Toxicological Sciences 56, 389-399 (2000)
Copyright © 2000 by the Society of Toxicology
Cellular and Molecular Mechanisms of Action of Linuron: An Antiandrogenic Herbicide that Produces Reproductive Malformations in Male Rats
* U.S. EPA, NHEERL, Research Triangle Park, North Carolina;
North Carolina State University/U.S. EPA Cooperative Training Program, Raleigh, North Carolina; and
Experimental Pathology Laboratories, Inc., Research Triangle Park, North Carolina
Antiandrogenic chemicals alter sex differentiation by several different mechanisms. Some, like flutamide, procymidone, or vinclozolin compete with androgens for the androgen receptor (AR), inhibit AR-DNA binding, and alter androgen-dependent gene expression in vivo and in vitro. Finasteride and some phthalate esters demasculinize male rats by inhibiting fetal androgen synthesis. Linuron, which is a weak competitive inhibitor of AR binding (reported Ki of 100 µM), alters sexual differentiation in an antiandrogenic manner. However, the pattern of malformations more closely resembles that produced by the phthalate esters than by vinclozolin treatment. The present study was designed to determine if linuron acted as an AR antagonist in vitro and in vivo. In vitro, we (1) confirmed the affinity of linuron for the rat AR, and found (2) that linuron binds human AR (hAR), and (3) acts as an hAR antagonist. Linuron competed with an androgen for rat prostatic AR (EC50 = 100–300 µM) and human AR (hAR) in a COS cell-binding assay (EC50 = 20 µM). Linuron inhibited dihydrotestosterone (DHT)-hAR induced gene expression in CV-1 and MDA-MB-453-KB2 cells (EC50 = 10 µM) at concentrations that were not cytotoxic. In short-term in vivo studies, linuron treatment reduced testosterone- and DHT-dependent tissue weights in the Hershberger assay (oral 100 mg/kg/d for 7 days, using castrate-immature-testosterone propionate-treated male rats; an assay used for decades to screen for AR agonists and antagonists) and altered the expression of androgen-regulated ventral prostate genes (oral 100 mg/kg/d for 4 days). Histological effects of in utero exposure to linuron (100 mg/kg/d, day 14–18) or DBP (500 mg/kg/d, day 14 to postnatal day 3) on the testes and epididymides also are shown here. Taken together, these results support the hypothesis that linuron is an AR antagonist both in vivo and in vitro, but it remains to be determined if linuron alters sexual differentiation by additional mechanisms of action.
Key Words: androgen binding; flutamide; linuron; reproductive malformations; androgen receptor (AR) antagonists.
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