High-fat diet-induced GAIT element-mediated translational silencing of mRNAs encoding inflammatory proteins in macrophage protects against atherosclerosis.

Previously, we identified a mechanism of inflammation control directed by ribosomal protein L13a and "GAIT" (Gamma Activated Inhibitor of Translation) elements in target mRNAs and showed that its elimination in myeloid cell-specific L13a knockout mice (L13a KO) increased atherosclerosis susceptibility and severity. Here, we investigated the mechanistic basis of this endogenous defense against atherosclerosis. We compared molecular and cellular aspects of atherosclerosis in high-fat diet (HFD)-fed L13a KO and intact (control) mice. HFD treatment of control mice induced release of L13a from 60S ribosome, formation of RNA-binding complex, and subsequent GAIT element-mediated translational silencing. Atherosclerotic plaques from HFD-treated KO mice showed increased infiltration of M1 type inflammatory macrophages. Macrophages from KO mice showed increased phagocytic activity and elevated expression of LDL receptor and pro-inflammatory mediators. NanoString analysis of the plaques from KO mice showed upregulation of a number of mRNAs encoding inflammatory proteins. Bioinformatics analysis suggests the presence of the potential GAIT elements in the 3'UTRs of several of these mRNAs. Macrophage induces L13a/GAIT-dependent translational silencing of inflammatory genes in response to HFD as an endogenous defense against atherosclerosis in ApoE-/- model.

L13a-dependent translational control in macrophages limits the pathogenesis of colitis.

Sustained inflammation from infiltrated immune cells plays a pivotal role in the pathogenesis of ulcerative colitis (UC). Previously, we established the role of ribosomal protein L13a in the regulation of an inflammation-responsive post-transcriptional operon in myeloid cells. However, the role of this protein as a molecular cue to control the severity of colitis is not known. Here, we examined whether L13a-dependent translational control in macrophages could serve as an endogenous defense against colitis. The administration of dextran sodium sulfate induced experimental colitis in myeloid-specific L13a-knockout (KO) and control mice. Pathological scoring and injury to the colon mucosa evaluated the severity of colitis. The steady-state levels of several pro-inflammatory cytokines and chemokines were determined through ELISA and polyribosome profile analysis. Rapid weight loss, severe rectal bleeding, shortening of the colon, and significantly reduced survival rate were observed in the KO mice. Histopathological analysis of the colons of KO mice showed a severe disruption of epithelial crypts with immune cell infiltrates. Elevated levels of several inflammatory cytokines and chemokines and abrogation of their naturally imposed translational silencing were observed in the colons of the KO mice. Higher serum levels of several pro-inflammatory cytokines and the release of gut bacteria and endotoxins into the blood streams of KO mice were detected, suggesting the amplification of the inflammatory response to septicemia. Taken together, these results reveal an essential role for L13a in the endogenous protection against UC and demonstrate the potential for new therapeutic opportunities through the deliberate promotion of this mechanism.

Extraribosomal l13a is a specific innate immune factor for antiviral defense.

UNLABELLED: We report a novel extraribosomal innate immune function of mammalian ribosomal protein L13a, whereby it acts as an antiviral agent. We found that L13a is released from the 60S ribosomal subunit in response to infection by respiratory syncytial virus (RSV), an RNA virus of the Pneumovirus genus and a serious lung pathogen. Unexpectedly, the growth of RSV was highly enhanced in L13a-knocked-down cells of various lineages as well as in L13a knockout macrophages from mice. In all L13a-deficient cells tested, translation of RSV matrix (M) protein was specifically stimulated, as judged by a greater abundance of M protein and greater association of the M mRNA with polyribosomes, while general translation was unaffected. In silico RNA folding analysis and translational reporter assays revealed a putative hairpin in the 3'untranslated region (UTR) of M mRNA with significant structural similarity to the cellular GAIT (gamma-activated inhibitor of translation) RNA hairpin, previously shown to be responsible for assembling a large, L13a-containing ribonucleoprotein complex that promoted translational silencing in gamma interferon (IFN-γ)-activated myeloid cells. However, RNA-protein interaction studies revealed that this complex, which we named VAIT (respiratory syncytial virus-activated inhibitor of translation) is functionally different from the GAIT complex. VAIT is the first report of an extraribosomal L13a-mediated, IFN-γ-independent innate antiviral complex triggered in response to virus infection. We provide a model in which the VAIT complex strongly hinders RSV replication by inhibiting the translation of the rate-limiting viral M protein, which is a new paradigm in antiviral defense. IMPORTANCE: The innate immune mechanisms of host cells are diverse in nature and act as a broad-spectrum cellular defense against viruses. Here, we report a novel innate immune mechanism functioning against respiratory syncytial virus (RSV), in which the cellular ribosomal protein L13a is released from the large ribosomal subunit soon after infection and inhibits the translation of a specific viral mRNA, namely, that of the matrix protein M. Regarding its mechanism, we show that the recognition of a specific secondary structure in the 3' untranslated region of the M mRNA leads to translational arrest of the mRNA. We also show that the level of M protein in the infected cell is rate limiting for viral morphogenesis, providing a rationale for L13a to target the M mRNA for suppression of RSV growth. Translational silencing of a viral mRNA by a deployed ribosomal protein is a new paradigm in innate immunity.

