Research progress on the role of IL‑37 in gynecological and obstetrics‑related diseases (Review)

Interleukin (IL)-37 was first identified in 2000 and officially designated interleukin-IL-37 in 2010 ( 1 ). The IL-1 family comprises 11 members, most of which are pro-inflammatory cytokines ( 1 ). In contrast to its pro-inflammatory counterparts, IL-37 exhibits potent anti-inflammatory ( 2 - 4 ) and metabolic regulatory effects ( 5 , 6 ) in various inflammatory diseases, autoimmune disorders and cancers. The human IL-37 gene is located on chromosome 2, spans ~3.617 kb and contains six exons. Alternative splicing generates five distinct splice variants (IL-37a-e) ( 2 ). Among these, IL-37b, which comprises exons 1, 2, 4, 5 and 6, with only exon 3 absent and encodes 218 amino acids, is the longest and best-characterized isoform and is generally regarded as the principal biologically active form; IL-37a lacks exons 1 and 2, IL-37c lacks exons 3 and 4, IL-37d lacks exons 2 and 3 and IL-37e lacks exons 2-4 ( 2 , 7 ). Exons 1 and 2 of IL-37b encode an N-terminal pro-domain that contains a caspase-1 cleavage site, which is removed upon cytokine maturation. Exons 4-6 encode a canonical 12 β-strand trefoil structure characteristic of the IL-1 family cytokine domain, which mediates binding to the IL-18 receptor α chain (IL-18Rα) ( 8 , 9 ), suggesting that the IL-37a, b and d subtypes all have biological functions, while IL-37c and e do not. Despite extensive sequencing and database searches of the IL-1 gene locus, no murine homolog of IL-37 has been identified to date. Consequently, IL-37 transgenic (IL-37tg) mice have become an essential model for investigating the role of IL-37 in disease pathophysiology. IL-37 exists in both monomeric and dimeric forms in vivo , with molecular weights of ~22 and 42 kDa, respectively ( 10 , 11 ). Native IL-37 forms a head-to-head homodimer that is considered autoinhibited ( 12 ), as dimerization restricts its interaction with IL-18Rα or IL-1R8. By contrast, the monomeric form exhibits significantly enhanced anti-inflammatory activity in various cell types ( 10 , 12 , 13 ). Based on these findings, it may be speculated that the paradox of elevated IL-37 alongside ongoing inflammation in certain inflammatory diseases may stem from partial homodimerization and self-inhibition of tissue-accumulated IL-37. Nevertheless, current studies merely quantify total IL-37 abundance, lacking sufficient evidence for its homodimerization and self-inhibitory effects in inflammatory diseases. In addition, the anti-inflammatory activity of IL-37 may also be regulated by microenvironmental factors. Can microenvironmental factors alter the balance between monomers and dimers? The above issues need to be experimentally verified. The biological activity of IL-37 is regulated at multiple levels, including its production, receptor binding, proteolytic processing and signal transduction. A comprehensive understanding of these fundamental characteristics will enhance the current insight into the therapeutic potential of IL-37 in gynecological and obstetric diseases. IL-37 is constitutively expressed in various human cell types, including monocytes, activated B cells, natural killer (NK) cells, adipocytes and epithelial cells, and in tissues such as the thymus, lung, lymph node, breast, intestine, placenta and uterus ( 9 ). This basal expression is attributed to a conserved 10-nucleotide sequence element within the IL-37 coding sequence that partially overlaps with known or putative protein-binding sites involved in mRNA destabilization. These instability elements promote rapid degradation of IL-37-specific mRNA in unstimulated cells ( 14 ). Experimental evidence has shown that in stably transfected RAW264.7 mouse macrophage cells, no IL-37 protein expression was