Insulin-like growth factor binding proteins (IGF-BPs) in bovine articular and ovine growth-plate chondrocyte cultures: their regulation by IGFs and modulation of proteoglycan synthesis

Altered IGF-I activity and accelerated bone elongation in growth plates precede excess weight gain in a mouse model of juvenile obesity

Nearly one-third of children in the United States are overweight or obese by their preteens. Tall stature and accelerated bone elongation are characteristic features of childhood obesity, which cooccur with conditions such as limb bowing, slipped epiphyses, and fractures. Children with obesity paradoxically have normal circulating IGF-I, the major growth-stimulating hormone. Here, we describe and validate a mouse model of excess dietary fat to examine mechanisms of growth acceleration in obesity. We used in vivo multiphoton imaging and immunostaining to test the hypothesis that high-fat diet increases IGF-I activity and alters growth plate structure before the onset of obesity. We tracked bone and body growth in male and female C57BL/6 mice (n = 114) on high-fat (60% kcal fat) or control (10% kcal fat) diets from weaning (3 wk) to skeletal maturity (12 wk). Tibial and tail elongation rates increased after brief (1-2 wk) high-fat diet exposure without altering serum IGF-I. Femoral bone density and growth plate size were increased, but growth plates were disorganized in not-yet-obese high-fat diet mice. Multiphoton imaging revealed more IGF-I in the vasculature surrounding growth plates of high-fat diet mice and increased uptake when vascular levels peaked. High-fat diet growth plates had more activated IGF-I receptors and fewer inhibitory binding proteins, suggesting increased IGF-I bioavailability in growth plates. These results, which parallel pediatric growth patterns, highlight the fundamental role of diet in the earliest stages of developing obesity-related skeletal complications and validate the utility of the model for future studies aimed at determining mechanisms of diet-enhanced bone lengthening.NEW & NOTEWORTHY This paper validates a mouse model of linear growth acceleration in juvenile obesity. We demonstrate that high-fat diet induces rapid increases in bone elongation rate that precede excess weight gain and parallel pediatric growth. By imaging IGF-I delivery to growth plates in vivo, we reveal novel diet-induced changes in IGF-I uptake and activity. These results are important for understanding the sequelae of musculoskeletal complications that accompany advanced bone age and obesity in children.

Breast cancer cells rely on environmental pyruvate to shape the metastatic niche

The extracellular matrix is a major component of the local environment-that is, the niche-that determines cell behaviour1. During metastatic growth, cancer cells shape the extracellular matrix of the metastatic niche by hydroxylating collagen to promote their own metastatic growth2,3. However, only particular nutrients might support the ability of cancer cells to hydroxylate collagen, because nutrients dictate which enzymatic reactions are active in cancer cells4,5. Here we show that breast cancer cells rely on the nutrient pyruvate to drive collagen-based remodelling of the extracellular matrix in the lung metastatic niche. Specifically, we discovered that pyruvate uptake induces the production of α-ketoglutarate. This metabolite in turn activates collagen hydroxylation by increasing the activity of the enzyme collagen prolyl-4-hydroxylase (P4HA). Inhibition of pyruvate metabolism was sufficient to impair collagen hydroxylation and consequently the growth of breast-cancer-derived lung metastases in different mouse models. In summary, we provide a mechanistic understanding of the link between collagen remodelling and the nutrient environment in the metastatic niche.

