p53 May positively regulate hepatocyte proliferation in rats

Molecular Pathways Governing the Termination of Liver Regeneration

The liver has the unique capacity to regenerate, and up to 70% of the liver can be removed without detrimental consequences to the organism. Liver regeneration is a complex process involving multiple signaling networks and organs. Liver regeneration proceeds through three phases: the initiation phase, the growth phase, and the termination phase. Termination of liver regeneration occurs when the liver reaches a liver-to-body weight that is required for homeostasis, the so-called "hepatostat." The initiation and growth phases have been the subject of many studies. The molecular pathways that govern the termination phase, however, remain to be fully elucidated. This review summarizes the pathways and molecules that signal the cessation of liver regrowth after partial hepatectomy and answers the question, "What factors drive the hepatostat?" SIGNIFICANCE STATEMENT: Unraveling the pathways underlying the cessation of liver regeneration enables the identification of druggable targets that will allow us to gain pharmacological control over liver regeneration. For these purposes, it would be useful to understand why the regenerative capacity of the liver is hampered under certain pathological circumstances so as to artificially modulate the regenerative processes (e.g., by blocking the cessation pathways) to improve clinical outcomes and safeguard the patient's life.

mTORC2 Facilitates Liver Regeneration Through Sphingolipid-Induced PPAR-α-Fatty Acid Oxidation

BACKGROUND & AIMS

During liver regeneration after partial hepatectomy, the function and metabolic pathways governing transient lipid droplet accumulation in hepatocytes remain obscure. Mammalian target of rapamycin 2 (mTORC2) facilitates de novo synthesis of hepatic lipids. Under normal conditions and in tumorigenesis, decreased levels of triglyceride (TG) and fatty acids (FAs) are observed in the mTORC2-deficient liver. However, during liver regeneration, their levels increase in the absence of mTORC2.

METHODS

Rictor liver-specific knockout and control mice underwent partial hepatectomy, followed by measurement of TG and FA contents during liver regeneration. FA metabolism was evaluated by analyzing the expression of FA metabolism-related genes and proteins. Intraperitoneal injection of the peroxisome proliferator-activated receptor α (PPAR-α) agonist, p53 inhibitor, and protein kinase B (AKT) activator was performed to verify the regulatory pathways involved. Lipid mass spectrometry was performed to identify the potential PPAR-α activators.

RESULTS

The expression of FA metabolism-related genes and proteins suggested that FAs are mainly transported into hepatocytes during liver regeneration. The PPAR-α pathway is down-regulated significantly in the mTORC2-deficient liver, resulting in the accumulation of TGs. The PPAR-α agonist WY-14643 rescued deficient liver regeneration and survival in mTORC2-deficient mice. Furthermore, lipidomic analysis suggested that mTORC2 deficiency substantially reduced glucosylceramide (GluCer) content. GluCer activated PPAR-α. GluCer treatment in vivo restored the regenerative ability and survival rates in the mTORC2-deficient group.

CONCLUSIONS

Our data suggest that FAs are mainly transported into hepatocytes during liver regeneration, and their metabolism is facilitated by mTORC2 through the GluCer-PPAR-α pathway, thereby establishing a novel role for mTORC2 in lipid metabolism.

Cellular plasticity balances the metabolic and proliferation dynamics of a regenerating liver

The adult liver has an exceptional ability to regenerate, but how it maintains its specialized functions during regeneration is unclear. Here, we used partial hepatectomy (PHx) in tandem with single-cell transcriptomics to track cellular transitions and heterogeneities of ∼22,000 liver cells through the initiation, progression, and termination phases of mouse liver regeneration. Our results uncovered that, following PHx, a subset of hepatocytes transiently reactivates an early-postnatal-like gene expression program to proliferate, while a distinct population of metabolically hyperactive cells appears to compensate for any temporary deficits in liver function. Cumulative EdU labeling and immunostaining of metabolic, portal, and central vein-specific markers revealed that hepatocyte proliferation after PHx initiates in the midlobular region before proceeding toward the periportal and pericentral areas. We further demonstrate that portal and central vein proximal hepatocytes retain their metabolically active state to preserve essential liver functions while midlobular cells proliferate nearby. Through combined analysis of gene regulatory networks and cell-cell interaction maps, we found that regenerating hepatocytes redeploy key developmental regulons, which are guided by extensive ligand-receptor-mediated signaling events between hepatocytes and nonparenchymal cells. Altogether, our study offers a detailed blueprint of the intercellular crosstalk and cellular reprogramming that balances the metabolic and proliferative requirements of a regenerating liver.

