Regulation of mitochondrial mRNA stability by RNase L is translation-dependent and controls IFNalpha-induced apoptosis

Interferons (IFNs) inhibit the growth of many different cell types by altering the expression of specific genes. IFNs activities are partly mediated by the 2'-5' oligoadenylates-RNase L RNA decay pathway. RNase L is an endoribonuclease requiring activation by 2'-5' oligoadenylates to cleave single-stranded RNA. Here, we present evidence that degradation of mitochondrial mRNA by RNase L leads to cytochrome c release and caspase 3 activation during IFNalpha-induced apoptosis. We identify and characterize the mitochondrial translation initiation factor (IF2mt) as a new partner of RNase L. Moreover, we show that specific inhibition of mitochondrial translation with chloramphenicol inhibits the IFNalpha-induced degradation of mitochondrial mRNA by RNase L. Finally, we demonstrate that overexpression of IF2mt in human H9 cells stabilizes mitochondrial mRNA, inhibits apoptosis induced by IFNalpha and partially reverses IFNalpha-cell growth inhibition. On the basis of our results, we propose a model describing how RNase L regulates mitochondrial mRNA stability through its interaction with IF2mt.

The 2-5A/RNase L/RNase L inhibitor (RLI) [correction of (RNI)] pathway regulates mitochondrial mRNAs stability in interferon alpha-treated H9 cells

Interferon alpha (IFNalpha) belongs to a cytokine family that exhibits antiviral properties, immuno-modulating effects, and antiproliferative activity on normal and neoplasic cells in vitro and in vivo. IFNalpha exerts antitumor action by inducing direct cytotoxicity against tumor cells. This toxicity is at least partly due to induction of apoptosis. Although the molecular basis of the inhibition of cell growth by IFNalpha is only partially understood, there is a direct correlation between the sensitivity of cells to the antiproliferative action of IFNalpha and the down-regulation of their mitochondrial mRNAs. Here, we studied the role of the 2-5A/RNase L system and its inhibitor RLI in this regulation of the mitochondrial mRNAs by IFNalpha. We found that a fraction of cellular RNase L and RLI is localized in the mitochondria. Thus, we down-regulated RNase L activity in human H9 cells by stably transfecting (i) RNase L antisense cDNA or (ii) RLI sense cDNA constructions. In contrast to control cells, no post-transcriptional down-regulation of mitochondrial mRNAs and no cell growth inhibition were observed after IFNalpha treatment in these transfectants. These results demonstrate that IFNalpha exerts its antiproliferative effect on H9 cells at least in part via the degradation of mitochondrial mRNAs by RNase L.

The 2'-5' oligoadenylate/RNase L/RNase L inhibitor pathway regulates both MyoD mRNA stability and muscle cell differentiation

The 2'-5' oligoadenylate (2-5A)/RNase L pathway is one of the enzymatic pathways induced by interferon. RNase L is a latent endoribonuclease which is activated by 2-5A and inhibited by a specific protein known as RLI (RNase L inhibitor). This system has an important role in regulating viral infection. Additionally, variations in RNase L activity have been observed during cell growth and differentiation but the significance of the 2-5A/RNase L/RLI pathway in these latter processes is not known. To determine the roles of RNase L and RLI in muscle differentiation, C2 mouse myoblasts were transfected with sense and antisense RLI cDNA constructs. Importantly, the overexpression of RLI in C2 cells was associated with diminished RNase L activity, an increased level of MyoD mRNA, and accelerated kinetics of muscle differentiation. Inversely, transfection of the RLI antisense construct was associated with increased RNase L activity, a diminished level of MyoD mRNA, and delayed differentiation. In agreement with these data, MyoD mRNA levels were also decreased in C2 cells transfected with an inducible RNase L construct. The effect of RNase L activity on MyoD mRNA levels was relatively specific because expression of several other mRNAs was not altered in C2 transfectants. Therefore, RNase L is directly involved in myoblast differentiation, probably through its role in regulating MyoD stability. This is the first identification of a potential mRNA target for RNase L.

