Vitamin D₃ metabolites enhance the NLRP3-dependent secretion of IL-1β from human THP-1 monocytic cells

Vitamin D3 has emerged as an important regulator of the immune system. With metabolic enzymes for vitamin D3 activation and vitamin D receptors (VDR) now identified in a variety of immune cells, the active vitamin D3 metabolite 1,25(OH)2D3, is thought to possess immunomodulatory properties. We examined whether 1,25(OH)2D3 might also enhance the NLRP3-dependent release of mature IL-1β from macrophages. PMA-differentiated THP-1 cells were stimulated with vitamin D3 metabolites and assessed for CYP27, CYP24, NLRP3, ASC, pro-caspase-1 expression by western blot and real-time qPCR as well as inflammasome activation with pro-inflammatory cytokine IL-1β release measured by ELISA. Exposure to 1,25(OH)2D3 had no effect on the basal expression levels of VDR; however, CYP27A1 transcript was suppressed and CYP24A1 transcript was substantively elevated. Both 1,25(OH)2D3 - and 25(OH)D3 induced IL-1β release from THP-1 cells, and these effects were blocked with application of the caspase-1 inhibitor YVAD and the NLRP3 inhibitors glyburide and Bay 11-7082. Interestingly, 1,25 (OH)2D3 exposure reduced NLRP3 protein expression but had no effect on ASC or pro-caspase-1 protein levels. The increase in mature IL-1β elicited by 1,25(OH)2D3 was modest compared to that found for ATP or C. difficile toxins. However, co-treatment of THP-1 cells with ATP and 1,25(OH)2D3 resulted in more IL-1β secretion than ATP or 1,25(OH)2D3 alone.