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.