Immunoproteomic identification and characterization of Ni²⁺-regulated proteins implicates Ni²⁺ in the induction of monocyte cell death

Nickel allergy is the most common cause of allergic reactions worldwide, with cutaneous and systemic effects potentially affecting multiple organs. Monocytes are precursors of not only macrophages but also dendritic cells, the most potent activators of nickel hypersensitivity. Monocytes are themselves important antigen-presenting cells, capable of nickel-specific T-cell activation in vivo and in vitro, in addition to being important for immediate innate immune inflammation. To elucidate early Ni²⁺-dependent inflammatory molecular mechanisms in human monocytes, a Ni²⁺-specific proteomic approach was applied. Quantitative two-dimensional (2D) differential gel electrophoresis and Delta2D software analyses coupled with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) revealed that Ni²⁺ significantly regulated 56 protein species, of which 36 were analyzed by MALDI-MS. Bioinformatics analyses of all identified proteins resulted in Ni²⁺-associated functional annotation clusters, such as cell death, metal ion binding, and cytoskeletal remodeling. The involvement of Ni²⁺ in the induction of monocyte cell death, but not T-cell death, was observed at Ni²⁺ concentrations at or above 250 μM. Examination of caspase activity during Ni²⁺-mediated cell death revealed monocytic cell death independent of caspase-3 and -7 activity. However, confocal microscopy analysis demonstrated Ni²⁺-triggered cytoskeletal remodeling and nuclear condensation, characteristic of cellular apoptosis. Thus, Ni²⁺-specific peripheral blood mononuclear cell stimulation suggests monocytic cell death at Ni²⁺ concentrations at or above 250 μM, and monocytic effects on immune regulation at lower Ni²⁺ concentrations.