Systemic inhibition or global deletion of CaMKK2 protects against post-traumatic osteoarthritis

OBJECTIVE

To investigate the role of Ca^2+/calmodulin-dependent protein kinase 2 (CaMKK2) in post-traumatic osteoarthritis (PTOA).

METHODS

Destabilization of the medial meniscus (DMM) or sham surgeries were performed on 10-week-old male wild-type (WT) and Camkk2^-/- mice. Half of the DMM-WT mice and all other cohorts (n = 6/group) received tri-weekly intraperitoneal (i.p.) injections of saline whereas the remaining DMM-WT mice (n = 6/group) received i.p. injections of the CaMKK2 inhibitor STO-609 (0.033 mg/kg body weight) thrice a week. Study was terminated at 8- or 12-weeks post-surgery, and knee joints processed for microcomputed tomography imaging followed by histology and immunohistochemistry. Primary articular chondrocytes were isolated from knee joints of 4-6-day-old WT and Camkk2^-/- mice, and treated with 10 ng/ml interleukin-1β (IL)-1β for 24 or 48 h to investigate gene and protein expression.

RESULTS

CaMKK2 levels and activity became elevated in articular chondrocytes following IL-1β treatment or DMM surgery. Inhibition or absence of CaMKK2 protected against DMM-associated destruction of the cartilage, subchondral bone alterations and synovial inflammation. When challenged with IL-1β, chondrocytes lacking CaMKK2 displayed attenuated inflammation, cartilage catabolism, and resistance to suppression of matrix synthesis. IL-1β-treated CaMKK2-null chondrocytes displayed decreased IL-6 production, activation of signal transducer and activator of transcription 3 (Stat3) and matrix metalloproteinase 13 (MMP13), indicating a potential mechanism for the regulation of inflammatory responses in chondrocytes by CaMKK2.

CONCLUSIONS

Our findings reveal a novel function for CaMKK2 in chondrocytes and highlight the potential for its inhibition as an innovative therapeutic strategy in the prevention of PTOA.

Enhanced repair of articular cartilage defects in vivo by transplanted chondrocytes overexpressing insulin-like growth factor I (IGF-I)

Traumatic articular cartilage lesions have a limited capacity to heal. We tested the hypothesis that overexpression of a human insulin-like growth factor I (IGF-I) cDNA by transplanted articular chondrocytes enhances the repair of full-thickness (osteochondral) cartilage defects in vivo. Lapine articular chondrocytes were transfected with expression plasmid vectors containing the cDNA for the Escherichia coli lacZ gene or the human IGF-I gene and were encapsulated in alginate. The expression patterns of the transgenes in these implants were monitored in vitro for 36 days. Transfected allogeneic chondrocytes in alginate were transplanted into osteochondral defects in the trochlear groove of rabbits. At three and 14 weeks, the quality of articular cartilage repair was evaluated qualitatively and quantitatively. In vitro, IGF-I secretion by implants constructed from IGF-I-transfected chondrocytes and alginate was 123.2+/-22.3 ng/10(7) cells/24 h at day 4 post transfection and remained elevated at day 36, the longest time point evaluated. In vivo, transplantation of IGF-I implants improved articular cartilage repair and accelerated the formation of the subchondral bone at both time points compared to lacZ implants. The data indicate that allogeneic chondrocytes, transfected by a nonviral method and cultured in alginate, are able to secrete biologically relevant amounts of IGF-I over a prolonged period of time in vitro. The data further demonstrate that implantation of these composites into deep articular cartilage defects is sufficient to augment cartilage defect repair in vivo. These results suggest that therapeutic growth factor gene delivery using encapsulated and transplanted genetically modified chondrocytes may be applicable to sites of focal articular cartilage damage.

Gene-based approaches for the repair of articular cartilage

Gene transfer technology has opened novel treatment avenues toward the treatment of damaged musculoskeletal tissues, and may be particularly beneficial to articular cartilage. There is no natural repair mechanism to heal damaged or diseased cartilage. Existing pharmacologic, surgical and cell based treatments may offer temporary relief but are incapable of restoring damaged cartilage to its normal phenotype. Gene transfer provides the capability to achieve sustained, localized presentation of bioactive proteins or gene products to sites of tissue damage. A variety of cDNAs have been cloned which may be used to stimulate biological processes that could improve cartilage healing by (1) inducing mitosis and the synthesis and deposition of cartilage extracellular matrix components by chondrocytes, (2) induction of chondrogenesis by mesenchymal progenitor cells, or (3) inhibiting cellular responses to inflammatory stimuli. The challenge is to adapt this technology into a useful clinical treatment modality. Using different marker genes, the principle of gene delivery to synovium, chondrocytes and mesenchymal progenitor cells has been convincingly demonstrated. Following this, research efforts have begun to move to functional studies. This involves the identification of appropriate gene or gene combinations, incorporation of these cDNAs into appropriate vectors and delivery to specific target cells within the proper biological context to achieve a meaningful therapeutic response. Methods currently being explored range from those as simple as direct delivery of a vector to a cartilage defect, to synthesis of cartilaginous implants through gene-enhanced tissue engineering. Data from recent efficacy studies provide optimism that gene delivery can be harnessed to guide biological processes toward both accelerated and improved articular cartilage repair.

