Pathogenesis of primary liver tumors induced by chemicals

Spongiosis Hepatis in a Wild Aquarium-Maintained Red Irish Lord

An aquarium-maintained female Red Irish Lord Hemilepidotus hemilepidotus presented with severe coelomic distension. The fish was anesthetized for ultrasonographic examination, which highlighted multiple cyst-like lesions in the liver and a distended ovary that was filled with follicles and an inspissated egg mass. Multiple exploratory celiotomies were performed for egg mass removal, liver biopsy, ovariosalpingectomy, and body wall rupture repair. Fourteen weeks after original presentation, and subsequent to 2 weeks of anorexia, the fish died. At necropsy, the liver was severely enlarged and distorted by multiple, coalescing, cyst-like spaces with no grossly normal liver parenchyma. The spleen also contained a raised cyst-like structure. Microscopically, the liver had well-demarcated foci of hepatocyte loss with retained meshworks of interconnected, perisinusoidal stellate cells. The fluid-filled spaces surrounded by stellate cells were not lined by epithelium or endothelium. The spleen had similar fluid-filled spaces formed of stellate cells. The cyst-like lesions in the liver were consistent with spongiosis hepatis; however, the concurrent development of a morphologically comparable lesion in the spleen is not typical of spongiosis hepatis cases. This case may represent the first report of spontaneously occurring spongiosis hepatis in a fish maintained in a public aquarium, as well as the first report in a fish of spongiosis hepatis-like lesions in an organ other than the liver.

Hepatotoxin N-nitrosomorpholine-induced carcinogenesis in rat liver: ex vivo exploration of preneoplastic and neoplastic hepatocytes

N-nitrosomorpholine (NNM) is a hepatotoxic and hepatocarcinogenic agent. This agent was administered in the form of drinking water which contained 200 mg of NNM/liter. Its time-dependent intake profile showed four phases over 20 weeks, followed by a fifth phase where only water was supplied. Most frequently, hepatocellular carcinoma appeared between the end of phase IV and the beginning of phase V. At 5 weeks of NNM administration, foci of altered hepatocytes (FAH) containing 100-1000 hepatocytes could be isolated together with free hepatocytes by the collagenase perfusion method. When these foci were grown on the William's Medium E containing hormonally defined medium, they were able to survive approximately twice as long as normal hepatocytes At 10 weeks of NNM administration, few FAH were isolated together with free hepatocytes. The hepatocytes which had been placed under extended chemical stress showed increased heat tolerance (7 to 8 h) at 43 degrees C, while normal hepatocytes could survive 3 to 4 h. At the neoplastic phase spanning the end of the 20 weeks of the NNM administration and water phase, the rats bearing hepatocellular carcinoma entered the terminal stage, where observable tumor masses could be isolated from the tumor bearing liver and tested for ex vivo growth in tissue culture. After stabilization of the isolated primary hepatoma cells through 10 passages of propagation on William's Medium E or minimal Eagle's medium containing 10% FBS, their gene expression profile was analyzed by DNA microarray and compared with the profile of normal hepatocytes. The comparison revealed that upregulation involved ribosome-dependent protein synthesis, including 40S ribosomal proteins (S4, S7, S18, S20), 60S ribosomal proteins (L6, L21, L32, L37, P1), initiation factor 4A, and elongation factor 1alpha.

Cystic degeneration/Spongiosis hepatis in rats

Cystic degeneration/spongiosis hepatis in rats has been proposed to be a preneoplastic and/or neoplastic lesion by some authors, because of its proliferative properties and persistent increased cell turnover rate in stop experiments using hepatocarcinogens, and the assumption that it can develop into a sarcoma. The neoplastic potential of cystic degeneration is questioned in this review article. Cystic degeneration, which appears to derive from altered Ito cells, does not have neoplastic histomorphologic characteristics, although it may be composed of cells with an increased mitotic index. In this regard, persistent proliferation is also seen with other nonneoplastic lesions. Arguments are presented to show that the induced, probably extremely rare sarcoma that was associated with cystic degeneration most likely derives from the very rare induced spherical Ito-cell aggregate with an unusually high cellular turnover rate in rats treated with hepatocarcinogens, and not from cystic degeneration. Also, in none of 12 referenced standard oncogenicity studies with chemically induced cystic degeneration was the lesion associated with mesenchymal (Ito-cell) tumors. Consequently, evidence is lacking that cystic degeneration in rats should be classified as a preneoplastic or neoplastic lesion. The 12 oncogenicity studies in rats with induced cystic degeneration showed a marked sex predilection, with males more likely to develop either spontaneous or chemically induced lesions. In these 12 studies, cystic degeneration was more often associated with hepatocellular hypertrophy or hepatotoxicity. rather than hepatocarcinogenicity. Thus, it is concluded that hepatocarcinogens induce cystic degeneration, not because they are carcinogenic. but because they have other effects on the liver, and that cystic degeneration may be a secondary/reparative change. Cystic degeneration in fish parallels the situation in rats in many respects, yet the existence of the lesion in other species, including man, is not as well supported. Based on the data presented in this review, spontaneous and induced cystic degeneration in rats and fish is not a preneoplastic or neoplastic lesion and risk assessment for man can be based on no-effect levels and safety margins, as for other nonneoplastic adverse effects that have no counterpart in man.