HIF-1α metabolically controls collagen synthesis and modification in chondrocytes

Endochondral ossification, an important process in vertebrate bone formation, is highly dependent on correct functioning of growth plate chondrocytes1. Proliferation of these cells determines longitudinal bone growth and the matrix deposited provides a scaffold for future bone formation. However, these two energy-dependent anabolic processes occur in an avascular environment1,2. In addition, the centre of the expanding growth plate becomes hypoxic, and local activation of the hypoxia-inducible transcription factor HIF-1α is necessary for chondrocyte survival by unidentified cell-intrinsic mechanisms3-6. It is unknown whether there is a requirement for restriction of HIF-1α signalling in the other regions of the growth plate and whether chondrocyte metabolism controls cell function. Here we show that prolonged HIF-1α signalling in chondrocytes leads to skeletal dysplasia by interfering with cellular bioenergetics and biosynthesis. Decreased glucose oxidation results in an energy deficit, which limits proliferation, activates the unfolded protein response and reduces collagen synthesis. However, enhanced glutamine flux increases α-ketoglutarate levels, which in turn increases proline and lysine hydroxylation on collagen. This metabolically regulated collagen modification renders the cartilaginous matrix more resistant to protease-mediated degradation and thereby increases bone mass. Thus, inappropriate HIF-1α signalling results in skeletal dysplasia caused by collagen overmodification, an effect that may also contribute to other diseases involving the extracellular matrix such as cancer and fibrosis.

Regulatory Functions of Insulin-like Growth Factor Binding Proteins in Osteoarthritis

Insulin-like growth factor binding proteins (IGFBPs) are a group of secreted proteins, which bind to IGF-I (and IGF-II) with high affinity and modulate the biological actions of IGFs. Abundant evidence points the importance of the IGF-I/IGFBP system on both cell growth and differentiation. A role for the IGF-I/IGFBP system in the regulation of normal human cartilage has been previously reported. In this context, recent studies suggest an emerging role for IGFBPs in the failure of cartilage during osteoarthritis (OA). Indeed, increased IGFBP levels have been reported in both the articular cartilage and synovial fluid from patients with OA. Overexpression of IGFBPs, by altering the bioavailability and function of IGFs, is likely to deliver IGFs-independent signals for chondrocyte survival. This, at least in part, might explain the degenerative changes of the cartilage in OA. Further studies are necessary to clarify the mechanisms that cause the overexpression of IGFBPs in patients with OA. Advances in our understanding of the relationship between osteoarthritis and the IGF-I/IGFBP system may lead to new treatment strategies for this degenerative disease.

Articular Cartilage Repair with Autografting Under the Influence of Insulin-Like Growth Factor-1 in Rabbits

Insulin-like growth factor (IGF)-1 has been successfully demonstrated to stimulate proteoglycan synthesis, slow down its catabolism and promote cartilage formation through well defined in vitro studies. It was therefore, assumed that IGF-1 would eventually serve to augment current cartilage repair techniques in vivo. Study was therefore, designed to determine the influence of IGF-1 in cartilage repair with or without autografting. For this purpose articular cartilage repair model was created in the left knee of 48 New Zealand white rabbits of either sex, 6-7 months old, weighing 1-2 kg. The articular cartilage defect was created in the femoral groove of femoro-patellar joint using hand held trephine under xylazine and ketamine anaesthesia in all the animals. The defect created was 3 mm in diameter and 2 mm in depth. For autografting, osteochondral tissues harvested from the proximal patellar groove of the femur were placed in the distal defect and vice versa. The experimental animals were divided mainly into four groups, i.e. Group A (control), Group B (autografting), Group C (control + IGF-1) and Group D (autografting + IGF-1). Animals of group A and B were provided only with collagen scaffolds at 10 mug/cm(2) whereas animals of treatment group C and D were provided with collagen scaffolds holding 30 ng/30 mul of IGF-1 into the defect. Evaluation of cartilage repair was done on days 15, 30 and 45 after ethically killing the animals. Initially IGF-1 had shown the tendency for either in the maintenance of autografted cartilage or helped in proliferation of chondroblast for the repair process. However, later in the process, cartilage formation apparently declined and appeared to converge to osseous tissue. Collectively, non-responsiveness of osteoarthritic chondrocytes to IGF-1 could be partially attributed to either increased IGF-binding proteins in the joint space, micromovement of the graft, lack of nutrition, dose of IGF-1 or its half life in the current study.

