The cytokine interleukin-6 (IL-6) plays an important role in inflammation where it functions as a pro-inflammatory cytokine. Signal transduction of IL-6 leads to activation of different signaling pathways, e.g. the JAK/STAT pathway. A prerequisite for signaling initiation is the formation of a complex consisting of the cytokine bound to the α-receptor interleukin-6 receptor (IL-6R), which induces homodimerization of the signal-transducing β-receptor glycoprotein 130 (gp130). Upon ligand binding the complex is endocytosed, but only little is known about the fate of both receptors and the cytokine itself. Surface expression of the IL-6R is furthermore regulated by limited proteolysis, which results in the release of a soluble form of the receptor and downregulation of its cellular expression.
Here, we show that the IL-6R, as well as gp130, is endocytosed constitutively but the rate of endocytosed receptor can be slightly enhanced by IL-6 stimulation. Further, we could demonstrate that the absence of gp130 leads to an increase in the half-life of surface IL-6R indicating dependence of IL-6R internalization on gp130. Whether the decrease of internalization is a result of the missing capability of complex formation or the absence of the gp130 C-terminal part, which contains several internalization motifs, needs to be elucidated. Treatment with clathrin or dynamin inhibitors revealed that both receptors are internalized in a clathrin- and dynamin-dependent manner. Furthermore, inhibition of clathrin leads to abrogated STAT3 signaling indicating the importance of the formation of vesicular structures at the plasma membrane for JAK/STAT signal initiation. Immunofluorescence microscopy showed that following internalization, both receptors are guided to lysosomes and recycling endosomes, and stimulation with IL-6 leads to an increase in IL-6R and gp130 recycling.