Adipose tissue reduction in mice lacking the translational inhibitor 4E-BP1.

All nuclear-encoded mRNAs contain a 5' cap structure (m7GpppN, where N is any nucleotide), which is recognized by the eukaryotic translation initiation factor 4E (eIF4E) subunit of the eIF4F complex. The eIF4E-binding proteins constitute a family of three polypeptides that reversibly repress cap-dependent translation by binding to eIF4E, thus preventing the formation of the eIF4F complex. We investigated the biological function of 4E-BP1 by disrupting its gene (Eif4ebp1) in the mouse. Eif4ebp1-/- mice manifest markedly smaller white fat pads than wild-type animals, and knockout males display an increase in metabolic rate. The males' white adipose tissue contains cells that exhibit the distinctive multilocular appearance of brown adipocytes, and expresses the uncoupling protein 1 (UCP1), a specific marker of brown fat. Consistent with these observations, translation of the peroxisome proliferator-activated receptor-gamma co-activator 1 (PGC1), a transcriptional co-activator implicated in mitochondrial biogenesis and adaptive thermogenesis, is increased in white adipose tissue of Eif4ebp1-/- mice. These findings demonstrate that 4E-BP1 is a novel regulator of adipogenesis and metabolism in mammals.

Translational repression by a novel partner of human poly(A) binding protein, Paip2.

The eukaryotic mRNA 3' poly(A) tail acts synergistically with the 5' cap structure to enhance translation. This effect is mediated by a bridging complex, composed of the poly(A) binding protein (PABP), eIF4G, and the cap binding protein, eIF4E. PABP-interacting protein 1 (Paip1) is another factor that interacts with PABP to coactivate translation. Here, we describe a novel human PABP-interacting protein (Paip2), which acts as a repressor of translation both in vitro and in vivo. Paip2 preferentially inhibits translation of a poly(A)-containing mRNA, but has no effect on the translation of hepatitis C virus mRNA, which is cap- and eIF4G-independent. Paip2 decreases the affinity of PABP for polyadenylate RNA, and disrupts the repeating structure of poly(A) ribonucleoprotein. Furthermore, Paip2 competes with Paip1 for PABP binding. Thus, Paip2 inhibits translation by interdicting PABP function.

Structure and genomic organization of the human AUF1 gene: alternative pre-mRNA splicing generates four protein isoforms.

The steady-state levels of many mRNAs are determined in part by their turnover rates. Turnover rates, in turn, are usually controlled by proteins that bind cis-acting sequence elements in mRNAs. One class of cis-acting instability determinants is composed of A + U-rich elements present in the 3'-UTRs of many labile mRNAs. Many A + U-rich elements are bound by the AUF1 family of RNA-binding proteins, which may target these mRNAs for rapid decay. cDNA cloning and immunoblot analyses suggest that the AUF1 family consists of at least four isoforms. Previous genomic cloning combined with FISH and Southern analyses of a panel of monochromosomal mouse/human or hamster/human somatic cell hybrids localized two AUF1 loci to human 4q21.1-q21.2 and Xq12 (B. Wagner et al., 1996, Genomics 34: 219-222). In the present study AUF1 gene organization was examined. The results suggest that the four known AUF1 isoforms are generated by alternative pre-mRNA splicing of a transcript encoded by the chromosome 4 locus. Functionally, this creates isoforms with different RNA-binding affinities and specificities. Thus, alternative pre-mRNA splicing may serve to create functional versatility within the AUF1 family of proteins.

Structural determinants in AUF1 required for high affinity binding to A + U-rich elements.

AUF1 is an RNA-binding protein that contains two nonidentical RNA recognition motifs (RRMs). AUF1 binds to A + U-rich elements (AREs) with high affinity. The binding of AUF1 to AREs is believed to serve as a signal to an mRNA-processing pathway that degrades mRNAs encoding many cytokines, oncoproteins, and G protein-coupled receptors. Because the ARE binding activity of AUF1 appears central to the regulation of many important genes, we analyzed the domains of the protein that are important for this activity. Examination of the RNA binding affinity of various AUF1 mutants suggests that both RRMs may be required for binding to the human c-fos ARE. However, the two RRMs together are not sufficient. Highest affinity binding of AUF1 to an ARE requires an alanine-rich region of the N terminus and a short glutamine-rich region in the C terminus. In addition, the N terminus is required for dimerization of AUF1. However, AUF1 binds an ARE as a hexameric protein. Thus, protein-protein interactions are important for high affinity ARE binding activity of AUF1.

