Cellular analysis of c-Ha-ras gene expression in rat liver after CCl4 administration

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.

Differential protooncogene expression in Sprague Dawley and Fischer 344 rats during 1,2-dichlorobenzene-induced hepatocellular regeneration

Significant differences in hepatotoxic injury of 1,2-dichlorobenzene (o-DCB) have been reported (Gunawardhana, L., Sipes, I.G., 1991. Dichlorobenzene hepatotoxicity strain differences and structure activity relationships. Adv. Exp. Med. Biol. 283, 731-734; Stine, E.R., Gunawardhana, L., Sipes, I.G., 1991. The acute hepatotoxicity of the isomers of dichlorobenzene in Fischer 344 and Sprague-Dawley rats: isomer specific and strain-specific differential toxicity. Toxicol. Appl. Pharmacol. 109, 472-481; Valentovic, M.A., Ball, J. G., Anestis, D., Madan E., 1993a. Acute hepatic and renal toxicity of dichlorobenzene isomers in Fischer 344 rats. J. Appl. Toxicol. 13, 1-7; Kulkarni, S.G., Duong, H., Gomila, R., Mehendale, H.M., 1996. Strain differences in tissue repair response to 1,2-dichlorobenzene. Arch. Toxicol. 70, 714-723. Kulkarni, S.G., Warbritton, A., Bucci, T., Mehendale, H.M., 1997. Antimitotic intervention with colchicine alters the outcome of o-DCB-induced hepatotoxicity in Fischer 344 rats. Toxicology. 120, 79-88). Although, hepatotoxic injury of o-DCB is greater in Fischer 344 (F344) when compared with Sprague Dawley (S-D) rats, this interstrain difference does not transcend into any difference in lethal effects of o-DCB. Interstrain difference in compensatory tissue repair has been suggested as the underlying mechanism for the lack of strain differences in lethality (Kulkarni et al., 1996; Kulkarni et al., 1997, see these refs. above). However, the mechanism(s) for this interstrain difference in tissue repair is (are) not currently understood. The objectives of the present study were (1) to investigate if the differences in compensatory tissue repair are reflected in differential protooncogene expression in S-D versus F344 rat livers and (2) to investigate if changes in protooncogene expression could explain the decrease and delay in tissue repair response beyond a threshold of 0.6 ml o-DCB/kg. Male S-D and F344 rats (8/9 weeks old) were administered either 0.6 or 1.2 ml o-DCB/kg and changes in expression of protooncogenes c-myc (immediate early) and Ha-ras (delayed early) were examined over a time course. Findings of this study indicate that the timing and extent of c-myc and Ha-ras expression varies in the two strains following administration of o-DCB. Thus, the timing and extent of compensatory liver regeneration that ensues following o-DCB administration in S-D and F344 rats is temporally concordant with the protooncogene expression in the two strains.

Toxicodynamics of low level toxicant interactions of biological significance: inhibition of tissue repair

Because of the complexity of studying the toxicological effects of mixtures of chemicals, much of the mechanistic information has become available through work with binary mixtures of toxic chemicals. Mechanisms derived from studies employing chemicals at individually nontoxic doses are more useful than the mechanisms of interactive toxicity at high doses from the perspective of environmental and public health. Several examples of chemical combinations and interactive toxicity at low doses are now available. Chlordecone-potentiated halomethane hepatotoxicity, where suppression of cell division and tissue repair response permits very high amplification of CCl4 injury culminating in animal mortality, is one such model. Phenobarbital-potentiated CCl4 injury does not lead to animal mortality in spite of much higher liver injury in comparison to the chlordecone+CCl4 model. Much higher stimulation of tissue repair allows the animals to survive despite higher liver injury. Similar interactions have been reported between alcohols and halomethane toxicants. These and other studies have revealed that infliction of toxicant-induced injury is accompanied by a parallel but opposing tissue repair stimulation response which allows the animals to overcome that injury up to a threshold dose. Beyond this threshold, tissue repair response is both diminished and delayed allowing unrestrained progression of injury. Large doses of chemicals can be predictably lethal owing to these two latter effects on tissue repair. Dose-response paradigms in which tissue repair response is measured as a parallel but opposing effect to toxic injury might be useful in more precise prediction of the ultimate outcome of toxic injury in risk assessment. Autoprotection experiments with CCl4, thioacetamide, 2-butoxyethanol and related chemicals as well as heteroprotection against acetaminophen-induced lethality with thioacetamide are examples where tissue repair stimulation has been shown to rescue the animals from massive and normally lethal liver injury. The concept of toxicodynamic interaction between inflicted injury and stimulated tissue repair offers mechanistic opportunity to fine-tune other aspects of human health risk assessment procedure. Tissue repair mechanisms may also offer a mechanistic basis to explain species and strain differences as well as to more accurately assess inter-individual differences in human sensitivity to toxic chemicals. Because tissue repair is affected by nutritional status, assessment of risk from exposure to chemicals without attention to nutritional status may be misleading. Finally, the concept of using maximum tolerated doses (MTDs) in long-term toxicity studies such as cancer bioassays may need to be re-examined. MTDs might be predictably expected to maximally stimulate cell division and it is known that increased cell division is likely to lead to increased number of errors in DNA replication thereby predisposing these animals to cancer. It is clear that detailed studies of toxicodynamic interaction between tissue injury and stimulated tissue repair are likely to yield significant dividends in fine-tuning risk assessment.

