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Quantitative analysis of the EGF receptor autocrine system reveals cryptic regulation of cell response by ligand capture

¹ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
² Newborn Medicine, Children’s Hospital, Boston, MA 02115, USA
³ Fundamental Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA
⁴ Division of Bioengineering and Environmental Health and Center for Cancer Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

*Author for correspondence (e-mail: lauffen{at}mit.edu)

Accepted March 29, 2001

Autocrine signaling is important in normal tissue physiology as well as pathological conditions. It is difficult to analyze these systems, however, because they are both self-contained and recursive. To understand how parameters such as ligand production and receptor expression influence autocrine activity, we investigated a human epidermal growth factor/epidermal growth factor receptor (EGF/EGFR) loop engineered into mouse B82 fibroblasts. We varied the level of ligand production using the tet-off expression system and used metalloprotease inhibitors to modulate ligand release. Receptor expression was varied using antagonistic blocking antibodies. We compared autocrine ligand release with receptor activation using a microphysiometer-based assay and analyzed our data using a quantitative model of ligand release and receptor dynamics. We found that the activity of our autocrine system could be described in terms of a simple ratio between the rate of ligand production (V_LT) and the rate of receptor production (V_R). At a V_LT/V_R ratio of <0.3, essentially no ligand was found in the extracellular medium, but a significant number of cell receptors (30-40%) were occupied. As the V_LT/V_R ratio increased from 0.3 towards unity, receptor occupancy increased and significant amounts of ligand appeared in the medium. Above a V_LT/V_R ratio of 1.0, receptor occupancy approached saturation and most of the released ligand was lost into the medium. Analysis of human mammary epithelial cells showed that a V_LT/V_R ratio of <5x10^-4was sufficient to evoke >20% of a maximal proliferative response. This demonstrates that natural autocrine systems can be active even when no ligand appears in the extracellular medium.

Key words: Autocrine, EGF, Computational model

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