Biochemical microanalysis of alpha-glucosidase activity in preneoplastic and neoplastic hepatic lesions induced in rats by N-nitrosomorpholine

Clear cell hepatocellular carcinoma: origin, metabolic traits and fate of glycogenotic clear and ground glass cells

Clear cell hepatocellular carcinoma (CCHCC) has hitherto been considered an uncommon, highly differentiated variant of hepatocellular carcinoma (HCC) with a relatively favorable prognosis. CCHCC is composed of mixtures of clear and/or acidophilic ground glass hepatocytes with excessive glycogen and/or fat and shares histology, clinical features and etiology with common HCCs. Studies in animal models of chemical, hormonal and viral hepatocarcinogenesis and observations in patients with chronic liver diseases prone to develop HCC have shown that the majority of HCCs are preceded by, or associated with, focal or diffuse excessive storage of glycogen (glycogenosis) which later may be replaced by fat (lipidosis/steatosis). In ground glass cells, the glycogenosis is accompanied by proliferation of the smooth endoplasmic reticulum, which is closely related to glycogen particles and frequently harbors the hepatitis B surface antigen (HBsAg). From the findings in animal models a sequence of changes has been established, commencing with preneoplastic glycogenotic liver lesions, often containing ground glass cells, and progressing to glycogen-poor neoplasms via various intermediate stages, including glycogenotic/lipidotic clear cell foci, clear cell hepatocellular adenomas (CCHCA) rich in glycogen and/or fat, and CCHCC. A similar process seems to take place in humans, with clear cells frequently persisting in CCHCC and steatohepatitic HCC, which presumably represent intermediate stages in the development rather than particular variants of HCC. During the progression of the preneoplastic lesions, the clear and ground glass cells transform into cells characteristic of common HCC. The sequential cellular changes are associated with metabolic aberrations, which start with an activation of the insulin signaling cascade resulting in pre-neoplastic hepatic glycogenosis. The molecular and metabolic changes underlying the glycogenosis/lipidosis are apparently responsible for the dramatic metabolic shift from gluconeogenesis to the pentose phosphate pathway and Warburg-type glycolysis, which provide precursors and energy for an ever increasing cell proliferation during progression.

Hepatocellular glycogenotic foci after combined intraportal pancreatic islet transplantation and knockout of the carbohydrate responsive element binding protein in diabetic mice

Aims

The intraportal pancreatic islet transplantation (IPIT) model of diabetic rats is an insulin mediated model of hepatocarcinogenesis characterized by the induction of clear cell foci (CCF) of altered hepatocytes, which are pre-neoplastic lesions excessively storing glycogen (glycogenosis) and exhibiting activation of the AKT/mTOR protooncogenic pathway. In this study, we transferred the IPIT model to the mouse and combined it with the knockout of the transcription factor carbohydrate responsive element binding protein (chREBP).

Methods

C57BL/6J Wild-type (WT) and chREBP-knockout (chREBP-KO) mice (n = 297) were matched to 16 groups (WT/ chREBP-KO, experimental/control, streptozotocine-induced diabetic/not diabetic, one/four weeks). Experimental groups received the intraportal transplantation of 70 pancreatic islets. Liver and pancreatic tissue was examined using histology, morphometry, enzyme- and immunohistochemistry and electron microscopy.

Results

CCF emerged in the liver acini downstream of the transplanted islets. In comparison to WT lesions, CCF of chREBP-KO mice displayed more glycogen accumulation, reduced activity of the gluconeogenic enzyme glucose-6-phosphatase, decreased glycolysis, lipogenesis and reduced levels of the AKT/mTOR cascade members. Proliferative activity of CCF was ∼two folds higher in WT mice than in chREBP-KO mice.

Conclusions

The IPIT model is applicable to mice, as murine CCF resemble preneoplastic liver lesions from this hepatocarcinogenesis model in the rat in terms of morphological, metabolic and molecular alterations and proliferative activity, which is diminished after chREBP knockout. chREBP appears to be an essential component of AKT/mTOR mediated cell proliferation and the metabolic switch from a glycogenotic to lipogenic phenotype in precursor lesions of hepatocarcinogenesis.

