Targeting the IL-6/JAK/STAT3 signalling axis in cancer

Abstract

The IL-6/JAK/STAT3 pathway is aberrantly hyperactivated in many types of cancer, and such hyperactivation is generally associated with a poor clinical prognosis. In the tumour microenvironment, IL-6/JAK/STAT3 signalling acts to drive the proliferation, survival, invasiveness, and metastasis of tumour cells, while strongly suppressing the antitumour immune response. Thus, treatments that target the IL-6/JAK/STAT3 pathway in patients with cancer are poised to provide therapeutic benefit by directly inhibiting tumour cell growth and by stimulating antitumour immunity. Agents targeting IL-6, the IL-6 receptor, or JAKs have already received FDA approval for the treatment of inflammatory conditions or myeloproliferative neoplasms and for the management of certain adverse effects of chimeric antigen receptor T cells, and are being further evaluated in patients with haematopoietic malignancies and in those with solid tumours. Novel inhibitors of the IL-6/JAK/STAT3 pathway, including STAT3-selective inhibitors, are currently in development. Herein, we review the role of IL-6/JAK/STAT3 signalling in the tumour microenvironment and the status of preclinical and clinical investigations of agents targeting this pathway. We also discuss the potential of combining IL-6/JAK/STAT3 inhibitors with currently approved therapeutic agents directed against immune-checkpoint inhibitors.

Fig. 1. IL-6 signalling pathways

a | In the classic IL-6 signalling pathway, IL-6 binds to the membrane-bound IL-6 receptor-α (subsequently referred to as IL-6R), thus inducing formation of a heterohexameric complex consisting of two molecules each of IL-6, IL-6R, and the IL-6 receptor subunit-β (gp130). Formation of this complex results in activation of the JAK/STAT3 signalling pathway, leading to the transcription of STAT3 target genes. The IL-6/IL-6R/gp130 complex can also activate the PI3K/AKT/mTOR (mechanistic target of rapamycin) and RAS/RAF/MEK/ERK pathways (not pictured). b | In the IL-6 trans-signalling pathway, soluble IL-6R (sIL-6R) binds to IL-6. sIL-6R can be generated by alternative splicing of IL6R mRNA or cleavage of membrane-bound IL-6R by disintegrin and metalloproteinase domain-containing protein 170(ADAM10) or ADAM17. When IL-6 binds with sIL-6R, this complex is then able to bind to and induce the dimerization of gp130, leading to the activation of downstream signalling pathways (as described above for classic IL-6 signalling pathways). gp130 is ubiquitously expressed, although the IL-6R is expressed in only a limited number of cell types. Trans-signalling via sIL-6R enables IL-6 to act on cells with limited or absent IL-6R expression. IL-6 trans-signalling can be negatively regulated by soluble gp130 (sgp130), which is generated by alternative splicing. This molecule competes with membrane-bound gp130 for binding to the IL-6–sIL-6R complex, thereby inhibiting IL-6 trans-signalling but not the classic IL-6 signalling pathway.

Fig. 2. Signalling downstream of the IL-6 receptor

JAK proteins bind to the Box 1 and Box 2 domains in the intracellular portion of IL-6 receptor subunit-β (gp130). This leads to JAK-mediated phosphorylation of gp130 at several tyrosine residues, including four C-terminal residues that serve as docking sites for STAT3. Once bound to gp130, STAT3 is phosphorylated by JAKs at tyrosine 705, leading to STAT3 dimerization and nuclear translocation, followed by STAT3-mediated transcription of target genes. The figure shows signalling initiated by classic signalling. Trans-signalling pathways initiate downstream signalling in the same fashion. Tyrosine phosphorylation of STAT3 can also be induced by other oncogenic proteins, including SRC and BCR–ABL1. IL-6/JAK/STAT3 signalling is negatively regulated by a number of mechanisms. Suppressor of cytokine signalling (SOCS) 1 and SOCS3 bind to and inhibit the kinase activity of JAKs. SOCS3 is a STAT3 target gene; following transcription, SOCS3 then acts as a component of a negative-feedback loop that maintains tight regulation of this pathway. The following phosphatases also have a role in the negative regulation of this pathway: tyrosine-protein phosphatase non-receptor type 6 (SHP1; also known as PTPN6); tyrosine-protein phosphatase non-receptor type 11 (SHP2); dual specificity protein phosphatase 22 (DUSP22); receptor-type tyrosine-protein phosphatase-δ (PTPRD); receptor-type tyrosine-protein phosphatase T (PTPRT); tyrosine-protein phosphatase non-receptor type 1 (PTPN1); tyrosine-protein phosphatase non-receptor type 2 (PTPN2). Protein inhibitor of activated STAT3 (PIAS3), an E3 SUMO protein ligase, as well as PDZ and LIM domain protein 2 (PDLIM2), an ubiquitin E3 ligase, are additional endogenous proteins that inhibit STAT3 by mediating STAT3 degradation. The expression of PIAS3 and PDLIM2 can be inhibited by oncogenic microRNAs (miRNAs): miR-18a targets PIAS3, whereas miR-221 and miR-222 target PDLIM2. Another miRNA, miR-551b-3p, promotes STAT3 gene expression. Other miRNAs act to negatively regulate the IL-6/JAK/STAT3 pathway: expression of IL6R is inhibited by miR-218 and miR-34a, while STAT3 expression is inhibited by miR-17-5p, miR-20a, and miR-124.

Fig. 3. Inhibitors of the IL-6/JAK/STAT3 signalling pathway

Various targeted agents that inhibit different nodes of the IL-6 signalling pathway have been developed. Siltuximab, sirukumab, olokizumab, clazakizumab, and MEDI5117 are anti-IL-6 monoclonal antibodies. Tocilizumab and sarilumab are monoclonal antibodies that target IL-6R. These antibodies inhibit both the classic and trans-signalling pathways. By contrast, the gp130–Fc fusion protein olamkicept inhibits IL-6 trans-signalling but not the classic signalling pathway. Tofacitinib, ruxolitinib, pacritinib, and AZD1480 are small-molecule tyrosine kinase inhibitors that target JAKs, preventing phosphorylation of STAT3. C188-9, OPB-31121, OPB-51602, and other Src homology domain 2 (SH2) domain inhibitors interfere with STAT3 dimerization. The STAT3 antisense oligonucleotide AZD9150 binds to and causes the destruction of STAT3 mRNA, thus decreasing STAT3 expression. The cyclic STAT3 decoy contains a nucleotide sequence derived from the promoter of the STAT3 target gene FOS. This decoy competitively inhibits STAT3 binding to genomic response elements in the promoter regions of target genes.