In order to avoid the complex dual effects of estrogen and antiestrogen, the attempt was made to establish the tumor lines in which estrogens show either stimulatory or inhibitory property in terms of the tumor growth. The administration of estrogen to host mice bearing one of the mouse Leydig cell tumor lines, called T 124958-R, resulted in marked enhancement of the tumor growth even at pharmacological levels of estrogen. On the other hand, estrogenization of host mice almost completely inhibited the growth of the other tumor line (T 22137) without detectable stimulatory effects. The physicochemical properties of the cytosol estrogen receptor in both sublines were found to be similar in relation to the affinity toward ligands, steroid specificity, sedimentation profile, and the dissociation rate kinetics.

Using these tumor lines, the action mechanism of tamoxifen on the tumor growth was examined. Daily administration of this compound (30 µg/mouse) led to enhanced tumor growth in T 124958-R, while the growth of T 22137 was inhibited by the same procedure. In the combination experiments, tamoxifen was found to be unable to antagonize estrogen-induced enhancement or inhibition of the growth in these tumors. In addition, both tumors contained similar levels of the antiestrogen binding sites. These results suggest that tamoxifen modulated the tumor growth through its estrogenic potency.

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This work was partly supported by grants-in-aid from the Ministry of Education, Tokyo, Japan; the Cancer Research Foundation; and the Hirai Cancer Research Fund.

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