Type II [3H]estradiol binding site antagonists: inhibition of normal and malignant prostate cell growth and proliferation

A number of studies from our laboratory and Others have shown that synthetic and naturally occurring bioflavonoids and related compounds have significant antiproliferative activity in the rat uterus and mouse mammary tumor model systems. This cell regulatory activity is attributed to the fact these compounds mimic methyl p-hydroxyphenyllactate (MeHPLA) as ligands for nuclear type II [3H]estradiol binding sites. The rodent prostate is also an estrogen target tissue which contains type II sites (1,2). Therefore, we assessed the effects of 2,6-bis((3-methoxy-4-hydroxyphenyl)-methylene)-cyclohexanone (BMHPC) on normal and malignant prostatic cell growth and proliferation in vitro and in vivo. This cyclovalone is designed to bind to type II sites with high affinity and mimic MeHPLA as a cell growth antagonist. Oral administration of BMHPC (9.5-38.0 mg/kg body weight per day) to intact adult male Balb/c mice for 14 days resulted in a dose dependent reduction (P<0.01) in prostatic weight relative to controls. No significant treatment effects of BMHPC on seminal vesicular, testicular or body weights were observed. BMHPC also competed for [3H]estradiol binding to type II sites in LNCaP and PC-3 human prostatic Cancer cell lines and this ligand inhibited the proliferation of these cells in a dose and time dependent fashion. A direct correlation between type II site occupancy by BMHPC and the inhibition of LNCaP or PC-3 cell proliferation was observed which was reversible (not shown) following removal of BMHPC from the medium. Flow cytometry studies revealed that the type II site antagonist significantly reduced (p<0.01-p<0.001) the numbers of LNCaP and PC-3 cells in G0/G1 and caused an accumulation (p<0.001) of these cells in S-phase and G2/M (p<0.01). These data suggest that BMHPC blocks mitosis. This is consistent with the observed cytostatic activity of BMHPC in a variety of model systems. Oral administration of BMHPC to nude mice bearing subcutaneous PC-3 cell xenografts impeded the growth of these solid tumors in vivo without significant signs of toxicity. These findings demonstrate that BMHPC possesses significant anti-proliferative activity in normal and malignant prostatic tissues and cells, and extend our hypothesis that MeHPLA regulation of cellular proliferation via type II binding site interactions is an important pathway involved in cell growth regulation.