Immune regulation of 25-hydroxyvitamin D-1α-hydroxylase in human monocytic THP1 cells:: Mechanisms of interferon-γ-mediated induction

Context: 25-Hydroxyvitamin D can be activated to 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] by the rate-limiting enzyme 1 alpha-hydroxylase in cells of the immune system under control of immune stimuli, such as interferon-gamma(IFN gamma). In pathological situations, such as sarcoidosis, this can lead to systemic excess of 1,25(OH)(2)D-3 and hypercalcemia. Objective: The aim of this study was to elucidate the intracellular pathways used by the immune system to tightly regulate 1,25(OH)(2)D-3 production in monocytes and macrophages. Design: Human monocytic THP1-cells were differentiated and activated by IFN gamma and a secondary stimulus, such as lipopolysaccharide or phorbol myristate acetate. 1 alpha-Hydroxylase mRNA levels were quantified by real-time RT-PCR. The involvement of different signaling pathways in the regulation of this enzyme was investigated using specific pharmacological inhibitors, whereas phosphorylation of signal transducer and activator of transcription 1 alpha and CCAAT/enhancer binding protein beta was investigated by Western blotting. Results: In undifferentiated monocytic THP1 cells, IFN gamma needs to be combined with a second stimulus, such as lipopolysaccharide, to induce 1 alpha-hydroxylase. In contrast, in phorbol myristate acetate-differentiated THP1 macrophages, IFN gamma alone induces 1 alpha-hydroxylase and to much higher levels. Many different signaling pathways need to be activated concurrently to allow immune-mediated 1 alpha-hydroxylase up-regulation. We show involvement of the Janus kinase-signal transducer and activator of transcription, MAPK, and nuclear factor-kappa B pathways, with a crucial role for the transcription factor CCAAT/enhancer binding protein beta. Furthermore, histone remodeling involving histone deacetylases and histone acetylase p300 is required. Conclusion: The present findings indicate that IFN gamma-mediated 1,25(OH)(2)D-3 production, as observed in granulomatous diseases such as sarcoidosis, will take place only under conditions where the necessary other signaling pathways are also activated.