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. T reatment with either the 29-kDa fragment of fibronectin or interleukin -1 alpha results in a time- and dose-dependent inhibition of proteogly can synthesis as well as a stimulation in the expression of CD44 mRNA level as determined by semiquantitative polymerase chain reaction foll owing reverse transcription, No noticeable effect at 6 h was observed, By 24 h, the major CD44 product (CD44H) from fibronectin fragment-tre ated cultures showed an 8-fold increase; CD44H from interleukin-1 alph a-treated cultures showed a B-fold increase as compared to control cul tures, In addition, a minor band, determined to be an isoform of CD44, was also shown to be up-regulated by both mediators. Stimulation of C D44 mRNA via interleukin-1 was also evident by in situ hybridization s tudies 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 i nterleukin-1 alpha. Fluorescein conjugated hyaluronan binding and inte rnalization studies indicate that the increase in CD44 protein, induce d by interleukin-1 alpha, closely correlates with an increase in funct ional hyaluronan receptors present at the chondrocyte cell surface, Ta ken together these results indicate that conditions that up-regulate c hondrocyte catabolism also up-regulate the expression of CD44, a cell surface hyaluronan receptor involved in hyaluronan endocytosis.