2. Summary of the IL-4/-13 Signaling Pathway

IL-4, first described in 1981, is a secreted cytokine that, in its physiologic function, can regulate antibody production, hematopoiesis, and inflammation, and is also involved in the development of effector T-cell responses [19]. The closely related IL-13, first described in 1993, is a human lymphokine that can regulate inflammatory and immune responses [20]. IL-4 and IL-13 are essential for the induction and persistence of the type 2 immune response, and they are associated with multiple atopic diseases, such as asthma and atopic dermatitis [21]. IL-4 and IL-13 are mainly produced by immune cells, such as CD4-T-cells, basophils, eosinophils, and natural killer T (NKT) cells [22].

The structure of IL-4 receptor (IL-4R), IL-13 receptor (IL-13R), and the positions of the intracellular signaling molecules of them have been summarized in several articles [23,24,25,26]. There are three different kinds of IL-4 receptors (Figure 1). IL-4 binds to the IL-4Rα chain, then recruits the IL-2Rγ-common (γc) chain (type I IL-4R) or the IL-13Rα1 chain (type II IL-4R) to form a receptor complex that can initiate signal transduction [25]. The type III IL-4R is formed by all the three chains [27]. IL-13 can also signal via three different receptors (Figure 1). The type II IL-13R complex has the same components as the type II IL-4R [28]. IL-13R type I (IL-4Rα/IL-13Rα1/IL-13Rα2) and type II (IL-4Rα/IL-13Rα1) are expressed in non-hematopoietic cells, while type III (IL-4Rα/IL-13Rα1/γc) is only expressed on the surface of hemocytes [27]. Overall, this results in a possible complex web of IL-4– and IL-13–mediated signaling pathways [29].

Figure 1. Receptor types and signal transduction of IL-4R and IL-13R. There are three different kinds of IL-4 receptors. IL-4 binds to the IL-4Rα chain, then recruits the IL-2Rγ-common (γc) chain (type I IL-4R) or the IL-13Rα1 chain (type II IL-4R) to form a receptor complex that can initiate signal transduction. The type III IL-4R consists of all three chains. IL-13 also have three different receptors. IL-13 receptor type I (IL-4Rα/IL-13Rα1/IL-13Rα2) and type II (IL-4Rα/IL-13Rα1) are expressed in solid tumors, while type III (IL-4Rα/IL-13Rα1/γc) is expressed in hemocytes only. IL-13 binds IL-13Rα1 with a low affinity and complexes with the IL-4Rα (type II receptor). IL-13 binds to the IL-13Rα2 with a high affinity. IL-13 can bind to a soluble IL-13Rα2 receptor, which has no downstream signaling, or bind to transmembrane IL-13Rα2 and activate AP-1. Figure sketch adapted from reference [27].

With regard to the IL-13Rα2 chain, IL-13 is not the sole ligand. For example, chitinase-3-like protein 1 (CHI3L1) could bind to IL-13Rα2 and regulate oxidant injury, apoptosis, and melanoma metastasis [30]. Transmembrane protein 219 and CD44 play an important role in IL-13Rα2 mediated signaling which is induced by CHI3L1 [31,32].

Altogether, IL-4R and IL-13R share two receptor chains (IL-4Rα and IL-13Rα1) and can mediate common, but also diverse biological functions [27]. Both IL-4 and IL-13 phosphorylate and activate signal transducer and activator of transcription (STAT) 6 [27]. STAT3, STAT5, and STAT1 can also be activated, but to a lesser degree [23]. IL-4 can signal through IRS-2 (generally expressed by hematopoietic cells) or IRS-1 (generally non-hematopoietically expressed) [23]. As mentioned above, IL-13 could bind to the IL-13Rα2 chain, which has a very high affinity for IL-13. The downstream signaling involves AP-1 family members c-jun and Fra-2 [33]. IL-13Rα2 can inhibit downstream signals of IL-13R and IL-4R through regulating STAT6 [34,35].

3. IL-13/IL-13R in Gastric Cancer

IL-13Rs are overexpressed in several human solid cancer cell lines [36,37]. Our group demonstrated that IL-13R and IL-4R were expressed in pancreatic cancer cell lines, such as PANC-1, MIAPaCa-2, and CAPAN-1 [38]. Their proliferation was inhibited by Pseudomonas exotoxin (PE) combined to IL-13 or IL-4, demonstrating the receptor’s functionality [38]. IL-13Rα2 is expressed in HS766T and MIAPaCa-2 pancreatic cancer cells, as well [36]. One recombinant chimeric protein IL-13PE was found highly cytotoxic to GC cell line CRL1739, which also expressed the type II IL-4R receptor (Figure 1) binding both IL-4 and IL-13 [39]. IL-13Rα2 is also expressed in GC cell lines MKN-45, AGS and MGC308 [32,40].

Gabitass et al. evaluated plasma IL-13 and IL-4 levels in 131 patients (46 pancreatic cancer, 25 GC, and 60 esophageal cancer) and 54 healthy controls [41]. IL-13 levels in patients’ plasma were significantly higher in all the three cancer patients compared with controls [41]. In another study, Lin et al. evaluated IL-13Rα2 expression in tissue microarrays of 507 GC patients [15]. They found the overexpression of the IL-13Rα2 chain in cancer tissue was associated with poor prognosis after gastrectomy [15].

Chen and coworkers showed that CHI3L1 secreted by M2 macrophage could promote the metastasis of GC cell lines MKN-45 and AGS by binding to the IL-13Rα2 chain [40]. The mechanism is mediated by activating the mitogen-activated protein kinase signaling pathway, which upregulates the matrix metalloproteinase genes [40]. Geng et al. found CD44v3 could bind to both CHI3L1 and IL-13Rα2 in GC cell lines AGS and MGC308 [32]. In this study, CHI3L1 expression was positively related to GC invasion depth and lymph node status in 100 GC tissues from patients [32].

4. IL-4/IL-4R in Gastric Cancer

Human GC cell lines such as CRL1739 express IL-4R [42]. IL-4 inhibited proliferation of HTB-135 GC cells by down-regulating G0-G1 cell cycle nuclear-regulating factors, including retinoblastoma gene product, c-myc, and cyclin D1 [43]. IL-4 could cause G1 phase arrest in the GC cell line CRL 1739 by binding to IL-4Rα and γc (type I IL-4R) [42]. IL-4 could also inhibit the growth of GC cells and this effect was positively related with IL-4R expression level of the respective cell lines [44]. The expression was detected by flow cytometry using biotin-labeled IL-4. It remains unclear, however, what type of IL-4R causing inhibition of GC cell growth was expressed (Figure 1).

Gabitass et al. found plasma IL-4 levels in 25 GC patients were significantly higher than in 54 healthy controls [41]. Cárdenas et al. found serum IL-4 levels in 17 GC patients were significantly elevated comparing with 30 healthy individuals by sandwich ELISA [45]. In their study, elevated serum levels of IL-4 indicated a higher risk of mortality, but there is no statistical association [45]. Orea and co-workers studied a total of 30 biopsies of GC by immunohistochemistry [46]. They found a significantly higher expression of IL-4 in stages I and II than in stages III and IV, pointing to a possible growth inhibitory effect of IL-4 in GC [46].