title: Tissue-selective regulation of aromatase expression by calcitriol: implications for breast cancer therapy.
abstract: Aromatase, the enzyme that catalyzes estrogen synthesis, is critical for the progression of estrogen receptor-positive breast cancer (BCa) in postmenopausal women. We show that calcitriol, the hormonally active form of vitamin D, regulates the expression of aromatase in a tissue-selective manner. Calcitriol significantly decreased aromatase expression in human BCa cells and adipocytes and caused substantial increases in human osteosarcoma cells (a bone cell model exhibiting osteoblast phenotype in culture) and modest increases in ovarian cancer cells. Calcitriol administration to immunocompromised mice bearing human BCa xenografts decreased aromatase mRNA levels in the tumors and the surrounding mammary adipose tissue but did not alter ovarian aromatase expression. In BCa cells, calcitriol also reduced the levels of prostaglandins (PGs), major stimulators of aromatase transcription, by suppressing the expression of cyclooxygenase-2 (which catalyzes PG synthesis) and increasing that of 15-hydroxyprostaglandin dehydrogenase (which catalyzes PG degradation). The mechanism of aromatase down-regulation by calcitriol in BCa cells is therefore 2-fold: a direct repression of aromatase transcription via promoter II through the vitamin D-response elements identified in this promoter and an indirect suppression by reducing the levels of PGs. Combinations of calcitriol with three different aromatase inhibitors (AIs) caused enhanced inhibition of BCa cell growth. The combination of calcitriol and an AI may have potential benefits for BCa therapy. In addition to augmenting the ability of AIs to inhibit BCa growth, calcitriol acting as a selective aromatase modulator that increases aromatase expression in bone would reduce the estrogen deprivation in bone caused by the AIs, thus ameliorating the AI-induced side effect of osteoporosis.
authors.affiliation: Department of Medicine, Division of Endocrinology, Stanford University School of Medicine, 300 Pasteur Drive, Stanford, California 94305-5103, USA.
authors.forename: Aruna V
authors.initials: AV
authors.lastname: Krishnan
authors.forename: Srilatha
authors.initials: S
authors.lastname: Swami
authors.forename: Lihong
authors.initials: L
authors.lastname: Peng
authors.forename: Jining
authors.initials: J
authors.lastname: Wang
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Moreno
authors.forename: David
authors.initials: D
authors.lastname: Feldman
mesh_terms: D041721:3T3-L1 Cells; D052436:Adipose Tissue, White / Q000187:drug effects / Q000473:pathology; D000818:Animals; D001141:Aromatase / Q000235:genetics* / Q000378:metabolism; D047072:Aromatase Inhibitors / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000235:genetics / Q000473:pathology; D019556:COS Cells; D002117:Calcitriol / Q000008:administration & dosage / Q000494:pharmacology*; D002522:Chlorocebus aethiops; D005260:Female; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D051379:Mice; D009928:Organ Specificity / Q000187:drug effects / Q000235:genetics; D010024:Osteoporosis / Q000139:chemically induced / Q000517:prevention & control; D011453:Prostaglandins / Q000378:metabolism; D014407:Tumor Cells, Cultured; D023041:Xenograft Model Antitumor Assays
chemical_list: D047072:Aromatase Inhibitors; D011453:Prostaglandins; D001141:Aromatase; D002117:Calcitriol