The Biological Effects of Interleukin-6 and Their Clinical Applications in Autoimmune Diseases and Cancers

Interleukin-6 (IL-6) is one of the pro-infl ammatory cytokines involved in pathogenesis of various autoimmune and chronic infl ammatory diseases. IL-6 through binding to its cellular receptor can transduce both classicaland trans-signaling pathways. Overproduction of circulating IL-6 can be detected in patients with different autoimmune diseases. Tocilizumab, a humanized monoclonal antibody against IL-6 receptor, can block IL-6-mediated signaling and has been approved for the treatment of rheumatoid arthritis and Castleman’s disease. Besides, expression of IL-6 may promote tumorigenesis and has been detected in various tumors, including multiple myeloma, colorectal cancer, breast cancer, lymphoma, breast cancer and lung cancer. Furthermore, increased levels of circulating IL-6 are associated with poor prognosis and cachexia in cancer patients. Monotherapy with IL-6 blockade or a combination therapy with both IL-6 blockade and conventional chemotherapy may reduce the progression of cancer and improve the status of cachexia in cancer patients. Finally, based upon the known biological effects of IL-6, diseases, other than autoimmune diseases and cancers, potentially to be anti-IL-6 candidates will be briefl y discussed. Review Article The Biological Effects of Interleukin-6 and Their Clinical Applications in Autoimmune Diseases and Cancers Deng-Ho Yang1, 2* 1Division of Rheumatology/Immunology/Allergy, Department of Internal Medicine, Taichung ArmedForces General Hospital, Taichung, Taiwan, Republic of China 2Division of Rheumatology/Immunology/Allergy, Department of Internal Medicine, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan, Republic of China Dates: Received: 15 February, 2017; Accepted: 01 March, 2017; Published: 02 March, 2017 *Corresponding author: Deng-Ho Yang, MD, PhD, Taichung Armed Forces General Hospital: No 348, Section 2, Chung-Shan Rd, Taiping Dist., Taichung 41152, Taiwan, Republic of China, Tel: 886-4-23934191; Fax: 886-4-23934192; E-mail:


Introduction
Interleukin-6 (IL-6), composed of 184 amino acids with a molecular weight of 26 kDa, is one of the proinfl mmatory cytokine and plays a major role in the progression of systemic and local infl ammation. Signifi cant elevation or excessive production of circulating IL-6 usually develops during the disease course of systemic infl ammation including infectious or non-infectious diseases [1]. Several studies revealed that IL-6 has multiple functions in immune, infl ammation and oncogenesis, and is associated with the promotion of synthesis of acute phase proteins including C-reactive protein (CRP), serum amyloid A, hepatoglobin, fi brinogen and hepcidin [2].
Persistent dysregulation of circulating IL-6 with overproduction may be found in the patients with different autoimmune diseases. Rheumatoid arthritis (RA), one of autoimmune diseases, is a systemic infl ammation of polyarthritis, fever, myalgia, malaise and anemia with increasing circulating proinfl ammatory cytokine including IL-6 [3]. Higher circulating IL-6 is observed in the RA patients and is associated with disease activity, erythrocyte sedimentation rate (ESR), CRP and bony structural damage [4,5]. Increased levels of serum and synovial may be found in the patients with juvenile idiopathic arthritis (JIA) [6,7]. After medication of humanized anti-IL-6 receptor monoclonal antibody (tocilizumab), the clinical manifestations of polyarthritis and serum acute phase proteins are improved. IL-6 targeting therapy serves as a novel therapeutic strategy for the patients with RA and JIA [3,8].
IL-6 can promote osteocalstogensis and induce systemic bone loss with structural bony erosion in the disease course of RA [9]. Among the patients with systemic lupus erythematosus (SLE), increased expression of IL-6 is found, and the clinical manifestations of arthritis and circulating acute phase protein is improved after medication of IL-6 blocker (10). Active systemic infl ammation and progressive renal involvement may develop after implication of IL-6 trans-signaling in lupus mice [11]. Elevated concentration of IL-6 is observed in the patients with Castleman's disease and the level is reduced after clinical improvement [12]. These fi ndings prove that IL-6 plays a major role in the pathogenesis of immunological abnormalities and the development of systemic infl ammation in different autoimmune disease.
