Adaptation of EGF receptor signal transduction to three‐dimensional culture conditions: Changes in surface receptor expression and protein tyrosine phosphorylation

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Abstract
A431 cells grown as three‐dimensional spheroids show growth stimulation in response to nanomolar concentrations of EGF in contrast to monolayer cultures that show inhibition. In investigating the alterations in EGF signal transduction that underlie this modification of the proliferative response, we have compared the expression of EGF receptors on A431 cells under these conditions and related our findings to tyrosine phosphorylation and the growth response. EGF receptors were measured by 125I‐EGF binding to trypsin‐dispersed cells. Unexpectedly, dispersion of the monolayers caused an 80% decrease in surface EGF receptor, although, after dispersion, EGF receptor was digested by trypsin with a half‐life of 69 ± 32 min. No evidence for a comparable loss of cellular EGF receptor was seen on trypsin dispersion of spheroids. After allowing for this effect, we found that the receptor density on nondispersed monolayers (5 × 106 per cell) was twentyfold greater than that on spheroids (0.25 × 106 per cell). EGF‐induced tyrosine phosphorylation was confined to the outermost cells of the spheroid, although the presence of surface‐expressed EGF pinding sites could be demonstrated throughout the structure and the number of EGF receptors/cell on dispersed spheroid cells showed a single distribution peak by flow cytometry, with no evidence for more than one population. Using RCM‐lysozyme as a substrate, tyrosine phosphatase activity in spheroids lay within the range observed in monolayer cultures. Autophosphorylation of the EGF receptor following EGF stimulation in monolayer cultures of A431 cells rose rapidly in the first 10 seconds and then slowly increased for at least 3 h. In spheroids, it reached a maximum within 10 seconds and then declined over 3 h. Since the microenvironment within a tumor resembles that in a spheroid, a similar reduction in surface EGF receptor expression may be expected in tumors relative to monolayer cultures, together with corresponding growth stimulation in response to EGF.