EVIENCE THAT IGF 2 INTERFERES WITH THE LYSOSOMAL ENZYME ACTIVITIES OF CARTILAGE CELLS IN VITRO

Abstract

In cartilage cells as well as in other cell types, IGF2 is considered as a growth factor mainly mimicking the effect of IGF1 through IGF1-receptor. Since cartilage cells contain both types I and II IGF-receptors, it is still unknown wether IGF2 may have specific effects mediated through the IGF2-Mannose-6-Phosphate receptor (IGF2/M6P-R). This bifunctional protein also binds glycosylated proteins such as newly synthesized acid protease enzymes being responsible of their targeting from me Golgi to the lysosomes. Our purpose was to investigate the possibility for IGF2, by comparison with IGF1, to interact with the storage of chondrocyte lysosomal enzymes. Cultured chondrocytes from prepuberial, fetal or adult rabbits were used and their content of acid phosphatase. cathepsin B and L activities was quantified by using a colorimetric reaction with appropriate substrates. In basal conditions, the acid protease activities localized by histochemistry and electron microscopy, were observed in the RER. in the Golgi vesicles and in the lysosomes of the chondrocytes. The addition of IGF2 into the culture medium during 60 min., significantly reduced chondrocyte acid protease activities in a dose dependent manner with maximum effect at 10-9M. There was respectively 25%, 33% and 21% decrease of acid phosphatase, cathepsin B and L activities as compared with chondrocyte activities evaluated in basal conditions (100%). By contrast, the addition of [ 1-34]-Parathyroid hormone (PTH), significantly increased these protease activities in a dose and time dependency. The maximum effect of [1-34]-PTH was observed at 10-8M with 40%, 89% and 60% increased activities above the respective basal levels. IGF2 inhibited the PTH effect and still decreased the chondrocyte acid protease content below control values. Such an inhibiting effect was not observed with similar concentrations of IGF1. In addition, the number of chondrocyte IGF2/M6P specific binding sites was significantly increased in cells treated with PTH as compared with non treated ones. Such a stimulating effect was not observed in the presence of [3-34]-PTH nor [1-34N3-34] PTH mixture. Finaly, the number of IGF1 binding sites was not increased by [1-34]-PTH, These data are in favour of a new role for IGF2 in cartilage, concerning the degradation of chondrocyte proteins through the mediation of IGF2/M6P-R.