detected despite the presence of an active CMV promoter ( 9 , 14 ). A similar phenomenon was observed in IL-37 transgenic mouse models ( 15 ). However, IL-37 expression is closely associated with the inflammatory microenvironment and the presence of inflammatory cells ( 16 ). Upon inflammatory stimulation, IL-37 expression is markedly upregulated ( 2 ). For instance, lipopolysaccharide (LPS) stimulation strongly induces IL-37 expression in monocytes and macrophages in vitro ( 1 ). In addition to LPS, pro-inflammatory mediators such as IL-1β, IL-6, TNF-α and TGF-β1, have also been shown to effectively induce endogenous IL-37 expression in tissue cells ( 1 ). Notably, despite low constitutive expression, IL-37 levels are sufficient to exert potent immunoregulatory and anti-inflammatory effects. This represents a protective mechanism that limits excessive tissue damage and uncontrolled inflammation ( 11 ). Further studies have demonstrated that picomolar concentrations of IL-37 elicit robust anti-inflammatory responses in mice ( 17 , 18 ), whereas higher concentrations promote dimerization and autoinhibition, thereby attenuating its biological activity ( 10 , 13 ). In various gynecological and obstetric diseases as well as gynecological tumors, IL-37 expression levels vary, being either upregulated or downregulated ( Table I ). For instance, IL-37 levels are elevated in endometriosis, HIV infection and non-metastatic breast cancer, whereas they are decreased in adenomyosis, polyendocrine metabolic ovarian syndrome and endometrial carcinoma ( Table I ), suggesting context-dependent regulatory roles. Similar to Toll-like receptors (TLRs), members of the IL-1 receptor (IL-1R) family are structurally characterized by extracellular immunoglobulin-like domains and an intracellular Toll-IL-1 receptor (TIR) domain ( 19 ). The canonical signaling mechanism involves an IL-1 family ligand binding to a primary IL-1 receptor chain, which subsequently recruits a second IL-1 receptor chain (co-receptor chain) to initiate signal transduction ( 19 ). For instance, IL-37 belongs to the IL-1 ligand family, whereas IL-18Rα (also known as IL-1R5) and IL-1R8 are members of the IL-1R family ( 20 ). The anti-inflammatory activity of IL-37 requires simultaneous engagement of both receptors: IL-18Rα serves as the ligand-binding chain for IL-37, while IL-1R8 functions as the co-receptor ( 21 , 22 ). IL-1R8 (also termed TIR8 or SIGIRR) is a cell-membrane receptor that is essential for the signal transduction of extracellular IL-37 ( 23 , 24 ). Upon proteolytic cleavage, this receptor yields soluble IL-1R8 (sIL-1R8), which functions as an IL-37 antagonist. It blocks IL-37 binding to membrane-bound receptor complexes, thereby attenuating IL-37-mediated immune regulation and anti-inflammatory activities. IL-18 binding protein (IL-18BP) is a high-affinity decoy receptor for IL-18 and functions as a natural antagonist of IL-18-mediated signaling ( 25 ). Notably, IL-18BP can bind not only to IL-18 but also to the anti-inflammatory cytokine IL-37 ( 26 , 27 ). Structurally, IL-37 shares two conserved amino acid residues with IL-18: Glu-35 and Lys-124 in IL-37 correspond to Glu-35 and Lys-89 in IL-18, respectively. These residues are critical for the interaction between IL-37 and IL-18BP ( 2 , 27 ). IL-37 binds to IL-18BP through these conserved sites, similar to IL-18, and subsequently associates with IL-18Rβ to form an IL-18BP-IL-37-IL-18Rβ ternary complex. This interaction prevents IL-18Rβ from participating in IL-18 signaling, thereby exerting an inhibitory effect ( 27 - 29 ). Small mother against decapentaplegic (SMAD) family 3 (SMAD3) is a member of the SMAD family of intracellular signal transducer proteins ( 30 ). In the canonical SMAD signaling pathway, phosphorylated Smad2 and Smad3 