Isolated interferon alpha-receptor complexes stabilized in vitro

Historical developments in the research of interferon receptors

Interferons (IFNs) were discovered 50 years ago independently by Isaacs and Lindemann and by Nagata and Kojima. When it was later realized that IFNs are active at very low concentrations, research began to determine how their powerful effects were generated from such a small initial signal. It has since been established that interferons, as well as all other cytokines, employ cell surface receptors to translate their presence in the serum to a potent cellular response to a viral infection. These receptor complexes are composed of multiple distinct glycosylated transmembrane polypeptides, a number of protein tyrosine kinases, and interact transiently with a large variety of other proteins including transcription factors, phosphatases, signaling repressors, and adaptor proteins coupling the receptor to alternative signaling pathways. Three major receptor complexes exist that are exclusive to each of three major classes of interferon. Even though the effects of each major class of interferon vary physiologically, each receptor complex interacts with its ligand in similar ways and activates similar signaling cascades. In this mini-review, we take a historical perspective at the major events in the characterization of interferon receptors, discussing interesting results that still need to be explained.

Interferons Alpha, Beta, and Omega

Interferon alpha (IFN-α) is a mixture of closely related proteins, termed “subtypes,” expressed from distinct chromosomal genes. Interferon β (IFN-β) is a single protein species and is molecularly related to IFN-α subtypes, although it is antigenically distinct from them. IFN omega (IFN-ω) is antigenically distinct from IFN-α and IFN-β but is molecularly related to both. The genes of three IFN subtypes are tandemly arranged on the short arm of chromosome 9. They are transiently expressed following induction by various exogenous stimuli, including viruses. They are synthesized from their respective mRNAs for relatively short periods following gene activation and are secreted to act, via specific cell surface receptors, on other cells. IFN-α subtypes are secreted proteins and as such are transcribed from mRNAs as precursor proteins, pre-IFN-α, containing N-terminal signal polypeptides of 23 hydrophobic amino acids (aa) mainly. Pre-IFN-β contains 187 aa, of which 21 comprise the N-terminal signal polypeptide and 166 comprise the mature IFN-β protein. IFN-ω contains 195 aa—the N-terminal 23 comprising the signal sequence and the remaining 172, the mature IFN-ω protein. At the C-terminus, the aa sequence of IFN-ω is six residues longer than that of IFN-α or IFN-β proteins. IFN-α, as a mixture of subtypes, and IFN-ω may be produced together following viral infection of null lymphocytes or monocytes/macrophages. The biological activities of IFNs are mostly dependent upon protein synthesis with selective subsets of proteins mediating individual activities. IFNs can also stimulate indirect antiviral and antitumor mechanisms, depending upon cellular differentiation and the induction of cytotoxic activity.

Configuration of the interferon-alpha/beta receptor complex determines the context of the biological response

Constituents of the Type 1 interferon (IFN) receptor (IFNABR) identified to date include the alpha and beta transmembrane subunits and the associated intracellular kinases, Jak 1 and Tyk 2. In this report, we demonstrate that a human cell type that expresses both subunits of IFNABR, together with Jak 1 and Tyk 2, exhibits a limited binding capacity for and is only partially sensitive to the effects of IFN-alpha/beta, despite adequate levels of the cytoplasmic transcription factors Stat1, Stat2, and Stat3. Specifically, a low affinity interaction between IFN-alpha/beta and cell surface receptors results in ISGF3 (Stat1:2) activation and an antiviral response, yet no IFN-inducible growth inhibition. Using a panel of murine cells that are variably configured with respect to the human IFNABR-alpha/beta subunits, we provide evidence that an additional component(s) encoded on human chromosome 21 is required to confer high affinity binding and IFN-inducible growth inhibition to cells that express the alpha and beta subunits of the IFNABR. The data indicate that transcriptional activation that leads to an antiviral response is mediated by IFN-alpha/beta activation of IFNABR-alpha and IFNABR-beta in the context of a low affinity interaction, yet a high affinity interaction is necessary for signal transducing events that mediate growth inhibition. We provide evidence that the extent of ISGF3 activation correlates directly with the magnitude of an antiviral but not a growth inhibitory response.

