Calcium effects on epidermal growth factor receptor‐mediated endocytosis in normal and SV40‐transformed human fibroblasts

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Abstract
Lowering of extracellular Ca2+ levels will reversibly arrest the growth of human fibroblasts (WI38). Simian virus40(SV40)‐transformed WI38 cells do not exhibit this Ca2+‐dependent arrest. One possibility for this difference in Ca2+ requirement is that extracellular or surface membrane‐bound Ca2+ may be required for growth factor receptor‐mediated endocytosis and this Ca2+ requirement may differ in normal versus transformed cells. In this study we have evaluated the role of Ca2+ in the binding, internalization, and degradation of epidermal growth factor (EGF) in the WI38 and SV40 WI38 cell. The binding of [125I]EGF to the cell surface is not significantly altered by lowering of Ca2+ to 10−5‐M levels in either the normal or transformed cell. At this Ca2+ level, growth of the normal cell is inhibited. The subsequent internalization of EGF is reduced nearly threefold in the normal cell but not in the transformed cell following Ca2+ deprivation. Degradation of the EGF‐receptor complex is also sensitive to Ca2+. A twofold reduction in the rate of release of acid‐soluble 125I occurs in the normal but not the transformed cell under conditions of lowered medium Ca2+. In contrast, 2‐chloro‐10‐3‐aminopropyl phenothiazine (CP), an inhibitor of the Ca2+‐dependent regulator protein calmodulin, causes an inhibition of [125I]EGF internalization and degradation in both the normal and transformed WI38 cell, and a marked inhibition of [125I]EGF binding to the cell surface receptor of the transformed cell but not the normal cell.