Growth Hormone Stops Excessive Inflammation After Partial Hepatectomy, Allowing Liver Regeneration and Survival Through Induction of H2-Bl/HLA-G.

BACKGROUND AND AIMS: Growth hormone (GH) is important for liver regeneration after partial hepatectomy (PHx). We investigated this process in C57BL/6 mice that express different forms of the GH receptor (GHR) with deletions in key signaling domains. APPROACH AND RESULTS: PHx was performed on C57BL/6 mice lacking GHR (Ghr-/- ), disabled for all GH-dependent Janus kinase 2 signaling (Box1-/- ), or lacking only GH-dependent signal transducer and activator of transcription 5 (STAT5) signaling (Ghr391-/- ), and wild-type littermates. C57BL/6 Ghr-/- mice showed striking mortality within 48 hours after PHx, whereas Box1-/- or Ghr391-/- mice survived with normal liver regeneration. Ghr-/- mortality was associated with increased apoptosis and elevated natural killer/natural killer T cell and macrophage cell markers. We identified H2-Bl, a key immunotolerance protein, which is up-regulated by PHx through a GH-mediated, Janus kinase 2-independent, SRC family kinase-dependent pathway. GH treatment was confirmed to up-regulate expression of the human homolog of H2-Bl (human leukocyte antigen G [HLA-G]) in primary human hepatocytes and in the serum of GH-deficient patients. We find that injury-associated innate immune attack by natural killer/natural killer T cell and macrophage cells are instrumental in the failure of liver regeneration, and this can be overcome in Ghr-/- mice by adenoviral delivery of H2-Bl or by infusion of HLA-G protein. Further, H2-Bl knockdown in wild-type C57BL/6 mice showed elevated markers of inflammation after PHx, whereas Ghr-/- backcrossed on a strain with high endogenous H2-Bl expression showed a high rate of survival following PHx. CONCLUSIONS: GH induction of H2-Bl expression is crucial for reducing innate immune-mediated apoptosis and promoting survival after PHx in C57BL/6 mice. Treatment with HLA-G may lead to improved clinical outcomes following liver surgery or transplantation.

TLR3 drives IRF6-dependent IL-23p19 expression and p19/EBI3 heterodimer formation in keratinocytes.

Interferon regulatory factor (IRF) family members impart cell-type specificity to toll-like receptor (TLR) signalling, and we recently identified a role for IRF6 in TLR2 signalling in epithelial cells. TLR3 has a well-characterized role in wound healing in the skin, and here, we examined TLR3-dependent IRF6 functions in human keratinocytes. Primary keratinocytes responded robustly to the TLR3 agonist poly(IC) with upregulation of mRNAs for interferon-ß (IFN-ß), the interleukin-12 (IL-12) family member IL-23p19 and the chemokines IL-8 and chemokine (C-C motif) ligand 5 (CCL5). Silencing of IRF6 expression enhanced poly(IC)-inducible IFN-ß mRNA levels and inhibited poly(IC)-inducible IL-23p19 mRNA expression in primary keratinocytes. Consistent with these data, co-transfection of IRF6 increased poly(IC)-inducible IL-23p19 promoter activity, but inhibited poly(IC)-inducible IFN-ß promoter activity in reporter assays. Surprisingly, poly(IC) did not regulate IL-12p40 expression in keratinocytes, suggesting that TLR3-inducible IL-23p19 may have an IL-23-independent function in these cells. The only other IL-12 family member that was strongly poly(IC) inducible was EBI3, which has not been shown to heterodimerize with IL-23p19. Both co-immunoprecipitation and proximity ligation assays revealed that IL-23p19 and EBI3 interact in cells. Co-expression of IL-23p19 and EBI3, as compared with IL-23p19 alone, resulted in increased levels of secreted IL-23p19, implying a functional role for this heterodimer. In summary, we report that IRF6 regulates a subset of TLR3 responses in human keratinocytes, including the production of a novel IL-12 family heterodimer (p19/EBI3). We propose that the TLR3-IRF6-p19/EBI3 axis may regulate keratinocyte and/or immune cell functions in the context of cell damage and wound healing in the skin.