Overexpression of human insulin-like growth factor-I promotes new tissue formation in an ex vivo model of articular chondrocyte transplantation

Articular cartilage, the tissue that forms the gliding surface of joints, has a poor regenerative capacity. Insulin-like growth factor-I (IGF-I) is a polypeptide that is anabolic and mitogenic for cartilage. Transfection of articular chondrocytes with an expression plasmid vector containing the cDNA for human IGF-I under the control of the cytomegalovirus promoter/enhancer led to expression of the transgene and synthesis of biologically relevant amounts of IGF-I protein. Transplantation of transfected articular chondrocytes on to the surface of articular cartilage explants led to the formation of a new tissue layer on the cartilage explant surface. The new tissue was characterized by the presence of type II collagen and proteoglycan and by the absence of type I collagen, consistent with hyaline-like cartilage. The tissue formed by the chondrocytes expressing IGF-I was thicker and contained more cells than controls transfected with an expression plasmid vector containing the Escherichia coli (E. coli) beta-galactosidase (lacZ) gene. Transplantation of articular chondrocytes that overexpress human IGF-I also increased DNA synthesis and the synthesis of glycosaminoglycans by the underlying explant cartilage chondrocytes. These results identify a mechanism by which IGF-I may simultaneously promote chondrogenesis and shift cartilage homeostasis in an anabolic direction. The data further suggest that therapeutic growth factor gene transfer may be applicable to articular cartilage.

The effect of dynamic compression on the response of articular cartilage to insulin-like growth factor-I

Articular cartilage is routinely subjected to mechanical forces and to cell-regulatory molecules. Previous studies have shown that mechanical stimuli can influence articular chondrocyte metabolic activity, and biochemical studies have shown that growth factors and cytokines control many of the same cell functions. Little is known, however, of the relationships or interplay, if any, between these two key components of the articular environment. This study investigated the comparative and interactive effects of low amplitude, sinusoidal, dynamic compression and insulin-like growth factor-I (IGF-I), a polypeptide in synovial fluid that is anabolic for cartilage. In bovine patellofemoral cartilage explants, IGF-I increased protein and proteoglycan synthesis 90% and 120%, respectively while dynamic compression increased protein and proteoglycan synthesis 40% and 90%, respectively. Stimulation by IGF-I was significantly greater than by dynamic compression for both protein and proteoglycan synthesis. When applied together, the two stimuli enhanced protein and proteoglycan synthesis by 180% and 290%, respectively, a degree greater than that achieved by either stimulus alone. IGF-I augmented protein synthesis with a time constant of 12.2 h. Dynamic compression increased protein synthesis with a time constant of 2.9 h, a rate significantly faster than that of IGF-I, suggesting that these signals act via distinct cell activation pathways. When used together, dynamic compression and IGF-I acted with a time constant of 5.6 h. Thus, dynamic compression accelerated the biosynthetic response to IGF-I and increased transport of IGF-I into the articular cartilage matrix, suggesting that, in addition to independently stimulating articular chondrocytes, cyclic compression may improve the access of soluble growth factors to these relatively isolated cells.

Mechanical and physicochemical regulation of the action of insulin-like growth factor-I on articular cartilage

The development and maintenance of healthy joints is a complex process involving many physical and biological stimuli. This study investigates the interaction between insulin-like growth factor-I (IGF-I) and static mechanical compression in the regulation of articular cartilage metabolism. Bovine cartilage explants were treated with concentrations of IGF-I from 0 to 300 ng/ml in the presence or absence of 0-50% static compression, and the transient and steady-state incorporation of [(3)H]proline and [(35)S]sulfate into matrix components were measured. In parallel studies, cartilage explants were treated with 0-300 ng/ml IGF-I at media pH ranging from 6.4 to 7.2 and the steady-state incorporation of [(3)H]proline and [(35)S]sulfate was measured. The effect of 50% static compression on IGF-I transport was determined by measuring the uptake of (125)I-labeled IGF-I into cartilage explants. Static compression decreased both [(3)H]proline and [(35)S]sulfate incorporation in a dose-dependent manner in the presence or absence of IGF-I. IGF-I increased [(3)H]proline and [(35)S]sulfate incorporation in a dose-dependent manner in the presence or absence of compression, but the anabolic effect of the growth factor was lessened when the tissue was compressed by 50%. The response of cartilage explants to IGF-I was similarly lessened in unstrained tissue cultured in media at pH 6.4, a condition which results in a similar intratissue pH to that when cartilage is compressed by 50%. The characteristic time constant (tau) for IGF-I stimulation of cartilage explants was approximately 24 h, while tau for inhibition of biosynthesis by static compression was approximately 2 h. Samples which were both compressed and treated with IGF-I demonstrated an initial decrease in biosynthetic activity at 2 h, followed by an increase at 24 h. Static compression did not alter tau for (125)I-labeled IGF-I transport into cartilage but decreased the concentration of (125)I-labeled IGF-I in the tissue at equilibrium.

Regulation by IGF-I and TGF-beta1 of Swarm-rat chondrosarcoma chondrocytes

The growth factors transforming growth factor-beta 1 and insulin-like growth factor-I influence a wide range of cellular actions, including the growth of several neoplastic cell types. Their role in the regulation of neoplastic chondrocytes remains unclear. We tested the hypotheses that transforming growth factor-beta 1 and insulin-like growth factor-I differentially regulate neoplastic chondrocytes and interact to modulate the mitotic and matrix synthetic activities of neoplastic chondrocytes. We used Swarm-rat chondrosarcoma chondrocytes to investigate the effect of each factor individually and of both factors in combination on [(3)H]thymidine incorporation into DNA and on [(35)S]sulfate incorporation into glycosaminoglycans. Each factor increased [(3)H]thymidine incorporation 2.7-fold: transforming growth factor-beta 1 achieved this effect at a 20-fold lower concentration than insulin-like growth factor-I. In contrast, insulin-like growth factor-I stimulated [(35)S]sulfate incorporation 3.5-fold; this was twice the maximal effect of transforming growth factor-beta 1. Transforming growth factor-beta 1 and insulin-like growth factor-I each decreased the proportion of newly synthesized glycosaminoglycans that were retained in the cells and pericellular matrix, indicating that the anabolic effect of these factors is only partly directed toward cell-associated matrix production. The mitogenic and matrix synthetic actions of insulin-like growth factor-I and transforming growth factor-beta 1 were synergistic. In concert, they increased [(3)H]thymidine incorporation approximately 12-fold, an effect three times greater than the sum of the maximal stimulation achieved by each factor individually. Similarly, transforming growth factor-beta 1 and insulin-like growth factor-I together increased glycosaminoglycan synthesis approximately two times more than the sum of their maximal individual effects. Taken together, these data indicate that these chondrosarcoma chondrocytes are positively regulated by insulin-like growth factor-I and transforming growth factor-beta 1 and that these growth factors interact to augment the mitotic and matrix synthetic actions of the chondrocytes. If supported in human models, the sensitivity to growth factors of these cells suggests that interventions directed toward growth factor inhibition may be of therapeutic value.

