INHIBITION OF CHONDROCYTE CATHEPSIN-B AND CATHEPSIN-L ACTIVITIES BY INSULIN-LIKE GROWTH FACTOR-II (IGF-II) AND ITS SER(29) VARIANT IN-VITRO- POSSIBLE ROLE OF THE MANNOSE 6-PHOSPHATE IGF-II RECEPTOR

Lysosomal enzymes and IGF-II both bind to the mannose 6-phosphate (M6P )/IGF-II receptor. This receptor targets newly synthesized lysosomal e nzymes to lysosomes. The functional meaning of IGF-II binding to this receptor is not well known. We have Postulated that IGF-II may affect the targeting of lysosomal enzymes in cartilage remodeling. We therefo re examined the effect of IGF-II, the Ser(29) IGF-II variant (vIGF-II) and IGF-I on lysosomal cathepsin B and L activities from post-natal r abbit chondrocytes in vitro. This effect was compared with the ability of each peptide to stimulate chondrocyte-sulfated proteoglycan synthe sis. The sulfating dose-response relationship of the IGF peptides corr esponded to their relative binding affinities for the type I-IGF recep tor (IGF-I > IGF-II > vIGF-II). The intracellular cathepsin B and L ac tivities were inhibited in a time- and dose-dependent manner by IGF-II or vIGF-II. Maximal inhibition of cathepsin B and L activities (40 an d 30% below controls, respectively) was found after an 8 h treatment w ith 100 ng/ml IGF-II or vIGF-II. By contrast, IGF-I up to 1 mu g/ml or insulin up to 2 mu g/ml had no inhibitory effect. The relative potenc y pattern corresponded to the binding profile of each ligand for the M 6P/IGF-II receptor. A treatment of chondrocytes with IGF-I or insulin transiently increased the binding of radiolabelled IGF-II at the cell surface to similar to 120% of controls, whereas IGF-II or vIGF-II had no effect. Thus, it is unlikely that the inhibition of lysosomal enzym e activities by IGF-II peptides could result from a redistribution of M6P/IGF-II receptors from intracellular compartments to the plasma mem brane. We hypothesize that internalized IGF-II peptides could occupy t he intracellular M6P/IGF-II binding sites required for the targeting o f cathepsins B and L to lysosomes.