Identification of AUF1 (heterogeneous nuclear ribonucleoprotein D) as a component of the alpha-globin mRNA stability complex.

mRNA turnover is an important regulatory component of gene expression and is significantly influenced by ribonucleoprotein (RNP) complexes which form on the mRNA. Studies of human alpha-globin mRNA stability have identified a specific RNP complex (alpha-complex) which forms on the 3' untranslated region (3'UTR) of the mRNA and appears to regulate the erythrocyte-specific accumulation of alpha-globin mRNA. One of the protein activities in this multiprotein complex is a poly(C)-binding activity which consists of two proteins, alphaCP1 and alphaCP2. Neither of these proteins, individually or as a pair, can bind the alpha-globin 3'UTR unless they are complexed with the remaining non-poly(C) binding proteins of the alpha-complex. With the yeast two-hybrid screen, a second alpha-complex protein was identified. This protein is a member of the previously identified A+U-rich (ARE) binding/degradation factor (AUF1) family of proteins, which are also known as the heterogeneous nuclear RNP (hnRNP) D proteins. We refer to these proteins as AUF1/hnRNP-D. Thus, a protein implicated in ARE-mediated mRNA decay is also an integral component of the mRNA stabilizing alpha-complex. The interaction of AUF1/hnRNP-D is more efficient with alphaCP1 relative to alphaCP2 both in vitro and in vivo, suggesting that the alpha-complex might be dynamic rather than a fixed complex. AUF1/hnRNP-D could, therefore, be a general mRNA turnover factor involved in both stabilization and decay of mRNA.

Adhesion-dependent regulation of an A+U-rich element-binding activity associated with AUF1.

Monocyte adherence results in the rapid transcriptional activation and mRNA stabilization of numerous mediators of inflammation and tissue repair. While the enhancer and promoter elements associated with transcriptional activation have been studied, mechanisms linking adhesion, mRNA stabilization, and translation are unknown. GROalpha and interleukin-1beta (IL-1beta) mRNAs are highly labile in nonadhered monocytes but stabilize rapidly after adherence. GROalpha and IL-1beta transcripts both contain A+U-rich elements (AREs) in the 3' untranslated region (UTR) which have been directly associated with rapid mRNA turnover. To determine if the GROalpha ARE region was recognized by factors associated with mRNA degradation, we carried out mobility gel shift analyses using a series of RNA probes encompassing the entire GROalpha transcript. Stable complexes were formed only with the proximal 3' UTR which contained the ARE region. The two slower-moving complexes were rapidly depleted following monocyte adherence but not direct integrin engagement. Deadherence reactivated the two largest ARE-binding complexes and destabilized IL-1beta transcripts. Antibody supershift studies demonstrated that both of these ARE RNA-binding complexes contained AUF1. The formation of these complexes and the accelerated mRNA turnover are phosphorylation-dependent events, as both are induced in adherent monocytes by the tyrosine kinase inhibitor genistein and the p38 MAP kinase inhibitor of IL-1beta translation, SK&F 86002. These results demonstrate that cell adhesion and deadhesion rapidly and reversibly modify both cytokine mRNA stability and the RNA-binding complexes associated with AUF1.

Structural determination in AUF1 required for high affinity binding to A + U-rich elements.

AUF1 is a RNA-binding protein that contains two non-identical RNA recognition motifs (RRMs). AUF1 binds to A + U-rich elements (AREs) with high affinity. The binding of AUF1 to AREs is believed to serve as a signal to a mRNA processing pathway which degrades mRNAs encoding many cytokines, oncoproteins and G protein-coupled receptors. Because the ARE-binding activity of AUF1 appears central to the regulation of many important genes, we analyzed the domains of the protein that are important for this activity. Examination of the binding affinity of various mutants indicates that both RRMs may be required for binding. However, they are not sufficient. Highest affinity binding requires an alanine/glycine-rich region of the N-terminus and a short glutamine-rich region in the C-terminus. The N-terminus is required for dimerization of AUF1. However, AUF1 binds an ARE as a hexameric protein. Thus protein-protein interactions are important for high affinity ARE-binding activity of AUF1.

Increased granulocyte-macrophage colony-stimulating factor mRNA instability in cord versus adult mononuclear cells is translation-dependent and associated with increased levels of A + U-rich element binding factor.

The level of granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA is fourfold lower in phorbol myristate acetate (PMA) + phytohemagglutinin (PHA)-activated mononuclear cells (MNC) from newborns compared with adults. The GM-CSF transcription rate is similar in umbilical cord and adult MNC, but transcript half-life is threefold lower in cord activated MNC. Interaction of RNA binding proteins, such as the cloned adenosine + uridine-rich element, binding factor, AUF1, with eight AUUUA motifs in the human GM-CSF mRNA 3'-untranslated region (GM-3'-UTR) has been implicated in regulating transcript stability. Translational inhibition by cycloheximide (CHX) significantly increased GM-CSF mRNA accumulation and half-life by three-fold in activated cord MNC, but had a minimal effect in activated adult MNC as compared with PMA + PHA alone. Electrophoretic mobility-shift assays with a 32P-labeled, 305-nucleotide RNA comprising the GM-3'-UTR revealed two RNaseT1-resistant, bound complexes that were almost twice as abundant in cord than in adult MNC extracts. Mobility-shift competition assays and RNaseT1 mapping localized the binding site of both complexes to a 52-nucleotide region containing seven of eight AUUUA motifs. Inclusion of AUF1 antiserum produced a supershifted complex at 35-fold higher levels in cord than in adult MNC extracts. Extracts from the carcinoma cell line 5637, with extended GM-CSF mRNA half-life, also had very low levels of anti-AUF1 supershifted complex. Anti-AUF1 immunoblotting showed significantly higher levels of two AUF1 protein isoforms and lower levels of one in cord than in adult MNC or 5637 extracts. These results suggest that destabilization of GM-CSF mRNA in cord MNC is translation-dependent and that increased levels of specific AUF1 isoforms in cord MNC may target transcripts for increased degradation, which could account in part for dysregulation of neonatal phagocytic immunity.

