The regulation of IL-6/gp130 signaling by PTPRF in colon cancer

Elevated Interleukin 6 (IL-6) levels and the activation of IL6/JAK/STAT3 signaling play an important role in promoting tumor growth and progression in colorectal cancer, making the IL-6 pathway a potential therapeutic target. Binding of IL-6 stimulates the formation of a receptor complex consisting of the IL-6 receptor and its co-receptor, gp130, which triggers the activation of JAK family kinases to phosphorylate tyrosine residues on the intracellular domain of gp130. The phosphorylated tyrosine residues act as docking sites for key signaling components, including transcription factor STAT3. Docking of STAT3 to gp130 allows for the phosphorylation of Tyr705 on STAT3 by JAK, the residue necessary for dimerization and nuclear translocation of STAT3 to promote gene transcription. While the phosphorylation-dependent activation of the IL-6 pathway has been intensively investigated, the regulation of signaling inactivation by protein phosphatases remains largely unexplored. Here we determined the role of protein tyrosine phosphatase receptor type F (PTPRF) in negatively regulating the tyrosine phosphorylation steps that control IL-6/gp130 signaling. The expression of PTPRF was depleted by CRISPR-mediated knockout or doxycycline inducible RNAi in 293T and colon cancer cells. We found that PTPRF downregulation resulted in an increase in total protein expression as well as tyrosine phosphorylation of gp130 basally. Additionally, IL-6 stimulation-induced activation of Jak family kinases, as indicated by Tyr1007/1008 phosphorylation, was largely increased and more sustained in PTPRF knockout cells compared to control. Furthermore, results from co-immunoprecipitation experiments showed that the association of STAT3 with gp130 was increased in PTPRF knockout cells, consistent with increased JAK activation. Functionally, the expression of SOCS3, a STAT3 target gene, was significantly elevated upon IL-6 stimulation as determined by RT-qPCR in PTPRF knockdown colon cancer cells. Taken together, this study identifies PTPRF as a novel regulator of the IL-6/gp130 signaling pathway in colon cancer.