Pharmacological disruption of insulin-like growth factor 1 binding to IGF-binding proteins restores anabolic responses in human osteoarthritic chondrocytes

Insulin-like growth factor 1 (IGF-1) has poor anabolic efficacy in cartilage in osteoarthritis (OA), partly because of its sequestration by abnormally high levels of extracellular IGF-binding proteins (IGFBPs). We studied the effect of NBI-31772, a small molecule that inhibits the binding of IGF-1 to IGFBPs, on the restoration of proteoglycan synthesis by human OA chondrocytes. IGFBPs secreted by human OA cartilage or cultured chondrocytes were analyzed by western ligand blot. The ability of NBI-31772 to displace IGF-1 from IGFBPs was measured by radiobinding assay. Anabolic responses in primary cultured chondrocytes were assessed by measuring the synthesis of proteoglycans in cetylpyridinium-chloride-precipitable fractions of cell-associated and secreted 35S-labeled macromolecules. The penetration of NBI-31772 into cartilage was measured by its ability to displace 125I-labeled IGF-1 from cartilage IGFBPs. We found that IGFBP-3 was the major IGFBP secreted by OA cartilage explants and cultured chondrocytes. NBI-31772 inhibited the binding of 125I-labeled IGF-1 to IGFBP-3 at nanomolar concentrations. It antagonized the inhibitory effect of IGFBP-3 on IGF-1-dependent proteoglycan synthesis by rabbit chondrocytes. The addition of NBI-31772 to human OA chondrocytes resulted in the restoration or potentiation of IGF-1-dependent proteoglycan synthesis, depending on the IGF-1 concentrations. However, NBI-31772 did not penetrate into cartilage explants. This study shows that a new pharmacological approach that uses a small molecule inhibiting IGF-1/IGFBP interaction could restore or potentiate proteoglycan synthesis in OA chondrocytes, thereby opening exciting possibilities for the treatment of OA and, potentially, of other joint-related diseases.

Growth stimulation of mandibular condyles and femoral heads of newborn rats by IGF-I

Primary and secondary cartilage differ in embryonic origin and are generally considered to have a different mode of growth. However, few experimental studies exist that directly compare the two types of cartilage and their growth regulation. The regulation of cartilage growth is a complex mechanism involving growth factors like insulin-like growth factor-I (IGF-I). The purpose of this study was to compare the growth of mandibular condyles of 4-day-old rats with that of femoral heads in vitro and to analyze the effects of IGF-I. Explants were cultured for up to 2 weeks with 0, 5, and 25 ng/ml IGF-1. Both, 5 and 25 ng/ml IGF-I significantly stimulated growth of the mandibular condyles while only 25 ng/ml IGF-I stimulated growth of the femoral heads. IGF-I increased glycosaminoglycan synthesis of both condylar and femoral cartilage. However, only the DNA synthesis of the mandibular condyles was significantly increased by IGF-I while that of the femoral heads was not affected. It is concluded that IGF-I stimulates growth of both secondary condylar cartilage and primary femoral cartilage. The mandibular condyle appears to be more sensitive to IGF-I than the femoral head, which may partly be due to the different developmental stage.

Regulation by glucocorticoids of cell differentiation and insulin-like growth factor binding protein production in cultured fetal rat nasal chondrocytes

