Lipocalin2 mediates anti-inflammatory functions through the inhibition of STAT3 and activation of STAT5 signaling in bone marrow derived macrophages

Abstract

Abstract Lipocalin2 (Lcn2), an innate immune protein, is known to be significantly up-regulated during inflammatory disorders including arthritis, however, its molecular mechanisms remain to be elucidated. We have shown that Lcn2-knockout (Lcn2KO) mice developed more severe serum-induced arthritis (STA) compared to wild-type (WT) mice with significantly reduced neutrophil infiltration but considerably more macrophage migration. Therefore, we have investigated the possible role of Lcn2 in regulating macrophages. Bone marrow cells were isolated from Lcn2KO and WT mice and polarized to classical or inflammatory (M1) and alternate or anti-inflammatory (M2) macrophages. The polarization of M1 (iNOS) and M2 (arginase-1) phenotypes was confirmed by Western blot analysis. We did not observe a difference in polarization of Lcn2KO and WT macrophages suggesting that Lcn2 may not alter the polarization of macrophages. However, anti-inflammatory cytokines such as TGF-β1 and IL-10 were significantly reduced in either LPS or IC-stimulated M2 phenotypes from Lcn2KO mice compared to WT mice. In agreement, we observed systemic elevated levels of pro-inflammatory cytokines in Lcn2KO compared to WT mice during STA. Further, we’ve demonstrated that Lcn2 deficient M1 macrophages displayed increased STAT3 activation compared to WT cells. In addition, WT M2 phenotype exhibited elevated STAT5 activation. Taken together, these results suggest that Lcn2 may promote the down-regulation of pro-inflammatory cytokines and up-regulate anti-inflammatory cytokines possibly through STAT3 and STAT5 signaling pathways as a potential negative feedback loop in order to limit the extent of inflammation during autoimmune arthritis conditions.