Disruption of Redox Homeostasis in the Transforming Growth Factor-α/c-myc Transgenic Mouse Model of Accelerated Hepatocarcinogenesis

Abstract

In previous studies we have demonstrated that transforming growth factor (TGF)-␣/c-myc double transgenic mice exhibit an enhanced rate of cell proliferation, accumulate extensive DNA damage, and develop multiple liver tumors between 4 and 8 months of age. To clarify the biochemical events that may be responsible for the genotoxic and carcinogenic effects observed in this transgenic model, several parameters of redox homeostasis in the liver were examined prior to development of hepatic tumors. By 2 months of age, production of reactive oxygen species, determined by the peroxidation-sensitive fluorescent dye, 2,7-dichlorofluorescin diacetate, was significantly elevated in TGF-␣/c-myc transgenic hepatocytes versus either wild type or c-myc single transgenic cells, and occurred in parallel with an increase in lipid peroxidation. Concomitantly with a rise in oxidant levels, antioxidant defenses were decreased, including total glutathione content and the activity of glutathione peroxidase, whereas thioredoxin reductase activity was not changed. However, hepatic tumors which developed in TGF-␣/c-myc mice exhibited an increase in thioredoxin reductase activity and a very low activity of glutathione peroxidase. Furthermore, specific deletions were detected in mtDNA as early as 5 weeks of age in the transgenic mice. These data provide experimental evidence that co-expression of TGF-␣ and c-myc transgenes in mouse liver promotes overproduction of reactive oxygen species and thus creates an oxidative stress environment. This phenomenon may account for the massive DNA damage and acceleration of hepatocarcinogenesis observed in the TGF-␣/c-myc mouse model.

Hepatocellular expression of a dominant-negative mutant TGF-β type II receptor accelerates chemically induced hepatocarcinogenesis

“…However, disruption of the TGF-b signaling pathway seems to be of minor importance in hepatocarcinogenesis in comparison to other epithelial carcinomas: Overexpression of DkTbRII in the lung, pancreas, mammary gland or skin led to much higher discrepancy in onset and incidence of tumor development between tissues of transgenic mice and wild type mice (BoÈ ttinger et al, 1997a,b;Amendt et al, 1998). This is in accordance with histopathological ®ndings giving a very heterogenous picture with respect to the presence or absence of functional TGF-b type II receptors in HCCs (Kiss et al, 1997;Bedossa et al, 1995;Factor et al, 1998;Grasl-Kraupp et al, 1998;Abou-Shady et al, 1999). In addition, a potential reason for this dierence may be once more that TGFb related or unrelated cytokines compensate for decreased biological eects of TGF-b in the liver during chemical hepatocarcinogenesis.…”

“…In addition, many cancers are associated with increased local and systemic levels of TGF-b (Gold, 1999). Thus, on one hand TGF-b may stimulate malignant progression itself, possibly by modulating cell-matrix interactions, by its immunosuppressive activity or by enhancing angiogenesis (Arrick et al, 1992, Factor et al, 1998. On the other hand, TGF-b can have tumor suppressor activity, most likely due to its growth inhibitory eects.…”

“…In accordance with these ®ndings, Lowsky and coworkers could recently show that lymphomas of MSH2 knockout mice (MSH27/7) ± mice lacking the mismatch repair system ± frequently exhibit mutations in the type II TGF-b receptor (Lowsky et al, 2000). Apart from mutations of the type II TGF-b receptor, downregulation or lack of expression of this receptor might be another means of escape from TGF-b mediated growth control as shown for prostate cancer (Cardillo et al, 2000), breast cancer (Gobbi et al, 2000) and also for hepatocellular carcinoma (Bedossa et al, 1995;Kiss et al, 1997;Factor et al, 1998). Some manifest hepatocellular carcinomas showed a marked reduction or loss of expression of TbRII (Bedossa et al, 1995;Kiss et al, 1997;Factor et al, 1998).…”

“…Apart from mutations of the type II TGF-b receptor, downregulation or lack of expression of this receptor might be another means of escape from TGF-b mediated growth control as shown for prostate cancer (Cardillo et al, 2000), breast cancer (Gobbi et al, 2000) and also for hepatocellular carcinoma (Bedossa et al, 1995;Kiss et al, 1997;Factor et al, 1998). Some manifest hepatocellular carcinomas showed a marked reduction or loss of expression of TbRII (Bedossa et al, 1995;Kiss et al, 1997;Factor et al, 1998). Since this ®nding was not con®rmed by others (GraslKraupp et al, 1998;Abou-Shady et al, 1999) the role of TGF-b signaling in hepatocarcinogenesis remains elusive.…”

Oxidative stress and regulation of glutathione in lung inflammation

“…TGF-b1 also downregulates c-GCS-HS mRNA and glutathione synthesis in human alveolar epithelial cells and pulmonary artery endothelial cells in vitro [109,110]. Interestingly, recent studies by FACTOR et al [156] showed decreased glutathione synthesis in a TGF transgenic (overexpression) mouse model and increased susceptibility to oxidant-mediated injury. Various workers have shown that c-GCS-HS mRNA expression is under the control of the AP-1 transcription factor [31, 67,68,86], and that TGF-b1 may decrease c-GCS-HS gene expression via an AP-1 mechanism [157].…”