Differential regulation of IGF-binding proteins in rabbit costal chondrocytes by IGF-I and dexamethasone.
Cartilage is a primary target tissue for the IGFs. The mitogenic activity of these peptides is regulated by a family of high-affinity IGF-binding proteins (IGFBP-1 to -6). We characterized the IGFBPs produced by cultured chondrocytes derived from rib cartilage of prepubertal rabbits. Culture medium, which had been conditioned by these cells for 48 h showed bands of 22 kDa, 24 kDa and a 31/32 kDa doublet by Western ligand blotting with [(125)I]IGF-II. When the cells were grown in the presence of increasing amounts of IGF-I or IGF-II, the 31/32 kDa doublet increased in intensity (reaching a plateau of about 11-fold stimulation between 2 and 10 nM IGF-I). The 22 kDa and 24 kDa bands increased only slightly while a 26 kDa band became faintly visible. By Western immunoblotting the 31/32 kDa doublet was identified as IGFBP-5. An IGF-I analog with reduced affinity for IGFBPs, Long-R3 IGF-I, also induced IGFBP-5, while insulin was less effective (2.2-fold stimulation at 10 nM). IGF-I protected IGFBP-5 against proteolytic degradation by conditioned medium. IGF-I also enhanced the level of IGFBP-5 mRNA. LY294002, a specific inhibitor of the intracellular signaling molecule phosphatidylinositol 3-kinase, inhibited stimulation of IGFBP-5 by IGF-I. Dexamethasone suppressed IGFBP-5 (by 70% at 20 nM) but, at the same time, a 39/41 kDa doublet (presumably IGFBP-3) was induced. IGFBP-5 has been shown in several cell types to enhance the mitogenic activity of IGF-I. IGFBP-3 generally acts as a growth inhibitor. Therefore, the differential effects of dexamethasone on these regulatory proteins could account, at least in part, for the growth-arresting effect of this glucocorticoid.
Koedam
J A
JA
Department of Pediatric Endocrinology, University Medical Center Utrecht, Utrecht, The Netherlands. j.koedam@wkz.azu.nl
Hoogerbrugge
C M
CM
Van Buul-Offers
S C
SC
D000818:Animals; D000893:Anti-Inflammatory Agents / Q000494:pharmacology*; D015152:Blotting, Northern; D015153:Blotting, Western; D018929:Cell Culture Techniques; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000187:drug effects / Q000378:metabolism; D018969:Insulin-Like Growth Factor Binding Proteins / Q000235:genetics / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D012333:RNA, Messenger / Q000235:genetics; D011817:Rabbits; D011994:Recombinant Proteins / Q000494:pharmacology; D012272:Ribs
D000893:Anti-Inflammatory Agents; D018975:Insulin-Like Growth Factor Binding Protein 5; D018969:Insulin-Like Growth Factor Binding Proteins; D012333:RNA, Messenger; D011994:Recombinant Proteins; D007334:Insulin-Like Growth Factor I; D003907:Dexamethasone