Serum factors, growth factors and UDP-sugar metabolism in bovine articular cartilage chondrocytes

Effects of physical stimulation with electromagnetic field and insulin growth factor-I treatment on proteoglycan synthesis of bovine articular cartilage

OBJECTIVE

To investigate the single and combined effects of electromagnetic field (EMF) exposure and the insulin growth factor-I (IGF-I) on proteoglycan (PG) synthesis of bovine articular cartilage explants and chondrocytes cultured in monolayers.

DESIGN

Bovine articular cartilage explants and chondrocyte monolayers were exposed to EMF (75Hz; 1.5mT) for 24h in the absence and in the presence of both 10% fetal bovine serum (FBS) and IGF-I (1-100ng/ml). PG synthesis was determined by Na(2)-(35)SO(4) incorporation. PG release into culture medium was determined by the dimethylmethylene blue (DMMB) assay.

RESULTS

In cartilage explants, EMF significantly increased (35)S-sulfate incorporation both in the absence and in the presence of 10% FBS. Similarly, IGF-I increased (35)S-sulfate incorporation in a dose-dependent manner both in 0% and 10% FBS. At all doses of IGF-I, the combined effects of the two stimuli resulted additive. No effect was observed on medium PG release. Also in chondrocyte monolayers, IGF-I stimulated (35)S-sulfate incorporation in a dose-dependent manner, both in 0% and 10% FBS, however, this was not modified by EMF exposure.

CONCLUSIONS

The results of this study show that EMF can act in concert with IGF-I in stimulating PG synthesis in bovine articular cartilage explants. As this effect is not maintained in chondrocyte monolayers, the native cell-matrix interactions in the tissue may be fundamental in driving the EMF effects. These data suggest that in vivo the combination of both EMF and IGF may exert a more chondroprotective effect than either treatment alone on articular cartilage.

Age-related changes in the response of human articular cartilage to IL-1alpha and transforming growth factor-beta (TGF-beta): chondrocytes exhibit a diminished sensitivity to TGF-beta

Cartilage glycosaminoglycan (GAG) synthesis and composition, upon which its structural integrity depends, varies with age, is modified by anabolic and catabolic stimuli, and is regulated by UDP-glucuronate availability. However, how such stimuli, prototypically represented by transforming growth factor-beta1 (TGF-beta1) and IL-1alpha, modify GAG synthesis during aging of normal human articular cartilage is not known. Using explants, we show that chondroitin sulfate (CS):total GAG ratios decrease, whereas C6S:C4S ratios increase with cartilage maturation, and that chondrocytes in the cartilage mid-zone, but not the superficial or deep zones, exhibit uridine 5'-diphosphoglucose dehydrogenase (UDPGD) activity, which is also increased in mature cartilage. We also show that IL-1alpha treatment reduces both total GAG and CS synthesis, decreases C6S:C4S ratios (less C6S), but fails to modify chondrocyte UDPGD activity at all ages. On the other hand, TGF-beta1 increases total GAG synthesis in immature, but not mature, cartilage (stimulates CS but not non-CS), age-independently decreases C6S:C4S (more C4S), and increases chondrocyte UDPGD activity in a manner inversely correlated with age. Our findings show that TGF-beta1, but not IL-1alpha, modifies matrix synthesis such that its composition more closely resembles "less mature" articular cartilage. These effects of TGF-beta1, which appear to be restricted to periods of skeletal immaturity, are closely associated although not necessarily mechanistically linked with increases in chondrocyte UDPGD activity. The antianabolic effects of IL-1alpha are, on the other hand, likely to be independent of any direct modification in UDPGD activity and manifest equally in human cartilage of all ages.

