Specific, high affinity receptors for 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] have been demonstrated in human breast cancer cells. In addition, 1,25-(OH)2D3 has been shown to inhibit replication in some human breast cancer cell lines, although the mechanism(s) of this antitumor activity remain undefined. There is currently considerable interest in the role of autocrine growth factors in the control of breast cancer cell proliferation and the effects of steroid hormones on their production, receptor binding, and action. Since the epidermal growth factor (EGF) receptor mediates the effects of both EGF and the autocrine growth factor, α-transforming growth factor, we investigated the effect of 1,25-(OH)2D3 on EGF receptor levels in several human breast cancer cell lines.
Preincubation of T-47D cells with 1,25-(OH)2D3 for 24 h resulted in a significant concentration-dependent decline in the specific binding of [125I]EGF. The effect was observed when EGF binding was assayed at either 0 or 37°C, both before and after treatment with acid to remove receptor bound endogenous ligand. This indicated that the effect on [125I]EGF binding was not due to effects of 1,25-(OH)2D3 on receptor internalization and degradation or receptor occupancy. The half-maximal inhibitory concentration of 1,25-(OH)2D3 was approximately 2 nm. The decrease in EGF binding was due to a decrease in receptor number from 2,900 sites/cell in control cultures to 2,330 and 1,730 sites/cell in cells treated for 24 h with 10-5 and 10-6 m 1,25-(OH)2D3, respectively. There was no change in the affinity of the receptor for EGF following treatment with 1,25-(OH)2D3 [Kd = 0.075 ± 0.006 nm (± SEM) for control and Kd = 0.083 ± 0.004 nm for treated cells. Decreased EGF receptor levels were also achieved with a number of analogues of 1,25-(OH)2D3 in accordance with their affinities for the 1,25-(OH)2D3 receptor, i.e., potencies for decreasing EGF binding in T-47D cells were in the order: 1,25-(OH)2D3 > 1,24,25-trihydroxyvitamin D3 > 1,25,26-trihydroxyvitamin D3 > 24,25-dihydroxyvitamin D3 ≥ 25-hydroxyvitamin D3. Specific, saturable EGF binding to MCF-7 cells was also reduced by 1,25-(OH)2D3 while binding to BT-20 and HBL-100 cells was unaffected by this treatment.
These results demonstrate that 1,25-(OH)2D3 treatment reduces EGF receptor levels in two breast cancer cell lines known to be growth inhibited by this hormone and suggest that one potential mechanism through which 1,25-(OH)2D3 could exert its known inhibitory effects on these cells is by reducing their sensitivity to autocrine growth factors that act via the EGF receptor.
Supported by the New South Wales State Cancer Council and the National Health and Medical Research Council of Australia.