Erik Meyer

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

The potent growth-inhibitory activity of cytokines of the transforming growth factor-β (TGF-β) superfamily and their widespread expression in epithelia suggest that they may play an important role in the maintenance of epithelial homeostasis. To analyse TGF-β mediated tumor suppressor activity in the liver, we generated transgenic mice overexpressing a dominant negative type II TGF-β receptor in hepatocytes under control of the regulatory elements of the human C-reactive protein gene promoter. Transgenic animals exhibited constitutive and liver-specific transgene expression. The functional inactivation of the TGF-β signaling pathway in transgenic hepatocytes was shown by reduced TGF-β induced inhibition of DNA synthesis in primary hepatocyte cultures. Liver morphology and spontaneous tumorigenesis were unchanged in transgenic mice suggesting that interruption of the signaling of all three isoforms of TGF-β in hepatocytes does not disturb tissue homeostasis in the liver under physiological conditions. However, following initiation with the carcinogen diethylnitrosamine and tumor-promotion with phenobarbital transgenic mice exhibited a moderate albeit significant increase in the incidence, size and multiplicity of both preneoplastic tissue lesions in the liver and of hepatocellular carcinomas. These results give in vivo evidence for a tumor suppressor activity of the endogeneous TGF-β system in the liver during chemical hepatocarcinogenesis.

Liver‐specific overexpression of matrix metalloproteinase 9 (MMP‐9) in transgenic mice accelerates development of hepatocellular carcinoma

Matrix metalloproteinase‐9 (MMP‐9) plays a central role in tumor invasion and development of metastases. Expression of MMP‐9 had been shown in human hepatocellular carcinomas (HCCs). However, it remained unclear whether MMP‐9 could influence development of HCC. In order to address this issue, we generated transgenic mice overexpressing MMP‐9 in the liver. In order to avoid embryonic lethality a Cre‐lox system was utilized for conditional overexpression of MMP‐9 under control of an albumin enhancer and promoter. Induction of MMP‐9 overexpression in transgenic mice was achieved by i.v. injection of an adenovirus coding for the Cre recombinase. Initiation of liver carcinogenesis was achieved by injection of diethylnitrosamine (DEN) followed by Phenobarbital administration in drinking water. Transgene expression was induced at the age of 6 wk. Four and six months later mice were sacrificed and examined macroscopically and microscopically in a blinded manner. Alb/Cre/MMP‐9‐transgenic mice showed liver specific overexpression of MMP‐9‐mRNA and protein after induction. At the age of 6 months livers of transgenic mice showed 15.7 ± 11.6 tumors (mean ± SD) in contrast to wildtype mice with only 7.9 ± 11.0 tumors (P < 0.03). By histopathology examination of the livers HCCs were identified in 42% of the transgenic mouse livers but only 8% in wildtype animals. In summary, we established a novel MMP‐9 transgenic mouse model, and report on a significantly increased susceptibility of MMP‐9 transgenic mice to chemically induced carcinogenesis. This is the first in vivo proof that MMP‐9 overexpression promotes liver tumor development.