| Popis: |
The regulatory effects of angiotensin-II (AngII) and several growth factors, including insulin-like growth factor 1 (IGF-1), basic fibroblast growth factor (bFGF), and transforming growth factor beta1 (TGFbeta1) on the AngII subtype 2 (AT2) receptor were studied using R3T3 cells, a mouse fibroblast cell line that expresses only AT2 receptors. AngII increased (in a time- and dose-dependent manner) AT2 binding sites but had no effects on AT2 messenger RNA (mRNA) levels. At maximal concentration (10(-7) M) AngII caused a 4-fold increase of AT2 receptor number. In contrast, IGF-1 increased (3- to 4-fold), whereas bFGF and TGFbeta1 decreased (by about 90% and 80%, respectively) AT2 receptor and mRNA levels. Moreover, AngII potentiated the effect of IGF-1 on receptor number, but not on AT2 mRNA levels, and significantly reduced the inhibitory action of bFGF and TGFbeta1 on AT2 binding sites but not on AT2 mRNA levels. None of these factors modified AT2 mRNA half-life. The potential effects of these factors on transcription of the AT2 gene were measured by means of nuclear run-on assays. IGF-1 increased the rate of transcription by about 2.5-fold, whereas bFGF and TGFbeta1 reduced it by 90 and 80%, respectively. In contrast, AngII did not modify either the basal or IGF-1-stimulated transcription rate. Finally, AngII alone or together with IGF-1, but not IGF-1 alone, increased the attachment of AT2 mRNA to polysomal fractions. The present findings demonstrate that the main mechanism by which AngII regulates the AT2 receptor is by increasing the rate of AT2 mRNA translation, whereas the stimulatory (IGF-1) or inhibitory (bFGF and TGFbeta1) effects of these growth factors on AT2 expression are exerted at the transcriptional level. |
