Increased expression of c-fos, c-jun and LRF-1 is not required for in vivo priming of hepatocytes by the mitogen TCPOBOP

Evaluation of the carcinogenicity of carbon tetrachloride

Carbon tetrachloride (CCl4) has been extensively used and reported to produce toxicity, most notably involving the liver. Carbon tetrachloride metabolism involves CYP450-mediated bioactivation to trichloromethyl and trichloromethyl peroxy radicals, which are capable of macromolecular interaction with cell components including lipids and proteins. Radical interaction with lipids produces lipid peroxidation which can mediate cellular damage leading to cell death. Chronic exposure with CCl4 a rodent hepatic carcinogen with a mode of action (MOA) exhibits the following key events: 1) metabolic activation; 2) hepatocellular toxicity and cell death; 3) consequent regenerative increased cell proliferation; and 4) hepatocellular proliferative lesions (foci, adenomas, carcinomas). The induction of rodent hepatic tumors is dependent upon the dose (concentration and exposure duration) of CCl4, with tumors only occurring at cytotoxic exposure levels. Adrenal benign pheochromocytomas were also increased in mice at high CCl4 exposures; however, these tumors are not of relevant importance to human cancer risk. Few epidemiology studies that have been performed on CCl4, do not provide credible evidence of enhanced risk of occurrence of liver or adrenal cancers, but these studies have serious flaws limiting their usefulness for risk assessment. This manuscript summarizes the toxicity and carcinogenicity attributed to CCl4, specifically addressing MOA, dose-response, and human relevance.

Exploration of the Transcriptional Landscape of ALPPS Reveals the Pathways of Accelerated Liver Regeneration

Background and Aims: ALPPS (associating liver partition and portal vein ligation for staged hepatectomy), a novel 2-staged hepatectomy, dramatically accelerates liver regeneration and thus enables extensive liver tumor resection. The signaling networks underlying the ALPPS-induced accelerated regeneration process are largely unknown.

Methods: We performed transcriptome profiling (TP) of liver tissue obtained from a mouse model of ALPPS, standard hepatectomy (68% model), and additional control surgeries (sham, PVL and Tx). We also performed TP using human liver biopsies (n = 5) taken from the occluded lobe and the future liver remnant (FLR) during the first step of ALPPS surgery (4–5 h apart). We used Oncofinder computational tools, which covers 378 ISPs, for unsupervised, unbiased quantification of ISP activity.

Results: Gene expression cluster analysis revealed an ALPPS specific signature: the IGF1R Signaling Pathway (Cell survival), the ILK Pathway (Induced cell proliferation), and the IL-10 Pathway (Stability determination) were significantly enriched, whereas the activity of the Interferon Pathway (Transcription) was reduced (p < 0.05). Further, the PAK- and ILK-associated ISPs were activated at an earlier time point, reflecting significant acceleration of liver regeneration (p < 0.001). These pathways, which were also recovered in human liver biopsies, control cell growth and proliferation, inflammatory response, and hypoxia-related processes.

Conclusions: ALPPS is not a straightforward addition of portal vein ligation (PVL) plus transection—it is more. The early stages of normal and accelerated liver regeneration are clearly discernible by a significantly increased and earlier activation of a small number of signaling pathways. Compounds mimicking these responses may help to improve the ALPPS method and further reduce the hospitalization time of the patient.

TCPOBOP-Induced Hepatomegaly and Hepatocyte Proliferation are Attenuated by Combined Disruption of MET and EGFR Signaling

TCPOBOP (1,4-Bis [2-(3,5-Dichloropyridyloxy)] benzene) is a constitutive androstane receptor (CAR) agonist that induces robust hepatocyte proliferation and hepatomegaly without any liver injury or tissue loss. TCPOBOP-induced direct hyperplasia has been considered to be CAR-dependent with no evidence of involvement of cytokines or growth factor signaling. Receptor tyrosine kinases (RTKs), MET and epidermal growth factor receptor (EGFR), are known to play a critical role in liver regeneration after partial hepatectomy, but their role in TCPOBOP-induced direct hyperplasia, not yet explored, is investigated in the current study. Disruption of the RTK-mediated signaling was achieved using MET knockout (KO) mice along with Canertinib treatment for EGFR inhibition. Combined elimination of MET and EGFR signaling [MET KO + EGFR inhibitor (EGFRi)], but not individual disruption, dramatically reduced TCPOBOP-induced hepatomegaly and hepatocyte proliferation. TCPOBOP-driven CAR activation was not altered in [MET KO + EGFRi] mice, as measured by nuclear CAR translocation and analysis of typical CAR target genes. However, TCPOBOP-induced cell cycle activation was impaired in [MET KO + EGFRi] mice due to defective induction of cyclins, which regulate cell cycle initiation and progression. TCPOBOP-driven induction of FOXM1, a key transcriptional regulator of cell cycle progression during TCPOBOP-mediated hepatocyte proliferation, was greatly attenuated in [MET KO + EGFRi] mice. Interestingly, TCPOBOP treatment caused transient decline in hepatocyte nuclear factor 4 alpha expression concomitant to proliferative response; this was not seen in [MET KO + EGFRi] mice. Transcriptomic profiling revealed the vast majority (~40%) of TCPOBOP-dependent genes primarily related to proliferative response, but not to drug metabolism, were differentially expressed in [MET KO + EGFRi] mice. Conclusion: Taken together, combined disruption of EGFR and MET signaling lead to dramatic impairment of TCPOBOP-induced proliferative response without altering CAR activation.

Constitutive androstane receptor (Car)-driven regeneration protects liver from failure following tissue loss

BACKGROUND & AIMS

Liver can recover following resection. If tissue loss is too excessive, however, liver failure will develop as is known from the small-for-size-syndrome (SFSS). The molecular processes underlying liver failure are ill-understood. Here, we explored the role and the clinical potential of Nr1i3 (constitutive androstane receptor, Car) in liver failure following hepatectomy.

