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

The α(1A)-AR is thought to couple predominantly to the Gα(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 Gα(q) coupling-defective variants of α(1A)-AR, as well as a combination of Ca(2+) channel blockers, uncovered cross-talk between α(1A)-AR and β(2)-AR that leads to potentiation of a Gα(q)-independent signaling cascade in response to α(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 β-AR-selective agonist, was examined with respect to activation of α(1A)-AR. α(1A)-AR selective antagonists were used to specifically block Iso evoked signaling in different cellular backgrounds and confirm its action at α(1A)-AR. Iso induced signaling at α(1A)-AR was further interrogated by probing steps along the Gα(q) /PLC, Gα(s) and MAPK/ERK pathways. In HEK-293/EBNA cells transiently transduced with α(1A)-AR, and CHO_α(1A)-AR stable cells, Iso evoked low potency ERK activity as well as Ca(2+) mobilization that could be blocked by α(1A)-AR selective antagonists. The kinetics of Iso induced Ca(2+) transients differed from typical Gα(q)- mediated Ca(2+) mobilization, lacking both the fast IP(3)R mediated response and the sustained phase of Ca(2+) 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 α(1A)-AR partial agonist with signaling bias toward MAPK/ERK signaling cascade that is likely independent of coupling to Gα(q).