Residues in the ligand binding domain that confer progestin or glucocorticoid specificity and modulate the receptor transactivation capacity.

To localize regions conferring ligand binding specificity of the human glucocorticoid (hGR) and progesterone (hPR) receptors, we constructed chimeras comprising the DNA-binding domain of the yeast transcription factor GAL4, linked to the ligand binding domain of hGR or hPR. Replacement of a sequence of hGR encompassing helices H6 and H7 with the homologous sequence from hPR creates a chimeric protein GP3, which binds the progestin RU 27987 with high affinity, and results in a concomitant loss of glucocorticoid binding [dexamethasone (DEX), RU 43044]. Moreover, GP3 is not able to mediate RU 27987-induced transactivation. A detailed mutational analysis of this sequence and the study of the recently solved hPR crystal structure revealed five residues that confer progestin responsiveness to GR or modulate ligand binding and transcriptional activation. Notably, the simultaneous presence of residues Ser637 and Phe639 on GP3, lining the ligand binding pocket, is specifically involved in RU 27987 binding. The absence of residues Asp641, Gln642, and Leu647 on GP3 is accountable for the lack of glucocorticoids binding on GP3. Unlike residue 642, residues 641 and 647 are not in direct contact with the ligand and most likely act through steric-mediated interactions. The presence of Gln642 and Leu647 are determinant for transcriptional activation in response to DEX and RU 27987, respectively. DEX-dependent transactivation is further enhanced by the presence of Leu647.

Conversion of big-endothelin-1 elicits an endothelin ETA receptor-mediated response in endothelial cells.

Functional conversion of big-endothelin-1 to endothelin-1 and characterization of endothelin receptor subtype were investigated in cultured rat aortic endothelial cells. Exogenous endothelin-1 and big-endothelin-1 both increased arachidonic acid release and inositol phosphate production dose dependently. Endothelin-1 was more potent than big-endothelin-1 as indicated by EC50 values: 0.5 +/- 0.1 nM and 10.0 +/- 2.0 nM for endothelin-1-induced arachidonic acid release and inositol phosphate formation, respectively, versus 1.0 +/- 0.4 nM and 35.0 +/- 6.0 nM for big-endothelin-1-induced responses. Big-endothelin-1, but not endothelin-1 actions were inhibited by phosphoramidon. Comparative studies of endothelin receptor agonists and antagonists showed that endothelin-3 but not sarafotoxin S6c stimulated arachidonic acid release and inositol phosphate formation. The responses to big-endothelin-1 and endothelin-1 were specifically inhibited by the selective endothelin ETA receptor antagonist, [cyclo-D-Trp-D-Asp-Pro-D-Val-Leu] (BQ-123) but not by the selective endothelin ETB receptor antagonist [N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma- methyl-Leu-D-Trp-(COMe)-D-NLeu-ONa] (BQ-788). [125I]Endothelin-1 binding was inhibited by endothelin-1, endothelin-3 and BQ-123 but not by BQ-788. These results indicate that the pharmacological responses to big-endothelin-1 in aortic endothelial cells are due to the extracellular phosphoramidon-sensitive conversion to endothelin-1. Endothelin effects are mediated through endothelin ETA receptors in these cells.

HR 720, a novel angiotensin receptor antagonist inhibits the angiotensin II-induced trophic effects, fibronectin release and fibronectin-EIIIA+ expression in rat aortic vascular smooth muscle cells in vitro.

The aim of this study was to evaluate the direct trophic effects of angiotensin II (AII) on rat vascular smooth muscle cells obtained from a single cellular isolate. Cell volume, protein synthesis, fibronectin (FN) release and FN-EIIIA+ mRNA isoform expression were analyzed in parallel. The effects of HR 720, a novel AT1 angiotensin receptor antagonist with some AT2 receptor affinity, were compared with those of selective AT1 antagonist EXP 3174. Both HR 720 and EXP 3174 inhibited in a concentration-dependent manner the maximum increase in cell volume induced by 10(-9) M Sar1-All (IC50 = 0.49 x 10(-9) M and 0.79 x 10(-9) M, respectively). Maximum [3H]leucine incorporation was also achieved at 10(-9) M All. HR 720 blocked the increase in protein synthesis with potency similar to EXP 3174; the respective IC50 values were 1.04 x 10(-9) M and 1.36 x 10(-9) M. All dose-dependently increased FN release, which was also equally inhibited by about 50% with both compounds at 10(-6) M. Furthermore, All enhanced FN-EIIIA+ mRNA in rat vascular smooth muscle cells (VSMC), which indicated a modulation of FN isoform expression which was inhibited by angiotensin II antagonists. In conclusion, All induced parallel and concentration-dependent increases in cell volume, protein synthesis, FN release and FN-EIIIA+ mRNA expression in vascular smooth muscle cells. These effects appeared to be essentially mediated by AT1 receptor stimulation as indicated by the equal inhibitory effects of HR 720 and EXP 3174.