Specific binding of benzodiazepines to human breast cancer cell lines

Binding of [H-3]Ro5-4864, a peripheral benzodiazepine receptor (PBR) agonis t, to BT-20 human, estrogen- (ER) and progesterone- (PR) receptor negative breast cancer cells was characterized. It was found to be specific, dose-de pendent and saturable with a single population of binding sites. Dissociati on constant (K-D) was 8.5 nM, maximal binding capacity (B-max) 339 fM/10(6) cells. Ro5-4864 (IC50 17.3 nM) and PK 11195 (IC50 12.3 nM) were able to co mpete with [H-3]Ro5-4864 for binding, indicating specificity of interaction with PER. Diazepam was able to displace [H-3]Ro5-4864 from binding only at high concentrations (>1 mu M), while ODN did not compete for PER binding. Thymidine-uptake assay showed a biphasic response of cell proliferation. Wh ile low concentrations (100 nM) of Ro5-4864, PK 11195 and diazepam increase d cell growth by 10 to 20%, higher concentrations (10 -100 mu M) significan tly inhibited cell proliferation. PK 11195, a potent PER ligand, was able t o attenuate growth of BT-20 cells stimulated by 100 nM Ro5-4864 and to reve rse growth reduction caused by 1 and 10 mu M Ro5-4864, but not by 50 mu M a nd 100 mu M. This indicates that the antimitotic activity of higher concent rations of Ro5-4864 is independent of PER binding. It is suggested, that PE R are involved in growth regulation of certain human breast cancer cell lin es, possibly by supplying proliferating cells with energy, as their endogen ous ligand is a polypeptide transporting Acyl-CoA.