Regulation of Insulin-Like Growth Factor-Binding Protein-5 by Insulin-Like Growth Factor I and Interleukin-1α in Ovine Articular Chondrocytes*

Abstract

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.

The Insulin-Like Growth Factor System in the GT1-7 GnRH Neuronal Cell Line

“…In granulosa cells and pituitary AtT-20 cells, IGF-I exerts its effect on IGFBP-5 by stimulation of its mRNA in the cells [32, 33]. In contrast, in osteoblast and chondrocyte cells, IGF-I showed no effect on IGFBP-5 mRNA but increased IGFBP-5 by reducing proteolysis [34, 35]. In GT1-7 cells, IGFs had no effects on IGFBP-5 mRNA (data not shown).…”

Homeostasis of the extracellular matrix of normal and osteoarthritic human articular cartilage chondrocytes in vitro

“…Most of the known effects of IGF-1 and -2 are mediated by IGFR1 15 . IGF binding proteins (IGFBP) that are produced by the chondrocytes and bind to IGF are important controlling factors of IGF activity [16][17][18][19] . The secretion of IGFBP is controlled by the cytokines IL-1 and TNF- 17 .…”

“…Besides capability to induce resorption of articular cartilage, IL-1 has been shown to directly depress the synthesis of aggrecan and collagen by the chondrocyte 22,23 . In addition, IL-1 may depress chondrocytes by reducing IGFI access to these cells by enhancing IGFBP synthesis 17,19 . The IL-1 effects are mediated through a high-affinity cell surface receptor, IL-1RI 24,25 .…”

One of two chondrocyte‐expressed isoforms of cartilage intermediate‐layer protein functions as an insulin‐like growth factor 1 antagonist

“…Insulin‐like growth factor 1 (IGF‐1) blocks this TGFβ response, and IGF‐1 alone promotes the lowering of extracellular PPi (7). However, IGF‐1 may be less able to restrain extracellular PPi levels in cartilage with established OA, where chondrocytes typically manifest varying degrees of IGF‐1 resistance (8–16).…”