Through its many ligand-binding functions, the insulin-like growth factor Il/mannose 6-phosphate receptor (IGF2R) is responsible for transporting mannose-6-phosphate (Man-6-P)-bearing lysosomal enzymes to their appropriate intracellular destination, for mediating uptake and subsequent degradation of the mitogen, insulin-like growth factor 11 (IGF-II), and for facilitating activation of the growth inhibitor, transforming growth factor-b (TGF-b). Each of these activities is consistent with the proposed role of the IGF2R as a tumor suppressor, yet the question of which ligand-binding functions of the IGF2R are responsible for the cell-growth suppressor activity have not been directly addressed. This project will test the hypothesis that the IGF2R's growth suppressor activity depends on the efficient operation of both its IGF-II and Man-6-F binding functions, by the following specific aims: 1) To measure the growth-suppressive effects of wild-type IGF2R vs. receptors mutated in the Man-6-P vs. IGF-II binding functions. IGF2R-deficient cell lines transfected with wild-type IGF2R cDNA expression constructs or IGF2R mutants defective in binding IGF-II or Man-6-P ligands will be analyzed for proliferation relative to vector-transfected controls. Our expectation is that increased wild-type IGF2R expression will inhibit cell growth, and that the mutants will show impairment of this growth-suppressive activity. 2) To determine the effects of cancer-associated M6P/IGF2R missense or truncation mutations on the growth-suppressive activity of the IGF2R. We expect that missense mutations will exhibit reductions in IGF2R function and that truncation mutants may have novel dominant-negative effects. 3) To assess the contribution to the receptor's growth-suppressive activity of pre- and post-receptor binding events that depend on IGF2R dimerization, i.e. development of high-affinity ligand binding and enhanced internalization. These studies will contribute to understanding the receptor's tumor suppressor function and permit rational design of strategies that exploit the IGF2R in cancer prevention and therapy.
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