The Neuropeptide Y Y.sub.1 Receptor: A Diagnostic Marker? Expression in MCF-7 Breast Cancer Cells Is Down-Regulated by Antiestrogens In Vitro and in Xenografts

The neuropeptide Y (NPY) Y.sub.1 receptor (Y.sub.1 R) has been suggested as a tumor marker for in vivo imaging and as a therapeutic target. In view of the assumed link between estrogen receptor (ER) and Y.sub.1 R in mammary carcinoma and with respect to the development of new diagnostic tools, we investigated the Y.sub.1 R protein expression in human MCF-7 cell variants differing in ER content and sensitivity against antiestrogens. ER and Y.sub.1 R expression were quantified by radioligand binding using [.sup.3 H]-17[beta]-estradiol and the Y.sub.1 R selective antagonist [.sup.3 H]-UR-MK114, respectively. The latter was used for cellular binding studies and for autoradiography of MCF-7 xenografts. The fluorescent ligands Cy5-pNPY (universal Y.sub.1 R, Y.sub.2 R and Y.sub.5 R agonist) and UR-MK22 (selective Y.sub.1 R antagonist), as well as the selective antagonists BIBP3226 (Y.sub.1 R), BIIE0246 (Y.sub.2 R) and CGP71683 (Y.sub.5 R) were used to identify the NPY receptor subtype(s) by confocal microscopy. Y.sub.1 R functionality was determined by mobilization of intracellular Ca.sup.2+ . Sensitivity of MCF-7 cells against antiestrogen 4-hydroxytamoxifen correlated directly with the ER content. The exclusive expression of Y.sub.1 Rs was confirmed by confocal microscopy. The Y.sub.1 R protein was up-regulated (100%) by 17[beta]-estradiol (EC.sub.50 20 pM) and the predominant role of ER[alpha] was demonstrated by using the ER[alpha]-selective agonist "propylpyrazole triol". 17[beta]-Estradiol-induced over-expression of functional Y.sub.1 R protein was reverted by the antiestrogen fulvestrant (IC.sub.50 5 nM) in vitro. Furthermore, tamoxifen treatment of nude mice resulted in an almost total loss of Y.sub.1 Rs in MCF-7 xenografts. In conclusion, the value of the Y.sub.1 R as a target for therapy and imaging in breast cancer patients may be compromised due to Y.sub.1 R down-regulation induced by hormonal (antiestrogen) treatment.