FUNCTIONAL ACTIVATION OF MUTANT HUMAN INSULIN-RECEPTOR BY MONOCLONAL-ANTIBODY

Background A mutant insulin receptor, Ser323Leu, has been reported in two severely insulin-resistant patients with Rabson-Mendenhall syndrom e. In both cases, extreme be controlled by conventional Ser323Leu muta nt insulin receptor is inserted normally in the plasma membrane but ha s very low binding affinity for insulin. A monoclonal antibody directe d against the extracellular domain of the insulin receptor (83.14) can mimic the natural ligand as far as the first step after ligand bindin g-autophosphorylation of the intracellular domain of the receptor. We have investigated whether antibody binding can imitate autophosphoryla tion of the Ser323Leu mutant receptor and lead to metabolic events wit hin the cell. Methods The effects of insulin and the insulin-receptor monoclonal antibody on receptor autophosphorylation and glycogen synth esis were compared in Chinese hamster ovary cells expressing the wild- type human insulin receptor, mock-transfected cells, cells expressing an insulin-receptor mutant without autophosphorylation capacity, and c ells expressing the Ser323Leu mutant receptor. Findings Cells expressi ng the Ser323Leu mutant receptor had very low specific insulin binding and, expressing wild-type insulin receptors, did autophosphorylation or stimulation of glycogen synthesis in response to insulin. However, exposure of cells expressing the Ser323Leu mutant receptor to monoclon al antibody 83.14 resulted in autophosphorylation and stimulation of g lycogen synthesis similar to that seen in cells expressing wild-type i nsulin receptors. Interpretation Although insulin does not bind to cel ls expressing the Ser323Leu mutation, insulin signalling can be mimick ed by exposure of the cells to an antibody to the extracellular domain of the insulin receptor. Activation by monoclonal antibodies of mutan t transmembrane receptors that show normal cell-surface expression but defective ligand binding may provide an approach to the therapy of so me subtypes of inherited hormone resistance for which little effective treatment is available.