An inducible autocrine cascade regulates rat hepatocyte proliferation and apoptosis responses to tumor necrosis factor-alpha

Structure of the EGF receptor transactivation circuit integrates multiple signals with cell context

Transactivation of the epidermal growth factor receptor (EGFR) is thought to be a process by which a variety of cellular inputs can be integrated into a single signaling pathway through either stimulated proteolysis (shedding) of membrane-anchored EGFR ligands or by modification of the activity of the EGFR. As a first step towards building a predictive model of the EGFR transactivation circuit, we quantitatively defined how signals from multiple agonists were integrated both upstream and downstream of the EGFR to regulate extracellular signal regulated kinase (ERK) activity in human mammary epithelial cells. By using a "non-binding" reporter of ligand shedding, we found that transactivation triggers a positive feedback loop from ERK back to the EGFR such that ligand shedding drives EGFR-stimulated ERK that in turn drives further ligand shedding. Importantly, activated Ras and ERK levels were nearly linear functions of ligand shedding and the effect of multiple, sub-saturating inputs was additive. Simulations showed that ERK-mediated feedback through ligand shedding resulted in a stable steady-state level of activated ERK, but also showed that the extracellular environment can modulate the level of feedback. Our results suggest that the transactivation circuit acts as a context-dependent integrator and amplifier of multiple extracellular signals and that signal integration can effectively occur at multiple points in the EGFR pathway.

NADPH oxidase NOX1 controls autocrine growth of liver tumor cells through up-regulation of the epidermal growth factor receptor pathway

FaO rat hepatoma cells proliferate in the absence of serum through a mechanism that requires activation of the epidermal growth factor receptor (EGFR) pathway. The aim of this work was to analyze the molecular mechanisms that control EGFR activation in these and other liver tumor cells. Reactive oxygen species production is observed a short time after serum withdrawal in FaO cells, coincident with up-regulation of the NADPH oxidase NOX1. NOX1-targeted knockdown, the use of antioxidants, or pharmacological inhibition of NADPH oxidase attenuates autocrine growth, coincident with lower mRNA levels of EGFR and its ligand transforming growth factor-alpha (TGF-alpha) and a decrease in phosphorylation of EGFR. EGFR-targeted knockdown induces similar effects on cell growth and downstream signals to those observed in NOX1-depleted cells. Early NOX1 activation induces both a feedback-positive loop via an Src-ERK pathway that up-regulates its own levels, and a parallel signaling pathway through p38 MAPK and AKT resulting in EGFR and TGF-alpha up-regulation. Human hepatocellular carcinoma cell lines, but not non-tumoral hepatocytes, show autocrine growth upon serum withdrawal, which is also coincident with NOX1 up-regulation that mediates EGFR and TGF-alpha expression. The use of antioxidants, or pharmacological inhibition of NADPH oxidase, effectively attenuates autocrine growth in hepatocellular carcinoma cell lines. In summary, results presented in this study indicate that NOX1 might control autocrine cell growth of liver tumor cells through regulation of the EGFR pathway.

Networks inferred from biochemical data reveal profound differences in toll-like receptor and inflammatory signaling between normal and transformed hepatocytes

Systematic study of cell signaling networks increasingly involves high throughput proteomics, transcriptional profiling, and automated literature mining with the aim of assembling large scale interaction networks. In contrast, functional analysis of cell signaling usually focuses on a much smaller sets of proteins and eschews computation but focuses directly on cellular responses to environment and perturbation. We sought to combine these two traditions by collecting cell response measures on a reasonably large scale and then attempting to infer differences in network topology between two cell types. Human hepatocytes and hepatocellular carcinoma cell lines were exposed to inducers of inflammation, innate immunity, and proliferation in the presence and absence of small molecule drugs, and multiplex biochemical measurement was then performed on intra- and extracellular signaling molecules. We uncovered major differences between primary and transformed hepatocytes with respect to the engagement of toll-like receptor and NF-κB-dependent secretion of chemokines and cytokines that prime and attract immune cells. Overall, our results serve as a proof of principle for an approach to network analysis that is systematic, comparative, and biochemically focused. More specifically, our data support the hypothesis that hepatocellular carcinoma cells down-regulate normal inflammatory and immune responses to avoid immune editing.