Efficient lipid-mediated gene transfer to articular chondrocytes

We examined nonviral, lipid-mediated gene transfer methods as potential tools for efficient transfection of articular chondrocytes. Transfection conditions were determined for primary cultures of normal human articular, osteoarthritic human articular and normal bovine articular chondrocytes using a lacZ reporter gene construct with the commercially available cationic liposomes Cellfectin, DMRIE-C, LipofectAmine, Lipofectin, LipoTaxi, TransFast and the lipid-based reagent FuGENE 6. Optimized conditions were then evaluated in an ex vivo model of chondrocyte transplantation. FuGENE 6 transfection produced the maximum levels of transgene expression. Transfection efficiency was cell type specific and affected by DNA concentration, lipid/DNA ratio and the presence of hyaluronidase, a matrix-degrading enzyme. Analysis of X-gal staining demonstrated an efficiency of 41.0% in normal bovine articular chondrocytes, 20.7% in normal human articular chondrocytes and 7.8% in osteoarthritic human chondrocytes. Transfected chondrocytes were found to successfully populate the articular cartilage surface in explant cultures. Transplanted genetically modified chondrocytes adhered to the articular cartilage and continued to produce beta-galactosidase for 2 weeks. This evaluation and optimization of lipid-based gene transfer into articular chondrocytes may serve as a useful tool in studies of genes involved in articular cartilage damage and repair and as a potential delivery method for therapeutic genes. Gene Therapy (2000) 7, 286-291.

The unmet anti-inflammatory needs in orthopedics

Approximately half of Americans 70 years of age or older suffer from arthritis. Nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most effective nonsurgical therapies for arthritis, but usage often causes harmful side effects, especially in the gastrointestinal tract. Such effects require supplemental therapy that adds an economic burden and may even cause death. The benefits derived from NSAIDs are believed to be due to suppression of cyclooxygenase-2 (COX-2), while the harmful side effects are believed to be due to suppression of cyclooxygenase-1 (COX-1). COX-2-specific inhibitors that do not inhibit COX-1 may meet arthritis sufferers' needs for therapies that are safe, convenient, and as effective as conventional NSAIDs.

Cartilage degeneration in relation to repetitive pressure: case study of a unilateral hip hemiarthroplasty patient

In vivo acetabular contact pressures were measured over 32 months in an elderly man with a pressure instrumented hemiarthroplasty. After death, left (hemiarthroplasty) and right (control) acetabula were explanted. Cartilage thickness and degeneration were quantified from magnetic resonance imaging and histological analysis. Highest repetitive in vivo contact pressures during gait (4.5 to 6.5 MPa) were measured in the superior dome of the acetabulum and decreased at a rate of approximately 1 MPa per year after implant (R2 = 0.48, P < .001). Contact pressure magnitudes measured during gait correlated positively with regional histology score (R2 = 0.34, P < .0001) and negatively with cartilage thickness (R2 = 0.35, P < .0001). Although histology scores were typical of early osteoarthritis (histological grade of 4-6), there were no significant differences in overall histology score for the left and right acetabula (P = .23). We conclude that acetabular cartilage degeneration was explained, in part, by repetitive stress, but the degeneration did not appear to be mediated solely by articulation with the metallic endoprosthesis.

Transport of tissue inhibitor of metalloproteinases-1 through cartilage: contributions of fluid flow and electrical migration

The preservation of the structure of articular cartilage depends on the availability of inhibitors of matrix-degrading enzymes. Tissue inhibitor of metalloproteinases-1 is thought to be an important contributor to the integrity of the matrix of articular cartilage, but the mechanisms that regulate its availability within the tissue are poorly understood. These studies elucidate the contributions of diffusion, fluid flow, and electrical migration to the transport of iodinated recombinant human tissue inhibitor of metalloproteinases-1 through explants of adult bovine articular cartilage under conditions relevant to the loading of cartilage. With use of measured partition coefficients of the cartilage explants, the diffusivity of the inhibitor was 0.5-1.6 x 10(-7) cm2/sec. Fluid velocities that were induced by applying an electrical current across the cartilage disks increased the flux of the inhibitor by approximately 20 to more than 150-fold compared with the effect of diffusion alone for the range of current densities that were applied. We examined the contribution of electrophoretic migration by titrating the charge on the inhibitor during measurements of flux and found that flux in the presence of the applied current decreased as the inhibitor became more negatively charged. Enhancements in the flux of the inhibitor were observed relative to the flux during diffusion alone even under conditions in which electrophoretic migration opposed the flux due to fluid flow, suggesting that of the transport mechanisms tested, fluid flow was dominant. These results suggest that the physical phenomena present during physiologic loading conditions (e.g., fluid flows and streaming currents) can affect the transport of tissue inhibitor of metalloproteinases-1 through the matrix of cartilage.