Age-related differences in TGF-alpha and proto-oncogenes expression in rat liver after a low dose of carbon tetrachloride

The resiliency of rats during early post-natal development to CCl4 or to an interactive hepatotoxicity of chlordecone (CD) + CCl4 has been shown to be due to an efficient stimulation of tissue repair. The objective of the current study was to investigate if this is due to efficient expression of transforming growth factor-alpha (TGF-alpha) and proto-oncogenes. Postnatally developing (20 day old) and adult (60 day old) male Sprague-Dawley rats were challenged with a single low dose of CCl4 (100 microL/kg, ip) or corn oil. Liver samples were collected during a time course (0-96 h) after the administration of CCl4 and used to examine TGF-alpha and early (c-fos) and late (H-ras and K-ras) proto-oncogenes mRNA expressions. Significant increases in TGF-alpha, H-ras, and K-ras gene expressions were evident as early as 12 hours after CCl4 and peaked between 24 and 48 hours in an age-dependent manner as detected by slot-blot analysis. Results of the study revealed three- and twofold increases in TGF-alpha gene expression in 20 and 60 day old rats, respectively, after CCl4. There were 3.5- and 2.5-fold increases in H-ras and 4.4- and 3.4-fold increases in K-ras in 20 and 60 day old rats, respectively. In contrast, a 10-fold increase in c-fos mRNA expression was evident in 20 day old rats 1 hour after CCl4 treatment, returning to the baseline value by 3 hours, whereas in 60 day old rats, this increase was less than twofold. The overall findings of this study indicate that TGF-alpha and the early and late proto-oncogene mRNA expressions were enhanced in an age- and time-dependent manner in response to a low dose of CCl4. These results further strengthen the view that the remarkable resiliency of rats to hepatotoxicants during early postnatal development is due to substantial increases in stimulation of hepatocellular regeneration and tissue repair mechanisms, leading to regression of liver injury and recovery.

Expression and phosphorylation of rat c-met/hepatocyte growth factor receptor during rat liver regeneration

Hepatocyte growth factor receptor is identified as a heterodimeric tyrosine kinase encoded by the c-met gene. This study was designed to determine how the c-met/hepatocyte growth factor receptor participates in the intracellular events involved in rat liver regeneration induced by administration of carbon tetrachloride. Expression of the rat c-met mRNA increased, peaking 24 h after carbon tetrachloride administration almost in parallel with MET protein expression. Histochemical studies demonstrated that expression of the rat c-met was enhanced in cells surrounding the damaged areas, and also that the distribution of cells expressing MET was almost in accordance with that of cells expressing proliferating cells nuclear antigen. The MET protein underwent intense tyrosine phosphorylation peaking at 12 h after carbon tetrachloride administration, and prior to DNA synthesis. Phospholipase C gamma and phosphatidylinositol 3-kinase, intracellular signal transducing molecules containing Src homology 2 domain, were associated with the MET protein following tyrosine phosphorylation in vivo. These observations suggest that expression and tyrosine phosphorylation of MET protein associated with signal transducing molecules may provide a mechanism whereby hepatocyte growth factor exerts its action on hepatocyte growth during rat liver regeneration induced by carbon tetrachloride administration.

Changes in DNA strand breaks in non parenchymal cells following hepatocyte regeneration in CCl4-induced rat liver injury

DNA strand breaks (nicks) in non-parenchymal cells (NPCs) in CCl4-induced acute or chronic liver injury in rats were detected using an in situ nick translation method; their dynamic changes were analysed in relation to the proliferation pattern of hepatocytes and NPCs, as revealed by bromodeoxyuridine (BrdU)-uptake. In acute injury, hepatocyte proliferation started before centrilobular necrosis had occurred, whereas BrdU-labeled sinusoidal NPCs markedly increased only after centrilobular necrosis was apparent. DNA breakages in NPCs paralleled the proliferation pattern of these cells, suggesting that nicks are physiological, and reflect proliferation and activated gene expression. In chronic injury, liver cirrhosis developed after 9 weeks, but BrdU-labeling of hepatocytes was almost the same level as that in untreated liver. The number of BrdU-labeled NPCs showed only a slight increase, while those with DNA breakages were much more frequent in the cirrhotic stage, suggesting a significant role for NPCs in the fibrotic process. These results indicate that DNA strand breaks in NPCs act as a marker for activation states such as proliferation, differentiation and/or activated gene expression.

