The effects of receptor density and cell shape on epidermal growth factor binding

In this paper we describe the effects of receptor density and cell shape on the binding of epidermal growth factor (EGF) to its receptor. Association kinetics of EGF binding to cells with a high receptor density was done using A431 cells. The association rate of EGF binding was apparently independent of the EGF concentration, most likely due to diffusion limited EGF binding as result of high receptor density. The effect of receptor density on EGF association rate was examined by reducing the number of functional EGF receptors on A431 cells. Preincubation of the cells with a monoclonal antibody directed against the EGF receptor and isolation of the cytoskeletons of A431 cells which both leaves only EGF binding to high affinity receptors revealed an EGF concentration dependent association rate. These results were confirmed in HeLa cells with 40 times less receptor numbers than A431 cells demonstrating the effect of receptor density on EGF binding. The influence of shape of the cell on EGF binding was examined by comparing the EGF association to monolayer cells with that of suspension cells. EGF association to suspended A431 cells was EGF concentration dependent. In conclusion we have shown that binding of EGF to A431 cells is dependent not only on the intrinsic rate constants but in addition on both receptor numbers per cell and the shape of cells. These results are in agreement with the hypothesis that EGF binding can be restricted by limited diffusion.