Co-localization of insulin-like growth factor binding protein 3 and fibronectin in human articular cartilage.
OBJECTIVE
The anabolic cytokine insulin-like growth factor I (IGF-I) stimulates chondrocyte synthesis of matrix macromolecules and several lines of evidence suggest that it has a major role in maintaining articular cartilage and possibly in cartilage repair. Despite the apparent importance of IGF-I in articular cartilage metabolism and its potential importance in joint diseases, little is known about the regulation of IGF-I activity within the tissue. Insulin-like growth factor binding proteins (IGFBPs) bind IGF-I and can modify its activity. At least three IGFBPs are expressed by chondrocytes: IGFBP-3, -4 and -5. Localization of IGFPBs in the articular cartilage extracellular matrix (ECM) could create reservoirs of IGF-I within the articular cartilage ECM and thereby regulate local IGF-I levels. We hypothesized that ECM molecules bind and concentrate IGFPBs in the pericellular/territorial matrix.


METHODS
Semi-quantitative immunohistological measures of co-localization were used to compare the spatial distribution of IGFBP-3, -4, and -5 with the distributions of three peri-cellularly-enriched matrix molecules fibronectin, tenascin-C, and type VI collagen in osteoarthritic and non-osteoarthritic human articular cartilage. Purified proteins were used in an agarose diffusion assay to compare IGFBP-3 binding to the same three matrix proteins.


RESULTS
IGFBP-3 associated with fibronectin in the pericellular/territorial matrix (approximately 40% co-localization) but not with tenascin-C, or type VI collagen (approximately 6% and approximately 15% co-localization respectively, P< 0.05). Neither IGFBP-4, nor IGFBP-5 were associated with any of the three ECM proteins (P< 0.05). In agarose diffusion assays IGFBP-3 interacted with fibronectin and heparan sulfate proteoglycan but not with type VI collagen or tenascin-C.


CONCLUSIONS
Direct binding between purified IGFBP-3 and fibronectin and the strong co-localization the two proteins in the cartilage matrix support the hypothesis that IGFPB-3 and fibronectin help regulate local IGF-I levels.
Iowa City Veterans Administration Medical Center and University of Iowa Department of Orthopaedics, Iowa City, USA.
J A
JA
Martin
B A
BA
Miller
M B
MB
Scherb
L A
LA
Lembke
J A
JA
Buckwalter
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002358:Cartilage, Articular / Q000737:chemistry*; D019902:Chondrocytes / Q000737:chemistry; D005353:Fibronectins / Q000032:analysis*; D006801:Humans; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000032:analysis*; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000032:analysis; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000032:analysis; D018613:Microscopy, Confocal; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000378:metabolism*
D005353:Fibronectins; D018972:Insulin-Like Growth Factor Binding Protein 3; D018974:Insulin-Like Growth Factor Binding Protein 4; D018975:Insulin-Like Growth Factor Binding Protein 5