METHODS

Activators of Car, various hepatectomies, Car(-/-) mice, humanized CAR mice, human tissue and ex vivo liver slice cultures were used to study Car in the SFSS. Pathways downstream of Car were investigated by in vivo siRNA knockdown.

RESULTS

Excessive tissue loss causing liver failure is associated with deficient induction of Car. Reactivation of Car by an agonist normalizes all features associated with experimental SFSS. The beneficial effects of Car activation are relayed through Foxm1, an essential promoter of the hepatocyte cell cycle. Deficiency in the CAR-FOXM1 axis likewise is evident in human SFSS. Activation of human CAR mitigates SFSS in humanized CAR mice and improves the culture of human liver slices.

CONCLUSIONS

Impaired hepatic Car-Foxm1 signaling provides a first molecular characterization of liver that fails to recover after tissue loss. Our findings place deficient regeneration as a principal cause behind the SFSS and suggest CAR agonists may bear clinical potential against liver failure.

LAY SUMMARY

The unique regenerative capacity of liver has its natural limits. Following tissue loss that is too excessive, such as through extended resection in the clinic, liver failure may develop. This is known as small-for-size-syndrome (SFSS) and represents the most frequent cause of death due to liver surgery. Here we show that deficient induction of the protein Car, a central regulator of liver function and growth, is a cause of liver failure following extended resection; reactivation of Car through pharmacological means is sufficient to prevent or rescue the SFSS.

Principles of liver regeneration and growth homeostasis

Liver regeneration is perhaps the most studied example of compensatory growth aimed to replace loss of tissue in an organ. Hepatocytes, the main functional cells of the liver, manage to proliferate to restore mass and to simultaneously deliver all functions hepatic functions necessary to maintain body homeostasis. They are the first cells to respond to regenerative stimuli triggered by mitogenic growth factor receptors MET (the hepatocyte growth factor receptor] and epidermal growth factor receptor and complemented by auxiliary mitogenic signals induced by other cytokines. Termination of liver regeneration is a complex process affected by integrin mediated signaling and it restores the organ to its original mass as determined by the needs of the body (hepatostat function). When hepatocytes cannot proliferate, progenitor cells derived from the biliary epithelium transdifferentiate to restore the hepatocyte compartment. In a reverse situation, hepatocytes can also transdifferentiate to restore the biliary compartment. Several hormones and xenobiotics alter the hepatostat directly and induce an increase in liver to body weight ratio (augmentative hepatomegaly). The complex challenges of the liver toward body homeostasis are thus always preserved by complex but unfailing responses involving orchestrated signaling and affecting growth and differentiation of all hepatic cell types.

Elucidating the metabolic regulation of liver regeneration

The regenerative capability of liver is well known, and the mechanisms that regulate liver regeneration are extensively studied. Such analyses have defined general principles that govern the hepatic regenerative response and implicated specific extracellular and intracellular signals as regulated during and essential for normal liver regeneration. Nevertheless, the most proximal events that stimulate liver regeneration and the distal signals that terminate this process remain incompletely understood. Recent data suggest that the metabolic response to hepatic insufficiency might be the proximal signal that initiates regenerative hepatocellular proliferation. This review provides an overview of the data in support of a metabolic model of liver regeneration and reflects on the clinical implications and areas for further study suggested by these findings.

SRC-3 is required for CAR-regulated hepatocyte proliferation and drug metabolism

BACKGROUND & AIMS

Nuclear receptors such as pregnane X receptor and constitutive androstane receptor (CAR) are important regulators of drug-metabolizing systems such as P450 enzymes and modulate xenobiotic metabolism as well as hepatocellular proliferation. Binding of CAR to NR response elements alone is not sufficient to activate gene expression. Here, we investigate the role of steroid receptor co-activator (SRC) family members in CAR-mediated hepatocyte proliferation and drug metabolism.

METHODS

The role of SRCs in CAR activation was assessed in cell-based transfection assays and protein-protein interaction assays. The in vivo role of SRCs in CAR-mediated hepatocyte proliferation and drug metabolism was examined by using mice deficient in SRCs.

RESULTS

SRC-3 displayed the highest co-activating activity to CAR compared with SRC-1 and SRC-2 in a cell-based reporter assay. Knockout of SRC-3 in mice attenuated hepatic hyperplasia induced by a CAR agonist 1,4-bis-[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP), which was associated with a reduced expression of c-Myc and Foxm-1. In contrast, knockout of SRC-1 or SRC-2 in mice did not affect TCPOBOP-induced hepatic hyperplasia. SRC-3-deficient mice were hypersensitive to zoxazolamine-induced paralysis, but were resistant to acetaminophen hepatotoxicity induced by TCPOBOP, whereas mutant mice deficient in SRC-1 or SRC-2 exhibited severe acetaminophen hepatotoxicity similar to wild-type controls. Accordingly, deficiency in SRC-3, but not SRC-1 or SRC-2, resulted in a reduced CAR-mediated expression of drug metabolism-related genes in the liver.

CONCLUSIONS

Our study demonstrates that SRC-3 is the predominant transcriptional co-activator among the three SRC family members for CAR activation to promote hepatocyte proliferation and drug metabolism.

Functional Relationships between Lipid Metabolism and Liver Regeneration

The regenerative capacity of the liver is well known, and the mechanisms that regulate this process have been extensively studied using experimental model systems including surgical resection and hepatotoxin exposure. The response to primary mitogens has also been used to investigate the regulation of hepatocellular proliferation. Such analyses have identified many specific cytokines and growth factors, intracellular signaling events, and transcription factors that are regulated during and necessary for normal liver regeneration. Nevertheless, the nature and identities of the most proximal events that initiate hepatic regeneration as well as those distal signals that terminate this process remain unknown. Here, we review the data implicating acute alterations in lipid metabolism as important determinants of experimental liver regeneration and propose a novel metabolic model of regeneration based on these data. We also discuss the association between chronic hepatic steatosis and impaired regeneration in animal models and humans and consider important areas for future research.

