Identification of a domain in the beta subunit of the type I interferon (IFN) receptor that exhibits a negative regulatory effect in the growth inhibitory action of type I IFNs

Advantage of IFN-beta/alpha2b same-day administration for ribavirin-intolerant patients with chronic hepatitis C

AIM

Although interferon (IFN)/ribavirin is the mainstream combination treatment for chronic hepatitis C in patients with a high viral load, ribavirin is problematic for women of childbearing age and patients with anemia. Therefore we needed to establish a new regimen without ribavirin.

METHODS

We devised a new regimen (same-day beta/alpha2b) to administer IFN-beta and alpha2b on the same-day, and compared it with IFN-alpha2b alone and IFN-alpha2b plus ribavirin. The cases were 36 patients (26.1%) in whom ribavirin could not be used (young women, anemia, etc.) among 138 patients who underwent IFN treatment after ribavirin release.

RESULTS

The percentages of patients withdrawing due to side-effects were 6.8%, 18.8% and 17.0% in the treatment with same-day beta/alpha2b, IFN-alpha2b alone and IFN-alpha2b plus ribavirin groups, respectively. In genotype 1b, the sustained viral response (SVR) was 28.6% (4/14), 13.6% (3/22) and 25.0% (8/32) with a high viral load, and 91.7% (11/12), 27.3% (3/11) and 57.1% (4/7) with a low viral load for the respective groups. According to a study on viral half-life during the early phase of IFN therapy, there was no difference among the regimens of same-day IFN-beta/alpha2b, beta alone, alpha2b alone and twice-daily treatment with IFN-beta. Same-day beta/alpha2b treatment showed a significantly higher SVR rate in moving type patients with a genotype 1b/high viral load.

CONCLUSIONS

Same-day beta/alpha2b treatment resulted in few cases where therapy was discontinued and showed a high SVR rate. This regimen is especially appropriate in cases where ribavirin has been deemed unsuitable.

Ingested (Oral) IFN-α Represses TNF-α mRNA in Relapsing-Remitting Multiple Sclerosis

In a phase II trial in relapsing-remitting multiple sclerosis (RRMS), patients ingesting 10,000 IU, but not 30,000 IU, interferon-alpha (IFN-alpha) showed fewer gadolinium enhancements at months 5 and 6, along with decreased proinflammatory tumor necrosis factor-alpha (TNF-alpha) protein secretion. Therefore, we examined MxA mRNA induction and TNF-alpha mRNA repression after 100, 300, 1,000, 3,000, and 10,000 IU doses of ingested IFN-alpha in 24 RRMS patients to determine the optimal dose for future clinical trials in MS. Maximal TNF-alpha repression occurs at 100, 1,000, and 3,000 IU. These data provide new optimal doses for additional clinical studies using ingested IFN-alpha in MS.

Stat2 binding to the interferon-alpha receptor 2 subunit is not required for interferon-alpha signaling

The interferon-alpha (IFNalpha) receptor consists of two subunits, the IFNalpha receptor 1 (IFNaR1) and 2 (IFNaR2) chains. Following ligand binding, IFNaR1 is phosphorylated on tyrosine 466, and this site recruits Stat2 via its SH2 domain. In contrast, IFNaR2 binds Stat2 constitutively. In this study we have characterized the Stat2-IFNaR2 interaction and examined its role in IFNalpha signaling. Stat2 binds the major IFNaR2 protein but not a variant containing a shorter cytoplasmic domain. The interaction does not require a STAT SH2 domain. Both tyrosine-phosphorylated and non-phosphorylated Stat2 bind IFNaR2 in vitro; however, relatively little phosphorylated Stat2 associates with IFNaR2 in vivo. In vitro binding assays defined IFNaR2 residues 418-444 as the minimal interaction domain and site-specific mutation of conserved acidic residues within this domain disrupted in vitro and in vivo binding. An IFNaR2 construct carrying these mutations was either (i) overexpressed in 293T cells or (ii) used to complement IFNaR2-deficient U5A cells. Unexpectedly, the activity of an IFNalpha-dependent reporter gene was not reduced but, instead, was enhanced up to 2-fold. This suggests that this particular IFNaR2-Stat2 interaction is not required for IFNalpha signaling, but might act to negatively inhibit signaling. Finally, a doubly truncated recombinant fragment of Stat2, spanning residues 136-702, associated with IFNaR2 in vitro, indicating that the interaction with IFNaR2 is direct and occurs in a central region of Stat2 marked by a hydrophobic core.

