By Charlotte O'Donnell PhD
As IL-33/ST2 is such a potent inducer of inflammation, it is not surprising that dysregulation of the pathway has been linked to the pathogenesis of several autoimmune disorders. In asthma, IL-33 was shown to be a key driver of lung inflammation, as lymphocyte and eosinophil recruitment and general lung inflammation were reduced in the absence of IL-33. This suggests that the IL-33/ST2 axis could be a potential therapeutic target in asthma. IL-33 is also implicated in rheumatic diseases. Indeed, IL-33 was detected in the serum of rheumatoid arthritis (RA) patients, while being completely absent in healthy controls. The level of circulating IL-33 correlated with IgM and RA-related antibodies. Research to date also suggests that IL-33 has a pro-inflammatory effect in various rheumatological diseases, activating synovial fibroblasts and mast cells in the joints. IL-33 was also increased in the skin lesions of patients with atopic dermatitis compared to non-inflamed skin, thus indicating a role for IL-33 in psoriatic pathology.
IL-33/ST2 in Inflammatory Bowel Disease
IL-33 and ST2 are also implicated in the pathogenesis of inflammatory bowel disease (IBD). Both IL-33 mRNA and protein were found to be upregulated in inflamed ulcerative colitis (UC) and Crohn’s disease (CD) . It has been reported that IL-33 expression was increased in the epithelium of mild to moderate UC. As the intestinal epithelium becomes damaged and mucosal lesions form in severe UC, IL-33 becomes increased in myofibroblasts, where it functions as an alarmin. The expression pattern of ST2 was also altered in IBD. ST2 expression becomes depleted in intestinal epithelial cells (IECs), and is prominent in the lamina propria of IBD patients. The expression of ST2 isoforms is also altered in UC, ST2L becomes reduced, while sST2 is increased compared to CD and control tissue. Circulating levels of both IL-33 and sST2 were also increased in UC patients compared to controls . Expression of IL-33 and ST2 were found to be regulated by TNF, as anti-TNF therapy reduced IL-33 levels and increased sST2 expression in UC patients.
Mouse models of IBD have shown a complex role for IL-33  and ST2 in this disease. Loss of IL-33 or ST2 improved symptoms and reduced intestinal inflammation in the early stage of dextran sodium sulphate (DSS) induced colitis. Contrastingly, however, a further study reported delayed resolution of DSS dependent tissue damage in IL-33-/- mice. Consistent with this finding, treatment with exogenous IL-33 ameliorated chronic DSS colitis. Therefore, IL-33 may have dual roles in intestinal pathology as well as in the maintenance of intestinal homeostasis.
ST2 as a Positive Prognostic Indicator
ST2 has been investigated in many diseases including obesity, atherosclerosis, and cardiovascular disease. Studies to date indicate that sST2 may be useful as a possible prognostic biomarker in cardiovascular disease , as sST2 levels are associated with heart failure severity and poor outcome. Circulating levels of sST2 also appear to decline with improved prognosis in patients with heart failure. Thus, sST2 was recently included in current AHA guidelines for determining risk in acute and chronic heart failure patients. However, concerns were raised regarding the specificity of sST2 as it is also increased in other disease types such as autoimmune disorders.
IL-33/ST2 in Cancer
A link between the IL-33/ST2 signalling axis and tumorigenesis has recently been identified. Immune infiltrates within tumours can positively or negatively influence patient mortality by altering angiogenesis, metastasis and response to therapeutics . Therefore, it is vital to determine the effect of specific cytokines on tumorigenesis. Initially the link between IL-33/ST2 and cancer was identified in breast cancer and most research to date has focused on this cancer type.
Early studies utilising ST2-/- mice demonstrated that ST2 deletion inhibited breast cancer progression and increased the intra-tumoural accumulation of both innate i.e. NK cell and acquired immune cells i.e. CD8+ T-cells and Th1/Th17cytokines, indicating a lack of IL-33 signalling through ST2L promotes a Th1 response. In addition, suppressing sST2 reduced ErbB2-induced cell motility in breast cancer cells. Moreover, breast cancer patients with metastatic disease showed increased levels of circulating sST2 compared to patients with primary tumours. Further studies in breast cancer also showed significantly higher levels of both IL-33 and sST2 in the serum of patients with ER positive breast cancer relative to healthy controls . Moreover, administration of IL-33 to breast cancer-bearing mice showed accelerated tumour growth and increased metastasis. The proposed mechanism responsible for the enhanced tumour growth was the increase in the number of infiltrating immunosuppressive immune cells and innate lymphoid cells, providing further evidence of the role of IL-33 in driving carcinogenesis.
