Agonist activity of the 3-hydroxy metabolites of tibolone through the oestrogen receptor in the mouse N20.1 oligodendrocyte cell line and normal human astrocytes.

17beta-oestradiol (E(2)) may have a beneficial impact on the development of age-related diseases, in part through alpha and beta oestrogen receptors (ER) in glia. Tibolone, a synthetic steroid, could influence glial-mediated neuroprotection if agonist oestrogenic activity is demonstrable. We used the N20.1 mouse oligodendrocyte cell line as a glial cell model to evaluate the response of ERalpha and ERbeta through oestrogen-response element (ERE) and AP-1-driven reporters to E(2), 4-hydroxytamoxifen (4OHT) and to two tibolone metabolites, 3alpha-hydroxytibolone (3alpha-OH-Tib) and 3beta-hydroxytibolone (3beta-OH-Tib). In addition, we tested the activity of these same ligands through the endogenous ERalpha in human normal astrocytes. Because endogenous ER was not detected in the N20.1 cells, we tested the ability of exogenous ER to activate transcription in response to ligands (100 nM) using a transient cotransfection assay with an ERalpha expression vector. To test the antagonist activity of 3alpha-OH-Tib and 3beta-OH-Tib, we used them in combination with E(2) (10(-8) M), at concentrations of 10(-7) M and 10(-6) M. The human normal astrocytes were treated similarly, with the exception that no ER-encoding DNA was used. Specific ER ligand mediated activity was shown using the E(2) antagonist ICI 182 780 and the pSG5 empty vector. E(2), 3alpha-OH-Tib, and 3beta-OH-Tib stimulated ERalpha on an ERE-promoter at each concentration (P < 0.001) but not at an AP-1-driven promoter. 4OHT was an effective antagonist, but did not exhibit agonist activity on the ERE-driven promoter. 4OHT was an effective agonist through ERalpha on an AP-1-driven promoter. 3alpha-OH-Tib and 3beta-OH-Tib were not effective antagonists of E(2). Both metabolites acted through the ER because the addition of an E(2) antagonist blocked their activity. These results show that 3alpha-OH-Tib and 3beta-OH-Tib exert agonist activity, yet lack antagonist or additive activity, through the ERalpha and ERbeta on an ERE-driven but not on an AP-1-driven promoter in a glial cell model and in normal human astrocytes. This contrasts with the effects of 4OHT, which exerted little or no agonist activity, but reduced E(2)-stimulated activity through ERalpha on the ERE, in the same cells.