associate with Smad4 to form a trimeric complex (Smad2/3/4), which translocates to the nucleus to regulate gene expression. Smad2 and Smad3 can also form functional dimers with Smad4 (Smad2/4 and Smad3/4) ( 31 ). Notably, although IL-37 possesses only a weak nuclear localization sequence, it can compete with Smad2 or Smad4 for binding to Smad3, thereby facilitating nuclear translocation of the IL-37/Smad3 complex ( 31 ). This complex promotes the production of anti-inflammatory factors and alleviates inflammatory responses ( 32 ), while simultaneously inhibiting multiple signaling pathways and the metastatic phenotype of tumor cells ( 31 ). In addition, a report have shown that intracellular IL-37 interferes with the TGF-β1 signaling cascade by interacting with Smad3, thereby attenuating liver inflammation and fibrosis ( 33 ). Another research group found that intervention with Smad3-specific small interfering RNA significantly weakened the inhibitory effect of IL-37 on IL-17, indicating that the TGF-β1-Smad3 pathway is involved in mediating the inhibitory action of IL-37 on IL-17 ( 1 ). Although the above studies have demonstrated that IL-37 can reduce the overall output of the TGF-β1 pathway, they have not provided direct experimental evidence that inhibition of the SMAD2/SMAD3/SMAD4 complex mediates IL-37-induced suppression of TGF-β1 signaling. Therefore, a direct causal link between Smad2/3/4 complex inhibition and IL-37-mediated suppression of TGF-β1 signaling remains to be experimentally established, and this represents an important direction for future research. A 2010 study published in Nature Immunology ( 1 ) performed immunofluorescence staining with phosphorylated Smad3 antibodies and immunoprecipitation assays with non-phosphorylated Smad3 antibodies. The authors verified that IL-37b is capable of binding simultaneously to both phosphorylated and non-phosphorylated Smad3 in A549 and THP-1 cells. Consistent with these findings, Luo et al ( 31 ) further validated the interaction between IL-37 and Smad3 via immunoprecipitation experiments using non-phosphorylated Smad3 antibodies. Multiple review studies have also clarified that the binding of IL-37 to intracellular phosphorylated Smad3 promotes the nuclear translocation of IL-37 ( 4 , 11 , 32 ). These existing findings support two plausible mechanistic hypotheses: First, mature IL-37 may bind to non-phosphorylated Smad3 in the cytoplasm, thereby suppressing Smad3 activation and subsequent nuclear translocation. Second, IL-37 may interact with phosphorylated Smad3 to translocate into the nucleus and inhibit the transcription of downstream target genes. Nevertheless, these hypotheses remain to be biochemically validated. Specifically, phosphorylation-specific Smad3 antibodies are required to precisely identify the exact binding form of Smad3 that interacts with mature IL-37 in cells. Caspase-1 is a key protease required for the nuclear translocation of IL-37. The caspase-1 cleavage site is located between Asp20 and Glu21 within exon 1 of IL-37 ( 2 , 34 ). Newly synthesized IL-37 is produced as a precursor peptide, which is subsequently cleaved by caspase-1 in the cytoplasm to generate the mature form ( 34 ). Both the precursor and mature forms of IL-37 are biologically active and can be released from cells via an unconventional secretory mechanism ( 35 ). IL-37 suppresses inflammation and modulates cellular function through three distinct pathways: Two extracellular and one intracellular ( Fig. 1 ) ( 4 , 19 , 28 ). The first pathway involves IL-37 binding to the IL-18Rα/IL-1R8 receptor complex. On the cell surface, IL-37 binds to IL-18Rα and subsequently recruits IL-1R8 to form a tripartite complex. This assembly prevents the recruitment of the proximal MyD88 innate immune signal transduction adaptor, which