Ligand-induced association of the type I interferon receptor components

Two transmembrane polypeptides, IFNAR and IFN-alpha/Beta R, were previously identified as essential components of the type I interferon (IFN) receptor, but their interrelationship and role in ligand binding were not clear. To study these issues, we stably expressed and characterized the two polypeptides in host murine cells. In human cells, native IFN-alpha/beta R is a 102-kDa protein but upon reduction only a 51-kDa protein is detected. In host murine cells human IFN-alpha/beta R was expressed as a 51-kDa protein. Host cells expressing IFN-alpha/beta R bound IFN-alpha 2 with a high affinity (Kd of 3.6 nM), whereas cells expressing IFNAR exhibited no ligand binding. Upon coexpression of IFNAR and the 51-kDa IFN-alpha/beta R, the affinity for IFN-alpha 2 was increased 10-fold, approaching that of the native receptor. We show by cross-linking that both the cloned (51-kDa) and native (102-kDa) IFN-alpha/beta R bind IFN-alpha 2 to form an intermediate product, while IFNAR associates with this product to form a ternary complex. Hence, IFNAR and IFN-alpha/beta R are components of a common type I IFN receptor, cooperating in ligand binding. Ligand-induced association of IFNAR and IFN-alpha/beta R probably triggers transmembrane signaling.

Structure of the human interferon alpha receptor

The structure of the IFN alpha receptor has been studied by methods such as affinity crosslinking and gel chromatography over the last 8 years. The recent development of monoclonal antibodies against the receptor, and the cloning of an IFN alpha receptor cDNA has provided new important tools to understand the IFN alpha receptor structure. Thus, it has become obvious that the IFN alpha receptor has a more complex structure than first anticipated, probably involving more than one subunit. This review analyzes the present knowledge about the structure of the IFN alpha receptor, as well as many unresolved issues concerning this topic.

Changes in interferon receptors on peripheral blood mononuclear cells from patients with chronic hepatitis B being treated with interferon

We studied the binding of 125I-labeled human interferon-alpha to peripheral blood mononuclear cells and the activity of 2',5'-oligoadenylate synthetase in peripheral blood mononuclear cells obtained from 21 patients with chronic hepatitis B who were treated with human interferon-alpha or interferon-beta. Fourteen patients were given interferon daily for 4 wk. Interferon receptors per cell decreased to about 50% of baseline but increased to baseline by 2 wk after therapy ended. The activity of 2',5'-oligoadenylate synthetase rose about fivefold during therapy, decreasing to baseline by 1 wk after the end of therapy. The seven other patients were given interferon daily for 2 wk, no interferon for 2 wk and then interferon daily for 2 wk more. During both periods of therapy on this schedule, interferon receptors decreased to about 50% but returned to baseline 1 wk after the interferon was stopped. The activity of 2',5'-oligoadenylate synthetase increased about fivefold during both the first and second periods of therapy and decreased to baseline 1 wk after interferon was stopped. Close negative correlation existed between the number of interferon receptors and the 2',5'-oligoadenylate synthetase activity. The results of interferon therapy could not be predicted by either the numbers of interferon receptors before therapy or by the decrease in this number during therapy.

