Enhanced proteolytic activity directed against the N-terminal of IGF-I in diabetic rats

We have recently identified in serum an acid protease which is capable of g enerating des(1-3)IGF-I from intact IGF-I. Here we have utilized a syntheti c substrate with the sequence, biotin-G-P-E-T-L-C-BSA which contains the N- terminal sequence of IGF-I, to investigate the levels of this protease acti vity in streptozotocin-diabetic rats. Protease activity, quantified in term s of the amount of the biotin label lost, was determined in serum and hepat ic extracts from normal control rats, diabetic rats and insulin-treated dia betic rats. Both the serum protease activity and protease activity in hepat ic extracts were significantly increased in diabetic rats compared with con trol rats (P<0.02 and P<0.005). Following acute administration of insulin, a rapid and marked reduction in serum protease activity was observed; with an similar to 50% reduction apparent at 30 min (P<0.001). Chronic insulin t reatment of diabetic rats also significantly reduced the serum and hepatic protease activity to the levels seen in control rats. A positive correlatio n between protease activity and serum glucose level was observed (r=0.58, P <0.005). The abundance of Spi 2.1 mRNA, a serine protease inhibitor, capabl e of inhibiting the IGF-I protease activity in vitro, was significantly dec reased in the liver of diabetic rats and insulin treatment of diabetic rats did not normalize Spi 2.1 mRNA levels. These data suggest that the conversion of TGF-I to the more active des(1-3) IGF-I variant may be enhanced in diabetic animals. Since serum IGF-I levels are reduced in diabetic rats, increased des(1-3)IGF-I-generating protease activity would enhance the functional activity of the circulating IGF-I.