Evidence for IL‐6/STAT3‐independent induction of lipocalin‐2 in the liver of mice infected with Escherichia coli
Authors: Aurelie Gineste, Patricia Martin, Eric Oswald, Hélène Coppin, Marie‐Paule Roth
Journal: Hepatology (2015)

Supported by grants from the FRM (DEQ2000326528) and ANR (ANR‐13‐BSV3‐0015‐01). Potential conflict of interest: Nothing to report. Author names in bold designate shared co‐first authorship. To the Editor: We read with great interest the article by Xu et al.,1 who demonstrated that hepatocytes are responsible for more than 90% of the highly elevated serum level of lipocalin‐2 (LCN2) after bacterial infection with Klebsiella pneumonia or Escherichia coli. During infection, bacteria acquire much of their iron from the host by synthesizing siderophores that scavenge iron and transport it to the pathogen. Upon encountering bacteria, the Toll‐like receptors on immune cells and hepatocytes stimulate the transcription, translation, and secretion of LCN2. Secreted LCN2 then limits bacterial growth by sequestrating the iron‐laden polycarboxylate‐type siderophores.2 Although Xu et al.1 conclude that hepatic interleukin‐6 (IL‐6) receptor and signal transducer and activator of transcription 3 (STAT3) activation play a key role in stimulating LCN2 production by hepatocytes after bacterial infection, they do not provide data supporting this mechanism of LCN2 induction after E. coli infection. We have shown that lipopolysaccharide (LPS) strongly induces Lcn2 messenger RNA in the liver of both Il‐6–/– and wild‐type (WT) mice (Fig. 1A), which suggests that induction of Lcn2 by LPS is not Il‐6‐dependent. In addition, despite the fact that Stat3 is not activated 4 hours after infection with E. coli (footpad injection of 108 colony‐forming units) in Il6–/– as it is in WT mice (Fig. 1B), Lcn2 messenger RNA is similarly induced in the liver of both WT and Il6–/– mice (Fig. 1C). As expected from this strong induction of Lcn2, WT and Il‐6–/– show comparable bacterial clearance in the blood 4 and 28 hours after infection (Fig. 1D). Altogether, these data demonstrate that IL‐6 activation of the STAT3 signaling pathway is unlikely to be the primary determinant of LCN2 induction after E. coli infection.Figure 1: (A) Expression of Lcn2 was measured by quantitative polymerase chain reaction in the liver of Il‐6 –/– and WT mice sacrificed at different time points (n = 3‐6/time point) after an LPS challenge (1 µg/g). Analysis of variance was used to test planned contrasts in Lcn2 expression. At each time point, Lcn2 expression was significantly induced by LPS in both WT and Il‐6 –/– mice (95% confidence intervals for the fold changes do not include 1). Notably, Lcn2 did not respond differently to LPS in Il‐6 –/– relative to WT mice (not significant, P > 0.05). (B) Immunoblots show that E. coli (108 colony‐forming units) transiently activates Stat3 in the liver of WT, but not Il6 –/–, mice. (C) However, induction of Lcn2 messenger RNA is not significantly lower in Il6 –/– than in WT mice (n = 6‐9/time point; not significant, P > 0.05). (D) Escherichia coli colony‐forming units in the blood of infected WT and Il6 –/– mice are not statistically different (not significant, P > 0.05). Abbreviation: NS, not significant.Earlier studies have shown that IL‐1β is a positive regulator of LCN2 expression through activation of nuclear factor‐κB. Mutations in the IL‐1β responsive elements in the LCN2 promoter abolish IL‐1β or LPS‐induced expression of LCN2.2 Interestingly, IL‐1β negatively regulates IL‐6‐induced STAT3 activation in hepatocytes in a p38MAPK‐dependent nuclear factor‐κB–independent manner involving phosphorylation of the IL‐6 receptor subunit gp130 and its subsequent internalization and degradation.3 Thus, a critical role of IL‐1β, rather than IL‐6/STAT3, in the induction of LCN2 after E. coli infection may well explain why Lcn2 messenger RNA remains high while Il‐6‐induced Stat3 activation is suppressed 28 hours postinfection in WT mice.