Macrophages and inflammatory mediators in chemical toxicity: a battle of forces

Macrophages function as control switches of the immune system, providing a balance between pro- and anti-inflammatory responses. To accomplish this, they develop into different subsets: classically (M1) or alternatively (M2) activated macrophages. Whereas M1 macrophages display a cytotoxic, proinflammatory phenotype, much like the soldiers of The Dark Side of The Force in the Star Wars movies, M2 macrophages, like Jedi fighters, suppress immune and inflammatory responses and participate in wound repair and angiogenesis. Critical to the actions of these divergent or polarized macrophage subpopulations is the regulated release of inflammatory mediators. When properly controlled, M1 macrophages effectively destroy invading pathogens, tumor cells, and foreign materials. However, when M1 activation becomes excessive or uncontrolled, these cells can succumb to The Dark Side, releasing copious amounts of cytotoxic mediators that contribute to disease pathogenesis. The activity of M1 macrophages is countered by The Force of alternatively activated M2 macrophages, which release anti-inflammatory cytokines, growth factors, and mediators involved in extracellular matrix turnover and tissue repair. It is the balance in the production of mediators by these two macrophage subpopulations that ultimately determines the outcome of the tissue response to chemical toxicants.

Monitoring safety of liposomes containing rifampicin on respiratory cell lines and in vitro efficacy against Mycobacterium bovis in alveolar macrophages

Rifampicin-encapsulated liposome suspensions were prepared by a chloroform-film method and converted to dry powders by freeze-drying with mannitol as a cryoprotectant. The liposome suspension had multilamellar nanovesicles with 50% rifampicin encapsulation. The liposome dry powder comprised particles with a mass median aerodynamic diameter of 3.4 mum, with 60% present as a fine particle fraction. Rifampicin-encapsulated liposomes were evidently nontoxic to respiratory associated cells, including bronchial epithelial cells, small airway epithelial and alveolar macrophages (AMs). Furthermore, the liposomes did not activate AMs to produce interleukin-1 beta, tumor necrosis factor-alpha, and nitric oxide at a level that would cascade to other inflammatory effects. The minimum inhibitory concentrations against Mycobacterium bovis was 0.2 and 0.8 microM for liposomes containing rifampicin and free rifampicin, respectively. The less negatively charged reconstituted liposome displayed the greatest activity against intracellular growth of M. bovis.

Synergistic effects of tethered growth factors and adhesion ligands on DNA synthesis and function of primary hepatocytes cultured on soft synthetic hydrogels

The composition, presentation, and spatial orientation of extracellular matrix molecules and growth factors are key regulators of cell behavior. Here, we used self-assembling peptide nanofiber gels as a modular scaffold to investigate how fibronectin-derived adhesion ligands and different modes of epidermal growth factor (EGF) presentation synergistically regulate multiple facets of primary rat hepatocyte behavior in the context of a soft gel. In the presence of soluble EGF, inclusion of dimeric RGD and the heparin binding domain from fibronectin (HB) increased hepatocyte aggregation, spreading, and metabolic function compared to unmodified gels or gels modified with a single motif, but unlike rigid substrates, gels failed to induce DNA synthesis. Tethered EGF dramatically stimulated cell aggregation and spreading under all adhesive ligand conditions and also preserved metabolic function. Surprisingly, tethered EGF elicited DNA synthesis on gels with RGD and HB. Phenotypic differences between soluble and tethered EGF stimulation of cells on peptide gels are correlated with differences in expression and phosphorylation the EGF receptor and its heterodimerization partner ErbB2, and activation of the downstream signaling node ERK1/2. These modular matrices reveal new facets of hepatocellular biology in culture and may be more broadly useful in culture of other soft tissues.

Integration of multiple signaling pathway activities resolves K-RAS/N-RAS mutation paradox in colon epithelial cell response to inflammatory cytokine stimulation

Colon tumors frequently harbor mutation in K-RAS and/or N-RAS, members of a GTPase family operating as a central hub for multiple key signaling pathways. While these proteins are strongly homologous, they exhibit diverse downstream effects on cell behavior. Utilizing an isogenic panel of human colon carcinoma cells bearing oncogenic mutations in K-RAS and/or N-RAS, we observed that K-RAS and double mutants similarly yield elevated apoptosis in response to treatment with TNFalpha compared to N-RAS mutants. Regardless, and in surprising contrast, key phospho-protein signals were more similar between N-RAS and dual mutants. This apparent contradiction could not be explained by any of the key signals individually, but a multi-pathway model constructed from the single-mutant cell line data was able to predict the behavior of the dual-mutant cell line. This success arises from a quantitative integration of multiple pro-apoptotic (pIkappaBalpha, pERK2) and pro-survival (pJNK, pHSP27) signals in manner not easily discerned from intuitive inspection.