The interferon-inducible Stat2:Stat1 heterodimer preferentially binds in vitro to a consensus element found in the promoters of a subset of interferon-stimulated genes

Regulated expression of type I interferon (IFN)-stimulated genes (ISG) requires the binding of the signal transducer and activator of transcription (Stat) complexes to enhancer elements located in the ISG promoters. These enhancer elements include the IFN-stimulated response element (ISRE) and the palindromic IFN-gamma activation site (GAS) element. Regulated expression of ISRE containing ISG depends on IFN-stimulated gene factor 3 (ISGF3), a heterodimer involving Stat1 and Stat2 in association with a DNA-binding adapter protein, p48/IFN regulatory factor-9 (IRF-9). Several GAS binding Stat complexes involving Stat1, Stat3, Stat4, and Stat5 have been described, but their contribution to GAS-dependent ISG expression remains to be established. We and others previously identified an IFN-alpha-inducible Stat2:1 heterodimer that exhibits binding to the GAS element of the IRF-1 gene. These previous studies raise the possibility that Stat2:1 may participate in the transcriptional activation of the subset of ISG containing GAS elements. To address this question, we performed a PCR-assisted binding site selection procedure to define the Stat2:1 recognition sequence. The data reveal that Stat2:1 preferentially binds to a palindromic sequence similar to the consensus GAS element found in the promoter of several ISG. Our results establish that in addition to the classic complex formation involving Stat2, Stat1, and p48 associations, Stat2:1 heterodimers are formed in response to IFN treatment that may play an important role in the transcriptional regulation of certain ISG.

Vanadate facilitates interferon alpha-mediated apoptosis that is dependent on the Jak/Stat pathway

Type I interferon (IFN)-dependent inhibition of cell growth can occur either in the absence or presence of apoptosis. The mechanisms that determine whether or not cells undergo apoptosis after exposure to IFN-alpha are not clear. This study shows that a variety of cell lines that display growth inhibition but not apoptosis in response to IFN-alpha will undergo programmed cell death when low concentrations of the protein-tyrosine phosphatase inhibitor vanadate are added with IFN-alpha. In contrast, the combination of tumor necrosis factor-alpha with vanadate did not trigger apoptosis in these cells. Caspase-3 activity was detected only in cells exposed to IFN-alpha and vanadate but not to IFN-alpha or vanadate alone. The ability of IFN-alpha and vanadate to induce apoptosis did not require expression of p53 and was blocked by N-acetyl-l-cysteine. Activation of the Jak/Stat pathway and expression of IFN-inducible genes was not altered by incubation of cells with IFN-alpha and vanadate compared with IFN-alpha alone. However, mutant cells lacking Stat1, Stat2, Jak1, or Tyk2, or cells expressing kinase inactive Jak1 or Tyk2 did not undergo apoptosis in the presence of IFN-alpha and vanadate. These results suggest that IFN-alpha stimulation of Stat-dependent genes is necessary, but not sufficient, for this cytokine to induce apoptosis. Another signaling cascade that involves the activity of a protein-tyrosine phosphatase and/or the generation of reactive oxygen species may play an important role in promoting IFN-alpha-induced apoptosis.