Glucocorticoids (GCs) modulate insulin-like growth factor action in cartilage through mechanisms that are complex and insufficiently defined, especially in the context of cranio-facial growth. Because the family of IGF-binding proteins (IGFBP-1 to -6) is important in the regulation of IGF availability and bioactivity, we examined the effect of GCs on chondrocyte differentiation in correlation with IGFBP production in cultured fetal rat chondrocytes isolated from nasal septum cartilage of fetal rat. Dexamethasone (DEX) effects were tested before and at the onset of extracellular matrix maturation. DEX induced a dose-dependent increase in the size of cartilage nodule formed, (45)Ca incorporation into extracellular matrix, alkaline phosphatase activity, and sulfatation of glycosaminoglycans, maximal effects being obtained with a 10-mM DEX concentration. The IGFBPs produced by cultured chondrocytes were characterized in culture medium which had been conditioned for 24 h under serum-free conditions by these cells. Western ligand blotting with a mixture of [(125)I]IGF-I and -II revealed bands of 20, 24, 29, a 31-32 kDa doublet and a 39-41 kDa triplet which were differently regulated by DEX. Immunoblotting showed that following DEX exposure, IGFBP-3 and -6 were up-regulated whereas IGFBP-2, -5, and the 24 kDa band were down-regulated. The effect of DEX on both differentiation and IGFBP production showed a same dependence, and developed when extracellular matrix maturation had been just induced. The results obtained in this chondrocyte culture system show that production of IGFBPs is modulated by DEX at physiological concentrations thus regulating IGF availability and action, a control which could promote the primordial role of the rat nasal septum in craniofacial growth.

Antiproliferative effects of insulin-like growth factor-binding protein-3 in mesenchymal chondrogenic cell line RCJ3.1C5.18. relationship to differentiation stage

Chondrogenesis results from a complex equilibrium between chondrocyte proliferation and differentiation. Insulin-like growth factors (IGFs) have a crucial role in chondrogenesis, but their mechanisms of action are not well defined. IGF-binding protein-3 (IGFBP-3) is the major carrier for circulating IGFs in postnatal life, and has been shown to have IGF-independent effects on proliferation of several cancer cell lines. In this study, we have evaluated the IGF-independent and -dependent effects of IGFBP-3 on chondrocyte proliferation and the relationship of these effects with chondrocyte differentiation stage. We used the RCJ3.1C5.18 nontransformed mesenchymal chondrogenic cell line, which, over 2 weeks of culture, progresses through the differentiation pathway exhibited by chondrocytes in the growth plate. We demonstrated that IGFBP-3 inhibited, in a dose-dependent manner (1-30 nm), the proliferation of chondroprogenitors and early differentiated chondrocytes, stimulated by des-(1-3)-IGF-I and longR(3)-IGF-I (IGF-I analogs with reduced affinity for IGFBP-3), and by insulin and IGF-I. In terminally differentiated chondrocytes, IGFBP-3 retained the ability to inhibit cell proliferation stimulated by IGF-I, but had no effect on cell growth stimulated by insulin, or des-(1-3)-IGF-I or longR(3)IGF-I. By monolayer affinity cross-linking, we demonstrated a specific IGFBP-3-associated cell-membrane protein of approximately 20 kDa. We determined that IGFBP-3 has an antiproliferative effect on chondrocytes and, that this effect is related to the differentiation process. In chondroprogenitors and early differentiated chondrocytes, antiproliferative effect of IGFBP-3 is mainly IGF-independent, whereas, following terminal differentiation this effect is IGF-dependent.

Expression and release of insulin-like growth factor binding proteins in isolated epiphyseal growth plate chondrocytes from the ovine fetus