Characterization of RNABP, an RNA binding protein that associates with RNase L

The 2',5'-oligoadenylate (2-5A)/RNase L pathway is one of several enzymatic pathways induced by interferons (IFN). RNase L is a latent endoribonuclease that is activated on its binding by 2-5A and inhibited by the ribonuclease L inhibitor (RLI). We have shown previously by coimmunoprecipitation that RNase L may be associated with a 90-kDa RNA binding protein (RNABP), identified with a monoclonal antibody (mAb) raised against an RNase L complex purified under native conditions on 2-5A-sepharose. Here we confirm, by gel-filtration and pull-down analysis, the association of RNase L and RNABP, and we demonstrate that this association is significantly increased in the presence of 2-5A. Moreover, we found that RNABP protein levels decrease during terminal differentiation in various cell lines but do not vary during vesicular stomatitis virus (VSV) or encephalomyocarditis virus (EMCV) infection or following IFN-alpha/beta treatment. In this latter case, although total cellular RNABP levels do not vary, the amount of RNABP found in the cytoplasm increases in comparison to that found in the nucleus, indicating a cytoplasmic localization of RNABP after IFN-alpha/beta treatment. Finally, we demonstrate the interaction between RNase L and RNABP in intact cells. Microinjection of an mAb against RNABP into HeLa cells inhibits RNase L antiviral activity and partially inhibits the IFN-alpha/beta-induced antiviral activity.

A 37 kDa 2-5A binding protein as a potential biochemical marker for chronic fatigue syndrome

PURPOSE

Recent studies have revealed abnormalities in the ribonuclease L pathway in peripheral blood mononuclear cells of patients with the chronic fatigue syndrome. We conducted a blinded study to detect possible differences in the distribution of 2-5A binding proteins in the cells of patients with chronic fatigue syndrome and controls.

PATIENTS AND METHODS

We studied 57 patients with chronic fatigue syndrome and 53 control subjects (28 healthy subjects and 25 patients with depression or fibromyalgia). A radioactive probe was used to label 2-5A binding proteins in unfractionated peripheral blood mononuclear cell extracts and to compare their distribution in the three groups.

RESULTS

A 37 kDa 2-5A binding polypeptide was found in 50 (88%) of the 57 patients with chronic fatigue syndrome compared with 15 (28%) of the 53 controls (P < 0.01). When present, the amount of 37 kDa protein was very low in the control groups. When expressed as the ratio of the 37 kDa protein to the 80 kDa protein, 41 (72%) of the 57 patients with chronic fatigue syndrome had a ratio > 0.05, compared with 3 (11%) of the 28 healthy subjects and none of the patients with fibromyalgia or depression.

CONCLUSION

The presence of a 37 kDa 2-5A binding protein in extracts of peripheral blood mononuclear cells may distinguish patients with chronic fatigue syndrome from healthy subjects and those suffering from other diseases.

RNase L inhibitor is induced during human immunodeficiency virus type 1 infection and down regulates the 2-5A/RNase L pathway in human T cells

The interferon-regulated 2-5A/RNase L pathway plays a major role in the antiviral and antiproliferative activities of these cytokines. Several viruses, however, have evolved strategies to escape the antiviral activity of the 2-5A/RNase L pathway. In this context, we have cloned a cDNA coding for the RNase L inhibitor (RLI), a protein that specifically inhibits RNase L and whose regulated expression in picornavirus-infected cells down regulates the activity of the 2-5A/RNase L pathway. We show here that RLI increases during the course of human immunodeficiency virus type 1 (HIV-1) infection, which may be related to the downregulation of RNase L activity that has been described to occur in HIV-infected cells. In order to establish a possible causal relationship between these observations, we have stably transfected H9 cells with RLI sense or antisense cDNA-expressing vectors. The overexpression of RLI causes a decrease in RNase L activity and a twofold enhancement of HIV production. This increase in HIV replication correlates with an increase in HIV RNA and proteins. In contrast, reduction of RLI levels in RLI antisense cDNA-expressing clones reverses the inhibition of RNase L activity associated with HIV multiplication and leads to a threefold decrease in the viral load. This anti-HIV activity correlated with a decrease in HIV RNA and proteins. These findings demonstrate that the level of RLI, via its modulation of RNase L activity, can severely impair HIV replication and suggest the involvement of RLI in the inhibition of the 2-5A/RNase L system observed during HIV infection.