Cellular plasticity balances the metabolic and proliferation dynamics of a regenerating liver

The adult liver has an exceptional ability to regenerate, but how it maintains its specialized functions during regeneration is unclear. Here, we used partial hepatectomy (PHx) in tandem with single-cell transcriptomics to track cellular transitions and heterogeneities of ∼22,000 liver cells through the initiation, progression, and termination phases of mouse liver regeneration. Our results uncovered that, following PHx, a subset of hepatocytes transiently reactivates an early-postnatal-like gene expression program to proliferate, while a distinct population of metabolically hyperactive cells appears to compensate for any temporary deficits in liver function. Cumulative EdU labeling and immunostaining of metabolic, portal, and central vein-specific markers revealed that hepatocyte proliferation after PHx initiates in the midlobular region before proceeding toward the periportal and pericentral areas. We further demonstrate that portal and central vein proximal hepatocytes retain their metabolically active state to preserve essential liver functions while midlobular cells proliferate nearby. Through combined analysis of gene regulatory networks and cell-cell interaction maps, we found that regenerating hepatocytes redeploy key developmental regulons, which are guided by extensive ligand-receptor-mediated signaling events between hepatocytes and nonparenchymal cells. Altogether, our study offers a detailed blueprint of the intercellular crosstalk and cellular reprogramming that balances the metabolic and proliferative requirements of a regenerating liver.

Cell cycle regulation in NAFLD: when imbalanced metabolism limits cell division

Cell division is essential for organismal growth and tissue homeostasis. It is exceptionally significant in tissues chronically exposed to intrinsic and external damage, like the liver. After decades of studying the regulation of cell cycle by extracellular signals, there are still gaps in our knowledge on how these two interact with metabolic pathways in vivo. Studying the cross-talk of these pathways has direct clinical implications as defects in cell division, signaling pathways, and metabolic homeostasis are frequently observed in liver diseases. In this review, we will focus on recent reports which describe various functions of cell cycle regulators in hepatic homeostasis. We will describe the interplay between the cell cycle and metabolism during liver regeneration after acute and chronic damage. We will focus our attention on non-alcoholic fatty liver disease, especially non-alcoholic steatohepatitis. The global incidence of non-alcoholic fatty liver disease is increasing exponentially. Therefore, understanding the interplay between cell cycle regulators and metabolism may lead to the discovery of novel therapeutic targets amenable to intervention.

Tissue repair brakes: A common paradigm in the biology of regeneration

To date, most attention on tissue regeneration has focused on the exploration of positive cues promoting or allowing the engagement of natural cellular restoration upon injury. In contrast, the signals fostering cell identity maintenance in the vertebrate body have been poorly investigated; yet they are crucial, for their counteraction could become a powerful method to induce and modulate regeneration. Here we review the mechanisms inhibiting pro-regenerative spontaneous adaptive cell responses in different model organisms and organs. The pharmacological or genetic/epigenetic modulation of such regenerative brakes could release a dormant but innate adaptive competence of certain cell types and therefore boost tissue regeneration in different situations.

Propofol attenuates cytokine-mediated upregulation of expression of inducible nitric oxide synthase and apoptosis during regeneration post-partial hepatectomy

Purpose:

To determine the effects of propofol and ketamine anesthesia on liver regeneration in rats after partial hepatectomy (PHT).