Insulin-like growth factor-II (IGF-II) is an autocrine modulator of epiphyseal chondrogenesis in the fetus. The cellular availability of IGFs are influenced by the IGF-binding proteins (IGFBPs). In this study, we investigated the control of expression and release of IGFBPs from isolated epiphyseal growth plate chondrocytes from the ovine fetus by hormones and growth factors implicated in the chondrogenic process. Chondrocytes were isolated from the proliferative zone of the fetal ovine proximal tibial growth plate and maintained in monolayer culture at early passage number. Culture media conditioned by chondrocytes under basal conditions released IGFBPs of 24, 34, and 29 kDa, and a less abundant species of 39-43 kDa that were identified immunologically as IGFBP-4, IGFBP-2, IGFBP-5, and IGFBP-3, respectively. Messenger RNAs encoding each species were identified by Northern blot analysis within chondrocytes, as was mRNA encoding IGFBP-6. Exposure to IGF-I or IGF-II (13 or 26 nM) caused an increase in expression and release of IGFBP-3. The release of IGFBP-2 and IGFBP-5 were also potentiated without changes to steady state mRNA, and for IGFBP-5 this was due in part to a release from the cell membrane in the presence of IGF-II. Insulin (16.7 or 167 nM) selectively increased mRNA and the release of IGFBP-3, while cortisol (1 or 5 microM) inhibited both mRNA and release of IGFBP-2 and IGFBP-5. Transforming growth factor-beta1 (TGF-beta1) (0.1 or 0.2 nM) increased the expression and release of IGFBP-3, and caused an increase in mRNAs encoding IGFBP-2 and IGFBP-5. Neither growth hormone (GH), fibroblast growth factor-2, nor thyroxine (T(4)) had any effect on IGFBP expression or release. The results suggest that IGFBP expression and release within the developing growth plate can be modulated by IGF-II and other trophic factors, thus controlling IGF availability and action.

Expression of the components of the insulin-like growth factor axis across the growth-plate

Linear bone growth occurs as the result of proliferation and differentiation of growth-plate chondrocytes. These two phases of chondrocyte growth are regulated separately, with insulin-like growth factor I (IGF-I) being the primary stimulator of proliferation. We studied the expression of the components of the growth hormone GH/IGF system to learn if this proliferative signal is altered as chondrocytes undergo differentiation. Growth-plate chondrocytes were isolated from fetal cows and fractionated on discontinuous Percoll gradients. Five populations were recovered, ranging from high density cells (proliferative chondrocytes) to low density cells (hypertrophic chondrocytes). Messenger RNAs (mRNAs) were analyzed by a reverse transcriptase/quantitative polymerase chain reaction (RT/qPCR) technique. Results showed that mRNA of IGF-I and IGF-II in proliferative chondrocytes was 32 and five fold more abundant, respectively, than in hypertrophic chondrocytes. Of the four major IGF-I mRNA transcripts, the class 1-Ea transcript was predominant. Messenger RNA levels for IGFBP-3, -4, and -5 were also reduced in hypertrophic chondrocytes. Levels of GH receptor, the type 1 IGF receptor, and IGF binding protein-2 (IGFBP-2) mRNAs were unchanged across the growth-plate. Since IGF-I and -II are potent stimulators of proliferation, the down-regulation of these genes may be necessary in order for hypertrophy to proceed.

Cellular localization and expression of insulin-like growth factors (IGFs) and IGF binding proteins within the epiphyseal growth plate of the ovine fetus: possible functional implications

The insulin-like growth factors (IGFs) are important in the regulation of normal fetal musculoskeletal growth and development, and their actions have been shown to be modulated by IGF binding proteins (IGFBPs). Because the anatomical distribution of IGFBPs is likely to dictate IGF bioavailability, we determined the cellular distribution and expression of IGF-I, IGF-II, and IGFBP-1 to IGFBP-6 in epiphyseal growth plates of the fetal sheep, using immunocytochemistry and in situ hybridization. Little mRNA for IGF-I was detectable within the growth plates, but mRNA for IGF-II was abundant in germinal and proliferative chondrocytes, although absent from some differentiating chondrocytes and hypertrophic cells. Immunohistochemistry for IGF-I and IGF-II showed a presence of both peptides in all chondrocyte zones, including hypertrophic cells. Immunoreactive IGFBP-2 to -5 were localized within the germinal and proliferative zones of chondrocytes, but little immunoreactivity was present within the columns of differentiating cells. IGFBP immunoreactivity again appeared in hypertrophic chondrocytes. IGFBP mRNA in chondrocytes of the epiphyseal growth plate was below the detectable limit of in situ hybridization. However, low levels of mRNAs for IGFBP-2 to -6 were detected by the reverse transcriptase polymerase chain reaction. A co-localization of IGFBPs with IGF peptides in intact cartilage suggests that they may regulate IGF bioavailability and action locally. To test this hypothesis, monolayer cultures of chondrocytes were established from the proliferative zone of the growth plate, and were found to release immunoreactive IGF-II and to express mRNAs encoding IGFBP-2 to -6. Exogenous IGFBP-3, -4, and -5 had an inhibitory action on IGF-II-dependent DNA synthesis. IGFBP-2 had a biphasic effect, potentiating IGF-II action at low concentrations but inhibiting DNA synthesis at equimolar or greater concentrations relative to IGF-II. Long R3 IGF-I, which has a reduced binding affinity for many IGFBPs, was more potent than native IGF-I in promoting DNA synthesis by chondrocytes. Our findings suggest that locally produced IGF-II and IGF-I derived from the circulation can influence fetal epiphyseal chondrogenesis, and that this may be modulated locally by multiple IGFBP expression.Key words: epiphyseal growth plate, insulin-like growth factor, insulin-like growth factor binding protein, chondrocyte, cartilage.

