In this study,we investigated the effects of nucleolin on lipopolysaccharide(LPS)-induced activation of MAPK and NF-KappaB(NF-κB)signaling pathways and secretion of TNF-α,IL-1β and HMGB1 in THP-1 monocytes.Immunofluorescence assay and Western blotting were used to identify the nucleolin expression in cell membrane,cytoplasm and nucleus of THP-1 monocytes.Inactivation of nucleolin was induced by neutralizing antibody against nucleolin.THP-1 monocytes were pretreated with anti-nucleolin antibody for 1 h prior to LPS challenge.The irrelevant IgG group was used as control.Secretion of inflammatory mediators(TNF-α,IL-1β and HMGB1)and activation of MAPK and NF-κB/I-κB signaling pathways were examined to assess the effects of nucleolin on LPS-mediated inflammatory response.Nucleolin existed in cell membrane,cytoplasm and nucleus of THP-1 monocytes.Pretreatment of anti-nucleolin antibody significantly inhibited the LPS-induced secretion of TNF-α,IL-1β and HMGB1.P38,JNK,ERK and NF-κB subunit p65 inhibitors could significantly inhibit the secretion of IL-1β,TNF-α and HMGB1 induced by LPS.Moreover,the phosphorylation of p38,JNK,ERK and p65(or nuclear translocation of p65)was significantly increased after LPS challenge.In contrast,pretreatment of anti-nucleolin antibody could significantly inhibit the LPS-induced phosphorylation of p38,JNK,ERK and p65(or nuclear translocation of p65).However,the irrelevant IgG,as a negative control,had no effect on LPS-induced secretion of TNF-α and IL-1β and phosphorylation of p38,JNK,ERK and p65(or nuclear translocation of p65).We demonstrated that nucleolin mediated the LPS-induced activation of MAPK and NF-κB signaling pathways,and regulated the secretion of inflammatory mediators(TNF-α,IL-1β and HMGB1).