Abstract 348: α1a-Adrenoceptor Genetic Variant-Triggered Hyperproliferation Is Mediated Through Aberrant Interaction With Spinophilin in Smooth Muscle Cells

Activation of α1-Adrenergic Receptors (α1ARs), members of the G protein-coupled receptor (GPCR) superfamily, in response to stimulation of the sympathetic nervous system by catecholamines plays a major role in regulating cardiovascular (CV) function and blood pressure (BP). Among three α1AR subtypes (α1a,α1b,α1d) expressed in vasculature α1aARs predominate in human vascular smooth muscle cells (SMC), particularly in resistant vessels most involved in BP control. Polymorphisms in the α1aAR gene are also related to hypertension (HTN), one of the major CV risk factors. Recently, we discovered a novel, unique mechanism of hypertension triggered by naturally occurring human α1aAR-G247R (247R) genetic variant, identified in the 3rd intracellular loop (3iL) of the receptor in hypertensive patient. In fibroblasts, 247R signals via constitutive active coupling to the βarrestin1/MMP7/EGFR pathway. Here we report that 247R expression in SMC triggers constitutive, Gq- and MMP/EGFR/ERK-dependent hyperproliferation. Agonist (phenylephrine) treatment of cells inhibits hyperproliferation and induces hypertrophy and α1aAR inverse agonist prazosin inhibits hyperproliferation and hypertrophy indicating Gq-dependent pathway. Expression of 247R in SMC also triggers upregulation of spinophilin (SPL), a ubiquitous multi-domain scaffold protein that binds to 3iL of several GPCRs and competes with βarrestins for 3iL binding. We hypothesized that SPL mediates constitutive signaling of 247R and examined whether SPL directly interacts with α1aAR-WT or 247R. Our preliminary results demonstrate a distinct interaction of α1aAR-WT and 247R with SPL, although the SPL-α1AR-WT interaction appears to be strongest. SPL 1-480aa fragment demonstrates stronger interaction with α1aAR-WT indicating that this domain is responsible for interaction with 3iL. Interestingly the SPL 480-817 fragment has stronger interaction with 247R. Thus, expression of the naturally occurring human α1aAR genetic variant in vascular cells activates distinct signaling pathways leading to aberrant hyperproliferation and hypertrophy, and may eventually result in hypertension and other CV diseases suggesting a possible novel mechanism underlying some forms of human hypertension.