Insulin-like growth factor binding protein-5 proteolytic activity in ovine articular chondrocyte culture

We have previously demonstrated that ovine articular chondrocytes synthesise and release insulin-like growth factor binding protein-5 (IGFBP-5) which subsequently undergoes proteolysis in the tissue culture medium. The IGFBP-5 proteolytic activity has now been characterised and its substrate specificity analysed using recombinant IGFBP-5 and purified chondrocyte-derived IGFBPs. Iodinated human recombinant IGFBP-5 was incubated with chondrocyte culture or conditioned medium in the presence or absence of various inhibitors. Serine protease inhibitors aprotinin and heparin effectively inhibited the breakdown of IGFBP-5. Furthermore, insulin-like growth factor-I (IGF-I) but not its structural analogues with reduced affinity for IGFBP-5, was also able to partially protect IGFBP-5 from degradation indicating that the association of IGF with the binding protein was required for the inhibition of the proteolytic activity. The inflammatory cytokine interleukin-1 did not have any effect on IGFBP-5 proteolysis. The proteolytic activity appears to be IGFBP-5-specific since the incubation of chondrocyte-derived IGFBPs with chondrocyte conditioned medium resulted in the loss of IGFBP-5 while the levels of the other two IGFBPs (IGFBP-2 and a 24 kDa IGFBP) remained unchanged. In conclusion, we show that IGFBP-5 is specifically cleaved by a serine protease released by primary cultures of ovine articular chondrocytes and also demonstrate the ability of IGF-I to inhibit the proteolytic activity both in cell culture and in cell-free conditions.

Regulation of insulin-like growth factor-binding protein-5 by insulin-like growth factor I and interleukin-1alpha in ovine articular chondrocytes

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.

Effects of insulin and insulin-like growth factors I and II on the growth of equine fetal and neonatal chondrocytes

The effects of insulin and insulin-like growth factors (IGFs) I and II on fetal and foal chondrocytes were investigated in vitro. Chondrocytes from the lateral trochlear ridge of the distal femur were obtained from 2 fetuses (280 and 320 days gestation) and one 4-day-old foal and cultured. Membrane proteins consistent with type 1 and type 2 IGF receptors were demonstrated by radioligand cross linking and equilibrium binding analysis. It was demonstrated that both IGF-I and IGF-II acted as mitogens for isolated equine chondrocytes when present as the sole mitogenic factor in monolayer culture. It was further shown that whereas insulin was able to promote the survival and expansion of cell populations of chondrocytes in culture there was significantly reduced mitogenic stimulation compared to the IGFs. These results suggest that the role of insulin in growth cartilage may be to promote chondrocyte survival, or to suppress differentiation/apoptosis. This supports the hypothesis that relative hyperinsulinaemia may be a contributory factor to equine dyschondroplasia (osteochondrosis). Understanding of contributory, and possibly triggering factors such as this may allow the development of modified methods of husbandry which minimise the risk of disease in populations with a known predisposition.

