Further characterisation of 'hepatotropin', a high molecular weight hepatotrophic factor in rat serum

Resistance of three immortalized human hepatocyte cell lines to acetaminophen and N-acetyl-p-benzoquinoneimine toxicity

BACKGROUND/AIMS

Acetaminophen toxicity in hepatocytes is attributed to generation of the toxic metabolite N-acetyl-p-benzoquinoneimine, leading to depletion of intracellular glutathione, alteration of redox potential and ultimately, cellular necrosis. We aimed to determine the effect of acetaminophen and N-acetyl-p-benzoquinoneimine on three human hepatocyte cell lines HH25, HH29 and HHY41, and for comparison, on primary rat hepatocytes, a cell type that is relatively resistant to acetaminophen-induced toxicity.

METHODS

We investigated the effect of incubation of rat hepatocytes and 3 hepatocyte cell lines with acetaminophen or N-acetyl-p-benzoquinoneimine on LDH release, glutathione status, mitochondrial function, CYP1A activity, albumin synthesis and DNA content.

RESULTS

We demonstrated that HH25, HH29 and HHY41 are resistant to the toxic effects of acetaminophen under conditions that induce cytotoxicity in rat primary hepatocytes, as indicated by maintenance of glutathione levels and basal LDH release. Incubation with N-acetyl-p-benzoquinoneimine caused a dose-dependent cytotoxicity in rat hepatocytes. Under comparable conditions N-acetyl-p-benzoquinoneimine had no effect on any of the hepatocyte cell lines. Nevertheless, when culturing the cells for a further 48 h, a decrease in glutathione levels, albumin synthesis, CYP1A activity, DNA content and mitochondrial function was apparent.

CONCLUSION

HH25, HH29 and HHY41 cells are highly resistant to acetaminophen and N-acetyl-p-benzoquinoneimine-induced toxicity. They tolerate a much higher concentration of both toxins for a longer period of time compared to rat primary hepatocytes. These results are of relevance in the use of these cell lines to investigate acetaminophen hepatotoxicity, and may be of importance in the choice of cells for use in bioartificial liver support systems.

Augmentation of the early phase of liver regeneration after 70% partial hepatectomy in rats following selective Kupffer cell depletion

BACKGROUND/AIMS

Kupffer cells are located in the liver sinusoids adjacent to hepatocytes and elaborate a range of growth regulatory molecules involved in regulating hepatocyte proliferation. In vitro observations imply the potential for Kupffer cells to exert both stimulatory and inhibitory influences on hepatocyte DNA synthesis. We aimed to determine the overall effect of Kupffer cell activity during the early regenerative processes after partial hepatectomy.

METHODS

We investigated hepatocyte DNA synthesis, induced by partial hepatectomy in rats, following selective elimination of Kupffer cells by liposome encapsulated dichlormethylene bisphosphonate (Cl2MBP).

RESULTS

We demonstrate that the early phase of liver regeneration was enhanced following Kupffer depletion, as indicated by a greater proportion of hepatocytes undergoing DNA synthesis, and a higher mitotic index. This was associated with an alteration in the balance of growth factors in the liver; HGF and TGFbeta mRNA were reduced in Kupffer cell-depleted animals, and IL-1beta mRNA was absent. In addition, in the absence of partial hepatectomy, the selective depletion of Kupffer cells leads to an increase in the proliferation of hepatocytes in resting liver undergoing DNA synthesis.

CONCLUSION

The overall effect of depleting the liver of Kupffer cells is to enhance the proliferation rate of hepatocytes, both after partial hepatectomy and in the resting state.

Gene expressions of c-met and hepatocyte growth factor in chronic liver disease and hepatocellular carcinoma

BACKGROUND/AIMS

The roles of c-met proto-oncogene and hepatocyte growth factor in human livers have not been shown.

METHODS

Gene expressions of both c-met and hepatocyte growth factor were quantified in livers with chronic active hepatitis and in cirrhotic livers with hepatocellular carcinoma as well as in normal controls, using competitive reverse transcription polymerase chain reaction.

RESULTS

C-met expression was significantly increased in chronic active hepatitis compared with control livers, and c-met expression in chronic active hepatitis correlated with serum alanine aminotransferase levels. Hepatocyte growth factor expression was increased in some patients with chronic active hepatitis compared with controls, and there was a significant correlation between c-met expression and hepatocyte growth factor expression. On the other hand, in hepatocellular carcinoma tissues, c-met expression was increased in some cases, while that in the surrounding non-carcinomatous tissues was similar to normal controls. Hepatocyte growth factor expression was not detected in the hepatocellular carcinoma tissues and was low in the surrounding non-carcinomatous tissues.

CONCLUSIONS

These findings suggest that hepatocyte growth factor may be involved in the regeneration of hepatocytes via paracrine mechanism in chronic active hepatitis, while in regulation of c-met expression in hepatocellular carcinoma tissues may be independent of hepatocyte growth factor stimulation.

