The estrogenic activity of cyanobacterial hepatotoxins microcystin-LR (MC-LR) and nodularin-R (NOD-R) was for the first time investigated in vitro in a stably transfect cell line with an estrogen-regulated luciferase gene. Treatment of cells with NOD-R caused a dose-dependent increase in the luciferase activity. NOD-R gave rise to an induction of luciferase activity with an EC50 value of 66.4 nM. whereas the positive control, 17 beta-estradiol (E-2) had an EC50 of 9.6 pM. This indicates that NOD-R is a 6900-fold weaker inducer of luciferase than E2. In contrast, only a slight but significant activation of the luciferase gene was observed with MC-LR between 2.01 and 60.1 nM, and a maximal-induced response was observed with 10.1 nM, approximately 25% of the maximal effect obtained with 1 nM E2. The decrease in the luciferase activity at high MC-LR concentrations can be explained by a cytotoxic effect. No synergistic estrogenic effect was observed when each toxin was co-administrated with E2. However, the induction of the luciferase activity by NOD-R and MC-LR was inhibited by co-treatment with 1 mu M of the pure estrogenic receptor (ER) antagonist ICI 182.780, thus proving the ER-dependency of the estrogenic effect. (C) 2009 Elsevier B.V. All rights reserved.