RAS mutations affect tumor necrosis factor-induced apoptosis in colon carcinoma cells via ERK-modulatory negative and positive feedback circuits along with non-ERK pathway effects

More than 40% of colon cancers have a mutation in K-RAS or N-RAS, GTPases that operate as central hubs for multiple key signaling pathways within the cell. Utilizing an isogenic panel of colon carcinoma cells with K-RAS or N-RAS variations, we observed differences in tumor necrosis factor-alpha (TNFalpha)-induced apoptosis. When the dynamics of phosphorylated ERK response to TNFalpha were examined, K-RAS mutant cells showed lower activation whereas N-RAS mutant cells exhibited prolonged duration. These divergent trends were partially explained by differential induction of two ERK-modulatory circuits: negative feedback mediated by dual-specificity phosphatase 5 and positive feedback by autocrine transforming growth factor-alpha. Moreover, in the various RAS mutant colon carcinoma lines, the transforming growth factor-alpha autocrine loop differentially elicited a further downstream chemokine (CXCL1/CXCL8) autocrine loop, with the two loops having opposite effects on apoptosis. Although the apoptotic responses of the RAS mutant panel to TNFalpha treatment showed significant dependence on the respective phosphorylated ERK dynamics, successful prediction across the various cell lines required contextual information concerning additional pathways including IKK and p38. A quantitative computational model based on weighted linear combinations of these pathway activities successfully predicted not only the spectrum of cell death responses but also the corresponding chemokine production responses. Our findings indicate that diverse RAS mutations yield differential cell behavioral responses to inflammatory cytokine exposure by means of (a) differential effects on ERK activity via multiple feedback circuit mechanisms, and (b) differential effects on other key signaling pathways contextually modulating ERK-related dependence.

Changes of plasma glutamine concentration and hepatocyte membrane system N transporters expression in early endotoxemia

BACKGROUND

Glutamine plays important roles in health and critical illness. During endotoxemia, glutamine metabolism, including its plasma level and transport, changes markedly. Previous studies have demonstrated that system N transporters in hepatocytes play a major role in hepatic glutamine transport. However, little is known about the changes of mRNA and protein expression of system N transporters in hepatocyte plasma membrane. Furthermore, the alteration of plasma glutamine concentration during endotoxemia is still controversial. In this study, we investigated the changes in early endotoxemic rats by intraperitoneal injection of lipopolysaccharide (LPS).

MATERIALS AND METHODS

Three, 6, 12 mg/kg body weight doses of LPS were injected intraperitoneally to establish the endotoxemic rat model; equal volume of 0.9% saline was used as the control. Before and 2, 4, 6, 12, 24h after injections, plasma glutamine concentration, mRNA, and protein expression of SNAT3 and SNAT5 transporters in hepatocyte plasma membrane were detected by high performance liquid chromatography, real-time PCR, and Western blot, respectively.

RESULTS

LPS injection resulted in a marked increase of the plasma glutamine concentration from 4 to 12h (3mg/kg) and 2 to 6h (6 mg/kg, 12 mg/kg) after the injection compared with its physiologic level, and a significant decrease in 6, 12 mg/kg groups at 24h. Both the mRNA and protein expression of SNAT3 and SNAT5 were enhanced by LPS in a time- and dose-dependent manner.

CONCLUSIONS

The plasma glutamine concentration in endotoxemic rat increased transiently during early endotoxemia but subsequently decreased over time. The effect of LPS on system N expression occurs not only at the protein level, but also at the mRNA level. It is reasonable to supplement glutamine for patients with sepsis or endotoxemia begin at 6 to 12h after the development of disease.

Monitoring of inflammatory mediators induced by silk sericin

Silk proteins have been shown to be good candidates for biomedical materials. However, there have been some reports regarding immunological and allergic responses to silk sericin. Our objective was to investigate the inflammatory mediators induced by sericin both in vitro and in vivo. Mouse monocyte and alveolar macrophage cell lines were used for monitoring levels of interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha generated after activation by sericin at concentrations of 0.2-1.0 mg/mL. The amounts of TNF-alpha and IL-1beta produced by both cell lines corresponded, in a dose-dependent manner, with the sericin concentration in the culture medium. The levels of TNF-alpha and IL-1beta generated after sericin activation by macrophage cells were higher than those generated by monocytes. However, these cytokine levels would not cascade to other inflammatory effects. Inflammatory mediators were also monitored from sericin-treated, cream base-treated and normal saline-soaked full-thickness rat excisions. Using wound size measurements and ELISA assays, sericin-treated wounds were shown to heal faster and had lower levels of inflammatory mediators, as compared with the cream base-treated and normal saline-soaked wounds. It can be concluded that sericin promotes the wound healing process without causing inflammation.

Na+/H+ exchanger mediates TNF-alpha-induced hepatocyte apoptosis via the calpain-dependent degradation of Bcl-xL

BACKGROUND AND AIM

It is well known that tumor necrosis factor-alpha (TNF-alpha) induces hepatocyte apoptosis and contributes to liver diseases. However, the exact mechanisms are not well understood.