Methods:

Male Wistar albino rats were assigned randomly to four groups of 10. Anesthesia was induced and maintained with propofol in groups 1 and 2, and with ketamine in groups 3 and 4. PHT was undertaken in groups 1 and 3. Rats in groups 2 and 4 (control groups) underwent an identical surgical procedure, but without PHT. At postoperative day-5, rats were killed. Regenerated liver was removed, weighed, and evaluated (by immunohistochemical means) for expression of inducible nitric oxide synthase (iNOS), endothelial NOS (eNOS), apoptosis protease-activating factor (APAF)-1, and proliferating cell nuclear antigen (PCNA). Also, blood samples were collected for measurement of levels of tumor necrosis factor (TNF)-α and interleukin (IL)-6.

Results:

Between groups 2 and 4, there were no differences in tissue levels of iNOS, eNOS, and APAF-1 or plasma levels of TNF-α and IL-6. eNOS expression was similar in group 1 and group 3. Expression of iNOS and APAF-1 was mild-to-moderate in group 1, but significantly higher in group 3. Groups 1 and 3 showed an increase in PCNA expression, but expression in both groups was comparable. Plasma levels of TNF-α and IL-6 increased to a lesser degree in group 1 than in group 3.

Conclusion:

Propofol, as an anesthetic agent, may attenuate cytokine-mediated upregulation of iNOS expression and apoptosis in an animal model of liver regeneration after partial hepatectomy.

Proliferation‑inhibiting pathways in liver regeneration (Review)

Liver regeneration, an orchestrated process, is the primary compensatory mechanism following liver injury caused by various factors. The process of liver regeneration consists of three stages: Initiation, proliferation and termination. Proliferation‑promoting factors, which stimulate the recovery of mitosis in quiescent hepatocytes, are essential in the initiation and proliferation steps of liver regeneration. Proliferation‑promoting factors act as the 'motor' of liver regeneration, whereas proliferation inhibitors arrest cell proliferation when the remnant liver reaches a suitable size. Certain proliferation inhibitors are also expressed and activated in the first two steps of liver regeneration. Anti‑proliferation factors, acting as a 'brake', control the speed of proliferation and determine the terminal point of liver regeneration. Furthermore, anti‑proliferation factors function as a 'steering‑wheel', ensuring that the regeneration process proceeds in the right direction by preventing proliferation in the wrong direction, as occurs in oncogenesis. Therefore, proliferation inhibitors to ensure safe and stable liver regeneration are as important as proliferation‑promoting factors. Cytokines, including transforming growth factor‑β and interleukin‑1, and tumor suppressor genes, including p53 and p21, are important members of the proliferation inhibitor family in liver regeneration. Certain anti‑proliferation factors are involved in the process of gene expression and protein modification. The suppression of liver regeneration led by metabolism, hormone activity and pathological performance have been reviewed previously. However, less is known regarding the proliferation inhibitors of liver regeneration and further investigations are required. Detailed information regarding the majority of known anti‑proliferation signaling pathways also remains fragmented. The present review aimed to understand the signalling pathways that inhbit proliferation in the process of liver regeneration.

Integrated analysis of microRNA and mRNA expression profiles highlights the complex and dynamic behavior of toosendanin-induced liver injury in mice

Triterpenoid Toosendanin (TSN) exhibits a plenty of pharmacological effects in human and great values in agriculture. However, the hepatotoxicity caused by TSN or Melia-family plants containing TSN used in traditional Chinese medicine has been reported, and the mechanisms of TSN-induced liver injury (TILI) still remain largely unknown. In this study, the dose- and time-dependent effects of TSN on mice liver were investigated by an integrated microRNA-mRNA approach as well as the general toxicological assessments. As the results, the dose- and time-dependent liver injury and alterations in global microRNA and mRNA expressions were detected. Particularly, 9-days 80 mg/kg TSN exposure caused most serious liver injury in mice, and the hepatic adaptation to TILI was unexpectedly observed after 21-days 80 mg/kg TSN administration. Based on the pathway analysis of the intersections between predicted targets of differentially expressed microRNAs and differentially expressed mRNAs at three time points, it revealed that TILI may be caused by glutathione depletion, mitochondrial dysfunction and lipid dysmetabolism, ultimately leading to hepatocytes necrosis in liver, while liver regeneration may play an important role in the hepatic adaptation to TILI. Our results demonstrated that the integrated microRNA-mRNA approach could provide new insight into the complex and dynamic behavior of TILI.

