Vitamin D deficiency promotes breast cancer growth in bone, mostly through changes in the bone microenvironment. Here we aim to further define the role of the vitamin D receptor (VDR) in systemic breast cancer spread to bone.
Following knock-down of VDR expression in the human breast cancer cell line, MDA-MB-231 (MDAVDR-/-), and subsequent luciferase gene transfection, MDAVDR-/- and non-target (NT) cells were injected intra-cardially into female nude mice (n=73). Systemic cancer cell spread and local tumour growth were monitored by sequential in-vivo bioluminescent and high-resolution X-ray imaging for 30 days. At endpoint, affected bones were analysed by µ-CT, histomorphometry and immunohistochemistry. Cancer cells in the bone marrow were quantified on days 3, 7, 14 and 21 post injection. VDR, E-cadherin and ß-catenin expression levels in MDAVDR-/- and NT cells were analysed in-vitro (Western) and in-vivo (IHC). In a translational approach, VDR, CYP24A1, E-cadherin and ß-catenin expression were measured by IHC in clinical breast cancer specimens (n=170) and correlated with tumour characteristics and disease progression over 5 years.
Compared to NT controls, MDAVDR-/- cells demonstrated increased cell migration and invasion in-vitro, associated with significantly reduced expression of β-catenin and E-cadherin protein in MDAVDR-/- comparde to NT cells. Following intra-cardiac injection, systemic spread occurred earlier and cancer cell numbers in the bone marrow were significantly greater in MDAVDR-/- injected mice. E-cadherin protein expression was significantly reduced in MDAVDR-/--derived tumours compared to NT-derived lesions. Analysis of human breast cancer tissue confirmed a strong association between VDR expression, tumour grade and prognosis; CYP24, β-catenin and E-cadherin protein levels were positively associated with VDR expression.
We conclude that loss of the VDR in human breast cancer promotes cell mobility, systemic spread and skeletal tumour burden by altering ß-catenin and E-cadherin expression.