Lactoferrin (LF) has been implicated in innate immunity. Here we reveal the signal transduction pathway responsible for human LF (hLF)-triggered
nuclear factor-kappaB (
NF-kappaB) activation.
Endotoxin-depleted hLF induces
NF-kappaB activation at physiologically relevant concentrations in the human monocytic
leukemia cell line, THP-1, and in mouse embryonic fibroblasts (MEFs). In MEFs, in which both
tumor necrosis factor receptor-associated factor 2 (
TRAF2) and
TRAF5 are deficient, hLF causes
NF-kappaB activation at a level comparable to that seen in wild-type MEFs, whereas TRAF6-deficient MEFs show significantly impaired
NF-kappaB activation in response to hLF.
TRAF6 is known to be indispensable in leading to
NF-kappaB activation in myeloid differentiating factor 88 (MyD88)-dependent signaling pathways, while the role of
TRAF6 in the MyD88-independent signaling pathway has not been clarified extensively. When we examined the hLF-dependent
NF-kappaB activation in MyD88-deficient MEFs, delayed, but remarkable,
NF-kappaB activation occurred as a result of the treatment of cells with hLF, indicating that both MyD88-dependent and MyD88-independent pathways are involved. Indeed, hLF fails to activate
NF-kappaB in MEFs lacking
Toll-like receptor 4 (TLR4), a unique TLR group member that triggers both MyD88-depependent and MyD88-independent signalings. Importantly, the
carbohydrate chains from hLF are shown to be responsible for TLR4 activation. Furthermore, we show that
lipopolysaccharide-induced
cytokine and
chemokine production is attenuated by intact hLF but not by the
carbohydrate chains from hLF. Thus, we present a novel model concerning the biological function of hLF: hLF induces moderate activation of TLR4-mediated innate immunity through its
carbohydrate chains; however, hLF suppresses
endotoxemia by interfering with
lipopolysaccharide-dependent TLR4 activation, probably through its
polypeptide moiety.