Activation of NF-κB/Rel occurs early during neoplastic transformation of mammary cells

NF-κB/Rel is a family of transcription factors which are expressed in all cells; however, in most non-B cells, they are sequestered in the cytoplasm in inactive complexes with specific inhibitory proteins, termed IκBs. We have recently shown that NF-κB/Rel factors are aberrantly activated in human breast cancer and rodent mammary tumors, and function to promote tumor cell survival and proliferation. Here, we have examined the time-course of induction of NF-κB/Rel factors upon carcinogen treatment of female Sprague-Dawley (S-D) rats in vivo and in human mammary epithelial cells (HMECs) in culture. We observed that NF-κB/Rel activation is an early event, occurring prior to malignant transformation. In S-D rats, increased NF-κB/Rel binding was detected in nuclear extracts of mammary glands from 40% of animals 3 weeks post-treatment with 15 mg/kg 7,12-dimethylbenz[a]anthracene (DMBA); this is prior to formation of tumors which normally begin to be detected after 7-9 weeks. In non-tumorigenic MCF-10F cells, in vitro malignant transformation upon treatment with either DMBA or benzo[a]pyrene (B[a]P) resulted in a 4- to 12-fold increase in activity of classical NF-κB (p65/p50). NF-κB induction was corrrelated with a decrease in the stability of the NF-κB-specific inhibitory protein IκB-α. Ectopic expression of the transactivating p65 subunit of NF-κB in MCF-10F cells induced the c-myc oncogene promoter, which is driven by two NF-κB elements, and endogenous c-Myc levels. Furthermore, reduction mammoplasty-derived HMECs, immortalized following B[a]P exposure, showed dysregulated induction of classical NF-κB prior to malignant transformation. Together these findings suggest that activation of NF-κB plays an early, critical role in the carcinogen-driven transformation of mammary glands.