Interleukin-13 Receptor Subunit Alpha-2 (IL-13Rα2)
Interleukin (IL)-13 is an immune regulatory cytokine that plays an important role in allergy and atopic diseases (Hershey 2003; Karp 2004). It has two receptors, IL-13 receptor (IL-13R)α1 and IL-13Rα2. IL-13Rα1 monomer binds IL-13 with low affinity and mediates IL-13 cellular functions by pairing with IL-4 receptor (IL-4R)α. The IL-13 binding to IL-13Rα1/IL-4Rα heterodimer leads to activation of signal transducer and activator transcription (STAT)6 by Janus kinases (JAKs) (Miloux et al. 1997). IL-13Rα2 binds to IL-13 with high affinity but doesn’t mediate IL-13 cellular functions due to having a short cytoplasmic tail and no signaling motifs in it (Donaldson et al. 1998). The cDNA for IL-13Rα2 has first been isolated from human renal carcinoma cell lines. In 1996, Caput et al. cloned the IL-13Rα2 cDNA from Caki-1 cell line (Caput et al. 1996). It is regarded as a decoy receptor since IL-13Rα2 prevents the binding of IL-13 to IL-13Rα1 and thereby inhibits IL-13 signaling when overexpressed (Zhang et al. 1997). However, some of the recent studies have illustrated IL-13-dependent and IL-13-independent functions for IL-13Rα2, indicating that the receptor may be more than just a decoy receptor (Cho et al. 2013; Fichtner-Feigl et al. 2006b). IL-13Rα2 is also known as cancer/testis antigen 19 (CT19) since it is one of the testis-specific antigens that show increased expression in high-grade cancers such as glioblastoma (Kawakami et al. 2006). The CT antigens have high immunogenicity and expression in cancers, and therefore they represent promising targets for immunotherapy for cancer.
Sites of Expression, Subcellular Localization, and Splice Variants of IL-13Rα2
Sites of Expression
IL-13Rα2 is narrowly expressed in human tissues, which expression at mRNA level has been found in the placenta, testis, liver, lung, thymus, and brain. The IL-13Rα2 transcript has been found in the spleen, liver, bone marrow, brain, colon, and skin in mice. There are several reports to suggest that IL-13Rα2 is highly expressed in several cancers, including glioblastoma, ovarian carcinoma, breast cancer, prostate cancer, pancreatic cancer, and Kaposi’s sarcoma (Kawakami et al. 2006). It has been shown that the expression of IL-13Rα2 can be stimulated by allergen or Th2 cytokines. In 2003, Zheng et al. showed that inflammatory mediators such as IL-4, IL-10, and IFN-γ (in addition to IL-13) can upregulate the IL-13Rα2 gene expression.
IL-13Rα2 exists as the membrane and soluble forms in mice and just as the membrane form in humans (Kasaian et al. 2011). The membrane form shows localization at the cell surface and on intracellular vesicles, whereas the soluble form shows extracellular localization. In monocytes and respiratory epithelial cells, IL-13Rα2, which exists mostly intracellularly, has been shown to mobilize to the cell surface following interferon (IFN)-γ stimulation (Daines and Hershey 2002). In mouse, the soluble form can be detected in circulation and has been shown to play an important role in regulation of IL-13 bioactivity. However, the soluble IL-13Rα2 has not been detected in human serum or bronchoalveolar lavage fluid (Kasaian et al. 2011).
The human IL-13Rα2 gene is located on chromosome X (Xq13.1-q28) and contains ten exons. Interestingly, murine IL-13Rα2 is also located on the same chromosome. In mouse, the IL-13Rα2 gene transcript undergoes alternate splicing to generate two discrete mRNAs, one encodes the membrane form (the full length) of the receptor and the other is a shorter transcript with a deletion of the exon 10, which encodes the soluble form. Although the homology between human and mouse IL-13Rα2 is conserved from exon 2 to exon 8, it is the downstream exons, which encode the transmembrane and the cytosolic domains, that are divergent. In fact, absence of the soluble form of IL-13Rα2 in humans could be due to low homology between human IL-13Rα2 exon 9 (encodes transmembrane domain) and mouse IL-13Rα2 exons 10 and 11. In contrast, the entire IL-13Rα2 gene is well conserved between human and monkey (Chen et al. 2009; Tabata and Khurana Hershey 2007).
IL-13Rα2 Structure and Domain Organization
The cytokine-binding region of IL-13Rα2 extracellular domain consists of an N-terminal S-type Ig region (D1 loop) followed by two fibronectin III-like regions (D2 and D3 loops) and a WXSWS box, which forms the “cytokine-binding homology region” (CHR) (Fig. 1). IL-13 interacts within the IL-13Rα2 extracellular domain at two sites, termed “site II” and “site III”. The site II is where the four side chains of D helix (K104, K105, F107, and R108) of IL-13 interact. The D3 and D2 loops of IL-13Rα2 form disulfide and hydrogen bonds with IL-13. The site III has an extension of the antiparallel β sheet of the IL-13Rα2 D1 loop by two strands formed by the AB and CD loops of IL-13 (Lupardus et al. 2010).
