Differences in activity between alpha and beta type I interferons explored by mutational analysis

Lack of interferon beta-induced radiosensitization in four out of five human glioblastoma cell lines

PURPOSE

To investigate the potential of interferon beta to enhance the cytotoxic activity of ionizing irradiation against glioma cells, and to elucidate the possible mechanisms responsible for conflicting clinical results.

METHODS AND MATERIALS

Five glioblastoma cell lines (U87MG, U118MG, U373MG, MO59K, MO59J) with different radiosensitivity and genetic background were used. Experiments were performed in exponentially growing cultures, and cell survival was measured by a colony-forming assay. Cells were incubated with natural interferon beta (n-IFN-beta; 30-3000 IU/mL) for 24 h followed by single dose irradiation with 1 to 6 Gy of gamma-rays.

RESULTS

Significant differences in n-IFN-beta sensitivity were found. The cell lines also differed in their radiation sensitivity, and there was no correlation between the n-IFN-beta and the radiation sensitivity. In three of five cell lines, the interaction of n-IFN-beta and irradiation was infra-additive; in one cell line, it was additive. For MO59J cells only, which are NHEJ-deficient, supra-additivity was observed.

CONCLUSION

Our results confirm the remarkable heterogeneity that is characteristic of malignant glioma. The combined effect of n-IFN-beta and radiation was mostly infra-additive or additive; synergistic interaction might occur in tumor cells that already have acquired repair deficiencies because of their genetic instability, as shown for the MO59J cell line.

Pharmacokinetic and pharmacodynamic studies of a human serum albumin-interferon-alpha fusion protein in cynomolgus monkeys

Interferon-alpha (IFN-alpha) is indicated for the treatment of certain viral infections including hepatitis B and C, and cancers such as melanoma. The short circulating half-life of unmodified IFN-alpha makes frequent dosing (daily or three times weekly) over an extended period (6-12 months or more) necessary. To improve the pharmacokinetics of IFN-alpha and decrease dosing frequency, IFN-alpha was fused to human serum albumin producing a new protein, Albuferon. In vitro comparisons of Albuferon and IFN-alpha showed similar antiviral and antiproliferative activities, although Albuferon was less potent on a molar basis than IFN-alpha. Pharmacokinetic and pharmacodynamic properties of the fusion protein were enhanced in monkeys. After a single intravenous injection (30 microg/kg,) clearance was 0.9 ml/h/kg, and the terminal half-life was 68 h. After 30 microg/kg subcutaneous injection, apparent clearance (clearance divided by bioavailability) was 1.4 ml/h/kg, the terminal half-life was 93 h, and bioavailability was 64%. The rate of clearance of Albuferon was approximately 140-fold slower, and the half-life 18-fold longer, than for IFN-alpha given by the subcutaneous route in other monkey studies. Sera from Albuferon-treated monkeys demonstrated dose-related antiviral activity for > or =8 days based on an in vitro bioassay, whereas antiviral activity from IFN-alpha-treated animals was only slightly elevated relative to vehicle on day 0. Significant increases in 2',5'-oligoadenylate synthetase mRNA relative to IFN-alpha- or vehicle-treated animals were maintained for > or =10 days after subcutaneous dosing. The improved pharmacokinetics of Albuferon are accompanied by an improved pharmacodynamic response suggesting that Albuferon may offer the benefits of less frequent dosing and a potentially improved efficacy profile compared with IFN-alpha.

Regulatory effect of IFN-kappa, a novel type I IFN, on cytokine production by cells of the innate immune system

