The effects of trans-4-hydroxytamoxifen (OHTam) on proliferation of cells of the Ishikawa human endometrial adenocarcinoma line were studied under serum-free, phenol red-free conditions and compared to those of estradiol. The addition of OHTam (1 µm) to basal medium (BM), consisting of equal parts of Dulbecco's modified Eagle's medium and Ham's F-12 with additional glutamine and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, resulted in significant increases in cell numbers relative to controls. These effects were even greater than those obtained with estradiol (10 nm- 1 µm) or 1% charcoal-treated fetal bovine serum (ctFBS). Addition of 1% ctFBS to BM containing 1 µm OHTam further increased cell numbers whereas addition of estradiol (10 nm) did not do so. The stimulation of growth was positively correlated with OHTam concentrations in the range of 10 nm to 1 µm. Dissociation of estradiol and OHTam proliferative effects was observed in a variant of Ishikawa cells in which estradiol did not increase proliferation while OHTam had a strong stimulatory effect.

The growth-promoting effects of OHTam were also observed in BM containing 5% or 15% ctFBS. In contrast, in parallel experiments in which BM was replaced by minimal essential medium (Eagle's) with Earle's salts, OHTam (1 µm) did not stimulate proliferation under these conditions and acted as an antiestrogen, inhibiting the proliferative effects of estradiol. These results illustrate marked effects of medium composition on proliferation and antiestrogenic actions of OHTam.

Alkaline phosphatase activity was strongly stimulated by estradiol (10 nm) but only very weakly affected by OHTam (1 µm); at these concentrations, OHTam inhibited the effect of estradiol, both in serum-free BM and in minimal essential medium plus 15% ctFBS, demonstrating dissociation in its actions on proliferation and on enzymatic activity.

These findings suggest that OHTam may stimulate the proliferation of particular clones of endometrial cancer cells in human tumors. They also suggest that OHTam can exert effects not mediated by the estrogen receptor system, or form OHTam-estrogen receptor agonistic complexes unlike those resulting from estradiol-estrogen receptor interactions. Clearly, Ishikawa cells provide a useful model to investigate mechanisms of action of antiestrogens.

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Supported by Grant CA-15684, awarded by the National Cancer Institute, Department of Health and Human Services.

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