Biased signaling as allosteric probe dependence
Introduction
Historically, agonist-mediated cellular signaling has been dealt with as a monotonic signal passed on to the cell by the agonist-receptor complex in varying strengths (according to the efficacy of the agonist as a molecular property). The reason for this was the limited number of readouts of cell response. Actually until the 1980's, most descriptions of pharmacological agonism were obtained from isolated tissues which are integrated signals from numerous signaling systems. However, once pharmacologists were able to differentiate the various signaling pathways activated by agonists, it became clear that the ‘stimulus' imparted to a cell from an agonist-activated receptor was quite different with respect to the agonist producing the signal. Specifically, there is now evidence to show that agonists preferentially produce stimulus to some signaling pathways at the expense of others, i.e. biased signaling. This paper will discuss this phenomenon in terms of the standard properties of allosteric proteins, namely allosteric probe dependence. Seen in these terms, agonist bias would be an expected property of all new synthetic agonists and not a specialized effect. The application of this behavior to drug therapy requires the translation to in vivo systems and there are legitimate pharmacological reasons why this may be difficult to predict from in vitro assays.. As a preface to these discussions, it is useful to put the concept of ‘bias' into historical context with respect to its relevance to modern pharmacology.
Section snippets
What is signaling bias?
Cells optimize the sensitivity of their signaling systems with respect to their extracellular portals (GPCRs) according to their physiologic needs. The concept of ‘bias’ with respect to how much stimulus is imparted to any one signaling pathway (over another) with receptor stimulation is totally dependent on the particular tissue or cell type being stimulated, i.e. the scale of relative stimulus will change with the system being considered. This being the case, a general scale of agonist ‘bias’
What is the molecular mechanism of signaling bias?
Signaling bias emerged in the late 1980's in the pharmacological literature as an apparently new phenomenon. Every new phenomenon begins with the assumption that it is rare if only because there are no prior data to document its existence. For example, when inverse agonism was first proposed by Costa and Herz [7] it was considered to be a rare phenomenon only because few groups had the appropriate assay (constitutive receptor systems) to demonstrate its prevalence in general pharmacology.
Does biased signaling denote biased agonist response?
A high degree of bias does not ensure a high selective agonism; the production of agonist response is still within the realm of the magnitude of the efficacy the agonist has for the signaling pathway. The estimate of bias is simply a ratio of the intensities of the two signals when the system has the appropriate sensitivity to return an agonist response to both. If a given bias depends on different efficacies, then a low sensitivity may not demonstrate a response expected from a high estimate
Quantitative measurement of signaling bias
There are a number of methods proposed to measure the relative stimulation by an agonist on receptors to produce selective signaling. These involve the relative responses to the agonist for each signaling pathway either on a scale of relative activation with concentration or relative maximal effect; these methods are reviewed in detail by Onaran et al. [25] and have been reviewed comprehensively and summarized elsewhere [19]. Specifically, bias can be calculated with Transducer Coefficients
Efficacy-based vs affinity-based bias
Theoretically, a bias ratio could occur from either a different efficacy or a different affinity (or both). For example, the biased angiotensin molecule TRV120027 produces biased activation of β-arrestin with no difference in the affinity for the receptor whether it interacts with Gq protein or β-arrestin. This is shown in data by Violin et al. [31] where the EC50 for β-arrestin partial agonism (a reasonable estimate of affinity) of pKB = 7.9 is not significantly different from the pKB of the
Translation of in vitro bias to in vivo bias
The main impetus to measuring agonist bias is to estimate the relative activation of various signaling systems in vivo for therapeutic advantage. The identification biased ligands in vitro is meant to select molecules to be tested in in vivo systems that may or may not show useful phenotypic signaling profiles. A major question to be considered here is the translation of the correspondence of the signaling bias measured in vitro to in vivo systems. Fig. 6A shows the effect varying the
Therapeutic application of biased signaling
To date there are no examples of successfully designed biased molecules tested in humans that are superior to their ‘unbiased’ counterparts. While on the surface this might be considered an indictment of the concept, it simply refers to the limited number of pro-actively designed biased ligands that have gone through the development process to the point where they have been tested in humans. Retrospective analyses on the other hand have demonstrated biased signaling in already known successful
Conclusions
There are methods available to quantify signaling bias in vitro that can be used to identify molecules that stabilize unique receptor states. These, in turn, can be tested in more physiologically relevant assays to identify possibly useful therapeutic phenotype activity. The fact that cell type, kinetics and the relative stoichiometry of receptor and signaling components can alter these in vitro estimates of bias further argue for the testing of suspected biased molecules in therapeutic natural
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