Potential role of insulinlike growth factors in fracture healing

Several lines of evidence suggest that the insulinlike growth factors play a role in fracture healing. They promote cell proliferation and matrix synthesis by chondrocytes and osteoblasts, the two cell types largely responsible for the formation of fracture callus. Circulating levels of insulinlike growth factor I and bone mineral density decrease with increasing age, and administration of insulinlike growth factor I increases bone turnover in patients with low bone mineral density. Insulinlike growth factor I may accelerate the normal healing of intramembranous bone defects, inducing the healing of defects that otherwise would not heal. An important role of insulinlike growth factor I is to mediate many of the actions of growth hormone on the skeleton. Considerable effort has been devoted to testing the effect of growth hormone and, thereby, indirectly that of insulinlike growth factor I on fracture healing. These studies have yielded such disparate results that no general conclusions regarding the effect of growth hormone (or of growth hormone dependent insulinlike growth factor I) on fracture healing currently can be drawn. Additional studies are needed to clarify the role of the insulinlike growth factors in the fracture healing process and to determine how their anabolic actions can be enlisted in the clinical enhancement of fracture healing.

Longitudinal analysis of the relationship between serum insulin-like growth factor-I and radiographic knee osteoarthritis

OBJECTIVE

To examine the relation between serum insulin-like growth factor I (IGF-I) levels and both incident and progressive radiographic knee osteoarthritis (OA) in the Framingham Osteoarthritis Study.

DESIGN

Subjects had bilateral weight-bearing, anterior-posterior knee radiographs performed in 1983-1985 and again in 1992-1993. IGF-I levels were measured from blood specimens obtained in 1988-1989 by a competitive binding radio-immunoassay (RIA) after separation with octadecasilyl-silica cartridges of serum IGF-I from binding proteins. Participants without baseline radiographic OA [Kellgren and Lawrence grades (K&L) = 0-1] were classified as having incident disease if they had K&L > or = 2 grades at follow-up. Progressive OA was defined as an increase in K&L score of > or = 1 in knees with baseline OA (K&L > or = 2). All analyses were knee-based and sex-specific. We examined IGF-I tertiles in relation to the risk of incident and progressive radiographic OA separately, adjusting for age, body mass index (BMI), and baseline K&L score, and used generalized estimating equations to adjust for the correlation between fellow knees.

RESULTS

Four hundred and forty-one participants had knee radiographs and serum IGF-I levels measured. No associations were found for serum IGF-I levels and incident [women: OR = 0.9 (0.6-1.7), men OR = 1.2 (0.6-2.6)] or progressive [women OR = 0.9 (0.6-1.6), men OR = 0.9 (0.3-3.0)] radiographic knee OA in either sex. Neither did we observe any association between IGF-I and worsening of individual radiographic features of OA (i.e., osteophyte growth and joint space loss).

CONCLUSION

In summary, this longitudinal study did not demonstrate any association of serum IGF-I and incident or progressive radiographic knee OA. Further studies are needed to clarify the role of IGF-I in OA.

Interaction of epidermal growth factor and insulin-like growth factor-I in the regulation of growth plate chondrocytes

The action of growth factors on the cells of the epiphyseal growth plate is an important mechanism in the regulation of skeletal growth. Insulin-like growth factor-I (IGF-I) is known to play a central role in the regulation of bone growth. In contrast, the role, if any, of epidermal growth factor (EGF) is not yet clear. In these studies, we tested the hypothesis that EGF interacts with IGF-I in the regulation of growth plate chondrocyte mitotic and metabolic activities. Chondrocytes isolated from bovine radioulnar growth plates and incubated in suspension culture were analyzed for their responsiveness to EGF with respect to synthesis of DNA, proteins, and proteoglycans, responsiveness to IGF-I, and ability to specifically bind [125I]IGF-I. Treatment of growth plate chondrocytes with maximally effective concentrations (10-100 ng/ml) of EGF produced a 16-27% increase in specific binding of [125I]IGF-I. Scatchard analysis indicated that this increase in specific binding was due to an increase in the number of receptors/cell with no change in receptor affinity. EGF stimulated protein synthesis by 30-35%. Pretreatment with EGF increased the responsiveness of chondrocytes to IGF-I, resulting in 90 and 60% augmentation of IGF-I-stimulated mitotic activity and proteoglycan synthesis, respectively. Given the prominent role of IGF-I in skeletal development and the presence of EGF in the growth plate, this study suggests an important role for interactions between these growth factors in the regulation of skeletal growth.

Differential effects of serum, insulin-like growth factor-I, and fibroblast growth factor-2 on the maintenance of cartilage physical properties during long-term culture

The effects of fetal bovine serum, insulin-like growth factor-I, and fibroblast growth factor-2 on the regulation of the functional physical properties of adult bovine cartilage explants during an incubation period of 18-20 days was determined, and the relationship between the measured functional properties of the cartilage and the tissue composition was assessed. Cartilage disks were tested in the uniaxial radially confined configuration by the application of low amplitude oscillatory displacement and measurement of the resultant load and streaming potential. For the control cartilage terminated just after explant, the modulus was 0.39 +/- 0.28 MPa, the open circuit hydraulic permeability was 2.0 +/- 1.0 x 10(-15) m2/(Pa.sec), and the electrokinetic (streaming potential) coefficient was -2.3 +/- 0.6 mV/MPa. Incubation of cartilage in medium supplemented with serum or insulin-like growth factor-I resulted in maintenance of the modulus and electrokinetic coefficient, whereas incubation in basal medium or medium supplemented with fibroblast growth factor-2 led to a marked decrease from control values in the modulus and the amplitude of the electrokinetic coefficient. All of the culture conditions examined resulted in an increase in permeability that was not statistically significant. The variation in the electromechanical properties of all the cartilage samples tested was related to the density of tissue proteoglycan and collagen (hydroxyproline). The modulus was correlated with both the density of tissue proteoglycan (+0.014 MPa/[mg/ml]) and the density of tissue hydroxyproline (+0.008 MPa/[mg/ml]). The electrokinetic coefficient was also correlated with the density of proteoglycan (-0.080 [mV/MPa]/[mg/ml]) and the density of hydroxyproline (+0.064 [mV/MPa]/[mg/ml]). These data indicate that the regulation of chondrocyte matrix metabolism by growth factors can significantly affect the physical properties and function of cartilage.

