Position-dependent activity of alpha -fetoprotein enhancer element III in the adult liver is due to negative regulation

ZBTB20 is a sequence-specific transcriptional repressor of alpha-fetoprotein gene

Alpha-fetoprotein (AFP) represents a classical model system to study developmental gene regulation in mammalian cells. We previously reported that liver ZBTB20 is developmentally regulated and plays a central role in AFP postnatal repression. Here we show that ZBTB20 is a sequence-specific transcriptional repressor of AFP. By ELISA-based DNA-protein binding assay and conventional gel shift assay, we successfully identified a ZBTB20-binding site at −104/−86 of mouse AFP gene, flanked by two HNF1 sites and two C/EBP sites in the proximal promoter. Importantly, mutation of the core sequence in this site fully abolished its binding to ZBTB20 in vitro, as well as the repression of AFP promoter activity by ZBTB20. The unique ZBTB20 site was highly conserved in rat and human AFP genes, but absent in albumin genes. These help to explain the autonomous regulation of albumin and AFP genes in the liver after birth. Furthermore, we demonstrated that transcriptional repression of AFP gene by ZBTB20 was liver-specific. ZBTB20 was dispensable for AFP silencing in other tissues outside liver. Our data define a cognate ZBTB20 site in AFP promoter which mediates the postnatal repression of AFP gene in the liver.

Pericentral activity of alpha-fetoprotein enhancer 3 and glutamine synthetase upstream enhancer in the adult liver are regulated by β-catenin in mice

We previously showed that mouse alpha-fetoprotein (AFP) enhancer 3 activity is highly restricted to pericentral hepatocytes in the adult liver. Here, using transgenic mice, we show that the upstream enhancer of the rat glutamine synthetase gene is also active, specifically in pericentral regions. Activity of both enhancers is lost in the absence of β-catenin, a key regulator of zonal gene expression in the adult liver. Both enhancers contain a single, highly conserved T-cell factor/lymphoid enhancer factor binding site that is required for responsiveness to β-catenin. We also show that endogenous AFP messenger RNA levels in the perinatal liver are lower when β-catenin is reduced.

CONCLUSION

These data identify the first distinct zonally active regulatory regions required for β-catenin responsiveness in the adult liver, and suggest that postnatal AFP repression and the establishment of zonal regulation are controlled, at least in part, by the same factors.

Zhx2 and Zbtb20: novel regulators of postnatal alpha-fetoprotein repression and their potential role in gene reactivation during liver cancer

The mouse alpha-fetoprotein (AFP) gene is abundantly expressed in the fetal liver, normally silent in the adult liver but is frequently reactivated in hepatocellular carcinoma. The basis for AFP expression in the fetal liver has been studied extensively. However, the basis for AFP reactivation during hepatocarcinogenesis is not well understood. Two novel factors that control postnatal AFP repression, Zhx2 and Zbtb20, were recently identified. Here, we review the transcription factors that regulate AFP in the fetal liver, as well as Zhx2 and Zbtb20, and raise the possibility that the loss of these postnatal repressors may be involved in AFP reactivation in liver cancer.

Organogenesis and development of the liver

Embryonic development of the liver has been studied intensely, yielding insights that impact diverse areas of developmental and cell biology. Understanding the fundamental mechanisms that control hepatogenesis has also laid the basis for the rational differentiation of stem cells into cells that display many hepatic functions. Here, we review the basic molecular mechanisms that control the formation of the liver as an organ.

Zinc finger protein ZBTB20 is a key repressor of alpha-fetoprotein gene transcription in liver

The alpha-fetoprotein (AFP) gene is highly activated in fetal liver but is dramatically repressed shortly after birth. The mechanisms that underlie AFP transcriptional repression in postpartum liver are not well understood. AFP enhancer, repressor region, and promoter are implicated to be involved in AFP postnatal repression, but the major transcriptional repressor remains undefined. We previously identified a zinc finger protein gene ZBTB20. To determine its physiological functions in vivo, we have generated hepatocyte-specific ZBTB20 knockout mice by the Cre/loxP approach and demonstrated here that ZBTB20 ablation in liver led to dramatic derepression of the AFP gene in entire liver throughout adult life, although the hepatocytes were normally under nonproliferating status. Furthermore, we found that ZBTB20 was a transcriptional repressor capable of specifically inhibiting AFP promoter-driven transcriptional activity. Liver chromatin immunoprecipitation and mobility shift assays showed that ZBTB20 bound to AFP promoter directly. ZBTB20 was developmentally activated in liver after birth and inversely correlated with its AFP gene expression, suggesting that activated ZBTB20 expression in liver mediated AFP gene repression. Our data point to ZBTB20 as a key regulator governing AFP gene transcription and postulate a new model for the postnatal gene repression of AFP in liver.

Hereditary persistence of alpha-fetoprotein is due to both proximal and distal hepatocyte nuclear factor-1 site mutations

BACKGROUND AND AIMS

The molecular mechanism of hereditary persistence of alpha-fetoprotein (HPAFP) has been previously described in a large Scottish family, consisting of a -119G>A substitution in the distal hepatocyte nuclear factor 1 (HNF-1) binding site of the alpha-fetoprotein (AFP) gene promoter. We report here the molecular mechanisms of HPAFP in 2 new unrelated families.

METHODS

Family 1 was of Bengali origin, and family 2 was Italian. Four of 5 subjects (family 1) and 3 of 9 (family 2) showed HPAFP. The AFP gene promoter was studied in all available family members.

RESULTS

All subjects with high AFP levels had mutated promoter sequences. Family 1 showed the reported -119G>A substitution. Family 2 showed -55C>A and -65C>T substitutions in the proximal putative HNF-1 binding region of the promoter. The -55C>A mutation increased the similarity of the proximal HNF-1 binding region to a consensus binding region. Gel shift assays confirmed its increased affinity toward HNF-1, and transfection experiments revealed an increased level of gene transcription. The -65C>T substitution theoretically created a CCAAT box. However, gel shift and transfection experiments failed to show any biological effect of this substitution that is associated with the -55C>A mutation.

CONCLUSIONS

Two different mutations localized in either HNF-1 binding sites of the AFP gene promoter may result in HPAFP. This highlights the importance of HNF-1 in AFP gene expression. Unexplained persistent AFP should lead to family study and/or AFP gene promoter sequencing to avoid inappropriate explorations and treatment decisions.