Nickel compounds are prime inducers of contact allergy reactions in humans. To identify the signal transduction pathways mediating the cellular responses to nickel and to elucidate their hierarchy, we performed Affymetrix gene profiling using human primary endothelial cells, which strongly respond to nickel stimulation. Overall, we found 258 significantly modulated transcripts, comprising 140 up-regulated and 118 down-regulated genes. The bulk of those genes were identified as targets of two distinct signaling cascades, the IKK2/NF-B pathway and a proangiogenic pathway mediated by HIF-1␣, which accumulates upon exposure to nickel. Using dominant-interfering mutants and retroviral RNA interference technology, we demonstrate that both pathways act independently to regulate expression of nonoverlapping gene pools. NF-B activation mediates most of the proinflammatory responses to nickel. Nickel-dependent HIF-1␣ activation primarily modulates expression of genes involved in proliferation, survival, metabolism, and signaling, albeit the induction of some proinflammatory nickel-response genes, most prominently IL-6, which we identified as novel bona fide HIF-1␣ target in this study, is also critically dependent on this pathway. Furthermore, we provide evidence that transactivation of both transcription factors partially depends on p38 MAPK activation that contributes to the intensity of at least some target genes. Taken
E ndothelial cells (ECs),3 located between blood and tissue compartments are important components of the innate immune system. Their activation by endogenous or environmental factors is a key event in the initiation of inflammatory responses. A strong activator of endothelium is the transition metal nickel that is widely distributed in a multitude of products. Contact with nickel is potentially hazardous for health: nickel compounds not only act as carcinogens in humans and animals (1) but also represent potent allergens (haptens). In industrialized countries, nickel is the most frequent cause for contact hypersensitivity reactions (2) with an estimated rate of 39% of young women being sensitized (3). Nickel is furthermore of great relevance in the context of biocompatibility of cardiovascular stents and orthopedic and dental biomedical alloys (4 -6). Nickel ions exert proinflammatory and irritant properties, which, in addition to their sensitizing capacity, provide a "second signal" in hypersensitivity reactions (7).Studying primary ECs, we earlier demonstrated that divalent nickel ions are capable of directly inducing endothelial adhesion molecules such as ICAM-1, VCAM-1, and E-selectin (8). Further analysis revealed that transcription of these genes is mediated via the IKK2/IB␣/NF-B signal transduction pathway, which is activated in response to soluble nickel compounds but not to other divalent cations (9, 10). NF-B activation is initiated by IKK2-mediated phosphorylation and subsequent proteasomal degradation of IB proteins, which allows nuclear translocation and binding of the transcription factor...