The RNase L inhibitor (RLI) is induced by double-stranded RNA

The (2-5A)-RNase L pathway is an important component of interferon (IFN) action. Its central role in the antiviral effect of IFN against Picornaviridae has been clearly demonstrated. We have characterized and cloned a new component of this pathway, the RNase L inhibitor (RLI). RLI is a cellular protein whose mRNA is not regulated by IFN but is induced by viruses, such as encephalomyocarditis virus (EMCV). RLI inhibits RNase L during the time course of EMCV infection, and overexpression of RLI in HeLa cells partially reverses the antiviral action of IFN against EMCV. The replicative complexes of several viruses consist of double-stranded RNA structures. These dsRNAs could activate gene transcription as demonstrated for IFNs and could be responsible for RLI induction. We describe the increased expression of RLI mRNA and RLI protein induced by synthetic dsRNAs, such as poly(I):poly(C). This induction gives rise to an inhibition of the 2-5A-binding activity of RNase L. The inhibition of RNase L activity is transcient, probably due to the rapid turnover of RLI protein.

RNase L inhibitor (RLI) antisense constructions block partially the down regulation of the 2-5A/RNase L pathway in encephalomyocarditis-virus-(EMCV)-infected cells

The interferon-(IFN)-inducible 2',5'-oligoadenylate (2-5A)/endoribonuclease L (RNase L) pathway plays a major role in the antiviral and antiproliferative effects of IFN. The 2-5A/RNase L pathway appears to be regulated by the cell-growth status or viral infection. Viruses, and picornaviruses in particular, have evolved strategies to escape the 2-5A/RNase L-pathway-associated antiviral activity. We have recently cloned a cDNA coding for RLI, a RNase-L-specific protein inhibitor. Its regulated expression by viral infection could provide a new strategy to modulate the 2-5A/RNase L pathway. Since RNase L had been shown to be down regulated upon encephalomyocarditis (EMCV) infection, we stably transfected HeLa cells with a RLI antisense cDNA expressing vector. Four independent clones named VAS1, VAS2, VAS3 and VAS4 and one clone transfected with the empty vector (VV) as control, were analyzed. The level of RLI was decreased by 20% for VAS1, 25% for VAS2, 75% for VAS3 and 50% for VAS4. The inactivation of RNase L observed during EMCV infection was decreased in these clones as compared to control HeLa cells. Here again the results vary between the four clones. The maximum inhibition of RNase L (90%) was observed in control cells and in VAS1 while 48% inhibition was observed in VAS4 and 25% in VAS3. The reversal in RNase L inhibition thus reflects closely the resulting RLI level, in keeping with a major role of RLI in EMCV-induced down regulation of 2-5A-binding activity of RNase L. Moreover, cells expressing a low level of RLI (VAS3 and VAS 4) are partially resistant to EMCV infection.

