Functional significance of 25-dihydroxy vitamin D3 in breast cancer chemoprevention.

A81

Although the efficacy of active metabolite of vitamin D, 1,25(OH)2D3 and its analogs in mammary carcinogenesis has been reporeted, the role of prohormone 25-Hydroxyvitamin D3 (25(OH)D3) has not been defined. Recently, the expression of 25-hydroxy vitamin D 1α-hydroxylase (CYP27B1) has been reported in multiple organs. This has resulted in renewed interest in possible efficacy of 25 (OH)D3 in cancer prevention and therapy. In the present study, we determined its chemopreventive efficacy using mouse mammary organ culture (MMOC) system and human breast epithelial cell lines MCF12F and T47D. In MMOC, 100-500 nM 25(OH)D₃ suppressed both mammary alveolar and ductal lesions induced by DMBA by more than 50%, while the active hormone 1α,25(OH)₂D₃ at 100 nM suppressed alveolar lesions by more than 80%, the vitamin D3 at 100 nM only suppressed alveolar lesions only by 20%. Effects of both 25(OH)D₃ and 1α,25(OH)₂D₃ was determined on the induction of CYP24, a vitamin D target gene in MCF12F and T47D cells. Results showed that both the prohormone and the active hormone significantly induced CYP24 transactivation in a dose-dependent manner as evaluated by real-time PCR and inhibited cell proliferation, although as expected 25(OH)D3 was less potent than 1α,25(OH)₂D₃. Moreover CYP27B1 mRNA was expressed in both cell lines but not regulated by 25(OH)D₃ and 1α,25(OH)₂D₃. 1α-hydroxylase protein was extensively expressed in the epithelial cells in mouse mammary organ as determined by immunostaining. Interestingly, co treatment of T47D cells with vitamin D and an inhibitor of 1α-hydroxylase clotrimazol (1 μM) significantly enhanced CYP24 transactivation induced by 25(OH)D₃, suggesting that 25(OH)D₃ might directly activate VDR and regulate VDR target genes. These results suggest that 25(OH)D₃ could be developed as a natural chemopreventive agent for breast cancer prevention and therapy. [Supported by NCI-R01-CA82316 and R03 CA121365].