Introduction

Over 50 years ago, Salmon and Daughaday discovered the insulin-like growth factor-I (IGF-I) that has insulin-like signaling properties and can mediate various biologic activities in response to changes in the growth hormone (GH) status (1). Since then, there have been a lot of reports on the actions of IGF-I and its receptor (IGF-IR) and binding proteins (IGFBPs) in regulating biological functions in both physiological and pathological states. Originally, IGF-I was considered a key component of the somatomedin hypothesis. Based on this hypothesis, GH, made by pituitary glands, regulates somatic growth by controlling the production of hepatic IGF-I (or somatomedin C), which is then distributed through the circulation to act on remote target organs/tissues (2). This hypothesis was recently challenged by a number of important discoveries, including a seminal study examining skeletal development in mice with liver-specific deletion of the Igf-I gene (LiverIGF-I−/−) (3,4). In the latter mice, the circulating IGF-I level was reduced to less than 25% of normal. Despite that, those KO mice developed and grew normally, and their skeletal changes were minimal (3), suggesting that local IGF-I production is sufficient for supporting general growth and skeletal development. In addition, circulating insulin-like growth factor-II (IGF-II), (another high-affinity ligand to IGF-IR), may also compensate for the loss of systemic IGF-I in these animals. However, the impact of IGF-II on skeletal development is GH-independent and more restricted to embryogenesis in mice (5,6). All skeletal cells, including chondrocytes, osteoblasts, osteocytes and osteoclasts, express significant levels of IGF-I, IGF-II and IGF-IR (7–10). Studies using cell cultures deprived of systemic IGF-I effects and tissue-specific gene knockout (KO) mouse models have begun to define the molecular actions of IGF-I and IGF-IR in different bone cell populations. Collectively, these studies indicate that these molecules constitute complex autocrine/paracrine networks to regulate cell proliferation, survival, and differentiation, controlling the pace of cartilage and bone development and remodeling. This review focuses on recent studies, demonstrating the actions of local IGF-I/IGF-IR signaling in (a) supporting skeletal development and remodeling, (b) cross-talk with other autocrine/paracrine pathways in the bone, (c) mediating cell-cell interactions between various types of skeletal cells, and (d) integration with systemic signals to produce skeletal anabolism.

IGF-I, IGF-II, and IGF-IR Signaling Cascades

IGF-I is a single polypeptide chain consisting of 70 amino acids, which has a binding affinity to IGF-IR 100-fold higher than insulin. IGF-II consists 180 amino acids, which also binds to IGF-IR with a comparable binding affinity as IGF-I. IGF-II also binds to the IGF-II receptor (also named the cation-independent mannose 6-phosphate receptor), which acts as a signaling antagonist to prevent IGF-II responses. IGF-IR consists of two α and two β subunits, which are linked by disulfide bonds (11). The α and β subunits are cleavage products of a 1367-amino acid IGF-IR precursor (12). The extracellular α-subunit forms a pocket for IGF-I binding, while the intracellular domain (amino acids 956–1256) of the membrane-spanning β-subunit contains a kinase domain (13,14). Upon activation, the tyrosine kinase activity of the IGF-IR entails sequential phosphorylation events at residues Y1135, Y1131, and Y1136, which produce conformational changes in the β subunits to create docking sites for downstream signaling molecules and their phosphorylation (14). Activation of IGF-IR triggers multiple signaling cascades in a cell context-dependent manner to regulate diverse cell functions ranging from proliferation and survival to differentiation into specialized cell lineages. For example, in bone cells and chondrocytes, activated IGF-IRs can recruit members of the insulin receptor substrate (IRS) family (IRS 1–4) directly to their phosphotyrosine-binding (PTB) and SRC homology 2 (SH2) domains, and indirectly to the growth receptor binding protein-2 (Grb2) and the p85 subunit of phosphatidyl inositol 3 kinase (PI3K) through specific motifs within the IRSs (15–17). Both IRS-1 and IRS-2 are required for a full action of IGF-I in bone, but they regulate different functions. Deletion of IRS-1 gene in mice decreases both bone formation and resorption (18), whereas IRS-2 KO mice have reduced bone formation, but increased bone resorption (19), suggesting that different IRSs may be used by different types of bone cells.

