Insulin-like growth factors (IGFs) contribute to the maintenance of the cartilage matrix by stimulating
proteoglycan synthesis. In contrast,
interleukin-1 (IL-1), an inflammatory
cytokine, suppresses the synthesis of
proteoglycans. In pathological conditions the chondrocytes' responsiveness to
IGF-I is decreased, and elevated levels of
IGF-binding proteins (IGFBPs) have been implicated as a possible cause. The aim of this study was to investigate the effects of
IGF-I and
IL-1 on
IGFBP production by ovine articular chondrocytes (OAC) and the roles of these IGFBPs in the regulation of
proteoglycan synthesis. As revealed by Western
ligand and immunoblotting, OACs secreted
IGFBP-2 and a 24-kDa
IGFBP in culture medium under basal conditions. Exposure of the cells to
IGF-I for 48 h resulted in the appearance of
IGFBP-5 in the medium.
Des(1-3)IGF-I, an
IGF-I analog with reduced affinity for IGFBPs, also increased the level of
IGFBP-5, but to a lesser extent than
IGF-I, whereas LR3IGF-I, which has virtually no affinity for IGFBPs, had no effect on
IGFBP-5. Furthermore,
IGFBP-5 underwent a time-dependent limited proteolysis when incubated with OAC-
conditioned medium, degrading into 22- and 16-kDa fragments. The degradation of
IGFBP-5 was significantly inhibited by
IGF-I, but not by
des(1-3)IGF-I or LR3IGF-I.
Basic fibroblast growth factor,
transforming growth factor-beta, and
platelet-derived growth factor had no effect on OAC IGFBPs. However, IL-1alpha increased the
IGFBP-5 level in a dose-dependent manner, showing maximum activity at 200 U/ml. Furthermore, IL-1alpha, but not
IGF-I, induced
IGFBP-5 messenger RNA expression, as assessed by Northern blot analysis. Coincubation of
IGF-I with IL-1alpha resulted in a substantially increased
IGFBP-5 protein level, suggesting a synergism between the mechanisms of action of these two factors.
Des(1-3)IGF-I and LR3IGF-I were 10 times more potent than
IGF-I in stimulating
proteoglycan synthesis, indicating inhibition of
IGF-I activity by endogenous IGFBPs. IL-1alpha reduced the
IGF-I bioactivity, but had no effect on the activities of the
IGF-I analogs, thus implying that locally produced IGFBPs, particularly
IGFBP-5, which was substantially increased when
IGF-I and IL-1alpha were coincubated, mediated the reduction of the
IGF-I activity. Our results demonstrate that
IGF-I and IL-1alpha synergistically increase the level of
IGFBP-5 in OAC by inhibiting the proteolysis and stimulating the expression of
IGFBP-5, respectively. Furthermore, the attenuation of
IGF-I-stimulated
proteoglycan synthesis by IL-1alpha in OAC appears to be mediated by chondrocyte IGFBPs. We conclude that locally produced IGFBPs, in particular
IGFBP-5, may play a critical role in the regulation of cartilage matrix degradation in inflammatory and
degenerative arthritides.