From the previous studies, IL-6 is also involved in the infl ammation related tumorigenesis [13]. These cancers associated with abnormal systemic or local elevation of IL-6 involving different organs include breast, lung, ovarian, pancrea, prostate, colon, kidney and hematology [14]. Serum concentrations of IL-6 and CRP may serve as an excellent predicting biomarker of severity of systemic infl ammation in malignancy. In pancreatic cancer patients, tumor associated macrophages may release IL-6 and it up-regulates cytokine expression, promotes proliferation, immune evasion, angiogenesis and resistance to apoptosis of cancer cells [15]. The levels of serum IL-6 are signifi cantly higher in the patients with active weight loss [16]. Higher levels of serum IL-6 is observed in the patients with aggressive metastatic or recurrent breast cancers [17][18][19]. Among the hematological cancers including lymphoma and multiple myeloma, elevated serum IL-6 levels are found during the disease course [20][21][22][23]. In the patients with prostate cancer, higher serum IL-6 levels are associated with clinical metastasis and hormonerefractory [24,25]. Elevated serum IL-6 levels are observed in other types of cancers including colon, lung, ovarin and kidney [26][27][28][29][30]. Therefore, the pathological involvement of IL-6 is not only infl ammation but also has the effect of cancer cells related immune response. IL-6 may be one of the pathogenesis in progression and migration of malignancy, and is essential for inducing cancer related infl ammation. IL-6 blockade can suppress the differentiation of Th17 cells and can ameliorate chronic systemic infl ammation [14]. The overall clinical and biological effects of IL-6 are shown as Table 1 In the   trans-signaling pathway, free IL-6 can bind to soluble IL-6   receptor to precede the downstream signal and amplify IL-6 signaling. The JAK signaling is through the activation of gp130.
The gene expression of cell differentiation or proliferation may be activated by transcriptional factors of signal transducer and activator factor 3 (STAT3) or mitogen activated protein kinase (MAPK). IL-6 may induce activation and proliferation of B cell [1]. IL-6 with TGF- can induce T cells for differentiation of Overproduction or overexpression of circulating IL-6 can be found in different types of cancers. There is a direct correlation between IL-6 levels in tumor-associated endothelial cells  [40]. Tumor associated macrophages can induce IL-6 and may contribute to tumor progression associated with the infl ammation related tumorigenesis. Tumor associated macrophages produce IL-6 and it promotes expansion of these caner stem cells. Levels of IL-6 in human hepatocellular carcinoma correlate with tumor stage and markers of caner stem cells [41]. IL-6 signaling may affect infl ammation related cancer by modulating the resistance of T cells against apoptosis [42]. The imbalance between regulatory T cells and Th17 cells is observed in this progression. Therefore, the biological effect of IL-6 is not only fi nd in autoimmune disorders, but certain malignancies.

The rationale of developing anti-IL-6 for the clinical therapeutic purposes
IL-6 plays a major role in the systemic and local infl ammation of autoimmune disease, including RA, JIA, SLE and Castleman's disease [3,43]. In other autoimmune disease, increased expression of serum IL-6 is found among the patients with dermatomyositis [44]. The IL-6 blocker may be sometimes useful in these disease, such as adult-onset Still's disease, amyloidosis, vasculitis, relapsing polychondritis, and systemic sclerosis [43]. These fi ndings suggest that IL-6 is not the only major pathogenesis among these autoimmune  [46]. The assays of RA disease activity include DAS28, ESR, CRP and health assessment questionnaire [4], and IL-6 is associated with these clinical assays. IL-6 is important for the development of systemic osteopenia in RA. In autoimmune disease, chronic disease related anemia is due to overproduction of hepcidin through reduction of intestinal iron absorption and blocking the release of iron from macrophages. IL-6 may contribute the induction of hepcidin in anemia of chronic disease. In clinical, the hemoglobin level is increased by the medication of anti-IL-6 receptor antibody in the RA patients [47]. Decreased circulating regular T cells are found among RA patients. After medication of anti-IL-6 receptor antibody, tocilizumb, the clinical disease activity of RA is improved and regular T cells are signifi cantly increased [48,49]. Clinical studies reveal that tocilizumb has effect on control of RA with or without medication of methotrexate. Tocilizumb is still useful for the medication of RA when these patients fail to response TNF- inhibitor [49]. Monotherapy of tocilizumab is not inferior to tocilizumab combined with methotrexate [50]. The clinical and radiographic effects of tocilizumab may maintain after monotherapy for one year [51]. The subsequent fl are will be found after discontinuation of tocilizumab, but the disease can be controlled by re-administration of biologics [52]. Therefore, tocilizumab is one of the most common biological agents for monotherapy [53]. The monotherapy of tocilizumab is superior to monotherapy of TNF- for reduction of clinical signs and symptoms of RA patients [54].