Growth factor actions on articular cartilage

Polypeptide growth factors play a major role in the regulation of cell behavior, including that of articular chondrocytes. Among the most influential of these factors identified for articular cartilage are insulin like growth factor I (IGF-I), basic fibroblast growth factor (bFGF), and transforming growth factor beta (TGF beta). IGF-I stimulates articular chondrocyte matrix synthetic and mitotic activity and inhibits chondrocyte mediated matrix catabolism. The role of bFGF as a potent mitogen for articular chondrocytes is well established. In contrast, this factor appears to play a complex role in matrix synthesis and degradation, promoting both anabolic and catabolic functions. Both IGF-I and bFGF have been shown to augment articular cartilage repair in vivo. TGF beta is particularly dependent upon the context in which it acts, eliciting seemingly opposite effects under different experimental conditions. These and other factors interact to modulate their respective actions, creating effector cascades and feedback loops of intercellular and intracellular events that control articular chondrocyte functions. Elucidation of the actions and interactions of these factors may be expected to clarify the etiopathogenesis of osteoarthritis and possibly offer novel methods for its treatment.

The combination of platelet-derived growth factor-BB and insulin-like growth factor-I stimulates bone repair in adult Yucatan miniature pigs

The combination of insulin-like growth factor-I and platelet-derived growth factor-BB has previously been shown to stimulate healing of soft tissue wounds and the formation of bone and ligament around teeth. The purpose of the present study was to evaluate the effects of platelet-derived growth factor-BB and insulin-like growth factor-I individually and in combination on the healing of osseous wounds. Four standardized cortical wounds were created in each tibia of 11 adult Yucatan miniature pigs. The wounds in one tibia per animal were treated with either purified recombinant human insulin-like growth factor-I, platelet-derived growth factor-BB, or both in a methylcellulose gel. The wounds in each contralateral tibia received placebo gel alone. Coded serial sections of each wound were evaluated by computer-aided histomorphometry 21 days after surgery. The area and perimeter of the newly formed mineralized callus, the thickness of the total callus, and the percentage of mineralized tissue within the callus were significantly increased compared with the values of matched controls only in wounds treated with a combination of insulin-like growth factor-I and platelet-derived growth factor-BB. No significant differences in the measured parameters of callus formation were found in wounds treated with either insulin-like growth factor-I or platelet-derived growth factor-BB alone. Cartilage was present only in sites treated with insulin-like growth factor-I alone. These results suggest that the combination of platelet-derived growth factor-BB and insulin-like growth factor-I stimulates bone formation in wounds in long bones of adult animals and that these growth factors act via different pathways during the repair process.

Insulin-like growth factor I regulation of Swarm rat chondrosarcoma chondrocytes in culture

Insulin-like growth factor I (IGF-I) is anabolic for chondrocytes and is thought to be important in regulating such normal cartilaginous tissues as the epiphyseal growth plate. In the present studies, we have investigated the role of IGF-I in the regulation of neoplastic cartilage. Chondrocytes cultured from a transplantable rat chondrosarcoma were analyzed for responsiveness to IGF-I with respect to DNA and glycosaminoglycan synthesis as determined by labeling with radioactive thymidine and sulfate, respectively. Stimulation of [3H]thymidine and [35S]sulfate incorporation by IGF-I was two to four times that in serum-free controls, with half-maximal stimulation at 1 x 10(-9) M. The efficacy of IGF-I was approximately one-half of that of serum in stimulating [3H]thymidine incorporation and was comparable to that of serum for [35S]sulfate incorporation. When Swarm rat chondrosarcoma chondrocytes were cultured in the presence of IGF-I and exposed to graded concentrations of anti-IGF-I antibody, [3H]thymidine incorporation and [35S]sulfate incorporation were attenuated in a dose-dependent fashion to 29 and 25% of antibody-free controls, respectively. Nonspecific antibody not raised against IGF-I was not inhibitory. These observations suggest that the majority of IGF-I action on these cells is susceptible to immunoinhibition. To estimate the contribution of IGF-I to the regulation of these cells by serum, Swarm rat chondrosarcoma chondrocytes were cultured with graded concentrations of either calf serum or fetal calf serum in the presence of anti-IGF-I antibody, nonspecific antibody, or no other additives. Specific antibody attenuated the effect of calf serum on both [3H]thymidine and [35S]sulfate incorporation with overall inhibition of 52% (P < 0.01) and 48% (P < 0.001), respectively. Nonspecific antibody super-imposed small, variably stimulatory or inhibitory effects on those of calf serum. When chondrosarcoma chondrocytes were incubated with fetal calf serum, anti-IGF-I antibody exerted a minimal inhibitory effect, reducing both [3H]thymidine and [35S]sulfate incorporation by less than 25%. The immunoinhibition of both pre- and postnatal serum could be overcome in a dose-dependent fashion by increasing serum concentrations. These results suggest that the factors influencing Swarm rat chondrosarcoma chondrocytes may be developmentally regulated and that the contribution of IGF-I to the action of serum increases between fetal and post-natal life. These data support the hypothesis that chondrosarcoma is a somatomedin-responsive neoplasm and suggest that this tumor may be susceptible to interventions directed toward mechanisms that block insulin-like growth factor action.

