Abstract

Expression of the angiotensin II type 1 receptor (AT₁-R) mRNA in vascular smooth muscle cells (VSMC) is down-regulated by a variety of agonists, including growth factors, agonists of Gα_q protein-coupled receptors, and activators of adenylyl cyclase. To determine whether cAMP-dependent protein kinases (PKA) participates in AT₁-R mRNA down-regulation controlled by multiple classes of receptors, a PKA inhibitor peptide (PKIα) was developed and expressed in rat VSMC as a fusion with the enhanced green fluorescent protein (eGFP). PKA activity elicited both by forskolin and angiotensin II is suppressed in cells expressing this fusion protein (PKIα-eGFP), but platelet-derived growth factor-BB does not stimulate PKA activity in this preparation. PKIα-eGFP expression fully inhibits the forskolin-stimulated down-regulation of AT₁-R mRNA levels and blocks 50% of the effect elicited by angiotensin II. This indicates that PKA plays a substantial role in angiotensin II-stimulated AT₁-R mRNA down-regulation. However, inhibition of PKA has no effect on AT₁-R mRNA down-regulation caused by platelet-derived growth factor-BB. These findings show how agonists such as angiotensin II that are not normally considered as activators of PKA can use PKA-dependent processes to modulate gene expression. These findings also provide definitive evidence that PKA-dependent pathways are involved in modulation of AT₁-R mRNA levels in VSMC.