Interactions of tamoxifen in the chicken

The triphenylethylene antiestrogens are very potent antagonists of estrogen action in the chicken and manifest little agonist activity compared to their action in other species. The estrogen antagonism is most probably mediated by the estrogen receptor, to which tamoxifen binds with a K_i of 2.6 nM. Tamoxifen is readily metabolized by liver to 4-hydroxytamoxifen, which binds the liver nuclear estrogen receptor with a K_i, of 0.1 nM. The K_d of the receptor is 0.7 nM. Estrogen receptor concentrations in liver from immature chickens are relatively low both in nuclear and cytosol fractions. Treatment with estradiol results in 10-fold up-regulation of the nuclear levels to give a total receptor concentration of about 2 pmol/g tissue. Tamoxifen can promote this up-regulation to a limited extent, but interpretation of experimental results is compromised by difficulties with exchange assays in the face of the very high binding affinity of 4-hydroxytamoxifen. Tamoxifen also binds with high affinity (K_d 2-4 nM) and distinctive specificity to antiestrogen binding sites (AEBS) present in a wide variety of chicken tissues and in the highest concentration in the liver (800 pmol/g tissue). Liver and serum contain ether-soluble components which can compete for binding of [³H]tamoxifen to the AEBS. The serum AEBS inhibitory activity is chromatographically heterogeneous and is associated with a sterol-like fraction as well as with a fatty-acid-containing fraction. Tamoxifen treatment of cockerels results in dose- and time-dependent decreases in serum free and esterified cholesterol, and in phospholipids and triglycerides. These changes may reflect estrogenreceptor-independent interactions of tamoxifen.