Cell biology is different in small volumes: endogenous signals shape phenotype of primary hepatocytes cultured in microfluidic channels

The approaches for maintaining hepatocytes in vitro are aimed at recapitulating aspects of the native liver microenvironment through the use of co-cultures, surface coatings and 3D spheroids. This study highlights the effects of spatial confinement-a less studied component of the in vivo microenvironment. We demonstrate that hepatocytes cultured in low-volume microfluidic channels (microchambers) retain differentiated hepatic phenotype for 21 days whereas cells cultured in regular culture plates under identical conditions de-differentiate after 7 days. Careful consideration of nutrient delivery and oxygen tension suggested that these factors could not solely account for enhanced cell function in microchambers. Through a series of experiments involving microfluidic chambers of various heights and inhibition of key molecular pathways, we confirmed that phenotype of hepatocytes in small volumes was shaped by endogenous signals, both hepato-inductive growth factors (GFs) such as hepatocyte growth factor (HGF) and hepato-disruptive GFs such as transforming growth factor (TGF)-β1. Hepatocytes are not generally thought of as significant producers of GFs–this role is typically assigned to nonparenchymal cells of the liver. Our study demonstrates that, in an appropriate microenvironment, hepatocytes produce hepato-inductive and pro-fibrogenic signals at the levels sufficient to shape their phenotype and function.

Induction of p53-dependent p21 limits proliferative activity of rat hepatocytes in the presence of hepatocyte growth factor

Background

Hepatocyte growth factor (HGF), a potent mitogen for hepatocytes, enhances hepatocyte function without stimulating proliferation, depending on the physiological conditions. p53, a transcription factor, suppresses the cell proliferation by expressing p21^WAF1/CIP1 in various tissues.

Aim

To investigate the mechanism through which the hepatocytes maintain mitotically quiescent even in the presence of HGF.

Methods

We studied the relationship between p53 and p21 expression and the effect of p53-p21 axis on hepatocyte proliferation in primary cultured rat hepatocytes stimulated by HGF. Hepatic p21 levels are determined serially after partial hepatectomy or sham operation in rats.

Results

DNA synthesis was markedly increased by HGF addition in rat hepatocytes cultured at low density but not at high density. Cellular p53 levels increased in the hepatocytes cultured at both the densities. p21 levels were increased and correlated with cellular p53 levels in hepatocytes cultured at high density but not at low density. When the activity of p53 was suppressed by a chemical inhibitor for p53, cellular p21 levels were reduced, and DNA synthesis was increased. Similarly, p21 antisense oligonucleotide increased the DNA synthesis. In rats after partial hepatectomy, transient elevation of hepatic p21 levels was observed. In contrast, in sham-operated rats, hepatic p21 levels were increased on sustained time scales.

Conclusion

p53-related induction of p21 may suppress hepatocyte proliferation in the presence of HGF in the setting that mitogenic activity of HGF is not elicitable.

An integrative model links multiple inputs and signaling pathways to the onset of DNA synthesis in hepatocytes

During liver regeneration, quiescent hepatocytes re-enter the cell cycle to proliferate and compensate for lost tissue. Multiple signals including hepatocyte growth factor, epidermal growth factor, tumor necrosis factor α, interleukin-6, insulin and transforming growth factor β orchestrate these responses and are integrated during the G₁ phase of the cell cycle. To investigate how these inputs influence DNA synthesis as a measure for proliferation, we established a large-scale integrated logical model connecting multiple signaling pathways and the cell cycle. We constructed our model based upon established literature knowledge, and successively improved and validated its structure using hepatocyte-specific literature as well as experimental DNA synthesis data. Model analyses showed that activation of the mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways was sufficient and necessary for triggering DNA synthesis. In addition, we identified key species in these pathways that mediate DNA replication. Our model predicted oncogenic mutations that were compared with the COSMIC database, and proposed intervention targets to block hepatocyte growth factor-induced DNA synthesis, which we validated experimentally. Our integrative approach demonstrates that, despite the complexity and size of the underlying interlaced network, logical modeling enables an integrative understanding of signaling-controlled proliferation at the cellular level, and thus can provide intervention strategies for distinct perturbation scenarios at various regulatory levels.

