Involvement of 1,25D3-MARRS (membrane associated, rapid response steroid-binding), a novel vitamin D receptor, in growth inhibition of breast cancer cells.

In addition to classical roles in calcium homeostasis and bone development, 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] inhibits the growth of several cancer types, including breast cancer. Although cellular effects of 1,25(OH)2D3 traditionally have been attributed to activation of a nuclear vitamin D receptor (VDR), a novel receptor for 1,25(OH)2D3 called 1,25D3-MARRS (membrane-associated, rapid response steroid-binding) protein was identified recently. The purpose of this study was to determine if the level of 1,25D3-MARRS expression modulates 1,25(OH)2D3 activity in breast cancer cells. Relative levels of 1,25D3-MARRS protein in MCF-7, MDA MB 231, and MCF-10A cells were estimated by real-time RT-PCR and Western blotting. To determine if 1,25D3-MARRS receptor was involved in the growth inhibitory effects of 1,25(OH)2D3 in MCF-7 cells, a ribozyme construct designed to knock down 1,25D(3)-MARRS mRNA was stably transfected into MCF-7 cells. MCF-7 clones in which 1,25D3-MARRS receptor expression was reduced showed increased sensitivity to 1,25(OH)2D3 ( IC(50) 56+/-24 nM) compared to controls (319+/-181 nM; P<0.05). Reduction in 1,25D3-MARRS receptor lengthened the doubling time in transfectants treated with 1,25(OH)2D3. Knockdown of 1,25D3-MARRS receptor also increased the sensitivity of MCF-7 cells to the vitamin D analogs KH1060 and MC903, but not to unrelated agents (all-trans retinoic acid, paclitaxel, serum/glucose starvation, or the isoflavone, pomiferin). These results suggest that 1,25D3-MARRS receptor expression interferes with the growth inhibitory activity of 1,25(OH)2D3 in breast cancer cells, possibly through the nuclear VDR. Further research should examine the potential for pharmacological or natural agents that modify 1,25D3-MARRS expression or activity as anticancer agents.

Mechanism of 24,25-dihydroxyvitamin D3-mediated inhibition of rapid, 1,25-dihydroxyvitamin D3-induced responses: role of reactive oxygen species.

In intestine, 24,25(OH)(2)D(3), which is made under conditions of calcium-, phosphate-, and 1,25(OH)(2)D(3) sufficiency, inhibits the stimulatory actions of 1,25(OH)(2)D(3) on phosphate and calcium absorption. In the current work, we provide evidence that 24,25(OH)(2)D(3)-mediated signal transduction occurs mechanistically through increased H(2)O(2) production which involves binding of 24,25(OH)(2)D(3) to catalase and resultant decreases in enzyme activity. Physiological levels of H(2)O(2) mimicked the action of 24,25(OH)(2)D(3) on inhibiting 1,25(OH)(2)D(3)-stimulated phosphate uptake in isolated enterocytes. Moreover, the molecular basis of such inhibition was suggested by the presence of two thioredoxin domains in the 1,25D(3)-MARRS protein/ERp57: Exposure of cells to either 24,25(OH)(2)D(3) or H(2)O(2) gradually reduced 1,25(OH)(2)D(3) binding to 1,25D(3)-MARRS protein, between 10 and 20 min of incubation, but not to VDR. Feeding studies with diets enriched in the antioxidants vitamins C and E showed that net phosphate absorption in vivo nearly doubled relative to chicks on control diet. Antioxidant diets also resulted in increased [(3)H]1,25(OH)(2)D(3) binding to both 1,25D(3)-MARRS and VDR, suggesting benefits to both transcription- and membrane-initiated signaling pathways. Intriguingly, phosphorous content of bones from birds on antioxidant diets was reduced, suggesting increased osteoclast activity. Because mature osteoclasts lack VDR, we analyzed a clonal osteoclast cell line by RT-PCR and found it contained the 1,25D(3)-MARRS mRNA. The combined data provide mechanistic details for the 1,25(OH)(2)D(3)/24,25(OH)(2)D(3) endocrine system, and point to a role for the 1,25D(3)-MARRS protein as a redox-sensitive mediator of osteoclast activity and potential therapeutic target.