Diagnostic criteria for degenerative, inflammatory, proliferative nonneoplastic and neoplastic liver lesions in medaka (Oryzias latipes): consensus of a National Toxicology Program Pathology Working Group

Diagnostic criteria are presented for degenerative, inflammatory, nonneoplastic proliferative, and neoplastic lesions in the liver of medaka (Oryzias latipes), a small fish species frequently used in carcinogenesis studies. The criteria are the consensus of a Pathology Working Group (PWG) convened by the National Toxicology Program. The material examined by the PWG was from Medaka exposed to N-nitrosodiethylamine for 28 days, removed to clean water, and sacrificed 4, 6, or 9 mo after initiation of exposure. Degenerative lesions included hepatocellular intracytoplasmic vacuolation, hepatocellular necrosis, spongiosis hepatis, hepatic cysts, and hepatocellular hyalinization. Inflammatory lesions consisted of granulomas, chronic inflammation, macrophage aggregates, and focal lymphocytic infiltration. Nonneoplastic proliferative lesions comprised foci of cellular alteration (basophilic focus, eosinophilic focus, vacuolated focus, and clear cell focus) and bile duct hyperplasia. Neoplastic lesions included hepatocellular adenoma, hepatocellular carcinoma, cholangioma, and cholangiocarcinoma. Two lesions composed mainly of spindle cells were noted, hemangiopericytoma and spindle cell proliferation. Rather than being an exhaustive treatment of medaka liver lesions, this report draws from the published literature on carcinogen-induced liver lesions in medaka and other fish species and attempts to consolidate lesion criteria into a simplified scheme that might be useful to pathologists and other researchers using medaka lesions for risk assessment or regulatory purposes.

The stem cells of the liver ? a selective review

The current status of the much-debated question of the still-hypothetical stem cells of the liver is reviewed, with an emphasis on their role in hepatocarcinogenesis. The widely held view of the primacy of the hepatocyte, notably of the mononuclear diploid type, in this process--the "hepatocytic theory"--has been compared with variants of the "stem cell hypothesis" based on the "non-parenchymal epithelial cells" of the liver--the "oval" or biliary ductular cells, the "nondescript periductular" cells and the "primitive" bipotential epithelial cells. An attempt has been made to concentrate mainly on the more recent publications, in an effort to balance the conflicting opinions expressed by comparing results obtained by the newer procedures currently in use. Despite some interesting and relevant findings it appears that the evidence in favour of the stem-cell hypothesis is still circumstantial and that the hepatocytic theory has not been invalidated. Presumably the question of the hepatic stem cells will be answered when the riddle of hepatocarcinogenesis has been solved.

Spongiosis hepatis: chemical induction, pathogenesis, and possible neoplastic fate in a teleost fish model

Spongiosis hepatis (SH), first reported as a distinct lesion associated with certain forms of hepatic neoplasia in rats, has also been induced with chemicals, in a predictable fashion, in small teleost fishes being studied as carcinogenesis research models. The sheepshead minnow (Cyprinodon variegatus), exposed to N-nitrosodiethylamine (DENA) in sea water, provided the model for this study. The fish developed SH and presented a spectrum of developmental or progressive stages of the lesion over a 140 week holding period following a 6 week exposure to / 57 mg/L DENA. The origin of SH in the fish model is homologous to that in the rat model, both species having the perisinusoidal cell (stellate cells of Ito) in the space of Disse as the cell of origin. Light (LM) and electron microscopy (EM) studies characterized the different pathogenetic stages of SH in liver of the sheepshead minnow and revealed a possible late transition of SH to putative polymorphic cell neoplasms. The possible preneoplastic or neoplastic nature of SH from its time of origin in chemically exposed fish to time of appearance of associated presumptive neoplasms is discussed. SH may be a bioindicator of exposure to certain chemicals in some vertebrate species, from fishes to mammals.