IL-13Rα2 Cellular Functions and Its Interacting Proteins
Since IL-13Rα2 binds IL-13 with 50 times higher affinity than IL-13Rα1 and lacks any signaling motifs in its short cytoplasmic tail, it acts as a decoy receptor and inhibits any cellular response to IL-13 through the IL-13Rα1/IL-4Rα complex (Donaldson et al. 1998). Upon IL-13 binding, IL-13Rα2 internalizes and recycles back to the cell surface after getting rid of IL-13 in the intracellular recycling compartment (Kawakami et al. 2001a). Thus, the receptor decreases the availability of IL-13 to IL-13Rα1/IL-4Rα complex and thereby suppresses IL-13 biological activities. Kawakami et al. further studied the mechanism of internalization of IL-13-bound IL-13Rα2 (Kawakami et al. 2001b). Their study concluded that a trileucine motif located in the transmembrane domain was essential for the internalization of IL-13 through IL-13Rα2 (Kawakami et al. 2001b).
In addition to functions as a decoy receptor, IL-13Rα2 is suggested of having both IL-13-dependent and IL-13-independent functions. In macrophages as well as mouse model of oxazolone-induced colitis and bleomycin-induced lung fibrosis, IL-13Rα2 has shown to transduce a signal through the activator protein (AP)-1 family members c-jun and Fra-2, which then activates the TGFβ1 promoter, leading to fibrosis (Fichtner-Feigl et al. 2006a). Using high IL-13Rα2-expressing cells (THP-1 and monomac6 cells), it has been found that TGFβ1 promoter activation requires AP-1 (Fichtner–Feigl et al. 2008). An increased expression of TGFβ1, insulin-like growth factor 1 (IGF-1), and early growth-response gene product 1 (EGR-1), which are involved in fibrogenic processes, leads to production of collagen by myofibroblasts, and the process is initiated by AP-1 (Warren et al. 2009). Fichtner-Feigl et al. later found that blocking IL-13Rα2 expression reduces TGFβ1 and collagen production and prevents bleomycin-induced lung fibrosis (Fichtner–Feigl et al. 2008).
It has been reported recently that the inflammatory cytokines TNF-α and IL-17, which are associated with asthma, synergize to induce upregulation of IL-13Rα2 in primary human lung fibroblasts and mouse lungs (Badalyan et al. 2014). In 2013, Andrews et al. showed that the cytoplasmic domain of IL-13Rα2 regulates IL-4 signaling through the IL-13Rα1/IL-4Rα complex. They further showed that IL-13Rα2 is able to inhibit the STAT6 signaling pathway by blocking IL-4 signaling but not IL-13-mediated signaling (Andrews et al. 2013).
Chitinase 3-like-1 (Chi3l1) has recently been demonstrated to bind and signal via IL-13Rα2. In 2013, He et al. showed that Chi3l1 forms a multimeric complex with IL-13Rα2 and IL-13, which regulates inflammasome activation, melanoma metastasis, TGF-β1 production, etc. via macrophage mitogen-activated protein kinase (MAPK), protein kinase B and Wnt/β-catenin signaling, and in IL-13Rα2-dependent manner (He et al. 2013). In 2016, Lee et al. demonstrated that TMEM219, a membrane protein, binds to IL-13Rα2 and recruits the receptor to the cell surface. This study also showed that TMEM219 inhibits IL-13-induced tissue responses just like the decoy receptor IL-13Rα2 (Lee et al. 2016). It has also been shown that IL-13Rα2 requires a scaffold protein FAM120A to signal to the focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI3K) pathways in colon cancer metastasis (Bartolomé et al. 2015).
The binding of IL-13 to IL-13Rα2 has been reported to enhance extracellular signal-regulated kinases 1 and 2 (ERK1/ERK2) and matrix metalloproteinase (MMP) in ovarian cancer. In 2012, Fujisawa et al. identified that stimulation of IL-13Rα2-positive ovarian cancer cells with IL-13 results in upregulation of three different types of MMPs (MMPs 9, 12, and 14), which are the key mediators of cancer invasion and metastasis (Fujisawa et al. 2012). In 2015, Zhao et al. found that the photoreceptor cell-specific nuclear receptor (PNR/NR2E3), an orphan nuclear hormone receptor, activates IL-13Rα2 gene expression, leading to an increase in migration and metastasis of breast cancer cells (Zhao et al. 2015). Also in 2015, Papageorgis et al.’s study indicated that IL-13Rα2 is overexpressed in breast tumors from patients who developed lung metastases and that targeting IL-13Rα2 high-expressing breast cancer cells leads to enhanced STAT6 phosphorylation and thereby the upregulation of TP63, a metastasis suppressor gene (Papageorgis et al. 2015).
