Control of cell growth. III. Direct mitogenic effect of thyroid hormones on an estrogen-dependent rat pituitary tumor cell line.

A possible direct estrogen requirement for growth of GH3/C14 rat pituitary tumor cells was evaluated in culture medium supplemented with estrogen-depleted serum prepared by a 56 degree C charcoal extraction procedure, and with serum obtained from ovariectomized sheep and ovariectomized adrenalectomized sheep. Growth of the GH3/C14 cells in culture medium in which the final estrogen concentration was 2 pg/ml or less was equal to growth in medium with normal serum and equal to growth in the presence of estrogen-depleted serum to which estradiol was added back at concentrations of 10-1,000 pg/ml. Under no conditions could a direct estrogen requirement for growth be demonstrated. The function of thyroid hormones in cell growth was examined in culture medium supplemented with serum from thyroidectomized sheep. In such medium the growth of the GH3/C14 cells was stimulated 3.5-fold by addition of 1.0 X 10(-8) M L-thyroxine (T4) or 1.0 X 10(-9) m L-triiodothyronine (T3). Investigation of the possible synergistic effects of estrogens and T4 revealed that combinations of estrogen and T4 or T3 did not stimulate growth over that seen with T4 or T3 alone. These data indicated that estrogens are not direct growth requirements for these cells but instead operate in vivo through secondary or indirect mechanisms; in contrast, thyroid hormones are directly mitogenic in vitro.