Arrestin Pathways as Drug Targets - Progress in Molecular Biology and Translational Science - page 463

unique signaling events produced by biased agonists. Rather, biased ligands
appear to activate a subset of the downstream second messenger and protein
kinase cascades regulated by their native counterparts. In their comprehen-
sive survey, Sauliere et al. 37 were able to discern relatively subtle and cell-
type specific differences in downstream signaling between AngII and SII.
For example, they found that SII produced G i/o -dependent inhibition of
forskolin-stimulated cAMP production in HEK293T cells, while AngII
caused G q/11 -dependent potentiation of cAMP forskolin-stimulated cAMP
production that was further enhanced by pertussis toxin treatment. Such dif-
ferences probably arise from signal strength effects, in that some responses,
for example, G i/o -mediated inhibition of adenylyl cyclase, are graded, while
others, for example, G q/11 -mediated calcium flux, have a discrete activation
threshold. In such cases, a weak partial agonist may measurably affect a
tightly coupled or graded response while failing activate a less well-coupled
response or one with a high activation threshold ( Fig. 18.1 A). Nonetheless,
these results at least admit the possibility that biased agonists can qualitatively
change information flow as signals propagate by coupling the receptor to
downstream effectors with different efficiencies.
Of course, the failure to find novel biased agonist responses may be
explained by the fact that most work to date has focused on determining
which effects of native hormones are arrestin dependent, an approach that
guarantees that biased agonist responses will be defined as a subset of the
whole. Still, the limited number of discovery driven comparisons of native
and arrestin-selective agonism to date support the idea that biased agonist
effects comprise a subset of native agonist responses. Using a gel-based whole
cell phosphoproteomic approach, Kendall et al. uncovered novel arrestin-
dependent AT 1A R pathways leading to regulation of protein phosphatase
(PP)2A-Akt-glycogen synthase kinase (GSK)3

signaling and stimulation
of prostaglandin E2 synthesis by prostaglandin E synthase type 3. 40 But val-
idation experiments demonstrated that AngII, as well as SII, activated these
novel pathways. The most comprehensive side-by-side global phospho-
proteomic comparison of AngII and SII, performed by Christensen et al. ,
detected over 1183 regulated protein phosphorylation sites out of 10,000 sites
surveyed using LTQ-Orbitrap high resolution mass spectrometry. 41 Of
these, 756 (64%) were unique to AngII, 369 (34%) were regulated by both
AngII and SII, and only 58 (5%) were unique to SII. While their study clearly
demonstrated that G proteins and arrestins regulate robust, and largely
distinct, kinase cascades, they confirm that, at least in the short term, biased
agonists work by selecting from within the menu of native ligand responses.

b