The Glypican 3 (Gpc3) gene is expressed abundantly in the fetal liver, is inactive in the normal adult liver, and is frequently reactivated in hepatocellular carcinoma (HCC). This reactivation in HCC has led to considerable interest in Gpc3 as a diagnostic tumor marker and its possible role in tumorigenesis. Despite this interest, the basis for Gpc3 regulation is poorly understood. On the basis of the similarities between Gpc3 and alpha-fetoprotein expression in the liver, we reasoned that common factors might regulate these 2 genes. Here we identify zinc fingers and homeoboxes 2 (Zhx2) as a regulator of Gpc3. Mouse strainspecific differences in adult liver Gpc3 messenger RNA levels and transgenic mouse studies indicate that Zhx2 represses Gpc3 expression in the adult liver. We also demonstrate that Gpc3 is activated in the regenerating liver following a carbon tetrachloride treatment and that the level of Gpc3 induction is controlled by alpha-fetoprotein regulator 2 (Afr2).
Conclusion:
We show that Zhx2 acts as a repressor of Gpc3 in the adult liver, and this raises the interesting possibility that Zhx2 might also be involved in Gpc3 reactivation in HCC. We also show that Gpc3 is activated in the regenerating liver in an Afr2-dependent manner. Zhx2 and Afr2 represent the first known regulators of Gpc3. (HEPATOLOGY 2007;46:1541-1547.) G lypicans are a small family of glycosyl phosphatidylinositol-anchored heparin sulfate proteoglycans that are found in species ranging from Drosophila to vertebrates. 1 This family of proteins is associated with cell growth, development, and responses to various growth factors. 2,3 Six glypicans exist in vertebrates, several of which are globally expressed, whereas others exhibit more restricted developmental and tissuespecific expression. 1,3,4 Of the different glypicans, the Xlinked Gpc3 has been the focus of considerable attention because of its association with certain diseases. Loss of functional GPC3 is associated with Simpson-Golabi-Behmel syndrome in humans, which is characterized by general overgrowth with additional clinical manifestations. 5 Changes in Gpc3 expression are also associated with certain tumors. For example, reduced Gpc3 expression is often found in breast and ovarian tumors and mesotheliomas. [6][7][8] In contrast, Gpc3 expression is frequently increased in hepatocellular carcinoma (HCC). 9 In this respect, Gpc3 behaves as an oncofetal gene, in that it is expressed in the fetal liver, normally silent in the adult liver, and reactivated in cancer. In fact, Gpc3 may be reactivated in HCC as frequently as alpha-fetoprotein (AFP), which has been used extensively as a marker of this cancer, and may be a more reliable early diagnostic marker than AFP. 10,11 Despite considerable interest in Gpc3 activation in HCC, factors involved in regulating Gpc3 in the liver have not yet been identified. Because the Gpc3 gene is ex-