Human spasmolytic polypeptide is a cytoprotective agent that stimulates cell migration

BACKGROUND/AIMS

Gastric epithelium is attacked by acid, pepsin, and ingested agents. When a mucosal lesion occurs, the defect is rapidly closed by cell migration. Because spasmolytic polypeptide is rapidly produced at sites of injury, we postulated that human spasmolytic polypeptide (hSP) was important in mucosal repair. Recombinant hSP was used to test this hypothesis.

METHODS

The ulcer healing effect of various doses of hSP administered orally and subcutaneously was examined using an indomethacin (20 mg/kg) restraint rat model of gastric damage. Stability of hSP in gastrointestinal juice was determined using size-exclusion chromatography. The effect of hSP on migration of human colonic carcinoma cell lines HT29 and SW480 was determined using collagen gel invasion and wounded monolayer assays. Proliferation was assessed using [3H]thymidine incorporation and toluidine blue staining.

RESULTS

Infusions of hSP at 25 and 50 micrograms.kg-1.h-1 subcutaneously decreased gastric damage by about 50% (P < 0.01) without changing acid secretion. Oral hSP was ineffective. hSP was stable in gastrointestinal juice. hSP stimulated migration of HT29 cells but did not affect proliferation and had no effect on SW480 cells.

CONCLUSIONS

hSP may play a key role in the early stages of mucosal repair by stimulating the initial re-epithelialization by cell migration.

Epidermal growth factor is digested to smaller, less active forms in acidic gastric juice

BACKGROUND/AIMS

Epidermal growth factor (EGF) is present in gastric juice and has potent mitogenic properties. The stability of EGF in gastric juice under various physiological and pathophysiological conditions was examined.

METHODS

Recombinant human EGF1-53 was incubated with HCl containing pepsin. We also determined the forms of EGF present in the gastric juice of patients under basal conditions, patients taking the acid suppressant omeprazole, patients with achlorhydria, and volunteers undergoing intragastric neutralization with NaHCO3 (n = 6 per group). Samples were analyzed using mass spectroscopy and/or high-pressure liquid chromatography followed by radioimmunoassay. The effect of acid and pepsin digestion on EGF bioactivity was determined using an in vitro hepatocyte bioassay and an in vivo cytoprotection assay in the rat stomach.

RESULTS

EGF1-53 was digested to the EGF1-49 and EGF1-46 forms in all samples containing pepsin when the pH was < 4. In gastric juice samples with pH > 4, the proportion of intact EGF increased to about 60%. For both methods of bioassay, intact EGF1-53 was about 3-4 times as potent as acid and pepsin-treated EGF.

CONCLUSIONS

EGF is produced in the 1-53 form but is rapidly cleaved to smaller, less active forms in acidic gastric juice. In contrast, only a small proportion of the EGF is cleaved if the pH is maintained above 4. This mechanism may be relevant to the healing process of acid suppressants.

Plasma-membrane calcium-pump isoforms in human and rat liver

Plasma-membrane Ca(2+)-pumping ATPases (PMCAs) extrude Ca2+ from the cytoplasm of all cells. Some previous studies of ATP-dependent Ca2+ transport by liver membranes suggested there exist specific properties of the hepatic PMCA, including regulation by hormones which affect calcium signalling. Multiple PMCA isoforms are now known to result from expression of four different genes (known as PMCA 1-4) and alternative RNA splicing at three possible sites (A, B and C). We investigated which isoforms are expressed in adult human and rat liver RNA using reverse-transcription polymerase chain reaction with mixed primers designed to amplify parts of all the known PMCA transcripts. In human liver, products were identified by sequencing from PMCA1, PMCA2 and PMCA4, but not from PMCA3 or from any new gene. In rat liver, by contrast, only PMCA1 and PMCA2 were detectable, although we confirmed that the primers were able to amplify from rat lung a new sequence which is part of rat PMCA4. Of the alternatively spliced variants, at site A in the PMCA2 sequences, all the exons were included in both adult and fetal human liver. In human liver, the exon at site B was excluded in some products from PMCA1 and PMCA4, and at site C, only PMCA1b and one form of PMCA4 were found. Blots of human liver RNA showed PMCA1 and PMCA4 were abundantly expressed, unlike PMCA2. On blots of rat liver RNA, PMCA1 was more abundant than PMCA2, and purified rat parenchymal cell RNA gave similar findings. In summary, no new hepatic PMCA isoforms have been demonstrated, but differences between the predominant human and rat isoforms may have consequences for Ca2+ signalling or the response to liver cell injury.

