Calcitriol (1,25-dihydroxyvitamin D3), the hormonally active metabolite of vitamin D, inhibits the growth and induces the differentiation of many malignant cells including breast cancer (BCa) cells. Calcitriol exerts its anti-proliferative activity in BCa cells by inducing cell cycle arrest and stimulating apoptosis. Calcitriol also inhibits invasion, metastasis and tumor angiogenesis in experimental models of BCa. Our recent studies show additional newly discovered pathways of calcitriol action to inhibit the growth of BCa cells. Calcitriol suppresses COX-2 expression and increases that of 15-PGDH thereby reducing the levels and biological activity of prostaglandins (PGs). Calcitriol decreases the expression of aromatase, the enzyme that catalyzes estrogen synthesis selectively in BCa cells and the breast adipose tissue surrounding BCa, by a direct repression of aromatase transcription via promoter II as well as an indirect effect due to the reduction in the levels and biological activity of PGE2, which is a major stimulator of aromatase transcription through promoter II in BCa. Calcitriol down-regulates the expression of estrogen receptor alpha and thereby attenuates estrogen signaling in BCa cells including the proliferative stimulus provided by estrogens. We hypothesize that the inhibition of estrogen synthesis and signaling by calcitriol and its anti-inflammatory actions will play an important role in the use of calcitriol for the prevention and/or treatment of BCa.
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