Spatial and temporal regulation of GH–IGF-related gene expression in growth plate cartilage

Abstract

Previous studies of the GH-IGF system gene expression in growth plate using immunohistochemistry and in situ hybridization have yielded conflicting results. We therefore studied the spatial and temporal patterns of mRNA expression of the GH-IGF system in the rat proximal tibial growth plate quantitatively. Growth plates were microdissected into individual zones. RNA was extracted, reverse transcribed and analyzed by real-time PCR. In 1-week-old animals, IGF-I mRNA expression was minimal in growth plate compared with perichondrium, metaphyseal bone, muscle, and liver (70-, 130-, 215-, and 400-fold less). In contrast, IGF-II mRNA was expressed at higher levels than in bone and liver (65-and 2-fold). IGF-II expression was higher in the proliferative and resting zones compared with the hypertrophic zone (P!0 . 001). GH receptor and type 1 and 2 IGF receptors were expressed throughout the growth plate. Expression of IGF-binding proteins (IGFBPs)-1 through -6 mRNA was low throughout the growth plate compared with perichondrium and bone. With increasing age (3-, 6-, 9-, and 12-week castrated rats), IGF-I mRNA levels increased in the proliferative zone (PZ) but remained at least tenfold lower than levels in perichondrium and bone. IGF-II mRNA decreased dramatically in PZ (780-fold; P!0 . 001) whereas, type 2 IGF receptor and IGFBP-1, IGFBP-2, IGFBP-3, and IGFBP-4 increased significantly with age in growth plate and/or surrounding perichondrium and bone. These data suggest that IGF-I protein in the growth plate is not produced primarily by the chondrocytes themselves. Instead, it derives from surrounding perichondrium and bone. In addition, the decrease in growth velocity that occurs with age may be caused, in part, by decreasing expression of IGF-II and increasing expression of type 2 IGF receptor and multiple IGFBPs.

The effect of GH and IGF1 on linear growth and skeletal development and their modulation by SOCS proteins

“…The type-2 IGF1R is expressed equally throughout all maturational zones of the growth plate, whereas the type-1 receptor is more highly expressed by proliferating chondrocytes (Trippel et al 1986, Parker et al 2007. These data are consistent with the concept that IGF1 has regulatory actions on all chondrocytes of the growth plate.…”

SOCS2 is the critical regulator of GH action in murine growth plate chondrogenesis

“…(68) Our studies did show that SOCS2 À/À metatarsals increased in growth in response to GH, suggesting that modulation of SOCS2 expression may be a critical method of altering GH insensitivity and raising local IGF-1 concentration. (70) Interestingly, these authors suggest that the source of IGF-1 interacting with its chondrocyte receptor may be derived from the surrounding perichondrium and/or bone. (70) The divergence in body length and weight in SOCS2 À/À mice was noted to occur at about 3 weeks of age, and this is consistent with the knowledge that GH is the major regulator of postnatal growth in mice, where peak GH activity occurs between postnatal days 20 and 40.…”

“…(70) Interestingly, these authors suggest that the source of IGF-1 interacting with its chondrocyte receptor may be derived from the surrounding perichondrium and/or bone. (70) The divergence in body length and weight in SOCS2 À/À mice was noted to occur at about 3 weeks of age, and this is consistent with the knowledge that GH is the major regulator of postnatal growth in mice, where peak GH activity occurs between postnatal days 20 and 40. (71) The reasons for the sexual dimorphic differences in growth of SOCS2 À/À mice noted here and elsewhere are unclear but are likely to involve estrogen's modulatory actions on GH signaling through the stimulation of SOCS2 expression.…”

RECENT RESEARCH ON THE GROWTH PLATE: Impact of inflammatory cytokines on longitudinal bone growth

“…This observation does not exclude the possibility that the direct effect might be mediated by an increase in the local production of IGF1, which in turn stimulates growth plate chondrocytes. The GH receptor can also be detected in the chondrocytes of all zones of the growth plate and, therefore, a direct effect of GH has been suggested (Parker et al 2007). Such a direct effect of GH is supported by a study where local injection of GH into the growth plate accelerated bone growth compared with the contralateral bone (Isaksson et al 1982).…”