E1A represses apolipoprotein AI enhancer activity in liver cells through a pRb- and CBP-independent pathway

Hepatic deficiency of the pioneer transcription factor FoxA restricts hepatitis B virus biosynthesis by the developmental regulation of viral DNA methylation

The FoxA family of pioneer transcription factors regulates hepatitis B virus (HBV) transcription, and hence viral replication. Hepatocyte-specific FoxA-deficiency in the HBV transgenic mouse model of chronic infection prevents the transcription of the viral DNA genome as a result of the failure of the developmentally controlled conversion of 5-methylcytosine residues to cytosine during postnatal hepatic maturation. These observations suggest that pioneer transcription factors such as FoxA, which mark genes for expression at subsequent developmental steps in the cellular differentiation program, mediate their effects by reversing the DNA methylation status of their target genes to permit their ensuing expression when the appropriate tissue-specific transcription factor combinations arise during development. Furthermore, as the FoxA-deficient HBV transgenic mice are viable, the specific developmental timing, abundance and isoform type of pioneer factor expression must permit all essential liver gene expression to occur at a level sufficient to support adequate liver function. This implies that pioneer transcription factors can recognize and mark their target genes in distinct developmental manners dependent upon, at least in part, the concentration and affinity of FoxA for its binding sites within enhancer and promoter regulatory sequence elements. This selective marking of cellular genes for expression by the FoxA pioneer factor compared to HBV may offer the opportunity for the specific silencing of HBV gene expression and hence the resolution of chronic HBV infections which are responsible for approximately one million deaths worldwide annually due to liver cirrhosis and hepatocellular carcinoma.

This study demonstrates the connection between FoxA expression and gene silencing by DNA methylation in vivo during liver maturation. Insufficient FoxA expression results in selective developmentally regulated hepatitis B virus (HBV) silencing by DNA methylation. To our knowledge, this is the first in vivo demonstration that pioneer factors such as FoxA function by mediating the developmental demethylation of their target genes, leading to their tissue specific gene expression. Furthermore, our results strongly imply that the marking of cellular target genes for subsequent transcription later in development is dependent upon the level and timing of FoxA expression plus its affinity for its target sequences within enhancer and promoter regions. Consequently, these findings suggest that the appropriate control of FoxA activity during development could lead to the transcriptional inactivation of nuclear HBV covalently closed circular DNA by methylation and hence resolution of chronic HBV infection. This represents a clinical goal that current therapies are unable to attain, and hence suggests a potential route to a cure for this chronic infection which kills approximately 1 million individuals annually.

Inhibition of MITF transcriptional activity independent of targeting p300/CBP coactivators

Microphthalmia-associated transcription factor (MITF) activates the expression of melanocyte-specific markers and promotes the survival of embryonic, adult and malignant melanocytes. Although numerous MITF-dependent downstream genes have been identified, the mechanisms by which the MITF activity is coregulated remain elusive. Here we used a non-melanocytic cell line U2-OS as a model in which MITF evokes transcription of a paradigmatic MITF target tyrosinase and show that the adenoviral E1A protein represses the MITF-driven transcription in these cells. The E1A CR1 domain (which alone is insufficient to bind p300) was sufficient for repression, while the N-terminus, through which E1A binds the p300/CBP proteins and other coactivators, was unable to repress. Correspondingly, CR1 inhibited colony formation of MITF-positive, but not MITF-negative, melanoma cells. The repression by CR1 was largely independent of the PCAF-binding motif, previously recognized to be necessary for suppression of muscle-specific enhancer. Interestingly, CR1 conferred transcriptional competence to the MITF-CR1 chimera in which the MITF portion was rendered transcription-deficient. Moreover, MITF mutants defective in binding to p300/CBP in vivo still activated transcription, further supporting a p300/CBP-independent coactivation of MITF targets. MITF is amplified in a subset of melanomas and is thought to be required for sustained proliferation of malignant melanocytes. Our results suggest that understanding how CR1 represses Mitf activity may reveal a route to melanoma therapy.