Abstract
Cells of the monocyte/macrophage lineage are capable of both nitric oxide (NO) and 1,25-dihydroxyvitamin D [1, 25-(OH)2D] production through expression of inducible nitric oxide synthase (iNOS) and a putative 25-hydroxyvitamin D (25-OHD)-1-hydroxylase, respectively. We have recently reported that 1,25-(OH)2D synthesis in the chick myelomonocytic cell line HD-11 is restricted by inhibition of iNOS. In the current set of experiments, measuring nitrite, a stable water-soluble secreted metabolite of NO as an index of iNOS activity and 1,25-(OH)2D3 in lipid extracts of cells incubated with 200 nM 25-OHD3 as an index of 1-hydroxylase activity, we demonstrate that NO and 1,25-(OH)2D production by HD-11 cells are temporally related, induced by the same kinds of activating agents, and coordinately regulated. NO and 1,25-(OH)2D3 production by HD-11 cells was stimulated severalfold by the macrophage stimulators interferon-gamma and lipopolysaccharide and by an autologous, nonlipid, heat-labile factor with an apparent molecular mass approximately 10,000 daltons. As expected NO synthesis was 1) dependent upon the presence of L-arginine in the extracellular medium, 2) subject to significant stimulation by Nw-hydroxy-L-arginine, an L-arginine-derived intermediate in NO biosynthesis, and by sodium nitroprusside, a non-L-arginine-dependent source of intracellular NO, and 3) inhibited by Nw-nitro-L-arginine methyl ester, a competitive inhibitor of iNOS. At high NO production rates, induced either by high-dose lipopolysaccharide or by sodium nitroprusside exposure, there was an apparent downturn in 1,25-(OH)2D3 synthesis, suggesting functional dependence of the 1-hydroxylase on NO but ultimate inhibition of 1,25-(OH)2D3 synthetic capacity at high levels of intracellular NO production. On the basis of these results we postulate that the macrophage 25-OHD-1-hydroxylation reaction may be dependent on iNOS-generated NO as a soluble source of electrons and regulated in an autocrine mode by a macrophage-derived NO stimulatory factor and NO itself.
Published Version
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