Transcriptional and translational regulation of angiotensin II type 2 receptor by angiotensin II and growth factors

The regulatory effects of angiotensin-II (AngII) and several growth factors , including insulin-like growth factor 1 (IGF-1), basic fibroblast growth f actor (bFGF), and transforming growth factor beta 1 (TGF beta 1) on the Ang II subtype 2 (AT(2)) receptor were studied using R3T3 cells, a mouse fibrob last cell line that expresses only AT(2) receptors. AngII increased (in a t ime- and dose-dependent manner) AT(2) binding sites but had no effects on A T(2) messenger RNA (mRNA) levels. At maximal concentration (10(-7) M) AngII caused a 4-fold increase of AT(2) receptor number. In contrast, IGF-1 incr eased (3- to 4-fold), whereas bFGF and TGF beta 1 decreased (by about 90% a nd 80%, respectively) AT(2) receptor and mRNA levels. Moreover, AngII poten tiated the effect of IGF-1 on receptor number, but not on AT(2) mRNA levels , and significantly reduced the inhibitory action of bFGF and TGF beta 1 on AT(2) binding sites but not on AT(2) mRNA levels. None of these factors mo dified AT(2) mRNA half-life. The potential effects of these factors on tran scription of the AT(2) 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 TGF beta 1 reduced it by 90 and 80%, respectively. In contrast, AngII did not modify either the basal or IGF-1-stimulated transcription rate. Fin ally, AngII alone or together with IGF-1, but not IGF-1 alone, increased th e attachment of AT(2) mRNA to polysomal fractions. The present findings dem onstrate that the main mechanism by which AngII regulates the AT(2) recepto r is by increasing the rate of AT(2) mRNA translation, whereas the stimulat ory (IGF-1) or inhibitory (bFGF and TGF beta 1) effects of these growth fac tors on AT(2) expression are exerted at the transcriptional level.