Taiwanese researchers have elucidated the signaling mechanism downstream of IL-17 receptor B (IL-17RB) and shown that its blockade using a neutralizing antibody reduced pancreatic cancer tumorigenesis and metastasis, while prolonging the life of tumor-bearing mice.

Reported in the March 3, 2021, edition of Science Translational Medicine, these new study findings provide valuable insights into how this signaling pathway represents a potential new therapeutic target for pancreatic cancer.

"The therapeutic potential of an IL-17RB-targeting neutralization antibody in pancreatic cancer treatment was first demonstrated in a 2015 study. However, IL-17RB is known to be critical in maintaining immune homeostasis," said lead researcher Wen-Hwa Lee, an affiliated professor in the Drug Development Center at China Medical University in Taichung.

"Therefore, our study proposes that an agent specifically targeting tumor, but not immune IL-17RB, is needed for the potential treatment of pancreatic cancer," said Lee, noting the urgent need for such treatments.

"Upon diagnosis, just 25% of pancreatic cancers are surgically resectable, with unresectable tumors being treated with chemotherapy and radiotherapy," said Lee, who is a professor of biological chemistry at the University of California in Irvine, where research into IL-17RB began 15 years ago.

"However, chemotherapy and radiotherapy have very limited efficacy in pancreatic cancer, hence the urgent need for novel targeted therapies and/or immunotherapies."

IL-17 receptors

The IL-17 family comprises five transmembrane protein receptors (RA to RE), with IL-17A, C, and E mainly mediating inflammation, while the inflammatory roles of IL-17B and IL-17D are less clear.

IL-17 receptors contain two extracellular domains and a cytoplasmic SEFIR (similar expression to fibroblast growth factor genes and IL-17R) domain, which triggers downstream signaling.

Each IL-17R has distinct characteristics, with IL-17RA containing approximately 100 additional residues beyond the SEFIR domain, which are also required for signaling.

Both domains have been shown to form a single structure, while IL-17RA's cytoplasmic tail contains a distinct activation domain (AD) that binds to tumor necrosis factor receptor-associated factor 3 (TRAF3).

However, unlike IL-17RA, the SEFIR region of IL-17RB has a different 3D structure and lacks the C-terminus AD, suggesting it operates differently to IL-17RA.

Cancer cells exploit cell proliferation, division, differentiation, and migration signaling pathways to grow, with cytokines being involved in tumor progression by modulating the inflammatory tumor microenvironment.

Nevertheless, altered cytokine signaling pathways in tumor cells rarely drive progression, although IL-17RB overexpression in pancreatic, breast and other cancers correlates with malignancy.

Moreover, IL-17RB depletion or antibody treatment has been shown to prevent tumor growth and metastasis, implying the importance of IL-17RB in these cancers.

Finding the kinase culprit

IL-17RB's potential oncogenic function is similar to that of receptor tyrosine kinases (RTKs), which are oncogenic in various cancers. RTKs are activated by receptor-specific ligands binding to their extracellular regions, with the receptor activated by ligand-specific dimerization or oligomerization.

For most RTKs, this allows autophosphorylation of the kinase domain and engages downstream mediators propagating key cellular signaling pathways.

However, IL-17RB lacks a kinase domain and is not an RTK, so how it responds to its ligand and transmits its oncogenic signal to downstream mediators remains unknown.

This prompted the new study, which found that IL-17RB forms a homodimer and recruits mixed-lineage kinase 4 (MLK4) to phosphorylate it at tyrosine 447 upon treatment with IL-17RB in vitro.

"Consequently, phosphorylation of IL-17RB by MLK4 is not only important to IL-17RB downstream oncogenic signaling activation, but also specific to the cancer cell," said Lee.

In addition, higher amounts of phosphorylated IL-17RB in tumors from pancreatic cancer patients were shown to correlate with a worse patient prognosis.

"The median survival of patients with high- and low-P-Y447 levels was approximately 11 and 16.5 months, respectively," said Lee.

Phosphorylated IL-17RB was shown to recruit the ubiquitin ligase motif containing 56 to add lysine 63-linked ubiquitin chains to lysine 470 of IL-17RB, which further assembles NF-?B activator 1 (ACT1) and other factors to propagate downstream oncogenic signaling.

"Phosphorylation of IL-17RB by MLK4 is the key step for IL-17RB-mediated oncogenic signaling in tumors," Lee told BioWorld.

"Therefore, the level of phosphorylation of IL-17RB can not only serve as biomarker for predicting the prognosis of pancreatic cancer patients, but also as a target specific to cancer cells, but not to immune cells."

Consequently, IL-17RB mutants with substitutions at either tyrosine 447 or lysine 470 were shown to lose oncogenic activity.

"We made mutations of the IL-17RB gene/protein at specific residues, which are phosphorylated, ubiquitinated, and bound by MLK4," noted Lee.

"These IL-17RB mutants can then be used to further validate the importance of IL-17RB phosphorylation, ubiquitination and interaction with MLK4 in IL-17B-mediated IL-17RB downstream oncogenic signaling."

Treatment with an IL-17RB derived peptide inhibited tumorigenesis and metastasis and to prolong the life span of mice with pancreatic cancer.

"We used two pancreatic mouse models and a loop peptide to prove the concept that targeting IL-17RB phosphorylation is feasible and specific for pancreatic cancer treatment," said Lee.

"Median survival of the pancreatic tumor-bearing mice was longer in the group treated with the loop peptide (151 days) than in controls (93 days), with statistical significance but limited effects, possibly due to the lower stability of the loop peptide."

"Based on this concept, we are now working on the development of a new IL-17RB-targeting peptide mimetic, which we believe will be more feasible and reliable for cancer treatment."

Moreover, "according to the underlying mechanism identified in this study, we are working on selecting small-molecule inhibitors to block the interaction between IL-17RB and MLK4, and to inhibit IL-17RB phosphorylation," said Lee.

"We are also working on establishing a non-invasive diagnostic test to select the patients whose tumors are potentially responsive to IL-17RB-targeting agents."