Characterization of interferon-alpha binding sites on human cell lines

The binding sites for human interferon-alpha (IFN-alpha) have been characterized on human lymphoblastoid, melanoma, rhabdomyosarcoma, and cervical carcinoma cells. Crosslinking of iodinated-recombinant DNA-derived IFN-alpha-Con1, an analog of the known IFN-alpha subtypes, to the cell surface with disuccinimidyl suberate yielded four IFN-receptor complexes of 118, 138, 159, and 260 kD on all cell lines that specifically bind IFN-alpha. Since IFN-alpha exists in solution as monomers, dimers, and trimers, and the three lower molecular weight IFN-alpha-receptor complexes differ by the molecular weight of IFN-alpha (20 kD), this suggests that the human IFN-alpha receptor of 100 kD binds more than one molecule of IFN-alpha. The higher molecular weight complex of 260 kD may result from dimerization of the receptor. None of these complexes was observed in a rhabdomyosarcoma subclone that does not specifically bind IFN-alpha. Pretreatment of cells with trypsin abolished the formation of these complexes. Pretreatment of cells with neuraminidase did not reduce IFN-alpha binding, but increased the electrophoretic mobility of all four IFN-alpha-receptor complexes. Other glycosidases (i.e., mannosidase, beta-galactosidase, and endoglycosidase F) had no effects on IFN-alpha binding or mobility of complexes. Thus, although the IFN-alpha receptor is a glycoprotein, the glycosylated portion is apparently not part of the IFN-alpha-binding domain. The formation of IFN-alpha-receptor complexes is independent of the duration of incubation with IFN (from 5 min to 1 h at 15 degrees C).(ABSTRACT TRUNCATED AT 250 WORDS)

Interferon removes its own receptors as it blocks the division of Daudi cells

The Burkitt-derived line, Daudi, whose proliferation is inhibited by human alpha-interferon (IFN-alpha), was treated with 125I-labelled recombinant human IFN-alpha A. After separation from unbound ligand, cell-bound IFN was extracted with the detergent digitonin yielding soluble and insoluble complexes of IFN and receptor, together with a certain amount of uncomplexed IFN. 1. Soluble complexes were stable enough to be separated from uncomplexed IFN by permeation chromatography. Treatment of soluble complexes with the bifunctional reagent, disuccinimidyl suberate, yielded a radioactive product separating with an Mr of 130,000 on electrophoresis in sodium dodecyl sulphate. Similar complexes could be recovered with sodium dodecyl sulphate from the digitonin-insoluble residue, treated with the bi-functional reagent. 2. The total (soluble and insoluble) of complexed IFN obtained after digitonin extraction was a constant fraction (0.62) of the total cell-bound radioactivity, being independent of the concentration of IFN added to the cells (less than pM to greater than nM), and of the time of incubation (1 min to 20 h). However, between 30 min and 3 h of incubation, the insoluble complex increased, at the expense of the soluble complex, and there appeared a cellular pool of degraded ligand. From 3 h to 20 h the distribution of ligand-derived radioactivity remained constant while the total amount decreased to less than 10% of its value at 30 min. This decrease in binding was matched by the appearance of an equivalent quantity of radiolabelled fragments in the culture medium. 3. The inhibition of cellular division due to IFN was shown to be coincident with the disappearance of cellular binding and with the cell-mediated degradation of receptor-complexed IFN. We propose that IFN removes its own receptor and, in doing so, blocks a linked function necessary for the stimulated growth of Daudi cells.

Regulation of cell growth by interferon

Interferons can regulate growth and differentiation in a wide range of cell types. These mechanisms are currently being examined. Interferons inhibit the growth of tumour cells and are thus potential anti-cancer agents. They can also inhibit normal cell growth in vitro, and stimulate tumour cell growth in vitro. They may also be involved in some autoimmune diseases. This review examines the effect of interferons on cell proliferation, function, and growth, focusing primarily on in vitro cell systems.

Differential human interferon alpha receptor expression on proliferating and non-proliferating cells

The expression of interferon-alpha (IFN-alpha) receptors was studied on a variety of human cells, using monoiodinated IFN-alpha 2 probes. Steady-state binding at 4 degrees C revealed a single class of non-interacting IFN receptor on peripheral blood lymphocytes, and tonsillar B lymphocytes, which are both known to be G0/G1 resting cell populations. The binding affinity of this class of receptor was found to be on the order of 5 X 10(-10) M, expressed as an apparent dissociation constant (Kd). However, cells proliferating either in culture or in vivo were found to express a heterogeneity in IFN-alpha 2 binding. Such binding could be objectively resolved (by a version of the LIGAND program of P. Munson) into a two-site receptor model. Hill plots of binding to proliferating cells indicated a negative cooperativity in the interaction of IFN and receptor. The high-affinity component, expressed on proliferating cells, typically exhibits a Kd of (1-10) X 10(-11) M, while the lower-affinity component indicates a Kd of (1-10) X 10(-9) M. Furthermore, the low-affinity component is apparently expressed on the order of 10-200 times the copy number, per cell, of the high-affinity site. Affinity-labeling experiments revealed that, in addition to the 140-160-kDa IFN-binding complex reported by others, both the proliferating and non-proliferating cell populations possess a novel IFN-binding component of 60 kDa.

