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. Conclusion 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.

Prolactin promotes normal liver growth, survival, and regeneration in rodents: effects on hepatic IL-6, suppressor of cytokine signaling-3, and angiogenesis

“…Because PRL reduces LPS-induced IL-6 synthesis in Kupffer cells (49), we tested whether PH-induced production of IL-6 is altered in PRLR Ϫ/Ϫ mice and may, thus, contribute to enhanced mortality. As expected (44), expression and circulating levels of IL-6 were elevated at 3 and 6 h after PH in wild-type mice (P Ͻ 0.01-0.001; Fig. 4, A and B).…”

Functional analysis of the relationship between intestinal microbiota and the expression of hepatic genes and pathways during the course of liver regeneration

“…Furthermore, LPS regulates innate immune response, which is intimately associated with liver disease and hepatic regeneration [35]. LPS binding to TLR4 for NF-kB activation, which is essential for the priming phase of liver regeneration [36]. Ampicillin-impaired liver regeneration is associated with increased of CD1d-dependent natural killer T (NKT) cells.…”

Impact of NKT Cells and LFA-1 on Liver Regeneration under Subseptic Conditions

“…This finding is supported by work of Riehle et al showing that SOCS3 negatively regulates STAT3 phosphorylation and hepatocyte proliferation [22]. Although presence of IL-6 has been clearly demonstrated to support liver regeneration, recent data suggests that IL-6-dependent signaling might also negatively regulate hepatocyte proliferation, and these data highlight a potential role of SOCS3 for liver regeneration [20, 49]. Whether NK cells are involved in this scenario is not yet fully understood.…”