Up-regulation of the estrogen receptor by triiodothyronine in rat pituitary cell lines.

The effects of thyroid hormones on estrogen receptor (ER) levels in four different rat pituitary clonal cell lines designated as MtT/Se, SM, S and E, were investigated. When T3 was added at 10(-7) M, a significant increase in ER was evident after 9 h with maximum levels reached after 24-48 h in MtT/Se (270% of control), MtT/SM (160%) and MtT/S (140%). No significant increase was noted in MtT/E cells in which the T3 receptor (T3R) level was only one-tenth of that in the other cells, suggesting a direct link between ER expression and T3R binding. ER levels began to increase at 10(-10) M and reached maxima at 10(-8) M in MtT/Se, SM and S cells. Up to 10(-5) M of retinoic acid (RA) did not change ER levels in any of the cell lines. When Se cells were treated with cycloheximide before T3 administration, the increase in ER was completely blocked. Northern blot analysis of total RNA isolated from T3-treated MtT/Se cells revealed a limited 1.4-fold increase in ER mRNA at 10(-7) M. In conclusion, thyroid hormones (but not RA) increase ER levels in pituitary cells by a process requiring protein synthesis, and which is accompanied by an increase in the mRNA.