Activation of IGF-IR triggers the Ras-Raf-MEK-ERK-MAPK signaling cascade by a series of IRS-dependent protein phosphorylations and complex formations (17,20). Through the SH2 domain in the receptor, IGF-IR binds to the SH2 domain of Grb2, which, in turn, complexes with Sos, a guanine nucleotide exchange factor that mediates GDP/GTP exchange in Ras GTPase, and leads to Ras activation. The activated Ras stimulates Raf kinase to phosphorylate and activate MAP kinase kinase (MEK), which, in turn, activates ERK1/2 (MAPK1/2) via serine/threonine phosphorylations of the latter proteins. Activated ERK1/2 enters the nucleus to phosphorylate and activate various transcription factors, leading to increased cyclin D1 expression and reduced p21cip and p27kip expression facilitating cell proliferation.

Activation of IGF-IR/IRS complex also recruits and stimulates the PI3K, which converts PIP2 to PIP3 in the membrane to recruit Akt and associated proteins PDK 1/2, leading to Akt phosphorylation and activation. The activated Akt phosphorylates and inactivates Bad, a pro-apoptotic member of the bcl-2 family, therefore promoting cell survival (17). Akt can also activate mTOR and p70S6 kinase to stabilize beta-catenin to enhance proliferation (21). It has also been shown that hypoxia stimulates PI3K/Akt/mTOR pathway to increase VEGF-A production and promote vascular invasion in the growth plate (22,23), likely in coordination with IGF-I.

The actions of IGF-I and IGF-IR in skeletal development

Skeletal development is a highly coordinated process, which requires initial recruitment and migration of relevant precursors to future bone sites, commitment of the precursors to chondrogenic and osteogenic lineages and their terminal differentiation to acquire a complete set of cartilage- or bone-forming functions (24,25). Skeletal patterning, which determines bone numbers and shapes, is controlled by progenitor recruitment and commitment, while bone growth rates mainly depend on the proliferation, survival and differentiation of chondrogenic and osteogenic cells (24). Recent studies of global and tissue-specific gene KO mice indicate that IGF-I signaling is more critical in the latter steps of the process, but less in the skeletal patterning.

Studies of global IGF-I and IGF-IR KO mice

In mouse embryos, the expression of IGF-I and IGF-IR in the skeleton first appears at 13.5 dpc (days post coitus), peaks at 14.5 dpc for IGF-I or 16.5 dpc for IGF-IR, and then, slightly declines as gestation proceeds (26). Ablating IGF-I expression in the global IGF-I KO mice causes severe growth retardation, which is evident as early as 13.5 dpc (27–29). The KO mice develop smaller skeletons with a significant delay in mineralization at 14.5 dpc and onward. Despite that, all of their skeletal elements are present (26), indicating normal skeletal patterning. The skeletal defects in the KO mice are caused, at least in part, by reduced proliferation, delayed differentiation and increased apoptosis in the growth plate (26). Most of the IGF-I KO mice die immediately after birth. Studies of the few surviving KO mice (less than 5%), however, show continually retarded bone growth (27,28), suggesting that IGF-I signaling is also essential for skeletal development during post-natal growth. Ablating IGF-IR in the global IGF-IR KO mice produce similar, but more severe growth retardation and skeletal defects than the IGF-I KO mice. Global IGF-I/IGF-IR double KO mice phenocopy the phenotypes of the global IGF-IR KO mice (28). These data suggest that IGF-IR likely mediates all skeletal responses to IGF-I and perhaps the responses to IGF-II during embryonic development.

IGF-I/IGF-IR signaling in chondrogenesis

Although the above global KO mice confirmed the importance of IGF-I/IGF-IR signaling in supporting skeletal development, they did not specify the action of IGF-I signaling in specific cell lineages in the bone or exclude the impact of other systemic derangements on the skeleton. To define IGF-I/IGF-IR actions in chondrocytes, cartilage-specific IGF-IR KO mouse models (cartIGF-IR−/− mice) were made by crossing floxed IGF-IR mice that carried loxP sequences flanking the exon 3 of the gene (30) with mice expressing Cre recombinase transgene, under the control of a type II collagen promoter (31). The growth phenotype of cartIGF-IR−/− mice is less severe than the global IGF-IR KO mice. The body weights of newborn cartIGF-IR−/− mice are ≈90% (vs ≈45% for the global IGF-1R KO mice) of their wild-type littermates. Most of the cartIGF-IR−/− mice also die shortly after birth with smaller and less mineralized skeletons. These skeletal phenotypes are caused by severe chondrocyte defects characterized by reduced proliferation, delayed differentiation and hypertrophy and increased apoptosis (32). These data confirm the role of IGF-I/IGF-IR signaling in modulating vital functions of chondrocytes and the overall pace of endochondral bone formation in embryos.