cDNA cloning and expression analysis of the murine ribonuclease L inhibitor

The 2-5A/RNase L system is one of the pathways induced by interferon (IFN). It plays a major role in the antiviral and antiproliferative activities of IFNs. Recently, we have shown that the activity of the RNase L could be inhibited by a proteic inhibitor, the RNase L Inhibitor (RLI). Human RLI (Hu-RLI) was cloned and characterized. We describe here the isolation and characterization of the cDNA encoding the murine RLI (Mu-RLI). Hu-RLI and Mu-RLI protein have 98% amino acid identity. Mu-RLI is functionally homologous to Hu-RLI, and all the structural features and amino acid sequence motifs of Hu-RLI are conserved in Mu-RLI. Moreover, reticulocyte lysate translated Mu-RLI protein is also able to inhibit 2-5A binding on 2-5A-dependent RNAse-L. Northern blot analysis revealed that Mu-RLI cDNA hybridizes with one mRNA of 3.5 kb except for the testis where two mRNA of 3.5 and 2.1 kb, respectively, are detected, suggesting a tissue-specific regulation.

Cloning and characterization of a RNAse L inhibitor. A new component of the interferon-regulated 2-5A pathway

The 2-5A/RNase L system is considered as a central pathway of interferon (IFN) action and could possibly play a more general physiological role as for instance in the regulation of RNA stability in mammalian cells. We describe here the expression cloning and initial characterization of RLI (for RNase L inhibitor), a new type of endoribonuclease inhibitor. RLI cDNA codes for a 68-kDa polypeptide whose expression is not regulated by IFN. Its expression in reticulocyte extracts antagonizes the 2-5A binding ability and the nuclease activity of endogenous RNase L or the cloned 2DR polypeptide. The inhibition requires the association of RLI with the nuclease and is dependent on the ratio between the two proteins. Likewise RLI is coimmunoprecipitated with the RNase L complex by a nuclease-specific antibody. RLI does not lead to 2-5A degradation or to irreversible modification of RNase L. The overexpression of RLI in stably transfected HeLa cells inhibits the antiviral activity of IFN on encephalomyocarditis virus but not on vesicular stomatitis virus. RLI therefore appears as the first described and potentially important mediator of the 2-5A/RNase L pathway.

2',5'-Oligoadenylate-dependent RNase L is a dimer of regulatory and catalytic subunits

The subunit composition of RNase L, a key enzyme in the interferon system, has been characterized. RNase L was purified from human Daudi cells on a column of 2-5A-Sepharose and used to immunize Balb/c mice. A specific monoclonal antibody which recognizes a protein of 80 kDa has been isolated. This protein has been characterized and shown to be an RNA-binding protein with nuclease activity which is associated with, but distinct from, the 80-kDa 2-5A-binding protein known previously as RNase L. It is therefore proposed that the 2-5A-dependent RNase L is a complex of two distinct subunits: an 80-kDa RNA-binding protein (i.e. the catalytic subunit) and an 80-kDa 2-5A-binding protein (i.e. the regulatory subunit).

Regeneration of enzyme activity after western blot: activation of RNase L by 2-5A on filter--importance for its detection

A rapid and convenient new procedure for detecting RNase L activity following Western blot by renaturation of the enzyme on the nitrocellulose sheets is described. This method allows the simultaneous analysis of enzymatic activity (e.g., cleavage of poly(uridylic acid)-3'-[32P]pCp) and RNase L binding to radioactivE probes (e.g., 2-5A-3'-[32P]pCp) in the same sample. Unlike previously published methods, this procedure eliminates interference from proteases or other RNases during the analysis of RNase L activity. The detection of RNase(s) L is also affected by the presence of endogenous 2-5A, 2-5A derivatives, or other possible "inhibitors" in cell extracts; this Western blot assay allows of RNase(s) L to be detected independently of intracellular 2-5A or analogs. Differences between the procedures used so far and this Western blot technique can indeed be demonstrated. It is shown with this Western blot assay that although RNase L has been described as a protein of 185-200 kDa under nondenaturating conditions, its 80-kDa (and 40-kDa) component is able to bind 2-5A and to cleave poly(uridylic acid) in a 2-5A-dependent way, independently of other subunit(s) or cofactor(s).