AUF1 binding affinity to A+U-rich elements correlates with rapid mRNA degradation.

Rapid degradation of many labile mRNAs is regulated in part by an A + U-rich element (ARE) in their 3'-untranslated regions. Extensive mutational analyses of various AREs have identified important components of the ARE, such as the nonamer motif UUAUUUAUU, two copies of which serve as a potent mRNA destabilizer. To investigate the roles of trans-acting factors in ARE-directed mRNA degradation, we previously purified and molecularly cloned the RNA-binding protein AUF1 and demonstrated that both cellular and recombinant AUF1 bind specifically to AREs as shown by UV cross-linking assays in vitro. In the present work, we have examined the in vitro RNA-binding properties of AUF1 using gel mobility shift assays with purified recombinant His6-AUF1 fusion protein. We find that ARE binding affinities of AUF1 correlate with the potency of an ARE to direct degradation of a heterologous mRNA. These results support a role for AUF1 in ARE-directed mRNA decay that is based upon its affinity for different AREs.

Regulation of the mRNA-binding protein AUF1 by activation of the beta-adrenergic receptor signal transduction pathway.

In both cell culture based model systems and in the failing human heart, beta-adrenergic receptors ( beta-AR) undergo agonist-mediated down-regulation. This decrease correlates closely with down-regulation of its mRNA, an effect regulated in part by changes in mRNA stability. Regulation of mRNA stability has been associated with mRNA-binding proteins that recognize A + U-rich elements within the 3'-untranslated regions of many mRNAs encoding proto-oncogene and cytokine mRNAs. We demonstrate here that the mRNA-binding protein, AUF1, is present in both human heart and in hamster DDT1-MF2 smooth muscle cells and that its abundance is regulated by beta-AR agonist stimulation. In human heart, AUF1 mRNA and protein was significantly increased in individuals with myocardial failure, a condition associated with increases in the beta-adrenergic receptor agonist norepinephrine. In the same hearts, there was a significant decrease (approximately 50%) in the abundance of beta1-AR mRNA and protein. In DDT1-MF2 cells, where agonist-mediated destabilization of beta2-AR mRNA was first described, exposure to beta-AR agonist resulted in a significant increase in AUF1 mRNA and protein (approximately 100%). Conversely, agonist exposure significantly decreased (approximately 40%) beta2-adrenergic receptor mRNA abundance. Last, we demonstrate that AUF1 can be immunoprecipitated from polysome-derived proteins following UV cross-linking to the 3'-untranslated region of the human beta1-AR mRNA and that purified, recombinant p37AUF1 protein also binds to beta1-AR 3'-untranslated region mRNA.

Purification, characterization, and cDNA cloning of an AU-rich element RNA-binding protein, AUF1.

The degradation of some proto-oncogene and lymphokine mRNAs is controlled in part by an AU-rich element (ARE) in the 3' untranslated region. It was shown previously (G. Brewer, Mol. Cell. Biol. 11:2460-2466, 1991) that two polypeptides (37 and 40 kDa) copurified with fractions of a 130,000 x g postribosomal supernatant (S130) from K562 cells that selectively accelerated degradation of c-myc mRNA in a cell-free decay system. These polypeptides bound specifically to the c-myc and granulocyte-macrophage colony-stimulating factor 3' UTRs, suggesting they are in part responsible for selective mRNA degradation. In the present work, we have purified the RNA-binding component of this mRNA degradation activity, which we refer to as AUF1. Using antisera specific for these polypeptides, we demonstrate that the 37- and 40-kDa polypeptides are immunologically cross-reactive and that both polypeptides are phosphorylated and can be found in a complex(s) with other polypeptides. Immunologically related polypeptides are found in both the nucleus and the cytoplasm. The antibodies were also used to clone a cDNA for the 37-kDa polypeptide. This cDNA contains an open reading frame predicted to produce a protein with several features, including two RNA recognition motifs and domains that potentially mediate protein-protein interactions. These results provide further support for a role of this protein in mediating ARE-directed mRNA degradation.