Cocarcinogenic Effects of Islet Hormones and N-Nitrosomorpholine in Hepatocarcinogenesis after Intrahepatic Transplantation of Pancreatic Islets in Streptozotocin-Diabetic Rats

In contrast to high local insulin levels obtained after low-number transplantation (n = 350) of islets of Langerhans into the livers of diabetic rats, low insulin levels after high-number transplantation (n = 1,000) do not suffice to induce hepatocarcinogenesis. Herein, we investigated the possible cocarcinogenic potential of high and, in particular, low insulin levels, combining this in vivo model with a chemical model of hepatocarcinogenesis after administration of N-nitrosomorpholine (NNM). In three main experiments, different schemes of single or continuous NNM administration were combined with different transplantation procedures in diabetic or nondiabetic animals, i.e., low-number and high-number islet transplantation, transplantation of polystyrene particles, and sham transplantation. Animals were sacrificed between 3 and 53 weeks after transplantation procedures. Evidence for the cocarcinogenic effects of NNM and insulin was provided in each main experiment. NNM treatment after low-number islet transplantation resulted in an increase in the number of preneoplastic hepatocellular foci, and a significant increase in the number and an earlier appearance of hepatocellular adenomas and carcinomas compared with controls. Most intriguing was the increase in preneoplastic foci after combined NNM treatment and high-number islet transplantation, proving that insulin, even in lower doses, has at least cocarcinogenic effects on the downstream hepatocytes and thus promotes an otherwise initiated hepatocarcinogenic process. Conclusively, intrahepatic transplantation of pancreatic islets acts as a strong cocarcinogenic factor together with NNM in streptozotocin-diabetic rats.

Hepadnaviral hepatocarcinogenesis: in situ visualization of viral antigens, cytoplasmic compartmentation, enzymic patterns, and cellular proliferation in preneoplastic hepatocellular lineages in woodchucks

BACKGROUND/AIMS

Hepadnaviral hepatocarcinogenesis induced in woodchucks with and without dietary aflatoxin B1 has been established as an appropriate animal model for studying the pathogenesis of human hepatocellular carcinoma in high-risk areas. Our aim in this study was the elucidation of phenotypic cellular changes in early stages of this process.

METHODS

Woodchucks were inoculated as newborns with woodchuck hepatitis virus (WHV), and partly also exposed to aflatoxin B1. Sequential hepatocellular changes in the expression of viral antigens, ultrastructural organization, cellular proliferation and apoptosis were studied in situ by electron microscopy, enzyme and immunohistochemistry.

RESULTS

A characteristic finding in WHV-infected animals (with and without aflatoxin B1) was proliferative areas of minimal structural deviation, which predominated periportally, comprised glycogen-rich, amphophilic, and ground-glass hepatocytes, and expressed the woodchuck hepatitis core and surface antigens. Two main types of proliferative foci emerged from minimal deviation areas, glycogenotic clear cell foci and amphophilic cell foci (being poor in glycogen but rich in mitochondria), giving rise to the glycogenotic-basophilic and the amphophilic preneoplastic hepatocellular lineages. A gradual loss in the expression of viral antigens appeared in both lineages, particularly early in the glycogenotic-basophilic cell lineage. Whereas glycogenosis was associated with an enzymic pattern suggesting an early activation of the insulin-signaling pathway, amphophilic cells showed changes in enzyme activities mimicking a response of the hepatocytes to thyroid hormone, which may also result from early changes in signal transduction.

CONCLUSION

Preneoplastic hepatocellular lineages in hepadnaviral and chemical hepatocarcinognesis show striking phenotypic similarities, indicating concordant and possibly synergistic early changes in signaling.

Allosteric activation of acid alpha-glucosidase by the human papillomavirus E7 protein

Changes in the cellular carbohydrate metabolism are a hallmark of malignant transformation and represent one of the earliest discernible events in tumorigenesis. In the early stages of certain epithelial cancers, a metabolic switch is regularly observed, in which slowly growing glycogenotic cells are converted to highly proliferating basophilic cells. This step is accompanied by a rapid depletion of the intracellular glycogen stores, which in liver carcinogenesis results from the activation of the enzyme acid alpha-glucosidase by an as yet unknown mechanism. We show here that acid alpha-glucosidase is a target for the E7 protein encoded by human papillomavirus type 16, a human tumor virus that plays a key role in the genesis of cervical carcinoma. We show that expression of E7 induces the catalytic activity of acid alpha-glucosidase in vivo and wild type E7, but not transformation-deficient mutants bind directly to acid alpha-glucosidase and increase the catalytic activity of the enzyme in vitro. The data suggest that the E7 protein encoded by human papillomavirus type 16 can act as an allosteric activator of acid alpha-glucosidase.

