Abstract 1054: Identification of progesterone receptor membrane component-1 as the putative sigma-2 receptor

Abstract

Abstract Sigma binding sites represent a unique class of receptors and are highly expressed in tumor cells. Two sigma binding site subtypes, known as sigma-1 and sigma-2 receptors, were distinguished based on differences in their pharmacological profiles and molecular weights. The sigma-1 receptor gene has been cloned and well characterized in the CNS and cancer biology. However, the sigma-2 receptor has not yet been cloned. The purpose of the current study is to identify and clone sigma-2 receptor. We have synthesized an irreversible sigma-2 receptor selective photoaffinity probe, WC-II-21, containing a fluorescein isothiocyanate (FITC) group. This ligand has high binding affinity for sigma-2 receptor (Ki = 13 nM) and relatively low binding affinity for sigma-1 receptor (Ki > 1,000 nM). This ligand irreversibly labeled sigma-2 receptors in the rat liver membrane fractions after UV irradiation. The ligand-labeled proteins were solubilized, enriched and then separated through sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The position of a predominant band of the ligand-labeled protein in the gel was determined by western blot analysis using FITC antibody. MALDI-mass spectrometry analysis for the ligand-labeled proteins indentified the protein progesterone receptor membrane component-1 (PGRMC-1) with a convincingly high score of 81. Based on the similarities of the reported biological characteristics and functions between PGRMC1 and sigma-2 receptor, PGRMC1 was investigated in the current study. Immunocytochemistry revealed that PGRMC1 colocalized with molecular markers of endoplasmic reticulum and mitochondria in human melanoma cell line MDA-MB435. The subcellular localization of sigma-2 receptor also colocalized with fluorescent markers of endoplasmic reticulum and mitochondria as reported previously from our group (Cancer Res 2007; 67, 6708-6716). Thus, PGRMC1 and sigma-2 receptor appear to localize in the same subcellular organelles in the cell. PGRMC1 was knocked down with PGRMC1 siRNA in MDA-MB435 cells. Sigma-2 radioligand binding activity was significantly reduced in the PGRMC1 knocked down cells relative to the nontargeting siRNA transfected cells. These data suggest that PGRMC1 is the putative sigma-2 receptor. Further validation of PGRMC1 as the sigma-2 receptor is ongoing in our group. (Supported by CA 102869). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1054.