Iron is essential for many cellular processes and is required by bacteria for replication. To acquire
iron from the host, pathogenic Gram-negative bacteria secrete
siderophores, including
enterobactin (Ent). However, Ent is bound by the host
protein lipocalin 2 (Lcn2), preventing bacterial reuptake of aferric or ferric Ent. Furthermore, the combination of Ent and Lcn2 (Ent+Lcn2) leads to enhanced secretion of
interleukin-8 (IL-8) compared to that induced by either stimulus alone. Modified or structurally distinct
siderophores, including
yersiniabactin (Ybt) and glycosylated Ent (GlyEnt, or
salmochelin), deliver
iron to bacteria despite the presence of Lcn2. We hypothesized that the robust immune response to Ent and Lcn2 requires
iron chelation rather than the Ent+Lcn2 complex itself and also can be stimulated by Lcn2-evasive
siderophores. To test this hypothesis, cultured respiratory epithelial cells were stimulated with combinations of purified
siderophores and Lcn2 and analyzed by gene expression microarrays, quantitative PCR, and
cytokine immunoassays. Ent caused HIF-1α
protein stabilization, induced the expression of genes regulated by
hypoxia-inducible factor 1α (HIF-1α), and repressed genes involved in cell cycle and DNA replication, whereas Lcn2 induced expression of proinflammatory
cytokines.
Iron chelation by excess Ent or Ybt significantly increased Lcn2-induced secretion of
IL-8,
IL-6, and CCL20. Stabilization of HIF-1α was sufficient to enhance Lcn2-induced
IL-6 secretion. These data indicate that respiratory epithelial cells can respond to bacterial
siderophores that evade or overwhelm Lcn2 binding by increasing proinflammatory
cytokine production.