We have developed a rapid, sensitive dot-blot assay to screen transfected cell lines for the production of insulin-like growth factor-II (IGF-II). This chemiluminescence dot-blot method detects picogram quantities of IGF-II which is 100 times more sensitive than conventional IGF-II radioimmunoassays and can be completed in 5-6 hrs. We have asked whether or not the IGF-I receptor utilizes the phosphatidylinositol pathway in MG63 human osteosarcoma cells by measuring levels of diacyglycerol (DAG) following addition of IGF-I. No increase in DAG over levels in unstimulated cells was seen at 2,5,10, and 30 min following addition of IGF-I. We conclude that the IGF-I receptor does not signal biologic responses to IGF-I by stimulating the hydrolysis of phosphatidylinositol or phosphatidylcholine. We have examined IGF-I receptor expression and function in fibroblasts from 2 patients with deletion of the distal long arm of chromosome 15. It had been proposed by others that the severe growth impairment seen in patients with 15 q deletion syndrome may be related to the loss of a single copy of the IGF-I receptor gene. Quantitative Southern blot analysis of Hind III digests of fibroblast DNA showed that the IGF-I receptor band was 41.3 plus/minus 13% and 43.5 plus/minus12% compared to the control fibroblasts, suggesting loss of one copy of the IGF-I receptor gene in the patients. Cell surface expression of the IGF-I receptor was assessed by binding of the IGF-I analog, 125I-long R3IGF-I to fibroblast monolayer cultures. Using the amount of tracer binding that was blocked by the IGF-I receptor monoclonal antibody, alphaIR-3, as a measure of binding to the IGF-I receptor, the patients fibroblasts showed significantly lower cell surface expression of the IGF-I receptor. To assess receptor function, stimulation of N-methyl-[14C]alpha-aminoisobutyric acid uptake by a full range of IGF-I concentrations was examined in serum-starved cultures. There were no significant differences between the patient and control fibroblasts. We conclude that although there is evidence for decreased cell surface expression of the IGF-I receptor in fibroblasts that are missing one copy of the IGF-I receptor gene, the stimulation of amino acid uptake by IGF-I is not impaired.
