Analysis of 25-Hydroxyvitamin D₃-1α-Hydroxylase in Normal and Malignant Breast Tissue

Abstract

Background: The presence of extra-renal 25-hydroxyvitamin D₃ [25(OH)D₃]-1α-hydroxylase (1α-OHase) has been reported in several cell types including prostate and colon cancer cells. Additionally, alterations in the local production of 1α,25-dihydroxyvitamin D [1α,25(OH)₂D₃] have been implicated in the tumorigenesis of these malignancies. The aim of this study was to analyze whether normal breast tissue or breast cancer cells expressed 1α-OHase and to evaluate whether breast tissue possessed the capacity to produce 1α,25(OH)₂D₃ from 25(OH)D₃. Materials and Methods: Total RNA was extracted from normal breast tissue (n=11), breast carcinomas (n=12) and cultured MCF-7 breast cancer cells for real-time (LightCycler using specific hybridization probes) and conventional PCR analysis. Results: mRNA for 1α-OHase was detected in breast cancer tissue and in MCF-7 breast cancer cells. Interestingly, the mRNA levels for 1α-OHase were significantly increased in breast cancer compared to normal breast tissue. When the MCF-7 cells were treated with 1α,25(OH)₂D₃, cell proliferation was inhibited in a dose-dependent manner. Incubation of the MCF-7 cells with [³H]-25(OH)D₃ resulted in its conversion to [³H]-1,25(OH)₂D₃. The 1α-OHase activity in the MCF-7 cells was blocked by a specific cytochrome P450 inhibitor, clotrimazole. Conclusion: The data suggest that at least breast cancer cells expressed 1α-OHase mRNA and, therefore, might have the ability to synthesize 1α,25(OH)₂D₃ within the cells. The local production of 1α,25(OH)₂D₃ might play an important role in regulating the proliferation and differentiation of breast cells. We hypothesize that alterations in the local production of 1α,25(OH)₂D₃ may be involved in the tumorigenesis of breast cancer. Additionally, breast cancer may be a target for treatment with precursors of biologically-active vitamin D analogs.