Instructive roles for agonist binding parameters in determining the functional bandwidth of cytokine receptor signaling

摘要

The affinity of cytokine-receptor complexes on the cell surface is often poorly predictive of functional potency. To address this conundrum, we explored the inter-relationships of receptor binding to a wide range of downstream functional metrics for a prototypical cytokine, Interleukin-13 (IL-13), through structure-based engineering of agonists covering a spectrum of binding strengths for IL-13Rα1. Surprisingly, engineered IL-13 agonists spanning a broad affinity range exhibited similar potencies of STAT6 phosphorylation, while delays in STAT6 activation and nuclear translocation were only apparent for ligands with dramatically lower affinities. From this data, we developed a mechanistic model that quantitatively reproduced the kinetics of STAT6 phosphorylation for the entire spectrum of binding affinities. Receptor endocytosis plays a key role in buffering STAT6 phosphorylation potencies, while the lifetime of signaling complexes at the plasma membrane determines the long-term functional potency. The surprisingly complex inter-relationships between extracellular ligand binding and function highlight the importance of feedback mechanisms in modulating receptor responsiveness, and suggest new mechanism-based strategies to enhance the therapeutic efficacy of cytokine therapy.