Background and purpose: Vitamin D-related compounds can inhibit cancer cell growth, but the biologic mechanism of this inhibition remains to be determined. We investigated the possibility that these compounds interfere with the activity of insulin-like growth factors. Such activity can be suppressed or otherwise modulated by specific insulin-like growth factor-binding proteins.
Methods: The human breast cancer cell line MCF-7 was used in this study. The effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and two related compounds, EB1089 and KH1060, on cell proliferation were assessed by monitoring cell numbers and by measuring cellular incorporation of [3H]thymidine. Changes in the accumulation of insulin-like growth factor-binding proteins in cell-conditioned media (i.e., culture fluids) were assessed by means of standard protein blotting techniques; ligand blots were probed with [125I]insulin-like growth factor I, and immunoblots were probed with antibodies raised against specific binding proteins. Binding protein messenger RNA levels were determined by use of RNA blotting methods and complementary DNA probes.
Results: At concentrations of 10(-8) M and 10(-9) M, EB1089 and KH1060 exhibited stronger antiproliferative activity than 1,25(OH)2D3. When each of the vitamin D-related compounds was used separately at a concentration of 10(-9) M, a 20- to 25-fold increase in the concentration of insulin-like growth factor-binding proteins in MCF-7 cell-conditioned media was observed; this binding capacity was increased nine-fold, ninefold, and threefold, respectively, in the presence of 10(-10) M EB1089, KH1060, and 1,25(OH)2D3. Immunoblotting experiments demonstrated that all three vitamin D-related compounds induced the accumulation of insulin-like growth factor-binding protein 5 in cell-conditioned media. The accumulation of this binding protein was associated with an increase in cellular expression of its messenger RNA. EB1089 and 1,25(OH)2D3 attenuated the growth-promoting activity of insulin-like growth factor I on MCF-7 cells; however, these compounds did not inhibit the growth-promoting activity of long R3 IGF-I, an insulin-like growth factor I analogue with greatly reduced affinity for insulin-like growth factor-binding proteins.
Conclusions and implications: Our results indicate that vitamin D-related compounds stimulate production of insulin-like growth factor-binding protein 5, thereby indirectly suppressing cell proliferation.