Activation of signaling pathways is considered to be critical for acute lung injury; however, only a few in vivo
studies have addressed which signaling factors are involved in the process. In previous studies, we have found that ablation of metallothionein-nitric oxide signaling (Wesselkamper et al., 2006
) and macrophage stimulating 1 receptor (c-met-related tyrosine kinase) (McDowell et al., 2002
) increases susceptibility to nickel-induced acute lung injury in mice. Conversely, transgenic expression of transforming growth factor α (Hardie et al., 2002
) and keratinocyte growth factor 7 (Tichelaar et al., 2007
) decreases susceptibility to nickel-induced acute lung injury in mice. Here, we have identified MAP3K1 as one of the targets that could be augmented to reduce acute lung injury because MAP3K1 ablation reduces mean survival time. MAP3K1 protein is expressed in mouse lung in the airway epithelium and blood vessel; its ablation therefore likely impairs the ability of the airway to counteract nickel toxicity.
MAP3K2, another MAP3K sharing similar activities with MAP3K1 in downstream pathway induction, is also highly expressed in the airway epithelium (Fanger et al., 1997
; Schlesinger et al., 1998
). It is possible that MAP3K1 and MAP3K2 are functionally redundant at least in some cells; therefore, lacking MAP3K1 causes only a 20% increase in lethality to the Map3k1ΔKD/ΔKD
mice during nickel exposure. Studies using MEFs, however, suggest that MAP3K2 may be dispensable for cytotoxicity because Map3k2−/−
cells do not differ from wild-type cells in response to nickel treatment. Interestingly, when both MAP3K1 and MAP3K2 are ablated, the Map3k1ΔKD/ΔKDMap3k2−/−
compound mutant cells are resistant to cytotoxicity. It is worth noting that molecular responses taking place in fibroblasts may not be quite the same as those in lung epithelial cells; hence, the precise role of MEKK2 in nickel-induced acute lung injury may have to be evaluated in vivo
, cell survival is inversely related to phospho-JNK levels in response to nickel; however, this relationship is complex. In comparison to wild-type cells, the Map3k1ΔKD/ΔKD
cells display higher JNK activation and reduced survival, whereas the Map3k1ΔKD/ΔKDMap3k2−/−
cells have lower JNK activation and are resistant to cytotoxicity. This observation is consistent with the notion that JNK activity is promoting cell death (Faris et al., 1998
; Lin and Dibling, 2002
). Such a role would be consistent with the increase in survival noted in other models of acute lung injury when JNK phosphorylation is inhibited pharmacologically (Ishii et al., 2004
; Syrkina et al., 2007
). In addition to its well-established roles in transcription factor activation (Davis, 2000
), JNK has recently been shown to mediate epigenetic histone modification in response to nickel (Ke et al., 2008
). It is hence reasonable to suggest that the JNK pathway activation by nickel may potentiate lung injury by the way of activating gene expression.
At this time, we cannot explain why the Map3k1ΔKD/ΔKD
cells differ from the Map3k1ΔKD/ΔKDMap3k2−/−
cells in regard to nickel-induced JNK activation. A likely scenario may be that nickel induces the JNK pathway through several MAP3Ks, including MAP3K1, MAP3K2, and others, and only when multiple members of the MAP3Ks are ablated, as in the Map3k1ΔKD/ΔKDMap3k2−/−
cells, there will be an obvious suppression of JNK activation. Furthermore, MAP3K1 may have additional negative feedback roles in downregulation of the JNK pathway; thus, Map3k1ΔKD/ΔKD
cells, lacking such negative inhibition, will display more abundant JNK activation during nickel exposure. A role of MAP3K1 in inhibiting its downstream pathways has been described before (Lu et al., 2002
; Witowsky and Johnson, 2003
); this function is mediated through the plant homeodomain domain located at the N-terminus of MAP3K1, which acts as an E3 ubiquitin ligase that catalyzes downstream signaling factors for degradation.
Taken together, our studies suggest that there is a complex molecular interplay between MAP3K1 and MAP3K2 in the signal transduction pathways responding to nickel exposure. The MAP3K1 exhibits an unexpected role in suppressing nickel-induced JNK activation and reducing cytotoxicity. Hence, pharmacological agents that increase MAP3K1 activity may be useful in the treatment of nickel-induced acute lung injury happening in an occupational setting.