One of two chondrocyte-expressed isoforms of cartilage intermediate-layer protein functions as an insulin-like growth factor 1 antagonist.
OBJECTIVE
Aging and osteoarthritic (OA) cartilage commonly demonstrate enhanced expression of the large, transforming growth factor beta (TGFbeta)-inducible glycoprotein cartilage intermediate-layer protein (CILP) as well as enhanced extracellular inorganic pyrophosphate (PPi) that promotes the deposition of calcium pyrophosphate dihydrate crystals. In normal chondrocytes, TGFbeta induces elevated chondrocyte extracellular PPi. Insulin-like growth factor 1 (IGF-1) normally blocks this response and reduces extracellular PPi. However, chondrocyte resistance to IGF-1 is observed in OA and aging. Because CILP was reported to chromatographically fractionate with PPi-generating nucleotide pyrophosphatase phosphodiesterase (NPP) activity, it has been broadly assumed that CILP itself has NPP activity. Our objective was to directly define CILP functions and their relationship to IGF-1 in chondrocytes.


METHODS
Using primary cultures of articular chondrocytes from the knee, we defined the function of the previously described CILP (CILP-1) and of a recently described 50.6% identical protein that we designated the CILP-2 isoform.


RESULTS
Both CILP isoforms were constitutively expressed by primary cultured articular chondrocytes, but only CILP-1 expression was detectable in cultured knee meniscal cartilage cells. Neither CILP isoform had intrinsic NPP activity. But CILP-1 blocked the ability of IGF-1 to decrease extracellular PPi, an activity specific for the CILP-1 N-terminal domain. The CILP-1 N-terminal domain also suppressed IGF-1-induced (but not TGFbeta-induced) proliferation and sulfated proteoglycan synthesis, and it inhibited ligand-induced IGF-1 receptor autophosphorylation.


CONCLUSIONS
Two CILP isoforms are differentially expressed by chondrocytes. Neither CILP isoform exhibits PPi-generating NPP activity. But, increased expression of CILP-1, via N-terminal domain-mediated inhibitory effects of CILP-1 on chondrocyte IGF-1 responsiveness, could impair chondrocyte growth and matrix repair and indirectly promote PPi supersaturation in aging and OA cartilage.
Johnson
Kristen
K
Department of Veterans Affairs Medical Center, San Diego, and University of California, San Diego, CA 92161, USA.
Farley
David
D
Hu
Shou-Ih
SI
Terkeltaub
Robert
R
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D002358:Cartilage, Articular / Q000166:cytology / Q000378:metabolism*; D002417:Cattle; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology / Q000378:metabolism*; D018076:DNA, Complementary / Q000032:analysis; D011756:Diphosphates / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D016326:Extracellular Matrix Proteins / Q000378:metabolism*; D006133:Growth Substances / Q000502:physiology*; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000502:physiology*; D051379:Mice; D018345:Mice, Knockout; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D020033:Protein Isoforms; D011755:Pyrophosphatases / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013264:Stifle; D014162:Transfection
D018076:DNA, Complementary; D011756:Diphosphates; D016326:Extracellular Matrix Proteins; D006133:Growth Substances; D036341:Intercellular Signaling Peptides and Proteins; D020033:Protein Isoforms; C065700:myotrophin; D000469:Alkaline Phosphatase; D010727:Phosphoric Diester Hydrolases; C114607:CILP protein, human; D011755:Pyrophosphatases; C405782:nucleotide pyrophosphatase - phosphodiesterase I