p21-activated kinase-1 signaling mediates cyclin D1 expression in mammary epithelial and cancer cells.

p21-activated kinase 1 (Pak1) has been shown recently to induce hyperplasia in the mammary epithelium, a phenotype also manifested by overexpression of cyclin D1, a known indicator of the proliferative stage. Here we investigated the role of the Pak1 pathway in the expression of cyclin D1 using tissue culture models and transgenic mice expressing activated Pak1 in mammary glands. We found that hyperplastic mammary glands from catalytically active Pak1 transgenic mice exhibit a 5- to 7-fold increased expression of cyclin D1 as compared with stage-matched wild-type mice. In addition, Pak1 levels were elevated in human breast tumors and also correlated well with increased cyclin D1 expression. Increased expression of Pak1 in breast cancer cells stimulated cyclin D1 promoter activity, elevated levels of cyclin D1 mRNA, protein, and nuclear accumulation of cyclin D1. Conversely, Pak1 inhibition by an auto-inhibitory peptide (amino acids 83-149) or Pak1 knockdown by short interference RNA markedly reduced the expression of cyclin D1, suggesting a requirement of a functional Pak1 pathway for optimal expression of cyclin D1. Results from deletion and mutant analysis indicate that Pak1 regulates cyclin D1 transcription by means of an NF-kappaB-dependent pathway. Together, these findings suggest a model wherein Pak1 regulation of cyclin D1 expression might involve an NF-kappaB-dependent pathway and that hyperplasia in the mammary glands of Pak1-TG mice may be associated, at least in part, with the up-regulation of cyclin D1, and that Pak1 is up-regulated in human breast tumors.