Signaling in Context: Parsing the Adhesion-Dependence of Growth Factor Signaling

Citation

Galownia, Niki Chiyomi (2007) Signaling in Context: Parsing the Adhesion-Dependence of Growth Factor Signaling. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Q5RF-8K30. https://resolver.caltech.edu/CaltechETD:etd-04222007-221859

Abstract

Intracellular signaling induced by adhesion and soluble growth factors is a significant contributor to cellular function. The serine/threonine kinase, extracellular signal-regulated kinase (Erk), is a prominent point of signaling crosstalk between adhesion and growth factors. Despite extensive effort, the effect of individual growth factors on adhesion-dependent Erk signaling remains unclear due to considerable protocol differences and qualitative analyses. To address these issues, we developed an experimental technique to compare systematically the crosstalk between adhesion and individual growth factors and a quantitative protocol for measuring the magnitude and dynamics of Erk signaling.

Using these methods, we demonstrate that: (1) Adhesion to fibronectin desensitizes Erk activation for cells stimulated by either PDGF (platelet-derived growth factor) or bFGF (basic fibroblast growth factor), but not by EGF (epidermal growth factor); (2) EGF, but not PDGF or bFGF, induces adhesion-dependent Erk activation enhancement; and (3) for adherent cells, either EGF or PDGF stimulation generates transient Erk activation, while bFGF stimulation mediates sustained Erk activation. This data reveal that there are significant differences in the adhesion-dependence of growth factor signaling. The most striking observation was that adhesion desensitizes cells to low doses of specific growth factors (PDGF and bFGF). Studies conducted to uncover the underlying mechanism(s) revealed that adhesion-mediated desensitization of Erk activation exhibits rapid kinetics and occurs at or above the level of Ras activation, but does not involve Sos hyperphosphorylation.

To further probe the mechanisms responsible for generating different Erk signaling dynamics, we constructed a simple coarse-grain model of Erk activation and deactivation pathways. These pathways are represented by four distinct motifs: activation, constitutive direct-deactivation, feedback-mediated direct-deactivation, and feedback-decoupling. Our model predicts that transient and sustained Erk signaling dynamics require specific combinations of the four signaling motifs. These predictions suggest that differences in the Erk activation motif are most likely responsible for the experimentally observed characteristics of adhesion-dependent EGF-mediated Erk signaling. Furthermore, the model indicates either feedback-decoupling deactivation or direct-deactivation as the mechanisms responsible for the observed transient-versus-sustained signaling dynamics induced by different growth factors.

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