Shedding of the Interleukin-6 (IL-6) Receptor (gp80) Determines the Ability of IL-6 to Induce gp130 Phosphorylation in Human Osteoblasts

• Published in: The Journal of biological chemistry 2002-05, Vol.277 (19), p.16879-16887
• Main Authors: Vermes, Csaba, Jacobs, Joshua J., Zhang, Jian, Firneisz, Gabor, Roebuck, Kenneth A., Glant, Tibor T.
• Format: Article
• Language: English
• Subjects: Antigens, CD - metabolism; Antigens, CD - physiology; Base Sequence; Blotting, Western; Cell Membrane - metabolism; Cell Survival; Cells, Cultured; Cytokine Receptor gp130; Enzyme Inhibitors - pharmacology; Flow Cytometry; Glycosylation; Humans; Interleukin-6 - metabolism; Membrane Glycoproteins - metabolism; Membrane Glycoproteins - physiology; Molecular Sequence Data; Osteoblasts - metabolism; Phosphorylation; Precipitin Tests; Protein Synthesis Inhibitors - pharmacology; Receptors, Interleukin-6 - metabolism; Receptors, Interleukin-6 - physiology; Reverse Transcriptase Polymerase Chain Reaction; Time Factors; Transcription, Genetic; Tyrosine - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M200546200

Human osteoblasts produce interleukin-6 (IL-6) and respond to IL-6 in the presence of soluble IL-6 receptor (sIL-6R), but the cell surface expression of IL-6R and the mechanism of sIL-6R production are largely unknown. Three different human osteoblast-like cell lines (MG-63, HOS, and SaOS-2) and bone marrow-derived primary human osteoblasts expressed both IL-6R and gp130 as determined by flow cytometry and immunoprecipitation. However, the membrane-bound IL-6R was nonfunctional, as significant tyrosine phosphorylation of gp130 did not occur in the presence of IL-6. Phorbol myristate acetate induced a dramatic increase of both IL-6R shedding (i.e. the production of sIL-6R) and IL-6 release in osteoblast cultures, but the cell surface expression of gp130 remained unchanged. IL-6 complexed with sIL-6R, either exogenously introduced or derived from the nonfunctional cell surface form by shedding, induced rapid tyrosine phosphorylation of gp130. This effect was inhibited by neutralizing antibodies to either sIL-6R or gp130, indicating that the gp130 activation was induced by IL-6/sIL-6R/gp130 interaction. Protein kinase C inhibitors blocked phorbol myristate acetate-induced and spontaneous shedding of IL-6R resulting in the absence of sIL-6R in the culture medium, which in turn also prevented the activation of gp130. In conclusion, human osteoblasts express cell surface IL-6R, which is unable to transmit IL-6-induced signals until it is shed into its soluble form. This unique mechanism provides the flexibility for osteoblasts to control their own responsiveness to IL-6 via the activation of an IL-6R sheddase, resulting in an immediate production of functionally active osteoblast-derived sIL-6R.