Caspase-8 acts as a tumor suppressor in chemically induced hepatocarcinogenesis in mice

Background & Aims: Caspase-8 (Casp8) is the apical protease essential for the initiation of extrinsic apoptosis. A role of Casp8 for preventing Hepatocellular Carcinoma (HCC) has been postulated. However, several studies revealed tumor suppressive, but also tumor promoting properties of Casp8 depending on the tissue environment. Therefore, the aim of this study was to investigate if Casp8 inhibits HCC-development in the chemical model of diethylnitrosamine (DEN)-induced hepatocarcinogenesis using conditional, hepatocyte-specific Casp8 knockout (Casp8Δhepa) mice.

Methods: Casp8Δhepa mice and wildtype (Casp8f/f) controls were treated with a single low dose of DEN at the age of two weeks and sacrificed after 24 and 40 weeks to monitor tumor initiation or tumor progression, respectively. Alternatively, six week old mice were treated with high dose of DEN to induce acute genotoxic liver injury and subsequent regeneration, which was analysed 24 – 96 hours after treatment.

Results: Loss of Casp8 did not substantially affect the immediate reaction towards DEN-induced genotoxic acute liver damage. However, the start of compensatory proliferation was slightly accelerated in Casp8Δhepa mice hinting at a shortened phase of DNA-repair in these animals. Interestingly, loss of Casp8 was not associated with reduced liver apoptosis after acute DEN treatment demonstrating that liver damage in the DEN model is Casp8-independent. Twenty-four weeks after DEN-mediated HCC induction, the frequency and size of dysplastic liver lesions were similar in Casp8f/f and Casp8Δhepa mice. Thus, Casp8 does not prevent tumor initiation in the DEN model. However, 40 weeks after DEN treatment HCC progression was substantially enhanced in Casp8Δhepa mice as measured by increase tumor number and size. Detailed analysis revealed that Casp8Δhepa mice showed increased hepatic inflammation and accelerated NF κB-activation after DEN treatment. Moreover, loss of Casp8 in advanced HCC resulted in decreased levels of the cell cycle inhibitors p21 and p27 and in an induction of the putative tumor stem cell marker CD133+.

Conclusion: Our data points to novel, non-apoptotic properties of Casp8 for the suppression of liver tumors potentially by regulating cell cycle inhibition, hepatic stem cell activation and NF-kB. Accordingly, therapeutical inhibition of Casp8 in hepatitis patients – which is currently tested in clinical trials – should be handled carefully as it could increase the risk of hepatocarcinogenesis.