Proteins cytokine-binding, soluble

Olsson, L, Gatanaga, T., Gullberg, U., Lantz, M., and Granger, G. A. (1993). Tumor necrosis factor (TNF) binding proteins (soluble TNF receptor forms) with possible roles in inflammation and malignancy. Eur. Cytokine Network 4, 169-180. 

Enbrel is a product now approved for medical use that is based upon this strategy. The product is an engineered hybrid protein consisting of the extracellular domain of the TNF p75 receptor fused directly to the Fc (constant) region of human IgG (see Box 13.2 for a discussion of antibody structure) The product is expressed in a CHO cell line from which it is excreted as a dimeric soluble protein of approximately 150 kDa. After purification and excipient addition (mannitol, sucrose and trometamol), the product is freeze-dried. It is indicated for the treatment of rheumatoid arthritis and is usually administered as a twice-weekly s.c. injection of 25 mg product reconstituted in WFI. Enbrel functions as a competitive inhibitor of TNF, a major pro-inflammatory cytokine. Binding of TNF to Enbrel prevents it from binding to its true cell surface receptors. The antibody Fc component of the hybrid protein confers an extended serum half-life on the product, increasing it by fivefold relative to the soluble TNF receptor portion alone. 

Table 5.7. Cytokines for which soluble receptors have been detected in biological fluids. The exact functional role played by these binding proteins, in modulating cytokine activity, remains to be elucidated in most cases...

Etanercept is a recombinant fusion protein produced in Chinese hamster ovary cells. It consists of the intracellular ligand-binding portion of the human p75 TNF receptor linked to the Fc portion of human immunoglobulin (Ig) Gi. Two p75 molecules are attached to each Fc molecule. Etanercept binds to soluble TNF-a and TNF-(3 and forms inactive complexes, effectively lowering circulating levels of these cytokines. It is administered subcutaneously, generally twice weekly. 

A variation on the basic theme of receptor Tyr kinases is seen in receptors that have no intrinsic protein kinase activity but, when occupied by their ligand, bind a soluble Tyr kinase. One example is the system that regulates the formation of erythrocytes in mammals. The cytokine (developmental signal) for this system is erythropoietin (EPO), a 165 amino acid protein produced in the kidneys. When EPO binds to its plasma membrane receptor (Fig. 12-9), the receptor dimerizes and can now bind the soluble protein kinase JAK (Janus kinase). This binding activates JAK, which phosphory-lates several Tyr residues in the cytoplasmic domain of the EPO receptor. A family of transcription factors, collectively called STATs (signal transducers and activators of transcription), are also targets of the JAK kinase activity. An SH2 domain in STATS binds (P)-Tyr residues in the EPO receptor, positioning it for this phosphorylation by JAK. When STATS is phosphorylated in re-... 

Artificial liver support systems aim at the extracorporeal removal of water soluble and protein-bound toxins (albumin being the preferential binding protein) associated with hepatic failure. Albumin contains reversible binding sites for substances such as fatty acids, hormones, enzymes, dyes, trace metals and drugs [26] and therefore helps elimination by kidneys of substances that are toxic in the unbound state. It should be noticed that the range of substances to be removed is broad and not completely identified. Clinical studies showed that the critical issue of the clinical syndrome in liver failure is the accumulation of toxins not cleared by the failing liver. Based on this hypothesis, the removal of lipophilic, albumin-bound substances, such as bilirubin, bile adds, metabolites of aromatic amino acids, medium-chain fatty acids, and cytokines, should be benefidal to the dinical course of a patient in liver failure. 

Two types of soluble binding protein have been described, nonspecific serum proteins and cytokine-specific soluble receptor proteins. The major nonspecific serum protein capable of binding cytokines appears to be a2-macroglobuIin (B57, B58, Mil). A number of soluble cytokine inhibitors related to the relevant receptor have been described. The soluble form of the IL-2R a chain has been found in the serum of apparently normal individuals and is increased in the serum of individuals with inflammatory diseases such as rheumatoid arthritis (W31). Similar molecules have been described for IL-1, IL-6, IFNy, and IL-7 (F7, N14, S62). The mechanism of release has not been properly established but appears to require proteolytic cleavage of the membrane-bound receptor. Soluble inhibitors for two other cytokines, IL-4 and TNF, appear to be derived by alternative RNA splicing sites that give rise to receptors lacking a transmembrane sequence and that are secreted (M50, S22). 

Cytokine receptors can be divided into two groups those whose intracellular domains exhibit intrinsic protein tyrosine kinase (PTK) activity and those whose intracellular domains are devoid of such activity. Many of the latter group of receptors, however, activate intracellular soluble PTKs upon ligand binding. 

The study of protein function as it applies to covalent and noncovalent molecular changes associated with specific binding is of fundamental interest to those involved with the discovery of therapeutic proteins. Discovering soluble receptors that bind the cytokines that elicit inflammatory immune responses is one of many... 

TNF-a and IL-1 are current targets of antiinflammatory drug therapy. A homotrimer of 17-kDa protein subunits whose effects include the activation of neutrophils and eosinophils, induction of COX-2, induction of proinflammatory cytokines (e.g., IL-1, IL-6), enhancement of endothelial layer permeabihty, induction of adhesion molecules by endothelial cells and leukocytes, stimulation of fibroblast proliferation, degradation of cartilage, and stimulation of bone reabsorption. Two receptors mediate these effects a 55-kDa receptor (p55) and a 75-kDa receptor (p75). Each of these receptors is found in both cell surface and soluble forms. The binding of two or three cell surface receptors to TNF-a initiates an inflammatory response. Soluble p55 also acts as a signaling receptor for inflammatory responses, whereas soluble p75 acts as an antagonist. 

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