Comparative study of toxicological and cell cycle effects of okadaic acid and dinophysistoxin-2 in primary rat hepatocytes

AIMS

To determine the relative toxicity and effects on the cell cycle of okadaic acid and dinophysistoxin-2 in primary hepatocyte cultures.

MAIN METHODS

Cytotoxicity was determined by the MTT method, caspase-3 activity and lactate dehydrogenase release to the medium. The cell cycle analysis was performed by imaging flow cytometry and the effect of the toxins on cell proliferation was studied by quantitative PCR and confocal microscopy.

KEY FINDINGS

We show that dinophysistoxin-2 is less toxic than okadaic acid for primary hepatocytes with a similar difference in potency as that observed in vivo in mice after intraperitoneal injection. Both toxins induced apoptosis with caspase-3 increase. They also inhibited the hepatocytes cell cycle in G1 affecting diploid cells and diploid bi-nucleated cells. In proliferating hepatocytes exposed to the toxins, a decrease of p53 gene expression as well as a lower protein level was detected. Studies of the tubulin cytoskeleton in toxin treated cells, showed nuclear localization of this molecule and a granulated tubulin pattern in the cytoplasm.

SIGNIFICANCE

The results presented in this work show that the difference in toxicity between dinophysistoxin-2 and okadaic acid in cultured primary hepatocytes is the same as that observed in vivo after intraperitoneal injection. Okadaic acid and dinophysistoxin-2 arrest the cell cycle of hepatocytes at G1 even in diploid bi-nucleated cells. p53 and tubulin could be involved in the cell cycle inhibitory effect.

Effect of porto-systemic shunting on NOS expression after extended hepatectomy in rats

AIM

Several surgical procedures have been developed for reducing portal vein pressure to prevent postoperative liver injury. Nitric oxide synthase expression (NOS) induced by elevation of portal vein pressure is thought to play an important role in liver regeneration, but the details are not well understood.

METHODS

Rats in the control group and in the subcutaneous splenic transposition (SST) group underwent 90% partial hepatectomy. Survival and portal vein pressure were analyzed. The serum IL-6 and TNF-alpha levels were measured by enzyme-linked immunosorbent assay (ELISA). Hepatocyte proliferation and apoptosis 12 hours after hepatectomy were analyzed immunohistochemically. The protein and messenger RNA expression of inducible and endothelial NOS were analyzed using Western blotting and quantitative reverse transcriptase polymerase chain reaction, respectively.

RESULTS

The survival rate of the SST group was significantly higher. Portal vein pressure, TNF-alpha level and the apoptotic index were significantly lower in the SST group. Twelve hours after surgery, liver inducible NOS (iNOS) protein expression was significantly lower in the SST group. However, protein expression of endothelial NOS was not significantly different between the groups.

CONCLUSION

Inducible NOS expression after extended hepatectomy is related to the effects of porto-systemic shunting on the splanchnic circulation. Also, iNOS induction and concomitant nitric oxide generation appear to participate in the cytotoxicity of excessive portal pressure after extended hepatectomy.

Impaired liver regeneration in acute liver failure: the significance of cross-communication of growth associated factors in liver regeneration

