title: Toll-like receptor 4 and myeloid differentiation factor 88 provide mechanistic insights into the cause and effects of interleukin-6 activation in mouse liver regeneration.
abstract: Partial hepatectomy (PH) consistently results in an early increase of circulating interleukin-6 (IL-6), which is thought to play a major role in liver regeneration. Activation of this cytokine after PH requires the adaptor protein, MyD88, but the specific MyD88-related receptors involved remain unidentified. It is also unknown whether the magnitude of IL-6 elevation determines the extent of subsequent hepatocyte proliferation. Here, we uncovered artifacts in the assessment of circulating IL-6 levels when using cardiac puncture in mice after PH. By using retro-orbital bleed sampling, we show that the circulating levels of IL-6 after PH were not directly correlated with the extent of hepatocyte DNA synthesis in individual mice. The IL-6 increase after PH was attenuated in all lipopolysaccharide-hyporesponsive mouse strains studied (e.g., C3H/HeJ, Tlr4 null, Cd14 null, Tlr2,4,9 null, and Tlr2,4-Caspase1 null) and was severely abrogated in Myd88 null mice. Despite attenuated IL-6 levels, Tlr4 null mice showed normal signaling downstream of IL-6 and normal hepatocyte proliferation. In contrast, Myd88 null mice showed severe impairments in signal transducer and activator of transcription 3 phosphorylation and Socs3 induction, but had enhanced and prolonged extracellular signal-related kinase 1 and 2 phosphorylation in the first 6 hours after PH. Unexpectedly, these changes were associated with accelerated initiation of hepatocyte proliferation, as assessed by hepatocyte bromodeoxyuridine incorporation, phospho-histone H3 immunostaining, and cyclin E and A protein expression.
  
  
  CONCLUSIONS
  TLR-4 signaling contributes to IL-6 activation after PH, but the Tlr4-independent component appears sufficient for ensuring intact signaling downstream of IL-6. The lack of correlation between IL-6 levels and hepatocyte proliferation after PH, and the accelerated start of hepatocyte proliferation in Myd88 null mice despite abrogated cytokine activation, may highlight relevant antiproliferative effects of IL-6 signaling, possibly via Socs3, in the regulation of liver regeneration.
authors.affiliation: Department of Pathology, University of Washington, Seattle, WA 98195, USA.
authors.forename: Javier
authors.initials: J
authors.lastname: Vaquero
authors.forename: Jean S
authors.initials: JS
authors.lastname: Campbell
authors.forename: Jamil
authors.initials: J
authors.lastname: Haque
authors.forename: Ryan S
authors.initials: RS
authors.lastname: McMahan
authors.forename: Kimberly J
authors.initials: KJ
authors.lastname: Riehle
authors.forename: Renay L
authors.initials: RL
authors.lastname: Bauer
authors.forename: Nelson
authors.initials: N
authors.lastname: Fausto
mesh_terms: D000818:Animals; D006498:Hepatectomy; D015850:Interleukin-6 / Q000502:physiology*; D008115:Liver Regeneration / Q000502:physiology*; D051379:Mice; D053594:Myeloid Differentiation Factor 88 / Q000502:physiology*; D051197:Toll-Like Receptor 4 / Q000502:physiology*
chemical_list: D015850:Interleukin-6; D053594:Myeloid Differentiation Factor 88; D051197:Toll-Like Receptor 4