Gender-specific interplay of signaling through β-catenin and CAR in the regulation of xenobiotic-induced hepatocyte proliferation

Aberrant signaling through the Wnt/β-catenin pathway is a critical determinant in human and rodent liver carcinogenesis and generally accepted to be a potent driver of proliferation. Xenobiotic agonists of the constitutive androstane receptor (CAR) induce massive acute hyperplasia of mouse liver and facilitate the outgrowth of hepatocellular carcinomas with activated β-catenin. In the present study, the interplay of β-catenin-dependent and CAR-dependent signaling in the liver and its effect on hepatocyte proliferation were analyzed in transgenic mice with hepatocyte-specific knockout of Ctnnb1 (encoding β-catenin) following treatment with two CAR agonists, 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP) and phenobarbital. Hepatocyte-specific knockout of β-catenin inhibited CAR agonists-induced hepatocyte proliferation in male mice. By contrast, the proliferative effect of CAR agonists was strongly augmented in female β-catenin knockout animals. This was due to prolonged proliferation of the knockout hepatocytes. CAR-mediated hepatocyte proliferation was, at least in part, dependent on estrogen signaling and was associated with enhanced expression of FoxM1 and elevated activity of the PDK1/p90RSK pathway. In conclusion, our study shows that gender-specific factors determine whether β-catenin signaling plays a pro- or an antiproliferative role in the regulation of mouse hepatocyte proliferation induced by CAR agonists.

Effects on coagulation factor production following primary hepatomitogen-induced direct hyperplasia

AIM: To investigate the molecular mechanisms involved in coagulation factor expression and/or function during direct hyperplasia (DH)-mediated liver regeneration.

METHODS: Direct hyperplasia-mediated liver regeneration was induced in female C57BL/6 mice by administering 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP), a representative hepatomitogen. Mice were weighed and sacrificed at various time points [Day 0 (D0: prior to injection), 3 h, D1, D2, D3, and D10] after TCPOBOP administration to obtain liver and blood samples. Using the RNA samples extracted from the liver, a comprehensive analysis was performed on the hepatic gene expression profiling of coagulation-related factors by real-time RT-PCR (fibrinogen, prothrombin, factors V, VII, VIII, IX, X, XI, XII, XIIIβ, plasminogen, antithrombin, protein C, protein S, ADAMTS13, and VWF). The corresponding plasma levels of coagulation factors (fibrinogen, prothrombin, factors V, VII, VIII, IX, X, XI, XII, XIII, and VWF) were also analyzed and compared with their mRNA levels.

RESULTS: Gavage administration of TCPOBOP (3 mg/kg body weight) resulted in a marked and gradual increase in the weight of the mouse livers relative to the total body weight to 220% by D10 relative to the D0 (control) ratios. At the peak of liver regeneration (D1 and D2), the gene expression levels for most of the coagulation-related factors (fibrinogen, prothrombin, factors V, VII, VIII, IX, XI, XII, XIIIβ, plasminogen, antithrombin, protein C, ADAMTS13, VWF) were found to be down-regulated in a time-dependent manner, and gradually recovered by D10 to the basal levels. Only mRNA levels of factor X and protein S failed to show any decrease during the regenerative phase. As for the plasma levels, 5 clotting factors (prothrombin, factors VIII, IX, XI, and XII) demonstrated a significant decrease (P < 0.05) during the regeneration phase compared with D0. Among these 5 factors, factor IX and factor XI showed the most dramatic decline in their activities by about 50% at D2 compared to the basal levels, and these reductions in plasma activity for both factors were consistent with our RT-PCR findings. In contrast, the plasma activities of the other coagulation factors (fibrinogen, factors V, VII, XIII, and VWF) were not significantly reduced, despite the reduction in the liver mRNA levels. Unlike the other factors, FX showed a temporal increase in its plasma activity, with significant increases (P < 0.05) detected at D1.

CONCLUSION: Investigating the coagulation cascade protein profiles during liver regeneration by DH may help to better understand the basic biology of the liver under normal and pathological conditions.

NFAT4 deficiency results in incomplete liver regeneration following partial hepatectomy

BACKGROUND

Liver regeneration following partial hepatectomy requires the orchestration of highly regulated molecular pathways; a change in the abundance or activity of a specific gene product has the potential to adversely affect this process. The nuclear factor of activated T-cells (NFAT) transcription factors represent a family of gene transcription signaling intermediates that translate receptor-dependent signaling events into specific transcriptional responses using the Ras/Raf pathway.

MATERIALS AND METHODS

Eight-week old NFAT4 knockout (KO) mice and their wild type counterparts (Balb-c) underwent two-thirds partial hepatectomy. The animals were sacrificed and their livers were harvested at specific time points during regeneration. Recovery of liver mass was measured for each time point. PCR analysis was used to analyze expression levels of the immediate early genes c-fos, c-jun and c-myc as well as downstream effectors of NFAT4 including FGF-18 and BMP-4.

