Isoproterenol Acts as a Biased Agonist of the Alpha-1A-Adrenoceptor that Selectively Activates the MAPK/ERK Pathway

The alpha(1A)-AR is thought to couple predominantly to the G alpha(q)/PLC pathway and lead to phosphoinositide hydrolysis and calcium mobilization, although certain agonists acting at this receptor have been reported to trigger activation of arachidonic acid formation and MAPK pathways. For several G protein-coupled receptors (GPCRs) agonists can manifest a bias for activation of particular effector signaling output, i.e. not all agonists of a given GPCR generate responses through utilization of the same signaling cascade(s). Previous work with G alpha(q) coupling-defective variants of alpha(1A)-AR, as well as a combination of Ca2+ channel blockers, uncovered cross-talk between alpha(1A)-AR and beta(2)-AR that leads to potentiation of a G alpha(q)-independent signaling cascade in response to alpha(1A)-AR activation. We hypothesized that molecules exist that act as biased agonists to selectively activate this pathway. In this report, isoproterenol (Iso), typically viewed as beta-AR-selective agonist, was examined with respect to activation of alpha(1A)-AR. alpha(1A)-AR selective antagonists were used to specifically block Iso evoked signaling in different cellular backgrounds and confirm its action at alpha(1A)-AR. Iso induced signaling at alpha(1A)-AR was further interrogated by probing steps along the G alpha(q)/PLC, G alpha(s) and MAPK/ERK pathways. In HEK-293/EBNA cells transiently transduced with alpha(1A)-AR, and CHO_alpha(1A)-AR stable cells, Iso evoked low potency ERK activity as well as Ca2+ mobilization that could be blocked by alpha(1A)-AR selective antagonists. The kinetics of Iso induced Ca2+ transients differed from typical G alpha(q)-mediated Ca2+ mobilization, lacking both the fast IP3R mediated response and the sustained phase of Ca2+ re-entry. Moreover, no inositol phosphate (IP) accumulation could be detected in either cell line after stimulation with Iso, but activation was accompanied by receptor internalization. Data are presented that indicate that Iso represents a novel type of alpha(1A)-AR partial agonist with signaling bias toward MAPK/ERK signaling cascade that is likely independent of coupling to G alpha(q).