The perinatal death of the cart.IGF-IR−/− mice, however, precluded studies of IGF-IR actions in the growth plates during postnatal growth. To address this issue, tamoxifen (Tam)-inducible cartilage-specific IGF-IR KO (TamCarIGF-I R−/−) mice were studied (33,34). At 2 weeks of age and 7 to 8 days after the induction of gene KO by Tam, the TamCarIGF-IR−/− mice show significant growth retardation with a body weight that was ≈70% of their control littermates, and demonstrate severely disorganized growth plates again, due to reduced cell proliferation and delayed differentiation, indicating comparable IGF-IR actions in growth plate development during embryonic and postnatal development (32).

Cross-talk between IGF-I/IGF-IR and parathyroid hormone–related protein (PTHrP) signaling in chondrocytes

The PTHrP/Indian Hedgehog (Ihh) feedback loop is a well-established autocrine/paracrine pathway that controls the pace of chondrocyte differentiation (35–37) (Figure 1). PTHrP is made by perichondrial and reserve cells in embryonic skeletons and diffuses into the proliferation zone to activate the type 1 PTH/PTHrP receptor (PTH1R) in proliferating chondrocytes to sustain their proliferation and delay their maturation (38). When proliferating chondrocytes mature, they increase the production of Ihh, which acts on its receptor Patched in the neighboring cells via unknown mechanisms to increase the production of PTHrP (39), thus slowing down cell differentiation to prevent early closure of the growth plate (Figure 1). During postnatal growth, PTHrP is expressed in maturing/prehypertrophic chondrocytes to support a local autocrine/paracrine interaction. Interestingly, global IGF-I KO, CartIGF-IR−/−, and TamCarIGF-IR−/− mice all show increased expression of PTHrP protein and RNA, but decreased Ihh expression in their growth plates, indicating a disrupted PTHrP/Ihh feedback loop (26,32). The increased PTHrP expression could readily contribute to the delayed chondrocyte differentiation and mineralization in these KO mice, as these chondrocyte defects are also apparent in transgenic mice overexpressing PTHrP (40). However, the latter mice show increased chondrocyte proliferation and decreased apoptosis in their growth plates. On the other hand, blocking IGF-IR signaling in chondrocytes prevents an increase in growth and survival of chondrocytes, which was otherwise expected with increased PTHrP expression. The increased PTHrP expression may represent a failed attempt of the affected chondrocytes to compensate for the reduced proliferation in the IGF-IR KO growth plates. It remains unclear how ablation of IGF-IR leads to the increased PTHrP expression in chondrocytes. Although, Ihh is a critical modulator that promotes PTHrP expression in the growth plate, our observations on the decreased Ihh expression in the CartIGF-IR−/− and TamCarIGF-IR−/− growth plates, however, support an Ihh-independent negative regulation of PTHrP expression and, perhaps, a direct effect on Ihh expression by IGF-IR signaling (41) (Figure 1).

Figure 1
figure 1

A schema illustrating the interplays between IGF-I/IGF-IR, PTHrP/Ihh, and Ca2+/CaSR signaling pathways in modulating chondrocyte differentiation in the growth plate. IGF-IR signaling promotes cell proliferation, maturation and terminal differentiation, directly (blue arrows) or indirectly, by inhibiting PTHrP expression (red lines). The inhibition of PTHrP could be a direct effect of IGF-I on PTHrP-producing cells, or an indirect effect on the ability of Ihh to increase PTHrP expression. The CaSR begins to express in maturing chondrocytes and its expression level increases as cell differentiation proceeds. Ca2+/CaSR signaling increases both IGF-I and IGF-IR. This increased gene expression is required for a full action of CaSR in promoting chondrocyte differentiation, in addition to other IGF-IR-independent effects (purple arrows). See the main text for detailed descriptions of these IGF-I actions.

