Escape of autocrine ligands into extracellular medium: Experimental test of theoretical model predictions

We have developed an experimental system for testing mathematical model predictions concerning escape of autocrine ligands into the extracellular bulk medium. This system employs anti‐receptor blocking antibodies against the epidermal growth factor receptor (EGFR)/transforming growth factor alpha (TGFα) receptor/ligand pair. TGFα was expressed under the control of a tetracycline‐repressed promoter, together with a constitutively expressed human EGFR in B82 mouse fibroblast cells. This expression system allowed us to vary TGFα synthesis rates over a roughly 300‐fold range by adjusting tetracycline concentration. TGFα accumulation in the extracellular bulk medium was then measured as a function of cell density, TGFα synthesis rate, and anti‐EGFR blocking antibody concentration. Consistent with model predictions, amounts of ligand in the medium on a per cell basis were found to diminish as cell density was increased but with reduced dependence on cell density at higher ligand synthesis rates. Similarly consistent with model predictions, higher ligand synthesis rates also decreased the effect of anti‐receptor blocking antibodies. Our investigation has established that we can successfully analyze and understand autocrine ligand secretion behavior from the basis of our theoretical model. ©1998 John Wiley & Sons, Inc. Biotechnol Bioeng 57: 571‐582, 1998.