IFN-kappa is a recently identified type I IFN that exhibits both structural and functional homology with the other type I IFN subclasses. In this study, we have investigated the effect of IFN-kappa on cells of the innate immune system by comparing cytokine release following treatment of human cells with either IFN-kappa or two recombinant IFN subtypes, IFN-beta and IFN-alpha2a. Although IFN-alpha2a failed to stimulate monocyte cytokine secretion, IFN-kappa, like IFN-beta, induced the release of several cytokines from both monocytes and dendritic cells, without the requirement of a costimulatory signal. IFN-kappa was particularly effective in inhibiting inducible IL-12 release from monocytes. Unlike IFN-beta, IFN-kappa did not induce release of IFN-gamma by PBL. Expression of the IFN-kappa mRNA was observed in resting dendritic cells and monocytes, and it was up-regulated by IFN-gamma stimulation in monocytes, while IFN-beta mRNA was minimally detectable under the same conditions. Monocyte and dendritic cell expression of IFN-kappa was also confirmed in vivo in chronic lesions of psoriasis vulgaris and atopic dermatitis. Finally, biosensor-based binding kinetic analysis revealed that IFN-kappa, like IFN-beta, binds strongly to heparin (K(d): 2.1 nM), suggesting that the cytokine can be retained close to the local site of production. The pattern of cytokines induced by IFN-kappa in monocytes, coupled with the unique induction of IFN-kappa mRNA by IFN-gamma, indicates a potential role for IFN-kappa in the regulation of immune cell functions.

Strong inhibition of Ewing tumor xenograft growth by combination of human interferon-alpha or interferon-beta with ifosfamide

Ewing sarcoma is the second most common bone tumor in childhood. Despite aggressive chemotherapy and radiotherapy strategies, the prognosis of patients with metastatic disease remains poor. We have recently reported that Ewing tumor cell proliferation was strongly inhibited by IFN-beta and to a lesser degree by IFN-alpha. Moreover, under IFN-beta treatment, some cell lines undergo apoptosis. Since the possibility of using IFNs for Ewing tumor treatments may be of interest, we have evaluated the efficacy of Hu-IFNs in a nude mice model of Ewing tumor xenografts. The results reported here show that human type I IFNs, Hu-IFN-alpha and Hu-IFN-beta impaired tumor xenograft take and displayed an anti-growth effect toward established xenografts. Furthermore, we have also shown that combined therapy with Hu-IFNs and ifosfamide (IFO), an alkylating agent widely used in high-dose chemotherapy of Ewing tumors, results in a strong antitumor effect. Pathological analysis showed that Hu-IFN-alpha/IFO and Hu-IFN-beta/IFO were characterized by a dramatic decrease in the mitotic index and marked necrosis, as well as extensive fibrosis associated with numerous calcifications. To our knowledge, this is the first demonstration of a potential antitumor effect of human type I IFNs and IFO on Ewing tumors, providing a rational foundation for a promising therapeutic approach to Ewing sarcoma.

IFN-beta induces serine phosphorylation of Stat-1 in Ewing's sarcoma cells and mediates apoptosis via induction of IRF-1 and activation of caspase-7

Four human cell lines derived from Ewing's sarcoma, EW-7, EW-1, COH and ORS, were investigated to establish the effects of human recombinant interferon-alpha2a and human recombinant interferon-beta on cell proliferation and apoptosis. All four cell lines were much more sensitive to the antiproliferative effects of IFN-beta than of IFN-alpha. Analysis of the early signals triggered by IFN-alpha and IFN-beta demonstrated that the two IFNs were similarly effective in inducing tyrosine phosphorylation of the Jak-1 and Tyk-2 kinases and the transcription factors Stat-1 and Stat-2. Interestingly, an additional rapid phosphorylation of Stat-1 on serine was observed after IFN-beta treatment, with concomitant activation of p38 mitogen-activated protein kinase. In these cells, Stat-1 Ser727 phosphorylation in response to IFN-beta was found to be impaired by p38 MAPkinase inhibitor (SB203580). IFN-beta induced the formation of the Interferon Stimulated Gene Factor 3 complex more efficiently than IFN-alpha, as well as sustained induction of IRF-1, which may account for its greater induction of 2'5'oligo(A)synthetase and greater inhibition of cell proliferation. IFN-beta, but not IFN-alpha, induced apoptosis in wild-type p53 EW-7 and COH cell lines, but not in the mutated p53 EW-1 or ORS cell lines. The apoptosis induced by IFN-beta in EW-7 and COH cell lines appeared to be mediated by IRF-1 and involved the activation of caspase-7. Ectopic expression of IRF-1 induced apoptosis in all four cell lines which correlated with the activation of caspase-7 and with the downregulation of the Bcl-2 oncoprotein, as observed for IFN-beta-induced apoptosis in parental EW-7 and COH cell lines.

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.

