Long-term treatment with hepatic tumor promoters inhibits mitogenic responses of hepatocytes to acidic fibroblast growth factor and hepatocyte growth factor

Liver-specific ablation of integrin-linked kinase in mice results in enhanced and prolonged cell proliferation and hepatomegaly after phenobarbital administration

We have recently demonstrated that disruption of extracellular matrix (ECM)/integrin signaling via elimination of integrin-linked kinase (ILK) in hepatocytes interferes with signals leading to termination of liver regeneration. This study investigates the role of ILK in liver enlargement induced by phenobarbital (PB). Wild-type (WT) and ILK:liver-/- mice were given PB (0.1% in drinking water) for 10 days. Livers were harvested on 2, 5, and 10 days during PB administration. In the hepatocyte-specific ILK/liver-/- mice, the liver:body weight ratio was more than double as compared to 0 h at day 2 (2.5 times), while at days 5 and 10, it was enlarged three times. In the WT mice, the increase was as expected from previous literature (1.8 times) and seems to have leveled off after day 2. There were slightly increased proliferating cell nuclear antigen-positive cells in the ILK/liver-/- animals at day 2 as compared to WT after PB administration. In the WT animals, the proliferative response had come back to normal by days 5 and 10. Hepatocytes of the ILK/liver-/- mice continued to proliferate up until day 10. ILK/liver-/- mice also showed increased expression of key genes involved in hepatocyte proliferation at different time points during PB administration. In summary, ECM proteins communicate with the signaling machinery of dividing cells via ILK to regulate hepatocyte proliferation and termination of the proliferative response. Lack of ILK in the hepatocytes imparts prolonged proliferative response not only to stimuli related to liver regeneration but also to xenobiotic chemical mitogens, such as PB.

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

EGF-Induced receptor autophosphorylation in primary hepatocytes isolated from phenobarbitone-treated mice

Phenobarbitone (PB) treatment of mice causes a decrease in the growth factor responsiveness of hepatocytes. Here, epidermal growth factor receptor (EGFR) expression and receptor autophosphorylation was determined in hepatocytes isolated from control and PB-treated mice. There was a decrease in the level of EGFR expression in hepatocytes isolated from mice following PB administration when compared to controls. EGF caused an approximate 20-fold increase of the 170 kD phosphotyrosine band in control hepatocytes, which was inhibited by the EGFR specific tyrosine kinase inhibitor 4, 5-dianilinopthalamide. Following PB treatment, the degree of basal receptor phosphorylation (in the absence of EGF) was significantly greater and therefore the fold rise in EGFR phosphorylation in isolated hepatocytes was lower than in controls. However, the overall extent of EGF-induced receptor phosphorylation was not diminished in hepatocytes isolated from PB-treated mice. Therefore the reduction in responsiveness to growth factors seen in hepatocytes ex vivo or the cessation of proliferation observed in vivo following PB administration is unlikely to be attributed to a decrease in ligand binding and subsequent receptor autophosphorylation.

Phenobarbital promotes liver growth in c-myc/TGF-alpha transgenic mice by inducing hypertrophy and inhibiting apoptosis

Phenobarbital (PB) is a non-genotoxic liver tumor promoter used extensively in initiation-promotion protocols. To determine the mode of PB action, double transgenic mice overexpressing both the c-myc and transforming growth factor (TGF)-alpha genes were treated with PB in the food for 10 weeks, from 3 weeks of age. After 3-4 weeks on PB a peak in liver mass was noted, which subsequently leveled off at a value approximately 30% above untreated animals. The mitotic index in mice given PB peaked at 1 week of treatment and was significantly elevated compared with untreated animals. No significant difference between treated and untreated animals was seen thereafter, although a trend of PB-associated mitotic suppression was noticeable. The apoptotic index also showed a trend of suppression compared with untreated animals, significant after prolonged PB administration. Dysplastic hepatocytes were more prominent in PB-treated mice than untreated animals, particularly pericentrally. Removal of PB from the diet at 4 weeks of treatment led to a dramatic increase in apoptosis. This accompanied a drop in the liver mass to the level of untreated controls by 10 days. Throughout the study, PB-treated animals showed markedly lower levels of TGF-beta1 ligand, coincident with an elevated level of the anti-apoptotic protein Bcl-2. On withdrawal of PB, the levels of all these proteins rapidly changed to mirror those seen in untreated mice. In all treatment groups, no change in the levels of epidermal growth factor receptor, TGF-beta receptors I and II or Bcl-xS/L were seen. We conclude from our data that PB stimulates liver growth in double transgenic c-myc/TGF-alpha mice by induction of liver hypertrophy and inhibition of apoptosis, brought about by both a decrease in signaling through the TGF-beta pathway and an increase in Bcl-2. The data support the hypothesis that PB promotes neoplastic development through a reduction in the incidence of cell death.

Responsiveness of hepatocytes from dichloroacetic acid or phenobarbital treated mice to growth factors in primary culture

Hepatocytes isolated from male B6C3F1 mice and maintained in primary culture were exposed to epidermal growth factor (EGF), hepatocyte growth factor (HGF), acidic fibroblast growth factor (aFGF) alone or in combination with the mitoinhibitory transforming growth factor beta 1 (TGF-beta 1). Groups of mice were exposed to 3.5 g/l dichloroacetic acid (DCA), 0.1% phenobarbital (PB) or the drinking water vehicle for 0, 2, 5, 10, 20, 30, 60, or 90 days. Following a 2 h attachment period, the growth factors with or without TGF-beta 1 were added together with [3H]thymidine. The cells were harvested 48 h later and the incorporation of the labeled thymidine into cellular DNA was determined. Basal DNA synthesis was enhanced following 2 days of PB treatment after which it declined to levels significantly below that in the untreated mice. No early time enhancement of DNA synthesis was measured in the hepatocyte cultures for animals exposed to DCA, but the late time inhibition was also seen. Primary cultures of hepatocytes isolated from control and DCA treated mice exhibited similarly enhanced DNA synthesis in response to eGF, HGF, or aFGF alone or in combination with TGF-beta 1. In contrast, hepatocytes from PB treated animals were refractory to the effects of the growth factors at all time periods. These data suggest that the early depression of cell proliferation we have seen during DCA induced hepatocellular cancer is not due to an impaired ability of hepatocytes to respond to growth factors and that the mechanisms of liver tumorigenesis in the mouse induced by PB and DCA are dissimilar.