is essential for signal transduction by IL-1, IL-18 and all TLRs. Consequently, downstream signaling cascades, including the MAPK, NF-κB and JNK pathways, as well as Fyn tyrosine protein kinase and TGF-β-activated kinase 1 activation, are inhibited. Simultaneously, this pathway activates anti-inflammatory mediators such as Mer tyrosine kinase and phosphatase and tensin homolog (PTEN), and induces p62 to further suppress inflammatory responses ( 19 ). Additionally, IL-37 promotes fatty acid oxidation and attenuates obesity progression by activating the AMPK signaling pathway ( 5 ). By attenuating the activation of ERK1/2, JNK and IKKα/β-kinases that promote insulin resistance by inhibiting insulin signaling-IL-37 enhances insulin sensitivity ( 5 ). IL-37 has also been shown to inhibit the growth of endometriotic lesions via the Akt and ERK1/2 signaling pathways, whereas IL-37 silencing produces the opposite effect ( 36 , 37 ). Studies in IL-1R8-deficient mice demonstrate that IL-37 fails to inhibit inflammasome activation, underscoring the essential role of IL-1R8 signaling in mediating the effects of IL-37 ( 19 , 37 ). The second pathway involves IL-37 binding to IL-18BP and subsequently recruiting IL-18Rβ to form an IL-37-IL-18BP-IL-18Rβ ternary complex ( 4 , 20 , 27 , 28 ). IL-18BP is present in the extracellular space, where it binds to soluble mature IL-18, thereby preventing IL-18 from engaging its receptors and effectively neutralizing IL-18 activity ( 11 , 26 ). Notably, IL-37 binding to IL-18BP does not impair IL-18BP-mediated inhibition of IL-18; rather, it potentiates this inhibitory effect ( 20 ). In human NK cell lines, IL-37 enhances IL-18BP-mediated neutralization of IL-18 activity by 25-30%, thereby downregulating IL-18-induced IFN-γ production ( 26 ). The formation of the IL-37-IL-18BP-IL-18Rβ complex sequesters IL-18Rβ, preventing its incorporation into the IL-18-IL-18Rα-IL-18Rβ trimeric signaling complex. This effectively blocks IL-18 signal transduction and facilitates the anti-inflammatory actions of IL-37 ( 27 ). The third pathway entails the binding of mature IL-37 to Smad3 in the cytoplasm ( 4 , 28 , 32 ). The C-terminal domain of mature IL-37 interacts with Smad3 and phosphorylated Smad3 (p-Smad3) facilitates the nuclear translocation of IL-37. Once in the nucleus, the IL-37/p-Smad3 complex suppresses multiple downstream signaling pathways, including ERK, JNK, p38 MAPK, PI3K, NF-κB and STAT3, thereby inhibiting inflammatory responses and exerting anti-tumor effects ( 31 ). IL-37 attenuates inflammation and mitigates sepsis-induced lung injury by modulating TGF-β and Smad3 signaling ( 38 ). Importantly, the D20A mutation (replacement of Asp20 with Ala) renders IL-37 resistant to caspase-1 cleavage, abolishes Smad3 binding and prevents nuclear translocation ( 39 ). Similarly, pharmacological inhibition of caspase-1 in IL-37tg mice impairs IL-37 function ( 39 ). Recent findings demonstrate that peritoneal macrophages from IL-37tg mice exhibit a 40-50% reduction in LPS-induced inflammatory cytokine production (IL-1β, IL-6, TNF-α and IFN-γ), whereas macrophages from IL-37-D20A-tg transgenic mice fail to exhibit this inhibitory effect ( 39 ). Furthermore, IL-37tg mice with Smad3 deficiency exhibit markedly attenuated anti-inflammatory responses following LPS stimulation compared to controls ( 2 ). Collectively, these findings confirm the critical role of Smad3-mediated nuclear translocation in the anti-inflammatory activity of IL-37. In summary, IL-37 modulates the expression, phosphorylation and dephosphorylation of key signaling molecules through three distinct pathways, leading to significant alterations in fundamental biological processes, including cell proliferation, differentiation, angiogenesis, metabolism and inflammation.