METHODS

In the present study, we reported that Na(+)/H(+) exchanger (NHE) is involved in TNF-alpha-induced hepatocyte apoptosis.

RESULTS

TNF-alpha time dependently induced an increase in NHE activity in hepatocytes, but cariporide, an NHE inhibitor, blocked the TNF-alpha-induced increase of NHE activity in a dose-dependent manner. Increased NHE activity induced by TNF-alpha was associated with increased intracellular calcium (Ca(2+)(i)) concentration and calpain activity. Cariporide reversed these effects induced by TNF-alpha. In addition, TNF-alpha downregulated Bcl-xL, an anti-apoptotic protein, but not mRNA levels. The inhibition of either calpain or NHE blocked the TNF-alpha-induced decrease of the Bcl-xL protein. TNF-alpha did not change the pro-apoptotic Bax and Bak protein levels. Cariporide, calcium remover 1,2-bis (2-aminophenoxy) ethane-N,N,N0,N0-tetraacetic acid, or calpain inhibitor benzyloxycarbonyl-leucyl-leucinal attenuated TNF-alpha-induced hepatocyte apoptosis.

CONCLUSION

TNF-alpha via NHE results in hepatocyte apoptosis through the calcium/calpain/Bcl-xL pathway.

Synergistic drug-cytokine induction of hepatocellular death as an in vitro approach for the study of inflammation-associated idiosyncratic drug hepatotoxicity

Idiosyncratic drug hepatotoxicity represents a major problem in drug development due to inadequacy of current preclinical screening assays, but recently established rodent models utilizing bacterial LPS co-administration to induce an inflammatory background have successfully reproduced idiosyncratic hepatotoxicity signatures for certain drugs. However, the low-throughput nature of these models renders them problematic for employment as preclinical screening assays. Here, we present an analogous, but high-throughput, in vitro approach in which drugs are administered to a variety of cell types (primary human and rat hepatocytes and the human HepG2 cell line) across a landscape of inflammatory contexts containing LPS and cytokines TNF, IFN gamma, IL-1 alpha, and IL-6. Using this assay, we observed drug-cytokine hepatotoxicity synergies for multiple idiosyncratic hepatotoxicants (ranitidine, trovafloxacin, nefazodone, nimesulide, clarithromycin, and telithromycin) but not for their corresponding non-toxic control compounds (famotidine, levofloxacin, buspirone, and aspirin). A larger compendium of drug-cytokine mix hepatotoxicity data demonstrated that hepatotoxicity synergies were largely potentiated by TNF, IL-1 alpha, and LPS within the context of multi-cytokine mixes. Then, we screened 90 drugs for cytokine synergy in human hepatocytes and found that a significantly larger fraction of the idiosyncratic hepatotoxicants (19%) synergized with a single cytokine mix than did the non-hepatotoxic drugs (3%). Finally, we used an information theoretic approach to ascertain especially informative subsets of cytokine treatments for most highly effective construction of regression models for drug- and cytokine mix-induced hepatotoxicities across these cell systems. Our results suggest that this drug-cytokine co-treatment approach could provide a useful preclinical tool for investigating inflammation-associated idiosyncratic drug hepatotoxicity.

Hepatitis B virus X protein modulates apoptosis in primary rat hepatocytes by regulating both NF-kappaB and the mitochondrial permeability transition pore

The hepatitis B virus (HBV) X protein (HBx) is a multifunctional protein that regulates numerous cellular signal transduction pathways, including those that modulate apoptosis. However, different HBx-dependent effects on apoptosis have been reported; these differences are likely the consequence of the exact conditions and cell types used in a study. Many of the previously reported studies that analyzed HBx regulation of apoptosis were conducted in immortalized or transformed cells, and the alterations that have transformed or immortalized these cells likely impact apoptotic pathways. We examined the effects of HBx on apoptotic pathways in cultured primary rat hepatocytes, a biologically relevant system that mimics normal hepatocytes in the liver. We analyzed the effects of HBx on apoptosis both when HBx was expressed in the absence of other HBV proteins and in the context of HBV replication. HBx stimulation of NF-kappaB inhibited the activation of apoptotic pathways in cultured primary rat hepatocytes. However, when HBx-induced activation of NF-kappaB was blocked, HBx stimulated apoptosis; blocking the activity of the mitochondrial permeability transition pore inhibited HBx activation of apoptosis. These results suggest that HBx can be either proapoptotic or antiapoptotic in hepatocytes, depending on the status of NF-kappaB, and confirm previous studies that link some HBx activities to modulation of the mitochondrial permeability transition pore. Overall, our studies define apoptotic pathways that are regulated by HBx in cultured primary hepatocytes and provide potential mechanisms for the development of HBV-associated liver cancer.