In autoimmune disease, IL-6 related abnormal presentation of hepcidin is central to anemia of chronic diseases including RA and SLE. Signifi cant elevation of serum IL-6 is found in breast cancer patients when compared with healthy controls [55,56]. Breast cancer cells may secrete IL-6 to support the proliferation of cancer, and reduction of proliferation of cancer is observed after IL-6 knockdown [57]. In another study about breast cancer, the expression of IL-6 is associated with the expression of estrogen receptor [58]. However, the abnormal expression of IL-6 also can be detected in cancer cells without expression of estrogen receptor [59]. Therefore, the increased expression of IL-6 plays a critical role in the development of breast cancer independently. Cancer-associated fi broblasts may promote tamoxifen resistance though IL-6-induced degradation of estrogen receptor- in breast cancer [60]. In colon cancer cells, mucin 2 can increase the secretion of IL-6 and progress the colonic carcinogenesis [61]. The presence of IL-6 is associated with activation of carcinogenesis and DNA damage by upregulation of CYP1B1 and CYP2E2 in the progression of colonic infl ammation [62]. In gastric cancer cells, IL-1 may enhance the invasiveness of cancer cells through the IL-6 expression [63]. The immunohistochemical expression of IL-2 receptor and IL-6 is higher in prostate cancer tissue than in normal tissue [64]. IL-6 gene polymorphism is associated with increased risk of bladder cancer [65]. Among female without history of smoking, genetic polymorphism of IL-6 is associated with the risk of lung cancer risk [66]. Increased expression of serum IL-6 is associated with poor survival in lung cancer [67]. After surgical resection of lung cancer cell, the levels of CRP, TNF- ad IL-6 are decreased [68]. The anti-IL-6 antibody, siltuximab, has more potent effects on tumor inhibition of lung cancer through infl uence of cancer-associated fi broblasts [69]. The pathogenesis of IL-6 in the development of cancer is about the direct effect from the cancer stem cells or indirect effect from the systemic infl ammation. Cancer stem cells may activate the IL-6 signaling pathway and amplify the expression of circulating IL-6 [70]. In the progression of malignancy, IL-6 is associated with angiogenesis and inhibiting apoptosis of tumors. In conclusion, adequate blocking the IL-6 signaling may disturbance the development of tumor related metastasis and migration. Besides, anti-IL-6 therapy severs as a biological effect for improvement of systemic infl ammation in cancer and autoimmune disease. In the disease course of malignancy, cachexia is usually found. The biological effect of IL-6 can damage the skeletal muscle, adipose, mucosa of intestine and liver tissue.

The recent advances of the experience (clinical benefi ts and adverse events) of applying anti-IL-6 therapeutics
Therefore, circulating IL-6 may affect the degree of cachexia.
At the same time, increased serum IL-6 is observed in different cancers including lung, liver, pancreas, colon, prostate, breast and bladder. Blocking IL-6-signaling pathway can relieve the clinical symptoms of cachexia in cancer patients [73]. IL-6 is one of major transcriptional regulators of hepcidin and chronic anemia with overproduction of hepcidin may contribute to the anemia [74]. Among the multiple myeloma patients with chronic anemia, IL-6 has the activity for acting synergistically with bone morphogenetic protein 2 on the induction of hepcidin [75]. After the blocking medication of IL-6, the anemia is improved by prevention of the induction of hepcidin [74,76]. In summary, different biological effects of IL-6 can be found in autoimmune diseases and cancers. These effects include arthritis, anemia, vascular angiogenesis, autoantibody production, lymphoadenopathy, tumor metastasis and cachexia. The different IL-6 related biological effects are shown as Table 2.
From different studies, IL-6 blockade therapy may be associated with adverse events including infections, infusion reactions, central demyelinating nerve and gastrointestinal perforation [77]. Laboratory abnormalities are found during the medication of IL-6 blockade, and these abnormalities include hyperlipidemia, elevation of liver function and neutropenia [78]. Therefore, adequate evaluation should be performed antibody is used to treat the patients with Castleman's disease, and the clinical manifestations and abnormal laboratory data are improved after three months therapy [79]. Siltuximab, a novel anti-IL-6 monoclonal antibody, has therapeutic benefi ts in the patients with Castelman's disease. After medication of siltuximab (12 mg/kg every 3 weeks), partial clinical benefi t response and tumor response can be found among these patients [80]. In another study, 86% patients with Castleman's disease have clinical and radiological response with signifi cant decreasing CRP after siltuximab therapy [81].
Multiple myeloma is a malignancy disease associated with B-cell proliferation and elevation of circulating IL-6. In vivo, humanized anti-IL6 monoclonal antibody can inhibit the disease activity of multiple myeloma [82]. In the phase 1 study, siltuximab (anti-IL-6 monoclonal antibody) is used in multiple myeloma for recommend dose of 11 mg/kg once every 3 weeks [83]. The addition of siltuximab in the medication of multiple myeloma has no signifi cant different when compared with traditional chemotherapy; but sustained suppression of circulating CRP is observed in the patients with medication of siltuximab [84].
