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Predictability of estrogenic mixture effects

The in vitro Estrogen Responsive Chemical Activated Luciferase Gene eXpression (ER-CALUX) reporter gene assay was used in this study to investigate combination effects. The ER-CALUX assay is a recombinant reporter gene assay which measures transactivation of the estrogen receptor following exposure to (xeno-) estrogens in T47D human breast cancer cells stably transfected with an estrogen-regulated luciferase reporter gene construct. In this study, 13 (xeno-) estrogenic compounds, including estradiol, estrone, ethinylestradiol, estriol, mestranol, diethylstilbestrol, bisphenol A, nonylphenol, octylphenol, resorcinol monobenzoate, benzophenone 3, 2,4-dihydroxybenzophenone, and, 4,4'-dihydroxybenzophenone, were tested. Compounds were first tested individually to examine dose-response relationships and determine NOEC, EC1 and EC50 levels. When tested together in ratios based on these levels, mixture effects could be well predicted by the concept of concentration addition, independent of the actual mixture ratio and the effect level under observation. The ratio between observed and predicted EC50 values never exceeded a factor of 1.2. In fact, significant mixture effects could be found even if every compound was present at concentrations that would provoke no significant effect if applied singly, e.g. individual EC1s or NOECs or fractions of these values. In this study, the ER-CALUX assay proved to be very sensitive and gave highly reproducible results, even for the multi-component mixtures.

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