Biologically active
vitamin D (
1,25-dihydroxycholecalciferol or 1,25(
OH)2D) is synthetized from inactive prohormone
25-hydroxycholecalciferol (25(
OH)D) by the
enzyme CYP27B1 1-α-hydroxylase in kidney and several extrarenal tissues including breast. Although the development of
breast cancer has been linked to inadequate
vitamin D status, the importance of bioactive
vitamin D production within
tumors themselves is not fully understood. To investigate the role of tumoral
vitamin D production in mammary epithelial cell progression to
breast cancer, we conducted a Cre-loxP-mediated
Cyp27b1 gene ablation in the mammary epithelium of the polyoma middle
T antigen-mouse mammary tumor virus (PyMT-MMTV) mouse
breast cancer model. Targeted ablation of
Cyp27b1 was accompanied by significant acceleration in initiation of spontaneous mammary
tumorigenesis. In vivo, cell proliferation, angiogenesis, cell cycle progression, and survival markers were up-regulated in
tumors by
Cyp27b1 ablation, and apoptosis was decreased. AK
thymoma (AKT) phosphorylation and expression of several components of nuclear factor κB (NF-κB),
integrin, and
signal transducer and activator of transcription 3 (STAT3) signaling pathways were increased in Cyp27b1-ablated
tumors compared with nonablated controls. In vitro, 1,25(
OH)2D treatment induced a strong antiproliferative action on
tumor cells from both ablated and nonablated mice, accompanied by rapid disappearance of NF-κB p65 from the nucleus and segregation in the cytoplasm. In contrast, treatment with the metabolic precursor 25(
OH)D was only effective against cells from nonablated mice. 25(
OH)D did not inhibit growth of Cyp27b1-ablated cells, and their nuclear NF-κB p65 remained abundant. Our findings demonstrate that in-
tumor CYP27B1 1-α-hydroxylase activity plays a crucial role in controlling early oncogene-mediated mammary
carcinogenesis events, at least in part by modulating tumoral cell NF-κB p65 nuclear translocation.