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DOI: 10.1055/s-2008-1046798
© Georg Thieme Verlag KG Stuttgart · New York
Levonorgestrel Antagonism on Estrogen-induced Pituitary Tumors Is Mediated by Progesterone Receptors
Publication History
received 07.05.2007
accepted 20.09.2007
Publication Date:
05 March 2008 (online)
Abstract
Using both in vitro and in vivo approaches, we studied the antagonism exerted by the synthetic progestin levonorgestrel on estrogen-induced prolactinomas, considering that levonorgestrel shows partial androgenic properties and that androgens inhibit estrogen-induced prolactin synthesis and secretion. In the tumors, binding of estrogens to their receptors was competed neither by progesterone receptor ligands nor by androgen receptor ligands, ruling out direct inhibitory effects of these drugs on tumor development. Progestin binding was competed by the progesterone receptor agonists progesterone and levonorgestrel, by the antagonist mifepristone, and also by the androgen dihydrotestosterone, whereas the androgen receptor antagonist hydroxyflutamide was a weak competitor. In addition, both progesterone receptor and androgen receptor ligands competed for binding to androgen receptors. In primary cultures of pituitary tumors, levonorgestrel decreased prolactin secretion, an effect that was blocked by mifepristone but not by hydroxyflutamide. In vivo results indicated that levonorgestrel inhibition of both estrogen-induced pituitary weight increment and hyperprolactinemia was reduced by mifepristone, whereas flutamide was unable to block levonorgestrel effects. Our results suggest that even when an interaction of levonorgestrel with androgen receptors in the tumors is possible, the antagonistic effects of levonorgestrel on tumor development and functionality are mediated by progesterone receptors.
Key words
prolactin - mifepristone - RU486 - flutamide - progestin
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Correspondence
G.G. Piroli
Department of Pharmacology
Physiology, and Neuroscience
University of South Carolina School of Medicine
6439 Garners Ferry Rd, Bldg 1, Rm D4
Columbia
South Carolina 29209
USA
Phone: +1/803/733 32 19
Fax: +1/803/733 15 23
Email: gpiroli@gw.med.sc.edu