The Emerging Role of IL-13Rα2 in Disease
A link between high concentration of the soluble IL-13Rα2 in blood serum and patients having parasitic infections has been reported. In 2004, Mentink-Kane et al. reported elevated levels of IL-13Rα2 in humans with Schistosomiasis mansoni infection, concluding that IL-13Rα2 can be a signal for chronic and pernicious inflammation in schistosomiasis (Mentink-Kane et al. 2004). A reduction in tissue fibrosis and collagen deposition has been observed in mice treated with soluble IL-13Rα2, indicating that inhibiting IL-13 may prevent damaging tissue fibrosis (Chiaramonte et al. 1999). Consistent with this, IL-13Rα2-deficient mice were found to be more sensitive to Schistosomiasis mansoni infection (Mentink-Kane et al. 2004).
The expression of IL-13Rα2 has been found to be significantly reduced in the airway fibroblast asthma patients (Plante et al. 2006). This may be one of the reasons why asthmatic patients are highly responsive to IL-13. Consistent with this, IL-13Rα2 knockout (KO) mice have been shown to exhibit enhanced IL-13 responsiveness (Wood et al. 2003). Wood et al. showed that IL-13Rα2-deficient mice exhibit increased levels of bone marrow macrophage progenitors and enhanced levels of serum immunoglobulin (Ig)A, IgE, IgG2a, and IgG2b, suggesting that IL-13 can enhance these Ig levels in vivo. IL-13Rα2 has also been associated with systemic sclerosis. A study has found that gene polymorphism of the receptor results in susceptibility to systemic sclerosis (Granel et al. 2006).
IL-13Rα2 has shown to be overexpressed in a variety of malignancies, including brain tumors, renal cell carcinoma, squamous cell carcinoma of the head and neck, ovarian cell carcinoma, pancreatic cancer, and breast and prostate cancer (Fujisawa et al. 2009; Gonzalez-Moreno et al. 2005; Jarboe et al. 2007; Kawakami et al. 2003; Kioi et al. 2006a, b; Puri et al. 1996; Zhao et al. 2014). Furthermore, it is one of the CT antigens (CT19) that are expressed in testis and highly immunogenic (Kawakami et al. 2006). Therefore, IL-13Rα2 has gathered a lot of interest as a possible drug target for treating cancer. Several previous studies found that IL-13Rα2 is overexpressed in ∼75% of WHO grade IV glioblastoma multiforme (GBM) patients (Wykosky et al. 2008). A cytotoxic drug composed of IL-13 and a modified bacterial toxin (Pseudomonas exotoxin 38) (IL-13-PE38) has been developed for GBM therapy (Kioi et al. 2004).
A goat polyclonal antibody (AF146) against human IL-13Rα2 has been raised and used in the recognition of protein by Western blotting (WB) by R&D Systems (Lee et al. 2016). There are numerous commercially available monoclonal and polyclonal antibodies to IL-13Rα2. A mouse monoclonal antibody (clone 2K8) of human IL-13Rα2 is available from Santa Cruz (sc-134363). This antibody can be used for detection of endogenous IL-13Rα2 by WB and immunocytochemistry (ICC) or immunofluorescence (IF). A mouse monoclonal antibody of human IL-13Rα2 is available from Sigma-Aldrich (WH0003598M1-100UG), which can also be used for WB.
IL-13Rα2 is a monomeric receptor that binds to IL-13 with high affinity. IL-13Rα2 is 380aa in length and has a molecular weight of approximately 56 kDa. Its gene is located on chromosome X (Xq13.1-q28). Structurally, this receptor consists of an N-terminal signal peptide followed by a large extracellular domain, a transmembrane domain, and a small cytoplasmic domain with no known signaling motifs. Although there is no evidence to suggest the existence of splice variants for human IL-13Rα2, there are two splice variants (membrane and soluble forms) for mice IL-13Rα2. The receptor is considered as a “decoy” receptor, which sequesters IL-13 from IL-13Rα1/IL-4Rα complex. However, recent reports have established IL-13-dependent and IL-13-independent functions for IL-13Rα2 and the importance of its interacting proteins for these cellular functions. It is overexpressed in various malignancies, including brain tumors, renal cell carcinoma, and pancreatic cancer, indicating IL-13Rα2 as a potential biomarker and therapeutic target for high-grade cancer. IL-13Rα2 has been shown to play a role in airway inflammation and also in inflammation associated with parasitic infections. In both occasions, IL-13Rα2 is upregulated and plays an important role in inflammation control, preventing further damage to tissues. There is no doubt that future studies on IL-13Rα2 will better our understanding of mechanisms of its expression in health and disease.
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