Our previous in vitro studies have demonstrated dose-dependent effects of CXCR2 ligands on hepatocyte cell death and proliferation. In the current study, we sought to determine if CXCR2 ligand concentration is responsible for the divergent effects of these mediators on liver regeneration after ischemia/reperfusion injury and partial hepatectomy.
Murine models of partial ischemia/reperfusion injury and hepatectomy were used to study the effect of CXCR2 ligands on liver regeneration.
We found that hepatic expression of the CXCR2 ligands, macrophage inflammatory protein-2 (MIP-2) and keratinocyte-derived chemokine (KC), was significantly increased after both I/R injury and partial hepatectomy. However, expression of these ligands after I/R injury was 30-100-fold greater than after hepatectomy. Interestingly, the same pattern of expression was found in ischemic versus non-ischemic liver lobes following I/R injury with expression significantly greater in the ischemic liver lobes. In both systems, lower ligand expression was associated with increased hepatocyte proliferation and liver regeneration in a CXCR2-dependent fashion. To confirm that these effects were related to ligand concentration, we administered exogenous MIP-2 and KC to mice undergoing partial hepatectomy. Mice received a “high” dose that replicated serum levels found after I/R injury and a “low” dose that was similar to that found after hepatectomy. Mice receiving the “high” dose had reduced levels of hepatocyte proliferation and regeneration whereas the “low” dose promoted hepatocyte proliferation and regeneration.
Together, these data demonstrate that concentrations of CXC chemokines regulate the hepatic proliferative response and subsequent liver regeneration.
Emerging information implies that the Ron receptor tyrosine kinase may play a role in the inflammatory response. However, the manner in which this receptor contributes to the response is not well understood. In the present studies, we investigated the role of the Ron receptor in the acute lung inflammatory response. Wild-type and mutant mice lacking the tyrosine kinase domain of Ron (Ron TK−/−) were subjected to acute lung injury induced by intranasal administration of bacterial lipopolysaccharide (LPS). Wild-type mice showed increased lung injury after LPS administration, as determined by the leakage of albumin into the lung and by histopathological changes. Ron TK−/− mice had more than twice the amount of albumin leak and much greater thickening of the alveolar septae. Lipopolysaccharide administration caused neutrophil recruitment into the lungs, as measured by myeloperoxidase. However, Ron TK−/− mice had much higher baseline levels of myeloperoxidase, which did not increase further after LPS. Lung injury in wild-type mice occurred with activation of the transcription factor, nuclear factor κB (NF-κB), and subsequent increases in intrapulmonary generation of tumor necrosis factor α. In TK−/− mice, there was far less IκB-α and IκB-β protein and greater activation of NF-κB. This was associated with substantially increased production of tumor necrosis factor α and the nitric oxide (NO) by-product, nitrite. The data suggest that the Ron receptor tyrosine kinase plays an important regulatory role in acute inflammatory lung injury by suppressing signals leading to activation of NF-κB.
Inflammation; transcription factors; cytokines; hepatocyte growth factor–like protein; receptor tyrosine kinase
Previous studies have shown that CXC chemokines containing Glu-Leu-Arg (ELR) in their amino-terminus stimulate hepatocyte proliferation and liver regeneration after partial hepatectomy. These ELR+CXC chemokines bind to two receptors, CXCR1 and CXCR2. Previous work has shown that CXCR2 is involved in the proliferative effects of CXC chemokines. However, the function of CXCR1 during the regenerative response has not been studied. The aim of the current study was to investigate the role of CXCR1 in liver regeneration after partial hepatectomy. C57BL/6 (wild type) or CXCR1 −/− mice were subjected to 70% partial hepatectomy or sham surgery and sacrificed on day 2 and 4 after operation. There were no significant differences in liver/ body weight ratio or hepatocyte proliferation. The data suggest that CXCR1 does not mediate the proliferative effects of ELR+ CXC chemokines during liver regeneration after partial hepatectomy.
