Interleukin-13 receptor


The interleukin-13 receptor is a type I cytokine receptor, binding Interleukin-13. It consists of two subunits, encoded by IL13RA1 and IL4R, respectively. These two genes encode the proteins IL-13Rα1 and IL-4Rα. These form a dimer with IL-13 binding to the IL-13Rα1 chain and IL-4Rα stabilises this interaction. This IL-13 receptor can also instigate IL-4 signalling. In both cases this occurs via activation of the Janus kinase /Signal Transducer and Activator of Transcription pathway, resulting in phosphorylation of STAT6. Phosphorylated STAT6 dimerises and acts as a transcription factor activating many genes, such as eotaxin.
There is also another receptor that can bind IL-13: IL-13Rα2 encoded by the IL13RA2 gene. This binds IL-13 with very high affinity but does not allow IL-4 binding. It acts as a negative regulator of both IL-13 and IL-4, however the mechanism of this is still undetermined.

Function

Interleukin 13 is an effector cytokine partially sharing the signaling pathways with IL-4 due to the utilization of a common receptor system. A “private” receptor system, binding specifically IL13 with high affinity, seems to use different signalling pathways and is increasingly studied for its interest as novel potential prognostic factor, biomarker or therapeutic target in different types of cancer.

The “shared” IL-4 / IL-13 receptor

IL-13 uses IL-4 receptor type II, a complex formed by an IL-4Rα chain and an IL-13Rα1 chain. Initially the ligand, IL-4 or IL-13, bind to IL-4Rα chain and IL-13Rα1 respectively; thereafter, a secondary chain will also bind, forming the complete IL-4RII. The complex IL-4/IL-4Rα however, can also bind a different secondary chain, the IL-2Rγc, forming the IL-4 receptor type I. In non-hematopoietic cells, IL-2Rγc is poorly expressed, on the other hand IL-13Rα1 is poorly expressed in lymphocytes but abundantly in all non-hematopoietic cells; myeloid cells express both of them to a certain degree. This different distribution of secondary chains accounts for the difference distribution of completed receptors, being IL-4RI prevalently expressed in lymphocyte, and IL-4RII prevalently in non-hematopoietic cells. Consequently, only IL-4, through IL-4R1, is able to modulate the function of lymphocytes inducing Th2 polarisation and B cells IgG1/IgE class switching, while IL-13 is mainly acting on myeloid cells and non-hematopoietic cells, having strong effects on mucus production, smooth muscle contraction, epithelium permeabilisation.
After the complete assemblage, the conformational changes in IL-4RI or IL-4RII tails leads to the intracellular signaling, starting with the auto and cross-phosphorylation of associated Jak kinases, and followed by phosphorylation of intracellular domains of IL-4Rα in critical Y residues which are therefore activated to form the docking sites for downstream signalling molecules endowed with SH domains. While the docking sites in IL-4R1 are able to efficiently activate both STAT6 and IRS2 signalling molecules, IL-4RII only activates effectively STAT6. Activated STAT6 molecules form dimers which translocate to the nucleus to bind responsive elements
The binding affinity of IL-4 for IL-4Rα is much higher than IL-13 for the IL-13Rα1, hence IL-4 would out-compete IL-13 for receptor availability within IL4R2 at parity of concentration.

The “private” IL-13 receptor

Besides IL-13Rα1 chain. It presents an extraordinary affinity to IL-13, but does not form complexes with any secondary chain.
Because of the apparent lack of signaling domain and the short tail, it has been initially thought not to have any signaling activity, and regarded as “decoy” receptor, that is its function would just consist in competing for IL-13 binding and neutralizing his effect.
Indeed, it has been shown that IL-13Rα2 blocks IL-13 driven STAT6 signalling by binding IL-13 with high affinity, however a partial block is also extending to IL-4 driven STAT6 signalling, presumably due to the cytoplasmic domain interfering with the assembling of IL-4/IL-4Rα with a secondary chain.
However, increasing evidences are accumulating that IL-13Rα2 is more than a “decoy”. IL-13 signalling through IL-13Rα2 and AP1-driven TGF-β production has been initially reported in monocytes and then confirmed in mouse models. According these studies, IL-13, through the over-expression of IL-13Rα2 would be able to activate AP-1 signalling and production of TGF-β, driving pro-fibrotic effects. Some recent works is evidencing how a wide range of signals can be actually activated by this receptor in normal or pathologic environments. How IL-13Rα2 might overcome the limitation of a 17 aminoacids short tail lacking any signalling motif, it is not clear yet but it has been shown that, at least in some cases, the association with other receptors or signalling adaptors can do the trick.