Ribosomal protein L13a deficiency in macrophages promotes atherosclerosis by limiting translation control-dependent retardation of inflammation.

OBJECTIVE: Unresolved inflammatory response of macrophages plays a pivotal role in the pathogenesis of atherosclerosis. Previously we showed that ribosomal protein L13a-dependent translational silencing suppresses the synthesis of a cohort of inflammatory proteins in monocytes and macrophages. We also found that genetic abrogation of L13a expression in macrophages significantly compromised the resolution of inflammation in a mouse model of lipopolysaccharide-induced endotoxemia. However, its function in the pathogenesis of atherosclerosis is not known. Here, we examine whether L13a in macrophage has a protective role against high-fat diet-induced atherosclerosis. APPROACH AND RESULTS: We bred the macrophage-specific L13a knockout mice L13a Flox(+/+) Cre(+/+) onto apolipoprotein E-deficient background and generated the experimental double knockout mice L13a Flox(+/+) Cre(+/+) apolipoprotein E deficient (apoE(-/-)). L13a Flox(+/+) Cre(-/-) mice on apolipoprotein E-deficient background were used as controls. Control and knockout mice were subjected to high-fat diet for 10 weeks. Evaluation of aortic sinus sections and entire aorta by en face showed significantly higher atherosclerosis in the knockout mice. Severity of atherosclerosis in knockout mice was accompanied by thinning of the smooth muscle cell layer in the media, larger macrophage area in the intimal plaque region and higher plasma levels of inflammatory cytokines. In addition, macrophages isolated from knockout mice had higher polyribosomal abundance of several target mRNAs, thus showing defect in translation control. CONCLUSIONS: Our data demonstrate that loss of L13a in macrophages increases susceptibility to atherosclerosis in apolipoprotein E-deficient mice, revealing an important role of L13a-dependent translational control as an endogenous protection mechanism against atherosclerosis.

Insights into the mechanism of ribosomal incorporation of mammalian L13a protein during ribosome biogenesis.

In contrast to prokaryotes, the precise mechanism of incorporation of ribosomal proteins into ribosomes in eukaryotes is not well understood. For the majority of eukaryotic ribosomal proteins, residues critical for rRNA binding, a key step in the hierarchical assembly of ribosomes, have not been well defined. In this study, we used the mammalian ribosomal protein L13a as a model to investigate the mechanism(s) underlying eukaryotic ribosomal protein incorporation into ribosomes. This work identified the arginine residue at position 68 of L13a as being essential for L13a binding to rRNA and incorporation into ribosomes. We also demonstrated that incorporation of L13a takes place during maturation of the 90S preribosome in the nucleolus, but that translocation of L13a into the nucleolus is not sufficient for its incorporation into ribosomes. Incorporation of L13a into the 90S preribosome was required for rRNA methylation within the 90S complex. However, mutations abolishing ribosomal incorporation of L13a did not affect its ability to be phosphorylated or its extraribosomal function in GAIT element-mediated translational silencing. These results provide new insights into the mechanism of ribosomal incorporation of L13a and will be useful in guiding future studies aimed at fully deciphering mammalian ribosome biogenesis.

An extraribosomal function of ribosomal protein L13a in macrophages resolves inflammation.

Inflammation is an obligatory attempt of the immune system to protect the host from infections. However, unregulated synthesis of proinflammatory products can have detrimental effects. Although mechanisms that lead to inflammation are well appreciated, those that restrain it are not adequately understood. Creating macrophage-specific L13a-knockout mice, we report that depletion of ribosomal protein L13a abrogates the endogenous translation control of several chemokines in macrophages. Upon LPS-induced endotoxemia, these animals displayed symptoms of severe inflammation caused by widespread infiltration of macrophages in major organs causing tissue injury and reduced survival rates. Macrophages from these knockout animals show unregulated expression of several chemokines (e.g., CXCL13, CCL22, CCL8, and CCR3). These macrophages failed to show L13a-dependent RNA binding complex formation on target mRNAs. In addition, increased polyribosomal abundance of these mRNAs shows a defect in translation control in the macrophages. Thus, to our knowledge, our studies provide the first evidence of an essential extraribosomal function of ribosomal protein L13a in resolving physiological inflammation in a mammalian host.

Requirement of rRNA methylation for 80S ribosome assembly on a cohort of cellular internal ribosome entry sites.

