Chemical carcinogen-induced rat mammary carcinogenesis is a potential model of p21-activated kinase positive female breast cancer.

The p21-activated kinase 1 (PAK1) gene encodes a serine/threonine kinase that is overexpressed in a subset of human breast carcinomas with poor prognosis. The laboratory rat (Rattus norvegicus) orthologous gene is located at Mammary carcinoma susceptibility 3 (Mcs3) QTL on rat chromosome 1. We used quantitative PCR to determine effects of Mcs3 genotype and 7,12-dimethylbenz(a)anthracene (DMBA) exposure on Pak1 expression. There was no effect of Mcs3 genotype; however, there was a 3.5-fold higher Pak1 level in DMBA-exposed mammary glands (MGs) than in unexposed glands (P < 0.05). Sequence variants in Pak1 exons did not alter amino acid sequence between Mcs3-susceptible and -resistant strains. Protein expression of PAK1/Pak1 in human breast carcinomas and DMBA-exposed rat mammary glands was detected using immunohistochemistry (IHC). Rat mammary glands from 12-wk-old females unexposed to DMBA were negative for Pak1, whereas 24% of carcinogen-exposed mammary glands from age-matched females stained positive for Pak1. The positive mammary glands exposed to carcinogen had no pathological signs of disease. Human breast carcinomas, used as comparative controls, had a 22% positivity rats. This was consistent with other human breast cancer studies of PAK1 expression. Similar frequencies of human/rat PAK1/Pak1 expression in female breast carcinomas and carcinogen-induced rat mammary glands, showing no visible pathogenesis of disease, suggests aberrant PAK1 expression is an early event in development of some breast cancers. Laboratory rats will be a useful experimental organism for comparative studies of Pak1-mediated mechanisms of breast carcinogenesis. Future studies of PAK1 as a diagnostic marker of early breast disease are warranted.