In addition to its anti-inflammatory and anti-metabolic disorder properties, IL-37 also exhibits anti-tumor activities. Recent studies have demonstrated progress in understanding the role of IL-37 in gynecological tumors, including breast cancer, cervical cancer, epithelial ovarian cancer and endometrial carcinoma (see Table III ). Breast cancer is one of the most common malignancies in women and a leading cause of cancer-related death ( 114 ). However, the precise immunopathological mechanisms underlying this disease remain elusive. Previous studies have shown that when recombinant adenoviral vectors containing human IL-37 cDNA fragments were transduced into 4T1 cells (a breast cancer cell line), the resulting IL-37-expressing 4T1 cancer cells formed smaller tumors upon implantation into the mammary fat pads compared with control groups ( 115 ). Further experiments indicated that IL-37 inhibits the growth of 4T1 breast cancer cells by promoting the activation and proliferation of CD4+ T cells and modulating the tumor microenvironment ( 115 , 116 ). Extensive research has demonstrated that breast cancer exhibits metastatic heterogeneity, resulting in variable prognosis and treatment response among patients. Studies have shown that estrogen receptor-positive/human epidermal growth factor receptor 2-positive (ER+/HER2+) and estrogen receptor-negative/HER2-positive (ER-/HER2+) breast cancers exhibit distinct metastatic patterns. Patients with ER-/HER2+ breast cancer had significantly higher rates of multiple metastases and lower survival rates than those with ER+/HER2+ breast cancer ( 117 ). Another study revealed that the relative expression level of IL-37 in blood samples of ER+/progesterone receptor+/HER2+ patients (non-metastatic breast cancer) was significantly higher than that in HER2+ patients ( 118 ). Furthermore, compared with healthy controls, both patients with metastatic and non-metastatic breast cancer had significantly lower CD8+ T-cell counts in blood samples ( 118 , 119 ). These findings suggest that measuring IL-37 levels and CD8+ T-cell counts may help elucidate their relationship with breast cancer metastasis and inform the development of novel treatment strategies. MDA-MB-231 and MCF-7 are two additional human breast cancer cell lines. The former is metastatic and exhibits low IL-37 expression, whereas the latter is non-metastatic and shows higher IL-37 expression ( 31 ). Low IL-37 expression in tumors is significantly associated with poor prognosis in cancer patients. MDA-MB-231 cells overexpressing IL-37b via transgenic technology exhibited significantly decreased expression of inflammatory factors, integrins and MMP-9, thereby suppressing tumor cell invasiveness and metastasis. Conversely, IL-37b knockdown in MCF-7 cells enhanced these malignant phenotypes ( 31 ). Therefore, IL-37 may represent a potential therapeutic target for tumors, particularly metastatic tumors. CC is the fourth most frequent cancer in women worldwide, second only to breast cancer ( 120 , 121 ). Human papillomavirus infection and chronic inflammation are considered major pathogenic factors in the development of CC ( 122 ). IL-37 expression is decreased in CC and this reduction is associated with poor prognosis of the disease. Previous studies have shown that IL-37 inhibits the proliferation and invasion of CC cells (HeLa and C33A cells) by suppressing STAT3 expression and STAT3 phosphorylation ( 123 ). Further research suggested that activation of runt-related transcription factor 2 (RUNX2) in cancer cells is associated with tumorigenesis, metastasis and invasion ( 124 ). In Siha and C33A CC cells transfected with IL-37, cancer cell invasion was significantly inhibited and RUNX2 expression (at both the mRNA and protein levels) was markedly reduced, indicating that the inhibitory effect of IL-37 on the invasion of these two cell lines is mediated through downregulation of RUNX2 expression ( 124 ). Cell apoptosis is considered a tumor-suppressive mechanism. Bim, a pro-apoptotic molecule consisting of 198 amino acids, plays a key role in this process ( 120 ). IL-37 has been shown to upregulate Bim expression and induce apoptosis in CC cell lines HeLa and C33A ( 120 ). Additionally, a recent study demonstrated that IL-37 can suppress tumor immune evasion by downregulating CD47 expression in CC cells, thereby enhancing macrophage-mediated phagocytosis of tumor cells ( 125 ). In summary, IL-37 inhibits the progression of CC by regulating the expression of factors involved in cancer cell proliferation, invasion, apoptosis and immune escape. Epithelial ovarian cancer is an aggressive malignancy, and one of the primary factors contributing to its high mortality rate is the advanced stage of the disease at the time of diagnosis ( 126 ). Therefore, exploring novel approaches for molecular diagnosis and molecular therapy is particularly important for early detection and treatment. Huo et al ( 127 ) reported that serum IL-37 protein levels in patients with epithelial ovarian cancer were significantly higher than those in healthy individuals, and patients with International Federation of Gynaecology and Obstetrics (FIGO) stage III/IV epithelial ovarian cancer exhibited markedly higher IL-37 levels than those with FIGO stage I/II disease. Elevated serum IL-37 levels in patients with ovarian cancer were associated with adverse survival outcomes. Furthermore, analysis revealed that IL-37 expression levels were positively correlated with tumor size, lymph node metastasis and recurrence ( 127 ). These findings suggest that IL-37 may serve as a clinically useful indicator of epithelial ovarian cancer progression and as a potential biomarker for predicting patient survival prognosis. However, to the best of our knowledge, to date, no reports have addressed the precise biological role of IL-37 in epithelial ovarian cancer. Endometrial carcinoma is one of the most common gynecological malignancies ( 128 ). Like other tumor cells, endometrial cancer cells possess migratory and invasive properties ( 129 ). In vitro experiments have demonstrated that IL-37bΔ1-45 (the mature form of IL-37b) significantly inhibits the malignant phenotype of endometrial cancer cells, including invasion, migration and metastasis; furthermore, in vivo experiments have yielded consistent results ( 124 ). MMP2 contributes to both tumor growth and metastasis. Further experiments have shown that IL-37bΔ1-45 markedly reduces MMP2 expression levels in endometrial cancer cells and suppresses their migration and invasion by targeting the Rac1/NF-κB/MMP2 signaling pathway ( 11 , 129 ).