Systematic mutational mapping of sites on human interferon-beta-1a that are important for receptor binding and functional activity

A systematic mutational analysis of human interferon-beta-1a (IFN-beta) was performed to identify regions on the surface of the molecule that are important for receptor binding and for functional activity. The crystal structure of IFN-beta-1a was used to design a panel of 15 mutant proteins, in each of which a contiguous group of 2-8 surface residues was mutated, in most instances to alanine. The mutants were analyzed for activity in vitro in antiviral and in antiproliferation assays, and for their ability to bind to the type I IFN (ifnar1/ifnar2) receptor on Daudi cells and to a soluble ifnar2 fusion protein (ifnar2-Fc). Abolition of binding to ifnar2-Fc for mutants A2, AB1, AB2, and E established that the ifnar2 binding site on IFN-beta comprises parts of the A helix, the AB loop, and the E helix. Mutations in these areas, which together define a contiguous patch of the IFN-beta surface, also resulted in reduced affinity for binding to the receptor on cells and in reductions in activity of 5-50-fold in functional assays. A second receptor interaction site, concluded to be the ifnar1 binding site, was identified on the opposite face of the molecule. Mutations in this region, which encompasses parts of the B, C, and D helices and the DE loop, resulted in disparate effects on receptor binding and on functional activity. Analysis of antiproliferation activity as a function of the level of receptor occupancy allowed mutational effects on receptor activation to be distinguished from effects on receptor binding. The results suggest that the binding energy from interaction of IFN-beta with ifnar2 serves mainly to stabilize the bound IFN/receptor complex, whereas the binding energy generated by interaction of certain regions of IFN-beta with ifnar1 is not fully expressed in the observed affinity of binding but instead serves to selectively stabilize activated states of the receptor.

Dimerization of the interferon type I receptor IFNaR2-2 is sufficient for induction of interferon effector genes but not for full antiviral activity

We constructed chimeric receptors wherein the extracellular domain of the erythropoietin receptor (EpoR) was fused to the transmembrane and intracellular domains of the interferon (IFN) type I receptor subunits, IFNaR1 or IFNaR2-2. Transfection into 2fTGH and Tyk2-deficient 11,1 cells showed that EpoR/IFNaR2-2 alone was able to transduce a signal upon stimulation with erythropoietin (Epo), as judged by induction of the interferon type I-inducible 6-16 promoter. In contrast, protection against infection with encephalomyocarditis virus or vesicular stomatitis virus was reduced or absent, respectively. To further investigate the role of IFNaR1 in the induction of an antiviral state, we analyzed the Epo- versus IFNalpha-induced transcription of a set of genes, involved in antiviral protection. Up to 24 h after stimulation with Epo or IFNalpha, comparable transcription of the p56, dsRNA-dependent protein kinase, 2'-5'A synthetase, and MxA genes was seen. However, at later time points, only in the case of Epo induction, a sharp decrease of mRNA levels was observed. Western blotting analysis of dsRNA-dependent protein kinase showed a similar pattern at the protein level. Taken together, our results imply a role for IFNaR1 in the induction of sustained mRNA and protein levels that are likely required for optimal antiviral activity.

Mutational and structural analysis of the binding interface between type I interferons and their receptor Ifnar2

Type I interferons (IFN) exert pleiotropic activities through binding to two cell surface receptors, ifnar1 and ifnar2. We are investigating the biophysical basis of IFN signaling by characterizing the complex of the extra-cellular domain of ifnar2 (ifnar2-EC) with IFNs on the level of purified recombinant proteins in vitro. Here, we present a detailed mutational study on the functional epitopes on both IFN and ifnar2. Kinetic and thermodynamic parameters were determined by label-free heterogeneous phase detection. On IFNalpha2, a relatively small functional epitope comprising ten amino acid residues was localized, which is nearly entirely formed by residues on the AB loop. Two hot-spot residues, L30 and R33, account for two-thirds of the total interaction energy. Comparing the anti-viral potency of the various mutants to the binding affinity towards ifnar2 revealed a proportional correlation between the two, suggesting a rate-limiting role of ifnar2 binding in IFN signaling. On ifnar2, residues T46, I47 and M48 were identified as hot-spots in the interaction with IFNalpha2. For another ten residues on ifnar2, significant contribution of interaction energy was determined. Based on these data, the functional epitope on ifnar2 was defined according to a homology model based on other members of the class II hCR family in good agreement with the complementary functional epitope on IFNalpha2. Although IFNalpha2 and IFNbeta bind competitively to the same functional epitope, mutational analysis revealed distinct centers of binding for these IFNs on ifnar2. This small shift of the binding site may result in different angular orientation, which can be critically coupled to cytoplasmic signaling.

