The effect of 1.25 dihydroxyvitamin D3 on binding and internalization of epidermal growth factor in cultured cells : Studies on BT-20 cells using quantitative electron microscope autoradiography

The biological effects of 1.25(OH)₂D₃ on epidermal growth factor receptor (EGF-R) and on EGF internalization were examined in human mammary carcinoma BT-20 cells. In this cell line, with known amplification of the epidermal growth factor receptor gene, EGF was not stimulatory for growth. Biological assay and quantitative EM autoradiography combined with iodinated ligand binding to specific receptors demonstrated that the number of binding sites per unit of length of plasma membrane was 2.48-fold higher in treated than in control cells. ¹²⁵I-EGF was progressively internalized in a time- and temperature-dependent manner after selective association with the membrane-coated pits. No modification of the time course of ¹²⁵I-EGF internalization was noted in the control and in the treated cells, but a different distribution of the labeling in the subcellular compartment was observed in treated cells. In 1.25(OH)₂D₃-treated batches, the grain density remained low in the receptosomes throughout the experiment, whereas it was high and occurred early in the lysosomes. On the other hand, in control cells, the grain density of the receptosomes was high, whereas it occurred late and was relatively low in the lysosomes. These data suggest that 1.25(OH)₂D₃ is a regulator of EGF-R level in BT-20 cell line, but it cannot be affirmed whether this effect is direct or mediated by other parameters.