In this study, we report a novel biological function of vitamin A metabolites in conversion of naive FoxP3− CD4+ T cells into a unique FoxP3+ regulatory T cell subset (termed “retinoid-induced FoxP3+ T cells”) in both human and mouse T cells. We found that the major vitamin A metabolite all-trans-retinoic acid induces histone acetylation at the FoxP3 gene promoter and expression of the FoxP3 protein in CD4+ T cells. The induction of retinoid-induced FoxP3+ T cells is mediated by the nuclear retinoic acid receptor α and involves T cell activation driven by mucosal dendritic cells and costimulation through CD28. Retinoic acid can promote TGF-β1-dependent generation of FoxP3+ regulatory T cells but decrease the TGF-β1- and IL-6-dependent generation of inflammatory Th17 cells in mouse T cells. Retinoid-induced FoxP3+ T cells can efficiently suppress target cells and, thus, have a regulatory function typical for FoxP3+ T cells. A unique cellular feature of these regulatory T cells is their high expression of gut-homing receptors that are important for migration to the mucosal tissues particularly the small intestine. Taken together, these results identify retinoids as positive regulatory factors for generation of gut-homing FoxP3+ T cells.

The X gene product encoded by the hepatitis B virus, termed pX, is a promiscuous transactivator of a variety of viral and cellular genes under the control of diverse cis-acting elements. Although pX does not appear to directly bind DNA, pX-responsive elements include the NF-cB, AP-1, and CRE (cAMP response element) sites. Direct proteinprotein interactions occur between viral pX and the CREbinding transcription factors CREB and ATF. Here we examine the mechanism of the protein-protein interactions occurring between CREB and pX by using recombinant proteins and in vitro DNA-binding assays. We demonstrate that pX interacts with the basic region-leucine zipper domain of CREB but not with the DNA-binding domain of the yeast transactivator protein Gal4. The interaction between CREB and pX increases the affinity of CREB for the CRE site by an order of magnitude, although pX does not alter the rate of CREB dimerization. Methylation interference footprinting reveals differences between the CREB DNA and CREB-pX DNA complexes. These experiments demonstrate that pX alters the way CREB interacts with the CRE DNA and suggest that the basic, DNA-binding region of CREB is the target of pX. Transfection assays in PC12 cells with the CREBdependent somatostatin promoter demonstrate a nearly 15-fold transcriptional induction after forskolin stimulation in the presence of pX. These results support the significance of the CREB-pX protein-protein interactions in vivo.The 3.2-kb hepatitis B virus (HBV) genome contains four recognized open reading frames, three of which encode virion structural proteins. These include the surface and core antigens and viral polymerase (1). The fourth frame, conserved among all mammalian hepadnaviruses, encodes a 16.5-kDa protein, termed X antigen (1). The X gene is expressed during viral infection, producing a 1-kb mRNA (2). In transgenic mice, the HBV X protein (pX) induces liver cancer (3).pX is a transactivator of transcriptional elements, such as those present within the HBV enhancer (4, 5), simian virus 40 enhancer (6), human immunodeficiency virus long terminal repeat (7-9), and the human interferon gene (10). X-responsive cis-acting elements include NF-KB (9), AP-1 (11), and cAMP response element (CRE) (12) sites. pX does not appear to bind DNA directly. Having multiple X-responsive cis-acting elements suggests that the mechanism of pX transactivation is pleiotropic (13). Activation by pX may involve targeting different regulatory steps in these signal transduction pathways or direct protein-protein interactions with diverse cellular transcription factors and the transcriptional machinery. Recent studies (14) with inhibitors of protein kinase C reported that pX acts indirectly by activating the protein kinase C transduction pathway, thereby increasing the activity of the cognate transcription factors. A more direct mechanism of pX transactivation is supported by earlier studies (11,15) showing that pX possesses an activation domain. pX stimulates tran-The publication costs of this articl...

Chronic hepatitis B virus (HBV) infection is a major factor in hepatocellular carcinoma (HCC) pathogenesis by a mechanism not yet understood. Elucidating mechanisms of HBV‐mediated hepatocarcinogenesis is needed to gain insights into classification and treatment of HCC. In HBV replicating cells, including virus‐associated HCCs, suppressor of zeste 12 homolog (SUZ12), a core subunit of Polycomb repressive complex2 (PRC2), undergoes proteasomal degradation. This process requires the long noncoding RNA, Hox transcript antisense intergenic RNA (HOTAIR). Intriguingly, HOTAIR interacts with PRC2 and also binds RNA‐binding E3 ligases, serving as a ubiquitination scaffold. Herein, we identified the RNA helicase, DEAD box protein 5 (DDX5), as a regulator of SUZ12 stability and PRC2‐mediated gene repression, acting by regulating RNA‐protein complexes formed with HOTAIR. Specifically, knockdown of DDX5 and/or HOTAIR enabled reexpression of PRC2‐repressed genes epithelial cell adhesion molecule (EpCAM) and pluripotency genes. Also, knockdown of DDX5 enhanced transcription from the HBV minichromosome. The helicase activity of DDX5 stabilized SUZ12‐ and PRC2‐mediated gene silencing, by displacing the RNA‐binding E3 ligase, Mex‐3 RNA‐binding family member B (Mex3b), from HOTAIR. Conversely, ectopic expression of Mex3b ubiquitinated SUZ12, displaced DDX5 from HOTAIR, and induced SUZ12 down‐regulation. In G2 phase of cells expressing the HBV X protein (HBx), SUZ12 preferentially associated with Mex3b, but not DDX5, resulting in de‐repression of PRC2 targets, including EpCAM and pluripotency genes. Significantly, liver tumors from HBx/c‐myc bitransgenic mice and chronically HBV‐infected patients exhibited a strong negative correlation between DDX5 messenger RNA levels, pluripotency gene expression, and liver tumor differentiation. Notably, chronically infected HBV patients with HCC expressing reduced DDX5 exhibited poor prognosis after tumor resection, identifying DDX5 as an important player in poor prognosis HCC. Conclusion: The RNA helicase DDX5, and E3 ligase Mex3b, are important cellular targets for the design of novel, epigenetic therapies to combat HBV infection and poor prognosis HBV‐associated liver cancer. (Hepatology 2016;64:1033‐1048)