Liver regeneration is considered to be retarded or suppressed in patients with acute liver failure. However, the mechanisms for this suppression remain to be elucidated. In order to evaluate deficiencies in liver regeneration in patients with acute liver failure, we focused on the cross-communication of growth-associated factors in experimental models. In primary cultured rat hepatocytes, the levels of cellular p53 and transforming growth factor-alpha (TGF-alpha), as well as DNA synthesis increased by the addition of hepatocyte growth factor (HGF). When p53 activity was suppressed, TGF-alpha expression and DNA synthesis were reduced. DNA synthesis was also reduced when TGF-alpha activity was suppressed. In rats after partial hepatectomy, hepatic HGF and p53 protein levels increased, followed by an increase of hepatic TGF-alpha levels and hepatocyte proliferation. Circulating levels of HGF and TGF-alpha correlated with changes in the hepatic levels. The suppression of TGF-alpha activity reduced the proliferation of hepatocytes in these rats. In patients suffering from acute hepatitis, serum HGF levels increased followed by an increase of serum TGF-alpha levels. In contrast, in patients with acute liver failure, the increase of serum TGF-alpha levels was suppressed depending on the severity of hepatic failure, even though serum HGF levels markedly increased. The patients' hepatic levels were consistent with serum levels. Cross-communication of growth associated factors may be important in the progression of liver regeneration, and impaired regenerative capacity in patients with acute liver failure may be attributable, at least in part, to a disruption of communication of growth-associated factors.

Protective effects of Ginkgo biloba against rat liver carcinogenesis

Ginkgo biloba (EGb) has been proposed as a promising candidate for cancer chemoprevention and has shown protective effects on the liver against chemically induced oxidative injury and fibrosis. The potential beneficial effects of EGb were investigated in two rat liver carcinogenesis bioassays induced by diethylnitrosamine (DEN). In a short-term study for anti-initiating screening, male Wistar rats were fed a basal diet or supplemented diet with 500 or 1000 ppm EGb and initiated 14 days later with a single dose of DEN (100 mg/kg i.p.). The respective groups were killed 24h or 2 weeks after DEN-initiation. Liver samples were collected for the analysis of proliferating cell nuclear antigen (PCNA), transforming growth factor alpha (TGF-alpha), p53, apoptosis and induction of single hepatocytes and minifoci positive for the enzyme glutathione S-transferase P-form (GST-P). In a medium-term study for anti-promoting screening, the animals received a single dose of DEN (200 mg/kg i.p.) and, 2 weeks later, were fed a basal diet or supplemented diet with 500 or 1000 ppm EGb for 6 weeks. All animals underwent 70% partial hepatectomy (PH) at week 3 and killed at week 8. Liver samples were collected to analyze development of preneoplastic foci of altered hepatocytes (FAH) expressing GST-P. In the short-term study, pretreatment of rats with 1000 ppm EGb significantly reduced the rates of cell proliferation, apoptosis and p53, TGF-alpha immunoreactivity and the number of GST-P-positive hepatocytes. In the medium-term study, EGb treatment during the post-initiation stage failed to reduce the development of DEN-induced GST-P-positive foci. Thus, EGb presented inhibitory actions during initiation but not promotion of rat liver carcinogenesis induced by DEN.

Activation of the p53-p21(Cip1) pathway is required for CDK2 activation and S-phase entry in primary rat hepatocytes

p53 plays a major role in the prevention of tumor development. It responds to a range of potentially oncogenic stresses by activating protective mechanisms, most notably cell-cycle arrest and apoptosis. The p53 gene is also induced during normal liver regeneration, and it has been hypothesized that p53 serve as a proliferative 'brake' to control excessive proliferation. However, it has lately been shown that p53 inhibition reduces hepatocyte growth factor-induced DNA synthesis of primary hepatocytes. Here we show that epidermal growth factor (EGF) activated p53 in a phosphatidylinositol-3 kinase-dependent way, and thus induced the cyclin-dependent kinase inhibitor p21(Cip1) in primary rat hepatocytes. p53 inactivation with a dominant-negative mutant (p53(V143A)) attenuated EGF-induced DNA synthesis and was associated with reduced CDK2 phosphorylation and retinoblastoma protein hyperphosphorylation. When p21(Cip1) was ectopically expressed in p53-inactivated cells, these effects were neutralized. In conclusion, our results demonstrate that in normal hepatocytes, EGF-induced expression of p53 is involved in regulating CDK2- and CDK4 activity, through p21(Cip1) expression.