Membrane-associated insulin-like growth factor-binding protein-3 inhibits insulin-like growth factor-I-induced insulin-like growth factor-I receptor signaling in ishikawa endometrial cancer cells

The function of cell surface-associated insulin-like growth factor-binding proteins (IGFBPs) is controversial. Both inhibition and facilitation of IGF action as well as IGF-independent effects have been reported. We examined the influence of endogenous cell surface-associated IGFBPs on IGF-I receptor (IGF-IR) function in Ishikawa endometrial cancer cells by comparing the effects of IGF-I and its truncated analog des-(1-3)-IGF-I on several components of the IGF-IR signal transduction pathway in the absence of significant amounts of soluble IGFBPs. IGF-I and des-(1-3)-IGF-I are known to have similar affinities for IGF-IR, although the affinity of des-(1-3)-IGF-I for IGFBPs is greatly reduced. Here we show that the two ligands were equipotent not only in IGF-IR binding but also in receptor activation in NIH 3T3 cells overexpressing IGF-IR and possessing a relatively small number of cell surface-associated IGFBPs. In contrast, des-(1-3)-IGF-I manifested a remarkably higher potency as compared with IGF-I in inducing short and middle term cellular responses in IGF-IR-transfected Ishikawa endometrial cancer cells possessing a high number of both the receptor and the cell membrane-bound IGFBP-3. Thus, this difference in the effects of IGF-I and des-(1-3)-IGF-I can be attributed to the attenuation of IGF-I-mediated IGF-IR signaling by membrane-bound IGFBP-3.

Platelet derived growth factor stimulates chondrocyte proliferation but prevents endochondral maturation

Platelet-derived growth factor (PDGF) is a cytokine released by platelets at sites of injury to promote mesenchymal cell proliferation. Since many bone wounds heal by endochondral bone formation, we examined the response of chondrocytes in the endochondral lineage to PDGF. Confluent cultures of rat costochondral resting zone cartilage cells were incubated with 0-300 ng/mL PDGF-BB for 24 h to determine whether dose-dependent changes in cell proliferation (cell number and [3H]-thymidine incorporation), alkaline phosphatase specific activity, [35S]-sulfate incorporation, or [3H]-proline incorporation into collagenase-digestible protein (CDP) or noncollagenase-digestible protein (NCP), could be observed. Long-term effects of PDGF were assessed in confluent cultures treated for 1, 2, 4, 6, 8, or 10 d with 37.5 or 150 ng/mL PDGF-BB. To determine whether PDGF-BB could induce resting zone chondrocytes to change maturation state to a growth zone chondrocyte phenotype, confluent resting zone cell cultures were treated for 1, 2, 3, or 5 d with 37.5 or 150 ng/ml PDGF-BB and then challenged for an additional 24 h with 1,25-(OH)2D3. PDGF-BB caused a dose-dependent increase in cell number and [3H]-thymidine incorporation at 24 h. The proliferative effect of the cytokine decreased with time. PDGF-BB had no effect on alkaline phosphatase at 24 h, but at later times, the cytokine prevented the normal increase in enzyme activity seen in post-confluent cultures. This effect was primarily on the cells and not on the matrix. PDGF-BB stimulated [35S]-sulfate incorporation at all times examined, but had no effect on [3H]-proline incorporation into either the CDP or NCP pools. Thus, percent collagen production was not changed. Treatment of the cells for up to 5 d with PDGF-BB failed to elicit a 1,25-(OH)2D3 responsive phenotype typical of rat costochondral growth zone cartilage cells. These results show that committed chondrocytes can respond to PDGF-BB with increased proliferation. The effect of the cytokine is to enhance cartilage matrix production, but at the same time to prevent progression of the cells along the endochondral maturation pathway.