Antitumour effects of
retinoids are attributed to their influence on cell proliferation, differentiation, apoptosis and angiogenesis. In our effort to develop useful agents for
breast cancer therapy, we evaluated the effects of four representative
retinoic acid metabolism blocking agents (RAMBAs,
VN/14-1, VN/50-1, VN/66-1 and VN/69-1) on growth inhibition of oestrogen receptor positive (ER +ve, MCF-7 and T-47D) and oestrogen receptor negative (ER -ve, MDA-MB-231) human
breast cancer cells. Additionally, we investigated the biological effects/molecular mechanism(s) underlying their growth inhibitory properties as well as their antitumour efficacies against MCF-7 and MCF-7Ca tumour xenografts in nude mice. We also assessed the effect of combining
VN/14-1 and
all-trans-retinoic acid (ATRA) on MCF-7 tumour xenografts. The ER +ve cell lines were more sensitive (IC(50) values between 3.0 and 609 nM) to the RAMBAs than the ER -ve MDA-MB-231 cell line (IC(50)=5.6-24.0 microM).
Retinoic acid metabolism blocking agents induced cell differentiation as determined by increased expression of
cytokeratin 8/18 and oestrogen receptor-alpha (ER-alpha). Similar to ATRA, they also induced apoptosis via activation of
caspase 9. Cell cycle analysis indicated that RAMBAs arrested cells in the G1 and G2/M phases and caused significant downregulation (>80%) of
cyclin D1 protein. In vivo, the growth of MCF-7 mammary tumours was dose-dependently and significantly inhibited (92.6%, P<0.0005) by
VN/14-1. The combination of
VN/14-1 and ATRA also inhibited MCF-7 breast tumour growth in vivo (up to 120%) as compared with single agents (P<0.025).
VN/14-1 was also very effective in preventing the formation of MCF-7Ca tumours and it significantly inhibited the growth of established MCF-7Ca tumours, being as effective as the clinically used
aromatase inhibitors,
anastrozole and
letrozole. Decrease in
cyclin D1 and upregulation of cytokeratins, Bad and Bax with
VN/14-1 may be responsible for the efficacy of this compound in inhibiting
breast cancer cell growth in vitro and in vivo. Our results suggest that our RAMBAs, especially
VN/14-1 may be useful novel
therapy for
breast cancer.