Autoregulation of 1,25-dihydroxyvitamin D synthesis in macrophage mitochondria by nitric oxide

Tissue macrophages from patients with granuloma-forming disease, most notably sarcoidosis, express a 25-hydroxyvitamin D-1-hydroxylase which can produce in vivo sufficient quantities of the active vitamin D metabolite 1,25-dihydroxyvitamin D to cause hypercalcemia. In contrast to the NADPH-dependent cytochrome P450-linked mixed function oxidase which is normally only expressed in significant quantity in proximal renal tubular cells and regulated in an endocrine fashion, the mitochondrial-based 1-hydroxylase in the macrophage [1] is stimulated in a paracrine mode by cytokines (i.e., IFN-gamma) and lipopolysaccharide (LPS) [2] requires an extracellular source of L-arginine for full basal expression and [3] can be regulated in an intracrine fashion by nitric oxide (NO). In these experiments we employed inducible nitric oxide synthase (iNOS)-free, intact mitochondria preparations from the avain macrophage-like cell line HD-11, which constitutively express the 1-hydroxylase, and nonenzymatically-generated NO to investigate NO-mediated autoregulation of the macrophage 1-hydroxylase. Sodium nitroprusside (SNP)- or S-nitroso-N-acetyl-penicillamine (SNAP)-induced up-regulation of the 1-hydroxylase required the presence of either NADPH or NADP in the reaction mixture, while NO-induced inhibition of mitochondrial 1,25-(OH)2D3 synthesis was NO-dependent and NADP/NADPH-independent. These data suggest NO has bifunctional effects on the macrophage 1-hydroxylase. At relatively high concentrations NO competes with O2 for enzyme binding, inhibiting hormone synthesis. At lower production levels, NO serves as a source of reducing equivalents for the enzyme by providing for the reduction of NADP to NADPH.