The expressions of oncogenes and liver-specific genes in Morris hepatomas

Adventures in hepatocarcinogenesis

Neoplasia is a heritably altered, relatively autonomous growth of tissue. Hepatocarcinogenesis, the pathogenesis of neoplasia in liver, as modeled in the rat exhibits three distinct, quantifiable stages: initiation, promotion, and progression. Simple mutations and/or epigenetic alterations may result in the irreversible stage of initiation. The stage of promotion results from selective enhancement of cell replication and selective inhibition of cellular apoptosis of initiated cells dependent on the genetic and/or epigenetic alterations of the latter. The irreversible stage of progression results from initial karyotypic alterations that evolve into greater degrees of genomic instability. The initial genomic alteration in the transition from promotion to progression may involve primarily epigenetic mechanisms driven by epigenetic and genetic alterations fixed during the stage of promotion.

Na+ :H+ exchange inhibition induces intracellular acidosis and differentially impairs cell growth and viability of human and rat hepatocarcinoma cells

Amiloride and its more potent analog, hexamethylene amiloride (HMA), inhibits Na+ :H+ exchange and decreases intracellular pH in a concentration-dependent way in two human hepatocarcinoma cell lines and in a rat hepatocarcinoma cell line that differs in its phenotypic characteristics, resembling the clinical situation encountered in human hepatocarcinomas. After 24 h of exposure, DNA synthesis and cell protein content of the cultures decreases according to the concentration of the drugs and in parallel to Na+ exchange inhibition and the drop in pHi promoted. RNA and protein syntheses are less sensitive to its action. The above effects induced by HMA are accompanied by an abrupt decrease in cell viability and lysosomal integrity at 24 h. These effects develop gradually with the exposure time as does the increase in free radical production. Decreased viability is totally or partially restored by N-acetylcysteine or deferoxamine, but the degree of intracellular acidification produced is not. These results tend to suggest that intracellular acidification can diminish cell growth and provoke cytotoxic cell death by diminishing reduced glutathione (GSH) levels and impairing lysosomal integrity, reflecting the sensitivity of hepatocarcinoma cells to Na+ exchange inhibition and intracellular acidosis.

Current pathogenetic and molecular concepts in viral liver carcinogenesis

Hepatocellular carcinoma (HCC) is one of the most frequent malignancies in humans and in most cases a consequence of chronic infection of the liver by hepatotropic viruses (Hepatitis B Virus (HBV) and possibly Hepatitis C Virus (HCV)). Formation of HCC results from a stepwise process involving different preneoplastic lesions that reflect multiple genetic events, like protooncogene activation, tumor suppressor gene inactivation, and growth factor over- or reexpression. Recent investigations have gained new insights into how these factors are activated and may interact. In addition, improved knowledge of the molecular biology of HBV has led to better understanding of its pleiotropic effects on induction and progression in hepatocarcinogenesis.

Immunolocalization of ras oncogene p21 in human liver diseases

Fifty-five cases representing a spectrum of disease states of the human liver and 10 normal liver controls were examined for the presence of the ras oncogene product p21. Conventional formalin-fixed, paraffin-embedded sections were immunostained by the avidin-biotin complex method with the broadly reactive ras p21 monoclonal antibody (Mab) RAP-5. The specificity of the reactions was confirmed by immunostaining selected samples with Mab Y13-259. In the normal liver, virtually no hepatocytic immunostaining was noted. Variable, often extensive, and convincing immunoreactions were noted in diverse forms of hepatitis, cirrhosis, and allograft rejection; the strongest immunostaining was found in samples of focal nodular hyperplasia. Hepatic adenomas and hepatocellular carcinomas showed unevenly distributed, moderate to weak reactions or no reaction at all; cholangiocarcinomas did not immunostain. In reactive but non-transformed liver cell populations, enhanced p21 ras reactions seemed to correlate with the severity of the injury and the intensity of the proliferative response. The uneven and comparatively weak ras p21 reactions noted in adenomas and carcinomas suggest that this oncogene product may be involved only transitorily in their transformation processes and possibly may not be involved in certain variants thereof.

