ENHANCED ANGIOTENSIN RECEPTOR-TYPE-1 MESSENGER-RNA DEGRADATION AND INDUCTION OF POLYRIBOSOMAL MESSENGER-RNA BINDING-PROTEINS BY ANGIOTENSIN-II IN VASCULAR SMOOTH-MUSCLE CELLS

Stimulation of cultured rat thoracic aorta vascular smooth muscle cell s (VSMCs) with 100 nM angiotensin II (Ang II) reduces angiotensin rece ptor type 1 (AT(1)-R) gene expression. mRNA levels are reduced to simi lar to 30% of control levels 4 hr after the addition of Ang II to the culture medium. The loss of mRNA remains sustained for up to 24 hr aft er the addition of Ang II. The half-life of the AT(1)-R mRNA is simila r to 2 hr in cells treated with a single dose of 100 nM Ang II. This r epresents a 3-fold reduction from its half-life of 6 hr in nonstimulat ed cells, as assessed by treatment with 5,6-dichlorobenzimidazole or a ctinomycin D to block transcription. Thus, the AT(1)-R mRNA is moderat ely unstable in VSMC and destabilized further by treatment with Ang II . Ang II-induced AT(1)-R mRNA destabilization is prevented by pretreat ment with transcriptional inhibitors or the protein synthesis inhibito r cycloheximide, suggesting that Ang II-induced AT(1)-R mRNA destabili zation requires the induction of an unknown factor or factors that are postulated to mediate this effect. AT(1)-R mRNA levels decrease more rapidly in vitro from a polyribosomal fraction isolated from VSMC expo sed for 2 hr to 100 nM Ang II compared with that from vehicle-treated cells, suggesting that polyribosomal-associated AT(1)-R mRNA is at lea st one site of action for the mRNA destabilization effect of Ang II. A ng II stimulation induces a complex of polyribosomal proteins that bin d specifically in the distal 350 bases of the AT(1)-R mRNA. Regulation of mRNA stability accounts in part for modulation of AT(1)-R gene exp ression by Ang II in VSMCs, and Ang II-induced AT(1)-R mRNA polyriboso mal binding proteins are associated with this phenomenon.