Role of the intracellular domain of the human type I interferon receptor 2 chain (IFNAR2c) in interferon signaling. Expression of IFNAR2c truncation mutants in U5A cells

A human cell line (U5A) lacking the type I interferon (IFN) receptor chain 2 (IFNAR2c) was used to determine the role of the IFNAR2c cytoplasmic domain in regulating IFN-dependent STAT activation, interferon-stimulated gene factor 3 (ISGF3) and c-sis-inducible factor (SIF) complex formation, gene expression, and antiproliferative effects. A panel of U5A cells expressing truncation mutants of IFNAR2c on their cell surface were generated for study. Janus kinase (JAK) activation was detected in all mutant cell lines; however, STAT1 and STAT2 activation was observed only in U5A cells expressing full-length IFNAR2c and IFNAR2c truncated at residue 462 (R2.462). IFNAR2c mutants truncated at residues 417 (R2. 417) and 346 (R2.346) or IFNAR2c mutant lacking tyrosine residues in its cytoplasmic domain (R2.Y-F) render the receptor inactive. A similar pattern was observed for IFN-inducible STAT activation, STAT complex formation, and STAT-DNA binding. Consistent with these data, IFN-inducible gene expression was ablated in U5A, R2.Y-F, R2.417, and R2.346 cell lines. The implications are that tyrosine phosphorylation and the 462-417 region of IFNAR2c are independently obligatory for receptor activation. In addition, the distal 53 amino acids of the intracellular domain of IFNAR2c are not required for IFN-receptor mediated STAT activation, ISFG3 or SIF complex formation, induction of gene expression, and inhibition of thymidine incorporation. These data demonstrate for the first time that both tyrosine phosphorylation and a specific domain of IFNAR2c are required in human cells for IFN-dependent coupling of JAK activation to STAT phosphorylation, gene induction, and antiproliferative effects. In addition, human and murine cells appear to require different regions of the cytoplasmic domain of IFNAR2c for regulation of IFN responses.

Impaired antiviral response and alpha/beta interferon induction in mice lacking beta interferon

We have generated mice lacking the gene for beta interferon and report that they are highly susceptible to vaccinia virus infection. Furthermore, in cultured embryo fibroblasts, viral induction of alpha interferon and of 2-5A synthetase genes is impaired. We also show that beta interferon does not prime its own expression.

Role of the cytoplasmic domains of the type I interferon receptor subunits in signaling

Type I interferons are imperative in maintaining a defense against viral infection. These cytokines also play an important role in the control of cell proliferation. These effects are triggered by ligand binding to a specific cell surface receptor. In the present article, we attempt to analyze the advances made in the last four years on type I interferon signaling. This review will focus on the contribution of the cytoplasmic domain of the alpha and betaL chains of the receptor to the activation of the Jak-Stat pathway. We also analyze the possible role of other pathways in interferon signaling.

Determination of Residues Involved in Ligand Binding and Signal Transmission in the Human IFN-α Receptor 2

The human IFN-α receptor (hIFNAR) is a complex composed of at least two chains, hIFNAR1 and hIFNAR2. We have performed a structure-function analysis of hIFNAR2 extracellular domain regions using anti-hIFNAR2 mAbs (1D3, 1F3, and 3B7) and several type I human IFNs. These mAbs block receptor activation, as determined by IFN-stimulated gene factor 3 formation, and block the antiviral cytopathic effects induced by type I IFNs. We generated alanine substitution mutants of hIFNAR2-IgG and determined that regions of hIFNAR2 are important for the binding of these blocking mAbs and hIFN-α2/α1. We further demonstrated that residues E78, W101, I104, and D105 are crucial for the binding of hIFN-α2/α1 and form a defined protrusion when these residues are mapped upon a structural model of hIFNAR2. To confirm that residues important for ligand binding are indeed important for IFN signal transduction, we determined the ability of mouse L929 cells expressing hIFNAR2 extracellular domain mutants to mediate hIFN signal. hIFN-α8, previously shown to signal a response in L929 cells expressing hIFNAR1, was unable to signal in L929 cells expressing hIFNAR2. Transfected cells expressing hIFNAR2 containing mutations at residues E78, W101, I104, or D105 were unresponsive to hIFN-α2, but remained responsive to hIFN-β. In summary, we have identified specific residues of hIFNAR2 important for the binding to hIFN-α2/1 and demonstrate that specific regions of the IFNAR interact with the subspecies of type I IFN in different manners.