
<ArticleTitle>Quantitative analysis of the <prot><prot>EGF</prot> receptor</prot> autocrine system reveals cryptic regulation of cell response by ligand capture.</ArticleTitle>
<AbstractText>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 <prot>epidermal growth factor</prot>/<prot>epidermal growth factor receptor</prot> (<prot>EGF</prot>/<prot>EGFR</prot>) 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 &#60;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 &#60; 5 x 10(-4)was 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.</AbstractText>