Protein syntheses mediated by cellular and viral internal ribosome entry sites (IRESs) are believed to have many features in common. Distinct mechanisms for ribosome recruitment and preinitiation complex assembly between the two processes have not been identified thus far. Here we show that the methylation status of rRNA differentially influenced the mechanism of 80S complex formation on IRES elements from the cellular sodium-coupled neutral amino acid transporter 2 (SNAT2) versus the hepatitis C virus mRNA. Translation initiation involves the assembly of the 48S preinitiation complex, followed by joining of the 60S ribosomal subunit and formation of the 80S complex. Abrogation of rRNA methylation did not affect the 48S complex but resulted in impairment of 80S complex assembly on the cellular, but not the viral, IRESs tested. Impairment of 80S complex assembly on the amino acid transporter SNAT2 IRES was rescued by purified 60S subunits containing fully methylated rRNA. We found that rRNA methylation did not affect the activity of any of the viral IRESs tested but affected the activity of numerous cellular IRESs. This work reveals a novel mechanism operating on a cohort of cellular IRESs that involves rRNA methylation for proper 80S complex assembly and efficient translation initiation.

Interleukin-1 receptor-associated kinase 2 is critical for lipopolysaccharide-mediated post-transcriptional control.

IRAK2, a member of the interleukin-1 receptor-associated kinase (IRAK) family, has been implicated in Toll-like receptor (TLR)-mediated signaling. We generated IRAK2-deficient mice to examine its function in detail. These mice are resistant to lipopolysaccharide-induced septic shock, because of impaired TLR4-mediated induction of pro-inflammatory cytokines and chemokines. Although IRAK2 deficiency did not affect TLR4-mediated NFkappaB activation, a reduction of lipopolysaccharide (LPS)-mediated mRNA stabilization contributed to the reduced cytokine and chemokine production observed in bone marrow-derived macrophages from IRAK2-deficient mice. Furthermore, the ratios of LPS-induced cytokine and chemokine mRNAs in translation-active (polysomal) versus translation-inactive (free ribosomes) pools were reduced in IRAK2-deficient macrophages compared with wild type macrophages. Importantly, LPS-induced phosphorylation of MKK3/6, MNK1, and eIF4E was significantly reduced in IRAK2-deficient macrophages compared with wild type macrophages. Moreover, LPS stimulation induced an interaction of IRAK2 with TRAF6, MKK3/6, and MK2, implicating a critical role for mitogen-activated protein kinase signaling in LPS-induced IRAK2-mediated post-transcriptional control. These results reveal that IRAK2 is required for LPS-mediated post-transcriptional control of cytokine and chemokine expression, which plays an essential role in TLR4-induced septic shock.

Human ribosomal protein L13a is dispensable for canonical ribosome function but indispensable for efficient rRNA methylation.

Previously, we demonstrated that treatment of monocytic cells with IFN-gamma causes release of ribosomal protein L13a from the 60S ribosome and subsequent translational silencing of Ceruloplasmin (Cp) mRNA. Here, evidence using cultured cells demonstrates that Cp mRNA silencing is dependent on L13a and that L13a-deficient ribosomes are competent for global translational activity. Human monocytic U937 cells were stably transfected with two different shRNA sequences for L13a and clonally selected for more than 98% abrogation of total L13a expression. Metabolic labeling of these cells showed rescue of Cp translation from the IFN-gamma mediated translational silencing activity. Depletion of L13a caused significant reduction of methylation of ribosomal RNA and of cap-independent translation mediated by Internal Ribosome Entry Site (IRES) elements derived from p27, p53, and SNAT2 mRNAs. However, no significant differences in the ribosomal RNA processing, polysome formation, global translational activity, translational fidelity, and cell proliferation were observed between L13a-deficient and wild-type control cells. These results support the notion that ribosome can serve as a depot for releasable translation-regulatory factors unrelated to its basal polypeptide synthetic function. Unlike mammalian cells, the L13a homolog in yeast is indispensable for growth. Thus, L13a may have evolved from an essential ribosomal protein in lower eukaryotes to having a role as a dispensable extra-ribosomal function in higher eukaryotes.

Nonstructural proteins of respiratory syncytial virus suppress premature apoptosis by an NF-kappaB-dependent, interferon-independent mechanism and facilitate virus growth.

