title: Receptorphin: a conserved peptide derived from the sequence of the opioid receptor, with opioid displacement activity and potent antiproliferative actions in tumor cells.
abstract: BACKGROUND
  In addition to endogenous opioids, a number of peptide sequences, derived from endogenous (hemorphins, alphaS1-casomorphin), and exogenous proteins (casomorphins, exorphins) have been reported, possessing opioid activity. In the present work, we report the identification of a new peptide, receptorphin (Tyr-Ile-Phe-Asn-Leu), derived from the sequence of the second transmembrane loop of the opioid receptor. This sequence is unique for the opioid receptor, and conserved in all species and receptor-types.
  
  
  CONCLUSIONS
  Receptorphin competes for opioid binding, presenting a kappa-receptor interaction, while it binds equally to delta- and mu- opioid and somatostatin-binding sites, and inhibits the cell proliferation of a number of human cancer cell lines, in a dose-dependent and reversible manner, at the picomolar or the nanomolar range. Receptorphin shows a preferential action on prostate cancer cells.
  
  
  CONCLUSIONS
  Our work identifies, for the first time a peptide, in a receptor sequence, possessing ligand-agonistic activities. A hypothesis, based on receptorphin liberation after cell death, is presented, which could tentatively explain the time-lag observed during opioid antiproliferative action.
authors.affiliation: Laboratory of Experimental Endocrinology, University of Crete, School of Medicine, PO Box 1393, Heraklion, GR-71110, Greece. kampa@med.uoc.gr
authors.forename: M
authors.initials: M
authors.lastname: Kampa
authors.forename: S
authors.initials: S
authors.lastname: Loukas
authors.forename: A
authors.initials: A
authors.lastname: Tsapis
authors.forename: E
authors.initials: E
authors.lastname: Castanas
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D002455:Cell Division / Q000187:drug effects; D006801:Humans; D009842:Oligopeptides / Q000494:pharmacology*; D010446:Peptide Fragments / Q000494:pharmacology; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism; D011957:Receptors, Opioid / Q000737:chemistry*; D017465:Receptors, Opioid, delta / Q000187:drug effects / Q000378:metabolism; D017473:Receptors, Opioid, kappa / Q000187:drug effects / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000187:drug effects / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D009842:Oligopeptides; D010446:Peptide Fragments; D011956:Receptors, Cell Surface; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; C480068:receptorphin