Exogenous interleukin 6 (IL-6), synthesized at the initiation of the acute phase response, is considered responsible for signaling hepatocytes to produce acute phase proteins. It is widely posited that IL-6 is either delivered to the liver in an endocrine fashion from immune cells at the site of injury, or alternatively, in a paracrine manner by hepatic immune cells within the liver. A recent publication showed there was a muted IL-6 response in lipopolysaccharide (LPS)-injured mice when nuclear NFκB was specifically inactivated in the hepatocytes. This indicates hepatocellular signaling is also involved in regulating the acute phase production of IL-6. Herein, we present extensive in vitro and in vivo evidence that normal hepatocytes are directly induced to synthesize IL-6 mRNAs and protein by challenge with LPS, a bacterial hepatotoxin, and by HGF, an important regulator of hepatic homeostasis. As the IL-6 receptor is found on the hepatocyte, these results reveal that induction of the acute phase response can be regulated in an autocrine as well as endocrine/paracrine fashion. Further, herein we provide data indicating that following partial hepatectomy (PHx), HGF differentially regulates IL-6 production in hepatocytes (induces) versus immune cells (suppresses), signifying disparate regulation of the cell sources involved in IL-6 production is a biologically relevant mechanism that has previously been overlooked. These findings have wide ranging ramifications regarding how we currently interpret a variety of in vivo and in vitro biological models involving elements of IL-6 signaling and the hepatic acute phase response.

Extracellular superoxide dismutase (EC-SOD) is abundant in the lung and limits inflammation and injury in response to many pulmonary insults. To test the hypothesis that EC-SOD has an important role in bacterial infections, wild-type and EC-SOD knockout (KO) mice were infected with Escherichia coli to induce pneumonia. Although mice in the EC-SOD KO group demonstrated greater pulmonary inflammation than did wild-type mice, there was less clearance of bacteria from their lungs after infection. Macrophages and neutrophils express EC-SOD; however, its function and subcellular localization in these inflammatory cells is unclear. In the present study, immunogold electron microscopy revealed EC-SOD in membrane-bound vesicles of phagocytes. These findings suggest that inflammatory cell EC-SOD may have a role in antibacterial defense. To test this hypothesis, phagocytes from wild-type and EC-SOD KO mice were evaluated. Although macrophages lacking EC-SOD produced more reactive oxygen species than did cells expressing EC-SOD after stimulation, they demonstrated significantly impaired phagocytosis and killing of bacteria. Overall, this suggests that EC-SOD facilitates clearance of bacteria and limits inflammation in response to infection by promoting bacterial phagocytosis.

Apoptosis-enabling neuronal potassium efflux is mediated by an enhancement of K+ currents. In cortical neurons, increased currents are triggered by dual phosphorylation of Kv2.1 by Src and p38 at channel residues Y124 and S800. It was recently shown that a K+ current surge is also present in hepatocytes undergoing apoptosis, and that the hepatitis C virus (HCV) nonstructural protein 5A (NS5A) could inhibit Kv2.1-mediated currents and block cell death. Here, we show that NS5A1b (from HCV genotype 1b) expression in rat neurons depresses delayed rectifier potassium currents, limits the magnitude of the K+ current surge following exposure to activated microglia, and is neuroprotective. In a non-neuronal recombinant expression system, cells expressing Kv2.1 mutated at residue Y124, but not S800 mutants, are insensitive to NS5A1b-mediated current inhibition. Accordingly, NS5A1b co-expression prevents phosphorylation of wild-type Kv2.1 by Src at Y124, but is unable to inhibit p38 phosphorylation of the channel at S800. The actions of the viral protein are genotype-selective, as NS5A1a does not depress neuronal potassium currents nor inhibit Src phosphorylation of Kv2.1. Our results indicate that NS5A1b limits K+ currents following injury, leading to increased neuronal viability. NS5A1b may thus serve as a model for a new generation of neuroprotective agents.

No abstract

IL‐6 is synthesized during the general acute phase response and, the hepatic response to IL‐6 is thought to be paracrine. For general injuries, provision of IL‐6 to the liver is thought to occur via NFkappaB‐dependent synthesis and release from immune cells at the injury site; IL‐6 is discharged into the circulation, travels to the liver, and stimulates the hepatocytes. When the site of injury is the liver itself, the IL‐6 is thought to be provided by resident hepatic immune cells. As LPS injection is used to simulate systemic bacterial infections, it was surprising to note that mice with hepatocellular‐specific inactivation of NFkappaB have significantly lower hepatic IL‐6 transcripts following LPS injection. Consequently, we hypothesized that hepatocytes can also synthesize IL‐6 in response to injury. Herein, we provide irrefutable evidence that normal hepatocytes in serum‐free cultures are capable of producing both IL‐6 mRNA and protein. Furthermore, injection of LPS into animals leads to production of hepatocellular IL‐6 and, hepatocytes from regenerating livers synthesize IL‐6 following hepatic resection. Finally we demonstrate that, HGF, an important monitor of hepatic function, regulates IL‐6 production by the hepatocytes both in vitro and in vivo. Combined, these results indicate that production of acute phase proteins in response to general injury can be autocrine.