Antiestrogens prevent the stimulatory effects of L-triiodothyronine on cell proliferation.
This paper studies the modulatory effects of two antiestrogens, the steroid ICI 164 384 and the nonsteroidal compound 4-hydroxytamoxifen (4-OH-tam), on the proliferation of L-T3-stimulated cell lines. Three cell lines known to be stimulated by thyroid hormones and to contain estrogen receptors but to have a different estradiol sensitivity to estradiol were used; F4Z2 and MCF-7 cells were stimulated, whereas GH3 cells were insensitive. Cells were counted 6-7 days after hormones and antihormones were added to the culture medium, separately or in association (a fixed hormone concentration and increasing antihormone concentrations and vice versa). In F4Z2 and MCF-7 cells, antiestrogens prevented noncompetitively the stimulatory effect of L-T3 and, as expected, competitively the stimulatory effect of estradiol. In GH3 cells, antiestrogens had proper inhibitory effects, but they did not prevent significantly the proliferative effect of L-T3. To investigate the mechanisms of the modulatory effects in F4Z2 cells we examined the consequences of antiestrogens on thyroid hormone receptors (nuclear binding of L-T3 and mRNAs of thyroid hormone receptors, i.e. c-erbA alpha and -beta) and insulin-like growth factor-I (IGF-I; secretion and mRNAs). Antiestrogens neither competed with L-[125I]T3 nor reproducibly decreased the number and affinity of thyroid hormone-binding sites. While 4-OH-tam frequently decreased the amount of c-erbA beta transcripts, ICI 164 384 did not alter the distribution of alpha and beta cDNA transcripts. Further, neither antiestrogen prevented the increases in IGF-I accumulation in conditioned medium and IGF-I mRNA concentrations induced by L-T3 (0.1 nM). In conclusion, 1) antiestrogens are potent noncompetitive inhibitors of the action of L-T3 on the proliferation of cells whose growth is responsive to estradiol (F4Z2 and MCF-7), but not of the action on a cell line whose growth is insensitive to estradiol (GH3). 2) The loss of L-T3 sensitivity is not due to a loss of thyroid receptors or a decrease in IGF-I production. 3) In addition to estrogen receptors, factors involved in the estradiol control of cell proliferation appear to be required for the antiestrogen inhibition of L-T3 action.