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Cross-talk between IGF-IR and extracellular Ca2+-sensing receptor (CaSR) signaling in chondrocytes

Ca2+ critically supports terminal differentiation of chondrocytes and normal growth plate development. Ca2+ deficiency causes childhood rickets (42) due to delayed chondrocyte differentiation and reduced matrix synthesis and mineralization in the growth plates. These defects could be reversed with adequate supplementations of dietary Ca2+ (43), indicating the importance of Ca2+ availability in normal growth plate development. Studies using chondrogenic cell lines or primary growth plate chondrocytes further, confirm direct actions of Ca2+ on the cells to stimulate G protein-mediated signaling responses, and promote terminal differentiation of the cells (44–47).

Studies of cartilage-specific CaSR KO mice and cultured chondrocytes suggest that the CaSR is the molecular basis for extracellular Ca2+-sensing in chondrocytes (44–47). In the growth plate, the CaSR is first expressed in maturing chondrocytes, and its level increases as cells hypertrophy (45). This expression pattern supports a role for the CaSR in mediating terminal differentiation of the cells. In support of this concept, knockdown of CaSR expression blocks the ability of high extracellular Ca2+ concentrations ([Ca2+]e) to promote cell differentiation and matrix mineralization in cultured chondrocytes (46). Furthermore, CartCaSR−/− mice with cartilage-specific ablation of CaSR gene develop shorter, undermineralized skeletons due to delayed differentiation of hypertrophic chondrocyte (44). Interestingly, the expression of IGF-I and IGF-IR is profoundly reduced in the growth plates of the CartCaSR−/− mice (44), suggesting that Ca2+/CaSR promotes chondrocyte differentiation, at least in part, by enhancing IGF-I signaling (Figure 1). In concordance with this regulatory scheme, ablating IGF-IR genes in cultured chondrocytes inhibits the ability of high [Ca2+]e (by ≈50%) to enhance terminal differentiation and matrix mineralization, supporting a role for IGF-I/IGF-IR signaling in mediating CaSR-induced chondrocyte differentiation (Figure 1).

Does the Ca2+/CaSR signaling pathway also interact with the PTHrP/Ihh feedback loop? In cultured chondrocytes, raising [Ca2+]e profoundly inhibits PTHrP and PTH1R expression and blocks the ability of PTHrP to delay cell differentiation and matrix mineralization [(47) and unpublished data]. Furthermore, ablating the IGF-IR gene specifically in growth plate chondrocytes in mice increases their expression of PTHrP, but not PTH1R (32). These observations support a paradigm, in which Ca2+/CaSR signaling counteracts PTHrP/PTH1R signaling by suppressing PTH1R expression, independently, of IGF1/IGF1R signaling and by inhibiting PTHrP expression via the IGF1/IGF1R pathway to support normal progression of chondrocyte differentiation and growth plate development (Figure 1).

IGF-I/IGF-IR actions in bone remodeling

Throughout life, bone is constantly remodeled by co-operative actions of bone-resorbing osteoclasts and bone-forming osteoblasts (48,49). The osteoclasts are derived from hematopoietic stem cells (HSC), while osteoblasts differentiate from bone marrow mesenchymal stem cells (MSC, Figure 2). Bone formation and resorption are tightly balanced to maintain structural integrity of the bone and to mobilize Ca2+ to meet systemic demands. Studies of global IGF-I KO mice (50,51), which survive to adulthood, show decreased bone formation rates, but higher trabecular bone mass (BV/TV), indicating reduction in both osteogenic and osteoclastogenic activities in the bones. These observations support a non-redundant role for IGF-I signaling in bone remodeling. Recent studies of cell-specific IGF-I and IGF-IR KO mice and in vitro cell cultures have begun to reveal detailed molecular actions of those molecules in each population of skeletal cells.

