Nuclear type II [3H]estradiol binding site ligands: inhibition of ER-positive and ER-negative cell proliferation and c-Myc and cyclin D1 gene expression

These studies assessed the effects of 3,4-dihydroxybenzalacetone (ZN-1) and 1-(3,4-dihydroxyphenyl)-2-propanol (ZN-2) on MCF-7 cell proliferation. The compounds blocked [3H]estradiol binding to nuclear type II sites, but did not compete for [3H]estradiol binding to recombinant ERα or ERβ. ZN-1 and ZN-2 inhibited the proliferation of ERα and ERβ positive (MCF-7) and negative (MCF-10A) breast cells, further ruling out direct binding to ER in the mechanism of action of these compounds. Pre-loading type II sites with ZN-1 or ZN-2 reduced [3H]estradiol exchange, strongly suggesting the drugs were binding covalently. ZN-1 treatment resulted in complete occupancy of type II sites and sustained (9 days) inhibition of MCF-7 cell proliferation following its removal from the tissue culture medium. This cell growth inhibition was not due to non-specific toxicity, as the numbers of viable, attached cells per dish (determined by trypan blue dye exclusion) remained constant throughout this 9-day period and eventually reversed by day 19. ZN-2 effects on cell proliferation reversed more rapidly following discontinuation of treatment, a response consistent with the inability of the compound to totally block type II binding. Both ZN-1 and ZN-2 blocked estradiol stimulation of c-Myc and cyclin D1 gene expression in MCF-7 cells, two events that are clearly coupled to cell cycle progression. We suspect this may occur through ZN-1 or ZN-2 modification of nucleosome function and/or chromatin remodeling since nuclear type II sites are localized to a complex of histones H3 and H4 (Shoulars et. al, J Steroid Biochem. Mol. Biol. 96: 19–30, 2005).