Autocrine control of vitamin D metabolism in synovial cells from arthritic patients

OBJECTIVE This study was designed to investigate whether 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), produced by activated synovial fluid macrophages, promotes its own catabolism by upregulating vitamin D-24-hydroxylase (24-OHase) in synovial fibroblasts through a vitamin D receptor (VDR) mediated mechanism. METHODS Synovial macrophages and fibroblasts were derived from patients with rheumatoid arthritis. Expression of VDR and 24-OHase mRNAs was determined using in situ hybridisation. Vitamin D hydroxylase activity was determined by incubating cells with [3H]-25-(OH)D3, or [3H]-1,25-(OH)2D3, and metabolite synthesis quantified using high performance liquid chromatography. RESULTS 1,25-(OH)2D3increased expression of mRNA for both VDR and 24-OHase in fibroblasts by approximately threefold over 24 hours. 1,25-(OH)2D3 increased fibroblast 24-OHase activity, yielding 24-hydroxylated, and more polar, metabolites. In co-culture, fibroblasts were able to catabolise macrophage derived 1,25-(OH)2D3. CONCLUSIONS 1,25-(OH)2D3is produced by macrophages in vitro at biologically relevant concentrations and can increase its own catabolism by synovial fibroblasts; this effect is probably mediated via upregulation of both synovial fibroblast VDR and 24-OHase.