Class IIa Histone Deacetylases Drive Toll-like Receptor-Inducible Glycolysis and Macrophage Inflammatory Responses via Pyruvate Kinase M2.

Histone deacetylases (HDACs) drive innate immune cell-mediated inflammation. Here we identify class IIa HDACs as key molecular links between Toll-like receptor (TLR)-inducible aerobic glycolysis and macrophage inflammatory responses. A proteomic screen identified the glycolytic enzyme pyruvate kinase M isoform 2 (Pkm2) as a partner of proinflammatory Hdac7 in murine macrophages. Myeloid-specific Hdac7 overexpression in transgenic mice amplifies lipopolysaccharide (LPS)-inducible lactate and promotes a glycolysis-associated inflammatory signature. Conversely, pharmacological or genetic targeting of Hdac7 and other class IIa HDACs attenuates LPS-inducible glycolysis and accompanying inflammatory responses in macrophages. We show that an Hdac7-Pkm2 complex acts as an immunometabolism signaling hub, whereby Pkm2 deacetylation at lysine 433 licenses its proinflammatory functions. Disrupting this complex suppresses inflammatory responses in vitro and in vivo. Class IIa HDACs are thus pivotal intermediates connecting TLR-inducible glycolysis to inflammation via Pkm2.

Growth hormone receptor; mechanism of action.

The growth hormone receptor has been an archetype for ligand-induced receptor dimerisation in cytokine receptor signalling. However, we now know that it exists as a constitutive dimer and is activated by a reorganisation of receptor subunits as a result of asymmetric placement of two receptor binding sites on the hormone monomer. This review highlights several topics including: current models of receptor activation; recent advances in the understanding of GH signalling demonstrating that ligand-induced signalling activates Src/ERK pathway in parallel to the classical JAK2-STAT5 signalling; and the nuclear localised growth hormone receptor correlates with high proliferation status and carcinogenesis.

Tmem2 Regulates Embryonic Vegf Signaling by Controlling Hyaluronic Acid Turnover.

Angiogenesis is responsible for tissue vascularization during development, as well as in pathological contexts, including cancer and ischemia. Vascular endothelial growth factors (VEGFs) regulate angiogenesis by acting through VEGF receptors to induce endothelial cell signaling. VEGF is processed in the extracellular matrix (ECM), but the complexity of ECM control of VEGF signaling and angiogenesis remains far from understood. In a forward genetic screen, we identified angiogenesis defects in tmem2 zebrafish mutants that lack both arterial and venous Vegf/Vegfr/Erk signaling. Strikingly, tmem2 mutants display increased hyaluronic acid (HA) surrounding developing vessels. Angiogenesis in tmem2 mutants was rescued, or restored after failed sprouting, by degrading this increased HA. Furthermore, oligomerized HA or overexpression of Vegfc rescued angiogenesis in tmem2 mutants. Based on these data, and the known structure of Tmem2, we find that Tmem2 regulates HA turnover to promote normal Vegf signaling during developmental angiogenesis.

Mechanism of activation of protein kinase JAK2 by the growth hormone receptor.

Signaling from JAK (Janus kinase) protein kinases to STAT (signal transducers and activators of transcription) transcription factors is key to many aspects of biology and medicine, yet the mechanism by which cytokine receptors initiate signaling is enigmatic. We present a complete mechanistic model for activation of receptor-bound JAK2, based on an archetypal cytokine receptor, the growth hormone receptor. For this, we used fluorescence resonance energy transfer to monitor positioning of the JAK2 binding motif in the receptor dimer, substitution of the receptor extracellular domains with Jun zippers to control the position of its transmembrane (TM) helices, atomistic modeling of TM helix movements, and docking of the crystal structures of the JAK2 kinase and its inhibitory pseudokinase domain with an opposing kinase-pseudokinase domain pair. Activation of the receptor dimer induced a separation of its JAK2 binding motifs, driven by a ligand-induced transition from a parallel TM helix pair to a left-handed crossover arrangement. This separation leads to removal of the pseudokinase domain from the kinase domain of the partner JAK2 and pairing of the two kinase domains, facilitating trans-activation. This model may well generalize to other class I cytokine receptors.