Early bioenergetic changes in hepatocarcinogenesis: preneoplastic phenotypes mimic responses to insulin and thyroid hormone

Biochemical and molecular biological approaches in situ have provided compelling evidence for early bioenergetic changes in hepatocarcinogenesis. Hepatocellular neoplasms regularly develop from preneoplastic foci of altered hepatocytes, irrespective of whether they are caused by chemicals, radiation, viruses, or transgenic oncogenes. Two striking early metabolic aberrations were discovered: (1) a focal excessive storage of glycogen (glycogenosis) leading via various intermediate stages to neoplasms, the malignant phenotype of which is poor in glycogen but rich in ribosomes (basophilic), and (2) an accumulation of mitochondria in so-called oncocytes and amphophilic cells, giving rise to well-differentiated neoplasms. The metabolic pattern of human and experimentally induced focal hepatic glycogenosis mimics the phenotype of hepatocytes exposed to insulin. The conversion of the highly differentiated glycogenotic hepatocytes to the poorly differentiated cancer cells is usually associated with a reduction in gluconeogenesis, an activation of the pentose phosphate pathway and glycolysis, and an ever increasing cell proliferation. The metabolic pattern of preneoplastic amphophilic cell populations has only been studied to a limited extent. The few available data suggest that thyromimetic effects of peroxisomal proliferators and hepadnaviral infection may be responsible for the emergence of the amphophilic cell lineage of hepatocarcinogenesis. The actions of both insulin and thyroid hormone are mediated by intracellular signal transduction. It is, thus, conceivable that the early changes in energy metabolism during hepatocarcinogenesis are the consequence of alterations in the complex network of signal transduction pathways, which may be caused by genetic as well as epigenetic primary lesions, and elicit adaptive metabolic changes eventually resulting in the malignant neoplastic phenotype.

Hepatic preneoplasia in hepatitis B virus transgenic mice

Hepatocarcinogenesis in hepatitis B virus transgenic mice was studied by means of a correlative cytomorphological and cytochemical approach at different time points in animals from 1 to 34 mo old. HBsAg-positive ground-glass hepatocytes emerged throughout the liver parenchyma in nearly all transgenic mice during the first 4 mo after birth. The panlobular expression of HBsAg persisted until foci of altered hepatocytes appeared (6 to 9 mo of age). Three different types of foci of altered hepatocytes-namely, glycogen-storage foci, mixed cell foci and glycogen-poor foci-developed. Hepatocellular adenomas and carcinomas appeared after 11 mo. Orcein staining revealed frequent transitions between ground-glass hepatocytes extensively expressing HBsAg and glycogen-storage (predominantly clear-cell) foci containing HBsAg-positive cytoplasmic components. Similar transitions between ground-glass hepatocytes and glycogenotic (clear) cells were often found in diffuse parenchymal glycogenosis at 11 or 12 mo. Remnants of HBsAg-positive material were also detected in mixed cell foci, glycogen-poor diffusely basophilic cell foci, hepatic adenoma and hepatocellular carcinoma. These findings suggest that ground-glass hepatocytes are the direct precursor of foci of altered hepatocytes and their neoplastic descendants. The extensive expression of HBsAg is gradually down-regulated during neoplastic transformation, just as the morphological the biochemical phenotypes of foci of altered hepatocytes, hepatic adenoma and hepatocellular carcinoma in transgenic mice resemble those described in chemical hepatocarcinogenesis. The predominant sequence of cellular changes leading from glycogen-storage (predominantly clear cell) foci to mixed cell foci, hepatic adenoma and hepatocellular carcinoma is characterized by a gradual decrease in the activities of glycogen synthase, phosphorylase, glucose-6-phosphatase and adenylate cyclase, whereas glucose-6-phosphate dehydrogenase and pyruvate kinase activities increase. These alterations indicate a shift from the glycogenotic state toward an increase in the pentose phosphate pathway and glycolysis.

