Genetic manipulations utilizing albumin and alpha-fetoprotein promoter/enhancers affect both hepatocytes and oval cells
Authors: Konrad L. Streetz, Torsten Wuestefeld, Michael P. Manns, Christian Trautwein
Journal: Hepatology (2004)

In our recent study,1 the contribution of interleukin 6 (IL-6)/gp130–dependent pathways during liver injury was investigated. We used an ubiquitous (Mx1-promoter, MxCre2, 3) and cell-specific (AlfpCre, alfa-fetoprotein [AFP] +albumin enhancer and albumin-promoter4, 5) approach to conditionally inactivate gp130 in different liver cell types, thereby showing that gp130-deleted mice (either selectively in hepatocytes or in all liver cells) had a blunted STAT3 activation and acute-phase response induction after CCl4 treatment. However, gp130 deficiency had no effect on the degree of acute CCl4-mediated liver injury and subsequent hepatocyte proliferation. In contrast, after repetitive CCl4 injections in 1x-Cre, but not in AlfpCre gp130-deleted mice, a more pronounced fibrosis progression was evident. From these experiments we concluded that deletion of gp130 in hepatocytes through the AlfpCre construct is not crucial for fibrosis progression in this model, as the phenotype was evident in mice expressing Cre under the control of the ubiquitous Mx1-promoter. Therefore, our main conclusion was that IL-6 acts via gp130 on cells other than hepatocytes within or even outside the liver. The interpretation of this observation would be independent of whether oval cells would also show a deletion of gp130 through the AlfpCre construct. The body of evidence for an involvement of oval cells during chronic liver injury is rapidly growing, but the nature of these cells itself is not yet fully understood. Oval cells have not been specifically investigated in our study, although it is likely that nonactivated oval cells might be within the pool of Mx1-mediated gp130-deleted cells of the liver. Therefore we are thankful for the comment made by Matthews et al. about this specific aim. As oval cells are suspected to express AFP and even albumin, they would be subjected to AlfpCre-mediated gp130 knockout in our system. This event would occur at the latest at the time point of their induction by potentially injuring processes. As the AlfpCre+/gp130loxP mice did not show any significant phenotype during acute and chronic CCl4 mediated liver injury, the role of gp130 in oval cells is most likely not essential under these circumstances. However, we cannot rule out that nonactivated oval cells might initially not express enough AFP/albumin to activate Cre transcription. We are fully aware that ectopic expression of AFP and albumin in cells other than hepatocytes has to be taken into consideration especially when using the construct to mediate Cre-recombinase specific gene-inactivation. There is evidence that these genes are activated during embryonic development in cells of the hepatoblast from the endoderm6 and can even be found within the gut, pancreas, and kidney, although at very low levels. To ensure only expression within the hepatic lineage, consideration was taken to limit Cre expression in the herein-used AlfpCre construct4 by the application of several regulatory elements (the mouse albumin enhancer, the albumin promoter, and 3 mouse AFP enhancers). Cell type–specific gene expression is always flawed with limitations. This is either due to the fact that most systems for transcriptional activation are leaky at some degree or that the achieved expression level might be too low to overcome certain barriers. Therefore, better and even more specific approaches are needed. The specific aim of gene deletion by an AFP/albumin-dependent Cre recombinase in oval cells cannot be successfully answered at this time point. It is, however, an interesting subject for further studies and should be considered in future approaches of hepatocyte-specific gene inactivation. Konrad L. Streetz M.D.*, Torsten Wuestefeld Ph.D.*, Michael P. Manns M.D.*, Christian Trautwein M.D.*, * Department of Gastroenterology, Hepatology & Endocrinology Medical School of Hannover Hannover, Germany.