Table of Contents:
HBV- and HCV-infections belong to the most important infectious diseases, taking over 500 million chronically infected patients worldwide and more than five million newly infected people per year in account. They also are responsible for more than 90% of all hepatocellular carcinomas, which is the third leading cause for cancer death. Though totally different, both viruses induce similar symptoms and similar courses of disease. During tumour development from chronic infection via liver cirrhosis to hepatocellular carcinoma the CD95 system is activated. The CD95 system is a death system. Binding of the CD95 ligand to the CD95 receptor which is ubiquitously expressed induces a signal transduction pathway that leads to apoptosis. Important elements of this pathway are caspases and the mitochondria, the latter working as signal enhancer. Regulatory proteins are, among others, Bax and p53. Bax regulates the signal transduction on the level of the mitochondria whereas p53 mainly acts through the regulation of the CD95- and the Bax-gene and the induction of apoptosis itself.
On the one hand chronic infection is a risk factor for hepatocellular carcinoma. On the other hand the CD95 system is activated during tumour development. This leads to the question whether the CD95 system also plays a role during progression from acute to chronic infection. To solve this question Black6 mice (displaying 100% of CD95 receptor on the surface of their cells) and lpr-mice (displaying only 10% of the CD95 receptor on their cells) were infected with hepatitis B and subsequently investigated. I could show that the CD95 system is essential for fighting the infection since only the Black6 mice showed an immune response with subsequent elimination of the virus.
For further investigation on cellular level I established a cell culture system that was near the clinical situation by optimizing the isolation of primary human hepatocytes out of resection material and by using adenoviral vectors for p53 as well as for HBV to transduct the cells. Using this system I could show that HBV induces apoptosis in hepatocytes in cooperation with p53. Responsible for the interaction with p53 was the X-Protein of HBV, shortly called HBx. This cooperation with p53 during induction of apoptosis led to the question whether it does also take place in the transcriptional activation of the proteins involved since the CD95 gene as well as the Bax gene is, among others, regulated by p53. Gene activation studies for both genes showed, that they could be transcriptionally activated by HBV. Co-activation with p53 even was synergistic and was mediated by HBx. Semi-quantitative co-PCR with densitometric evaluation also proved induction of CD95 ligand gene expression.
In contrast to HBV, the Hepatitis C virus is an RNA virus. Comparison concerning the induction of apoptosis and the transactivation of the CD95- and the Bax-gene showed similar results. HCV can induce apoptosis and in that cooperates with p53. This cooperation also takes place in the transactivation of the CD95- and the Bax-gene though not as strong as between p53 and HBV. One of the strongest activators in HCV is the structural protein HCV-Core. Deletion studies revealed that this effect is due to the first 123 of 191 amino acids of the Core-protein.
In summary, the CD95 system is essential for the elimination of HBV and the protection of hepatocytes against enduring damage. Next: a cell culture model near to the real situation could be established by using primary human hepatocytes together with adenoviral vectors. Use of this system showed that HBV as well as HCV could transcriptionally activate the CD95- and the Bax-gene in cooperation with p53 and by that induce apoptosis.