Shedding of the interleukin-6 (IL-6) receptor (gp80) determines the ability of IL-6 to induce gp130 phosphorylation in human osteoblasts.
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.
Vermes
Csaba
C
Department of Orthopedic Surgery, Rush University, Rush-Presbyterian-St. Luke's Medical Center, Chicago, Illinois 60612, USA.
Jacobs
Joshua J
JJ
Zhang
Jian
J
Firneisz
Gabor
G
Roebuck
Kenneth A
KA
Glant
Tibor T
TT
D015703:Antigens, CD / Q000378:metabolism* / Q000502:physiology*; D001483:Base Sequence; D015153:Blotting, Western; D002462:Cell Membrane / Q000378:metabolism; D002470:Cell Survival; D002478:Cells, Cultured; D050822:Cytokine Receptor gp130; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005434:Flow Cytometry; D006031:Glycosylation; D006801:Humans; D015850:Interleukin-6 / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism* / Q000502:physiology*; D008969:Molecular Sequence Data; D010006:Osteoblasts / Q000378:metabolism*; D010766:Phosphorylation; D011233:Precipitin Tests; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D019947:Receptors, Interleukin-6 / Q000378:metabolism* / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors; D014158:Transcription, Genetic; D014443:Tyrosine / Q000378:metabolism
D015703:Antigens, CD; D004791:Enzyme Inhibitors; C494199:IL6ST protein, human; D015850:Interleukin-6; D008562:Membrane Glycoproteins; D011500:Protein Synthesis Inhibitors; D019947:Receptors, Interleukin-6; C113250:interleukin-6 receptor alpha; D050822:Cytokine Receptor gp130; D014443:Tyrosine