Lipid storage in cultured articular chondrocytes due to prostanoid precursors and a prostanoid synthesis inhibitor

PPARgamma2 expression in growth plate chondrocytes is regulated by p38 and GSK-3

Although peroxisome proliferator activated receptor (PPAR)γ remains a critical regulator of preadipocyte differentiation, new roles have been discovered in inflammation, bone morphogenesis, endothelial function, cancer, longevity and atherosclerosis. Despite the demonstration of PPARγ expression in chondrocytes, its role and the pathways affecting its expression and activity in chondrocytes remain largely unknown. We investigated the effects of PPARγ activation on chondrocyte differentiation and its participation in chondrocyte lipid metabolism. PPARγ2 expression is highly regulated during chondrocyte differentiation in vivo and in vitro PPARγ activation with troglitazone resulted in increased Indian hedgehog expression and reduced collagen X expression, confirming previously described roles in the inhibition of differentiation. However, the major effect of PPARγ2 in chondrocytes appears to be on lipid metabolism. During differentiation chondrocytes increase expression of the lipid-associated metabolizing protein, Lpl, which is accompanied by increased gene expression of PPARγ. PPARγ expression is suppressed by p38 activity, but requires GSK-3 activity. Furthermore, Lpl expression is regulated by p38 and GSK-3 signalling. This is the first study demonstrating a relationship between PPARγ2 expression and chondrocyte lipid metabolism and its regulation by p38 and GSK-3 signalling.

The association of lipid abnormalities with tissue pathology in human osteoarthritic articular cartilage

Articular cartilage is one of very few body tissues uniquely characterized as having substantial stores of lipid deposits. Lipid droplets are naturally accumulated by chondrocytes and individual fatty acids have been shown to have protective as well as deleterious effects on cartilage degradation in animal models of degenerative joint disease. As a means to better assess the role of lipids in human joint pathology, a comparative analysis of fatty acids was undertaken in small segments of osteoarthritic articular cartilage. The data were assessed in terms of chondrocyte synthetic activity and histological determination of disease severity. The distribution profile of individual fatty acids in normal and osteoarthritic specimens remained constant, with palmitic, oleic, and linoleic acids representing 85% of the total fatty acids. In contrast, levels of total fatty acids were markedly increased in association with increasing degree of lesion severity. Compared with tissue from normal-aged joints, grade 0 to 1 mild lesions had elevated levels of total fatty acids, essential fatty acids, and chondrocyte synthetic activity of 80%, 312%, and 393%, respectively. More severe tissue involvement (grade 6 to 9), was associated with even greater increases of 440%, 1,100%, and 1,150%, respectively. No change was noted in cholesterol content in any tissue. The accumulation of arachidonic acid was greater than the proportional increase in total fatty acid content and was primarily distributed into the neutral lipid fraction, where it constituted almost 62% of the fatty acid level in tissues of moderate lesion severity. There was an association of lipid accumulation in general and arachidonic acid in particular with histological severity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of extracellular fatty acids on lipid metabolism in cultured rabbit articular chondrocytes

Rabbit articular chondrocytes were cultured for 8 h in the presence of various concentrations (5-500 microM) of [14C] oleic, [14C] linoleic, and [3H] arachidonic acids. The radioactive unsaturated fatty acids were incorporated into triacylglycerol (TG) and phosphatidylcholine (PC) in a concentration-dependent manner; more fatty acids were incorporated into TG than into PC, at higher concentrations of extracellular fatty acids. Among these fatty acids, arachidonic acid was incorporated into TG much more than into PC, in spite of a very low concentration of arachidonic acid in TG. After transfer of the labeled cells to maintenance medium, the radioactivity in TG declined rapidly and [3H] arachidonic acid radioactivity in PC increased continuously during the chase time periods. Palmitoyl-unsaturated species were mainly formed in PC when cultured at a concentration of 5 microM of each fatty acid. However, when cultured at 500 microM, unsaturated-unsaturated species, specific for each unsaturated fatty acid were actively formed. These findings indicate that (1) fatty acid composition of TG and PC in articular chondrocytes is influenced by the degree of fatty acid supply, (2) formation and turnover of TG plays a role in fatty acid metabolism of cells, and (3) fatty acid pairing in PC is modulated by extracellular fatty acid concentrations.

Metabolic and ultrastructural changes in articular cartilage of rats fed dietary supplements of omega-3 fatty acids

A "marginally deficient" essential fatty acid state was produced in male Sprague-Dawley rats by dietary supplementation with omega 3 fatty acids. Animals fed diets containing the highest amounts of these fatty acids (10% menhaden fish oil) demonstrated a 70% maximum decrease in the linoleic and arachidonic acid content of articular cartilage, a 30-40% decrease in cartilage hexosamine content, with little effect on hydroxyproline levels, and a 32% inhibition of proteoglycan synthesis. Histologic analysis revealed an occasional surface irregularity and localized depletion of Safranin O and toluidine blue staining of articular cartilage on the femoral heads from animals taking the higher doses. Electron microscopic analysis revealed a marked decrease in "dark-staining" chondrocytes relative to "light-staining" cells in all animals fed menhaden fish oil. The cartilaginous changes noted in this study reflect a causal relationship between chondrocyte metabolism and an altered unsaturated fatty acid content. The observed responses of chondrocytes to omega 3 fatty acids may be similar to those commonly associated with the development of early osteoarthrosis. It is not known whether similar changes are induced in other species, including humans, but these observations suggest that some caution must be taken in the long-term administration of menhaden fish oil or other omega 3 fatty acid-containing preparations in rheumatoid arthritis patients.

Alterations in articular chondrocyte growth and proteoglycan synthesis due to prostanoid precursors

The effects of the prostaglandin precursors, dihomo-gamma-linolenic acid and arachidonic acid, on chondrocyte proliferation, proteoglycan synthesis and morphological structure were studied using lapine articular chondrocytes in vitro. Neither substance exerted a cytotoxic effect on chondrocytes. Dihomo-gamma-linolenic acid caused a dose-dependent inhibition of chondrocyte proliferation (8% and 35% reduction at 10 and 100 mumol/l respectively) (P less than 0.01), whereas arachidonic acid failed to cause any significant alteration: 10 mumol/l of dihomo-gamma-linolenic acid stimulated proteoglycan synthesis by 14% (P less than 0.01), whilst 100 mumol/l elicited a reduction of 14% (P less than 0.01); 100 mumol/l of arachidonic acid also caused a statistically significant inhibition (31%) (P less than 0.001) of 35SO4 incorporation into proteoglycans. The inhibitory effects on proteoglycan synthesis may be mediated by intracellularly synthesized prostaglandins, which are known to exert this effect. This may also help explain the susceptibility of articular cartilage to damage with increasing age, as arachidonic acid is found in increasing concentrations in the superficial layers of articular cartilage.