P29 Bivalent effects of hydrogen sulfide on IL-6 and IL-1β expression in the human monocyte cell line U937

Abstract

Background and objectives Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disorder, primarily affecting the articular structures and synovial membranes of multiple joints. Fibroblast-like synoviocytes and macrophages are key players in the development and progression of RA. The cytokines IL-6 and IL-1β play a crucial role in the degradation of cartilage and bone. IL-1β is expressed as a propeptide and is cleaved by caspase-1 into the mature form to be secreted into the extracellular space. The role of hydrogen sulfide (H2S) in inflammatory processes is controversial, with both pro- and anti-inflammatory effects discussed. Therefore, we investigated the effects of the H2S donor sodium hydrogen sulfide (NaHS) on IL-6 and IL-1β expression in the human leukemic monocyte cell line U937. Methods U937 cells were differentiated for 24 h with phorbol 12-myristate 13-acetate (PMA) before being stimulated for 6 and 24 h with lipopolysacharide (LPS) in the absence or presence of NaHS (0.125–1.0 mM). Intracellular and extracellular levels of IL-6 and IL-1β were quantified by enzyme-linked immunosorbent assays (ELISAs). Results H2S inhibited LPS-induced IL-6 expression in U937 cells in a dose-dependent manner. Incubation of the cells with LPS plus NaHS (1.0 mM) reduced IL-6 production at about 40–50%. Similar inhibitory effects could also be seen when the cells were stimulated with LPS for 24 h. Vice versa, significant inhibition of IL-6 expression was also observed when NaHS was added to the cells 6 h post-LPS treatment although this inhibitory effect was much less marked. In contrast to IL-6, the release of IL-1β was significantly induced when the cells were stimulated with LPS plus NaHS. In cells stimulated with LPS without NaHS, IL-1β secretion was not detectable. IL-1β release could be completely blocked by the caspase-1 inhibitor Q-VD-OPh and glibenclamide, an inhibitor of ATP-dependent K+ ion channels. Conclusion H2S showed bivalent effects on cytokine expression in LPS-stimulated U937 cells. On the one hand it significantly diminished IL-6 expression for up to 24 h, on the other hand it strongly enhanced the release of IL-1β. This let us suggest that H2S in addition to its known effect on ERK1/2 activation might also activate caspase-1 which in turn would lead to cleavage of the IL-1β propeptide.