Inhibition of aberrant proliferation and induction of apoptosis in HER-2/neu oncogene transformed human mammary epithelial cells by N-(4-hydroxyphenyl)retinamide

Epithelial cells from non-cancerous mammary tissue in response to exposure to chemical carcinogens or transfection with oncogenes exhibit hyperprolife ration and hyperplasia prior to the development of cancer. Aberrant prolife ration may, therefore, represent a modifiable early occurring preneoplastic event that is susceptible to chemoprevention of carcinogenesis. The synthe tic retinoid N-(4-hydroxyphenyl)retinamide (HPR), has exhibited preventive efficacy in several in vitro and in vivo breast cancer models, and represen ts a promising chemopreventive compound for clinical trials. Clinically rel evant biochemical and cellular mechanisms responsible for the chemopreventi ve effects of HPR, however, are not fully understood. Experiments were perf ormed on preneoplastic human mammary epithelial 184-B5/HER cells derived fr om reduction mammoplasty and initiated for tumorigenic transformation by ov erexpression of HER-2/neu oncogene, to examine whether HPR inhibits aberran t proliferation of these cells and to identify the possible mechanism(s) re sponsible for the inhibitory effects of HPR, Continuous 7-day treatment wit h HPR produced a dose-dependent, reversible growth inhibition, Long-term (2 1 day) treatment of 184-B5/HER cells with HPR inhibited anchorage-dependent colony formation by similar to 80% (P < 0.01) relative to that observed in the solvent control, A 24 h treatment with cytostatic 400 nM HPR produced a 25% increase (P = 0.01) in G(0)/G(1) phase, and a 36% decrease (P = 0.01) in S phase of the cell cycle. HPR treatment also induced a 10-fold increas e (P = 0.02) in the sub-Go (apoptotic) peak that was down-regulated in the presence of the antioxidant N-acetyl-L-cysteine, Treatment with HPR resulte d in a 30% reduction of cellular immunoreactivity to tyrosine kinase, where as immunoreactivity to p185(HER) remained essentially unaltered. HPR exposu re resulted in time-dependent increase in cellular metabolism of the retino id as evidenced by increased formation of the inert metabolite N-(4-methoxy phenyl) retinamide (MPR) and progressive increase in apoptosis, Thus, HPR-i nduced inhibition of aberrant proliferation may be caused, in part, by its ability to inhibit HER-2/neu-mediated proliferative signal transduction, re tard cell cycle progression and upregulate cellular apoptosis.