Increased expression of CD44 in bovine articular chondrocytes by catabolic cellular mediators

Bovine articular chondrocytes cultured in alginate beads were used to study the effect of catabolic cellular mediators on CD44 expression. Treatment with either the 29-kDa fragment of fibronectin or interleukin-1 alpha results in a time- and dose-dependent inhibition of proteoglycan synthesis as well as a stimulation in the expression of CD44 mRNA level as determined by semi-quantitative polymerase chain reaction following reverse transcription. No noticeable effect at 6 h was observed. By 24 h, the major CD44 product (CD44H) from fibronectin fragment-treated cultures showed an 8-fold increase; CD44H from interleukin-1 alpha-treated cultures showed a 6-fold increase as compared to control cultures. In addition, a minor band, determined to be an isoform of CD44, was also shown to be up-regulated by both mediators. Stimulation of CD44 mRNA via interleukin-1 was also evident by in situ hybridization studies of bovine as well as human articular cartilage in organ culture. The increased in CD44 mRNA is matched by an increase at the protein level as determined by Western blot analysis. The Western blot reveals a doublet protein band at 80-90 kDa that corresponds to the molecular mass of CD44H. Cultures incubated with fibronectin fragments for 24 h had an 8.0-fold increase in CD44, while a 6.6-fold was observed for interleukin-1 alpha. Fluorescein-conjugated hyaluronan binding and internalization studies indicate that the increase in CD44 protein, induced by interleukin-1 alpha, closely correlates with an increase in functional hyaluronan receptors present at the chondrocyte cell surface. Taken together these results indicate that conditions that up-regulate chondrocyte catabolism also up-regulate the expression of CD44, a cell surface hyaluronan receptor involved in hyaluronan endocytosis.

Heparin modulates conformational states of plasma fibronectin: an electron spin resonance spin label approach

We have examined the interaction between heparin and human plasma fibronectin using electron spin resonance (ESR) spin label methods. The titratable sulfhydryl groups of plasma fibronectin were modified with a maleimide spin label [Lai and Tooney (1984) Arch. Biochem. Biophys. 228, 465-473]. Addition of heparin resulted in a decrease in the maximum splitting value of the ESR spectrum of spin-labeled fibronectin from 66.8 to 64.3 G, suggesting that heparin induces a conformational alteration of plasma fibronectin. This heparin effect was noticeable at a heparin-to-fibronectin ratio of 20 to 1 and reached a plateau at about 100 to 1. Other sulfated carbohydrates were tested; dextran sulfate was found to be as effective as heparin but chondroitin sulfates were ineffective. The results presented suggest that the binding of heparin changes the molecular conformation of plasma fibronectin to a more relaxed or flexible state.