Figure 2
figure 2

A schema illustrating the actions of IGF-I/IGF-IR signaling in the differentiation of osteoblast and osteoclast in bone. IGF-IR signaling can promote the proliferation, survival, and differentiation of osteoblast (OB), osteocyte (OCY) and osteoclast (OCL), directly (blue arrows) or indirectly, by enhancing cell-cell interactions/couplings between OBs and OCLs, by increasing the expression of critical components in the c-fms/M-CSF, RANK/RANKL and EphrinB2/EphB4 signaling pathways (red boxes and arrows). Red arrows indicate uni- or bi-directional signaling responses mediated by these cell-cell interactions. IGF-IR signaling is also required for mechanosensing in osteocytes. See the main text for detailed descriptions of these IGF-I actions. HSC: hematopoietic stem cell; MSC: mesenchymal stem cells.

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IGF-I signaling in the osteoblastic lineage

The osteoblastic lineage begins with mesenchymal progenitors that progress through stages of preosteoblasts (Pre-OB), mature osteoblasts (OB) and, finally, osteocytes (OCY, Figure 2). At each stage, cells express specific markers and unique regulators of cell differentiation. Osteoprogenitors and preosteoblasts express RUNX2 and osterix (OSX), two critical transcriptional factors that specify cell fates. Preosteoblasts produce large amounts of type I collagen as a major component of their surrounding matrix. Mature or fully differentiated osteoblasts, which express osteocalcin (OCN), acquire functions to mineralize the matrix and produce hardened bone. Some mature osteoblasts are, subsequently, embedded in the matrix as osteocytes, which express the dentin matrix protein-1 (DMP-1). Some osteoblasts become quiescent on bone surface and function as lining cells, or otherwise, die by apoptosis (52,53).

In vitro and in vivo studies support a role for the IGF-I signaling in modulating growth, survival, differentiation and mineralizing functions of osteoblasts (Figure 2). In cultures of primary osteoblasts or osteogenic cell lines, IGF-I treatment promotes cell proliferation, differentiation and matrix production (54), and inhibits their apoptosis (55,56). Deleting IGF-IR at the stage of osteoprogenitor development by Cre-lox recombination using an osterix promoter-driven Cre mouse line, results in delayed osteoblast maturation, matrix synthesis and mineralization, without affecting the differentiation or function of osteoclasts (57). When IGF-IR is deleted in mature osteoblasts, using OCN promoter-driven Cre line, the KO mice produce less trabecular bone, with an increased proportion of osteoid in the bone, indicating impaired mineralizing functions in OCNIGF-IR−/− osteoblasts (58,59). In contrast, overexpressing IGF-I in mature osteoblasts increases bone formation, with a significant decrease in the mineralization lag time (the time for osteoid to be mineralized), but without a change in osteoblast number (60). These data are consistent with a role for IGF-I signaling in promoting osteoblast differentiation in early osteoblasts and enhancing mineralizing activities in mature osteoblasts (60).

Osteocytes make up over 90% of the cells in bone (61). In addition to their roles in bone development and remodeling, osteocytes are also a source of hormonal factors, like FGF23, which modulate mineral homeostasis by acting on distant organs, such as kidney, muscle, parathyroid glands and other tissues (62). DMPIGF-I KO mice, in which the IGF-I gene is deleted in osteocytes using DMP-1 promoter-driven Cre, do not have altered serum IGF-I, calcium, or phophorus levels, but do show decreased serum levels of the bone formation maker procollagen type I N-terminal propeptide (PINP) and the bone resorption marker, c-telopeptide (CTx). These KO mice also have reduced trabecular and cortical bone mineral content and decreased osteoclast surface and numbers, but have no change in trabecular bone volume. Interestingly, these mice also showed growth plate defects due to unknown causes (63). Nevertheless, these KO mice fail to respond to mechanical loading in promoting periosteal and endosteal bone formation, and the expression of early mechanoresponsive genes (IGF-I, Cox-2, c-fos), osteogenic markers (RUNX2, and osteocalcin) and canonical Wnt signaling genes (Wnt10b, Lrp5, Dkk1, sFrp2). The lack of osteogenic response is not due to a reduced osteocyte density (64), suggesting that osteocyte-derived IGF-I not only mediates bone osteoblast/osteocyte differentiation, but also the mechanosensitivity of the cells (Figure 2).