Expression patterns of cytokine, growth factor and cell cycle-related genes after partial hepatectomy in rats with thioacetamide-induced cirrhosis

AIM: To examine the differences in the responses of normal and cirrhotic livers to partial hepatectomy in relation to the factors influencing liver regeneration.

METHODS: Cirrhosis was induced in rats by admini-stration of thioacetamide. Untreated rats were used as controls. The control rats as well as the cirrhotic rats were subjected to 70% partial hepatectomy. At different time points after hepatectomy, the livers were collected and the levels of cytokines, growth factors and cell cycle proteins were analyzed.

RESULTS: After hepatectomy, the cirrhotic remnant expressed significantly lower levels of cyclin D1, its kinase partner, cdk4, and cyclin E as compared to the controls up to 72 h post hepatectomy. Significantly lower levels of cyclin A and cdk2 were also observed while the cdk inhibitor, p27 was significantly higher. In addition, the cirrhotic group had lower IL-6 levels than the control group at all time points up to 72 h following resection.

CONCLUSION: The data from our study shows that impaired liver regeneration in cirrhotic remnants is associated with low expression of cyclins and cdks. This might be the consequence of the low IL-6 levels in cirrhotic liver remnant which would in turn influence the actions of transcription factors that regulate genes involved in cell proliferation and metabolic homeostasis during the regeneration process.

Pifithrin-alpha induced p53 inhibition does not affect liver regeneration after partial hepatectomy in mice

BACKGROUND/AIMS

Beside its well-known function as tumour suppressor gene, p53 is supposed to positively regulate cell division and cell differentiation. Because hepatocyte proliferation has been reported to be reduced by blockade of p53 function in vitro, we examined in the present study the impact of p53 inhibition on hepatocyte proliferation in vivo.

METHODS

Mice treated with either pifithrin-alpha (PFT), a p53-inactivating agent, or the equivalent volume of vehicle, were subjected to 70% hepatectomy. In addition to assessment of liver mass restitution we examined p53 and p21 protein expression as well as PCNA expression and BrdU incorporation by using Western blot and immunohistochemical techniques. Extent of apoptosis was assessed by TUNEL assay.

RESULTS

PFT lowered nuclear but not cytoplasmic p53, and did not inhibit protein expression of regeneration-associated p21. PCNA protein expression as well as PCNA and BrdU immunohistochemistry did not differ between regenerating livers of either PFT- or vehicle-treated animals. Moreover, TUNEL analysis of regenerated liver tissue revealed comparable numbers of apoptotic cells in both groups.

CONCLUSIONS

Pharmacological inhibition of p53 did not impair liver regeneration in mice, implying that p53 is functionally redundant in that p53-independent pathways compensate for the blockade of p53 and sufficiently support the process of hepatocyte replication in liver regeneration.

Hepatocellular carcinoma--survival and clinical characteristics in relation to various histologic molecular markers in Western patients

BACKGROUND

Many genes participate in the regulation of cell proliferation and growth of tumor cells. Altered expression and loss of function of some of these gene products have been found in malignant tumors and correlated with progression and poor prognosis.

AIMS

Our aim was to correlate the expression of various molecular histologic markers with tumor characteristics and survival time of patients with hepatocellular carcinoma (HCC).

PATIENTS AND METHODS

Tumor tissues of 81 patients with HCC were investigated immunohistochemically for the expression of cellular proliferation markers Mib1 (Ki67) and c-erbB-2 (HER2/neu), cellcycle markers (p53, mdm2 and p21), CD81 (TAPA1), a marker shown to be associated with metastasis, and human leukocyte antigen (HLA)-DR expression, involved in immunological antigen presentation.

RESULTS

p21 was expressed in a higher percentage (83.3 vs. 50%, P=0.014) in undifferentiated histological tumor grades (Edmondson Steiner G3 vs. G1/G2). HCC in patients with enlarged lymph nodes expressed HLA-DR in a higher percentage (28.6%) than tumors without lymph node enlargement (5.7%, P=0.006). Patients with distant metastases were less likely to express CD 81 (11.1%) on tumor cells than patients without distant metastases (38.3%, P=0.0335). No other correlation with clinical or tumor characteristics or molecular histologic markers investigated was found. P53 accumulating patients showed a worse survival than patients with tumors p53 non-accumulating (median 4.1 months vs. median 9.3 months, P=0.01798). Neither the expression nor the non-expression of proliferation, cell cycle, immunologic or cell adhesion markers was associated with differences in survival. However, patients with a low expression of cell cycle marker mdm2 survived significantly longer (median 9.4 months) as compared with patients with high expression (median 3.9 months).

