Subcellular distribution of the insulin‐like growth factor (IGF) binding proteins (IGFBPs) 2 and 3 in articular chondrocytes

Abstract

The insulin-like growth factor (IGF) is a major anabolic regulator in articular cartilage. The IGF-binding proteins (IGFBPs) are increased during osteoarthritis (OA), but the function of the later proteins remains unknown. In general, the IGFBPs are pluripotential effectors capable of IGF regulation and of acting on their own to control key cell functions, including survival and proliferation. The independent functions are often associated with their cell location, and therefore this study explores the distribution of IGFBP-2 and IGFBP-3 in articular chondrocytes. Immunohistochemistry was used to localize IGFBP-2 in normal human articular cartilage. Bovine chondrocytes were used for subcellular fractionation (hypotonic cell lysis) under nonreducing conditions and nuclear purification (centrifugation on sucrose cushions). Cell fraction markers and IGFBPs were assayed in the subcellular fractions by Western immunoblot. The IHC results showed association of IGFBP-2 with chondrocytes, but not with the nuclei. Subcellular fractionation of isolated chondrocytes yielded intact nuclei as assessed at the light microscopic level; the nuclear marker histone H1 was exclusively associated with this fraction. More than 90% of the cytoplasmic marker GAPDH and all the detectable IGFBP-2 were in the cytoplasmic fraction. Immunoreactive IGFBP-3 was found in the cytoplasmic and peri-nuclear/nuclear fractions. Chondrocytes contain intracellular IGFBP-2 and IGFBP-3 but only IGFBP-3 is associated with nuclei. This suggests the hypothesis that the actions of these IGFBPs in articular cartilage extend beyond the classic modulation of IGF receptor action. ß

Differential expression of IGF1, IGFR1 and IGFBP3 in mandibular condylar cartilage between male and female rats applied with malocclusion

“…The production of IGFBP3 in chondrocytes can be increased by the elevated expression of inflammatory cytokines in OA, such as IL1α, TNFα and prostaglandin E2 (28, 29). In the present study, we confirmed the increased expression of IGFBP3 in OA‐like degenerative regions as being reported previously (9, 11) may contribute to neutralisation of IGF1 activity in condylar cartilage. Furthermore, the expression of IGFBP3 also showed gender differences.…”

“…In normal cartilage, IGFBPs extend the half‐life of IGF1, which makes the latter available for receptor binding. However, elevated IGFBP levels, such as the higher levels of IGFBP3 in OA, inhibit the binding of IGF1 to IGFR1, because of the higher affinity of IGF1 to IGFBP than that to IGFR1 (6, 9, 11).…”

Nuclear export and mitochondrial and endoplasmic reticulum localization of IGF-binding protein 3 regulate its apoptotic properties

“…Nuclear localization of IGFBP3 is a well-described phenomenon and has been demonstrated in a variety of cellular models (Jaques et al 1997, Li et al 1997, Wraight et al 1998, Liu et al 2000, Sun et al 2008). IGFBP3 possesses a consensus bipartite NLS (Radulescu 1994), and nuclear transport is facilitated by importin-β factor (Schedlich et al 2000).…”

Comparative Analyses of the Secretome from Dedifferentiated and Redifferentiated Adult Articular Chondrocytes

“… 17 IGFBP-2 increases proteoglycan production by mediating TGF-β effect on proteoglycan synthesis. 18 Enhanced expression of this factor in MLs and not in spheroids could have connections with the high proliferation rate of cells in ML. On the other hand, spheroids showed enhanced expression of vascular endothelial growth factor (VEGF), CKb 8-1, and macrophage colony–stimulating factor (M-CSF).…”