RESULTS

Hepatocyte proliferation and thus liver regeneration following hepatectomy was suppressed in NFAT4 knockout (KO) mice. Statistical significance was reached at 1 h, 7 d, and 10 d (P < 0.05) with a 22% median reduction in regeneration of liver mass in the NFAT4 KO mice by 10 d, at which time liver regeneration should be complete in mice. The immediate early gene c-fos was elevated in NFAT4 KO mice during early regeneration with a median value at 1 h and 1 d of 1.60E-08 and 1.09E-08 versus 6.10E-09 and 1.55E-09 in the Balb-c mice. C-jun, in contrast, was elevated during late regeneration in the NFAT4 KO mice (3.40E-09 and 5.67E-09 at 7 and 10 d, respectively) in comparison with the Balb-c mice (7.76E-10 and 1.24E-09, respectively.). NFAT2 was also up-regulated in the NFAT4 KO mice; however, no changes were detected in its downstream effectors, CCR1 and CCL3.

CONCLUSIONS

We demonstrated that NFAT4 deficiency impairs hepatic regeneration in a murine model proving that NFAT4 plays an important yet unclear role in liver regeneration; its absence may be compensated by c-fos, c-jun, and NFAT2 expression changes.

Influence of aflatoxin B1 on mRNA levels of acute-phase proteins and oncoproteins in albino rat liver

Background: The effect of aflatoxin B1 (AFB1) administration on expression of genes coding for acute-phase proteins, and nuclear protooncogenes c-fos and c-jun, and alpha-fetoprotein gene has been studied in rats. Methods: AFB1 was administered to male Albino Oxford (AO) rats as a single intraperitoneal dose (1 mg/kg body weight). The expression of genes for albumin, cystein protease inhibitor, fibrinogen, haptoglobin, a1-acid glycoprotein and for c-Fos, c-Jun and alpha-fetoprotein in rat liver was measured by Northern hybridization. Results: The mild increase in the levels of mRNA for acute-phase proteins after AFB1 administration was observed during the first 24 hours. The exceptions were the mRNA levels in liver for cystein protease inhibitor, which were 50%, decreased as compared to the control values. In addition, mild increase of the expression of c-fos protooncogene with two peaks were noted at three (1.3 fold) and 72 hours (1.5 fold) after injection AFB1 to rats. The expression of nuclear protooncogene c-jun at 1 hour and 72 hour after acute poisoning was 2.6 fold and 3.7 fold increased as compared to control values, respectively. The mRNA levels in liver for the alpha-fetoprotein reached a maximum at 1 hour after AFB1 injection and it was 1.8 times higher than the levels in the livers of nontreated animals. Conclusion: Single administration of AFB1 induced increased transcription of c-jun and c-fos genes while typical acute-phase response was not found.

Microarray analysis of mitogenic effects of T3 on the rat liver

BACKGROUND AND AIM

A single dose of the thyroid hormone tri-iodothyronine, T3, can enhance both size and function of normal rodent liver, which is potentially of value in the treatment of liver disease. However the mechanism of this has not been fully elucidated, and it cannot be modeled in vitro. We therefore investigated the transcriptome response to T3 in rat liver in vivo.

METHODS

After adult rats were administered 5 microg T3 subcutaneously, a whole rat genome microarray comparing global hepatic gene expression against vehicle-only treated liver after 3 h was performed.

RESULTS

Informative transcripts which had identifiable gene ontology biological processes were grouped according to function, broadly reflecting general metabolic effects and those linked to cell-proliferation control. We then compared the transcriptome response after 5-microg T3 initiating hepatocyte DNA synthesis (mitogenic) with that after 0.1 microg T3, a supraphysiological amount not initiating hepatocyte DNA synthesis.

CONCLUSIONS

We compared the results with published results of the response to other primary mitogens, and identified the Gadd45beta/MyD118 gene as a common early factor upregulated during proliferation.

Liver regeneration

Liver regeneration after partial hepatectomy is a very complex and well-orchestrated phenomenon. It is carried out by the participation of all mature liver cell types. The process is associated with signaling cascades involving growth factors, cytokines, matrix remodeling, and several feedbacks of stimulation and inhibition of growth related signals. Liver manages to restore any lost mass and adjust its size to that of the organism, while at the same time providing full support for body homeostasis during the entire regenerative process. In situations when hepatocytes or biliary cells are blocked from regeneration, these cell types can function as facultative stem cells for each other.

Liver regeneration

Liver regeneration after partial hepatectomy is a very complex and well-orchestrated phenomenon. It is carried out by the participation of all mature liver cell types. The process is associated with signaling cascades involving growth factors, cytokines, matrix remodeling, and several feedbacks of stimulation and inhibition of growth related signals. Liver manages to restore any lost mass and adjust its size to that of the organism, while at the same time providing full support for body homeostasis during the entire regenerative process. In situations when hepatocytes or biliary cells are blocked from regeneration, these cell types can function as facultative stem cells for each other.

During thioacetamide-induced acute liver failure, the proliferative response of hepatocytes to thyroid hormone is maintained, indicating a potential therapeutic approach to toxin-induced liver disease

In toxic liver injury, proliferation of preexisting hepatocytes helps restore liver mass and function. While loss of liver mass per se stimulates hepatocyte proliferation, exogenous mitogens have a potential role in enhancing liver regeneration. The aim of this study was to characterize the effects of the mitogen, tri-iodothyonine, on the regenerative capacity of hepatocytes during thioacetamide-induced liver failure. Rats received (two) thioacetamide injections and, 12 hr later, either tri-iodothyonine or vehicle-only control. Liver cell proliferation was assessed and comparison made with other control groups receiving tri-iodothyonine or vehicle only. In rats with thioacetamide-induced hepatitis the proportion of hepatocytes in S-phase was greater in the tri-iodothyonine group (27+/-3.5%) compared to the vehicle-only group (20+/-2.5%; P < 0.05), with, notably, a greater number of midzonal (BrdU) positive hepatocytes in the tri-iodothyonine group. We conclude that the ability of hepatocytes in the midzonal areas of rat liver to proliferate in response to tri-iodothyonine is maintained during severe acute toxic injury.

