The glucocorticoid receptor (GR) is a ligand-dependent transcription factor mediating the genomic effects of glucocorticoids. Two activation functions (AFs) are present in the GR. While the N-terminal AF1 is ligand independent, the C-terminal AF2 overlaps with the ligand-binding domain and is ligand dependent. In this study, we have mapped AF1 in duplicated rainbow trout GRs, called rtGR1 and rtGR2, showing a limited homology (24.5%) in the N-terminal domain. Ablation of this domain from rtGR1 or rtGR2 resulted in a marked decrease (>97%) in maximal hormone-dependent transactivation, but did not affect dexamethasone-binding activity or expression levels. This suggested that, similar to the situation in the human GR (hGR), AF1 is the main AF in the trout GRs. Sequence alignments with hGR suggested a localisation of AF1 to residues 70–230 of rtGR1 and 1–119 of rtGR2. These assignments were generally confirmed in the transactivation experiments with rtGR1- and rtGR2-derived mutants showing partial deletions of their N-terminal domains. In dexamethsone-treated cells (10−7 M, 2 h), the subcellular distribution of rtGR1 and rtGR2 mutants lacking the entire N-terminal domain, as well that of an rtGR1 mutant lacking the most N-terminal 234 amino acids, was similar to that of the corresponding wild-type GRs, suggesting that the disruption of transactivation activity was not caused by impairment of nuclear access of the mutants. Bioinformatic analyses predicted the presence of potential helical segments in the core of AF1 of rtGR1 and rtGR2, and further revealed that AF1 in rtGR1, rtGR2, and hGR shares a motif composed of hydrophobic and acidic amino acids.
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Armin Sturm, James E Bron, Darren M Green, and Nic R Bury
C J Newton, T Trapp, U Pagotto, U Renner, R Buric, and G K Stalla
The GH3 pituitary cell line has been used to investigate the role of the oestrogen receptor (ER) as a modulator of mitogenic signals in tumour cells in the absence of exogenous oestrogen. Using a chemically defined, serum- and oestrogen-free medium, we have demonstrated that the pure steroidal anti-oestrogens ICI 182780 and ICI 164384 are capable of blocking growth by more than 50% after 5 days of culture. Studies with conditioned medium have indicated that the basal growth is due to the secretion of autocrine growth stimulatory substances. Under serum- and oestrogen-free conditions, insulin and IGF-I increased the growth rate of these cells by twofold over a 5-day treatment period, and this effect was also blocked by the anti-oestrogens ICI 182780 and ICI 164384 (50% of maximum inhibition at 0·6 and 6 nM respectively). To explore the potential mechanism by which the ER apparently facilitates the growth factor effects under oestrogen-free conditions, GH3 cells were transiently transfected with a plasmid reporter containing the vitellogenin oestrogen response element (ΔMTV-ERE-LUC). We have shown that as well as oestradiol (OE2), insulin and IGF-I induce luciferase activity by between two- and sevenfold (four experiments), and these effects were completely blocked by ICI 182780. In contrast, growth factors and OE2 were unable to induce luciferase expression when transfections were performed with a plasmid reporter lacking the oestrogen response element. The studies presented here strongly suggest that, in the absence of oestrogen, the ER in these pituitary tumour cells has a role in growth, as peptide factors are able to induce its conversion to a state which is capable of up-regulating the transcription of key growth-promoting genes.