[Insulin-like growth factor II/mannose-6 phosphate receptors in a rat fibrosarcoma cell line]

Insulin-like growth factor II (IGF II)/mannose 6-phosphate (Man-6-P) receptor, which has two binding sites, was efficiently purified using a phospho-pentamannan affinity column. The molecular mass of the receptor purified from rat fibrosarcoma cells (KMT-17) was 240,000 (240K), whereas that of the receptor from normal rat liver and fibroblasts (NRK-49F) was 250K. However, when the receptor was subjected to deglycosylation by treatment with tunicamycin, the receptors of the KMT-17 and MRK-49F cells gave the same molecular mass of 225K, indicating that the difference in molecular mass between two cell lines resulted from the different oligosaccharide sizes. Analysis of the number of receptor site and binding affinity with IGF II on cell surface showed that KMT-17 cells expressed about 10 times more sites than in NRK-49F cells, whereas the binding affinity of KMT cells was lower than that of NRK-49F cells. Since it is known that asparagine-linked oligosaccharides in the IGF II/Man-6-P receptor are essential for IGF II binding, the types of oligosaccharides in the receptor were investigated by concanavalin A affinity chromatography. It was revealed that a ratio (38%) of tetra- and triantennary (multiantennary) complex-type oligosaccharides in the receptor of KMT-17 cells was lower than that (61%) in NRK-49F cells. These results indicate that the receptor of KMT-17 cells possesses predominantly less branched complex-type oligosaccharides and that multiantennary complex-type oligosaccharides of the receptor are most likely to contribute to higher affinity binding of IGF II. To determine whether IGF II activates phosphatidyl inositol-specific phospholipase C (PLC) in KMT-17 cells, the production of inositol trisphosphate (IP3) was measured. Treatment with IGF II increased the level of IP3 in a concentration-dependent manner. The level of IP3 reached the maximum after 15 seconds of incubation. When the cells were pretreated with anti-receptor antibody, no activation of the PLC was observed. These findings suggest that IGF II stimulates PLC through binding to the IGF II/Man-6-P receptor.