Endocrine Abstracts

Studies in vitro have highlighted the potent anti-cancer effects of the active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25D₃). However, the in vivo application of 1,25D₃ and its synthetic analogs remains problematical due to persistent hypercalcaemic side-effects. Recent studies have shown that it may be possible to overcome this in an autocrine fashion via extra-renal synthesis of 1,25D₃. In particular, analysis of prostate and colon cancer has highlighted the capacity of these tissues to generate their own 1,25D₃ via expression of the enzyme 1alpha-hydroxylase. To assess the possible impact of this on breast cancer, we have characterized expression and activity of 1alpha-hydroxylase in three different breast cancer cell lines: MCF-7 (ER+ve), MCF-7/VD^R (ER+ve) (Leo Pharmaceuticals) and MDA-MB-231 (MDA) (ER-ve). RT-PCR and Western blot analyses demonstrated the presence of mRNA and protein for 1alpha-hydroxylase in all three cell lines. Enzyme activity assays using ³H-25-hydroxyvitamin D₃ as substrate showed similar levels of 1,25D₃ synthesis in each cell line (50 fmoles/hr/mg protein). Parallel analysis of the companion enzyme 24-hydroxylase indicated that this was also expressed (approximately 100 fmoles/hr/mg protein for each line). Proliferation studies using ³H-thymidine incorporation and colony formation assays showed sensitive inhibition of cell proliferation in MCF-7 cells by 1,25D3 (70% inhibition at 10 nM for 72 hrs). In contrast both MCF-7/VD^R and MDA were both completely resistant to the hormone despite showing 1,25D₃-induced 24-hydroxylase assays. Data presented here highlight the capacity for local generation of active 1,25D₃ within breast tumours. We postulate that this may provide a novel, 'calcium friendly' approach to the use of vitamin D in the prevention and treatment of breast cancer. However, the success of this approach will also depend on overcoming apparent vitamin D resistance in many breast cancer cell lines.

This abstract was awarded the Michael White prize