Cellular localization of PRL-1 and PRL-2 gene expression in normal adult human tissues

Recent evidence suggests that the PRL-1 and -2 phosphatases may be multifunctional enzymes with diverse roles in a variety of tissue and cell types. Northern blotting has previously shown widespread expression of both transcripts; however, little is known about the cell type-specific expression of either gene, especially in human tissues. Therefore, we investigated expression patterns for PRL-1 and -2 genes in multiple normal, adult human tissues using in situ hybridization. Although both transcripts were ubiquitously expressed, they exhibited strikingly different patterns of expression. PRL-2 was expressed heavily in almost every tissue and cell type examined, whereas PRL-1 expression levels varied considerably both between tissue types and between individuals. Widespread expression of PRL-1 and -2 in multiple organ systems suggests an important functional role for these enzymes in normal tissue homeostasis. In addition, the variable patterns of expression for these genes may provide distinct activities in each tissue or cell type.

Gadd45beta is induced through a CAR-dependent, TNF-independent pathway in murine liver hyperplasia

We previously observed that Gadd45/MyD118, a member of the Gadd45 family of inducible factors, showed the strongest immediate-early induction common to two distinctive proliferation responses of the liver: (1) regeneration induced by surgical partial hepatectomy and (2) hyperplasia induced by the primary mitogen TCPOBOP, a ligand of the constitutive androstane receptor (CAR). Gadd45 is known to be stimulated by nuclear factor (NF) B, which is activated by tumor necrosis factor alpha (TNF) in the early response to partial hepatectomy. We therefore investigated whether TNF and NFB also stimulated Gadd45 as part of the response to CAR ligands, or whether activation occurred by an alternative pathway. TCPOBOP effects were characterized in three mouse genotypes: wild-type, TNFR1-/-, and TNFR1-/-TNFR2-/-. The results showed that TCPOBOP did not activate NFB in any of the mice, but a strong induction of Gadd45 messenger RNA was observed in all three genotypes, where TCPOBOP also induced CyP2b10, a classical target gene of activated CAR, and cyclin D1, a proliferation linked gene. Thus, the absence of TNFR signaling and induction of NFB did not impair CAR-mediated gene induction. Moreover, hepatocyte proliferation was strongly induced, and at significantly higher levels than wild type, in both TNFR1-/- and TNFR1-/-TNFR2-/- mice. Further studies evaluated TCPOBOP-induced gene expression in CAR-/- mice, by microarray expression profiling and Northern blot. The induced changes in gene expression, including the stimulation of Gadd45, were almost completely abolished--hence all were mediated via CAR activation. In conclusion, in the liver, Gadd45 can be induced by a distinctive pathway that requires CAR and is independent of TNF-NFB. The greater induction of proliferation in TNFR-null mice suggests negative cross-talk between the CAR and TNF-NFB controls that regulate proliferation.

Liver tissue engineering at extrahepatic sites in mice as a potential new therapy for genetic liver diseases

Liver tissue engineering using hepatocyte transplantation has been proposed as an alternative to whole-organ transplantation or liver-directed gene therapy to correct various types of hepatic insufficiency. Hepatocytes are not sustained when transplanted under the kidney capsule of syngeneic mice. However, when we transplanted hepatocytes with the extracellular matrix components extracted from Engelbreth-Holm-Swarm cells, hepatocytes survived for at least 140 days and formed small liver tissues. Liver engineering in hemophilia A mice reconstituted 5% to 10% of normal clotting activity, enough to reduce the bleeding time and have a therapeutic benefit. Conversely, the subcutaneous space did not support the persistent survival of hepatocytes with Engelbreth-Holm-Swarm gel matrix. We hypothesized that establishing a local vascular network at the transplantation site would reduce graft loss. To test this idea, we provided a potent angiogenic agent before hepatocyte transplantation into the subcutaneous space. With this procedure, persistent survival was achieved for the length of the experiment (120 days). To establish that these engineered liver tissues also retained their native regeneration potential in vivo, we induced two different modes of proliferative stimulus to the naive liver and confirmed that hepatocytes within the extrahepatic tissues regenerated with activity similar to that of naive liver. In conclusion, our studies indicate that liver tissues can be engineered and maintained at extrahepatic sites, retain their capacity for regeneration in vivo, and used to successfully treat genetic disorders.

Aging does not reduce the hepatocyte proliferative response of mice to the primary mitogen TCPOBOP

It has been shown that the magnitude of DNA synthesis and the time at which maximal DNA synthesis occurs after two-thirds partial hepatectomy (PH) is greatly reduced in the liver of aged rodents compared to young animals. This reduction could represent an intrinsic defect in proliferation or a more specialized change in the response to PH. We therefore evaluated the proliferative capacity of hepatocytes in aged animals, following treatment with primary liver mitogens. We show that treatment of 12-month-old CD-1 mice with the hepatomitogen 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) caused an increase in hepatocyte proliferation similar to that seen in young (8-week-old) mice. The labeling index was 82% in the livers of aged mice versus 76% in young animals. Histological observation demonstrated that the number of hepatocytes entering mitoses was similar in both groups; the mitotic indices were 2.5 per thousand and 2.7 per thousand, respectively. Additional experiments showed that the timing of DNA synthesis and M phase were nearly identical in both aged and young mice. Stimulation of hepatocyte DNA synthesis was associated with increased expression of several cell cycle-associated proteins (cyclin D1, cyclin A, cyclin B1, E2F, pRb, and p107); all were comparable in aged mice and young mice. TCPOBOP treatment also increased expression of the Forkhead Box transcription factor m1b (Foxm1b) to a similar degree in both groups. In conclusion, hepatocytes retain their proliferative capacity in old age despite impaired liver regeneration. These findings suggest that therapeutic use of mitogens would alleviate the reduction in hepatocyte proliferation observed in the elderly.

