title: The cytoplasmic 60 kDa progesterone receptor isoform predominates in the human amniochorion and placenta at term.
abstract: BACKGROUND
  The mechanism that initiates human parturition has been proposed to be 'functional progesterone withdrawal' whereby the 116 kDa B-isoform of the progesterone receptor (PR-B) switches in favour of the 94 kDa A-isoform (PR-A) in reproductive tissues. Recently, other PR isoforms, PR-S, PR-C and PR-M generated from the same gene have been identified and partially characterised.
  
  
  RESULTS
  Using immunohistochemical, western blotting and RT-PCR techniques, evidence is provided that indicates the major PR isoform present in human term fetal membranes (amnion and chorion) and syncytiotrophoblast of the placenta is neither of the classical nuclear PR-B or PR-A isoforms but is the N-terminally truncated 60 kDa PR-C isoform. Evidence is also provided that this 60 kDa isoform resides in the cytoplasm of the expressing cell types. Data are also presented to show that PR-B, PR-A and PR-S isoforms are essentially absent from the amnion and chorion, whereas PR isoforms A, B, C and S are all present in the decidua, with PR-A being the major isoform. The syncytiotrophoblast of the placenta contains the cytoplasmic 60 kDa isoform, but not isoforms PR-A, PR-B or PR-S.
  
  
  CONCLUSIONS
  The major PR isoform in the amnion, chorion and placenta is a 60 kDa protein that could be PR-C, suggesting that the cytoplasmic isoform has a specific role in extra-embryonic tissues and may be involved in the regulation of human parturition.
authors.affiliation: Preterm Birth Research Group, Reproductive Sciences, Department of Cancer Studies and Molecular Medicine, University of Leicester, Leicester, Leicestershire, LE2 7LX, UK. aht3@le.ac.uk
authors.forename: Anthony H
authors.initials: AH
authors.lastname: Taylor
authors.forename: Penny C
authors.initials: PC
authors.lastname: McParland
authors.forename: David J
authors.initials: DJ
authors.lastname: Taylor
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Bell
mesh_terms: D000650:Amnion / Q000378:metabolism*; D002823:Chorion / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism*; D005260:Female; D006801:Humans; D036801:Parturition / Q000378:metabolism / Q000502:physiology; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D020033:Protein Isoforms / Q000032:analysis / Q000737:chemistry / Q000378:metabolism; D012313:RNA / Q000378:metabolism; D011980:Receptors, Progesterone / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D020033:Protein Isoforms; D011980:Receptors, Progesterone; D012313:RNA