The two nonstructural (NS) proteins NS1 and NS2 of respiratory syncytial virus (RSV) are abundantly expressed in the infected cell but are not packaged in mature progeny virions. We found that both proteins were expressed early in infection, whereas the infected cells underwent apoptosis much later. Coincident with NS protein expression, a number of cellular antiapoptotic factors were expressed or activated at early stages, which included NF-kappaB and phosphorylated forms of protein kinases AKT, phosphoinositide-dependent protein kinase, and glycogen synthase kinase. Using specific short interfering RNAs (siRNAs), we achieved significant knockdown of one or both NS proteins in the infected cell, which resulted in abrogation of the antiapoptotic functions and led to early apoptosis. NS-dependent suppression of apoptosis was observed in Vero cells that are naturally devoid of type I interferons (IFN). The siRNA-based results were confirmed by the use of NS-deleted RSV mutants. Early activation of epidermal growth factor receptor (EGFR) in the RSV-infected cell did not require NS proteins. Premature apoptosis triggered by the loss of NS or by apoptosis-promoting drugs caused a severe reduction of RSV growth. Finally, recombinantly expressed NS1 and NS2, individually and together, reduced apoptosis by tumor necrosis factor alpha, suggesting an intrinsic antiapoptotic property of both. We conclude that the early-expressed nonstructural proteins of RSV boost viral replication by delaying the apoptosis of the infected cell via a novel IFN- and EGFR-independent pathway.

Purification and characterization of a 43 kD hepatoprotective protein from the herb Cajanus indicus L.

Cajanus indicus L, a herb, is popularly known for its hepatoprotective activity. Aqueous extract of the leaves of this plant contains hepatoprotective and hepatostimulatory molecule(s). Present study was aimed to isolate, purify and characterize the active principle(s) responsible for that activity. A hepatoprotective protein molecule has been purified to homogeneity (approximately 300 fold). Homogeneous preparation of the protein was achieved by homogenization, (NH(4))(2)SO(4) precipitation, ion-exchange chromatography, gel filtration and high performance liquid chromatography. The protein purified is composed of a single polypeptide chain having an apparent molecular mass of 43 kD as determined by SDS-PAGE and gel filtration through sephadex G-75 column. The isoelectric point of the protein determined was 4.8. Loss of biological activity after heat and protease treatment confirmed that the active molecule is a protein. Peptide fragments of the protein generated by trypsin cleavage were subjected to MALDI-TOF as well as LC-MS analyses and among the various fragments, four were very prominent and used for the determination of the amino acid sequence of the hepatoprotective protein. While one of the peptide fragment revealed strong sequence homology with plastocyanin, another fragment showed some similarity with a tomato protein present in the NCBI non-redundant database. The third peptide, on the other hand, is unique as it did not show any sequence homology with any known protein in the database. The protein showed maximum hepatoprotective activity when administered at a dose of 2 mg/kg body weight for five days after CCl(4 )administration. Histopathological studies also supported the hepatoprotective nature of the protein. Along with its curative property, the protein also possesses preventive role against a number of toxin induced hepatic damages.

Translational control of ceruloplasmin gene expression: Beyond the IRE

Translational control is a common regulatory mechanism for the expression of iron-related proteins. For example, three enzymes involved in erythrocyte development are regulated by three different control mechanisms: globin synthesis is modulated by heme-regulated translational inhibitor; erythroid 5-aminolevulinate synthase translation is inhibited by binding of the iron regulatory protein to the iron response element in the 5'-untranslated region (UTR); and 15-lipoxygenase is regulated by specific proteins binding to the 3'-UTR. Ceruloplasmin (Cp) is a multi-functional, copper protein made primarily by the liver and by activated macrophages. Cp has important roles in iron homeostasis and in inflammation. Its role in iron metabolism was originally proposed because of its ferroxidase activity and because of its ability to stimulate iron loading into apo-transferrin and iron efflux from liver. We have shown that Cp mRNA is induced by interferon (IFN)-ã in U937 monocytic cells, but synthesis of Cp protein is halted by translational silencing. The silencing mechanism requires binding of a cytosolic inhibitor complex, IFN-Gamma-Activated Inhibitor of Translation (GAIT), to a specific GAIT element in the Cp 3'-UTR. Here, we describe our studies that define and characterize the GAIT element and elucidate the specific trans-acting proteins that bind the GAIT element. Our experiments describe a new mechanism of translational control of an iron-related protein and may shed light on the role that macrophage-derived Cp plays at the intersection of iron homeostasis and inflammation.

Regulation of macrophage ceruloplasmin gene expression: one paradigm of 3'-UTR-mediated translational control.

Ceruloplasmin (Cp) is a copper protein with important functions in iron homeostasis and in inflammation. Cp mRNA expression is induced by interferon (IFN)-g in U937 monocytic cells, but synthesis of Cp protein is halted after about 12 h by transcript-specific translational silencing. The silencing mechanism requires binding of a 4-component cytosolic inhibitor complex, IFN-gamma-activated inhibitor of translation (GAIT), to a defined structural element (GAIT element) in the Cp 3'-UTR. Translational silencing of Cp mRNA requires the essential proteins of mRNA circularization, suggesting that the translational inhibition requires end-to-end mRNA closure. These studies describe a new mechanism of translational control, and may shed light on the role that macrophage-derived Cp plays at the intersection of iron homeostasis and inflammation.