Receptor-mediated hepatocarcinogenesis: role of hepatocyte proliferation and apoptosis

The rodent liver is a target organ for the action of several non-genotoxic carcinogens. These include dioxins, polychlorinated biphenyls, phenobarbital, peroxisome proliferators and organochlorine pesticides. These chemicals disrupt the homeostasis of the liver by perturbing hepatocyte cell death and proliferation, causing hyperplasia leading to tumour formation. Significant progress has been made towards elucidating the mechanisms of action of these toxicants since the discovery of receptors that bind specific classes of xenobiotics. Dioxins and polychlorinated biphenyls bind to the aryl hydrocarbon receptor, phenobarbital binds to the constitutive androstane receptor and peroxisome proliferators act via the their activated receptor alpha. These three receptors have ligand-dependent transcription activities and therefore mediate changes in gene expression in response to toxicant exposure. The development of transgenic mouse strains where the genes for these receptors are disrupted has demonstrated that receptor activity is essential for the toxicity of these carcinogens. This implies that changes in the expression of key target genes control proliferation and apoptosis in the xenobiotic-induced hepatocyte phenotype.

Immediate early genes and p21 regulation in liver of rats with acute hepatic failure

It has been observed that liver regeneration in acute hepatic failure (AHF) is suppressed [Eguchi et al. Hepatology 1996;24(6):1452-9]. The molecular mechanism regulating this inhibition is not known. We previously reported that in AHF rats, hepatocyte proliferation was significantly impaired with elevation in serum IL-6, TGF-beta1, and HGF [Kamohara et al. Biochem Biophys Res Commun 2000;273(1):129-35]. Following either 70% partial hepatectomy (PH) or liver injury, quiescent mature hepatocytes are "primed" to re-enter the cell cycle. The process of "priming" appears to be triggered by extracellular cytokines (IL-6 and TNF-alpha) and is characterized by expression of immediate early genes. Under the stimulation of growth factors such as HGF, "primed" hepatocytes exit the G1 phase of the cell cycle. G1-associated cyclins and their inhibitors play a pivotal role in G1/S cell cycle transition. Here, we demonstrate that immediate early gene (i.e. c-myc, c-fos) expression and AP-1 activity are preserved in AHF rat livers despite absence of hepatocyte proliferation. In contrast, p21 mRNA and protein are both over-expressed in AHF livers compared to livers from rats undergoing PH; this elevation leads to inhibition in Cdk2 activity, resulting in G1 cell cycle arrest and inhibition of regeneration.

Role of hepatocyte direct hyperplasia in lentivirus-mediated liver transduction in vivo

Lentiviral vectors have been used for gene transfer into the liver, but the ability of these vectors to efficiently transduce quiescent hepatocytes remains controversial. Regardless, lentivirus-mediated gene transfer is greatly enhanced when delivered during hepatocellular cycling. For this reason, the present study was designed to determine the role of hepatocyte proliferation in the enhancement of lentiviral transduction by using three different modes of liver regeneration: (1) compensatory regeneration stimulated by two-thirds partial hepatectomy, (2) direct hyperplasia after intragastric administration of the primary mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP), and (3) a combination of modes 1 and 2. Vesicular stomatitis virus glycoprotein (VSV-G)-pseudotyped lentiviral vector expressing beta-galactosidase was administered to mice via the peripheral circulation after a regeneration stimulus. Gene transfer as measured by 5-bromo-4-chloro-3-indolyl-beta-D-galactoside (X-Gal) staining showed 30-fold higher levels of liver transduction in groups 1 and 2 as compared with the non-liver-manipulated control group (p < 0.005). The combination of TCPOBOP and partial hepatectomy (group 3) resulted in an ~80-fold increase in transduction efficiency compared with the control animals. The enhanced transduction was consistent with higher levels of hepatocellular proliferation observed in animals that received both treatments compared with either single treatment alone. Importantly, the hepatocytes were the predominant cell type transduced, although transgene expression was observed in a low number of nonparenchymal cells regardless of which liver stimulus was received. Biodistribution studies confirmed that most of the gene transfer was limited to the liver and spleen. Taken together, this study suggests that disease-induced cellular proliferation in the liver will enhance the utility of this vector in treating diseases such as viral hepatitis, liver cirrhosis, and cancer.

Differential effects of transforming growth factor-beta1, a fibrogenic factor, on macrophage-like cells (HS-P) and myofibroblastic cells (MT-9) in vitro

Transforming growth factor-beta1 (TGF-beta1) produced by infiltrating macrophages plays a role in fibrotic disorders through the induction of myofibroblasts. To explore possible mechanisms by which TGF-beta1 may act in this context, we investigated effects of TGF-beta1 on macrophage-like (HS-P) and myofibroblastic (MT-9) cells, two novel cell lines developed by us. Immunocytochemically, the addition of TGF-beta1 (0, 0.1, 0.5, and 1.0 ng/ml) dose-dependently suppressed the expressions of antigens recognized by macrophage/histiocyte-specific antibodies (ED1 and ED2) in HS-P cells, whereas the addition concomitantly increased the number of anti-alpha-smooth muscle actin antibody-positive myofibroblastic cells, suggesting a possible phenotypical modulation of macrophages into myofibroblasts in the fibrotic lesions. By contrast, MT-9 cells did not show such immunophenotypical changes following TGF-beta1 addition. DNA synthesis, measured by tritiated thymidine-incorporation, was inhibited in a dose-dependent manner in MT-9 cells by TGF-beta1 addition (0, 0.1, 0.2, 0.5, 1.0, 5, and 10 ng/ml), but that in HS-P cells was unchanged. Northern blot analysis revealed that expressions of cell cycle-related early genes, c-jun and c-myc, were increased in HS-P cells after TGF-beta1 (1 ng/ml) addition, with c-jun showing peak expression prior to c-myc. By contrast, the peak expressions of c-jun and c-myc were delayed in TGF-beta1 (1 ng/ml)-added MT-9 cells, and their levels were less in MT-9 cells than in HS-P cells. Furthermore, TGF-beta1 (1 and 10 ng/ml) induced DNA laddering in MT-9 cells, but did not in HS-P cells. Based on these findings, it was speculated that TGF-beta1 could have induced G1 arrest in cell cycle and apoptosis in MT-9 cells. The present study showed that there were significant differences in the effects of TGF-beta1 between macrophage-like HS-P cells and myofibroblastic MT-9 cells, presumably depending on divergent susceptibilities to TGF-beta1 between both cell types. Because such cell types are key cells in the fibrogenesis, HS-P and MT-9 might be useful models for investigating the pathogenesis of fibrosis in vitro.

