The thyroid hormone and retinoid X receptors form a heterodimer with each other and mediate thyroid hormone (T3)-dependent transcription. Retinoid X receptor, in addition, forms a homodimer and mediates 9-cis-retinoic acid-dependent transcription. Here, recombinant thyroid hormone receptor and recombinant retinoid X receptor beta expressed from baculovirus vectors have been studied for ligand-mediated activation of transcription in vitro. We show that the two recombinant receptors, most likely as a heterodimer, cooperatively enhance transcription in vitro from a template containing functional T3 responsive elements. The enhancement was specific for the T3 responsive element and was greatest when T3 was added to the reaction (approximately 14-fold increase). Albeit to a lesser degree, the two receptors also directed transcription in the absence of T3. Template competition experiments suggest that the two receptors enhance formation of the preinitiation complex and that activation by T3 occurs when the ligand binds the receptor prior to (or during), but not after, the formation of the preinitiation complex. Although 9-cis-retinoic acid had no effect on the T3-dependent transcription, this ligand activated transcription in vitro directed by recombinant retinoic X receptor beta, most likely as a homodimer. This activation was observed when using nuclear extracts from embryonal carcinoma cells as a source of basal transcription factors, but not those from B lymphocytes. These results demonstrate that transcriptional activation mediated by T3 and 9-cis-retinoic acid can be reconstituted in vitro with the respective recombinant receptors.
[1]
B. O’Malley,et al.
Mechanism of estrogen receptor-dependent transcription in a cell-free system
,
1990,
Molecular and cellular biology.
[2]
B. O’Malley,et al.
Progesterone enhances target gene transcription by receptor free of heat shock proteins hsp90, hsp56, and hsp70
,
1991,
Molecular and cellular biology.
[3]
H. Samuels,et al.
The NF-kappa B and Sp1 motifs of the human immunodeficiency virus type 1 long terminal repeat function as novel thyroid hormone response elements
,
1993,
Molecular and cellular biology.
[4]
K. Umesono,et al.
Convergence of 9-cis retinoic acid and peroxisome proliferator signalling pathways through heterodimer formation of their receptors
,
1992,
Nature.
[5]
K. Dahlman-Wright,et al.
Direct interaction of the tau 1 transactivation domain of the human glucocorticoid receptor with the basal transcriptional machinery
,
1993,
Molecular and cellular biology.
[6]
B. Raaka,et al.
The conserved ninth C-terminal heptad in thyroid hormone and retinoic acid receptors mediates diverse responses by affecting heterodimer but not homodimer formation
,
1993,
Molecular and cellular biology.