Assessment of lysosomal function by quantitative histochemical and cytochemical methods

Quantitative histochemistry and cytochemistry enables a direct link to be made between metabolic functions such as the activity of lysosomal enzymes and the morphology of a tissue or a type of cell. Several approaches exist such as microchemistry based on (bio)chemical analysis of a single cell or a small piece of tissue dissected from a freeze-dried section. This technique has been routinely used for prenatal diagnosis of inherited enzyme defects and especially of lysosomal storage diseases. Other approaches are cytofluorometry or cytophotometry, which are based on the principle that a fluorescent or coloured final reaction product is precipitated at the site of the enzyme. The amount of final reaction product is analysed per cell or per unit volume of tissue using either a microscope cytofluorometer or flow cytometer for fluorescence measurements or an image analysing system or scanning and integrating cytophotometer for absorbance measurements. In principle, fluorescence methods are to be preferred over chromogenic methods because they are more sensitive and enable multiparameter analysis. However, only a limited number of fluorogenic methods are at hand that give a final reaction product which is sufficiently water-insoluble to guarantee good localisation. The best results have been obtained with methods based on naphthol AS-TR derivatives and with methods for the demonstration of protease activity using methoxynaphthylamine derivatives as substrates and 5'-nitrosalicylaldehyde as coupling reagent. Chromogenic methods are far better with respect to localisation properties and, therefore, most commonly used for quantitative histochemical analysis of lysosomal enzyme activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential changes in growth kinetics and cellular phenotype during hepatocarcinogenesis

Sequential changes in cell proliferation and cellular phenotype during hepatocarcinogenesis induced in rats with N-nitrosomorpholine were investigated by autoradiographic determination of the [3H]thymidine-labelling index in morphologically defined focal lesions and extrafocal hepatic tissue at different times between 4 and 48 weeks after withdrawal of the carcinogen (stop model). The labelling index was found to be significantly increased in all types of preneoplastic and neoplastic hepatic lesions as compared to both the liver tissue of untreated controls and the extrafocal parenchyma of N-nitrosomorpholine-treated rats. However, the extent of the increase in labelling index differed in the phenotypically diverse types of preneoplastic and neoplastic lesions. There was a significant but relatively small increase in the labelling index in clear and acidophilic cell foci. A much stronger elevation of cell proliferation was characteristic of mixed and basophilic cell foci. The development of hepatocellular adenomas and carcinomas from preneoplastic hepatic foci was further characterized by an additional increase in cell proliferation. Each specific cellular phenotype was associated with a rather uniform proliferation rate, which remained elevated at all time points studied, suggesting that the rate of cell proliferation in the phenotypically diverse preneoplastic hepatic foci mainly reflects the intrinsic growth potential of the respective cellular phenotypes. The results support the concept that the predominant sequence of cellular changes in hepatocarcinogenesis induced by the stop model leads from the clear and acidophilic cell foci, storing glycogen in excess, through mixed and basophilic cell foci to hepatocellular adenomas and carcinomas. The fact that the labelling index of the extrafocal liver tissue of N-nitrosomorpholine-treated rats was also significantly higher than that of the normal parenchyma of untreated controls might indicate an involvement of extrafocal hepatocytes, in addition to that of foci of altered hepatocytes, in hepatocarcinogenesis.

Significance of sequential cellular changes inside and outside foci of altered hepatocytes during hepatocarcinogenesis

A variety of phenotypic cellular changes emerge in the liver of different species prior to the appearance of hepatocellular adenomas and carcinomas induced by carcinogenic agents (chemicals, radiation, hepadna viruses) or develop "spontaneously." Foci of altered hepatocytes have been studied most extensively in rats treated with chemical carcinogens; they are considered preneoplastic lesions and have been used in several laboratories as endpoints in carcinogenicity testing. The principles and problems of the morphological classification of foci of altered hepatocytes are presented. In addition to the 4 types of foci generally accepted (clear, acidophilic, basophilic and mixed cell foci), further subtypes (intermediate cell foci) or other types of foci, namely tigroid cell foci and amphophilic cell foci, have more recently been separated as distinct pathomorphological entities. Whereas the amphophilic foci might result from a modulation of clear and acidophilic cell foci, the tigroid cell foci apparently represent a stage in a separate cell lineage leading to hepatocellular adenomas. It remains open whether the tigroid cell foci may also progress to carcinomas. Extrafocal phenotypic changes of hepatocytes might also be involved in hepatocarcinogenesis. The cellular phenotypes within foci also depend strongly, among many other factors, on the dose and duration of the carcinogenic treatment. Cytomorphological, cytochemical, microbiochemical and stereological studies suggest that the predominant sequence of cellular changes during hepatocarcinogenesis leads from the clear and acidophilic cell foci storing glycogen in excess through mixed cell foci and nodules to basophilic cell populations prevailing in hepatocellular carcinomas. A multitude of metabolic aberrations is associated with the sequential cellular changes. Aberrations in carbohydrate metabolism are particularly prominent and might be causally related to the neoplastic transformation of the hepatocytes.