Differential effects of bFGF and IGF-I on matrix metabolism in calf and adult bovine cartilage explants

The effects of basic fibroblast growth factor (bFGF) and insulin-like growth factor-I (IGF-I) on cell and matrix metabolism in calf and adult bovine cartilage explants were examined. In calf cartilage, bFGF elicited dose-dependent and bi-directional effects on mitotic activity and anabolic processes. Addition of bFGF at 3 ng/ml stimulated cell mitotic activity (total DNA) and synthesis of proteoglycan ([35S]sulfate incorporation), protein ([3H]proline incorporation), and collagen (formation of [3H]hydroxyproline), and resulted in a slight increase in proteoglycan deposition compared to basal medium. However, 30-300 ng/ml of bFGF inhibited mitotic activity and synthetic processes, accelerated [35S]proteoglycan release compared to basal medium, and resulted in an inhibition of proteoglycan deposition during the culture period. In contrast, treatment of adult cartilage with 3-300 ng/ml of bFGF did not affect the DNA content but did stimulate synthetic processes in a dose-dependent manner. Basic FGF also had bidirectional effects on matrix catabolism in adult cartilage, with 3 ng/ml accelerating [35S]proteoglycan release, but 30-300 ng/ml of bFGF resulting in release rates comparable to that in basal medium. Nonetheless, even with maximal bFGF stimulation, adult bovine cartilage suffered a net loss of proteoglycan during culture. Addition of 3-300 ng/ml of IGF-I to either calf or adult bovine cartilage stimulated synthetic processes and shifted the metabolic balance toward a net deposition of proteoglycan. Neither bFGF nor IGF-I altered the low basal rate of [3H]hydroxyproline release from either calf or adult bovine cartilage. Thus, (i) the regulatory effects of bFGF and IGF-I on bovine articular cartilage appear age-dependent, and (ii) bFGF is capable of promoting either anabolic or catabolic processes, and may therefore serve a dual role in the regulation of cartilage metabolism.

Expression of collagens I, II, X, and XI and aggrecan mRNAs by bovine growth plate chondrocytes in situ

The cells responsible for skeletal growth are the chondrocytes of the cartilaginous growth plate. These cells differentiate through a series of maturational stages, establishing different zones in the growth plate. Among the major functions of these cells is the production of appropriate extracellular matrix, primarily composed of collagens and proteoglycans. To determine whether matrix synthesis varies with respect to maturational stage and in which cell populations different collagens are expressed, bovine growth plates were analyzed by in situ hybridization to mRNA and by Northern blot hybridization. The most abundant collagen mRNA in the growth plate was type-II collagen. This mRNA was present at relatively low levels in the most immature cells of the growth plate but increased several-fold as cells entered the proliferative stage and remained high through subsequent phases of maturation. Type-XI collagen mRNA and mRNA for the cartilage-characteristic proteoglycan, aggrecan, were codistributed with the type-II collagen mRNA; however, both were present in much smaller quantities. Type-X procollagen mRNA was localized to chondrocytes late in their maturation and was expressed at levels similar to the expression of type-II collagen. In situ hybridization of serial sections revealed that growth plate chondrocytes in their more mature stages contain both type-II and type-X collagen mRNA. Type-I collagen mRNA was not observed in growth plate chondrocytes at any maturational stage; rather, it was localized to a morphologically distinct population of cells attached to calcifying cartilage septa in the region of vascular invasion.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of growth-plate chondrocytes by insulin-like growth-factor I and basic fibroblast growth factor

A study was performed in order to investigate the possible functional roles of insulin-like growth-factor I (IGFI) and basic fibroblast growth factor (bFGF) in the regulation of mitotic and metabolic activity of growth-plate chondrocytes. Chondrocytes from the distal radial growth plates of calves and the costal physeal cartilage of rats were exposed to these factors, individually and in combination, in primary monolayer culture, to assess their effects. The data showed that bFGF had both a greater potency and a greater efficacy as a mitogen for bovine growth-plate chondrocytes than did IGF-I. The maximum incorporation of 3H-thymidine by bFGF was 8.3 times that in serum-free (control) cultures; the maximum stimulation of incorporation by IGF-I was 2.5 times that in the control medium. In contrast, IGF-I stimulated a maximum incorporation of 35S-sulfate into glycosaminoglycans that was 2.6 times that in the IGF-I serum-free control cultures, while bFGF had no effect or was mildly inhibitory. When used together, these two factors acted synergistically. Incorporation of 3H-thymidine was more than two times greater than the sum of the effects of the growth factors when used alone and 20.5 times greater than that of the growth factor-free control cultures. Physeal chondrocytes from six-day-old rats were mitotically more responsive to bFGF than to IGF-I, but they were more responsive to IGF-I when they had been derived from twenty-eight-day-old rats. Interaction between bFGF and factors in the serum enhanced the mitotic activity of the rat chondrocytes, but bFGF did not interact with IGF-I under the same experimental conditions. In the presence of bFGF, there was a reduction in the stimulation by IGF-I of incorporation of 35S-sulfate and a decrease in the percentage of chondrocytes containing alkaline phosphatase. These growth factors also influenced cellular morphology in culture. In the presence of IGF-I or serum, the rat chondrocytes manifested the polygonal morphology typical of chondrocytes in culture, while bFGF promoted a more elongated spindle shape. Removal of bFGF and replacement by IGF-I restored the polygonal morphology, indicating that this transition is reversible.

