We have recently identified in serum an acid protease which is capable of generating des(1-3)IGF-I from intact IGF-I. Here we have utilized a synthetic 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 activity in streptozotocin-diabetic rats. Protease activity, quantified in terms of the amount of the biotin label lost, was determined in serum and hepatic extracts from normal control rats, diabetic rats and insulin-treated diabetic rats. Both the serum protease activity and protease activity in hepatic extracts were significantly increased in diabetic rats compared with control 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 approximately 50% reduction apparent at 30 min (P<0.001). Chronic insulin treatment of diabetic rats also significantly reduced the serum and hepatic protease activity to the levels seen in control rats. A positive correlation 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, capable of inhibiting the IGF-I protease activity in vitro, was significantly decreased 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 IGF-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.
H Yamamoto, C Maake and LJ Murphy
K Rajkumar, T Modric and LJ Murphy
Differentiation of precursor cells into mature fat cells is accompanied by enhanced expression of insulin-like growth factor (IGF)-I and is stimulated by multiple hormones including growth hormone, glucocorticoids, IGF-I and insulin. We used transgenic mice that overexpress insulin-like growth factor binding protein-1 to investigate the role of IGF-I in the accumulation of fat tissue. In response to a sucrose-enriched diet, transgenic mice gained significantly less body weight and the epididymal fat mass was significantly reduced compared with wild-type mice. The increase in adipocyte size was also significantly reduced in transgenic mice compared with wild-type mice. Fewer colonies were generated from adipose tissue from transgenic mice and the mitogenic response of these cells to IGF-I was significantly reduced compared with those from wild-type mice. Induction of glycerol-3-phosphate dehydrogenase, a measure of adipocyte differentiation, by IGF-I but not insulin, was reduced in preadipocytes from transgenic mice. These data indicate that IGF-I has a critical role in the proliferation of adipocyte precursors, the differentiation of preadipocytes and the development of obesity in response to calorie excess.
ST Dheen, K Rajkumar and LJ Murphy
Transgenic mice which overexpress insulin-like growth factor binding protein-1 (IGFPB-1) demonstrate fasting hyperglycemia, hyperinsulinemia and glucose intolerance in adult life. Here we have examined the ontogeny of pancreatic endocrine dysfunction and investigated islet cell proliferation and apoptosis in this mouse model. In addition we have examined pancreatic insulin content in transgenic mice derived from blastocyst transfer into non-transgenic mice. Transgenic mice were normoglycemic at birth but had markedly elevated plasma insulin levels, 56.2 +/- 4.5 versus 25.4 +/- 1.5 pmol/l, p < 0.001, and pancreatic insulin concentration, 60.5 +/- 2.5 versus 49.0 +/- 2.6 ng/mg of tissue, P < 0.01, compared with wild-type mice. Transgenic mice derived from blastocyst transfer to wild-type foster mothers had an elevated pancreatic insulin content similar to that seen in pups from transgenic mice. There was an age-related decline in pancreatic insulin content and plasma insulin levels and an increase in fasting blood glucose concentrations, such that adult transgenic mice had significantly less pancreatic insulin than wild-type mice. Pancreatic islet number and the size of mature islets were increased in transgenic animals at birth compared with wild-type mice. Both islet cell proliferation, measured by 5-bromo-2'-deoxyuridine labeling, and apoptosis, assessed by the in situ terminal deoxynucleotidyl transferase and nick translation assay, were increased in islets of newborn transgenic mice compared with wild-type mice. In adult mice both islet cell proliferation and apoptosis were low and similar in transgenic and wild-type mice. Islets remained significantly larger and more numerous in adult transgenic mice despite a reduction in pancreatic insulin content. These data suggest that overexpression of IGFBP-1, either directly or indirectly via local or systemic mechanisms, has a positive trophic effect on islet development.