Cathepsin B as a markerofthededifferentiated chondrocyte phenotype

SUMMARY Rabbitarticular cartilage doesnotsecretecathepsin B inorgan culture. By established methods formodulating thechondrocyte phenotype invitro, however, thesynthesis, intracellular storage, andsecretion ofcathepsin B were followed up over aperiod oftwomonths. Withchondrocytes grown inmonolayer cultures boththeintracellular pooloftheenzyme andits secretion were verysmall initially, butincreased progressively toafactor of110after eight weeks. Thesecretion ofcathepsin B was strongly depressed after transferring thecells frommonolayer tocollagen gelcultures. Incontrast, collagenase was secreted inalmost thesame amountsduring thewholeperiod inbothmonolayer andcollagen gelcultures. Thecells cultured incollagen gels secreted more collagenase thanthose grown inmonolayers. Thereversible switch ofcathepsin B secretion suggests thatthis enzyme,unlike collagenase, isa markerofthededifferentiated chondrocyte phenotype. Cathepsin B waslocalised within cultured chondrocytes using antibodies raised against rabbit liver cathepsin B andshared withitmany catalytic properties. ItsMr, however, was higher (34000compared with27000)andshowed an unusual resistance to denaturation atneutral-alkaline pH,which may confer on this enzyme an important roleinthe degradation ofcartilage matrix.