CONCLUSION

Our results suggest that p53 nuclear accumulation and mdm2 high expression are associated with poor survival in patients withHCC. Furthermore, patients with enlarged lymph nodes had HLA-DR-positive tumors more frequently and patients with distant metastases had tumors with CD81 expression less often.

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Role of nitric oxide increase on induced programmed cell death during early stages of rat liver regeneration

We analysed the possible cellular mechanism involved in the NO action in the balance between apoptosis and cell proliferation in liver regeneration process. We determined p53, proapoptotic protein Bax, antiapoptotic Bcl-xL, proliferating cell nuclear antigen (PCNA) and apoptotic index at the early stages of regenerative process after NO increase by lipopolysaccharide-induction (LPS) of inducible-type nitric oxide synthase (iNOS) and by direct NO donor (sodium nitroprusside, SNP). Male Wistar rats were randomised in four experimental groups: sham operated control (Sh), partial hepatectomised control (PH-C), partial hepatectomised pretreated with LPS (2 mg/kg body weight, i.p.) (PH-LPS), and partial hepatectomised pretreated with SNP (2.5 mg/kg body weight, i.v. at a rate of 1 ml/h) (PH-SNP). Animals were killed 5 h post-surgery. Hepatic cytosolic iNOS showed an increase of 34% in PH-C animals with respect to Sh, and LPS-treatment increased iNOS protein levels 30% compared with PH-C. Bax and p53 protein levels showed significant increases in LPS- and SNP-treated hepatectomised rats with respect to PH-C. The apoptotic indexes were increased 75% in both, PH-LPS and PH-SNP rats versus PH-C. The increase of NO did not show any change in the proliferation process. These results suggest that NO is involved in apoptosis via p53 and Bax proteins after PH, showing a tightly regulated growth process in liver regeneration.

Significance of branched chain amino acids as possible stimulators of hepatocyte growth factor

Amino acids can serve as regulatory molecules that modulate numerous cellular functions. Branched chain amino acids (BCAAs) are known to exert influences on cellular metabolism, amino acid transport, protein turn over, and gene expression. However, the mechanisms involved in the specific effect of BCAAs have not been clarified. BCAA supplementation therapy is a current treatment for patients with liver cirrhosis, therefore, specific BCAA activities should be examined. Hepatocyte growth factor (HGF) is considered to be a pleiotropic factor, and is reported to modulate gene expression and to stimulate the proliferation and functions of many cell types, including hepatocytes. A potential application of HGF for several types of diseases has been postulated. Here, we describe the potential of BCAAs as a therapeutic agent that acts through the induction of HGF production in the liver.

A novel mitochondrial carnitine-acylcarnitine translocase induced by partial hepatectomy and fasting

The carnitine-dependent transport of long-chain fatty acids is essential for fatty acid catabolism. In this system, the fatty acid moiety of acyl-CoA is transferred enzymatically to carnitine, and the resultant product, acylcarnitine, is imported into the mitochondrial matrix through a transporter named carnitine-acylcarnitine translocase (CACT). Here we report a novel mammalian protein homologous to CACT. The protein, designated as CACL (CACT-like), is localized to the mitochondria and has palmitoylcarnitine transporting activity. The tissue distribution of CACL is similar to that of CACT; both are expressed at a higher level in tissues using fatty acids as fuels, except in the brain, where only CACL is expressed. In addition, CACL is induced by partial hepatectomy or fasting. Thus, CACL may play an important role cooperatively with its homologue CACT in a stress-induced change of lipid metabolism, and may be specialized for the metabolism of a distinct class of fatty acids involved in brain function.