Manganese-induced potentiation of in vitro proinflammatory cytokine production by activated microglial cells is associated with persistent activation of p38 MAPK

Previous studies that investigated the role of inflammation in the neurotoxicity of manganese (Mn) found that Mn enhanced the production of inflammogen (lipopolysaccharide; LPS)-induced proinflammatory cytokines such as IL-6 and TNF-α. Although we have shown that the enhanced cytokine production occurs via a NF-κB-dependent mechanism, the role of upstream kinases in this Mn-induced enhancement has not been explored. As other studies have demonstrated that p38 mitogen activated protein kinase (p38) is necessary for LPS-induced, NF-κB-dependent expression of proinflammatory cytokines, we hypothesized that Mn enhancement of LPS-induced production of IL-6 and TNF-α may be associated with p38 activation and conducted a series of experiments to address our hypothesis. We found that pre-treatment of microglial cells with a p38-inhibitor (SB203580) prevented Mn + LPS-induced production of IL-6 and TNF-α. Moreover, potentiation of IL-6 and TNF-α production, which occurred in both concurrent and sequential (3 h apart) exposures to Mn and LPS, was inhibited by inhibition of p38. Additionally, Mn exposure enhanced the phosphorylation and activity of p38 and this effect was persistent. Although p38 activity declined over time LPS-exposed cells, it persisted in cells exposed to Mn or Mn + LPS. Thus, the increased production of proinflammatory cytokines by LPS-activated microglia exposed to Mn is associated with increased and persistent activation of p38.