The Insulin-like growth factors (IGFs) are key regulators of matrix homeostasis in articular cartilage, and it has been proposed that dysregulation of metabolism during osteoarthritis (OA) is due to IGF insensitivity. The IGF binding proteins (IGFBPs) can either promote or inhibit IGF activity or may act independently of IGFs. Preliminary studies by the applicant indicate that IGFBP-3 is increased during OA, as assessed directly in extracts of uncultured OA cartilage. The working hypothesis states that IGFBP-2 is Iocated on the cell membrane and acts to maintain matrix homeostasis. IGFBP-3 content in cartilage, but not IGFBP-2, increases 2-3 fold during OA. IGFBP-3 can bind to its own specific, high affinity proteins on the chondrocyte and can also bind to abundant extracellular anchoring sites. The latter include sites in the fibronectin (FN) matrix, where anchored IGFBP-3 acts to sequester IGFs. The overall effect of IGFBP-3 is to restrict IGF action and repair of cartilage.
The Specific Aims are: (1) the localization of IGFBP-3, IGFBP-2, IGF-I and II and the IGF receptor (IGF-IR) during OA progression in human cartilage (immunohistochemical studies) and, as appropriate, the possible co-localization of IGF with BP-3 and FN (by immunoelectron microscopy); to examine selected interactions of IGFBP-3 and IGF in vitro; and to test isolated chondrocytes for the presence of high affinity IGFBP-3 association proteins (putative receptors); (2) changes in the levels and synthesis of the components of the IGF axis (IGFBPs, IGFs, IGFIR), as well as in the putative IGFBP-3 anchor, fibronectin, during OA; and (3) the function of the IGFBPs, as assessed by their effects on matrix synthesis and cell division. The overall function of resident IGFBPs will be determined during OA by culturing human cartilage slices and examining the role of IGFBPs by addition of IGFs and various IGF analogs that have been mutated to selectively lose their capacity to bind IGFBPs, or to lose only their ability to bind to the IGF receptor. In addition, antisense oligodeoxynucleotides will be used to block IGFBP-3, as well as blocking antibodies following permeabilization of the cartilage matrix. The long-term goal is to design plausible protocols to shift the IGF axis towards a repair mode so as to define therapeutic candidates for human OA management.
|Sun, Tiezheng; Hunziker, Ernst B; Morales, Teresa I (2008) Subcellular distribution of the insulin-like growth factor (IGF) binding proteins (IGFBPs) 2 and 3 in articular chondrocytes. J Orthop Res 26:1421-7|
|Hunziker, E B; Kapfinger, E; Martin, J et al. (2008) Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) is closely associated with the chondrocyte nucleus in human articular cartilage. Osteoarthritis Cartilage 16:185-94|
|Morales, Teresa I (2008) The quantitative and functional relation between insulin-like growth factor-I (IGF) and IGF-binding proteins during human osteoarthritis. J Orthop Res 26:465-74|