Ni(II) and LPS Trigger Nrf2 Accumulation in Peripheral Blood Monocytes

Objectives: Although the full effects of Ni(II) on oral tissues are not known, Ni(II) amplifies the secretion of pro-inflammatory cytokines from cells such as monocytes. Previous studies in THP1 monocytes suggest that the mechanisms governing this amplification involve changes in regulation of nuclear events in the NFkB signaling pathway. Nrf2 is a transcription factor that regulates expression of redox management proteins that are known to modulate NFkB nuclear activity. In this study, we assessed whether Ni(II) alters Nrf2 signaling in human peripheral blood monocytes (PBM), extending our previous work in THP1 monocytes. Methods: PBM (Cambrex) were exposed to Ni(II) (0, 30, 50 µM, n = 2) for 72 h, then activated with LPS (P. ging, 2 µg/mL) for 30 min or 6 h. Immunoblots of proteins from whole cell lysates or cytosolic/nuclear fractions were probed for Nrf2, NFkBp65, IkBa, and b-actin, then analyzed using Odyssey infrared scanning and quantified. Controls received no Ni(II) or no LPS. Results: LPS controls verified that Ni(II) activated IkBa degradation and NFkBp65 nuclear translocation at 30 min, but that pathway activation was largely complete by 6 h as expected. Ni(II) had no effect on these events, as expected. However Ni(II) increased whole-cell levels of Nrf2, which was enhanced by 2-3-fold after 6 h (but not 30 min) of LPS activation (sig., p < 0.05). Nuclear accumulation of Nrf2 also was enhanced 2-3 fold by the combination of 50 µM Ni(II) and LPS for 6 h (p < 0.05). These effects were congruent with results reported for human THP1 monocytes. Conclusion: Ni(II) activates Nrf2 signaling, but its primary effect is to enhance LPS-induced whole-cell levels and nuclear accumulation of Nrf2. These effects may be one mechanism by which Ni(II) enhances NFkB nuclear activity and LPS-induced secretion of pro-inflammatory cytokines. (funding: MCG Starr Program; MCG Dental Research)