IL-33/ST2 and Breast Cancer
Since the link between IL-33/ST2 in carcinogenesis was first identified in breast cancer, this pathway has now been examined in numerous cancer types. Consistent with a role for IL-33 and ST2 in promoting tumour metastasis and invasion, inhibition of IL-33 and ST2 in glioma cells resulted in reduced tumour growth and colony formation in vitro, and reduced tumour size in vivo. In contrast, an anti-tumorigenic role for IL-33 has been reported in other studies, with IL-33 reduced in the serum of non-small cell lung cancer patients relative to controls , and circulating IL-33 negatively correlating with tumour stage in multiple myeloma patients.
Over-expression of IL-33 in breast and melanoma tumour cells was also observed to result in reduced tumour growth in vivo. Moreover, IL-33 overexpression induced recruitment of CD8+ cells and NK cells to the site of the tumour. Tumour metastasis was similarly attenuated in metastatic models of B16 melanoma and Lewis lung carcinoma cells transplanted into transgenic mice overexpressing IL-33. The mechanism proposed was through the activation of NF-κB signalling, which induced the proliferation, activation and recruitment of CD8+ cells and NK cells, which in turn attenuated pulmonary metastasis in melanoma and lung carcinoma models .
IL-33/ST2 and CRC
In relation to CRC, the IL-33/ST2 pathway has recently been investigated in a number of studies, with these studies being published during the course of my PhD studies. IL-33 was shown to be increased in the epithelium and stroma of CRC as compared to adjacent normal tissue and healthy volunteers. Expression of both IL-33 and ST2 was increased in intestinal adenomas. However, expression of both proteins was lower in the colon tumour cells compared to the intestinal adenoma cells . A recent study by Mertz et al. demonstrated that IL-33 and ST2 expression decreased with increasing tumour grade. In vitro studies have demonstrated that IL-33 stimulation enhanced invasion of primary CRC cells .
Overexpression of ST2 followed by stimulation with IL-33 further increased invasion in a dose dependent manner. Correspondingly, reducing IL-33 and ST2 expression using shRNA targeting both proteins inhibited the increased invasion observed upon IL-33 stimulation. Overexpression of IL-33 by SW620 cells in a nude mouse model reduced survival time. This was reversed using shRNA targeting IL-33 . Furthermore, ST2-/- mice treated with AOM and DSS showed a reduced tumour load compared to WT mice. This is a model of colitis-associated cancer. In an APCMin mouse model IL-33, sST2 and ST2L were increased in polyps compared to normal intestinal mucosa of WT mice. Knocking out IL-33 in the APC Min model reduced polyp number. Therefore, this data suggests that IL-33 may be active during polyp development. The majority of studies to date in CRC point to a pro-tumorigenic role for IL-33/ST2 signalling in CRC.
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- Lu, D.-p., et al., Serum soluble ST2 is associated with ER-positive breast cancer. BMC Cancer, 2014. 14(1): p. 1-8.
- Barrera, L., et al., Cytokine profile determined by data-mining analysis set into clusters of non-small-cell lung cancer patients according to prognosis. Ann Oncol, 2015. 26(2): p. 428-35.
- Gao, K., et al., Transgenic expression of IL-33 activates CD8+ T cells and NK cells and inhibits tumor growth and metastasis in mice. Cancer Letters, 2013. 335(2): p. 463-471.
- Cui, G., et al., Dynamics of the IL-33/ST2 network in the progression of human colorectal adenoma to sporadic colorectal cancer. Cancer Immunology, Immunotherapy, 2015. 64(2): p. 181-190.
- Liu, X., et al., IL-33/ST2 pathway contributes to metastasis of human colorectal cancer. Biochem Biophys Res Commun, 2014. 453(3): p. 486-92.
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