Soluble cytokine receptors

Soluble receptors leave cells producing them and can be active in the cellular microenvironment as well as in body fluids (17, 18). The discovery that membrane bound receptors are also released in body fluids has dramatically changed our understanding of the ligand-receptor interactions as well as, more in general, of the mode of action of hormones and cytokines. Ligand concentrations can be modified by soluble receptors, by down-regulation of the number of membrane bound receptors which are released, and may compete with membrane bound receptors for the ligands, thus effectively reducing the level of free active ligand. In addition, soluble receptors can render cells in tissues able to respond to ligands for which they express an incomplete set of receptor molecules.

Soluble receptors are generated by expression of an alternative mRNA splice which generates an appropriate transcript encoding a soluble isoform, by proteolytic cleavage, or by the action of phospholipase C (i'abie 2). The latter enzyme is active on PI linked receptors and is only involved, to date, in the release of the soluble ciliary neurotrophic factor (CNTF) receptor (19).

Table 2 Mechanisms of generation of soluble cytokine receptors

Proteolytic cleavage

TNF, RI and II

IL-lRti

IL-2R«

IL-6R«

M-CSFR (fms)

C-klt

Alternative splicing9

(IL-lRllj

IL-4Ra

1L-5ROL

GM-CSFRct

(IL-6Ru)

IL-7R

IL-9

LIF-R

IFNu/(3R

c-kit

aThe parenthesis for IL-1RII and L-6R0 indicates that, while an alternative spliced RNA has bean demonstrated, available in wrro information suggests a major role for proteolytic shedding. The soluble form of c-Kit originates from proteolytic cleavage of an alternatively spliced form.

aThe parenthesis for IL-1RII and L-6R0 indicates that, while an alternative spliced RNA has bean demonstrated, available in wrro information suggests a major role for proteolytic shedding. The soluble form of c-Kit originates from proteolytic cleavage of an alternatively spliced form.

Alternative splicing regulates the generation of mRNA encoding soluble forms of various cytokine receptors, including receptors for IL-4, IL-5, IL-6, IL-7, the type IIIL-1 receptor, the interferon a/p receptor, and the GM-CSF receptor.

Alternative splicing yields different soluble isoforms of cytokine receptors via different mechanisms. The simplest mechanism involves exclusion of the exon encoding the transmembrane portion of the receptor. This is the most common mechanism and is exemplified by the GM-CSF receptor a chain. A second mechanism consists of the inclusion in the mature mRNA transcript of an exon called the 'soluble exon', which causes protein chain termination before the transmembrane exon. This is the mechanism of generation of soluble receptors for IL-4, IL-5, and LIF.

The first cytokine receptor for which differential splicing was shown to generate membrane bound and soluble forms of the receptor was the mouse IL-4 receptor. The mRNA encoding soluble version of the IL-4R contains a 114 bp insertion upstream of the transmembrane domain, which resulted in extra six amino acids and premature termination. The resulting soluble receptor lacks the transmembrane and cytoplasmic domains. For the IL-7 receptor, a deletion of the sequences encoding the transmembrane domain alters the translational reading frame, resulting in 27 novel amino acids and premature termination.

Proteolytic cleavage is involved in the generation of soluble forms of the TNF receptor p55 (type I) and p75 (type II), of the IL-1 type II decoy receptor, and of the IL-2 receptor a chain. IL-6 receptor a chain can be released both by proteolysis of the membrane bound isoform or by alternative splicing. The relative contribution of these two mechanisms to the soluble IL-6 receptor found in biological fluids under normal and pathological conditions is unknown. The soluble form of c-kit is generated by both proteolytic cleavage and alternative splicing.

There is evidence that expression of the membrane bound versus soluble isoforms of cytokine receptors can be differentially regulated (20). Release of the type II IL-1 decoy receptor has been extensively investigated. Anti-inflammatory agents (glucocorticoid hormones, IL-4, IL-13) augment expression of the type II decoy receptor and consequently its release (20, 21). It has been calculated that in monocytes exposed to dexamethasone, the number of surface receptors increased from 3 x 103/cell to ~ 12 x 103 over a period of 24 hours and that over the same time ~ 20 x 103 receptors are released (21).

Thus, under these conditions, augmented release is associated with, and dependent on, augmented expression of the membrane bound isoforms and it is gene expression- and protein synthesis-dependent. A second rapid pathway of regulation of the type II receptor release is shared with the TNF receptors. TNF causes rapid shedding of the p55 and p75 receptor as well as of the type II IL-1 decoy receptor. It is of interest that work with specific blocking antibody suggest that the proteolytic cleavage of the p75 receptor is induced by binding of TNF to the p55 receptor.

Chemoattractants and agents which mimic elements in the signal transduction pathway of G-protein coupled receptors (phorbol esters, calcium ionophores) as well as TNF but not other pro- or anti-inflammatory cytokines, cause rapid release of the type 11 decoy receptors as well as of the TNF receptors (22).

The enzyme systems involved in the regulation of shedding of cytokine receptors have not been molecularly identified. There is evidence for the TNF and IL-1 type II decoy receptors that spontaneous release is to some extent dependent upon enzymes belonging to the serine protease group (23, 24). However, recent evidence using inhibitors strongly suggests that matrix metalloproteinases play a majoT role in the activation of release of the type II IL-1 receptor as well as of the IL-6 and TNF receptors (23).

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