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Characterization and Localization of Lipocortin 1-Binding Sites on Rat Anterior Pituitary Cells by Fluorescence-Activated Cell Analysis/Sorting and Electron Microscopy¹

Department of Neuroendocrinology, Division of Neuroscience and Psychological Medicine, Imperial College School of Medicine, Charing Cross Hospital, London, United Kingdom W6 8RF; the Department of Biochemical Pharmacology, The William Harvey Research Institute, St. Bartholomew’s and the Royal London School of Medicine and Dentistry at Queen Mary and Westfield College (R.J.F.), London, United Kingdom EC1M 6BQ; and the Department of Human Anatomy, University of Oxford (J.F.M.), Oxford, United Kingdom OX1 3QX

Address all correspondence and requests for reprints to: Prof. Julia Buckingham, Department of Neuroendocrinology, Division of Neuroscience and Psychological Medicine, Imperial College School of Medicine, Charing Cross Hospital, Fulham Palace Road, London, United Kingdom W6 8RF. E-mail: j.buckingham{at}cxwms.ac.uk

Lipocortin 1 (LC1) is an important mediator of glucocorticoid action in the anterior pituitary gland, where it appears to act via cell surface binding sites to suppress peptide release. We have exploited a combination of fluorescence-activated cell (FAC) analysis/sorting and electron microscopy to detect, characterize, and localize LC1-binding sites on the surface of dispersed rat anterior pituitary cells, using human recombinant LC1 (hu-r-LC1) as a probe. High affinity (K_d = 14 ± 3 nM) hu-r-LC1-binding sites were detected on approximately 80% of anterior pituitary cells dispersed with collagenase. The binding characteristics of the ligand resembled those observed in leukocytes, in that it was saturable; concentration, Ca²⁺, and temperature dependent; and abolished by trypsin. Functional studies demonstrated an excellent correlation between the presence of the cell surface binding protein and the capacity of an anti-LC1 monoclonal antibody to abrogate the inhibitory actions of dexamethasone (10 nM) on the release of ACTH initiated in vitro by CRH-41 (1 nM). Morphological analysis of cells harvested by FAC sorting showed that 1) somatotrophs, corticotrophs, lactotrophs, thyrotrophs, and gonadotrophs were all included in the population expressing LC1 binding sites; and 2) the LC1-binding sites assume a punctate distribution across the cell surface. These data show that anterior pituitary cells express high affinity surface LC1-binding protein(s); they thus provide further evidence for a specific membrane mechanism of action of LC1 in regulating the endocrine function of the anterior pituitary.

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