Li Laine Ooi, Yu Zheng, Hong Zhou, Trupti Trivedi, Arthur D. Conigraveb, Markus J. Seibel, Colin R. Dunstan
Breast cancer metastases to bone are common in advanced stage disease. We have recently demonstrated that vitamin D deficiency enhances breast cancer growth in an osteolytic mouse model of breast cancer metastasis. In this study, we examined the effects of vitamin D deficiency on tumor growth in an osteosclerotic model of intra-skeletal breast cancer in mice. The effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on proliferation and apoptosis of MCF-7 breast cancer cells, and changes in the expression of genes within the vitamin D metabolic pathway (VDR, 1α- and 24-hydroxylase) were examined in vitro. MCF-7 breast cancer cells were injected intra-tibially into vitamin D deficient and vitamin D sufficient mice co-treated with and without osteoprotegerin (OPG). The development of tumor-related lesions was monitored via serial X-ray analysis. Tumor burden and indices of proliferation and apoptosis were determined by histology along with markers of bone turnover and serum intact PTH levels. In vitro, MCF-7 cells expressed critical genes for vitamin D signalling and metabolism. Treatment with 1,25(OH)2D3 inhibited cell growth and proliferation, and increased apoptosis. In vivo, osteosclerotic lesions developed faster and were larger at endpoint in the tibiae of vitamin D deficient mice compared to vitamin D sufficient mice (1.49±0.08mm2 versus 1.68±0.15mm2, P<0.05). Tumor area was increased by 55.8% in vitamin D deficient mice (0.81±0.13mm2 versus 0.52±0.11mm2 in vitamin D sufficient mice). OPG treatment inhibited bone turnover and caused an increase in PTH levels, while tumor burden was reduced by 90.4% in vitamin D sufficient mice and by 92.6% in vitamin D deficient mice. Tumor mitotic activity was increased in the tibiae of vitamin D deficient mice and apoptosis was decreased, consistent with faster growth. Vitamin D deficiency enhances both the growth of tumors and the tumor-induced osteosclerotic changes in the tibiae of mice following intratibial implantation of MCF-7 cells. Enhancement of tumor growth appears dependent on increased bone resorption rather than increased bone formation induced by these tumors.