Catalytically active TYK2 is essential for interferon-beta-mediated phosphorylation of STAT3 and interferon-alpha receptor-1 (IFNAR-1) but not for activation of phosphoinositol 3-kinase

TYK2, a Janus kinase, plays both structural and catalytic roles in type I interferon (IFN) signaling. We recently reported (Rani, M. R. S., Gauzzi, C., Pellegrini, S., Fish, E., Wei, T., and Ransohoff, R. M. (1999) J. Biol. Chem. 274, 1891-1897) that catalytically active TYK2 was necessary for IFN-beta to induce the beta-R1 gene. We now report IFN-beta-mediated activation of STATs and other components in U1 (TYK2-null) cell lines that were complemented with kinase-negative (U1.KR930) or wild-type TYK2 (U1.wt). We found that IFN-beta induced phosphorylation on tyrosine of STAT3 in U1.wt cells but not in U1.KR930 cells, whereas STAT1 and STAT2 were activated in both cell lines. Additionally, IFN-beta-mediated phosphorylation of interferon-alpha receptor-1 (IFNAR-1) was defective in IFN-beta treated U1.KR930 cells, but evident in U1.wt cells. In U1A-derived cells, the p85/p110 phosphoinositol 3-kinase isoform was associated with IFNAR-1 but not STAT3, and the association was ligand-independent. Further, IFN-beta treatment stimulated IFNAR-1-associated phosphoinositol kinase activity equally in either U1.wt or U1.KR930 cells. Our results indicate that catalytically active TYK2 is required for IFN-beta-mediated tyrosine phosphorylation of STAT3 and IFNAR-1 in intact cells.

The type I interferon receptor: structure, function, and evolution of a family business

Recent results indicate that coherent models of how multiple interferons (IFN) are recognized and signal selectively through a common receptor are now feasible. A proposal is made that the IFN receptor, with its subunits IFNAR-1 and IFNAR-2, presents two separate ligand binding sites, and this double structure is both necessary and sufficient to ensure that the different IFN are recognized and can act selectively. The key feature is the duplication of the extracellular domain of the IFNAR-1 subunit and the configurational geometry that this imposes on the intracellular domains of the receptor subunits and their associated tyrosine kinases.

Shared receptor components but distinct complexes for alpha and beta interferons

The type I interferon family includes 13 alpha, one omega and one beta subtypes recognized by a complex containing the receptor subunits ifnar1 and ifnar2 and their associated Janus tyrosine kinases, Tyk2 and Jak1. To investigate the reported differences in the way that alpha and beta interferons signal through the receptor, we introduced alanine-substitutions in the ifnar2 extracellular domain, and expressed the mutants in U5A cells, lacking endogenous ifnar2. A selection, designed to recover mutants that responded preferentially to alpha or beta interferon yielded three groups: I, neutral; II, sensitive to alpha interferon, partially resistant to beta interferon; III, resistant to alpha interferon, partially sensitive to beta interferon. A mutant clone, TMK, fully resistant to alpha interferon with good sensitivity to beta interferon, was characterized in detail and compared with U5A cells complemented with wild-type ifnar2 and also with Tyk2-deficient 11.1 cells, which exhibit a similar alpha-unresponsive phenotype with a partial beta interferon response. Using anti-receptor antibodies and mutant forms of beta interferon, three distinct modes of ligand interaction could be discerned: (i) alpha interferon with ifnar1 and ifnar2; (ii) beta interferon with ifnar1 and ifnar2; (iii) beta interferon with ifnar2 alone. We conclude that alpha and beta interferons signal differently through their receptors because the two ligand subtypes interact with the receptor subunits ifnar 1 and ifnar2 in entirely different ways.