CXC chemokines mediate hepatic inflammation and injury following ischemia/reperfusion (I/R). More recently, signaling through CXC chemokine receptor-2 (CXCR2) was shown to delay liver recovery and repair after I/R injury. The chemokine receptor, CXCR1 shares ligands with CXCR2, yet nothing is known about its potential role in liver pathology. In the present study, we examined the role of CXCR1 in the injury and recovery responses to I/R using a murine model. CXCR1 expression was undetectable in livers of sham-operated mice. However, after ischemia CXCR1 expression increased 24 hours of reperfusion and was maximal after 96 hours of reperfusion. CXCR1 expression was localized largely to hepatocytes. In order to assess the function of CXCR1, CXCR2-/- mice were treated with the CXCR1/CXCR2 antagonist, repertaxin. Prophylactic treatment with repertaxin had no effect on acute inflammation or liver injury. However, when repertaxin was administered 24 hours post-reperfusion there was a significant increase in hepatocellular injury and a delay in recovery compared to control-treated mice. CXCR1-/- mice also demonstrated delayed recovery and regeneration after I/R when compared to wild-type mice. In vitro, hepatocytes from CXCR2-/- mice that were stimulated to express CXCR1 showed increased proliferation in response to ligand. Hepatocyte proliferation was decreased in CXCR1-/- mice in vivo.
This is the first report to show that CXCR1 expression is induced in hepatocytes after injury. Furthermore, the data suggest that CXCR1 has divergent effects from CXCR2 and appears to facilitate repair and regenerative responses after I/R injury.
chemokines; chemokine receptors; liver repair; hepatocyte proliferation; regeneration
Hepatic ischemia/reperfusion (I/R) injury is a principal consideration of trauma, resectional liver surgery and transplantation. Despite improvements in supportive care hepatic I/R injury continues to negatively impact patient outcomes due to significant tissue damage and organ dysfunction. CXC chemokines have been implicated as key mediators in the deleterious inflammatory cascade following hepatic I/R and also as important, beneficial regulators of liver recovery and regeneration. As such, their potential to mediate both beneficial and detrimental effects on hepatocytes makes them a key target for therapy. Herein, we provide a review of the inflammatory mechanisms of hepatic I/R injury, with a focus on the divergent functions of CXC chemokines in this response compared to other liver insults, and offer an explanation of this apparent paradox.
MEDLINE and PubMed
CXC chemokines are key mediators of both the inflammatory response to hepatic I/R as well as the recovery from this injury. Their contrasting functions in the regeneration of liver mass after an ischemic insult indicates that therapeutic manipulation of these mediator pathways should differ depending on the surgical milieu.
Ischemia/reperfusion; chemokines; inflammation; liver regeneration
Our previous work suggested an important role for the peptidyl-prolyl isomerase, Pin1, in hepatic NF-κB activation and liver injury during ischemia/reperfusion (I/R). In this study, we sought to determine the function of Pin1 in the injury response to hepatic I/R.
Wild-type and Pin1-/- mice were subjected to partial hepatic I/R. In addition, hepatocytes and Kupffer cells were isolated from these mice.
Pin1-/- mice had reduced hepatic NF-κB activation and more liver injury after I/R than wild-type mice. The increased injury was not a result of enhanced inflammation as Pin1-/- mice had the same level of proinflammatory cytokine production and less neutrophil accumulation in the liver. The reduced NF-κB activation was not a result of a defect in nuclear translocation of NF-κB. In fact, hepatic nuclear p65 protein expression was higher in Pin1-/- mice than wild-type mice. This suggests that Pin1 is important for NF-κB-DNA binding. This effect was specific to hepatocytes as isolated Kupffer cells from wild-type and Pin1-/- mice were identical in their activation of NF-κB and production of cytokines after stimulation. In contrast, hepatocytes stimulated with TNFα had greatly reduced NF-κB activation, reduced production of the CXC chemokine, MIP-2, and increased cell death.
These data suggest that Pin1 is a critical regulator of NF-κB activation in hepatocytes and its role in these cells appears to confer direct protective effects.
liver; hepatocytes; ischemia/reperfusion; Pin1; NF-κB
Hepatic ischemia/reperfusion (I/R) leads to liver injury and dysfunction through the initiation of a biphasic inflammatory response that is regulated by the transcription factor, NF-κB. We have previously shown that there is an age-dependent difference in the injury response to hepatic I/R in mice that correlates with divergent activation of NF-κB such that young mice have greater NF-κB activation, but less injury than old mice. In the present study, we investigated the mechanism by which age alters the activation of NF-κB in the liver during I/R. Young (4-5 weeks) and old (12-14 months) mice underwent partial hepatic ischemia/reperfusion. Livers were obtained for RNA microarray analysis and protein expression assays. Using microarray analysis, we identified age-dependent differences in the expression of genes related to protein ubiquitinylation and the proteasome. In old mice, genes that are involved in the ubiquitin-proteasome pathway were significantly down-regulated during I/R. Consistent with these findings, expression of a critical proteasome subunit, non-ATPase 4 (PSMD4), was reduced in old mice. Expression of the NF-κB-inhibitory protein, IκBα, was increased in old mice and was greatly phosphorylated and ubiquitinylated. The data provide strong evidence that the age-related defect in hepatic NF-κB signaling during I/R is a result of decreased expression of PSMD4, a proteasome subunit responsible for recognition and recruitment of ubiquitinylated substrates to the proteasome. It appears that decreased PSMD4 expression prevents recruitment of phosphorylated and ubiquitinylated IκBα to the proteasome, resulting in a defect in NF-κB activation.