Chronic prostatitis is associated with the progression and development of prostate cancer. One of the proinfl ammatory cytokines, IL-6, is important for systemic infl ammation and may link the aggressivness of prostate cancer [85]. Elevation of IL-6 level is usually found in prostate cancer patients with metastasis or refractory for hormone or radiation therapy through multiple signal pathways including JAK, STAT and MAPK [86][87][88]. Inhibition of IL-6 can enhance the radiation sensitivity of prostate cancer [87]. IL-6 has a major role in prostate cancer stem cells and may regulate down signaling of STAT3 activation [89]. In the patients with prostate cancer, bone metastasis may develop during the disease course.
IL-6 can increase the expression of RANKL and activate the abnormal bone resorption by osteoclasts in vitro. Blocking the IL-6 signaling by tocilizumab may inhibit skeletal tumor growth in vivo and reduce serum RANKL levels in prostate cancer cells [90]. Therefore, the conogenicity and bone metastasis of prostate cancer can be suppressed by anti-IL-6 antibody. Novel therapy about targeting IL-6 seems to fi ght against prostate cancer. IL-6 can promote the breast cancer migration and metastasis with activation of STAT3 signaling pathway [91]. From different clinical studies, increased serum IL-6 concentration is associated with advanced tumor stages of various studies including multiple myeloma, lung cancer, colorectal cancer, prostate cancer and breast cancer [92]. Elevation of serum IL-6 is associated with clinical symptoms of depression, anxiety and tumor progression [93]. The initiation of cancers is associated with the activation of cancer stem cells from normal cells.
IL-6 can induce the conversion of normal cells to cancer stem cells through the regulation of OCT-4 gene expression [94].
Blocking IL-6 signaling seems to be a potential therapeutic strategy for some cancers with elevation of circulating IL-6.
At the same time, resistant of multiple drugs and targeting cancers develop in some patients with breast cancers. In these patients, abnormal elevation and activation of IL-6 are found. Neutralizing antibodies against IL-6 may reverse resistance of these chemotherapies (paclitaxel and doxorubicin) [95].
Active IL-6 infl ammatory feedback loop may lead to aggressive expansion of breast cancer stem cells, and medication of IL-6 receptor antibody can interrupt this infl ammatory loop [96].
In colon carcinoma cells, clonogenic proliferation and invasiveness may be induced by IL-6 through the pathways of MAPK and PI3K [97]. This clonogenic effect can be blocked by IL-6 receptor neutralization antibody. The abnormal elevation of IL-6 has the ability to progress the growth of colon cancers.
The liver metastasis of colon cancer is associated with the activated macrophages inducing IL-6 secretion of colon cancer cells in vitro [98]. At the same, overexpression of IL-6 is found the metastatic colon carcinoma cells [99]. Therefore, the pathological effect of circulating IL-6 may be amplifi ed by metastatic tumor cells. Chronic colitis related infl ammation may induce the tumorgenesis by mediating IL-6/STAT3 signal pathway. In other study, polysaccharide peptidoglycan complex on Lactobacillus has the effect of inhibiting IL-6 production and may block the development of colitis-associated cancer through inhibition of STAT3 signaling [100].
The elevation of circulating IL-6 develops in the patients with lung cancer. The pathogenesis of lung cancer is associated with activation of macrophage with elevation of IL-6 and the development of epithelial to mesenchymal transition [101].

Active migration of lung cancer cells can be induced by IL-6
though JAK/STAT3 pathway [102]. Therefore, IL-6 is important for the metastasis of lung cancer. In vitro, the medication of anti-IL-6 antibody may reverse the induction of epithelial to mesenchymal transition in lung cancer cells [101]. Among the patients with lung cancer, serum IL-6 levels are higher in the patients with cachexia and poor response for chemotherapy [103]. The blocking agent for activity of IL-6 has the potential of improving disease prognosis and cachexia in lung cancer. A humanized anti-IL-6 antibody (ALD518) can improve the lung cancer related anemia and cachexia in Phase I and II clinical trial [104]. A chimeric anti-IL-6 monocolonal antibody, siltuximab, is used in the patients with advanced solid tumors including colorectal, ovarian and pancreatic cancers. Improvement of chronic anemia and systemic infl ammation are observed, but adverse events of hepatic function abnormalities, fatigue and neutropenia are also found [76]. The clinical responses of IL-6 blockade in autoimmune diseases and cancers are shown as Table 3.   [76,112] Anti-IL-6 receptor antibody Reduction of clonogenic growth and invasiveness by blocking MAPK and PI3K pathway in vitro Inhibition of tumor metastasis Colon cancer [97] 28