Insulin-like growth factor I-dependent regulation of prolidase activity incultured human skin fibroblasts

Prolidase [E.C.3.4.13.9] is a cytosolic exopeptidase that catalyses the hyd rolysis of C-terminal proline containing dipeptides or tripeptides. The enz yme plays an important role in the recycling of proline for collagen synthe sis. Increase in enzyme activity is correlated with increased rates of coll agen turnover but the mechanism and endpoints by which this enzyme is regul ated remain largely unknown. We have found that insulin-like growth factor- I (IGF-I), potent stimulator of collagen biosynthesis, induces prolidase ac tivity in cultured human skin fibroblasts. Supporting evidence comes from t he following observations: (1) Serum of fasted rats, (IGF-I, 72 +/- 16 ng/m l) showed about 50% reduced ability to stimulate prolidase activity and col lagen biosynthesis in confluent fibroblasts in comparison to the effect of control rat serum (IGF-I, 168 +/- 29). (2) An addition of IGF-I (100 ng/ml) to fasted rat serum restored its ability to stimulate prolidase activity a nd collagen biosynthesis to control values. (3) In confluent human skin fib roblasts, cultured for 48 h with serum free medium prolidase activity was d ecreased to 50% of control cells, cultured in the presence of normal rat se rum. Supplementation of serum free medium with EGF, PDCF and IGF-I (factors that can replace growth promoting activity of serum) stimulated prolidase activity to control values while the medium deprived IGF-I had no such effe ct. (4) The relative differences in prolidase activity due to specific trea tment of confluent cells with above growth factors were accompanied by para llel differences in the amount of the enzyme protein recovered from these c ells as shown by western immunoblot analysis. Thus we conclude that prolida se activity is regulated by IGF-I in confluent fibroblasts.