Interaction of basic fibroblast growth factor with bovine growth plate chondrocytes

The basic fibroblast growth factor (bFGF) family of peptides influences a wide range of cellular actions. To better understand the possible role of bFGF in the growth plate, we have characterized the interaction of this growth factor with isolated bovine growth plate chondrocytes. Basic FGF interacts with two classes of binding sites on these cells. One is consistent with high-affinity bFGF receptors and the other with low-affinity heparin-like binding sites on the chondrocyte surface. Radiolabeled bFGF binding studies revealed approximately 4 x 10(6) binding sites per cell, with a Kd of approximately 42 nM. Graded concentrations of heparin or NaCl competed with [125I]-labeled bFGF in a dose-dependent fashion, reducing [125I]-labeled bFGF binding by 75 and 97%, respectively. The data suggest the presence of a high-capacity, low-affinity class of binding sites with the properties of a heparin-like moiety. Affinity cross-linking of [125I]-labeled bFGF to chondrocytes labeled two principal species with apparent molecular masses of 135 and 160 kDa. Labeled bFGF was specifically displaced from both species by subnanomolar concentrations of unlabeled bFGF. These high-affinity, low-capacity binding sites are characteristic of classical bFGF receptors. Binding of [125I]-labeled bFGF to these sites was also influenced by heparin, consistent with coregulation of binding to the two classes of binding sites. The data suggest that bFGF participates in the regulation of skeletal growth at the growth plate and that this regulation may involve bFGF interaction with at least two distinct classes of binding sites.

Failure of growth hormone to alter the biomechanics of fracture-healing in a rabbit model

Standardized tibial osteotomies were created and stabilized with external fixation in twenty-seven skeletally mature rabbits. Fourteen animals were treated with a daily injection of human growth hormone (150 micrograms per kilogram of body weight), and thirteen received a daily injection of saline solution. Serial non-destructive biomechanical tests, radiography, and determinations of the levels of serum insulin-like growth-factor I were performed for all of the animals. Destructive strength-testing of the sites of osteotomy was performed at four, six, or eight weeks. Twenty-five of the twenty-seven osteotomies healed uneventfully. There were no significant differences in the serial biomechanical measurements at the sites of the healing osteotomies, in the radiographic measurements, or in the ultimate strength of the sites of the osteotomy between the two groups. The mean level of serum insulin-like growth-factor I increased 33 per cent relative to the preoperative baseline level in the group that received growth hormone and increased 10 per cent in the control group. This difference was not statistically significant. There was no significant correlation between the biomechanical properties at the sites of the osteotomies and the levels of serum insulin-like growth-factor I. Administration of growth hormone had no measurable effect on fracture-healing in this model of normal animals. It remains to be determined whether injection of growth hormone might affect healing when there is a state of deficiency of endogenous growth hormone or when there is a non-union of a fracture.

Growth factors and articular cartilage

Studies of articular cartilage over the decades have demonstrated a surprisingly brisk rate of synthesis of the matrix proteins which appears to vary considerably with metabolic, physicochemical and pathological state of the tissue. It has become evident that much of this activity is directed by low molecular weight protein mediators which act at specific receptor sites. Platelet derived growth factor (PDGF) is of limited action in normal cartilage, but insulin and its analogues, insulin growth factor-I and II are powerful stimulants of DNA synthesis. Basic fibroblast growth factor stimulates both DNA and protein synthesis and works synergistically with other factors. Transforming growth factor beta potentiates the action of the mitogens and enhances and regulates proteoglycan synthesis. These actions may be of special importance in osteoarthritis and lacerative injury to cartilage.

Growth factor stimulation of adult articular cartilage

We have examined the effect of peptide growth factors on DNA and proteoglycan synthesis by adult bovine articular cartilage in organ culture. The actions of somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I), insulin, epidermal growth factor (EGF), and fibroblast growth factor (FGF) from bovine pituitary were investigated individually and in combination. FGF stimulated a 10-fold increase in tritiated thymidine incorporation while other factors used individually did not influence mitotic activity. Used in concert, insulin with EGF and insulin with FGF acted synergistically in stimulating DNA synthesis 20-fold and 40-fold, respectively. All of these growth factors, acting individually, significantly enhanced radiosulfate incorporation. This stimulation was additive for Sm-C/IGF-I in combination with EGF or FGF, but not with insulin. These data indicate that adult bovine articular chondrocytes possess the capacity to augment both mitotic and differentiated cell functions in response to growth factors. The data further suggest that, with the exception of insulin and Sm-C/IGF-I, which appear to share a common mechanism of action, these factors produce their cellular effects via different receptor or postreceptor pathways.

Effect of somatomedin-C/insulin-like growth factor I and growth hormone on cultured growth plate and articular chondrocytes

To determine whether growth hormone has a direct effect on skeletal tissues not mediated by somatomedins, and to better define the role of somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) in skeletal development, bovine growth plate, and rabbit articular and growth plate chondrocytes in primary culture were evaluated under a variety of experimental conditions designed to elicit growth hormone and Sm-C/IGF-I stimulation. Under none of these conditions did bovine growth plate chondrocytes respond to either homologous bovine growth hormone or heterologous hGH. Under the same conditions, these cells were highly responsive to human Sm-C/IGF-I with respect to both [3H]thymidine and [35S]sulfate incorporation, indices of mitotic and differentiated cell functions, respectively. Similarly, both rabbit articular and growth plate chondrocytes showed enhanced incorporation of [3H] thymidine and [35S]sulfate in the presence of Sm-C/IGF-I, but did not respond to either native or recombinant hGH. Cells at different stages of maturation within the bovine growth plate differed in their reaction to Sm-C/IGF-I with proliferative zone cells manifesting a greater response to the peptide than cells of the reserve zone. These results suggest that the action of Sm-C/IGF-I on growth plate and articular chondrocytes is direct and that the effect of GH on these cells is indirect. The data further suggest that within the growth plate, the transition from reserve to proliferative status is associated with an increased Sm-C/IGF-I responsiveness, a change which may contribute to the functional differences in these cells.