Effects of lipopolysaccharide on a macrophage-like cell line (HS-P) from a rat histiocytic sarcoma

Lipopolysaccharide (LPS) is a major modulator of macrophage functions. To characterize a newly established rat histiocytic sarcoma-derived cell line (HS-P), immunophenotypic changes and cellular growth responses of HS-P cells exposed to LPS were investigated and compared with those of MT-9 cells isolated from a rat malignant fibrous histiocytoma. MT-9 cells have somewhat histiocytic features, because occasional cells react to rat macrophage-specific antibodies. Addition of LPS to cultured HS-P cells increased the numbers of cells immunopositive to ED1 (rat macrophage-specific antibody) and ED2 (rat histiocyte-specific antibody) and stimulated the phagocytosis of latex beads, whereas LPS-treated MT-9 cells did not show such immunophenotypic changes. LPS-treated HS-P cells showed enhanced immunolabelling of alpha-smooth muscle actin, suggesting a possible modulation of macrophages towards myofibroblastic cells. To evaluate cellular growth after the addition of LPS or fetal bovine serum, DNA synthesis was examined by measuring tritiated thymidine incorporation, and the mRNA expression of c- jun and c- myc (immediate early genes in the cell cycle) was examined by Northern blot analysis. In HS-P cells, the addition of serum greatly increased DNA synthesis and induced high expression of c- jun and c- myc; in contrast, LPS markedly depressed DNA synthesis and reduced the expression of c- jun and c- myc. HS-P cells were more sensitive than MT-9 cells to the growth-promoting effect of serum and the growth-inhibiting effect of LPS. The study demonstrated that HS-P cells are highly LPS-responsive, indicating that they would be useful for studies of macrophage functions.

Cyclin D1 is an early target in hepatocyte proliferation induced by thyroid hormone (T3)

The thyroid hormone (T3) affects cell growth, differentiation, and regulates metabolic functions via its interaction with the thyroid hormone nuclear receptors (TRs). The mechanism by which TRs mediate cell growth is unknown. To investigate the mechanisms responsible for the mitogenic effect of T3, we have determined changes in activation of transcription factors, mRNA levels of immediate early genes, and levels of proteins involved in the progression from G1 to S phase of the cell cycle. We show that hepatocyte proliferation induced by a single administration of T3 to Wistar rats occurred in the absence of activation of AP-1, NF-kappa B, and STAT3 or changes in the mRNA levels of the immediate early genes c-fos, c-jun, and c-myc. These genes are considered to be essential for liver regeneration after partial hepatectomy (PH). On the other hand, T3 treatment caused an increase in cyclin D1 mRNA and protein levels that occurred much more rapidly compared to liver regeneration after 2/3 PH. The early increase in cyclin D1 expression was associated with accelerated onset of DNA synthesis, as demonstrated by a 20-fold increase of bromodeoxyuridine-positive hepatocytes at 12 h after T3 treatment and by a 20-fold increase in mitotic activity at 18 h. An early increase of cyclin D1 expression was also observed after treatment with nafenopin, a ligand of a nuclear receptor (peroxisome proliferator-activated receptor alpha) of the same superfamily of steroid/thyroid receptors. T3 treatment also resulted in increased expression of cyclin E, E2F, and p107 and enhanced phosphorylation of pRb, the ultimate substrate in the pathway leading to transition from G1 to S phase. The results demonstrate that cyclin D1 induction is one of the earlier events in hepatocyte proliferation induced by T3 and suggest that this cyclin might be a common target responsible for the mitogenic activity of ligands of nuclear receptors.

Early increase in cyclin-D1 expression and accelerated entry of mouse hepatocytes into S phase after administration of the mitogen 1, 4-Bis[2-(3,5-Dichloropyridyloxy)] benzene

We have previously demonstrated that hepatocyte proliferation induced by the mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) is independent of changes in cytokines, immediate early genes, and transcription factors that are considered to be necessary for regeneration of the liver after partial hepatectomy (PH) or necrosis. To further investigate the differences between mitogen-induced mouse hepatocyte proliferation and liver regeneration after PH, we have measured the expression of cyclin D1, cyclin D3, cyclin E, and cyclin A and of the cyclin-dependent kinases CDK2, CDK4, and CDK6. The involvement of the cyclin-dependent kinase inhibitors p21 and p27 and of the oncosuppressor gene p53 was also examined at different times after stimulation of hepatocyte proliferation. Results showed that a single administration of TCPOBOP caused a very rapid increase in the levels of cyclin D1, a G1 protein, when compared with two thirds PH (8 hours versus 30 hours). The early increase in cyclin D1 protein levels was associated with a faster onset of increased expression of S-phase-associated cyclin A (24 hours versus 36 hours with PH mice). Accordingly, measurement of bromodeoxyuridine (BrdU) incorporation revealed that, although approximately 8% of hepatocytes were BrdU-positive as early as 24 hours after TCPOBOP, no significant changes in BrdU incorporation were observed at the same time point after two thirds PH. The expression of other proteins involved in cell cycle control, such as cyclin-dependent kinases (CDK4, CDK2, CDK6), was also analyzed. Results showed that expression of CDK2 was induced much more rapidly in TCPOBOP-treated mice (2 hours) than in mice subjected to PH (36 hours). A different pattern of expression in the two models of hepatocyte proliferation, although less dramatic, was also observed for CDK4 and CDK6. Expression of the CDK inhibitors p21 and p27 and the oncosuppressor gene p53 variably increased after two thirds PH, whereas basically no change in protein levels was found in TCPOBOP-treated mice. The results demonstrate that profound differences in many cell cycle-regulatory proteins exist between direct hyperplasia and compensatory regeneration. Cyclin D1 induction is one of the earlier events in hepatocyte proliferation induced by the primary mitogen TCPOBOP and suggests that a direct effect of the mitogen on this cyclin may be responsible for the rapid onset of DNA synthesis observed in TCPOBOP-induced hyperplasia.