Substantial evidence indicates that chronic inflammatory stimuli and the resulting inflammatory microenvironment may promote the development of gynecological and obstetric diseases as well as gynecological tumors. Therefore, understanding the occurrence and progression of these related diseases is crucial for developing interventions to promote women's health. IL-37 plays an important role in the diagnosis and treatment of gynecological and obstetric diseases and gynecological tumors by inhibiting the expression of pro-inflammatory cytokines, blocking excessive inflammatory responses and regulating gene transcription, cellular metabolism and cell proliferation through both intracellular and extracellular mechanisms. For instance, endometriosis exhibits tumor-like features and IL-37 suppresses excessive inflammatory responses through multiple signaling pathways, significantly reducing the number, size and weight of ectopic lesions. In patients with PMOS, IL-37 inhibits inflammation, reduces body weight, increases circulating adiponectin levels and improves insulin resistance, glucose tolerance and ovulatory dysfunction. In spontaneous preterm birth, IL-37 plays a protective role by maintaining local immune balance, thereby helping to prevent preterm delivery or pathological uterine contractions, cervical ripening and membrane activation/rupture. In HIV-infected individuals, IL-37 inhibits viral replication. In several gynecological tumors, including breast cancer, CC and endometrial carcinoma, IL-37 suppresses tumor progression by regulating the biological functions of cancer cells, inhibiting proliferation, migration, invasion and angiogenesis. Compared with healthy individuals, the expression levels of IL-37 are significantly altered in patients with gynecological diseases and tumors. This characteristic may serve as a valuable biomarker for disease diagnosis and prognostic prediction. In recent years, research on IL-37 has garnered increasing attention in this field. After a systematic literature search, no existing evidence was found regarding the expression or regulation of other IL-37 subtypes in gynecological diseases. Owing to limitations in current detection technologies, almost all clinical specimens obtained from gynecological patients-including serum, peritoneal fluid, eutopic/ectopic endometrium and cell culture supernatants-are only capable of detecting total IL-37. Similarly, almost all mouse specimens, derived from disease models injected with rhIL-37b protein, rhIL-37 transgenic mice and mouse tissues or cells treated with rhIL-37b protein, have been analyzed for IL-37b exclusively. These findings collectively demonstrate the roles of IL-37 and IL-37b in various disease contexts. It is well established that IL-37b is the most structurally complete and biologically active isoform among all IL-37 subtypes. Although the detection of total IL-37 can indirectly reflect the levels and functions of IL-37b, the potential contributions of other IL-37 subtypes cannot be excluded. To specifically identify distinct IL-37 subtypes, the use of subtype-specific primers, subtype-specific antibodies and subtype-specific ELISA kits would be required. With continued advances in detection methodologies, this limitation is expected to be overcome in the future. Future efforts should focus on expanding the scope of clinical studies, further validating the clinical utility of IL-37 as both a biomarker and a therapeutic target and providing robust evidence to support its development as a novel therapeutic agent.