IGF-I/IGF-IR signaling in osteoclasts

Studies of global IGF-I KO mice showed fewer (76% of WT control) and smaller osteoclasts with fewer nuclei in the bone (65), indicating impaired osteoclastogenesis and bone resorption (Figure 2). These osteoclast defects could explain, at least in part, the increased bone fraction, despite a decreased bone formation rate, in the KO mice described earlier. These osteoclast defects appear to be cell-autonomous, as they could be recapitulated by culturing osteoclast precursors from the global IGF-I KO mice, which showed reduced ability to differentiate into mature osteoclasts (65). Interestingly, co-culturing IGF-I+/+ osteoclast precursors (spleen cells from WT mice), with IGF-I−/− osteoblasts (bone marrow stromal cells from IGF-I KO mice), produces significantly fewer (by ≈90%) multi-nucleated osteoclasts than coculturing IGF-I+/+ osteoclast precursors with IGF-I+/+ osteoblasts, suggesting that osteoblasts may be a source of IGF-I to promote osteoclastogenesis. Similarly, co-culturing IGF-I−/− osteoclast precursors, with IGF-I+/+ osteoblast precursors, also produces less (by ≈50%) multi-nucleated osteoclasts than co-culturing IGF-I+/+ osteoclast precursors with IGF-I+/+ osteoblasts (65), suggesting that the expression of IGF-I in osteoclast precursors is also required to support the full-scale osteoclastogenesis.

To further define the action of IGF-I signaling on osteoclast differentiation, mice (OCLIGF-IR KO) with deletion of IGF-IR gene in osteoclast precursors, were studied. These KO mice have increased trabecular bone fraction and reduced osteoclast number, as seen in the global IGF-I KO mice. Similarly, ablation of IGF-IR in cultured osteoclast precursors reduces the number and size of osteoclasts and their expression of RANK, c-fms, and NFATc1 (unpublished data). Taken together, these in vitro and in vivo data, confirm an essential role for the IGF-I/IGF-IR signaling in promoting both early- and late-stage osteoclastogenesis (Figure 2).

IGF-I signaling in skeletal cell-cell interactions

In addition to their interactions through hormonal factors, like osteoblastic M-CSF, which activates c-fms in osteoclasts, osteoblasts, osteoclasts and chondrocytes, can also communicate with each other through physical interactions using their cell-surface ligands and receptors. These exert uni- or bi-directional signals in the corresponding cells (Figure 2). For example, osteoblasts express receptor activator of NF-kB (RANK) ligand (RANKL) to activate the RANK on the surface of osteoclast precursors, and induce the differentiation of the latter cells (66). More recent studies have shown that osteoblasts express EphB4, a member of the tyrosine kinase receptor Eph family, as well as its ligand ephrin B2, supporting interactions between osteoblasts. Osteoclasts express ephrin B2, but not EphB4, which enable osteoblast-osteoclast interactions (67). IGF-I stimulates RANKL, ephrin B2 and EphB4 expression in osteoblasts and RANK and EphrinB2 expression in osteoclasts. Changes in the expression of those signaling molecules could be part of the underlying mechanism modulating osteoclastogenesis in response to IGF-I (Figure 2). This concept is supported by the reduced ability (by ≈90) of IGF-I−/− osteoblasts to induce osteoclastogenesis of spleen cells cultured from WT mice (65). Furthermore, our recent studies demonstrate that IGF-I treatment enhances the ability of osteoblasts and chondrocytes to induce osteoclast formation in the corresponding co-cultures (osteoblasts/osteoclasts or chondrocytes/osteoclasts), and this effect could be blocked by a specific peptide that interferes with the EphB4-ephrinB2 interactions (manuscript in submission). These data support a role for IGF-I signaling in skeletal cell-cell interactions by stimulating RANKL/RANK and ephrin B2/EphB4 pathways in the cells (Figure 2).