Polyclonal antibodies against RNase L. Subcellular localization of this enzyme in mouse cells

RNase L activated by 2-5A (a series of 2'-5'-linked adenylic oligoribonucleotides) is a key enzyme of the interferon system. To study RNase L (endonuclease L) in intact cells independently of intracellular 2-5A and of its activity, we have developed polyclonal antibodies against RNase L. RNase L from mouse spleen was purified on a column of 2-5A-Sepharose and used to immunize rabbits in co-injection with polyadenylic-polyuridylic acid as adjuvant. Antibodies were purified by chromatography on Affi-Gel blue and 2-5A-Sepharose-immobilized RNase L. These polyclonal antibodies immunoprecipitate the 80- and 40-kDa forms of RNase L in mouse spleen. In Western blot, only the 80-kDa form of RNase L is recognized by these antibodies. These purified antibodies were used to localize RNase L in the cytoplasm of intact mouse NIH 3T3 cells by immunofluorescence. The cytoplasmic localization of RNase L was confirmed by its 2-5A binding activity after cellular fractionation.

Phosphorothioate analogues of (2'-5')(A)4: agonist and antagonist activities in intact cells

Metabolically stable phosphorothioate tetramer analogues of (2'-5')(A)n with Rp and/or Sp chirality in the 2'-5'-phosphodiester linkages constitute a new class of antiviral agents since they mimic the effects of interferons. Three of the diastereomeric 5'-monophosphates (i.e., pRpRpRp, pSpRpRp, and pRpSpSp) bind to and activate RNase L from extracts of HeLa cells. However, the pSpSpSp (2'-5')-(A)4-phosphorothioate is unique in that it binds to, but cannot activate, RNase L to cleave rRNA. When microinjected into the cytoplasm of HeLa cells followed by virus infection, the pRpRpRp, pSpRpRp, and pRpSpSp (2'-5')(A)4-phosphorothioates demonstrate antiviral activity, as does (2'-5')(A)4ox-red, an active (2'-5')(A)n analogue. When microinjected simultaneously with (2'-5')(A)nox-red, an active the pSpSpSp (2'-5')(A)4-phosphorothioate inhibits activation of RNase L in HeLa cells, thereby blocking direct protection of vesicular stomatitis virus. The agonist and antagonist properties of pRpRpRp and pSpSpSp, respectively, are transient probably as a consequence of the hydrolysis of the 5'-monophosphate and formation of the less active (2'-5')(A)4-phosphorothioate cores. The possible use of these (2'-5')(A)4-phosphorothioates as tools for dissecting the biological significance of the (2'-5')(A)n system or in antiviral chemotherapy is discussed.

Characterization of two murine (2'-5')(A)n-dependent endonucleases of different molecular mass

RNase L is considered as the major if not unique target of (2'-5')(A)n and therefore as an important intracellular mediator of interferon action. It behaves as an 185-kDa species in various cell extracts when analyzed by gel filtration. SDS/PAGE analysis of the polypeptides covalently labelled with a (2'-5')(A)4-3'-[32P]pCp probe reveals a single 80-kDa species, thus attesting a multimeric form of the 185-kDa protein. At variance with such data, mouse spleen extracts reveal an additional 40-kDa polypeptide with (2'-5')(A)n-dependent ribonucleolytic activity. This seemingly new form of RNase L migrates as a 40-kDa polypeptide when analyzed under native or denaturing conditions. It bears some structural similarity with the larger-molecular-mass RNase L as revealed by partial proteolysis. It is probably not generated through proteolytic degradation of the 185-kDa RNase L during extract preparation, although its physiological significance is unknown. Indeed various protease inhibitors do not significantly alter the ratio of 40-kDa and 185-kDa (2'-5')(A)n-dependent ribonucleases; moreover, the (2'-5')(A)n-binding capacity of the 40-kDa polypeptide is more stable than that of the 185-kDa one.

