Purified apolipoprotein B gene regulatory factor-3 is DNA topoisomerase I

Regulation of human apolipoprotein B gene expression at multiple levels

Apolipoprotein B is a large, amphipathic protein that plays a central role in lipoprotein metabolism. Because its overproduction and deficiency leads to metabolic and pathologic disorders, much effort has been paid to investigate the mechanisms of how its homeostasis is achieved. Earlier and recent studies have showed that apoB gene locus might reside in different chromatin domains in the hepatic and intestinal cells, and two sets of very distinct regulatory elements operate to control its transcription. Posttranscriptional modification of apoB mRNA is performed by a multicomponent enzyme complex, several possible pathways regulate the editing efficiency. Understanding of the mechanism responsible for apoB mRNA editing will provide the basis for C-to-U editing in gene therapy. In addition to apoB mRNA abundance and stability, its translation can be also regulated at the steps of elongation. The translocation of apoB into the ER is an important and complicated process that is less understood. Successful transport and correct folding of apoB may lead to its final secretion, otherwise subject to intracellular degradation, which is accomplished by proteasomal and nonproteasomal pathways at multiple levels and may differ among cell types.

2B4 (CD244)-mediated activation of cytotoxicity and IFN-gamma release in human NK cells involves distinct pathways

2B4 (CD244), a member of the CD2 subset of the Ig superfamily receptors, is expressed on all human NK cells, a subpopulation of T cells, basophils and monocytes. 2B4 activates NK cell mediated cytotoxicity, induces secretion of IFN-gamma and matrix metalloproteinases, and NK cell invasiveness. Although there have been several molecules shown to interact with 2B4, the signaling mechanism of 2B4-mediated activation of NK cells is still unknown. In this study, we found cross-linking of 2B4 on YT cells, a human NK cell line, results in the increased DNA binding activity of activator protein-1 (AP-1), an important regulator of nuclear gene expression in leukocytes. We investigated the possible role of various signaling molecules that may be involved in the activation of lytic function of YT cells via 2B4. Treatment of YT cells with various specific inhibitors indicate that 2B4-stimulation of YT cells in spontaneous and Ab-dependent cytotoxicity is Ras/Raf dependent and involves multiple MAPK signaling pathways (ERK1/2 and p38). However, only inhibitors of transcription and p38 inhibited 2B4-mediated IFN-gamma release indicating distinct pathways are involved in cytotoxicity and cytokine release. In this study we also show that 2B4 constitutively associates with the linker for activation of T cells (LAT) and that 2B4 may mediate NK cell activation via a LAT-dependent signaling pathway. These results indicate that 2B4-mediated activation of NK cells involves complex interactions involving LAT, Ras, Raf, ERK and p38 and that cytolytic function and cytokine production may be regulated by distinct pathways.

A prominent role for activator protein-1 in the transcription of the human 2B4 (CD244) gene in NK cells

The cell surface glycoprotein 2B4 (CD244) of the Ig superfamily is involved in the regulation of NK and T lymphocyte functions. We have recently identified CD48 as the high affinity counterreceptor for 2B4 in both mice and humans. The cytoplasmic domain of 2B4 associates with src homology 2 domain-containing protein or signaling lymphocyte activation molecule-associated protein, whose mutation is the underlying genetic defect in the X-linked lymphoproliferative syndrome. In this study, we report the molecular cloning and characterization of the human 2B4 (h2B4) promoter. Through primer extension analysis, we found that the transcription of the h2B4 gene initiates at multiple start sites. We isolated h2B4 genomic clones and PCR amplified the 5' untranslated region containing the promoter elements. We have identified a functional AP-1 site that lies between (-106 to -100) through transient transfection analysis in YT cells, a human NK cell line. EMSAs with Abs specific for various protein factors of the AP-1 family revealed that multiple members of the Jun family are involved in the regulation of the h2B4 gene. Mutation of the AP-1 site not only abolishes protein/DNA interactions but also promoter activity. These results demonstrate a significant role for AP-1 in the transcriptional regulation of the h2B4 gene.