Effect of luminal growth factor preservation on intestinal growth

Intestinal atrophy contributes to the clinical difficulties of patients who cannot eat normally. Atrophy is prevented by luminal food proteins but not by the equivalent aminoacids. This observation is not explained by current theories of intestinal physiology. Epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha) are secreted into the gut lumen. We speculated that these are digested by pancreatic enzymes in fasting juice, but preserved when food proteins block the active sites of these enzymes. Studies based on molecular size and bioactivity confirmed that fasting human jejunal juice destroys EGF and TGF alpha. EGF, but not TGF alpha, was preserved when the milk protein casein or an enzyme inhibitor were present; elemental diets were ineffective. Diversion of pancreatic juice to the mid point of the small intestine in rats significantly increased luminal EGF-like bioactivity and all variables of growth in the proximal enzyme-free segment. Our findings support a novel mechanism of control of intestinal growth, which has important clinical implications. The addition of enzyme-inhibiting proteins such as casein to elemental diets may preserve intestinal integrity and function.

Apoptosis of serum-free C2.8 mouse embryo hepatocytic cells caused by hepatocyte growth factor deprivation

C2.8 mouse embryo hepatocytic cells, acutely required exogenous hepatocyte growth factor (HGF) to survive and proliferate in serum-free Dulbecco's modified Eagle's medium supplemented with insulin, transferrin and Na-selenite. Greater than 90% of cultured C2.8 cells died within 48 hours from plating in the absence of HGF. Conversely, HGF prolonged maintenance of life and stimulated cell proliferation. Removal of HGF from the medium of cultures that had grown to confluency, also resulted in a rapid decreased cell survival. In the last circumstance, light microscopic observations revealed, with high frequency, morphological features characteristic of apoptosis. DNA within the affected cells underwent rapid fragmentation, revealed as a ladder of DNA fragments in multiples of about 200 base pairs. HGF prevented loss of cell viability, morphological damages and retarded DNA fragmentation in confluent C2.8 cells. Cycloheximide delayed cell death caused by HGF deprivation.

Processing of hepatocyte growth factor to the heterodimeric form is required for biological activity

Hepatocyte growth factor is a plasminogen-like molecule with diverse biological effects. Although it is synthesized as a single chain polypeptide, it was originally purified as a disulfide-linked heterodimer which was generated by an internal proteolytic event. Subsequent work indicated that preparations consisting largely of the monomeric form also exhibited potent activity. By using a combination of protease inhibition and site-directed mutagenesis, we established that conversion of the single chain polypeptide to the heterodimer occurred during the bioassay and was required for mitogenic and motogenic activity.

Galactosamine induced hepatitis induces a reduction in hepatocyte epidermal growth factor receptors

The rapid regenerative response of the rat liver to partial hepatectomy is associated with a decline in liver epidermal growth factor receptor numbers which implies that ligand epidermal growth factor receptor interactions maybe important in initiating and/or modulating this process. The proliferative process in toxic hepatitis (where in contrast with partial hepatectomy the majority of hepatocytes have been exposed to damaging influences) has been less widely investigated. We studied the DNA synthetic response of rat livers to toxic injury induced by a 350 or 800 mg/kg ip injection of galactosamine and that caused by 70% hepatectomy, comparing the changes in epidermal growth factor receptor status. Both resulted in down regulation of epidermal growth factor receptors, suggesting similar ligand epidermal growth factor receptor binding occurs during the proliferative response after galactosamine administration and after partial hepatectomy. In vitro studies on isolated hepatocytes showed that epidermal growth factor receptor down regulation was not a direct effect of galactosamine on hepatocyte membranes.

Effect of in vivo administration of an antibody to epidermal growth factor on the rapid increase in DNA synthesis induced by partial hepatectomy in the rat

Recent reports indicate that transforming growth factor alpha (TGF-alpha) is produced within the liver and acts as the natural ligand of the epidermal growth factor (EGF) receptor causing the EGF receptor down regulation and the hepatocyte proliferation observed after partial hepatectomy. The reported phenomenon that an antibody to EGF inhibits the regenerative response to partial hepatectomy was therefore re-investigated. The IgG fraction of an anti-rat EGF antibody was injected intravenously at the time of partial hepatectomy, and its effects on regenerative DNA synthesis were compared with those of non-immune IgG. Injection of IgG reduced the DNA synthetic response to partial hepatectomy, assessed 24 hours after resection by 3H-thymidine incorporation, but the effects of normal and anti-EGF IgG were not statistically different, despite the presence of excess anti-EGF IgG in the circulation throughout the experimental period. However, anti-EGF IgG could completely block the proliferative response of hepatocytes in culture to EGF. These results support the suggestion that EGF is not the major mediator of hepatocyte DNA synthesis in the early stages of liver regeneration (less than 24 hours).

