To the Editor:
An interesting report by Streetz et al. published in HEPATOLOGY described the production of hepatocyte conditional gp130 knockout mice subjected to chronic liver injury by repeated carbon tetrachloride (CCl4) treatment.1 Chronic injury induces liver regeneration by recruitment of cells from a precursor, either from a stem cell or a progenitor cell known as the oval cell. Numerous mouse studies such as that described by Streetz et al.1 utilize albumin-alpha-fetoprotein (AFP) promoter and/or enhancer sequences to achieve hepatocyte specific gene deletions by expressing Cre recombinase.2 The albumin-AFP promoter and/or enhancers are used, as AFP is an early marker of hepatocyte commitment. First detected at 8.5 days gestation in hepatoblasts, it is highly expressed in perinatal liver and diminishes to low or undetectable levels in the adult mouse.3 Albumin production commences around 9.5 days gestation and is maintained throughout development.4
We have previously shown that the choline-deficient, ethionine-supplemented diet (CDE) induces liver progenitor oval cells in rodents and that these cells express albumin and AFP.5, 6 Oval cells induced by CCl4 will express albumin and AFP.7 We therefore raise the possibility that the interpretation of many hepatocyte-specific gene targeting experiments may be compromised by this situation. A quiescent pool of oval cells exists in the liver and will respond to promitogenic signals initiated by chronic liver damage. In the study by Streetz et al.,1 repeated CCl4 treatment may activate oval cell proliferation.7 Therefore, oval cells derived from the endogenous pool will also be deficient for gp130. The protective role of gp130 stimulation as observed by Streetz et al.1 might well be explained by gp130 stimulation of oval cells, which results in cellular proliferation (see below). This phenomenon has not been considered in the publication. We wish to alert your readership that conditional gene targeting utilizing Cre recombinase driven by the albumin-AFP promoter/enhancer sequences will inactivate the gene of interest in hepatocytes and oval cells.
We recently produced novel data implicating the combination of interleukin (IL)-6 and its soluble IL-6R (sIL-6R) (hyper IL-6)8 in oval cell activation (Fig. 1). IL-6 signalling via the sIL-6R requires gp130 to exert its effect. Therefore, in the publication by Streetz et al.,1 it is likely that the knockout of gp130 in progenitor oval cells may compromise their activation during chronic liver injury. Although few oval cells are present in normal liver, their presence can confound experiments that do not involve chronic liver injury. In a mouse model where simian virus 40 (SV40) large T antigen is overexpressed under the control of the albumin promoter, hepatocellular carcinoma occurs9. It is accepted that SV40 large T antigen expressed in hepatocytes is responsible for the hepatocellular carcinoma. Since overexpression of SV40 large T antigen induces oval cell proliferation and carcinoma, which are positive for the oval cell marker A6,10 it's possible that the tumorigenic process is initiated in an oval cell.
In conclusion, experiments utilizing the albumin and AFP promoter/enhancers to activate or inactivate genes need to consider that oval cells as well as hepatocytes may be affected.