Inhibition of cyclic AMP dependent kinase by expression of a protein kinase inhibitor enhanced green fluorescent fusion protein attenuates angiotensin II induced type 1 AT1 receptor mRNA down-regulation in vascular smooth muscle cells

Expression of the angiotensin II type 1 receptor (AT(1)-R) mRNA in vascular smooth muscle cells (VSMC) is down-regulated by a variety of agonists, including growth factors, agonists of G alpha(q) protein-coupled receptors, and activators of adenylyl cyclase. To determine whether cAMP-dependent protein kinases (PKA) participates in AT(1)-R mRNA down-regulation controlled by multiple classes of receptors, a PKA inhibitor peptide (PKI alpha) 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 (PKI alpha-eGFP), but platelet-derived growth factor-BB does not stimulate PKA activity in this preparation. PKI alpha-eGFP expression fully inhibits the forskolin-stimulated down-regulation of AT(1)-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 AT(1)-R mRNA down-regulation. However, inhibition of PKA has no effect on AT(1)-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 AT(1)-R mRNA levels in VSMC.