Understanding the kinetics of the relation between the ligand and receptor in autocrine signaling pathways can be a difficult task because the levels of the ligand cannot be easily measured because of efficient "capture" of the ligand by the cell surface receptors, and these ligand-bound receptors are frequently internalized. DeWitt et al. engineered an artificial autocrine system to study how cell surface receptor density and ligand production levels influence autocrine signal transduction. Ligand production levels were controlled by using a tetracycline-inducible expression of epidermal growth factor (EGF) and metalloprotease inhibitors to block EGF release. Available EGF receptor (EGFR) levels were controlled through application of an EGFR antibody against the receptor that blocked ligand access. Mathematical modeling and kinetic analysis were used to show that the rate of ligand production to receptor production (VL/VR) can be used to define the activity of an autocrine system. In the engineered system, activity of the system and receptor occupancy were correlated with the extracellular acidification rate. The ligand capture was extremely efficient when the ratio of the rate of ligand production to receptor production (VL/VR) was less than 0.3. The level of receptor occupancy was only 30 to 40% under these conditions. When VL/VR approaches 1, receptor occupancy saturates, but ligand capture is much less efficient. This same type of analysis was applied to a natural system, the proliferation of mammary epithelial cells through EGFR activation in response to tumor necrosis factor-α (TNF-α). In that case, receptor production greatly exceeded ligand production and so ligand production was the rate-limiting factor controlling signaling through this pathway. Signaling through the EGFR by TNF-α in the mammary epithelial cells occurred when VL/VR was 5×10–4 and thus, the ligand production is "invisible"; that is, it is not detectable in the extracellular medium.
A. E. DeWitt, J. Y. Dong, H. S. Wiley, D. A. Lauffenburger, Quantitative analysis of the EFT receptor autocrine system reveals cryptic regulation of cell response by ligand capture. J. Cell Sci. 114, 2301-2313 (2001). [Online Journal]