Activation of nuclear protooncogenes and alpha-fetoprotein gene in rat liver during the acute inflammatory reaction

Nuclear protooncogene and alpha-fetoprotein gene expression is stimulated in hepatocytes during liver regeneration and by various growth factors in vitro. Metabolic adaptation of hepatocytes has been implicated in such gene reprogrammation. We examine here whether induction of an acute inflammation, a physiological situation of important metabolic adjustments, also triggers activation of nuclear oncogenes and of the AFP gene in rat liver. C-fos, c-jun and c-myc mRNA accumulated on Northern blots between 4-12 h of inflammation and the steady-state level of two small alpha-fetoprotein transcripts characteristic of the adult liver increased at 4 h and 24 h of inflammation. In situ hybridization showed accumulation of the mRNA of the four genes studied in all hepatocytes, without any zonal lobular heterogeneity. 3H-histoautoradiography and mitotic counts indicated an inhibition of DNA synthesis and mitosis, prolonged for at least 48 h after inflammation. Thus acute inflammation triggers the activation of nuclear protooncogenes and alpha-feto-protein gene in hepatocytes, but this activation is not followed by passage into the replicative cycle.

Timing of protooncogene expression varies in toxin-induced liver regeneration

Hepatic expression of the protooncogenes c-fos and c-myc occurs within 2 h after partial hepatectomy, and these immediate early genes are thought to prime the hepatocytes for subsequent proliferation. To examine whether such gene activation occurred in the setting of hepatocyte proliferation after toxic liver injury, protooncogene expression was examined during the regenerative response following liver injury from carbon tetrachloride (CCl4) or galactosamine (GalN). The pattern of protooncogene expression after CCl4 mirrored that seen after partial hepatectomy, with rises in c-fos and c-myc mRNA content within 2 h, and then a rapid return to baseline levels. In contrast, early c-fos and c-myc expression did not occur after GalN injury. Instead GalN-induced regeneration led to a delayed, and prolonged c-fos and c-myc activation which peaked 24-48 h after injury. Increases in c-jun, jun-B, and jun-D mRNA levels also occurred in both models at times similar to the rises of c-fos and c-myc expression. Although the timing of DNA synthesis was identical after GalN or CCl4 treatment, the proliferative response after GalN injury was significantly less than that of CCl4, and marked by the histologic appearance of oval cells. The coadministration of 2-acetylaminofluorene, an inhibitor of differentiated hepatocyte proliferation, together with CCl4 altered the usual pattern of post-CCl4 protooncogene expression to one resembling that seen after GalN injury. Thus, the timing of protooncogene expression during liver regeneration may vary considerably. These variations may influence the nature of the proliferative response in terms of which cell type(s) proliferates, and the amount of regeneration that ensues.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Hepatocyte growth factor (hepatopoietin A) rapidly increases in plasma before DNA synthesis and liver regeneration stimulated by partial hepatectomy and carbon tetrachloride administration

An enzyme-linked immunosorbent assay was used to measure the level of hepatocyte growth factor in rat plasma at various times after two-thirds partial hepatectomy or CCl4 administration. An initial 17-fold rise and 13-fold rise in the level of hepatocyte growth factor was observed 2 hr after partial hepatectomy and CCl4 treatment, respectively, well before the onset of DNA synthesis in the liver. The peaks of DNA synthesis in remnant livers and livers exposed to CCl4 occurred at 24 hr and 48 hr, respectively, as determined by 5-bromo-2'-deoxyuridine labeling and [3H]thymidine uptake by the liver. A later peak level (17-fold above control) of hepatocyte growth factor at 24 hr after CCl4 treatment coincided with strong immunostaining of damaged or necrotic hepatocytes around central veins with an antibody to hepatocyte growth factor. This suggests a later intrahepatic origin of the signals for liver regeneration after hepatotoxic injury subsequent to the early extrahepatic production of hepatocyte growth factor at 2 hr after CCl4 administration. The absence of staining in the liver remnants in partially hepatectomized rats implies that the increase in hepatocyte growth factor seen in the plasma is caused by production at extrahepatic site(s). Possible sources include the pancreas, brain, thyroid and salivary glands, and Brunner's glands of the duodenum. Norepinephrine also increases in plasma as early as 2 hr after hepatectomy. In vitro, [3H]thymidine incorporation into hepatocyte DNA in the presence of hepatocyte growth factor is greater if 10(-5) mol/L norepinephrine is also present in the media.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential protooncogene expression during regeneration in rat stomach

Cellular protooncogenes such as c-myc and c-Ha-ras may play important roles in the control of regeneration of the stomach. In this study, in situ hybridization histochemistry and immunohistochemistry were used to determine how these protooncogenes and the corresponding oncoproteins are expressed at the cellular level during gastric regeneration after mucosal injuries caused by indomethacin. In addition, cells in the S-phase were immunohistochemically detected by means of 5-bromo-2'-deoxyuridine incorporation. Expression of the c-myc gene was localized to nuclei and reached a maximum at 3 h, and that of the c-Ha-ras gene was localized to cytoplasm with a peak at 6-12 h after treatment on the mucous neck, parietal, chief, and enterochromaffinlike cells around the lesions. The distribution of cells in the S-phase roughly coincided with that of cells in which expression of the protooncogenes was detected. In conclusion, various types of gastric mucosal cells participated in the sequential regulated expression of cellular protooncogenes during regeneration of the rat stomach.