Integration of local IGF-I/IGF-IR signaling with systemic hormones

Integration with GH

Although it is well established that GH stimulates hepatic gene expression to make and supply IGF-I to the circulation, recent studies suggest that changes in circulating IGF-I levels may be an indicator of GH status, but may not be directly involved in controlling local functions in non-hepatic tissues, including bone (68). The fact that chondrocytes and osteoblasts also express growth hormone receptors (GHRs) (69) suggests direct actions of GH on these cells. Indeed, GH can exert both IGF-I-dependent and IGF-I-independent actions to regulate bone cell and chondrocyte functions. For example, GH could stimulate ERK and Akt phosphorylation and inhibit apoptosis in cultures of osteoblasts lacking IGF-IR expression, but fail to induce cell proliferation in the same cultures (70). Moreover, we (50) showed that GH could stimulate bone formation even in the global IGF-I KO mice, although not to the extent of IGF-I replacement. Studies by Gan and colleagues showed that GH can promote the formation of GHR/JAK2/IGF-IR complex to facilitate GHR signaling in osteoblasts (71), indicating a novel complementation between GHR and IGF-IR signaling, which could be a basis for some IGF-IR-dependent GH actions. The fact that OCNIGF-IR KO mice failed to respond to GH treatment to increase bone mass suggests that IGF-IR-dependent actions of GH are the predominant responses to promote skeletal anabolism (70). It, however, remains unclear regarding the sources of IGF-I that respond to GH and promote osteoblast functions in the bone.

Wu and colleagues generated a compound KO/transgenic mouse model (GHRKO-HIT) to assess the importance of local IGF-I production for skeletal development (72). In this model, GHR is globally ablated and an IGF-I transgene is expressed in liver to restore serum IGF-I levels. The skeletal IGF-I gene expression was blunted in these mice due to the lack of GHR signaling in the bone. Interestingly, IGF-I produced by the liver failed to reach bone to compensate for local IGF-I deficiency. As a result, GHRKO-HIT mice were growth retarded and had reduced bone growth. These data confirm the importance of locally produced IGF-I in supporting overall skeletal development

Integration with PTH

PTH is a critical regulator of mineral and skeletal homeostasis. Its effects on bone are complex. Intermittent administration of PTH produces skeletal anabolism (73,74) by increasing the growth (75), survival (76) and differentiation of osteoblasts (77). Continuous infusion of PTH, however, is catabolic to bone due to a net increase in bone resorption (78,79). Abundant studies support a non-redundant role for IGF-I signaling in mediatitng anabolic actions of PTH. In osteoblast cultures, PTH stimulates IGF-I production. Neutralizing antibodies for IGF-I block the stimulatory effects of PTH on collagen synthesis (80) and osteoblast differentiation (81). Intermittent PTH fails to elicit skeletal anabolism in global IGF-I KO mice (51,82,83), or in mice lacking the expression of IRS-1 (84). Similarly, ablation of IGF-IR in mature osteoblasts prevents the anabolic effects of intermittent PTH in OCNIGF-IR−/− mice (59). Interestingly, ablating IGF-I in the liver does not affect PTH-induced anabolism in bone (85). Taken together, these data support the concept that intermittent PTH produces skeletal anabolism by enhancing local IGF-I/IGF-IR signaling in bone.

Conclusions

IGF-I signaling is critical for all aspects of skeletal development and remodeling. The major signaling pathways for IGF-I are mediated by IGF-IR, which initiates both the MAPK and PI3K pathways that regulate diverse cellular functions. During embryonic and postnatal bone development, IGF-I signaling critically modulates chondrocyte growth, survival and differentiation, at least partly, by inhibiting PTHrP production and enhancing CaSR signaling. In bone, IGF-I is a critical regulator of growth, survival, differentiation and mineralizing functions of osteoblasts. IGF-I also regulates the mechanosensitivity of osteocytes. IGF-I signaling is required for the skeletal cell communications through RANKL/RANK, M-CSF/c-fms and ephrin B2/EphB4 signaling. Moreover, local IGF-I production and IGF-IR signaling are essential for the anabolic actions of PTH and GH.

Future directions

At least five important questions regarding local IGF-I/IGF-IR signaling warrant future investigation: 1) What are the local sources of IGF-I controlling chondorcyte, osteoblast and osteoclast functions? 2) How do systemic GH and PTH signals mediate IGF-I expression and IGF-IR signaling? 3) How does IGF-I signaling interact with other autocrine/paracrine signals, like Wnt, ephrins, BMP, TGFß and other bone regulating molecules? 4) How does a change in IGF-I signaling contribute to disease states, like osteoporosis, osteoarthritis, bone cancer metastasis and fracture healing? 5) Finally, and perhaps most importantly, how can we translate those basic research findings into clinical uses to prevent and treat skeletal diseases?