liver injury; inflammation; ageing; genomics; microarray
CXC chemokines and their receptor, CXCR2, are important components of the hepatic inflammatory response to ischemia/reperfusion. However, direct effects of CXC chemokines on hepatocytes during this response have not been studied. Wild-type and CXCR2-/- mice were subjected to 90 minutes of partial hepatic ischemia followed by up to 96 hours of reperfusion. CXCR2-/- mice had significantly less liver injury at all reperfusion times compared to wild-type mice. Early neutrophil recruitment (12 hours) was diminished in CXCR2-/- mice, but within 24 hours was the same as wild-type mice. Hepatocyte proliferation and regeneration was accelerated in CXCR2-/- mice compared to wild-type mice. These effects were associated with increased activation of NF-κB and STAT3, despite there being no difference in the expression of proliferative factors such as TNFα, IL-6, and HGF. To establish whether the accelerated proliferation and regeneration observed in CXCR2-/- mice was due to effects on hepatocytes rather than just a generalized decrease in acute inflammatory injury, mice were treated with the CXCR2 antagonist, SB225002, after neutrophil recruitment and injury were maximal (24 hours after reperfusion). SB225002 treatment increased hepatocyte proliferation and regeneration in a manner identical to that observed in CXCR2-/- mice. Treatment of primary wild-type hepatocytes with MIP-2 showed that low concentrations protected against cell death whereas high concentrations induced cell death. These effects were absent in hepatocytes from CXCR2-/- mice.
The data suggest that hepatocyte CXCR2 regulates proliferation and regeneration after I/R injury and reveal important differences in the role of this receptor in liver regeneration and repair induced under different conditions that may be related to ligand concentration.
liver injury; chemokines; regeneration; inflammation; proliferation
Hepatic ischemia/reperfusion (I/R) injury is a complication of liver surgery, transplantation and shock and is known to be age-dependent. Our laboratory has recently shown that peroxisome proliferator-activated receptor-gamma (PPARγ) is downregulated during hepatic ischemia and that this exacerbates injury. Here we examined whether activation of PPARγ during ischemia was age-dependent. Male mice of different ages (young: 4–5 weeks; adult: 10–12 weeks; old: 10–12 months) were subjected to up to 90 minutes of hepatic ischemia. PPARγ activation occurred throughout ischemia in young mice, whereas activation in adult and old mice was lost after 30 minutes. No significant differences were noted in PPARγ ligand expression amongst the age groups. However, in young mice we observed a predominance of PPARγ1 in the nucleus, whereas in old mice this isoform remained largely in the cytoplasm. Finally, the degree of PPARγ activation was associated with autophagy in the liver, a mechanism of self-preservation.
PPARγ activation is prolonged in young mice as compared to older mice. This appears to be mediated by a selective retention of PPARγ1 in the nucleus and is associated with increased autophagy. The data suggest that PPARγ activation is an important component of the age-dependent response to hepatic I/R injury.
liver injury; transcription; autophagy; hepatocytes
Sepsis patients may die either from an overwhelming systemic immune response and/or from an immunoparalysis-associated lack of anti-bacterial immune defence. We hypothesized that bacterial superantigen-activated T cells may be prevented from contribution into anti-bacterial response due to the inhibition of their effector functions by the hypoxia inducible transcription factor (HIF-1α) in inflamed and hypoxic areas.
Using the Cre-lox-P-system we generated mice with a T–cell targeted deletion of the HIF-1α gene and analysed them in an in vivo model of bacterial sepsis. We show that deletion of the HIF-1α gene leads to higher levels of pro-inflammatory cytokines, stronger anti-bacterial effects and much better survival of mice. These effects can be at least partially explained by significantly increased NF-κB activation in TCR activated HIF-1 α deficient T cells.
T cells can be recruited to powerfully contribute to anti-bacterial response if they are relieved from inhibition by HIF-1α in inflamed and hypoxic areas. Our experiments uncovered the before unappreciated reserve of anti-bacterial capacity of T cells and suggest novel therapeutic anti-pathogen strategies based on targeted deletion or inhibition of HIF-1 α in T cells.