MODULATION OF ANGIOTENSIN-II RECEPTOR (AT(2)) MESSENGER-RNA LEVELS INR3T3 CELLS

R3T3 cells, a mouse fibroblast cell line, express the type 2 angiotens in II receptor (AT(2)), but not the AT(1) subtype. We previously repor ted that expression of AT(2) sites in these cells were regulated by va rious conditions: 1. The number of AT(2) sites increased considerably when cells were contact-inhibited; 2. Stimulation of R3T3 cells with v arious mitogens caused a rapid decline of AT(2) binding sites; and 3. Stimulation of cells with angiotensin ligands resulted in upregulation of the AT(2) sites. In this study, to determine if altered AT(2) expr ession is under transcriptional, posttranscriptional, or translational control, we examined the level of AT(2) mRNA in R3T3 cells in respons e to various treatments. There was a 200-fold increase in AT(2) mRNA l evels in quiescent cells as compared to growing cells. Results from nu clear run-on assays suggested that the differences in AT(2) mRNA level s were primarily caused by changes in the rate of AT(2) gene transcrip tion. Stimulation of cells with fibroblast growth factor caused an app roximate threefold reduction of AT(2) mRNA levels, and also increased the rate of degradation of AT(2) mRNA, which correlated with the decre ase in AT(2) binding activity seen under these conditions. However, wh ereas treatment with angiotensin ligands increased AT(2) binding activ ity, the level of AT(2) transcripts did not increase. This pattern of expression implies that regulation of AT(2) receptors occurs at multip le levels, involving translational and/or posttranslational as well as transcriptional control, and further affords the cell the ability to rapidly modulate the number of AT(2) binding sites in response to chan ging extracellular conditions.