Increased epidermal growth factor receptor level in breast cancer cells treated by 1,25-dihydroxyvitamin D3

The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on epidermal growth factor receptor (EGF-R) level regulation was investigated on a human breast cancer cell line (BT-20). These cells display a great number of EGF-R (1 +/- 0.4 x 10(6) sites per cell). Scatchard analysis of binding data indicates the presence of two classes of sites: one of high affinity (Kd = 0.48 +/- 0.2 nM), the other of low affinity and higher capacity (Kd = 2.24 +/- 0.93 nM). Treatment of cells by 1,25(OH)2D3 for several days induces a progressive increase of the EGF-R number of sites per cell. This effect is dose and time dependent and results in a 3-fold enhancement of EGF-R binding in cells treated for 15 days, which involves the two classes of binding sites in the same proportion. This effect is not due to modifications of the EGF internalization and degradation processes. Inhibition of the effect of 1,25(OH)2D3 by cycloheximide suggests that it is dependent on new protein synthesis. There are no data in favor of a reduced occupancy of EGF-R sites by EGF-like substance accounting for the increase of unoccupied sites. Analysis of 125I binding by quantitative transmission electron microscope autoradiography illustrates the observation of a higher number of sites on the plasma membrane in treated than in control cells.