Interferons and oncogenes in the control of cell growth and differentiation: working hypothesis and experimental facts

This short review summarizes available evidence for (i) growth regulatory properties of exogenous as well as recently described autocrine IFNs, (ii) down-regulation of cellular oncogene expression with emphasis on c-myc and (iii) the possible involvement of the IFN-regulated 2-5A pathway at these levels. Initially described as a part of the IFN-induced antiviral mechanism, this double-stranded RNA-activated pathway leads to the preferential degradation of viral mRNAs in IFN-treated virus-infected cells probably through localized activation at the site of virus replication. Such mechanisms could be involved in the regulation of the stability of rapidly turning over mRNAs as for instance c-myc mRNA in IFN-treated cells. Whatever the elegance of the concept, however, experimental evidence is essentially circumstantial; tools developed in our group to strengthen the demonstration are briefly described.

5'-modified agonist and antagonist (2'-5')(A)n analogues: synthesis and biological activity

Two 5'-modified (2'-5')(A)4 oligomers with an increased resistance to phosphatase degradation were synthesized and evaluated for their ability to develop an antiviral response when introduced into intact cells by microinjection or by chemical conjugation to poly(L-lysine). The enzymatic synthesis of 5'-gamma-phosphorothioate and beta,gamma-difluoromethylene (2'-5')(A)4 from adenosine 5'-O-(3-thiotriphosphate) and adenosine beta,gamma-difluoromethylenetriphosphate by (2'-5')-oligoadenylate synthetase is described. The isolation and characterization of these (2'-5')(A)4 analogues were achieved by high-performance liquid chromatography. The structures of 5'-modified tetramers were corroborated by enzyme digestion. These two 5'-modified tetramers compete as efficiently as natural (2'-5')(A)4 for the binding of a radiolabeled (2'-5')(A)4 probe to ribonuclease (RNase) L. Nevertheless, at the opposite to 5'-gamma-phosphorothioate (2'-5')(A)4, beta,gamma-difluoromethylene (2'-5')(A)4 failed to induce an antiviral response after microinjection in HeLa cells. In addition, it behaves as an antagonist of RNase L as demonstrated by its ability to inhibit the antiviral properties of 5'-gamma-phosphorothioate (2'-5')(A)4 when both are microinjected in HeLa cells. The increased metabolic stability of 5'-gamma-phosphorothioate (2'-5')(A)4 as compared to that of (2'-5')(A)4 was first demonstrated in cell-free extracts and then confirmed in intact cells after introduction in the form of a conjugate to poly(L-lysine). Indeed, 5'-gamma-phosphorothioate (2'-5')(A)4-poly(L-lysine) conjugate induces protein synthesis inhibition and characteristic ribosomal RNA cleavages for longer times than unmodified (2'-5')(A)4-poly(L-lysine) in the same cell system.(ABSTRACT TRUNCATED AT 250 WORDS)

Requirement for endocytosis of poly(rI).poly(rC) to generate toxicity on interferon-treated LM cells

Poly(rI).poly(rC) induces a lytic reaction in interferon-treated mouse LM cells. We have attempted to determine whether the intracellular penetration of poly(rI).poly(rC) is a prerequisite for cell lysis and to gain some insight into the pathway followed. We found that poly(rI).poly(rC) coupled to Sepharose beads was unable to generate lysis of interferon-treated cells whereas the cells underwent lysis after microinjection of poly(rI).poly(rC). Some inhibitors of endocytosis were found to inhibit the development of the lytic reaction. Lysosomotropic amines or a low temperature (19 degrees C) blocked endocytosis of poly(rI).poly(rC) but did not prevent its uptake. The internalization of poly(rI).poly(rC) was energy-dependent and was blocked when sodium azide and 2-deoxyglucose were added simultaneously. We conclude that poly(rI).poly(rC) is internalized and reaches an acidic compartment before triggering the lytic reaction in the cell.