Immediate early gene expression in dog thyrocytes in response to growth, proliferation, and differentiation stimuli

In dog thyroid cells, insulin or IGF-1 induces cell growth and is required for the mitogenic action of TSH through cyclic AMP, of EGF, and of phorbol esters. HGF per se stimulates cell proliferation and is thus the only full mitogenic agent. TSH and cAMP enhance, whereas EGF phorbol esters and HGF repress differentiation expression. In this study, we have investigated for each factor and regulatory cascade of the intermediate step of immediate early gene induction, that is, c-myc, c-jun, jun D, jun B, c-fos, fos B, fra-1, fra-2, and egr1; fra-1 and fra-2 expressions were very low. TSH or forskolin increased the levels of c-myc, jun B, jun D, c-fos, and fos B while decreasing those of c-jun and egr1. Phorbol myristate ester stimulated the expression of all the genes. EGF and HGF stimulated the expression of all the genes except jun D and for EGF fos B. All these effects were obtained in the presence and in the absence of insulin, which shows that insulin is not necessary for the effects of the mitogens on immediate early gene expression. The definition of the repertoire of early immediate genes inductible by the various growth cascades provides a framework for the analysis of gene expression in tumors. (1) Insulin was able to induce all the protooncogenes investigated except fos B. This suggests that fos B could be the factor missing for insulin to induce mitogenesis. (2) No characteristic pattern of immediate early gene expression has been observed for insulin, which induces cell hypertrophy and is permissive for the action of the other growth factors. These effects are therefore not accounted for by a specific immediate early gene expression. On the other hand, insulin clearly enhances the effects of TSH, phorbol ester, and EGF on c-myc, junB, and c-fos expression. This suggests that the effect of insulin on mitogenesis might result from quantitative differences in the transcription complexes formed. (3) c-myc, c-fos, and jun B mRNA induction by all stimulating agents, whether inducing cell hypertrophy, or growth and dedifferentiation, or growth and differentiation, suggests that, although these expressions are not sufficient, they may be necessary for the various growth responses of thyroid cells. (4) The inhibition of c-jun and egr1 mRNA expression, and the marked induction of jun D mRNA appear to be specific features of the TSH cAMP pathway. They might be related to its differentiating action. (5) fos B, which is induced by TSH, forskolin, phorbol ester, and HGF but not by insulin, could be involved in the mitogenic action of the former factors.

The Ras/Rac1/Cdc42/SEK/JNK/c-Jun cascade is a key pathway by which agonists stimulate DNA synthesis in primary cultures of rat hepatocytes

The ability of signaling via the JNK (c-Jun NH2-terminal kinase)/stress-activated protein kinase cascade to stimulate or inhibit DNA synthesis in primary cultures of adult rat hepatocytes was examined. Treatment of hepatocytes with media containing hyperosmotic glucose (75 mM final), tumor necrosis factor alpha (TNFalpha, 1 ng/ml final), and hepatocyte growth factor (HGF, 1 ng/ml final) caused activation of JNK1. Glucose, TNFalpha, or HGF treatments increased phosphorylation of c-Jun at serine 63 in the transactivation domain and stimulated hepatocyte DNA synthesis. Infection of hepatocytes with poly-L-lysine-coated adenoviruses coupled to constructs to express either dominant negatives Ras N17, Rac1 (N17), Cdc42 (N17), SEK1-, or JNK1- blunted the abilities of glucose, TNFalpha, or HGF to increase JNK1 activity, to increase phosphorylation of c-Jun at serine 63, and to stimulate DNA synthesis. Furthermore, infection of hepatocytes by a recombinant adenovirus expressing a dominant-negative c-Jun mutant (TAM67) also blunted the abilities of glucose, TNFalpha, and HGF to stimulate DNA synthesis. These data demonstrate that multiple agonists stimulate DNA synthesis in primary cultures of hepatocytes via a Ras/Rac1/Cdc42/SEK/JNK/c-Jun pathway. Glucose and HGF treatments reduced glycogen synthase kinase 3 (GSK3) activity and increased c-Jun DNA binding. Co-infection of hepatocytes with recombinant adenoviruses to express dominant- negative forms of PI3 kinase (p110alpha/p110gamma) increased basal GSK3 activity, blocked the abilities of glucose and HGF treatments to inhibit GSK3 activity, and reduced basal c-Jun DNA binding. However, expression of dominant-negative PI3 kinase (p110alpha/p110gamma) neither significantly blunted the abilities of glucose and HGF treatments to increase c-Jun DNA binding, nor inhibited the ability of these agonists to stimulate DNA synthesis. These data suggest that signaling by the JNK/stress-activated protein kinase cascade, rather than by the PI3 kinase cascade, plays the pivotal role in the ability of agonists to stimulate DNA synthesis in primary cultures of rat hepatocytes.