Inhibition of endosomal insulin-like growth factor-I processing by cysteine proteinase inhibitors blocks receptor-mediated functions

The receptor for the type 1 insulin-like growth factor (IGF-D has been impl icated in cellular transformation and the acquisition of an invasive/metast atic phenotype in various tumors. Following ligand binding, the IGF-I recep tor is internalized, and the receptor ligand complex dissociates as the lig and is degraded by endosomal proteinases, In the present study we show that the inhibition of endosomal IGF-I-degrading enzymes in human breast and mu rine lung carcinoma cells by the cysteine proteinase inhibitors, E-64 and C A074-methyl ester, profoundly altered receptor trafficking and signaling. I n treated cells, intracellular ligand degradation was blocked, and although the receptor and two substrates, Shc and Insulin receptor substrate, mere hyperphosphorylated on tyrosine, IGF-I-induced DNA synthesis, anchorage-ind ependent growth, and matrix metalloproteinase synthesis were inhibited. The results suggest that ligand processing by endosomal proteinases is a key s tep in receptor signaling and function and a potential target for therapy.