Characterization of the [125I]endomorphin-2 binding sites in the MCF7 breast cancer cell line

In the present study, the expression of the μ-opioid receptor on protein level has been demonstrated in MCF7 breast cancer cells. Binding of the [125I]-labeled μ-opioid receptor selective ligand endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) was examined in vitro using a cross-linking assay followed by a Western blot technique. The radioactive complex had a molecular weight of about 65 kDa and was detectable by anti-μ-opioid receptor antibody, indicating the presence of μ-opioid receptors in MCF7 cell membranes. Characterization of endomorphin-2 binding to the membranes obtained from MCF7 cells was performed. Cold saturation experiments with [125I]endomorphin-2 showed biphasic binding curves in Scatchard coordinates. One component represents a high affinity and low capacity, and the other low affinity and higher capacity binding sites. The obtained Bmax values for [125I]endomorphin-2 binding to MCF7 membranes were much higher than those obtained for mouse brain. Pharmacological characterization of the [125I]endomorphin-2 binding sites was made using endomorphin-2 and two other μ selective ligands, morphiceptin, and [d-1-Nal3]morphiceptin on MCF7 cell membrane preparations and whole MCF7 cells. In both cases, the rank order of potency was [d-1-Nal3]morphiceptin > endomorphin-2 > morphiceptin, but in case of whole MCF7 cells the IC50 values were about 40 times higher.