Spatiotemporal changes in Ha-ras p21 expression through the hepatocyte cell cycle during liver regeneration

The protein product of the ras oncogene, Ha-ras (p21), is thought to be an important regulator of cell growth. The cytoplasmic relocalization of p21 in the cell during the cell cycle suggests a transient signaling role for this protein in association with its signal transduction function. Because of the importance of this role we examined spatial patterns in vivo of p21 expression at the protein and mRNA levels in hepatocytes during compensatory growth in rat liver following partial hepatectomy. A low level of p21 was immunolocalized on the cytoplasmic membrane of nonregenerating hepatocytes. The level of hepatic p21 increased significantly and without spatial restriction within the liver from 36 to 60 hr after partial hepatectomy (PH). p21 was localized in the cytoplasm of dividing hepatocytes and on the hepatic cytoplasmic membrane. The elevated p21 level decreased and was found mainly on hepatocyte plasma membranes by 96 hr after PH. Immunogold electron microscopy showed p21 localized over mitochondrial membranes and nuclei in nondividing regenerating hepatocytes. Approximately 50% of nonregenerating hepatocytes show nuclear localization of p21. This percentage changes with time following PH. The decrease in nuclear localization was accompanied with an increase in the low number of hepatocytes which demonstrated cytoplasmic localization in nondividing hepatocytes in regenerating liver. Flow cytometric analysis revealed a significant increase of p21 at 36 hr after PH which was 12 hr after the initial induction of ras mRNA. ras mRNA level increased 1.5-fold at 24 hr after PH and a maximum twofold induction was observed at 48 hr. Cell-cycle analysis of regenerating hepatocytes indicated a synchronized first peak of cell division 36-40 hr after PH. Dual parameter flow cytometry revealed that the level of p21 in hepatocytes in S phase and G2/M phase of the cell cycle was significantly higher than that in G0/G1 phase during regeneration. These findings suggest that p21 is important for the progression of regenerating hepatocytes to S phase and then to G2/M phase.

Increased expression of c-myc and c-Ha-ras in dichloroacetate and trichloroacetate-induced liver tumors in B6C3F1 mice

The expression of c-myc and c-H-ras in hyperplastic nodules and hepatocellular carcinomas induced in male B6C3F1 mice after chronic administration of dichloroacetate (DCA) and trichloroacetate (TCA) was studied using in situ hybridization. Expression of c-myc and c-H-ras mRNA was increased in both nodules and carcinomas relative to surrounding tissue and tissues obtained from control animals. Myc expression was similar in hyperplastic nodules and carcinomas induced by DCA, but was significantly higher in TCA-induced carcinomas than in hyperplastic nodules and carcinomas produced by DCA. In carcinomas from animals whose TCA treatment was suspended at 37 weeks, c-myc expression remained high relative to control and surrounding liver tissue at 52 weeks. In contrast, the expression of c-H-ras was consistently elevated in carcinomas from both treatments relative to hyperplastic nodules and non-tumor tissue. Within carcinomas from both treatments, focal areas could be located which expressed even higher levels of c-myc. This heterogeneity was not observed in carcinomas hybridized to c-H-ras-probes. These data suggest that elevated expression of c-H-ras and c-myc might play an important role in the development of hepatic tumors in B6C3F1 mice. Elevated expression of c-H-ras was closely associated with malignancy. Increased c-myc expression does not seem necessary for progression to the malignant state. On the other hand, the increased expression of c-myc appears related to the earlier progression of TCA-induced tumors to the malignant state.

Changes in gap junction protein (connexin 32) gene expression during rat liver carcinogenesis

A rat liver gap junction (GJ) cDNA probe that detects mRNA encoding the 32 Kd GJ-protein (connexin 32) was employed to study GJ-protein gene expression in rat liver tumors induced by a single exposure to diethylnitrosamine (DEN) followed by exposure to 2-acetylaminofluorene (AAF)/CCl4/AAF or induced by systemic administration of N-ethyl-N-hydroxyethylnitrosamine (EHEN). All carcinomas generated by these carcinogens showed markedly reduced levels of GJ-protein mRNA. This may indicate that GJ-protein levels and gap-junctional intercellular communication (GJIC) capacity are also severely compromised. Moreover, all hyperplastic nodules also showed a reduced level of GJ-protein mRNA. Taken together with our earlier finding that the liver tumor promoter phenobarbital inhibits GJ-protein gene expression, these results suggest that deranged GJIC is a relatively early event in liver multistage carcinogenesis. A range of other cDNA probes was also used to characterize gene expression in the DEN-induced tumors. Induction of expression was seen for glutathione S-transferase (placental form) (GST-P), gamma-glutamyltranspeptidase (GGT), and c-raf but not for c-Ha-ras or c-myc.

c-myc gene amplification during hepatocarcinogenesis by a choline-devoid diet

Liver tumors arise in rats fed a choline-devoid diet without added carcinogens. We found amplification of the c-myc gene in 13/13 of these tumors. The amplification ranged from 2- to 70-fold and was accompanied by an increase in c-myc gene expression. Amplification of c-myc was larger in tumors of rats fed a choline-devoid diet followed by a choline-supplemented diet than in tumors from animals fed a choline-devoid diet exclusively. In the former animals, low levels of c-myc gene amplification were also detected in nontumorous regions of tumor-bearing livers. The choline-devoid diet provides an in vivo experimental model for the induction of gene amplification in the rat liver. In this setting, amplification of the c-myc gene may be an early and critical event in carcinogenesis.