1 alpha,25-dihydroxyvitamin D3 regulates in vivo production of the third component of complement (C3) in bone

We previously reported that 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3] specifically stimulates production of the third component of complement (C3) by murine osteoblastic cells and marrow-derived stromal cells (ST2) in vitro. In the present study we examined tissue-specific production of C3 in vivo in vitamin D-deficient mice, some of which received supplemental 1 alpha,25-(OH)2D3. Western blot analysis indicated that the C3 protein band in bone was undetectable in vitamin D-deficient mice, but became distinct 48 h after 1 alpha,25-(OH)2D3 administration. The mRNA expression of C3 in bone was also undetectable in vitamin D-deficient mice and appeared as early as 24 h after 1 alpha,25-(OH)2D3 administration. mRNA expression apparently preceded the appearance of C3 protein. In contrast, there was no significant difference in the expression of hepatic C3 mRNA among normal mice fed laboratory chow and vitamin D-deficient mice with and without 1 alpha,25-(OH)2D3 administration. The serum concentration of C3 in vitamin D-deficient mice was almost identical to that in normal mice and was unchanged after 1 alpha,25-(OH)2D3 administration. 1 alpha,25-(OH)2D3 receptor (VDR) mRNAs were detected in the kidney and intestine, whereas no appreciable mRNA expression of VDR occurred in the liver. Osteopontin mRNA was expressed in response to 1 alpha,25-(OH)2D3 in the kidney, but not in the intestine. Immunohistochemical studies showed that in normal mice, the C3 protein was located mainly in the periosteal regions of calvaria and on the surfaces of bone trabeculae in the tibial metaphyses. These results demonstrate that 1 alpha,25-(OH)2D3 tissue-specifically regulates in vivo production of C3 in bone. The production of bone C3 cannot be attributed to the presence of VDR alone, and we speculate that other tissue-specific factors are required.