Comparative effects of epidermal growth factor, an insulin-glucagon combination, and a hepatocyte growth factor preparation on epidermal growth factor receptors

We investigated the changes in cell surface epidermal growth factor (EGF) receptors in the liver after partial hepatectomy, and in primary adult rat hepatocyte cultures following stimulation with either EGF, or a preparation of hepatocyte growth factor, or an insulin-glucagon combination. We confirmed a reduction in EGF receptors on hepatocytes after partial hepatectomy and a rapid down-regulation of EGF receptors on normal hepatocytes in vitro following exposure to EGF. Insulin and glucagon and hepatocyte growth factor, whilst initiating hepatocyte DNA synthesis, had only slight effects on their EGF binding capacity and EGF-receptor affinity. These results indicate that changes in cell membranes early in proliferation have only non-specific effects on EGF receptors, and, therefore, support the role of ligand binding to the EGF receptor as an important component of hepatocyte proliferation in vivo.

Partial purification and characterisation of an inhibitor of hepatocyte proliferation derived from nonparenchymal cells after partial hepatectomy

We have investigated the influences that nonparenchymal cells from regenerating rat liver exert on hepatocyte proliferation. When primary adult rat hepatocytes isolated from resting liver were co-cultured with nonparenchymal cells (NPCs) from resting liver of a different syngeneic animal, the proliferative response of hepatocytes to epidermal growth factor (EGF) was unaffected by the presence of NPCs. In the presence of NPCs taken from livers that had undergone partial hepatectomy 24 hours before (regen-NPCs), the response of hepatocytes from resting liver to EGF, TGF-alpha, and hepatocyte growth factor (HGF) was markedly inhibited. Inhibitory activity was not dependent on cell-to-cell contact, and conditioned-medium from regen-NPCs, but not normal NPCs, inhibited EGF-induced hepatocyte DNA synthesis by approximately 50%. After concentration by gel chromatography and lyophilisation, inhibition was 98%. The inhibitory activity migrated on SDS-PAGE gel electrophoresis with an apparent molecular weight of 14 to 17 kDa and was trypsin-sensitive but relatively heat-stable. The effects of blocking antibodies established that it was not TGF-beta 1, IL1-beta, or IL6. Investigations of regen-NPCs taken at different time points demonstrated that inhibitory activity was released into conditioned medium of cells harvested at 24 and 48 hours after partial hepatectomy, but not 10 or 72 hours. This powerful inhibitor of hepatocyte response to proliferogens is released by cultures of NPCs with a time course suggesting that it may be involved in terminating the surge of hepatocyte replication induced by partial hepatectomy.

Dimethyl sulphoxide induces a reduced growth rate, altered cell morphology and increased epidermal-growth-factor binding in Hep G2 cells

Culture of Hep G2 cells in medium containing 2% (v/v) dimethyl sulphoxide (DMSO) resulted in a slowing of growth and a marked change in morphological appearance. By day 6, cultures containing DMSO had one-third the number of cells compared with parallel control cultures. Measurement of 125I-epidermal-growth-factor (EGF) binding to DMSO-treated cells revealed a striking time-dependent elevation in specific EGF binding to their cell surface. Increased binding was detectable within 24 h of the start of DMSO treatment, reaching, by 6 days, levels almost 25 times greater than those for control cells. Addition of EGF to DMSO-treated cells caused a rapid down-regulation of the EGF receptor, but did not alter their proliferation rate. Slowing of growth by other means, such as serum starvation, growth to confluence or culture in the presence of sodium butyrate, did not affect 125I-EGF binding, indicating a specific effect of DMSO on these cells.

Mitogenic effects of coagulation factor XII and factor XIIa on HepG2 cells

The structure of coagulation factor XII (Hageman factor), inferred from its DNA sequence, includes two epidermal growth factor (EGF)-homologous domains in its amino-terminal region. This suggests that factor XII may exhibit EGF-like activities. Reciprocal antigenic cross-reactivity between factor XII and EGF was shown by exposing purified human factor XII or mouse EGF to anti-mouse EGF or anti-human factor XII. Western blot analysis showed that anti-mouse EGF recognized intact factor XII at 80 kDa. Together, these results suggest that the EGF-homologous domains are accessible for anti-EGF binding in native factor XII. To determine whether factor XII has mitogenic activity, HepG2 or L cells (10(4) cells per well) were grown in serum-free medium in the presence or absence of factor XII or kaolin-activated factor XII (factor XIIa). Both factors XII and XIIa (6.0 micrograms/ml) enhanced cell proliferation by approximately 2-fold (P less than 0.001 and P less than 0.005, respectively). In contrast, L cells, which are not EGF target cells, were not affected by either factor XII or factor XIIa. Various doses of factor XII enhanced cell proliferation, [3H]thymidine incorporation, and [3H]leucine incorporation in HepG2 cells cultured under the same conditions. These data indicate that factor XII, like EGF, is a mitogen for HepG2 cells and suggest a possible autocrine role in the liver.