Demonstration of type I and type II somatomedin receptors on bovine growth plate chondrocytes

The chondrocytes of the epiphyseal growth plate are the presumed target cells for hormones regulating skeletal growth. The somatomedins, a family of low molecular weight peptides, are thought to play a stimulatory role in this regulation. The cellular actions of the somatomedins are themselves determined by binding to specific receptors on target cells. Previous studies have characterized a specific receptor for somatomedin-C (Sm-C) or insulin-like growth factor I (IGF-I) on bovine growth plate chondrocytes (GPCs). We now report the characterization of a second type of somatomedin receptor on these cells that is more specific for another class of somatomedin represented by multiplication-stimulating activity (MSA) or rat insulin-like growth factor II (rIGF-II). Binding of [125I]MSA/rIGF-II to isolated GPCs was time dependent and saturable. Unlabeled Mr 7,100 MSA/rIGF-II and Sm-C/IGF-I were approximately equipotent in competing with [125I]MSA/rIGF-II for binding, while Mr 8,600 MSA/rIGF-II was an order of magnitude less potent. Low levels of competition by insulin appeared in some studies at concentrations of 10(-7) M and higher, suggesting displacement of [125I]MSA/rIGF-II binding to the Sm-C/IGF-I receptor. In affinity-labeling studies, [125I]MSA/rIGF-I labeled a complex of Mr greater than 300,000 (unreduced) and of Mr 140,000 (reduced), consistent with a type I somatomedin receptor composed of disulfide-linked subunits. [125I]MSA/rIGF-II labeled a Mr 240,000 moiety (unreduced) and Mr 260,000 (reduced), consistent with a type II somatomedin receptor. Both affinity-labeling and kinetic data revealed cross-binding of MSA/rIGF-II and insulin with the type I receptor and of Sm-C/IGF-I with the type II receptor. In contrast, the type II receptor did not recognize insulin. These data suggest a complex pattern of graded specificity of these receptors for their ligands. These data are consistent with the hypothesis that IGF-II as well as Sm-C/IGF-I participate in the stimulation of skeletal growth.

Localization of somatomedin-C binding to bovine growth-plate chondrocytes in situ

The somatomedins are a family of low-molecular-weight peptides that are thought to mediate the stimulatory effect of growth hormone on skeletal growth. The cells that are directly responsible for skeletal growth are the chondrocytes of the epiphyseal growth plate, and these are the presumed skeletal target cells for somatomedin. As with other peptide growth factors, the cellular effects of the somatomedins are initiated by the interaction of the growth factor with specific receptors on target-cell surface membranes. Chondrocytes that have been isolated from bovine growth plates were previously found to possess specific surface receptors for the principal growth-hormone-dependent somatomedin, somatomedin-C. These studies indicated that the interaction of growth-plate chondrocytes with somatomedin-C involves specific receptor-binding followed by somatomedin-C internalization by the cell, a process identical to that identified in the mechanism of action of other peptide growth factors in other cells. These studies, however, left unanswered the questions of whether there are differences in binding of somatomedin-C by the different cell populations within the physis and whether somatomedin-C has access to cells in intact tissues. The current studies address these issues and indicate that bovine physeal chondrocytes in situ are accessible to exogenous somatomedin-C, that they specifically bind somatomedin-C in situ, and that cells of different physeal zones bind somatomedin-C differently. Labeled somatomedin-C is specifically bound by cells of all physeal zones. However, the binding is greatest for those cells undergoing active synthesis of DNA in the proliferative zone.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a specific somatomedin-c receptor on isolated bovine growth plate chondrocytes

The interaction of somatomedin (Sm) with growth plate chondrocytes (GPCs) is believed to be the primary stimulus of skeletal growth. Using techniques designed to disrupt as little as possible the phenotypic characteristics of GPCs, we have been able to obtain 3-4 x 10(8) viable cells from the major physes of one newborn calf. The availability of these cells plus essentially pure Sm-C/insulin-like growth factor I, the most GH-dependent Sm, has now made possible detailed studies of the interaction of this radiolabeled peptide with the GPC receptor and of the subsequent processing of this hormone by these cells. The enzymatic methods required to free GPCs from their matrix led to loss of receptors, followed by rapid receptor regeneration by de novo synthesis in suspension cultures. Binding of [125I]iodo-Sm-C to GPCs was time dependent and saturable, with optima at 15 C and pH 7.8. At 37 C, binding peaked at 90 min and declined thereafter. Multiplication-stimulating activity, insulin, and nerve growth factor were less potent than unlabeled Sm-C in competition with [125I]iodo-Sm-C for its receptor. Human GH, epidermal growth factor, and fibroblast growth factor failed to show competition even at 10(-6) M. Analysis of the fate of [125I]iodo-Sm-C bound to GPCs at 37 C provided evidence that this hormone is internalized and extruded from the cell in a partially degraded form. Scatchard analysis of [125I]iodo-Sm-C binding to GPCs and to chondrocytes isolated from articular cartilage revealed similar Ka values, but reproducibly 2-6 times more receptors on growth plate than on articular chondrocytes.

Characterization of chondrocytes from bovine articular cartilage: I. Metabolic and morphological experimental studies

Articular chondrocytes were enzymatically isolated from mature bovine cartilage and separated by density zone-velocity sedimentation in a gradient. For cells from different levels of the gradient, size and staining characteristics were determined and rates of incorporation of tritiated cytidine and radiosulphate were measured as indicators of RNA and sulphated glycosaminoglycan synthesis, respectively. The data clearly show that the chondrocyte population is composed of cells that vary continuously in size and metabolic activity from one limit to another. The largest cells also demonstrated the greatest RNA production while the smallest cells had the least. There was, however, no such differentiation of sulphated proteoglycan production.