cellular functions including lamellipodia formation, actin cytoskeleton organization, cell growth, apoptosis, and neuronal development. Recent studies have implicated Rac1 in cytoskeletal abnormalities, production of reactive oxygen species, and generation of the amyloid beta-peptide (Abeta) observed in Alzheimer's disease. In this study, we examined the relationship between Rac1 and amyloid precursor protein (APP), because the abnormal proteolytic processing of APP is a pathologic feature of Alzheimer's disease. In primary hippocampal neurons, the Rac1-specific inhibitor NSC23766 decreased both Rac1 activity and APP protein levels in a concentration-dependent manner. To elucidate how NSC23766 decreases APP protein levels, we examined the effects of NSC23766 on APP processing, degradation, and biosynthesis. NSC23766 did not increase the levels of the proteolytic products of APP, sAPPalpha, Abeta40, and Abeta42. The proteasome inhibitor lactacystin did not reverse the NSC23766-induced decrease in APP protein levels. NSC23766 did, however, decrease the levels of both APP mRNA and APP protein. Decreased levels of APP mRNA and protein were also observed when HEK293 cells were transfected with an expression vector containing a dominant-negative Rac1 mutant or with siRNA targeting Rac1. By overexpressing progressively deleted fragments of the APP promoter in HEK293 cells, we identified a Rac1 response site at positions -233 to -41 bp in the APP promoter. Taken together, our results suggest that Rac1 regulates transcription of the APP gene in primary hippocampal neurons.