Molecular cloning and characterization of the human and mouse UDP-glucose dehydrogenase genes

The enzyme UDP-glucose dehydrogenase (Udpgdh) (EC 1.1.1.22) converts UDP-glucose to UDP-glucuronate, a critical component of the glycosaminoglycans, hyaluronan, chondroitin sulfate, and heparan sulfate. Although Udpgdh is a comparatively well characterized enzyme, no vertebrate genes encoding this enzyme have been reported to date. We report the cloning and characterization of the human and mouse UDP-glucose dehydrogenase genes. Mouse and human cDNAs predicted proteins of 493 and 494 amino acids, 24-25 residues longer at their carboxyl termini than the previously reported bovine Udpgdh sequence. The mouse Ugdh gene is composed of 10 exons, spanning 15 kilobases. Northern analyses indicated widespread expression of the gene in embryo and adult. Through interspecific backcross analyses, we localized the Ugdh gene to mouse chromosome 5 at approximately 39 centimorgans, suggesting that the human UGDH gene is localized to chromosome 4p13-15. Results from Southern analyses strongly suggest that Udpgdh is encoded by a single gene in the mouse. Transfection of mouse Ugdh expression vectors led to an increase in detectable Udpgdh activity in mammalian cells. Preliminary expression studies indicated that proinflammatory cytokines, such as interleukin 1beta, can substantially increase the expression of human UGDH in cultured human fibroblasts, suggesting that glycosaminoglycan biosynthesis may be partly regulated by the availability of activated UDP-glucuronate, as determined by relative Udpgdh expression levels.

Differential effects of Staphylococcus aureus alpha-hemolysin on the synthesis of hyaluronan and chondroitin sulfate by rat chondrosarcoma chondrocytes

The plasma membranes of rat chondrosarcoma chondrocytes were permeabilized by treatment with alpha-hemolysin, the major toxin produced by Staphylococcus aureus, which forms small, stable, heptameric, transmembrane pores (1-2 nm in diameter) permitting influx/efflux of low-molecular-mass molecules (< or = 2000 Da). Treated chondrocytes were permeable to entry of trypan blue and exit of ATP. We describe the effects of alpha-hemolysin on the synthesis of hyaluronan (HA) and chondroitin sulfate (CS) by chondrocytes using the simple sugar [3H]glucosamine as a metabolic precursor. Chondrocytes permeabilized with alpha-hemolysin in serum-free media decreased intracellular ATP and synthesis of CS to approximately 5% of control within 2-4 h, but synthesized HA (80% of control for 8 h; approximately 65% of control at 24 h). Adding fresh medium (with or without serum) to permeabilized cells increased ATP significantly and increased HA synthesis to near initial control values. Under the same conditions, the recovery of CS synthesis approached initial levels in control but not permeabilized cells. Our model demonstrates that the biosynthesis of HA by these cells in vitro is remarkably stable to cellular perturbations which drastically inhibit synthesis of CS on proteoglycans.

Chondrocyte cytokine and growth factor expression in murine osteoarthritis

Eighty-five percent of male STR/ort mice develop osteoarthritic lesions of the knee joint by 35 weeks of age. We have developed a non-radioactive in-situ hybridization method using digoxigenin-labeled oligonucleotide probes to study the expression of the cytokines interleukin (IL) 1 alpha, Il-1 beta and IL-6 and the growth factors insulin-like growth factor-1 (IGF-1) and transforming growth factor beta (TGF beta 1) during the development of osteoarthritis (OA) in this model. Age- and sex-matched CBA mice, which do not develop OA, showed no detectable expression of any of the cytokines or growth factors studied. In contrast, 20-week-old STR/ort mice with no OA lesions showed positive expression [positive: (+)] for all the cytokines and growth factors studied. At 35 weeks of age, STR/ort mice with varying grades of OA showed positive (+) or strong (++) signals for both cytokines and growth factors throughout the tibial articular cartilage. The strongest signal was seen in areas where OA lesions were present. In areas of histologically-normal cartilage adjacent to the lesions, the signals were still positive but weaker. Fifty-week-old STR/ort mice with OA lesions showed a similar pattern of expression to 35-week-old mice. Thirty-five or 50-week-old STR/ort mice with no OA lesions had much reduced expression compared with those with OA lesions. These mice may be indicative of those STR/ort mice which do not develop OA. The results seen in the STR/ort together with previous biochemical analyses are consistent with an up-regulation of anabolic growth factors and catabolic cytokines in the prelesional stages of OA with anabolic effects predominating. At later stages of OA, the effects of catabolic factors appear to predominate and osteoarthritic lesions become evident.