Hepatopathology of diethylnitrosamine in the medaka (Oryzias latipes) following short-term exposure

The effect of short-term exposure (48 hr) of 14-day-old medaka (Oryzias latipes) to diethylnitrosamine (DEN) was tested on 4 groups of fish at 0, 100, 200, and 400 mg/L, with interim sacrifices of up to 6 months post exposure. Many of the lesions seen were similar to those seen in other fish DEN carcinogenicity studies using longer term, adult exposures, including a cytotoxic phase with cystic degeneration of the hepatic parenchyma, cellular vacuolation, globular acidophilic cytoplasmic inclusions, spongiosis hepatis, foci of cellular alteration, adenomas, and cholangiomas. There were also several proliferative lesions of uncertain origin which occurred throughout the study. There appeared to be a direct relationship between exposure level and the incidence and severity of the lesions seen, including aggressiveness and invasiveness of the neoplasms seen. However, there was a possible difference from other studies in the types of neoplasms seen with this protocol, which may be related to age at exposure, exposure level, duration of exposure, or a combination of these factors. Similarities in lesions seen in this study and in rodent and other fish studies is an indication of promise for the use of this model in carcinogenicity testing. Differences indicate the variety of ways the fish liver can respond to toxic injury, and lend flexibility to the model.

Observations on altered hepatocellular foci in National Toxicology Program two-year carcinogenicity studies in rats

Retrospective characterization of morphological and stereological features of altered hepatocellular foci (AHF) in hematoxylin & eosin (H&E)-stained sections was performed on 6 conventional 2-yr carcinogenicity studies conducted in Fischer 344 (F344) rats by the National Toxicology Program (NTP). In 3 of these studies where there was clear evidence of hepatocarcinogenicity [1-amino-2,4-dibromoanthraquinone (ADBAQ), C.I. Acid Red 114, methyl carbamate], there was greater morphological variability in AHF than in the studies of chemicals that were not hepatocarcinogenic [4-hydroxyacetanilide, epinephrine, dimethoxane]. In addition to having the expected types of AHF, rats treated with ADBAQ, C.I. Acid Red 114, and methyl carbamate had atypical basophilic AHF. In addition, atypical eosinophilic AHF were present in rats treated with ADBAQ. Both types of atypical AHF showed a morphological spectrum and sequential changes suggesting they could develop into hepatocellular neoplasms. For the 3 liver tumor positive studies, there were dose and time-dependent increases in stereological parameters for the atypical as well as commonly occurring clear, vacuolated, and mixed cell AHF. Consistent stereological changes were not found for commonly occurring basophilic and eosinophilic AHF. Aside from some decreases in stereological measurements in some rats treated with 4-hydroxyacetanilide and epinephrine, there were no significant quantitative changes in AHF in the three liver tumor negative studies. These results show that hepatocarcinogens may induce unique types of AHF in conventional 2-yr carcinogenicity/toxicity studies in rats and may cause quantitative increases in commonly occurring clear, vacuolated, and mixed cell AHF. Such qualitative and quantitative changes are potentially useful predictors of hepatic neoplasia.

Viral versus chemical hepatocarcinogenesis

Almost all experimental studies on hepatocarcinogenesis have been performed on models induced by chemicals. The human hepatocellular carcinoma (HCC), a highly malignant tumor, particularly frequent in the Far East and in Africa, is overwhelmingly associated with hepatitis B virus (HBV) infections and only very rarely is it caused by chemicals. This raises the question as to what degree observations on the preferentially studied chemical carcinogenesis apply to human HCC. This question is timely, since a woodchuck model has been developed in which laboratory infection with a hepadna virus, biologically and genomically similar to HBV, regularly produces HCC in prolonged surface antigen carrier woodchucks. Permanent surface antigen carriage by itself may also be a risk factor. A comparison, based on available observations of chemical and hepadna viral carcinogenesis, was attempted on four aspects: (1) histologic features, (2) growth patterns, (3) growth factors, and (4) molecular biological observations. At this time, only few conclusions can be drawn, mainly on a morphological and molecular biological basis. In the initial stage, chemicals cause mutations of very few bases, while in hepadna viral infections long sequences are inserted into both strands of chromosomal DNA. Additional studies appear to be indicated, to the greatest degree with existing methods and primarily on hepadna viral carcinogenesis. A key question is whether the active necroinflammation, preceding or observed at the time the HCC is recognized, in woodchuck and also in human chronic HBV infections is a promoting event or a secondary reaction. Additional therapeutic strategies, specific for hepadna viral carcinogenesis, can be considered, particularly elimination of hepatocytes carrying integrated viral sequences and prevention or suppression of the promoting necroinflammation. Success in HCC may be applicable to cancers related to papilloma virus in other organs, for instance, the female genital tract.