The interferon receptors

Early studies on the mode of action of interferons have indicated that a receptor system on the cell surface is involved in its action. The first direct evidence to a high-affinity binding site was found only after pure interferon was available. Two different receptors, one specific for interferons-alpha and beta, and the other for interferon-gamma were recognized. A correlation between affinity to the receptor and specific activity was established. Cross-linked complexes of labeled interferons with their receptors were visualized on gel electrophoresis and even partially purified. Internalization of interferons after binding to the receptor was reported. The role of gangliosides as helpers of interferon binding was recently investigated. Fragments of interferons which still retained binding capacity were described and helped in elucidating the binding site on the interferon molecule.

The gene for the human immune interferon receptor is located on chromosome 6

When 32P-labeled human recombinant immune interferon gamma (Hu-[32P]IFN-gamma) is crosslinked to human cells with disuccinimidyl suberate, a complex with a molecular size of approximately equal to 117,000 Da was identified by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The formation of this complex is inhibited when the binding is performed in the presence of excess unlabeled Hu-IFN-gamma. The specific formation of the 117,000-Da complex is not observed in mouse L cells or Chinese hamster ovary cells. This complex shows all of the criteria that identify it as the Hu-IFN-gamma receptor or its binding subunit. The same complex can be formed following binding and covalent crosslinking of Hu-[32P]IFN-gamma to some hamster-human or mouse-human somatic cell hybrids. The presence of human chromosome 6 in the hybrids is necessary and sufficient for the formation of this complex. More specifically, the long arm of chromosome 6 seems sufficient. Therefore, we have localized the gene for the Hu-IFN-gamma receptor (or its binding subunit) to the long arm of human chromosome 6. The presence of this chromosome in the somatic cell hybrids is not adequate, however, to confer antiviral resistance to the hybrids in the presence of Hu-IFN-gamma.

Receptor dynamics of closely related ligands: "fast' and "slow' interferons

Two related human alpha interferons with 83% homology in their primary sequences show a similar specific activity on nonhuman cells, but a striking difference on human cells, on which alpha-1 shows 1-5% of the specific molar activity displayed by alpha-2. Both interferons were labelled with 125I, and their binding kinetics followed on growing cultures of the human Burkitt line Daudi. Binding of alpha-1 showed slower rates of association and faster rates of dissociation implying that differences in apparent binding affinity were responsible for the differences in specific molar activity. However, binding was shown to reach steady-state rather than an equilibrium, so differences in the dynamics of the ligand-receptor complexes may represent amplification of differences in the initial binding constant. alpha-2, but not alpha-1, induces a marked loss of binding sites leading to a high affinity steady-state binding. Inhibition of cell multiplication by both interferons depends on a continued stimulation by free ligands at steady-state. It is proposed that the differences in specific molar activity are, in the main, kinetic and cause alpha-1 and alpha-2 to behave respectively as "slow' and "fast' interferons.

Receptor mediated pathways for interferon action: in vivo implications

High affinity receptors for human interferon (IFN) are a common surface component of human nucleated cells. A model culture system has been used to mark the activation of surface receptors by IFN as the first step in several, possibly separate, pathways of action, some of which depend on the continued presence of IFN. Two types of equilibrium binding can be seen: an initial one of short duration corresponding to receptor activation and a second of longer duration corresponding to activation of a second site by IFN. Various different responses to IFN are seen to be determined by one or other of these equilibria. In vivo, implications are discussed in terms of the pharmacokinetic distribution of IFN, with particular reference to intravenous injection. Desensitization of target cells with respect to late equilibrium effects is seen as a possible consequence of receptor activation against the background of changing IFN concentration in vivo. A modulating effect of combined IFN species is proposed.